Phasonic Spectroscopy of a Quantum Gas in a Quasicrystalline Lattice
Phasonic degrees of freedom are unique to quasiperiodic structures and play a central role in poorly understood properties of quasicrystals from excitation spectra to wave function statistics to electronic transport. However, phasons are challenging to access dynamically in the solid state due to their complex long-range character and the effects of disorder and strain. We report phasonic spectroscopy of a quantum gas in a one-dimensional quasicrystalline optical lattice. We observe that strong phasonic driving produces a nonperturbative high-harmonic plateau strikingly different from the effects of standard dipolar driving. Tuning the potential from crystalline to quasicrystalline, we identify spectroscopic signatures of quasiperiodicity and interactions and map the emergence of a multifractal energy spectrum, opening a path to direct imaging of the Hofstadter butterfly.
S.V. Rajagopal et al., Phys. Rev. Lett. 123, 223201 (2019)
Emergent Quasicrystalline Symmetry in Light-Induced Quantum Phase Transitions
The discovery of quasicrystals with crystallographically forbidden rotational symmetries has changed the notion of the ordering in materials, yet little is known about the dynamical emergence of such exotic forms of order. Here we theoretically study a nonequilibrium cavity-QED setup realizing a zero-temperature quantum phase transition from a homogeneous Bose-Einstein condensate to a quasicrystalline phase via collective superradiant light scattering. Across the superradiant phase transition, collective light scattering creates a dynamical, quasicrystalline optical potential for the atoms. Remarkably, the quasicrystalline potential is “emergent” as its eightfold rotational symmetry is not present in the Hamiltonian of the system, rather appears solely in the low-energy states. For sufficiently strong two-body contact interactions between atoms, a quasicrystalline order is stabilized in the system, while for weakly interacting atoms the condensate is localized in one or few of the deepest minima of the quasicrystalline potential.
F. Mivehvar et al., Phys. Rev. Lett. 123, 210604 (2019)
Quantifying and Controlling Prethermal Nonergodicity in Interacting Floquet Matter
Periodic driving has become a key element in the experimental efforts to synthesize interesting many-body quantum states. Importantly, this depends crucially on the existence of a prethermal regime, which exhibits drive-tunable properties while forestalling the effects of undesirable heating. This dependence motivates the search for direct experimental probes of the underlying localized nonergodic nature of the wave function in this metastable regime. We report experiments on a many-body Floquet system consisting of atoms in an optical lattice subjected to ultrastrong sign-changing amplitude modulation. In this work, we measure an inverse participation ratio quantifying the degree of prethermal localization of the many-body wave function as a function of tunable drive parameters and interactions. We obtain a complete prethermal map of the drive-dependent properties of Floquet matter spanning four square decades of parameter space. Following the full time evolution, we observe sequential formation of two prethermal plateaux, interaction-driven ergodicity, and strongly frequency-dependent dynamics of long-time thermalization. The quantitative characterization of the prethermal Floquet matter realized in these experiments, along with the demonstration of control of its properties by variation of drive parameters and interactions, opens a new frontier for probing far-from-equilibrium quantum statistical mechanics and new possibilities for dynamical quantum engineering.
Cavity-Quantum-Electrodynamical Toolbox for Quantum Magnetism
The recent experimental observation of spinor self-ordering of ultracold atoms in optical resonators has set the stage for the exploration of emergent magnetic orders in quantum-gas-cavity systems. Based on this platform, we introduce a generic scheme for the implementation of long-range quantum spin Hamiltonians composed of various types of couplings, including Heisenberg and Dzyaloshinskii-Moriya interactions. Our model is composed of an effective two-component Bose-Einstein condensate, driven by two classical pump lasers and coupled to a single dynamic mode of a linear cavity in a double $\Lambda$ scheme. Cavity photons mediate the long-range spin-spin interactions with spatially modulated coupling coefficients, where the latter ones can be tuned by modifying spatial profiles of the pump lasers. As experimentally relevant examples, we demonstrate that by properly choosing the spatial profiles of the pump lasers achiral domain-wall antiferromagnetic and chiral spin-spiral orders emerge beyond critical laser strengths. The transition between these two phases can be observed in a single experimental setup by tuning the reflectivity of a mirror. We also discuss extensions of our scheme for the implementation of other classes of spin Hamiltonians.
F. Mivehvar et al., Phys. Rev. Lett. 122, 113603 (2019)
Genes lost during the transition from land to water in cetaceans highlight genomic changes associated with aquatic adaptations
While whales and dolphins spend their entire life in the ocean, these air-breathing mammals actually evolved from terrestrial species. The transition from land to water in the ancestors of modern whales and dolphins about 50 million years ago was accompanied by profound anatomical, physiological, and behavioral adaptations that facilitated life in water. However, which changes in the DNA underlie these adaptations remains incompletely understood. To reveal them, researchers at the Max Planck Institute of Molecular Cell Biology and Genetics (MPI-CBG), the MPI for the Physics of Complex Systems (MPI-PKS), and the Center for Systems Biology Dresden (CSBD) systematically searched for genes that were lost in the ancestor of today’s whales and dolphins. The research team discovered 85 gene losses, some of which likely helped whales to thrive in their new habitat.
M. Huelsmann et al., Science Advances 5, eaaw6671 (2019)
Dynamical Structure Factor of the Three-Dimensional Quantum Spin Liquid Candidate NaCaNi$_2$F$_7$
We study the dynamical structure factor of the spin-1 pyrochlore material NaCaNi$_2$F$_7$, which is well described by a weakly perturbed nearest-neighbour Heisenberg Hamiltonian, Our three approaches—molecular dynamics simulations, stochastic dynamical theory, and linear spin wave theory—reproduce remarkably well the momentum dependence of the experimental inelastic neutron scattering intensity as well as its energy dependence with the exception of the lowest energies. We discuss two surprising aspects and their implications for quantum spin liquids in general: the complete lack of sharp quasiparticle excitations in momentum space and the success of the linear spin wave theory in a regime where it would be expected to fail for several reasons.
S. Zhang et al., Phys. Rev. Lett. 122, 167203 (2019)
Congratulations to Markus on receiving a starting grant of the European Research Council (ERC)! The starting grants can be awarded to scientists who conduct their research at an EU-based research organisation and who have two to seven years of experience since completion of their PhD. The grants are awarded on an annual basis and valued at up to 1.5 million euros each.
Bath-Induced Decay of Stark Many-Body Localization
Recently, Stark-localized systems of interacting fermions in tilted lattices were predicted to show signatures of many-body localization similar to Anderson-localized interacting fermions in disorder potentials. Investigating the decay of such Stark many-body localization induced by the coupling to a dephasing bath (as it can be engineered for ultracold atoms in optical lattices), we find qualitative differences in comparison to disordered systems. For instance, the bath-induced growth of the total von Neumann entropy is not found to be logarithmically slow. Our results can directly be tested in systems of ultracold atoms in optical lattices.
How to Directly Measure Floquet Topological Invariants in Optical Lattices
The classification of topological Floquet systems with time-periodic
Hamiltonians transcends that of static systems. For example, spinless
fermions in periodically driven two-dimensional lattices are not
completely characterized by the Chern numbers of the quasienergy bands,
but rather by a set of winding numbers associated with the gaps. We
propose a feasible scheme for measuring these winding numbers in a
periodically driven optical lattice efficiently and directly. It is
based on measuring the topological charges of band-touching
singularities while adiabatically connecting the system of interest to
the topologically trivial high-frequency limit. As a by-product, we also
propose a method for probing spectral properties of time evolution
operators via a time analog of crystallography.
F. Nur Ünal et al., Phys. Rev. Lett. 122, 253601 (2019)
Avoided quasiparticle decay from strong quantum interactions
Quantum states of matter—such as solids, magnets and topological phases—typically exhibit collective excitations (for example, phonons, magnons and anyons). These involve the motion of many particles in the system, yet, remarkably, act like a single emergent entity—a quasiparticle. Known to be long lived at the lowest energies, quasiparticles are expected to become unstable when encountering the inevitable continuum of many-particle excited states at high energies, where decay is kinematically allowed. Although this is correct for weak interactions, we show that strong interactions generically stabilize quasiparticles by pushing them out of the continuum. This general mechanism is straightforwardly illustrated in an exactly solvable model. Using state-of-the-art numerics, we find it at work in the spin-1/2 triangular-lattice Heisenberg antiferromagnet (TLHAF). This is surprising given the expectation of magnon decay in this paradigmatic frustrated magnet. Turning to existing experimental data, we identify the detailed phenomenology of avoided decay in the TLHAF material Ba3CoSb2O9, and even in liquid helium, one of the earliest instances of quasiparticle decay. Our work unifies various phenomena above the universal low-energy regime in a comprehensive description. This broadens our window of understanding of many-body excitations, and provides a new perspective for controlling and stabilizing quantum matter in the strongly interacting regime.
R. Verresen et al, Nature Physics (2019), DOI: 10.1038/s41567-019-0535-3