Highlights

In a number of different behaviour, so-called non-Fermi-liquid behaviour appears due to strong correlations between electrons. The standard theoretical scenario relies on emergent collective bosonic modes with strong critical fluctuations that destroy the electronic quasiparticles. Due to the complexity of the actual material, it is difficult to determine the microscopic origin of the relevant bosonic modes systematically. Peng Rao and Francesco Piazza of the Max Planck institute for the Physics of Complex Systems have now shown that cavity quantum electrodynamics within two-dimensional materials is ideal to implement non-Fermi-liquid behaviour. The emergent bosonic modes belong here to the vacuum electromagnetic field, a microscopic degree of freedom of which the dynamics and coupling with electrons can be controlled by cavity engineering.

P. Rao and F. Piazza, Phys. Rev. Lett. 130, 083603 (2023)

Preservation of coherence is a fundamental, yet subtle, phenomenon in open systems. We uncover its relation to symmetries respected by the system Hamiltonian and its coupling to the environment. We discriminate between local and global classes of decoherence-free subspaces for many-body systems through the introduction of “ghost variables”. The latter are orthogonal to the symmetry and the coupling to the environment depends solely on them. Constructing them is facilitated in classical phase space and can be transferred to quantum mechanics through the equivalent role that Poisson and Lie algebras play for symmetries in classical and quantum mechanics, respectively. Examples are given for an interacting spin system.

J. Dubois, U.Saalmann, and J.M. Rost, Phys. Rev. Research 5, L012003 (2023)

We cordially welcome the arrival of our new group at the institute, headed by Alexander Wietek, who joins us from the Flatiron Institute. Alex's group is interested in the way quantum particles, like electrons or atoms, organize themselves while interacting with one another. This way, Alex and his colleagues aim at understanding how the macroscopic behavior of materials, like various forms of magnetism or superconductivity, emerges. Besides trying to explain existing experimental phenomena in solid-state physics, they investigate under which circumstances entirely new states of matter, like quantum spin liquids, can occur. To solve these questions, the new group is developing numerical technology to simulate quantum many-body systems. The quantum many-body problem is considered to be exponentially hard in the number of particles. One approach Alex is pursuing is to push the limits of exact simulations by developing high-performance computing software and distributed parallel algorithms for quantum many-body systems. Furthermore, the team is also embracing tensor network methods to reduce computational complexity by representing data efficiently. Welcome at MPI-PKS!!

Time crystals are a freshly discovered nonequilibrium phase of matter without an equilibrium counterpart, stabilized by external periodic drives and characterized by broken spatiotemporal symmetry. Scientists from the Nonequilibrium Quantum Dynamics group at the Max Planck Institute for the Physics of Complex Systems, together with collaborators at the KTH Royal Institute of Technology and at UC Berkeley, created a critical time crystal in a system of long-range interacting nuclear spins. Designing a novel two-frequency external driving protocol allowed the scientists to monitor the time-crystalline behavior continuously (avoiding the wave function collapse), and take real-time movies displaying the formation, lifetime, and meltdown of this exotic phase of matter. The experimental platform used offers unprecedented clarity and measurement throughput, which turned out fundamental for determining the boundaries of the time-crystalline phase, and investigating in detail the melting dynamics of the time crystal as it gradually heats up.

W. Beatrez et al., Nat. Phys. 19, 407 (2023)

A new type of fractal has been discovered in a class of materials called spin ices—famous, among other reasons, for their emergent magnetic monopole excitations. Spin ice materials are some of the most researched and best understood topological magnets. Nevertheless, the unusual dynamical properties of spin ice have been puzzling scientists for almost two decades. An international research team, including Jonathan N. Hallén and Roderich Moessner of the Max Planck Institute for the Physics of Complex Systems, has now shown that the dynamical rules governing the motion of the magnetic monopoles constrain these to move on fractal structures. By hosting the monopole motion, the fractals cause the peculiar dynamical behaviours observed in spin ice materials.
The discovery was surprising because the fractals were seen in a clean three-dimensional crystal, where they would not be expected conventionally. Even more remarkably, the fractals are visible in dynamical properties of the crystal, and hidden in static ones. The capacity of spin ice to exhibit such striking phenomena makes the team hopeful that spin ice will allow further surprising discoveries in the cooperative dynamics of even simple topological many-body systems. More details can be found in a press release (PDF).

J. N. Hallén et al., Science 378, 1218 (2022)
See also the related Science Perspective article by F. Flicker.

An international team of scientists including Roderich Moessner of the Max Planck Institute for the Physics of Complex Systems has observed an exotic quantum state of matter: a spin-orbital liquid formed on the pyrochlore oxide Pr₂Zr₂O₇. Here, both spin and orbital degrees of freedom remain dynamic down to extremely low temperature. It is known from the long history of condensed matter physics that suppressing orbital order down to low temperatures is extremely difficult, a precondition for the next tricky step of obtaining a spin-orbital liquid state. Pr₂Zr₂O₇ serves as a rare counterexample in which spins and orbitals are interlocked, so that fluctuation of one necessitates fluctuation of the other. More details can be found in a press release (PDF).

N. Tang, Y. Grisenko, K. Kimura et al., Nat. Phys. (2022)
See also the related Nat. Phys. News & Views article by V. S. Zapf, M. Lee, and P. F. S. Rosa.

Amorphous solids, which include colloidal glasses, dense emulsions, foams, and granular materials, are ubiquitous and important in both engineering and industry. When subjected to a suddenly imposed stress, they can exhibit a transient flow known as creep during which the flow rate decays as a power law over time. This power law is characterized by a quantity called the creep exponent. If the stress inducing the creep flow is low, the material eventually stops moving. But if this stress is sufficiently high, the power-law decay can be followed by sudden fluidization. Together with colleagues from École polytechnique fédérale de Lausanne (EPFL) and Université Paris-Saclay, Marko Popović of the Max Planck Institute for Physics of Complex Systems developed a theory of creep flow that can predict both the creep exponent and the time at which sudden fluidization occurs, as well as the temperature dependence of these two quantities. These predictions have been tested in numerical simulations and are consistent with previously published experimental observations. The key ingredient of the proposed theory is the new concept of a transient yield stress, which reflects the dynamics of the maximal stress that the material could sustain without flowing while it undergoes creep flow. Remarkably, the scaling of the creep exponent and the time of fluidization then follow from generic properties of the transient yield stress for both athermal and thermal systems. The success of the transient yield stress concept opens new exciting questions: What is the origin of the transient yield stress and what controls its dynamics? Can the concept of a transient yield stress be employed to describe other characteristic behaviours associated with the yielding of amorphous solids, such as shear banding instabilities?

Marko Popović, et al., Phys. Rev. Lett. 129, 208001 (2022), Editors' Suggestion
Selected for a Synopsis in Physics

The mechanism behind high-temperature superconductors has long been a great mystery to physicists. Even though the fundamental physical equations of interacting electrons in these materials are well-known, their solution has proved challenging. Whether or not superconductivity or the so-called "stripe order", where the density of electrons forms regular waves called "stripes", is realized has been an open question and these two states of matter have been considered in competition to one another. This study now shows that stripe order and superconductivity can, in fact, get along with one another quite well. By performing exact numerical simulations on a minimal model for cuprate superconductors, the author demonstrates that there exist states of matter with exactly one superconducting condensate per stripe. Since there are multiple stripes in the systems, there are also multiple condensates, a phenomenon called "fragmentation". The physical picture proposed in this study agrees well with experimental observations and demonstrates the predictive power of modern numerical techniques to study quantum many-body systems.

A. Wietek, Phys. Rev. Lett. 129, 177001 (2022)

Application deadline: 17 November 2022. Distinguished PKS postdoctoral fellows appear personally along with the departments and groups on the main research page of the institute and are expected to have at least one year of postdoctoral experience at an institution other than the one at which their PhD was awarded. Applications for this fellowship directly after completion of the PhD might be considered in exceptional cases. Please click on the link- button to see the full advertisement!

The different microbial species in complex ecological communities like the human microbiome often have different subpopulations called phenotypes, between which they can switch stochastically or in response to environmental cues, such as toxins released by competitors or antibiotics. Pierre Haas of the Max Planck Institute for the Physics of Complex Systems and the Max Planck Institute of Molecular Cell Biology and Genetics and collaborators at the University of Cambridge have analysed the ecological implications of such responsive switching. They combined a statistical analysis of many-species systems, a numerical study of a minimal two-species model, and analytical results for still simpler mathematical models. While responsive switching to a rare phenotype is destabilising on average, they could show that responsive switching to a rare "attack" phenotype is stabilising on average. A similar "attack" subpopulation was recently observed experimentally, which underlines the importance of responsive switching for ecological stability.

P. A. Haas, M. A. Gutierrez, N. M. Oliveira, and R. E. Goldstein, Phys. Rev. Research 4, 033224 (2022)