AC Josephson effect between two superfluid time crystals

Fermionic superfluid properties in a one-dimensional optical lattice

We discuss various superfluid properties of a two-component Fermi system in the presence of a tight one-dimensional periodic potential in a three-dimensional system. We use a zero temperature mean field theory and derive analytical expressions for the Josephson current, the sound velocity and the center of mass oscillations in the BCS-Bose Einstein condensation crossover region.

Absence of superfluidity in a parahydrogen film intercalated within a crystal of Na atoms

A recent claim of possible superfluid behaviour of parahydrogen films intercalated within a crystalline matrix of Na atoms is examined. Quantum Monte Carlo simulations at finite temperature yield strong numerical evidence that the system forms at low temperature a non-superfluid crystalline phase, commensurate with the underlying impurity lattice. The physics of this system is therefore qualitatively identical to that observed in similar settings, extensively studied in precedence. Comparison of numerical results obtained here, with those of the reference in which the prediction of superfluidi…

On the transmission of crystallisation waves across the edge between the rough and faceted crystalline surfaces in superfluid 4He

The wavelike processes of crystallisation and melting or crystallisation waves are well-known to exist at the crystal 4He surface in its rough state. Below the roughening transition temperature the crystal surface experiences the transition to the smooth faceted state and the crystallisation waves represent the propagation of a train of crystalline steps at the velocity depending on the crystal step height. Here we analyse the transmission and reflection of crystallisation waves propagating across the crystal edge separating the crystal surface in the rough and faceted states.

Direct Observation of Tunneling and Nonlinear Self-Trapping in a single Bosonic Josephson Junction

We report on the first realization of a single bosonic Josephson junction, implemented by two weakly linked Bose-Einstein condensates in a double-well potential. In order to fully investigate the nonlinear tunneling dynamics we measure the density distribution in situ and deduce the relative phase between the two condensates from interference fringes. Our results verify the predicted nonlinear generalization of tunneling oscillations in superconducting and superfluid Josephson junctions. Additionally we confirm a novel nonlinear effect known as macroscopic quantum self-trapping, which leads to…

The effect of the interphase kinetics on the motion of a quantum crystal in superfluid liquid

The fall of a quantum crystal in the state of "burst-like growth" in a superfluid liquid is considered. The experimental data of the pressure variation in the container during the fall of a crystal are discussed. The model of the motion of the crystal is suggested taking the interface dynamics into account. The results for the numerical simulation of the fall of a crystal are consistent with the experimental data. We find a significant effect of the liquid-crystal interface kinetics on the hydrodynamics of the liquid flow encircling a crystal.

Dynamics of a $^4$He quantum crystal in the superfluid liquid

The motion of helium crystals has been experimentally studied when the crystals fall in the superfluid liquid owing to gravity at temperatures above the roughening transitions where the whole crystal surface is in the atomically rough state. The rate of crystal fall at T = 1.25 K is higher than at T = 1.54 K. This is proof of the essential role of the normal component of superfluid helium in the deceleration of crystal motion. The pressure measurements in the container have shown the effect of surface kinetics on the motions of the crystal and its size. The fall of crystals with the low surfac…

Spin Vortex in Magnon BEC of Superfluid 3He-B

The phenomenon of the spontaneous phase-coherent precession of magnetization in superfluid 3He and the related effects of spin superfluidity are based on the true Bose-Einstein condensation of magnons. Several different magnon BEC states have been observed: homogeneously precessing domain (HPD); BEC condensation in the spin-orbit potential trap (Q-balls); coherent precession with fractional magnetization; and two modes of the coherent precession in squeezed aerogel. The spin superfluidity effects, like spin Josephson phenomena, spin current vortices, spin phase slippage, long distance magnetiz…

Observation of time quasicrystal and its transition to superfluid time crystal

We report experimental realization of a quantum time quasicrystal, and its transformation to a quantum time crystal. We study Bose-Einstein condensation of magnons, associated with coherent spin precession, created in a flexible trap in superfluid $^3$He-B. Under a periodic drive with an oscillating magnetic field, the coherent spin precession is stabilized at a frequency smaller than that of the drive, demonstrating spontaneous breaking of discrete time translation symmetry. The induced precession frequency is incommensurate with the drive, and hence the obtained state is a time quasicrystal.…

Quantum crystal oscillations in a superfluid liquid

We study the hydrodynamics of quantum 4He crystal oscillations in a superfluid liquid with involving the dynamics of atomically rough surfaces. It is shown that, due to enhancement of the kinetic growth coefficient as the temperature lowers, the reaction force of the liquid applied to the 4He crystal changes its character from the inertial to the viscous one vanishing as T->0. The model is confirmed by the experiments on the oscillations of the 4He crystal within the temperature range 0.54 - 1.43 K at frequencies 484 and 211 Hz. New type of a hydrodynamic instability is found. The instability …

Hexatic, Wigner Crystal, and Superfluid Phases of Dipolar Bosons

The finite temperature phase diagram of two-dimensional dipolar bosons versus dipolar interaction strength is discussed. We identify the stable phases as dipolar superfluid (DSF), dipolar Wigner crystal (DWC), dipolar hexatic fluid (DHF), and dipolar normal fluid (DNF). We also show that other interesting phases like dipolar supersolid (DSS) and dipolar hexatic superfluid (DHSF) are at least metastable, and can potentially be reached by thermal quenching. In particular, for large densities or strong dipolar interactions, we find that the DWC exists at low temperatures, but melts into a DHF at …

DC Josephson Effect with Fermi gases in the Bose-Einstein regime

We show that the DC Josephson effect with ultracold fermionic gases in the BEC regime of composite molecules can be described by a nonlinear Schrodinger equation (NLSE). By comparing our results with Bogoliubov-de Gennes calculations [Phys. Rev. Lett. 99, 040401 (2007)] we find that our superfluid NLSE, which generalizes the Gross-Pitaevskii equation taking into account the correct equation of state, is reliable in the BEC regime of the BCS-BEC crossover up to the limit of very large (positive) scattering length. We also predict that the Josephson current displays relevant beyond mean-field ef…

Superfluidity of a perfect quantum crystal

In recent years, experimental data were published which point to the possibility of the existence of superfluidity in solid helium. To investigate this phenomenon theoretically we employ a hierarchy of equations for reduced density matrices which describes a quantum system that is in thermodynamic equilibrium below the Bose-Einstein condensation point, the hierarchy being obtained earlier by the author. It is shown that the hierarchy admits solutions relevant to a perfect crystal (immobile) in which there is a frictionless flow of atoms, which testifies to the possibility of superfluidity in i…

Bound state of a He-3 atom at the interface of crystal and superfluid He-4

The bound state of a He-3 atom at the interface between coexisting crystalline and superfluid phases of He-4 is studied theoretically by means of first principle Quantum Monte Carlo simulations. We consider both the case of a solid/superfluid He-4 interface, as well as that of a few solid layers of He-4 forming on an attractive substrate. It is observed that the He-3 bound state is sharply localized in a well-defined, quasi-2D layer of He-4 in the region intermediate between the solid and the liquid. The layer in which the He-3 atom resides displays an intriguing interplay of atomic localizati…

利用超流$^4$He陀螺仪对引力参考系拖拽的量子传感

Quantum Sensing of Gravitational Frame-Dragging with a Superfluid $^4$He Gyrometer

本文提出利用超流$^4$He约瑟夫森结陀螺仪在实验室尺度测量地球引力参考系拖拽效应，理论分析表明在10 mK下灵敏度足以在1秒内解析0.2%的拖拽速率，对应40亿年1圈的旋转灵敏度。