Institute of Laser Physics
Institute
of Laserphysics
of Laserphysics
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Theory Group Ultra-cold atoms and solid state systems
Prof. Dr. Ludwig Mathey
We investigate the theory of ultracold atom and solid state systems, with particular emphasis on many-body effects. One of our primary research interest is many-body dynamics, such as dynamic phase transitions and dynamic control of many-body systems, as well as properties of superfluidity and superconductivity. Furthermore, we study the equilibrium phase diagrams of ultra-cold atoms, at both finite and zero temperature. Another objective is the advancement of the technology of ultra-cold atom systems, for example by investigating new cooling and detection methods.
Recent activities:
Time crystals in a shaken atom-cavity system
Jayson G. Cosme, Jim Skulte, Ludwig Mathey
We demonstrate the emergence of a time crystal of atoms in a high-finesse optical cavity driven by a phase-modulated transverse pump field, resulting in a shaken lattice. This shaken system exhibits macroscopic oscillations in the number of cavity photons and order parameters at noninteger multiples of the driving period, which signals the appearance of an incommensurate time crystal. The subharmonic oscillatory motion corresponds to dynamical switching between symmetry-broken states, which are nonequilibrium bond ordered density wave states. Employing a semiclassical phase-space representation for the driven-dissipative quantum dynamics, we confirm the rigidity and persistence of the time crystalline phase. We identify experimentally relevant parameter regimes for which the time crystal phase is long-lived, and map out the dynamical phase diagram. We compare and contrast the incommensurate time crystal with the commensurate Dicke time crystal in the amplitude-modulated case.
Phys. Rev. A 100, 053615 (2019)
An ideal Josephson junction in an ultracold two-dimensional Fermi gas
Niclas Luick, Lennart Sobirey, Markus Bohlen, Vijay Pal Singh, Ludwig Mathey, Thomas Lompe, Henning Moritz
Two-dimensional structures are present in almost all known superconductors with high critical temperatures, but the role of the reduced dimensionality is still under debate. Recently, ultracold atoms have emerged as an ideal model system to study such strongly correlated 2D systems. Here, we report on the realisation of a Josephson junction in an ultracold 2D Fermi gas. We measure the frequency of Josephson oscillations as a function of the phase difference across the junction and find excellent agreement with the sinusoidal current phase relation of an ideal Josephson junction. Furthermore, we determine the critical current of our junction in the crossover from tightly bound molecules to weakly bound Cooper pairs. Our measurements clearly demonstrate phase coherence and provide strong evidence for superfluidity in a strongly interacting 2D Fermi gas.
Emergent limit cycles and time crystal dynamics in an atom-cavity system
Hans Keßler, Jayson G. Cosme, Michal Hemmerling, Ludwig Mathey, Andreas Hemmerich
We propose an experimental realization of a time crystal using an atomic Bose-Einstein condensate in a high finesse optical cavity pumped with laser light detuned to the blue side of the relevant atomic resonance. By mapping out the dynamical phase diagram, we identify regions in parameter space showing stable limit cycle dynamics. Since the model describing the system is time-independent, the emergence of a limit cycle phase indicates the breaking of continuous time translation symmetry. Employing a semiclassical analysis to demonstrate the robustness of the limit cycles against perturbations and quantum fluctuations, we establish the emergence of a time crystal.
Phys. Rev. A 99, 053605 (2019)
Influence of electron-phonon coupling on low temperature phases of metallic single-wall carbon nanotubes
Junichi Okamoto, Ludwig Mathey, Wen-Min Huang
We investigate the effect of electron-phonon coupling on low temperature phases in metallic single-wall carbon nanotubes. We obtain low-temperature phase diagrams of armchair and zigzag type nanotubes with screened interactions with a weak-coupling renormalization group approach. In the absence of electron-phonon coupling, two types of nanotubes have similar phase diagrams. A D-Mott phase or d-wave superconductivity appears when the on-site interaction is dominant, while a charge-density wave or an excitonic insulator phase emerges when the nearest neighbor interaction becomes comparable to the on-site interaction. The electron-phonon coupling, treated by a two-cutoff scaling scheme, leads to different behavior in two types of nanotubes. For strong electron-phonon interactions, phonon softening is induced and a Peierls insulator phase appears in armchair nanotubes. We find that this softening of phonons may occur for any intraband scattering phonon mode. On the other hand, the effect of electron-phonon coupling is negligible for zigzag nanotubes. The distinct behavior of armchair and zigzag nanotubes against lattice distortion is explained by analysis of the renormalization group equations.
Phys. Rev. B 98, 205122 (2018)
Squeezed field path integral description of second sound in Bose-Einstein condensates
Ilias. M. H. Seifie, Vijay Pal Singh, L. Mathey
We propose a generalization of the Feynman path integral using squeezed coherent states. We apply this approach to the dynamics of Bose-Einstein condensates, which gives an effective low energy description that contains both a coherent field and a squeezing field. We derive the classical trajectory of this action, which constitutes a generalization of the Gross Pitaevskii equation, at linear order. We derive the low energy excitations, which provides a description of second sound in weakly interacting condensates as a squeezing oscillation of the order parameter. This interpretation is also supported by a comparison to a numerical c-field method.
Phys. Rev. A 100, 013602 (2019)
Press coverage
A new path to understanding second sound in Bose-Einstein condensates CUI UHH
Electron trimer states in conventional superconductors
Ali Sanayei, Pascal Naidon, Ludwig Mathey
We expand the Cooper problem by including a third electron in an otherwise empty band. We demonstrate the formation of a trimer state of two electrons above the Fermi sea and the third electron, for sufficiently strong inter-band attractive interaction. We show that the critical interaction strength is the lowest for small Fermi velocities, large masses of the additional electron and large Debye energy. This trimer state competes with the formation of the two-electron Cooper pair, and can be created transiently via optical pumping.
Light-induced coherence in an atom-cavity system
Christoph Georges, Jayson G. Cosme, Ludwig Mathey, Andreas Hemmerich
We demonstrate light-induced formation of coherence in a cold atomic gas system that utilizes the suppression of a competing density wave (DW) order. The condensed atoms are placed in an optical cavity and pumped by an external optical standing wave, which induces a long-range interaction mediated by photon scattering and a resulting DW order above a critical pump strength. We show that light-induced temporal modulation of the pump wave can suppress this DW order and restore coherence. This establishes a foundational principle of dynamical control of competing orders analogous to a hypothesized mechanism for light-induced superconductivity in high-Tc cuprates.
Phys. Rev. Lett. 121, 220405 (2018)
Dynamical Control of Order in a Cavity-BEC System
Jayson G. Cosme, Christoph Georges, Andreas Hemmerich, Ludwig Mathey
We demonstrate dynamical control of the superradiant transition of cavity-BEC system via periodic driving of the pump laser. We show that the dominant density wave order of the superradiant state can be suppressed, and that the subdominant competing order of Bose-Einstein condensation emerges in the steady state. Furthermore, we show that additional, non-equilibrium density wave orders, which do not exist in equilibrium, can be stabilized dynamically. Finally, for strong driving, chaotic dynamics emerges.
Phys. Rev. Lett. 121, 153001 (2018)
Disrupting crystalline order to restore superfluidity CUI UHH
