Strong correlations and angle-resolved photoemission

A family of Kagome metals AV3Sb5 (A= Cs, K, Rb) has gained significant attention due to its unique properties of charge density wave, superconductivity and electronic structure, characterized by flat bands, Dirac points, and saddle points [1, 2]. These features give rise to a diverse range of exotic quantum phenomena [3]. We have investigated the changes of the geometric and electronic structure at low temperature in the Nb-doped CsV3Sb5 Kagome metal across the transition to a charge density wave (CDW) state. With Nb doping, the CDW transition temperature TCDW is suppressed, while the superconducting transition temperature Tc increases [4]. X-ray photoelectron diffraction measurements reveal the emergence of a chiral atomic structure in the CDW phase, while circularly polarized X-ray photoemission indicates a pronounced non-trivial circular dichroism in the angular distribution of valence band photoemission [5]. I will discuss the atomic structure of Nb-doped CsV3Sb5 across the charge density wave transition as determined from the X-ray photoemission diffraction pattern comparison to simulated patterns obtained with the Bloch wave approach. Reference: [1] Stephen D. Wilson, Brenden R. Ortiz, AV3Sb5 kagome superconductors, Nature Reviews Materials 9, 420–432 (2024) [2] Brenden R. Ortiz et al., New kagome prototype materials: discovery of KV3Sb5, RbV3Sb5, and CsV3Sb5, Phys. Rev. Materials 3, 094407 (2019) [3] Yong Hu et al., Electronic landscape of kagome superconductors AV3Sb5 (A = K, Rb, Cs) from angle-resolved photoemission spectroscopy, npj Quantum Materials 8, 67 (2023) [4] Yongkai Li et al., Tuning the competition between superconductivity and charge order in the kagome superconductor Cs(V1-xNbx)3Sb5, Physical Review B 105, L180507 (2022) [5] H. J. Elmers et al., Chirality in the Kagome Metal CsV3Sb5, arXiv: 2408.03750v1 (2024)

The question of whether electron-phonon coupling leads to superconducting pairing has been a topic of considerable debate in condensed matter physics. Various t-J models have been proposed to explain the phenomenon of high temperature superconductivity without addressing the electron phonon interaction mechanisms. In this study, we decided to the superconductivity in layered square-lattice bilayer system and to revise the problem of possible superconductivity in such a prototype system by considering the intralayer electron-phonon coupling mechanism. The Hamiltonian in our model consists of two parts: the non-interacting part, that includes the free phonon Hamiltonian, nearest-neighbors and next-nearest neighbors hopping terms, and interlayer hopping term; and an interacting part encompassing local intralayer and interlayer Hubbard interactions, intralayer electron-phonon coupling, and coupling with an external interlayer electric field potential. Our starting point is the discretization of elementary single particle excitations as a product of two simultaneous quasiparticle excitations with the same energy. Additionally, we considered possible excitonic states between the layers of the metallic bilayer. By applying functional field integration techniques, we derive the effective electron-phonon interaction action and obtain a set of self-consistent equations for the spin-triplet superconducting gap, excitonic order parameter, chemical potential, and average charge-density imbalance between the layers. We numerically solve this system using finite difference approximation techniques employing the Newton’s fast convergence algorithm. The temperature dependence of aforementioned physical quantities is investigated across different electron doping regimes. Furthermore, we plot the superconducting critical phase transition diagram as a function of electron doping. In our case, the undoped system corresponds to the Anderson localization limit with fully occupied bands. Moreover, the superconducting gap is plotted for different limits of the electron-phonon interaction parameter within the half-filling regime. We demonstrate the coexistence of superconducting and excitonic states over a wide range of temperatures and doping levels. At the fixed temperature, we calculated numerically the excitonic and superconducting order parameters as a function of electron and hole doping in the system. Notably, at hole doping the excitonic order parameter enhances, whereas for electron doping, the superconducting order parameter is much larger and persists over a broader interval of doping. We also show that for both electron and hole doping, charge imbalance is maximized at the half-filling regime, while charge neutrality is achieved in the Anderson localization limit and at very high doping levels. In the limit of low electron-phonon coupling, the superconducting transition temperature is on order of $T_C$=25.76 K, while in the strong coupling limit, we obtain a transition temperature as high as room temperature, on the order of $T_C$=287.3 K. Moreover, we identify a range of values for the next-nearest neighbor hopping parameter, within which the superconducting gap is sustained. The proposed model may serve as a new mechanism for superconductivity in high-$T_C$ cuprate superconductors due to intralayer electron-phonon coupling.

The $Eu(Ga_{1-x}Al_{x})_4$ system, crystallizing in the tetragonal space group $I4/mmm$, has attracted considerable attention due to its remarkable magnetic properties. While $\mathrm{EuAl}_4$ exhibits intriguing magnetic phenomena, including multiple magnetic skyrmion phases and spontaneous helicity reversal, the microscopic mechanism underlying its helical magnetic order remains elusive. Here, we systematically investigated the three-dimensional electronic structure of the $Eu(Ga_{1-x}Al_{x})_4$ system using soft X-ray angle-resolved photoemission spectroscopy (SX-ARPES). Our findings reveal composition-dependent changes in the Fermi surface topology, suggesting its crucial role in governing the emergence of helical magnetism in this system.

We are setting up a new user facility for time-resolved terahertz (THz) pump – photoemission probe spectroscopy at TELBE at Helmoltz-Zentrum Dresden Rossendorf (HZDR). The unique combination of high-field terahertz excitation and the tr-ARPES probe will allow groundbreaking experiments on solid surfaces and interfaces. THz pumping allows for selective excitation of low energy fundamental modes such as lattice vibrations or magnons, as well as free carriers, while avoiding parasitic higher energy excitation of the electronic system. The direct access to the electronic band structure of a THz-driven solid E(k) provided by tr-ARPES allows an unprecedented view on the dynamics of emergent phenomena in complex solids or molecular interfaces, which is particularly interesting for collective phenomena, e.g. oscillations of the order parameter manifesting in Higgs or CDW modes. The tr-ARPES consists of a UHV system and a PHOIBOS 150 hemispherical electron analyzer equipped with a fast 2D delay line detector. A high harmonic generation based extreme ultraviolet (XUV) light source with 100 kHz repetition rate is driven by a >350 W average power sub-picosecond Yb:YAG InnoSlab amplifier in combination with a spectrally tunable optical parametric chirped pulse amplifier (OPCPA). The laser system provides a wide range of simultaneous outputs: (i) a 1030 nm wavelength output with 2 mJ pulse energy and 600 fs pulse duration for generating high average power THz radiation. A Multi Pass Cell (MPC) enables tunable pulse durations as low as 70 fs with 90% transmission. THz radiation is then generated using the tilted pulse-front scheme in lithium niobate. The THz output can be characterized as a function of input pulse duration, bandwith and chirp. (ii) The XUV output driven by the second harmonic of the OPCPA, provides XUV harmonics at 21.7 eV at a photon flux > 1×1013 photons/sec at the source. This unique combination of tunable outputs will enable new types of experiments combining extreme photon energy ranges in a table-top experiment. Further, the planned integration into the accelerator-based TELBE THz facility will allow the utilization of a broad range of THz parameters, such as bandwidth and central frequency.

We present the evolution of the electronic structure of CrSBr from its antiferromagnetic ground state to the paramagnetic phase above TN = 132 K, in both experiment and theory. The ground state angle-resolved photoemission spectroscopy (ARPES) results, obtained using a novel method to overcome sample charging issues, are very well reproduced by our QSGW calculations including Bethe-Salpeter Equations (BSE) self-consistently. By tracing band positions as a function of temperature, we identify certain bands at the X points to be exchange-split pairs of states with mainly Br and S character, with the splitting disappearing above TN. Our results lay firm foundations for the interpretation of the many other intriguing physical and optical properties of CrSBr.

Discovery of novel superconductors can be complex and time-consuming. This is due to the incomplete understanding of the non-conventional superconductivity mechanism and the trial-and-error nature of material synthesis. In this research, we propose a generative machine learning framework to learn the statistical distribution of crystal structure maps to search for new unseen superconductors. The developed variational autoencoder (VAE) encodes crystal structures to a latent space and samples from the latent space to decode novel lattices. This process inherently conforms to the space-group symmetries in lattices and the permutational, translational and rotational invariances.

J. Całka1, R. Kurleto2, M. Rosmus3, T. Romanova4, D. Kaczorowski4, P. Starowicz1 1Marian Smoluchowski Institute of Physics, Jagiellonian University, Prof. S. Łojasiewicza 11, 30-348 Kraków, Poland 2Solaris National Synchrotron Radiation Centre, Jagiellonian University, Czerwone Maki 98, 30-392 Kraków, Poland 3Université Paris-Saclay, CNRS, Institut des Sciences Moléculaires d’Orsay, 91405 Orsay, France 4Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okólna 2, 50-422 Wrocław, Poland PrBi is a simple monopnictide, which crystallizes in a rock salt structure. It was theoretically predicted that PrBi is a strongly correlated Dirac semimetal with symmetry protected Dirac nodes [1]. It is also expected that a non-Fermi liquid state is realized in this compound [1]. In addition to the intriguing theoretical predictions, it was found experimentally that PrBi has a high carrier mobility, small effective masses and a very large [2] and anisotropic [3] magnetoresistance. We present angle-resolved photoemission studies of PrBi crystals cleaved along (100) plane. Band structure and Fermi surface is mapped systematically for photon energies between 12 eV and 100 eV. It appears that the observed band structure has an important contribution of quasi two-dimensional features. Prominent hole pockets around the G point are well visible. Dirac cones at ???? and nodal lines along  Γ - ???? are clearly recognized. The spectra are also analyzed to test the predicted absence of Landau quasiparticles. [1] H. Hu, et al., “Gapless electronic topology without free-electron counterpart”, ArXiv2110.06182 [2] A. Vashist, et al., “Fermi surface topology and large magnetoresistance in the topological semimetal candidate PrBi”, Phys. Rev. B 99, 245131 (2019). [3] F. Tang, et al., “Anisotropic magnetoresistance and electronic features of the candidate topological compound praseodymium monobismuthide”, Phys. Chem, Chem. Phys. 25, 25573 (2023).

Uranium-based compounds exhibit a complex interplay of electronic correlations, magnetism, and band topology due to the 5f electrons. These materials serve as an intriguing platform to explore the competition between Kondo screening, magnetic interactions, and itinerant-electron behavior [1-2]. Using angle-resolved photoemission spectroscopy (ARPES), we report the band structure of UAsS for the first time, revealing well-defined band dispersions along with nearly flat bands near the Fermi level that exhibit hybridization. Temperature-dependent measurements further reveal the disappearance of a flat band just below the Fermi level. Our results provide new insights into the electronic properties of UAsS and contribute to the broader understanding of the electronic properties of actinide materials. [1] Chen et. al., Phys. Rev. Lett. 123, 106402. [2] Fujimori et. al., Phys. Rev. B 91, 174503.

Transparent conducting metal oxides (TCOs) associate the properties of high optical transparency and electrical conductivity, which makes them suitable for variety of applications such as window layer in liquid crystal and electroluminescent display devices as well as in solar cells. Ytterbium (Yb) doped CdO films prepared using a simple and effective spray pyrolysis technique. The structural, morphological and optoelectronic properties of Yb doped CdO thin films as a function of Yb concentration (1–3 at.%) have been studied. Yb doped CdO thin films exhibit excellent optical transparency, with an average transmittance over 75% in the visible region. It is found that Yb doping widens the optical band gap from 2.63 to 2.88 eV, via a Moss–Burstein shift and further decreases to 2.77 eV. The maximum reflectivity of 97.24% achieved for 2.5 at.% Yb:CdO film. The better values of resistivity, carrier concentration, mobility and figure of merit have been obtained for 2.5 at.% Yb:CdO, which are 2.6×10^−4 Ω cm, 13.9×10^20 cm^−3, 17.39 cm^2/Vs and 45.87×10^−3 (Ω)^−1 respectively. The obtained results revealed that Yb dopant has a significant influence on the optoelectronic properties of CdO-based TCO compound.

Magnetic bimerons, solitonic spin textures with the same topology as skyrmions, have attracted attention for their potential in spintronic applications. In this work, we explore the stabilization conditions and energy characteristics of bimerons in a circular nanodot through micromagnetic simulations and analytical calculations. We examine the dependence of the size, position, and orientation of the meron and antimeron cores on the anisotropy-induced easy-axis. Our results demonstrate that the bimeron orientation relative to the surrounding homogeneous state is strongly influenced by the Dzyaloshinskii–Moriya interaction type. Additionally, we show a non-reciprocal energy dependence on the bimeron's position within the nanodot. We also obtain that the bimeron size decreases with increasing anisotropy, while its equilibrium position is displaced from the nanodot center. Furthermore, an analysis of energy barriers reveals that bimeron contraction is the dominant annihilation mechanism under thermal fluctuations. These insights are valuable for developing magnetic devices that require precise control of topological spin textures.

Bismuth is the most metallic element of group V, with strong spin-orbit-coupling (SOC) and high carrier mobility, specially at its surfaces, making bismuth unique. In the last years thin films have attracted a lot of interest due to the different structures they can grow and their topological properties [1,2]. Bismuth is an interesting case of topological material, with predicted unconventional high order topology [1], and several film orientations exhibiting topological 1D states at the edges. At the monolayer limit, two different bismuthene allotropes have been grown (α and β), both showing topological edge states [2,3]. We grew Bi monolayers on Au(111) and Ag(111) substrates. Their atomic structure was investigated by Scanning Tunneling Microscopy (STM) and Low Energy Electron Diffraction (LEED), while electronic structure was probed by Angle Resolved Photoemission Spectroscopy (ARPES) and Scanning Tunneling Spectroscopy (STS). The growth of Bi on both substrates is very similar with a (110)-like structure, with electronic structure dominated by the Moire pattern imposed by the substrate and Bi lattices. The electronic structure revealed by ARPES indicated a strong coupling between the monolayers and the substrate, that totally changes the electronic structure of Bi monolayers. Alkali metal (Lithium and Potassium) were deposited in order to intercalate and decouple the monolayers from the substrate. Different surface reconstructions by LEED for each system. Corresponding ARPES measurements revealed modified band structures in all cases. Notably, for one system, the emergence of new bands near the Fermi level suggests decoupling of the Bi monolayer. The resulting band structure resembles to theoretical calculations for freestanding buckled β-Bismuthene available in the literature. References: [1] Schindler F. et al., Nature Phys 14, 918–924 (2018). [2] F. Reis et al., Science 357 (6348) (2017). [3] S. Salehitaleghani et al., 2D Mater. 10(1) (2023).

CrSBr is a bulk antiferromagnet, but is proposed to become ferromagnetic upon electron doping, or with thickness reduction to a monolayer. Motivated by this, we investigated the thickness-dependent electronic structure of CrSBr exfoliated onto a template-stripped gold substrate. Using micro-ARPES, we find a thickness-dependent renormalisation of the band gap, being smallest for the thinnest sample, likely a monolayer, where we also find significant population of the conduction band. The conduction band is nearly one-dimensional in character, different from the quasi-2D valence bands. On the other hand, the thickness dependence has minimal effect on the Br 3d core levels. We interpret the data in terms of a charge transfer mechanism at the CrSBr-Au interface, combined with layer-dependent screening.

EuO in the stochiometric rock salt structure is a wide band ferromagnetic semiconductor. With electron doping, either by Oxygen vacancies or by substitution of Eu with Gd, the Curie temperature rises but saturates at high doping. No consensus exists about the precise mechanism of Tc enhancement, but while it appears to scale with the free carrier concentration, which also saturates, it is not explained by a Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction model. We have investigated the electronic structure of EuO by angle-resolved photoemission (ARPES). An occupied conduction band minimum is readily observed at low temperature and identiied as having Eu 5d character with exchange splitting. The latter is clearly observed with both, the majority and the minority spin band occupied, at very high doping, thus supporting the highly spin polarized character of the band at low doping, albeit above the insulator to metal transition. With increasing temperature, shifts and intensity variations are observed in these bands, while a moderately bound impurity state with a complicated momentum structure balances the intensity changes of the conduction band state, thus suggesting a shift of spectral weight from one to the other.

Superlattices from twisted graphene mono- and bi-layer systems give rise to on-demand many-body states such as Mott insulators and unconventional superconductors. These phenomena are ascribed to a combination of flat bands and strong Coulomb interactions. However, a comprehensive understanding is lacking because the low-energy band structure strongly changes when the electron filling is varied. Here, we directly access the filling-dependent low energy bands of twisted bilayer graphene (TBG) and twisted double bilayer graphene (TDBG) by applying microfocused angle-resolved photoemission spectroscopy to in situ gated devices. Our findings for the two systems are in stark contrast: The doping dependent dispersion for TBG can be described in a simple model, combining a filling-dependent rigid band shift with a many-body related bandwidth change. In TDBG, on the other hand, we find a complex behaviour of the low-energy bands, combining non-monotonous bandwidth changes and tuneable gap openings. Our work establishes the extent of electric field tunability of the low energy electronic states in twisted graphene superlattices and can serve to underpin the theoretical understanding of the resulting phenomena.

Intercalated transition metal dichalcogenides (TMDCs) have recently garnered significant attention from the condensed matter community due to the demonstration of exotic phenomena that depend on the intercalated transition metal [1]. We investigate the electronic structure of V-intercalated TMDC, V1/3NbS2, using Circular Dichroic Angle-Resolved Photoemission Spectroscopy (CD-ARPES) in combination with the one-step model of photoemission as implemented in the SPR-KKR package [2]. CD-ARPES is a powerful technique that combines the capabilities of Angle-Resolved Photoemission Spectroscopy (ARPES) with circularly polarized light, allowing the orbital character and symmetry of electronic states to be revealed. This is especially crucial for materials with complex band structures, such as TMDCs. However, the intensity asymmetry in CD contains a well-known dichroism component related to the measurement geometry, as well as a component associated with magnetic ordering, complicating the extraction of spin information from CD-ARPES data. To address this challenge, our group has developed a model of dichroism based on the one-step model of photoemission (SPR-KKR code). By incorporating the measurement geometry, the ab initio calculations enable the separation of geometric and spin contributions to the CD signal [3,4]. [1] B. Edwards et al, Nature Materials 22, 459–465 (2023) [2] H. Ebert, D. Ködderitzsch and J. Minár, Rep. on Prog. in Phys. 74, 096501 (2011) [3] O. Fedchenko et al., Sci. Adv. 10, eadj4883 (2024) [4] S. Beaulieu et al, Phys. Rev. Lett. 125, 216404 (2020)

This talk addresses the problem of constructing a proper bosonized description of the collective modes in strongly interacting (non-)Fermi liquids which is specific to two spatial dimensions. Although, in a mild form, this subtlety exists in the Fermi liquid as well, the discussion focuses on the effects of long-ranged and/or retarded interactions which can completely destroy the fermionic quasiparticles. The present analysis also provides a further insight into the nature and properties of the collective bosonic modes in such systems.

Many unusual and promising properties have been reported recently for the transition metal trichalcogenides of the type $TM$PS$_3$ ($TM$=V, Mn, Fe, Ni ...), such as maintaining magnetic order to the atomically thin limit, ultra-sharp many-body excitons, metal-insulator transitions and giant linear dichroism among others. Here we conduct a detailed investigation of the electronic structure of NiPS$_3$ and FePS$_3$ using angle-resolved photoemission spectroscopy, \textbf{q}-dependent electron energy loss spectroscopy, optical spectroscopies and density functional theory (DFT). FePS$_3$ is a Mott insulator with a gap of $E_{gap}\approx 1.4$\,eV and $zigzag$ antiferromagnetism below $T_N=119$\,K. The low energy excitations are dominated by Fe 3$d$ states. Large and sign-changing linear dichroism is observed. We provide a microscopic mechanism explaining key properties of the linear dichroism based on the correlated character of the electronic structure, thereby elucidating the nature of the spin-charge coupling in FePS$_3$ and related materials. An ultra-sharp photoluminescence line intimately related to antiferromagnetic order has been found in NiPS$_3$, a correlated van-der-Waals material, opening prospects for magneto-optical coupling schemes and spintronic applications. Here we unambiguously clarify the singlet origin of this excitation, confirming its roots in the spin structure. Based on a comprehensive investigation of the electronic structure we develop, in a first step, an adequate theoretical understanding using DFT. In a second step the DFT is used as input for a dedicated multiplet theory by which we achieve excellent agreement with available multiplet spectroscopy. Our work connects the understanding of the electronic structure and of optical processes in NiPS$_3$ and related materials as a prerequisite for further progress of the field.

Using the combination of a new effective Hamiltonian approach and hybrid Monte-Carlo simulations, we unveil a variety of partially magnetically ordered (PMO) phases in the Kondo lattice model. Our approximation is motivated by two crucial features of the Hamiltonian: (i) formation of Kondo singlets leading to vanishing local magnetic moments, and (ii) spatially correlated nature of the effective single-particle kinetic energy. We discover PMO phases with fractional values 1/4, 3/8, and 1/2 of Kondo-screened sites. A common understanding of these states emerges in terms of a non-local ordering mechanism. The concept of site-selective spontaneous symmetry breaking introduced here provides a new general approach to study models of interacting fermions in the intermediate coupling regime.

R. Kurleto1, E.D.L. Rienks2, J. Sanchez-Barriga2, O.J. Clark2, M. Yao3, J. Bannies3, F. Roth4, D.V. Potorochin4, A. Fedorov5, J. Fink5,6 1Department of Physics, University of Colorado, Boulder, CO, 80309, USA 2Helmholtz-Zentrum Berlin, Albert-Einstein-Straße 15, 12489 Berlin, Germany 3Max Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany 4Institute of Experimental Physics, TU Bergakademie Freiberg, Leipziger Straße 23, 09599 Freiberg, Germany 5Leibniz Institute for Solid State and Materials Research Dresden, Helmholtzstr. 20, D-01069 Dres-den, Germany 6Insitut für Festkörperphysik, Technische Universität Dresden, D-01062 Dresden, Germany Author Email: rafal.kurleto@uj.edu.edu Electron correlation effects are reflected in macroscopic properties of quantum materials. In particular, electrical resistivity measurement probes scattering rates, which are mostly related to electron-electron interaction at low temperature. Complementary insight into interactions at microscopic level can be gained from angle-resolved photoelectron spectroscopy (ARPES). This method gives a direct information about a band structure in momentum space. Shape of the dispersion and widths of spectral lines can be used to calculate self-energy operator. In this contribution we are discussing the results of two-dimensional analysis on ARPES data which allows us to extract dispersion and quasiparticle lifetimes for selected quantum materials. This approach gives more accurate results in comparison to standard analysis methods [1]. The band structure of Na(110) deposited on W has been studied [2]. The effective mass of this prototypical nearly-free electron metal is slightly enhanced and close to those reported in previous reports, while the scattering rates do not match some many-body calculations predicting a strong coupling to spin fluctuations. Moreover, we analyzed the inner hole pocket of electron and hole- doped pnictide superconductors: BaFe2As2, BaCr2As2, and BaCo2As2 [3]. Large, linear-in-energy scattering rates have been obtained. The extracted slope of the scattering rate (Γ/E) depends on 3d count in a similar way as transition temperature (TC) to the superconducting state. Near optimally doped compounds we found scattering rates which are bigger than the binding energy. This indicates for the charge carriers a behavior beyond the Planckian limit. References [1] R. Kurleto, J. Fink, Journal of Electron Spectroscopy and Related Phenomena 253, 147127 (2021). [2] D.V. Potorochin, R. Kurleto, O.J. Clark, E.D.L. Rienks, J. Sanchez-Barriga, F. Roth, V. Voroshnin, A. Fedorov, W. Eberhardt, B. Buechner, J. Fink, arXiv:2112.00422 [3] J. Fink, E.D.L. Rienks, M. Yao, R. Kurleto, J. Bannies, S. Aswartham, I. Morozov, S. Wurmehl, T. Wolf, F. Hardy, C. Meingast, H.S. Jeevan, J. Maiwald, P. Gegenwart, C. Felser, B. Buchner, Physical Review B 103, 155119 (2021).

Undoped quasi-one-dimensional edge-shared cuprates are charge-transfer insulators [1,2]. In the vicinity of the Fermi energy, the electronic structure of these compounds is defined by Cu $3d$ and oxygen $2p$ states in CuO$_2$ chains. In-chain magnetic interactions between localized $S=1/2$ magnetic moments of Cu$^{2+}$ ions are described by the one-dimensional $J_1$-$J_2$ model that is generic for frustrated magnets. This is the reason for the interest in these compounds [1-11]. Magnetic interactions determine the thermodynamics of the compounds and those low-energy excitations on the scale of tens meV. At an energy scale of several eV, a generalized 5-band Hubbard model is used to interpret the experiments that probe the charge-transfer excitations (optics [3-6], EELS [6,7], RIXS [8-11], etc.). Because of the complexity of this many-body model only numeric calculations for small clusters are available that provide only restricted information about the excitation dynamics. We show that  the model may be reduced to the Kondo-lattice model within reasonable approximations. This allows us to derive the analytic expressions for the photocurrent, the optical conductivity, the loss function and RIXS spectrum. Temperature dependence of these spectra is discussed. References: [1] S.-L. Drechsler, et al., J. Magn. Magn. Mater. 316, 306–312 (2007). [2] A. Vasiliev, et al., npj Quant Mater 3, 18 (2018). [3] Y. Mizuno, et al., Phys. Rev. B 57, 5326 (1998). [4] J. Málek, et al., Phys. Rev. B 78, 060508(R) (2008). [5] Y. Matiks, et al., Phys. Rev. Lett. 103, 187401 (2009). [6] S.-L. Drechsler, et al., Phys. C 470, S84–S85 (2010). [7] S. Atzkern, et al., Phys. Rev. B 64, 075112 (2001). [8] F. Vernay, et al., Phys. Rev. B 77, 104519 (2008). [9] C. Monney, et al., Phys. Rev. Lett. 110, 087403 (2013). [10] S. Johnston, et al., Nat. Commun. 7, 10563 (2016). [11] E. Paris, et al., npj Quantum Mater. 6, 51 (2021).

Len E.G.$^{1,2}$, Shatnii T.D.$^1$, Len T.S.$^3$, Tsapko Ye.A.$^1$, Galstian I.Ye.$^1$ $^1$ G.V. Kurdyumov Institute for Metal Physics, NAS of Ukraine, 36 Academician Vernadsky Blvd., UA-03142 Kyiv, Ukraine. E-mail: len.evgeniy@gmail.com $^2$ Kyiv Academic University, NAS and MES of Ukraine, 36 Academician Vernadsky Blvd., UA-03142 Kyiv, Ukraine. $^3$ National Aviation University, 1 Kosmonavt Komarov Ave., UA-03058 Kyiv, Ukraine. Many observable physical properties of solids, exhibited at macroscopic level, define by quantum and collective phenomena at nanometre scale in atomic and electron subsystems. Occurrence of such many-particles phenomena in solids determines the most promising quantum materials for modern technologies. One of the powerful methods, which allows an experimental observation of corresponding effects in the electronic structure, is the method of positron spectroscopy, namely, the method of angular correlation of electron-positron annihilation radiation (ACAR). In addition, this method allows to determine the size and concentration of defects in crystal and to study their electronic properties. Thus, the theoretical consideration of the influence of some many-particles effects, including strong electron-electron correlations and nanoclusters formation on the momentum distributions of conduction electrons recorded by the ACAR method is important. The nanoclusters formation in binary substitutional alloys with strong electron correlations as well as the effects of electron correlations themselves are investigated in the single-band Hubbard model at the temperature of 0 K. Such effects significantly change the momentum distributions of electrons as well as the equilibrium values of magnetic moments at the atoms of different types (atoms in clusters are considered as the third component of alloy) and the parameters of pair correlations in the arrangement of atoms of different types and in the orientation of electron magnetic moments at nearest sites. Because of differences in the spectral function summation in the reciprocal space, the ACAR dependences frequently demonstrate a higher sensitivity to the effects of nanoclusters formation and to the effects of electron correlations than corresponding dependences of electronic density of states. Inaccuracies in the interpretation of experimental data of positron spectroscopy within the framework of the free electron model, traditional for this method, are analysed.

We present an investigation of the ultrafast electron dynamics and orbital-dependent effective mass evolution in 1T-TiSe$_{2}$ using time-resolved angle-resolved photoemission spectroscopy (Tr-ARPES). Our study focuses on the microscopic mechanisms driving charge density wave (CDW) formation and suppression, providing real-time insights into many-body interactions in quantum materials. The Tr-ARPES system at NSRRC, featuring a high-power Yb-KGW laser and high-harmonic generation (HHG) EUV probe pulses, offers 100 kHz repetition rate, 184 fs temporal resolution, and photon energies ranging from 33 eV to 70 eV. This setup enables precise tracking of nonequilibrium electronic states and their evolution under external excitation. Our results reveal that the Ti-$\textit{3d}$ orbital exhibits significantly greater effective mass variations than the Se-4p$^*$ orbital, underscoring its pivotal role in CDW formation. At low pump fluences, the CDW order remains largely intact. In contrast, at higher fluences, a transient melting of the CDW phase is observed, followed by a multi-stage recovery process governed by electron-electron and electron-phonon interactions. The fluence-dependent dynamics highlight the interplay between exciton condensation, periodic lattice distortions, and phonon bottleneck effects in shaping the CDW phase. By capturing these transient electronic structures, our study demonstrates the capability of Tr-ARPES in probing ultrafast phase transitions and provides new insights for controlling quantum states in advanced materials.

The \ce{Nb3X8} family (X=Cl, Br, I) is a novel group of breathing kagome materials. [1] Despite interest in these materials, their electronic properties are still unclear and are likely to differ among the family members. For instance, they could exhibit characteristics of either a strongly correlated Mott insulator or a weakly correlated obstructed atomic insulator. [2, 3] Moreover, the kagome lattice can naturally host flat bands, which give rise to exotic properties such as spin-liquids and high-temperature superconductivity. [4, 5] Here, I will present our recent experimental investigation of the band structure in bulk \ce{Nb3X8} kagome materials by means of angle-resolved photoemission spectroscopy (ARPES). With ARPES we directly image the flat bands in the \ce{Nb3X8} systems, and are able to verify their Mott insulator character with the support of theoretical calculations. [1] Y. Zhang et al., Physical Review B 3, 107 (2023); https://link.aps.org/doi/10.1103/PhysRevB.107.035126 [2] S. Grytsiuk et al. npj Quantum Materials 9, 8 (2024); https://doi.org/10.1038/s41535-024-00619-5 [3] Y. Xu et al., Physical Review B 16, 109 (2024); https://link.aps.org/doi/10.1103/PhysRevB.109.165139 [4] T. H. Han et al., Nature 492, 402 (2012); https://doi.org/10.1038/nature11659 [5] B. R. Ortiz et al., Physical Review Letters 125, 247002 (2020); https://doi.org/10.1103/PhysRevLett.125.247002

By using angle-resolved photoemission spectroscopy, we have studied the entanglement of different emergent quantum states in two kagome materials: the kagome superconductor CsV3Sb5 and the kagome ferromagnet Fe3Ge. In CsV3Sb5, we investigated the electronic properties of the charge density wave (CDW) and superconductivity. The CDW-related features can be well explained by Fermi surface nesting. The reconstructed Fermi surface in the CDW state has been determined. The superconducting gaps were detected for the first time by ARPES. The intriguing interplay between CDW and superconductivity has been discussed. In Fe3Ge, we studied the magnetic impact on the intrinsic kagome band structures. Two kagome Dirac fermions were found to respond quite differently to the spin-reorientation transition: the one containing 3dz2 orbitals evolves from gapped to nearly gapless, while the other embracing the 3dxz/3dyz components remains intact, suggesting that the effect of spin reorientation on the Dirac fermions is orbital-selective. Moreover, we demonstrated that there is no signature of charge order formation in Fe3Ge, in contrast to its sibling compound FeGe.

Nearly 40 years after the discovery of high-temperature superconductivity in cuprates, the microscopic origins of this phenomenon remain elusive. Overdoped Tl$_2$Ba$_2$CuO$_{6+x}$ (Tl2201) has emerged as a model system for probing the superconducting pairing mechanism, as its single crystals can be clean enough to exhibit quantum oscillations and display a simple Fermi liquid normal state. However, thermodynamic studies have so far been largely limited to polycrystalline samples, where specific heat data intriguingly suggest a vanishing superfluid fraction in this clean limit. Progress with single crystals has been hindered by the experimental challenges posed by their diminutive size. Here, I will outline our efforts to overcome these challenges by adapting membrane nanocalorimetry techniques for specific heat measurements on these single crystals. These measurements aim to provide new insights into the interplay between doping, superfluid density, and the pairing mechanism in the overdoped regime.

David R. Bacon\textsuperscript{1,3\textdagger}, Xing Zhu\textsuperscript{1\textdagger}, Vivek Pareek\textsuperscript{1\textdagger}, \\ Kenji Watanabe\textsuperscript{2}, Takashi Taniguchi\textsuperscript{2}, Michael K. L. Man\textsuperscript{1}, \\ Julien Madéo\textsuperscript{1}, and Keshav M. Dani\textsuperscript{1*} \noindent \textsuperscript{1} Femtosecond Spectroscopy Unit, Okinawa Institute of Science and Technology Graduate University, \\ Onna-son, Okinawa, Japan 904-0495 \\ \noindent \textsuperscript{2} National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan \\ \noindent \textsuperscript{3} Department of Chemistry, University College London, 20 Gordon Street, London \\ \noindent \textsuperscript{\textdagger} Equal contribution \\ Email: michael.man@oist.jp Abstract Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have emerged as promising platforms for valleytronics applications due to their strong Coulomb interactions and the presence of an easily accessible valley degree of freedom, which can be controlled via circularly polarized light. However, the valley polarization of bright excitons is rapidly lost due to various scattering mechanisms, including intervalley exchange interactions and phonon-mediated scattering. Additionally, the complex exciton landscape, which includes both bright and spin- or momentum-forbidden dark excitons, has hindered a comprehensive understanding of valley depolarization dynamics. Time- and angle-resolved photoemission spectroscopy (TR-ARPES) has proven to be a powerful technique for directly imaging excitons in energy and momentum space while simultaneously resolving their constituent electron and hole species [1-4]. In this presentation, we will discuss our latest momentum-resolved study on valley-polarized excitons in monolayer \text{WS}_2\ using TR-ARPES. By tracking exciton populations and their evolution across the Brillouin zone, we identify distinct valley depolarization mechanisms that dominate at different temperatures. Furthermore, we reveal excitation conditions under which intervalley exchange scattering is suppressed, leading to the formation of a momentum-dark exciton that maintains its valley selectivity for orders of magnitude longer than its bright exciton counterpart. These insights offer new strategies for manipulating valley polarization, advancing the potential of TMDCs for future valleytronic applications. References [1] J. Madéo et al., Science 370, 1199 (2020). [2] M. K. L. Man et al., Science Advances 7, eabg0192 (2021). [3] O. Karni et al., Nature 603, 247 (2022). [4] D. Schmitt et al., Nature 608, 499 (2022).

Physical properties can change significantly when bulk materials are thinned down to a few atomic layers. Here, we study the intriguing example of the metallic charge density wave system 1T-TaS$e_2$. Monolayer (ML) 1T-TaS$e_2$ was proposed to be a Mott insulator and is a candidate quantum spin liquid [1]. Previous Scanning tunneling microscopy (STM) experiments on Molecular beam epitaxial (MBE) grown islands of ML 1T-TaS$e_2$ coupled to metallic 1H-TaS$e_2$ found a sharp peak near the Fermi level, which was interpreted as a Kondo resonance originating from the screening of local moments of 1T-TaS$e_2$ by itinerant electrons in the metallic 1H-TaS$e_2$ layer [1]. We perform Angle resolved photoelectron spectroscopy (ARPES) measurements on ultra clean exfoliated few-layer 1T-TaS$e_2$, encapsulated with graphene, to study the evolution of the electronic structure with thickness. Reference: [1] Ruan et al., Nat. Phys. 17, 1154–1161 (2021)

Among the materials hosting Charge Density Wave (CDW) phases, transition metal tri-chalcogenides have attracted considerable attention thanks to their quasi-one-dimensional (q1D) nature, making them the ideal platform to study the Peierls transition. $ZrTe_3$ is a unique member of this family because its Fermi surface comprises both a 3D hole-like pocket centered at Γ and q1D bands at the zone edges [1,2], placing this material at the crossroad of different dimensionalities. Extensive ARPES studies have shown that the CDW phase, setting in at 63 K [3], is compatible with Fermi surface nesting (FSN) of the q1D bands. However, some of the observed changes in the band structure with temperature suggest the need for a larger view [4]. We performed time and angle resolved photoemission spectroscopy (trARPES) measurements, probing the material with 6 eV and 20.9 eV photon energy, revealing interesting dynamical features. First, a transient photo-induced shift of the whole band structure, and second, the excitation of coherent oscillations compatible with two $A_g$ phonon modes, modulating the 3D and q1D bands in different ways [5]. Our experiment indicates the presence of a strong electron-phonon coupling that could play a central role in the CDW formation. Supported by theoretical calculations of the Lindhard function, we are able to disentangle the electronic and lattice contributions to the CDW transition, providing a complete picture on this class of interesting materials with entangled dimensionalities. References [1] T. Yokoya, T. Kiss, A. Chainani, S. Shin, and K. Yamaya, Phys Rev B Condens Matter Mater Phys, vol. 71, no. 14, (2005). [2] M. Hoesch, X. Cui, K. Shimada, C. Battaglia, S. I. Fujimori, and H. Berger, Phys Rev B Condens Matter Mater Phys, vol. 80, no. 7, (2009). [3] S. Takahashi, T. Sambongi and S. Okada, J. Phys. Colloques 44 (1983). [4] S. P. Lyu et al., Chinese Physics B, vol. 27, no. 8, (2018). [5] N. Mignani, et al., in preparation (2025)

Stacking low-dimensional layered materials together provides a powerful route towards controlling the properties of quantum materials. In particular, differences in lattice constant or finite interlayer rotation can result in the emergence of new periodic potentials, in turn modulating the electronic structure. Here, we synthesised monolayer 1$H$-NbSe$_2$ on graphite substrates. Despite the large lattice mismatch of ~40%, our micro-ARPES measurements show clear signatures of interlayer interactions, including the formation of replica bands, interlayer charge transfer, and the observation of band hybridisation between states of the substrate and epilayer. Our measured electronic structure also reveals well-resolved spin-split Nb bands, indicating a significant role of spin–orbit coupling down to the monolayer limit in NbSe$_2$. I will discuss these results in terms of the electronic states of the NbSe$_2$ and the effect of resonant interlayer coupling.

The divergence of the electron density of states (DOS) plays an important role in enhancing many-body interactions and inducing various quantum phases in low-dimensional systems. However, such unique electronic structures remain experimentally elusive in three-dimensional (3D) systems, particularly those with strong spin-orbit coupling (SOC). Using angle-resolved photoemission spectroscopy and first-principles calculations for a Laves-phase superconductor Cs$\textrm{Bi}_{2}$, which features a Bi-pyrochlore 3D network with strong SOC, we identify two characteristic electronic structures, a dispersionless topological flat band saddle points, which cooperatively produce large DOS. Our findings suggest a novel mechanism for achieving a divergent DOS and lay a foundation for exploring exotic phenomena driven by the interplay of multiple singularities with a large DOS, nontrivial topology, and strong SOC in 3D pyrochlores.

E. Młyńczak 1, A. Surendran 1, S. Shaju 1, E. Beyer 2, T. Sobol 2, I. Aguilera 3, E. Madej 1, D. Wilgocka-Ślęzak 1, K. Freindl 1, J. Korecki 1, N. Spiridis 1 1 Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Krakow, Poland 2 National Synchrotron Radiation Centre SOLARIS, Jagiellonian University, Krakow, Poland 3 Institute for Theoretical Physics, University of Amsterdam, and European Theoretical Spectroscopy Facility (ETSF), Science Park 904, 1098 XH Amsterdam, The Netherlands Email: ewa.mlynczak@ikifp.edu.pl Quantum materials with a kagome lattice structure are a promising platform for condensed matter research due to their peculiar topological, electronic, and magnetic properties. Intermetallic compounds of tin and iron with a general formula of FexSny are formed by alternatively stacking along the c-axis Fe3Sn kagome bilayers and stanene layers [1]. These kagome materials host novel electronic properties like topological flat bands, Dirac cones, and fractionalized topological phases due to the interplay of spin-orbit coupling, strong electron correlations, and magnetic order [2][3]. Using molecular beam epitaxy, we have grown Fe-Sn epitaxial thin films of Fe3Sn2 stoichiometry on Pt(111) layers deposited on Al2O3 (0001) single crystals. The low energy electron diffraction showed sharp diffraction spots indicating (2x2) periodicity with respect to the Pt(111) substrate. The scanning tunneling microscopy presented clear hexagonal arrangement of atoms, characteristic of stanene and the Fe3Sn kagome layer. The samples have been transferred to the Solaris synchrotron using a vacuum suitcase for further studies of the electronic properties using x-ray photoelectron spectroscopy as well as angle-resolved photoemission. We have observed clear electronic dispersions in the valence band with the signatures of the flat bands close to the Fermi level. We will discuss the experimental results in comparison to the calculations of the electronic band structure performed using density functional theory within the generalized gradient approximation (GGA) as well as using the GW method. In addition, the films have been characterized ex-situ using x-ray diffraction and conversion electron Mössbauer spectroscopy. References [1] Li, Hang, et al. Appl. Phys. Lett. 114, 192408 (2019) [2] Ren, Z., Li, H., Sharma, S. et al. npj Quantum Mater. 7, 109 (2022) [3] Guo, H-M., and M. Franz Physical Review B 80, 113102 (2009). Acknowledgments This research was funded by the National Science Centre, Poland (NCN), grant number 2022/46/E/ST3/00184.

Bipolaronic superconductivity is an exotic pairing mechanism proposed for materials like BKBO; however, conclusive experimental evidence for a (bi)polaron metallic state in this material remains elusive. Here, we combine Angle-resolved photoemission, resonant inelastic X-ray, and neutron total scattering techniques with advanced modeling to study the local lattice distortions, electronic structure, and electron-phonon coupling in BKBO as a function of doping. Data for the parent compound x=0 indicates that the electronic gap opens in predominantly oxygen-derived states strongly coupled to a long-range ordered breathing distortion of the oxygen sublattice. Upon doping, short-range breathing distortions and sizable e-ph coupling persist into the superconducting regime x=0.4. Comparisons with exact diagonalization and determinant quantum Monte Carlo calculations further support this conclusion. Our results provide compelling evidence that the BKBO metallic phase hosts a liquid of small (bi)polarons derived from local breathing distortions of the lattice, with implications for understanding the low-temperature superconducting instability.

Autors: Natalia Olszowska, Dawid Wutke, Marcin Rosmus, Rafał Kurleto, Jacek J. Kołodziej We will present the configuration and recent upgrades of the URANOS (Ultra Resolved ANgular phOtoelectron Spectroscopy) beamline, now equipped with spin detectors and with a new 6-axis cryogenic manipulator set to be installed in the near future. This manipulator will be capable of operating at very low temperatures (4.3 K at normal pressure) on the sample block. Our focus will be on demonstrating the Spin-ARPES measurement geometry and the calibration of two orthogonal VLEED spin detectors. We will discuss the capabilities enabled by this measurement geometry, particularly in relation to different photon polarizations. By optimizing the beamline and end-station configuration, we have achieved an optimal geometry for detailed band structure analysis, allowing both dichroic and spin-resolved measurements across a broad photon energy range (8–150 eV). The proposed system geometry and a broad photon energy range also enables measurements of the entire Brillouin zone without requiring changes in the sample position. This minimizes variations in transition matrix elements caused by geometry changes, ensuring consistent dichroic and spin measurements under the same conditions. Calibration measurements—including energy resolution, reflectivity, FOM, the Sherman function, and spin measurements—will also be demonstrated for two spin detectors using a gold polycrystal and a Rashba-split Au(111) single crystal. More information about the URANOS beamline can be found on the NSRC Solaris website: https://synchrotron.uj.edu.pl/en_GB/linie-badawcze/uranos.

ANTARES is the nano-ARPES beamline at Synchrotron SOLEIL, designed for photoemission studies on small-scale samples. The beamspot size at ANTARES can reach 700 nm using a zone plate or 5 μm with a capillary mirror. Operando measurements are available, including control over bias voltage, current, and magnetic field. This poster will present recent developments at ANTARES, including: 1.Instrument Updates 2.Magneto-ARPES Commissioning Results (collabrated with Yi’s group at Rice University) Results of ARPES measurements with a small magnetic field (~10 mT) on TMD samples will be presented. Effects of spectrum rotation, sample heating, influence on the chamber and analyzer, along with other related effects, will be discussed. 3.Glove Box Integration The design for integrating a glove box into the vacuum system will be presented. In the near future, users at ANTARES will be able to prepare air-sentive samples and exfolicate samples onsite.

Observation of the Dirac Dispersions in Co-doped CaFe$_2$As$_2$ Marcin Rosmus$^1$, Natalia Olszowska$^2$, Rafa\l{} Kurleto$^2$, Zbigniew Bukowski$^3$, Pawe\l{} Starowicz$^4$ $^1$ Universit\'e Paris-Saclay, CNRS, Institut des Sciences Mol\'eculaires d’Orsay, 91405, Orsay, France $^2$ Solaris National Synchrotron Radiation Centre, Jagiellonian University, Czerwone Maki 98, 30-392 Krak\'ow, Poland $^3$ Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wroc\l{}aw, Poland $^4$ Marian Smoluchowski Institute of Physics, Jagiellonian University, Prof. S. {\L}ojasiewicza 11, PL-30348 Krak\'ow, Poland We performed an angle-resolved photoemission spectroscopy (ARPES) study of the electronic structure of the parent compound CaFe$_2$As$_2$ and two cobalt-doped iron-based superconductors: CaFe$_{1.93}$Co$_{0.07}$As$_2$ and CaFe$_{1.85}$Co$_{0.15}$As$_2$. The investigation covered the phase diagram, focusing on the transition from the orthorhombic spin density wave (SDW) phase to the tetragonal paramagnetic phase. Our measurements revealed characteristic features of the electronic structures associated with the SDW phase in the parent compound and the lightly doped sample ($x = 0.07$). In the heavily doped system ($x = 0.15$), we identified a band structure typical of the tetragonal paramagnetic superconducting phase. Notably, we report the first observation of Dirac cones located 30 meV below the Fermi energy in the orthorhombic SDW phase for both the nonsuperconducting CaFe$_2$As$_2$ and the superconducting CaFe$_{1.93}$Co$_{0.07}$As$_2$. Additionally, we did not observe a significant band shift in the parent and lightly doped compounds. These findings enhance our understanding of the electronic structure of CaFe$_{2-x}$Co$_x$As$_2$ and the possible connection between SDW-related phenomena and the existence of Dirac states in the 122 family of compounds.

Transition Metal Dichalcogenides (TMD) are layered materials obtained by stacking two-dimensional sheets weakly bonded by van der Waals interactions. In bulk TMD, band dispersions are observed in the direction normal to the sheet plane ($z$-direction) due to the hybridization of out-of-plane orbitals but no $k_z$-dispersion is expected at the single-layer limit. Using angle-resolved photoemission spectroscopy, we precisely address the two-dimensional to three-dimensional crossover of the electronic band structure of epitaxial WSe$_2$ thin films. Increasing number of discrete electronic states appears in given $k_z$-ranges while increasing the number of layers. The continuous bulk dispersion is nearly retrieved for 7-sheet films. These results are reproduced by calculations going from a relatively simple tight-binding model to a sophisticated KKR-Green's function calculation. This two-dimensional system is hence used as a benchmark to compare different theoretical approaches.

A complete picture of a material system’s electronic properties requires an understanding of its electronic dispersion in all three dimensions. This is true not only for covalently bonded bulk materials, but even for “quasi-two-dimensional” layered materials, where interlayer interactions can be nontrivial. As another example, many of the newly discovered Weyl systems exhibit their exotic physics in bulk-dispersing states, and these can exist alongside complex surface physics (e.g., [1]). To study these systems by angle-resolved photoemission spectroscopy (ARPES), one must be able to make photon-energy-dependent measurements. This has long been possible at synchrotrons, but for pump-probe studies has only recently become feasible at free-electron lasers and in tabletop laser systems based on high-harmonic generation (HHG). With sophisticated beamline control and optimisation, we can now achieve on-the-fly scanning between probe energies generated by HHG and use these to investigate electron dynamics and complex light-matter interactions in optically pumped three-dimensional solid-state systems [2,3]. These tools are available to the community via our Artemis user facility. The facility is also undergoing major end station upgrades that will offer new detection and measurement capabilities (including real-space mapping with small beam spot and hemispherical analyser, and imaging via momentum microscopy) from 2026 onward. [1] Kuibarov, et al., Nature 626 (2024) 294. [2] Majchrzak, et al., Rev. Sci. Instr. 95 (2024) 063001. [3] Majchrzak, et al., Phys. Rev. Research 7 (2025) 013025.

Among the many two-dimensional (2D) materials that have gained attention since the discovery of graphene, transition metal dichalcogenides (TMDCs) stand out as promising candidates for electronic and optoelectronic applications. TMDCs, with the general formula MX$_{2}$ (where M = Mo, W, and X = S, Se, Te), typically exhibit a distinct bandgap and spin-polarized bands. Numerous artificial methods have been proposed to engineer these properties, including chemical doping, strain induction, and external electric fields. Recent advancements in the synthesis of TMDCs have led to the emergence of 2D TMDC alloys [1]. This study focuses on the band structure of A$_{x}$B$_{1-x}$Se$_{2}$ (where A, B = Cr, Mo, W) alloys with varying composition fractions (x). The coherent potential approximation (CPA) [2] effectively models the average scattering properties in homogeneous random alloys and, within the KKR formalism, ensures no additional scattering when embedding an alloy component. Theoretical investigations using the SPR-KKR [3] package, employing CPA, reveal novel families of TMDC alloys that do not exhibit disorder effects in their band structure across different composition fractions. Experimentally, we examined the band structure of Mo$_{x}$W$_{1-x}$Se$_{2}$ alloys (x = 0 to 1) using Angle-Resolved Photoemission Spectroscopy (ARPES). The experimental results align well with the one-step model photoemission calculations within the SPR-KKR framework. Investigating these disordered systems provides fundamental insights, enhancing their potential applications. [1] Zhou, J., Lin, J., Huang, X., et al. Nature, 556, 355-359 (2018). [2] Soven, P., Phys. Rev., 156, 809 (1967). [3] Braun, J., Minar, J., Ebert, H. Physics Reports, 740 (2018).

Dichroic techniques are highly relevant in the field of topological materials, layered systems, and spin-polarized electronic states. Dichroism in angle-resolved photoemission is per se a matrix element effect, which depends on the initial and final states as well as on the perturbation by the light field. Although matrix element effects in ARPES such as dichroism are important for addressing properties of the initial state wave functions, the results can strongly depend on experimental geometry or final state effects. Combining experimental data on bulk WSe2 taken at soft x-ray photon energies with state-of-the-art photoemission calculations, we demonstrate that a dichroic observable called time-reversal dichroism remains unaffected against variation of photon energy, light polarization, and the angle of incidence. We demonstrate a direct link of TRDAD obtained with both linearly and circularly polarized photons to the initial state properties indicating its broad applicability. The robustness of this matrix element effect indicates a considerable benefit over other dichroic techniques and encourages further experimental and theoretical investigations.

We experimentally measure Tg for various Ge-Se compositions and explore different fitting models. We provides valuable insights into the kinetic and static properties of Ge-Se glasses, showcasing an innovative application of the modified Gibbs-Di-Marzio equation. Through this analysis, we are able to ascertain that the constant β in the Gibbs-Di-Marzio equation is dependent on the ratio of two temperatures and that the value of β (0.74) derived from the fit, while the value of β = 0.72134 derived from the theory for the Ge-Se binary system. We are extending this empirical Gibbs-Di-Marzio form in another way to enrich our scientific research for Ge-Se binary system. Email: deepak22s@yahoo.com

Two-dimensional (2D) van der Waals ferromagnets have gained enormous interest due to their potential applications as next-generation spintronic devices and provide a platform to explore the fundamental physics behind 2D magnetism. With Curie temperature of about 220 K, ferromagnetic Fe$_3$GeTe$_2$ having layered structure exhibits highly-anisotropic electronic and magnetic properties, persisting down to the monolayer limit. Here, we present the temperature evolution of electronic structure and its relation with complex magnetism in Fe$_3$GeTe$_2$ using high-resolution photoemission spectroscopy in conjunction with density functional theory and dynamical mean field theory. We unveil the appearance of quasiparticle peak and its evolution in the vicinity of Fermi energy, and empirical evidence of incoherent-coherent crossover at around 125 K and further shed light on the long-standing issue of irreproducible large Sommerfeld coefficient obtained from specific heat measurements.

The orbital angular momentum (OAM) of electron states is essential for topological and quantum geometric quantities in solids. Angle-resolved photoemission spectroscopy (ARPES) with variable circular light polarization, known as circular dichroism (CD), has been assumed to be a direct probe of OAM and, by proxy, of the Berry curvature of electronic bands in energy- and momentum-space. Indeed, topological surface states have been shown to exhibit angle-dependent CD (CDAD), and more broadly, CD is often interpreted as evidence of spin-orbit coupling. Meanwhile, CD originates from the photoemission matrix elements, which can have extrinsic contributions related to the experimental geometry and the inherently broken inversion symmetry at the sample surface. Therefore, it is crucial to broadly examine CD-ARPES to determine the scenarios in which it provides a robust probe of intrinsic material physics. We performed CD-ARPES on the canonical topological insulator Bi2Se3, observed CDAD in the surface states, and found a similar magnitude CD in virtually all bulk bands. Since OAM is forbidden by inversion symmetry in the bulk, we conclude this originates from symmetry-breaking in the photoemission process. Comparison with theoretical calculations supports this view and suggests that hidden OAM - localized to atomic sites within each unit cell - contributes significantly. Additional effects, including inter-atomic interference and final-state resonances, are responsible for the observed rapid variation of the CDAD signal with photon energy.

Currently, the highest superconducting critical temperatures (T$_c$) under ambient conditions are found in the trilayer cuprates, making them a subject of significant interest. In this study, we utilize time- and angle-resolved photoemission spectroscopy (tr-ARPES) to examine the low-energy electronic structure and the intertwined orders of Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+\delta}$ (Bi2223). We reveal a striking difference in the pump-induced spectral weight response on the underdoped (p = 0.08) and overdoped (p = 0.25) Fermi surfaces, that arise from the inner and outer layers of the trilayer, respectively. The momentum and energy dependencies of this spectral weight response are consistent with a short-range, fluctuating charge order ($Q \sim 0.33$ r.l.u.) on the inner CuO$_2$ plane. To further validate this, we perform resonant x-ray scattering on the same samples, confirming the presence of such a short-range translational symmetry-breaking state that coexists with superconductivity at low temperatures. We track the evolution of the spectral weight with electronic temperature, and use it to extract the quasiparticle residue Z and the single particle lifetime on both Fermi surfaces. Comparing this to conventional expectations of their doping dependence, the results are suggestive of interlayer interactions between the strongly correlated inner plane and the denser charge fluid of the outer CuO$_2$ planes, possibly boosting the superconducting critical temperature of the overall system.

Spin-orbit coupling in 3D topological insulators and the spin-momentum locking in the topological states present unique opportunities for control of electronic states via light coupling. For example, photocurrent excitation and control with circularly polarized light has been demonstrated, as well as generation of polarized excitons linked to topological states. In this talk I will discuss how we use time-resolved ARPES to study coupling of light to topological electronic states, and in particular gain better understanding of photocurrent generation in topological insulators. Moreover, using circular polarized light in two-photon photoemission from Bi$_2$Se$_3$ allowed us to identify interference of different photoemission pathways. By analyzing the rise times of the population following the optical excitation, we gained a complete view of the occupied and unoccupied electronic states, and how they are coupled by the light. We resolve the bands involved in the interfering photoemission paths, and demonstrate control of the interference via changes in photon energy and polarization. Resolving and controlling interference pathways in photoemission paves the way to phase sensitivity in ARPES as well as coherent control of photoexcited states in solids.

LOREA (flower in basque language), the ninth beamline of the ALBA synchrotron radiation source, started its operation in 2021 and is dedicated to electronic structure investigation of quantum materials by means of Angle Resolved Photo-Emission Spectroscopy (ARPES). The beamline covers the photon energy range of 10-1000 eV, with continuously variable polarization, resolving power of more than 104 in the whole range, and a spot size of about 10x10 μm2. The beamline allows to perform high resolution VUV ARPES as well as Soft X-ray ARPES and resonant PES. The MBS A-1 hemispherical electron analyser can be used in the deflector map mode to get isoenergy maps in the energy range 10-600eV, while ARPES is available in the 10-850eV range. The analyser includes an MBS spin manipulator and a Focus-VLEED spin detector to perform Spin-Resolved ARPES in all 6 components at once. The 6-axes cryo-manipulator can reach low temperatures better than 7.5K and is provided with 4 electrical contacts for characterizing the ARPES samples with transport measurements and to perform "operando" experiments on simple devices with dimensions of the order of about 20 micrometers. The beam positioning is facilitated by a microscope looking at the sample surface in the direction collinear with the beam and with a spatial resolution of about 10μm. The two preparation chambers allow to sputter clean surfaces and to deposit thin films of a wide range of materials, from noble metals to alkali and rare earth elements. The presence of a plasma source gives a further possibility of cleaning surface and to grow oxides or nitrides. Atomic layer deposition (ALD) of oxides is also possible thanks to a customised ALD reactor attached to the main preparation chamber. In this contribution we will show some results on topological insulators measured at low energy, on charge density waves materials measured with soft X-ray ARPES, on spin polarised surface and bulk states, and preliminary results of gated oxide heterostructures used in "operando" resonant PES experiments.

$ZnAs_2$, a binary II-V semiconductor with monoclinic crystal structure characterized by semi-spiral As chains and two in-equivalent Zn atoms ~\cite{{fleet1974crystal}}, has emerged as a promising material for optoelectronic applications due to its strong optical anisotropy \cite{ran2021integrated}. A detailed understanding of its electronic structure is crucial to pursue this potential. Here, we present novel results based on Angle-Resolved Photoemission Spectroscopy (ARPES) measurements, revealing a remarkable in-plane electronic anisotropy. \\ Exploiting light-polarization dependent measurements and density functional theory (DFT) calculations, we establish that the valence band is dominated by $p_z$ orbital, which plays a pivotal role in shaping the in-plane anisotropy ratio between electronic effective mass in different high symmetry directions $r \sim21.27$, a value significantly higher than the one observed in the well-known anisotropic semiconductor black phosphorus, with $r \sim 6.25$~\cite{{liu2016mobility}}. \\ Our results disclose the physical origin of the strong polarization-dependent absorption and reflectivity data reported in literature ~\cite{{turner1961physical},{sobolev1972optical}} and provide crucial insights into the electronic properties of $ZnAs_2$, strengthening its potential for future optoelectronic applications, such as sensors and polarization-sensitive photodetectors. References: @article{fleet1974crystal, title={The crystal structure of ZnAs2}, author={Fleet, ME}, journal={Acta Crystallographica Section B: Structural Crystallography and Crystal Chemistry}, volume={30}, number={1}, pages={122--126}, year={1974}, publisher={International Union of Crystallography} } @article{ran2021integrated, title={Integrated polarization-sensitive amplification system for digital information transmission}, author={Ran, Wenhao and Ren, Zhihui and Wang, Pan and Yan, Yongxu and Zhao, Kai and Li, Linlin and Li, Zhexin and Wang, Lili and Yang, Juehan and Wei, Zhongming and others}, journal={Nature Communications}, volume={12}, number={1}, pages={6476}, year={2021}, publisher={Nature Publishing Group UK London} } @article{turner1961physical, title={Physical properties of several II-V semiconductors}, author={Turner, WJ and Fischler, AS and Reese, WE}, journal={Physical Review}, volume={121}, number={3}, pages={759}, year={1961}, publisher={APS} } @article{sobolev1972optical, title={Optical spectra and energy band structure of the monoclinic crystals ZnP2 and ZnAs2}, author={Sobolev, VV and Syrbu, NN}, journal={physica status solidi (b)}, volume={51}, number={2}, pages={863--872}, year={1972}, publisher={Wiley Online Library} } @article{liu2016mobility, title={Mobility anisotropy in monolayer black phosphorus due to scattering by charged impurities}, author={Liu, Yue and Low, Tony and Ruden, P Paul}, journal={Physical Review B}, volume={93}, number={16}, pages={165402}, year={2016}, publisher={APS} }

In this work, we utilise the matrix element effect of the ARPES intensity to deduce \textit{k}-space initial state orbital texture featuring different symmetries in type-II Dirac semimetal $PtTe_2$. Our spin- and angle-resolved photoemission data were augmented by the one-step model of the photoemission within the spin-polarized relativistic Korringa-Kohn-Rostoker (SPR-KKR) Green’s function method of the Munich band structure software package. To extract information about the different contributions to the resulting spectral weight and spin polarization, the matrix element used in our one-step model of photoemission calculations includes all experimental parameters such as photon energy, light polarization and geometry configurations. Via such control over the experimental parameters, to investigate the orbital wavefunction above and below the Dirac point, we performed light polarization-dependent ARPES calculations.

Marieke Visscher, Lea Richter, Sebastian Buchberger, Bruno Saika, Shu Mo, Andy Mackenzie, Phil King Ruthenium dioxide has a complex band structure, underpinning a variety of phenomena including superconductivity under strain and a Dirac nodal line network [1, 2]. It has also been proposed as a candidate altermagnet [3], and although recent studies suggest it lacks the requisite magnetic order [4], it has been shown to host unusual spin-polarised states in its band structure [5]. These phenomena motivate the need for further studies into its electronic structure. Angle-resolved photoemission spectroscopy (ARPES) would be an ideal probe for this, and while there have been several pioneering studies to date [2, 5, 6], the three-dimensional structure of ruthenium dioxide makes it difficult to prepare the requisite clean and flat surfaces with conventional methods. We have therefore investigated a fabrication method based on Focused Ion Beam (FIB) structuring to stimulate sample cleavage along desired crystallographic planes [7]. With this method, we were able to obtain high quality surfaces, on which we performed ARPES measurements. This capability to tailor the sample cleavage leads to a significant increase in ARPES data quality, allowing new resolution of subtle band splitting and additional resolution of bulk vs. surface states in this system. 1. J. Ruf, H. Paik, N. Schreiber, H. Nair, L. Miao, J. Kawasaki, J. Nelson, B. Faeth, Y. Lee, B. Goodge, B. Pamuk, C. Fennie, L. Kourkoutis, D. Schlom, and K. Shen, “Strain-stabilized superconductivity,” Nature Communications, vol. 12, no. 1, p. 59, 2021. 2. V. Jovic, R. J. Koch, S. K. Panda, H. Berger, P. Bugnon, A. Magrez, K. E. Smith, S. Biermann, C. Jozwiak, A. Bostwick, E. Rotenberg, and S. Moser, “Dirac nodal lines and flat-band surface state in the functional oxide RuO2,” Physical Review B, vol. 98, no. 24, 2018. 3. L. Šmejkal, J. Sinova, and T. Jungwirth, “Beyond conventional ferromagnetism and antiferro- magnetism: Nonrelativistic spin and crystal rotation symmetry,” Physical Review X, vol. 12, no. 3, p. 031042, 2022. 4. M. Hiraishi, H. Okabe, R. K. A. Koda, T. Muroi, D. Hirai, and Z. Hiroi, “Nonmagnetic ground state in RuO2 revealed by muon spin rotation,” Physical Review Letters, vol. 132, no. 16, p. 166702, 2024 5. Liu, J., Zhan, J., Li, T., Liu, J., Cheng, S., Shi, Y., ... & Shen, D. (2024). Absence of Altermagnetic Spin Splitting Character in Rutile Oxide RuO 2. Physical Review Letters, 133(17), 176401 6. Osumi, T., Yamauchi, K., Souma, S., Shubhankar, P., Honma, A., Nakayama, K., ... & Sato, T. (2025). Spin-Degenerate Bulk Bands and Topological Surface States of RuO2. arXiv preprint arXiv:2501.10649. 7. A. Hunter, C. Putzke, I. Gaponenko, A. Tamai, F. Baumberger, and P. Moll, “Controlling crystal cleavage in focused ion beam shaped specimens for surface spectroscopy,” Review of Scientific Instruments, vol. 95, no. 3, 2024.

A Weyl semimetal is a new matter state possessing Weyl fermions near the Fermi level with several unique physical properties and it is confirmed by the existence of Fermi arc surface states [1]. In this work we study tantalum arsenide (TaAs) which is a prototypical Weyl semimetal compound. The electronic structure properties have been studied by soft and hard X-ray angle-resolved photoemission spectroscopy (ARPES) at energies of 440 eV and 2150 eV, respectively. For the first time, TaAs is experimentally investigated by the bulk sensitive photoemission in the hard X-ray regime. In order to interpret experimental data we performed one-step model of photoemission calculation which includes all matrix elements and final state effects [2,3,4]. Due to the strong photon momentum effects and uncertainty in the tilt of experimental geometry we used a so-called machine learning algorithm combined with a free-electron final-state model to find best possible experimental parameters. Our findings re-emphasize the overwhelming accuracy of hard X-ray ARPES (HARPES) compared to the traditional ultraviolet and soft X-ray one in case of bulk electronic structure, motivating further material discoveries [5]. [1] S. Xu et al., Science 349, 6248 (2015). [2] H. Ebert et al., Rep. Prog. Phys. 74, 96501 (2011). [3] H. Ebert et al., The Munich SPR-KKR Package, version 7.7, http: //olymp.cup.uni-muenchen.de/ak/ebert/SPRKKR (2017). [4] J. Braun et al., Phys. Rep. 740, 1 (2018). [5] A. X. Gray et al., Nat. Mater. 10, 759 (2011).

Authors: Andela Zivanovic $^{1}$, Brendan Edwards $^{1}$, Tommaso Antonelli $^{1}$, Dmitry A. Sokolov $^{2}$, Craig Polley $^{3}$, Andrew P. Mackenzie $^{2}$, Philip D. C. King $^{1}$ $^{1}$ School of Physics and Astronomy, University of St Andrews, St Andrews KY16 9SS, United Kingdom $^{2}$ Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Strasse 40, 01187 Dresden, Germany $^{3}$ MAX IV Laboratory, Lund University, 221 00 Lund, Sweden Ca$_3$Ru$_2$O$_7$, a bilayer compound in the Ca$_{n+1}$Ru$_{n}$O$_{3n+1}$ Ruddlesden-Popper series, is known for its polar metal state and orthorhombic structure, which leads to the formation of 90$^o$ rotational domains. Studies by Lei et al.$^{[1]}$ have revealed additional 180$^o$ rotated domains using scanning transmission electron microscopy and optical second harmonic generation imaging. In this work, we used angle-resolved photoemission spectroscopy (ARPES) to investigate the electronic structure of this system, using the small beam spot of the Bloch beamline at Max-IV. While we find that the electronic states are uniform across a single orthorhombic domain, we find a strong spatial dependence of the measured band intensity, with a marked momentum-dependent asymmetry in the measured intensity. Such intensity variations persist as the sample is warmed through its structural and magnetic phase transitions, while spatial mapping indicates well defined domains of opposite intensity variation, with a spatial extent of $\sim\!100~\mu$m. We tentatively attribute this to a strong dependence of the photoemission matrix element on subtle structural distortions that occur due to the polar distortions of Ca$_3$Ru$_2$O$_7$, enabling ARPES to provide new insight into the polar domain structure at the surface. References [1] S. Lei et al., “Observation of Quasi-Two-Dimensional Polar Domains and Ferroelastic Switching in a Metal, Ca3Ru2O7,” Nano Lett., vol. 18, no. 5, pp. 3088–3095, 2018.