Flat Bands, Strong Correlations, and Heavy Fermions

The proximity effect between a ferromagnetic insulator like EuS and a superconductor has received much attention recently. An exchange split BCS density of states has been observed in macroscopic tunneling experiments, suggesting the presence of triplet correlations at the surface. The relationship between the magnetic domain distribution and the local superconducting density of states remains however poorly understood. Scanning tunneling microscopy experiments have revealed signatures of magnetism in the density of states of a proximitized superconductor. The spatial dependence remained however, ill-understood. Here we have performed a detailed study of the local superconducting density of states in a EuS/Nb heterostructure with a millikelvin Scanning Tunneling Microscope (STM). We resolve the superconducting gap and study the modification of the superconducting density of states induced by ferromagnetic EuS. We discuss in detail the magnetic field dependence, demonstrating the role of the tip-surface interaction in resolving spatial structures in thin film samples. We then demonstrate that the superconducting density of states is affected by the domain structure, with a decrease in the zero-bias density of states when domains are oriented with the magnetic field.

In the present work, we investigate the excitonic effects in twisted bilayer graphene (tBLG) using the rotated bilayer Hubbard model. Our theory is evaluated across the entire momentum spectrum, avoiding low-energy approximations. We consider both intralayer and interlayer Coulomb interactions and impose a partial-filling condition for the average electronic densities in both layers of the bilayer. We calculate the excitonic gap parameter and chemical potential for different twist angles and various values of the interlayer Coulomb interaction parameter. Additionally, we demonstrate the emergence of electronic flat bands in the band structure, mediated by excitonic effects. We observe a doubling effect of Dirac's K-point at low interaction limits, where one Dirac node remains stable while the other shifts position as a function of the rotation angle. In the large twist angle limit, we find two additional Dirac-like nodes at the M-point in the Brillouin zone. Furthermore, we highlight the excitonic red-shift effect of the principal Dirac point K in the low interaction regime, while at strong interactions, we also observe a blue-shift effect at the M-point. Moreover, we reveal a metal-semiconductor transition in the tBLG system as the interlayer Coulomb interaction parameter increases. The theory developed in this work yields significant results and provides essential insights for observing excitonic effects in twisted bilayer graphene. These findings are of considerable interest for nano- and optoelectronic applications involving twisted bilayer systems.

Kagome metals are important for exploring emergent phenomena due to the interplay between band topology and electron correlation. Motivated by the recent discovery of charge density waves in the kagome lattice antiferromagnet FeGe, we investigate the impact of Sb doping on the structural, charge, and magnetic order of FeGe. The superlattice distortion induced by charge order disappears with only slight Sb doping (∼1.5%) down to 80 K. The antiferromagnetic ordering temperature gradually shifts to 280 K for FeGe0.7Sb0.3. For FeGe1−xSbx with x _ 0.1, crystal structures with a slightly distorted Fe kagome lattice are formed. A significant change in magnetic anisotropy from easy axis to easy plane with increasing x is identified from magnetization measurements. Interestingly, neutron diffraction reveals noncollinear 120 degree antiferromagnetic structures widely exist below TN for all samples with x _ 0.1. So far, either ferromagnetic or non-frustrated collinear antiferromagnetic order has been reported. The 120 degree order due to frustration discovered in our works is rare.

In conventional superconductors, electron pairs with opposite spins, known as Cooper pairs, form due to phonon-mediated attractive interactions, resulting in s-wave superconductivity. However, in layered materials with strong correlations and additional degrees of freedom, new pairing mechanisms can emerge, potentially giving rise to unconventional superconductivity, correlated insulating states. These systems are still less explored theoretically, because the modles can not solve exactly, which serves as a major motivation to pursue them through experimental techniques, aiming to uncover new physical insights and mechanisms. In twisted homo-bilayer of WSe2, ultra flat moire super lattice minibands form at twist angle 4-degree, such bands are a promising source of superconductivity near Van Hove singularity, which arises from saddle point of electronic band structure, notably enhancing strong correlation and gives rise to the emergence of novel electronic phases of matter, which are governed by quantum statistics, the cornerstone of Condensed matter physics. From the experimental overview, we observed insulating states at half-band filling, -1; with a twisted angle of 3.38 degrees. These insulating states arise from the antiterromagnetic-metal phase.

Helical trilayer graphene features three layers of graphene successively twisted in the same direction, resulting in a supermoiré pattern and a real-space Chern mosaic [1]. At the so-called chiral limit, perfect flat bands arise with an ideal quantum geometry and Landau level-like properties. We find [2] that a finite magnetic field induces topological phase transitions in these ideal flat bands giving higher Chern flat bands. Consequently, magnetic fields modify the Chern mosaic. We also find the analytical expressions of the flat band wavefunctions that elucidate the mechanism of the magnetic field induced topological transitions. [1] L-Q. Xia et al., Nat. Phys. (2025), T. Devakul et. al., Sci. Adv. 9, eadi6063 (2023), D. Guerci, Y Mao, C. Mora, PRR 6, L022025 (2024) [2] A. Datta, D. Guerci, M. O. Goerbig, C. Mora, PRB 110, 075417 (2024), Editors’s suggestion

The moiré superconductor magic-angle twisted bilayer graphene (MATBG) shows exceptional properties, with an electron (hole) ensemble of only $\sim 10^11$ carriers per square centimeter, which is five orders of magnitude lower than traditional superconductors (SCs). This results in an ultralow electronic heat capacity and a large kinetic inductance of this truly two-dimensional SC, providing record-breaking parameters for quantum sensing applications, specifically thermal sensing and single-photon detection. To fully exploit these unique superconducting properties for quantum sensing, here, we demonstrate a proof-of-principle experiment to detect single near-infrared photons by voltage biasing an MATBG device near its superconducting phase transition. We observe complete destruction of the SC state upon absorption of a single infrared photon even in a 16–square micrometer device, showcasing exceptional sensitivity. Our work offers insights into the MATBG-photon interaction and demonstrates pathways to use moiré superconductors as an exciting platform for revolutionary quantum devices and sensors.

The unconventional superconductor \text{UTe}_2 has been known as a promising platform for exploring topological superconductivity, strong correlations, and exotic quasiparticle phenomena including flat bands and nesting features. In this study, we have employed highly accurate density functional theory (DFT) calculations based on the Elk code to identify possible signatures of Fermi surface nesting in \text{UTe}_2. By combining these results with a real-space Bogoliubov-de Gennes (BdG) formalism, we examine topological features of the superconducting state, including the possibility of a skyrmion lattice in high magnetic fields.

We present new findings on the synthesis of thin films of two intermetallic heavy-fermion compounds using molecular beam epitaxy (MBE) growth, each exhibiting unique quantum properties. $YbRh_2Si_2$, a material that can be tuned to a Kondo destruction quantum critical point under a modest magnetic field, exhibits strange metal behavior in this regime. Notably, the entanglement properties of $YbRh_2Si_2$ are of particular interest, as they are directly linked to its quantum critical behavior. At the quantum critical point, entanglement is predicted to become long-ranged, offering intriguing opportunities for exploring fundamental quantum phenomena. Our thin films demonstrate electronic transport properties strongly influenced by surface roughness, and we show that growth temperature precisely controls the roughness, providing a novel approach to explore the material's properties in the thin-film limit. $Ce_3Bi_4Pd_3$, a Weyl-Kondo semimetal synthesized in thin-film form for the first time, is distinguished by its giant spontaneous Hall effect observed in bulk. Recently, it has been proposed that this spontaneous Hall effect could be enhanced through gating of the thin film, further expanding its potential for applications in microwave circuit technologies. These findings not only facilitate the investigation of emergent quantum phenomena and quantum criticality in reduced dimensions but also lay the groundwork for incorporating heavy-fermion materials into heterostructures and devices for quantum technologies.

I will first discuss transport of interacting bosons through an Aharonov-Bohm cage - a building block of flat band networks. In the absence of interactions the cage is insulating due to destructive interference. The cage stays insulating up to a critical value of the pump strength in the presence of mean field interactions, while the quantum regime induces particle pair transport and weak conductance below the critical pump strength. A swift crossover from quantum into the classical regime upon further pump strength increase is observed. I will then take the interaction to infinity and discuss trapping of hard core bosons in flat band networks both in one and two dimensions. The trapping leads to Hilbert space fragmentation and survives in the presence of suitable correlated disorder. References [1] A. R. Kolovsky, P. S. Muraev and S. Flach, Phys. Rev. A 108 (2023) L010201. [2] S. Lee, A. Andreanov, T. Sedrakyan and S. Flach, Phys. Rev. B 109 (2024) 245137.

Metals, contrary to usual belief, can show coherent spatial modulations in the electronic density, called charge density waves (CDWs). In superconductors, CDWs can be related to modulations in the spatial dependence of the Cooper pair density. Such modulations have been found in cuprates, pnictides, dichalcogenides, Kagomé materials, and in the heavy fermion UTe$_2$. Contrasting many other compounds, in UTe$_2$ the CDW modulation has only been observed in surface sensitive Scanning Tunneling Microscopy (STM) experiments. Furthermore, there is an apparent dissimilarity between the CDW wavevectors and known bulk properties. For this reason, a decoupling between electronic properties of bulk and surface has been suggested. We resolve, through experiments revealing new CDW wavevectors and a detailed comparison with density functional theory, the relationship between the CDW and bulk band structure.

Long-range magnetic order in heavy-fermion compounds can be suppressed via two fundamentally different mechanisms: either by screening the magnetic moments through the Kondo effect or by modifying the magnetic interactions between moments via geometric frustration or reduced dimensionality. The interplay between these different quantum-critical fluctuations is expected to give rise to novel electronic states. The heavy-fermion compound CePdAl crystallizes in a distorted kagome structure, placing it in close proximity to both instabilities. Recent transport and µSR measurements under hydrostatic pressure have revealed that, at the border of long-range antiferromagnetic order, an anomalous quantum-critical state with spin-liquid-like properties emerges [1,2]. To investigate the impact of these quantum-critical instabilities on the phase diagram of CePdAl, we conducted thermal-expansion and magnetostriction measurements under small uniaxial pressure across a wide temperature and field range. By analyzing the uniaxial-pressure, field, and temperature dependence of entropy, we inferred the underlying characteristic energy scales and disentangled their effects on the different phases. The resulting extended phase diagram is compared with those of related compounds and discussed in the context of the global phase diagram of heavy-fermion systems.

In this contribution I will present, crystal growth, characterisation and high-resolution single crystal x-ray diffraction study of Kagome superconductor CsV$\mathrm{_3}$Sb$\mathrm{_5}$}. We discover that at low temperatures, the structural modulations of the electronic superlattice, commonly associated with charge-density-wave order, undergo a transformation around $p \sim$ 0.7 GPa from the familiar $2\times2$ pattern to a long-range-ordered modulation at wavevector $q=(0, 3/8, 1/2)$. This shift is accompanied by a monoclinic distortion and persists until the pressure exceeds about 1.7 GPa. Our observations align with inferred changes in the CDW pattern from prior transport and nuclear-magnetic-resonance studies, providing new insights into these transitions. Interestingly, the pressure-induced variations in the electronic superlattice correlate with two peaks in the superconducting transition temperature as pressure changes, hinting that fluctuations within the electronic superlattice could be key to stabilizing superconductivity. regarding the crystallographic structure, will be discussed.

Kondo insulators acquire their incompressible character via a spin exchange coupling between itinerant charge carriers and local moments. Motivated by recent advances in tunable 2D materials, we focus on a class of Kondo insulator models where both the bandwidth and the quantum geometry of the itinerant electrons can be tuned. We argue that already in the simplest such model interesting physics arises. In particular, we present evidence that in a 1D model tuning the kinetic energy of the itinerant electrons drives a second order phase transition where the critical fluctuations are carried by the spin degrees of freedom. Moreover, the transition falls outside the usual Landau paradigm for symmetry-breaking transitions, and has connections to generalized Lieb-Schultz-Mattis obstructions.

Recent muon spin rotation ($\mu$SR) experiments on elementary rhenium have shown that its superconducting phase spontaneously breaks time-reversal symmetry [T. Shang et al., Phys. Rev. Lett. 121, 257002 (2018)]. Ab initio calculations further indicate that finite magnetic moments of opposite directions arise on the two atoms in the crystal’s elementary cell, coexisting with the superconducting state [G. Csire et al., Phys. Rev. B 106, L020501 (2022)]. The observed activated specific heat can be accounted for by a mixture of spin-singlet and spin-triplet Cooper pairs. To elucidate these findings, we performed a comprehensive symmetry classification of all possible superconducting and magnetic order parameters in the nonsymmorphic crystal structure of rhenium. We employed double-group theory to classify the spin-orbit coupled electronic states. We identified the relevant symmetry channels for time-reversal symmetry breaking and determined the corresponding terms in effective interaction. Our results shed light on the interplay between spin and orbital degrees of freedom in rhenium and provide a theoretical framework for exploring magnetism and superconductivity in other nonsymmorphic materials.

The quasi-1D conductor Li$_{0.9}$Mo$_6$O$_{17}$ (LMO) possesses a remarkably complex electronic character [1]. At high $T$, LMO exhibits many signatures of a Tomonaga-Luttinger liquid (TLL) [2,3], while at low T, the in-chain resistivity tends first towards an insulating ground state [4] before transitioning into a superconductor. The associated high upper critical field suggests a possible triplet pairing state [5]. While LMO lies close to \textonequarter filling, the non-stoichiometry of the Li prevents the filling from becoming truly commensurate. As a result, LMO sits on the boundary between a gapped (Mott) insulating state and a gapless metallic state (that ultimately becomes superconducting). It has been proposed that as temperature is reduced, the chain carriers interact progressively more strongly with \lq dark' excitons located near the Fermi level to produce an enhanced scattering amplitude $\gamma$ that drives the system ever closer to the Mott state [1,6]. This near-degeneracy between the SC and Mott states at low temperatures has been interpreted as a rare, if not unique, example of emergent symmetry [7]. The origin of the resistive upturn in LMO has been the subject of debate now for over 40 years. One piece of evidence in favour of the dark exciton scenario is the observation that the resistive upturn can be strongly suppressed by a large magnetic field [4], but that this suppression is minimized when the field is oriented along the poles of the MoO$_6$ octahedra [6]. This is due to the fact that for any other field orientation, the magnetic field mixes the angular momenta of these exciton states and causes the exciton Wigner lattice to melt, thereby reducing the resistive upturn [6]. In order to shed more light on the nature of the resistive upturn, we have studied the effect of uniaxial strain on the LMO ground state by studying the resistivity and associated high-field magnetoresistance (MR) of single crystals strained along either the $a$-axis or $b$-axis in fields up to 30 T and temperatures down to 1.3 K. Applying uniaxial strain simultaneously tunes the inter- and intrachain hopping parameters in the crystal in an anisotropic fashion. The strength of $\gamma$ depends sensitively on these hopping probabilities. We find that the zero-field resistivity and the low-$T$ MR are strongly tuned for strain along the $a$-axis, while strain applied along the $b$-axis induces almost no response, consistent with the quasi-1D nature of LMO. As we will argue, these findings support the picture of an excitonic origin to the low-$T$ upturn in LMO. Moreover, we show that uniaxial strain provides a novel way to tune the balance between the Mott and SC states in LMO without the need for high field strengths. [1] Chudzinski, \textit{EPJB} \textbf{90}, 148 (17) [2] Dudy et al., \textit{JPCM} \textbf{25}, 014007 (13) [3] Wakeham et al., \textit{Nat. Commun.} \textbf{2}, 396 (11) [4] Xu et al., \textit{PRL} \textbf{102}, 206602 (09) [5] Mercure et al., \textit{PRL} \textbf{108}, 187003 (12) [6] Lu et al., \textit{Sci. Adv.} \textbf{5}, eaar8027 (19) [7] Chudzinski et al., \textit{Science} \textbf{382}, 792 (23)

We have uncovered a novel phase transition of the celebrated Laughlin Fractional quantum Hall wave-function from its topologically ordered fluid phase onto a power-law-correlated vortex crystal in flat Chern bands with ideal quantum geometry. We will present a theory of ground state correlations and collective modes of these states across this transition and discuss their potential relevance to anomalous fractional quantum Hall phenomena in platforms such as moiré MoTe2, twisted bilayer graphene and pentalayer graphene.

Twisted graphene systems form an exciting platform to study the effects of strong electronic correlations arising due to flat bands. Members of the twisted graphene family like magic-angle twisted bilayer graphene and magic-angle twisted trilayer graphene (MATTG) exhibit unconventional superconductivity. The origin of this unconventional superconductivity largely remains elusive. In our experiments with the MATTG, we observe evidence of MATTG being an inhomogeneous superconductor due to the presence of moiré solitons and twistons; creating weak links in the superconductor which behave like an array of Josephson junctions. We gather large statistics on the switching from the superconducting to the normal state - the switching distributions. These distributions are studied with the variation of temperature and in-plane magnetic field providing evidence of a competing magnetic order in the ground state. Our experiments also provide credence for the heavy fermion description for the MATTG with the localized moment. We observe additional evidence for magnetic order in hysteresis and magnetoresistance with in-plane magnetic field, in normal regions doped slightly away from the superconducting pockets. The normal state hysteretic magnetic response could arise from localized moments. Also, our DC I-V characteristics, on analysis, reveal a broadened BKT transition as we extract the superfluid stiffness Js~0.15 K in MATTG.

Recent scanning tunneling spectroscopy (STS) experiments on the surface of the predicted topological Kondo insulator SmB$_{6}$ revealed that the peak in the local surface density of states (LDOS) is suppressed by a Gd impurity at the surface [1]. Surprisingly, this suppression extends spatially to a distance of several nm from the impurity, much larger than the range of the impurity potential, which is still not fully understood. In the present work, we model the SmB$_{6}$ bulk-surface heavy-fermion system by a layered auxiliary boson mean-field theory at zero temperature, while the Gd impurity possesses a static spin $S = 7/2$ in the 4f shell. This magnetic spin breaks time-reversal symmetry locally. We show that this leads to a suppression of the topological surface states in the vicinity of the impurity on the scale of nm in agreement with the experimental observations. Furthermore, the suppression length scales inversely with the heavy-fermion lattice Kondo temperature, indicating that the suppression of the Kondo effect is responsible for the destruction of the surface states observed over a large spatial range. Additionally, we examine the impact of the impurity on the edge states, revealing a curling of the edge states into the bulk around the impurity. [1] L. Jiao et al., Magnetic and defect probes of the SmB6 surface state. Sci. Adv. 4 (2018)

In heavy-fermion (HF) systems the interplay of the local Kondo exchange interaction and the long-range RKKY interaction remains a difficult problem. We propose a perturbative renormalization group technique which which can treat the Kondo- and RKKY-coupling on the ordered site of the Doniach phase diagram above the ordering temperature as well as on the heavy Fermi-liquid site. The Kondo screening of the local moments and the RKKY-induced ordering of the same are calculated in a selfconsistent way. We predict the coexistence of Kondo effect and magnetic order in a system where the RKKY-wavelength is commensurate with the lattice spacing, which was recently observed in a CeCoGe_{3} compound [1]. Furthermore we calculate the Kondo quasiparticle weight and predict the reduced spectral weight far above the ordering temperature in CeCu_{6-x}Au_{x} due to the frustrated RKKY interaction [2].\\ [1] P. Li \textit{et al.}, Phys. Rev. B 107, L201104 (2023) [2] J. Li \textit{et al.}, Phys. Rev. B 111, 035117 – (2025)

Electrons in gapless flat bands show markedly distinct responses both in noninteracting and interacting scenarios. Even the simplest setting is rich: band degeneracies involving one gapless flat band and a quadratically dispersive band can be classified into singular and non-singular categories based on the underlying quantum geometry and the singularity of the Bloch wavefunction at the degeneracy point. For a singular flat band, the application of a uniform magnetic field induces Landau levels with an anomalous energy quantization E ∼ −1/n below the energy of the flat band. We discuss the response of these anomalous Landau levels to periodic flux patterns that can manipulate the width of this anomalous spreading and lead to a degenerate manifold of zero modes (featuring an emergent supersymmetry) which is fertile to host exotic interaction-induced phases such as a fractional quantum Hall state and a generalized Wigner crystal with plausible phase transitions in between. In the end, we will illuminate the possibility of emulating the effect of anomalous Landau levels of a flat-band lattice model in a superconducting quantum processor where an adjustable synthetic gauge field can be realized by applying continuous modulation tones to all qubits.

Hydrides are famous for enabling high-$T_C$ conventional superconductivity. Here we propose ternary nickel hydrides $\ce{MNiH_2}$ ($\ce{M}=\ce{Na},\ce{Li}$) as a novel class of materials that mimic key aspects of unconventional superconducting cuprates and nickelates while exhibiting important differences. Compared to $\ce{Ni}$ oxides, density functional theory (DFT) computations show that the $\ce{Ni}-\ce{H}$ bands are wider due to shorter bond lengths and show a smaller charge transfer energy. This leads to a larger scale of magnetic interactions than for \ce{LaNiO_2}, which previous works on cuprates suggest should lead to a higher $T_C$. Based on the DFT computations, we develop a $t-J$ model specific for this class of materials and compute the superconducting order parameter phase diagram as a function of doping. Similar to cuprates and nickelates, the physics is mainly determined by the $\ce{NiH_2}$ layers for both the 2D and 3D cases. We find that 3D effects are stronger than in cuprates and tend to depress $T_C$. We discuss the prospect of stabilizing purely two-dimensional versions of this material in appropriate subtrates. [1] Ternary nickel hydrides: a new platform for unconventional superconductivity and quantum magnetism, Mateusz Domanski, Antonio Santacesaria, Paolo Barone, José Lorenzana, Wojciech Grochala, arXiv:2409.0669 Work done in collaboration with: Mateusz Domanski, Paolo Barone, Wojciech Grochala, José Lorenzana

The magnetic anisotropy of a typical crystalline compound is often attributed to the combined effect of crystal electric fields and spin-orbit coupling. We show that this simple picture is transformed in heavy-fermion compounds by the development of coherent electron scattering from local spin degrees of freedom. As the latter fractionalise and delocalise, they frustrate the magnetic anisotropy of the crystal by generating competing anisotropies in the effective moments and Curie-Weiss constants. Collective and local forces oppose each other in determining the direction of dominant magnetic fluctuations. This frustration manifests itself through the reorientation of the crystal's magnetic easy axis as it transitions from the high-temperature paramagnetic regime to the low-temperature magnetically correlated regime. We present an extension of Read-Newns theory to the underscreened Kondo lattice which we confirm agrees well with recent experimental results. In particular, that the temperature at which the reorientation takes place tracks the coherence energy scale across a wide range of actinide and lanthanide compounds. In some compounds the frustration can be the beginning of a cascade of decreasing energy scales. Once the easy-axis changes, further growth of magnetic correlations with cooling can lead to a rapid quenching of the hard-axis susceptibility, which we capture theoretically. At even lower temperatures, the high-temperature easy axis is revealed via a metamagnetic jump when a high magnetic field is applied along it, and in some cases the jump leads to the destruction of superconductivity.

We discuss a formation mechanism of the interlayer paired electronic superfluid state due to repulsive interaction between layers in graphene double layers. We study the competition between several possible pairing channels, from which a particularinterlayer order parameter emerges, ultimately responsible for the superconductivity in the system. The effect of the interlayer twist on this state is elucidated using perturbative renormalization group. The collective quantum state corresponding to that macroscopic phase is entangled with the order parameter being the measure of entanglement. We discuss the measurable consequences of the predicted effects and suggest possible experimental and technological setups.

Skyrmions are magnetic spin texture carrying a non-trivial topological charge and a well-defined topological charge density. In this contribution, we show that their dynamics is governed by the conservation of the topological dipole associated with the topological charge density. The conservation owes its origin to the translation invariance and the fact that the topological charge density is the generator of an area-preserving diffeomorphism. We discuss three remarkable implications. Firstly, the notion of skyrmion mass, which is often debated and appearing non-universal, can be formulated to be identically zero. Secondly, the topological dipole conservation brings about a correspondence between skyrmions and fractons. The latter are exotic quasiparticles exhibiting constrained mobility and a variety of unusual behaviors, owing to a similar conservation of their dipole moment and, in some cases, also their multipole moments. However, their physical realization is scarce in quantum-material platforms. Therefore, skyrmions provide a realistic setting to explore fractonic phenomena in experiments. Lastly, the dynamics of skyrmion is analogous to the guiding center of the Landau orbit in a quantum Hall problem. In particular, the topological charge density obeys the Girvin-MacDonald-Platzman algebra, which suggests an exciting prospect of realizing highly nontrivial quantum liquids of skyrmion, akin to the fractional quantum Hall states.

The collapse of the Kondo regime, followed by a delayed THz pulse emission upon its recovery has been observed in recent THz spectroscopy experiments on heavy-fermion compounds such as CeCu$_{6-x}$Au$_{x}$ [1]. In this work [2], a theoretical framework is developed to describe the non-equilibrium dynamics by employing the Anderson lattice model, time-dependent non-equilibrium dynamical mean-field theory, and the non-crossing approximation . We identify two key non-equilibrium mechanisms that play pivotal roles in the collapse and subsequent revival of Kondo coherence. First, due to the pulse intensity the hybridization between localized $f$-electrons and conduction electrons increases, shifting the system to a mixed-valence regime, leading to a rapid destruction of the Kondo state. Second, while the distribution function and the single-particle peak recover quickly, the Kondo peak requires significantly more time due to the intrinsic low-energy effects associated with Kondo physics. Additionally, we confirm the system's ability to emit a non-superradiant delayed pulse upon the recovery of the Kondo regime, confirming the many-body origin of the experimentally observed delayed pulse. The formalism developed paves the way for examining transient light-pulse interactions with many-body systems. [1] C. Wetli, S. Pal, J. Kroha, K. Kliemt, C. Krellner, O. Stockert, H. v. Löhneysen, and M. Fiebig, Nature Phys 14, 1103 (2018). [2] F. Meirinhos, M. Turaev, M. Kajan, T. Bode and J. Kroha /to be published/

Fixed-point annihilation is a generic mechanism that generates an extremely slow RG flow and has been suggested to explain the occurrence of a weak first-order transition instead of a deconfined critical point in two-dimensional quantum magnets. Here we explore this phenomenon in a (0+1)-dimensional spin-boson model which can be solved with unprecedented numerical accuracy using the recently-developed wormhole quantum Monte Carlo method. We find a tunable transition between two ordered phases that can be continuous or first-order, and even becomes weakly first-order in an extended regime close to the fixed-point collision. We provide direct numerical evidence for pseudo-critical scaling on both sides of the collision manifesting in an extremely slow drift of critical exponents. We also find scaling behavior at the symmetry-enhanced first-order transition as described by a discontinuity fixed point. Our study motivates future work in higher-dimensional quantum dissipative spin systems.

In the past few years, UTe₂, a heavy fermion superconductor, has been proposed as a potential spin-triplet superconductor [1], with an anisotropic and rich phase diagram. A prominent feature of the phase diagram, where much of the scientific effort has been focused, is a re-entrant superconducting phase up to 35 tesla for magnetic fields aligned near the b-axis [2]. This superconducting phase ends when a metamagnetic transition appears [3], suggesting that magnetism and superconductivity are interconnected. We set out to understand the high field magnetic anisotropy surrounding this superconducting phase by exploring the normal state of UTe₂ with resonant torsion magnetometry [4] in pulsed magnetic fields. High-quality crystals with Tc = 2 K were measured up to 60 T in the a-c and b-c planes at T = 4 K. In both cases, we found a sharp decrease in the magnetotropic susceptibility that sets in around 20 T when the field is aligned near the c-axis. At high fields, our measurements uniquely access the magnetic susceptibility transverse to the applied magnetic field [5]. The drop in the magnetotropic susceptibility for fields along the c-axis thus suggests a significant increase in χaa and χbb -- or an easy plane anisotropy -- at high fields. This behavior is likely related to anisotropic fluctuations of the U-dimers in strong magnetic fields. [1] Ran, Sheng, et al. "Nearly ferromagnetic spin-triplet superconductivity." Science 365.6454 (2019): 684-687. [2] Ran, Sheng, et al. "Extreme magnetic field-boosted superconductivity." Nature physics 15.12 (2019): 1250-1254. [3] Miyake, Atsushi, et al. "Metamagnetic transition in heavy fermion superconductor UTe2." journal of the physical society of japan 88.6 (2019): 063706. [4] Modic, Kimberly A., et al. "Resonant torsion magnetometry in anisotropic quantum materials." Nature Communications 9.1 (2018): 3975. [5] Shekhter, A., et al. "Magnetotropic susceptibility." Physical Review B 108.3 (2023): 035111.