Second sound in the crossover from the Bose-Einstein condensate to the Bardeen-Cooper-Schrieffer superfluid

Second sound is an entropy wave which propagates in the superfluid component of a quantum liquid. Because it is an entropy wave, it probes the thermodynamic properties of the quantum liquid. Here, we study second sound propagation for a large range of interaction strengths within the crossover between a Bose-Einstein condensate (BEC) and the Bardeen-Cooper-Schrieffer (BCS) superfluid, extending previous work at unitarity. In particular, we investigate the strongly-interacting regime where currently theoretical predictions only exist in terms of an interpolation in the crossover. Working with a quantum gas of ultracold fermionic 6Li atoms with tunable interactions, we show that the second sound speed varies only slightly in the crossover regime. By varying the excitation procedure, we gain deeper insight on sound propagation. We compare our measurement results with classical-field simulations, which help with the interpretation of our experiments.

Electronic Floquet gyro-liquid crystal

Floquet engineering uses coherent time-periodic drives to realize designer band structures on-demand, thus yielding a versatile approach for inducing a wide range of exotic quantum many-body phenomena. Here we show how this approach can be used to induce non-equilibrium correlated states with spontaneously broken symmetry in lightly doped semiconductors. In the presence of a resonant driving field, the system spontaneously develops quantum liquid crystalline order featuring strong anisotropy whose directionality rotates as a function of time. The phase transition occurs in the steady state of the system achieved due to the interplay between the coherent external drive, electron-electron interactions, and dissipative processes arising from the coupling to phonons and the electromagnetic environment. We obtain the phase diagram of the system using numerical calculations that match predictions obtained from a phenomenological treatment and discuss the conditions on the system and the external drive under which spontaneous symmetry breaking occurs. Our results demonstrate that coherent driving can be used to induce non-equilibrium quantum phases of matter with dynamical broken symmetry.

Three-body correlations in nonlinear response of correlated quantum liquid

Behavior of quantum liquids is a fascinating topic in physics. Even in a strongly correlated case, the linear response of a given system to an external field is described by the fluctuation-dissipation relations based on the two-body correlations in the equilibrium. However, to explore nonlinear non-equilibrium behaviors of the system beyond this well-established regime, the role of higher order correlations starting from the three-body correlations must be revealed. In this work, we experimentally investigate a controllable quantum liquid realized in a Kondo-correlated quantum dot and prove the relevance of the three-body correlations in the nonlinear conductance at finite magnetic field, which validates the recent Fermi liquid theory extended to the non-equilibrium regime.

The path to scientific heights (to the 90th anniversary of Academician of NAS of Ukraine S.V. Peletminskii)

February 14 marks the 90th anniversary of the outstanding Ukrainian theoretical physicist in the field of statistical physics, quantum liquid and crystal physics, plasma physics, magnetic phenomena, quantum field theory, whose work is recognized by the world scientific community, twice winner of the State Prize of Ukraine in Science and Technology (1986, 1996), Honored Worker of Science and Technology of Ukraine (1998), winner of the nominal prizes of the NAS of Ukraine named after K.D. Synelnykov (1978), M.M. Krylov (1986), M.M. Bogolyubov (2002), O.I. Akhiezer (2018), Chief Researcher at the Institute of Theoretical Physics of the National Science Center Kharkov Institute of Physics and Technology, Doctor of Physics and Mathematics (1966), Professor (1969), Academician of the NAS of Ukraine (1990) Sergey V. Peletminskii.

Three-body correlations in nonlinear response of correlated quantum liquid

Understanding the properties of correlated quantum liquids is a fundamental issue of condensed matter physics. Even in such a correlated case, fascinatingly, we can tell that the equilibrium fluctuations of the system govern its linear response to an external field, relying on the fluctuation dissipation relations based on the two-body correlations. Going beyond, up to the three-body correlations, is of importance for van der Waals force [1], the three-body force in nuclei [2], the Efimov state [3, 4], the ring exchange interaction in solid 3He [5, 6], and frustrated spin systems [7]. In our work, we have used a quantum dot in the Kondo regime, which is a controllable realization of such a correlated quantum liquid [8–11]. Thanks to the quality of our sample, where the Kondo effect in the unitary limit was achieved, we could quantitatively measure the three-body correlations and their role in the non-equilibrium regime, in perfect agreement with recent results of the Fermi liquid theory [12– 15]. In particular, we have demonstrated its importance when time-reversal symmetry is broken, solving a long-standing puzzle of the Kondo systems under the magnetic field [13]. The demonstrated method to relate three-body correlation and non-equilibrium transport opens up a way for further investigation of the dynamics of quantum many-body systems.