Breakdown Of Superfluidity Research Articles

Critical velocity and arrest of a superfluid in a pointlike disordered potential

Superfluid flow past a potential barrier is a well studied problem in ultracold Bose gases, however, fewer studies have considered the case of flow through a disordered potential. Here we consider the case of a superfluid flowing through a channel containing multiple point-like barriers, randomly placed to form a disordered potential. We begin by identifying the relationship between the relative position of two point-like barriers and the critical velocity of such an arrangement. We then show that there is a mapping between the critical velocity of a system with two obstacles, and a system with a large number of obstacles. By establishing an initial superflow through a point-like disordered potential, moving faster than the critical velocity, we study how the superflow is arrested through the nucleation of vortices and the breakdown of superfluidity, a problem with interesting connections to quantum turbulence and coarsening. We calculate the vortex decay rate as the width of the barriers is increased, and show that vortex pinning becomes a more important effect for these larger barriers. Published by the American Physical Society 2024

Coupled superfluidity of binary Bose mixtures in two dimensions

We consider a two-component Bose gas in two dimensions at low temperature with short-range repulsive interaction. In the coexistence phase where both components are superfluid, inter-species interactions induce a nondissipative drag between the two superfluid flows (Andreev-Bashkin effect). We show that this behavior leads to a modification of the usual Berezinskii-Kosterlitz-Thouless (BKT) transition in two dimensions. We extend the renormalization of the superfluid densities at finite temperature using the renormalization group approach and find that the vortices of one component have a large influence on the superfluid properties of the other, mediated by the nondissipative drag. The extended BKT flow equations indicate that the occurrence of the vortex unbinding transition in one of the components can induce the breakdown of superfluidity also in the other, leading to a locking phenomenon for the critical temperatures of the two gases.

Transitions and excitations in a superfluid stream passing small impurities

We analyze asymptotically and numerically the motion around a single impurity and a network of impurities inserted in a two-dimensional superfluid. The criticality for the breakdown of superfluidity is shown to occur when it becomes energetically favorable to create a doublet---the limiting case between a vortex pair and a rarefaction pulse on the surface of the impurity. Depending on the characteristics of the potential representing the impurity, different excitation scenarios are shown to exist for a single impurity as well as for a lattice of impurities. Depending on the lattice characteristics it is shown that several regimes are possible: dissipationless flow, excitations emitted by the lattice boundary, excitations created in the bulk, and the formation of large-scale structures.

Non-Abelian Gauge Fields in Photonic Cavities and Photonic Superfluids

We show that the TE-TM modes splitting and the structure anisotropy of a semiconductor microcavity combine into a non-Abelian gauge field for exciton-polaritons or cavity photons. The field texture can be tuned simply by rotating the sample and ranges continuously from a Rashba to a monopolar field. In the noninteracting regime, the latter leads to remarkable focusing and conical diffraction effects. In the interacting regime, the spin-orbit coupling induces a breakdown of superfluidity. The spatially homogeneous flows become unstable and dynamically evolve into spin textured states, such as stripes or domain walls.

Vortex–Antivortex Unbinding in Inhomogeneous 2D Atomic Condensates

The breakdown of superfluidity in a two-dimensional Bose gas is linked with the unbinding of vortex–antivortex pairs, which has been observed directly in a recent experiment with trapped atomic gases. The phenomenological Kosterlitz–Thouless (KT) description of the dissociation process, originally set up in the context of a uniform Bose system (such as superfluid helium), is no longer suited in the case of a strongly inhomogenous atomic gas in which, moreover, the vortex core size is a non-negligible fraction of the cloud size. Using the Gross–Pitaevskii energy functional we extend the original KT argument to take into account inhomogeneity and vortex core size in the calculation of both the binding energy of the vortex–antivortex pair, and the entropy per vortex in the cloud. We derive the resulting shift in the KT temperature, and find that it is in good agreement with experiment, in contrast to the value obtained in the context of the uniform Bose gas. Finally, we look at different intermediate levels of approximation for the binding energy and the entropy and investigate their effect on the shift in the KT temperature.

Superfluidity of spinor Bose-Einstein condensates

We consider a spin-1/2 Bose-Einstein condensate prepared initially in a single spin projection. The two channels of excitations existing in such a system (namely density and spin waves) are discussed and we show how pure spin waves can be excited in the presence of local magnetic defects. We analyze the role played by spin excitations on the Landau superfluidity criterion and demonstrate the absence of absolute superfluidity for the antiferromagnetic condensate. In the ferromagnetic case, we identify two critical velocities for the breakdown of superfluidity.

Dynamics of vortex assisted metal condensation in superfluid helium

Laser ablation of copper and silver targets immersed in bulk normal and superfluid (4)He was studied through time-resolved shadowgraph photography. In normal fluid, only a sub-millimeter cavitation bubble is created and immediate formation of metal clusters is observed within a few hundred microseconds. The metal clusters remain spatially tightly focused up to 15 ms, and it is proposed that this observation may find applications in particle image velocimetry. In superfluid helium, the cavitation bubble formation process is distinctly different from the normal fluid. Due to the high thermal conductivity and an apparent lag in the breakdown of superfluidity, about 20% of the laser pulse energy was transferred directly into the liquid and a large gas bubble, up to several millimeters depending on laser pulse energy, is created. The internal temperature of the gas bubble is estimated to exceed 9 K and the following bubble cool down period therefore includes two separate phase transitions: gas-normal liquid and normal liquid-superfluid. The last stage of the cool down process was assigned to the superfluid lambda transition where a sudden formation of large metal clusters is observed. This is attributed to high vorticity created in the volume where the gas bubble previously resided. As shown by theoretical bosonic density functional theory calculations, quantized vortices can trap atoms and dimers efficiently, exhibiting static binding energies up to 22 K. This, combined with hydrodynamic Bernoulli attraction, yields total binding energies as high as 35 K. For larger clusters, the static binding energy increases as a function of the volume occupied in the liquid to minimize the surface tension energy. For heliophobic species an energy barrier develops as a function of the cluster size, whereas heliophilics show barrierless entry into vortices. The present theoretical and experimental observations are used to rationalize the previously reported metal nanowire assembly in both superfluid bulk liquid helium and helium droplets, both of which share the common element of a rapid passage through the lambda point. The origin of vorticity is tentatively assigned to the Zurek-Kibble mechanism. Implications of the large gas bubble formation by laser ablation to previous experiments aimed at implanting atomic and dimeric species in bulk superfluid helium are also discussed, and it is proposed that the developed visualization method should be used as a diagnostic tool in such experiments to avoid measurements in dense gaseous environments.

Superfluidity breakdown of periodic matter waves in quasi-one-dimensional annular traps via resonant scattering with moving defects

We investigate, both analytically and numerically, the quasi-superfluidity properties of periodic Bose-Einstein condensates (BECs) in a quasi-one-dimensional (1D) ring with optical lattices (OL) of different kinds (linear and nonlinear) and with a moving defect of an infinite mass inside. To study the dynamics of the condensate we used a mean-field approximation describing the condensate by use of the Gross-Pitaevskii equation for the order parameter. We show that the resonant scattering of sound Bloch waves with the defect profoundly affect BEC superfluidity. In particular, a moving defect always leads to the breakdown of superfluidity independently of the value of its velocity. For weak periodic potentials the superfluidity breakdown may occur on a very long time scale (quasisuperfluidity) but the breakdown process can be accelerated by increasing the strength of the OL. Quite remarkably, we find that when the length of the ring is small enough to imply the discreteness of the reciprocal space, it becomes possible to avoid the resonant scattering and to restore quasi-superfluidity.

Theory of superfluidity and drag force in the one-dimensional Bose gas

The one-dimensional Bose gas is an unusual superfluid. In contrast to higher spatial dimensions, the existence of non-classical rotational inertia is not directly linked to the dissipationless motion of infinitesimal impurities. Recently, experimental tests with ultracold atoms have begun and quantitative predictions for the drag force experienced by moving obstacles have become available. This topical review discusses the drag force obtained from linear response theory in relation to Landau's criterion of superfluidity. Based upon improved analytical and numerical understanding of the dynamical structure factor, results for different obstacle potentials are obtained, including single impurities, optical lattices and random potentials generated from speckle patterns. The dynamical breakdown of superfluidity in random potentials is discussed in relation to Anderson localization and the predicted superfluid-insulator transition in these systems.

Hydrodynamic nucleation of quantized vortex pairs in a polariton quantum fluid

Quantized vortices appear in quantum gases at the breakdown of superfluidity. In liquid helium and cold atomic gases, they have been indentified as the quantum counterpart of turbulence in classical fluids. In the solid state, composite light‐matter bosons known as exciton polaritons have enabled studies of non-equilibrium quantum gases and superfluidity. However, there has been no experimental evidence of hydrodynamic nucleation of polariton vortices so far. Here we report the experimental study of a polariton fluid flowing past an obstacle and the observation of nucleation of quantized vortex pairs in the wake of the obstacle. We image the nucleation mechanism and track the motion of the vortices along the flow. The nucleation conditions are established in terms of local fluid density and velocity measured on the obstacle perimeter. The experimental results are successfully reproduced by numerical simulations based on the resolution of the Gross‐Pitaevskii equation. H ydrodynamic instabilities in classical fluids were studied in the pioneering experiments of BOnard in the 1910’s. Convective BOnardRayleigh flows and BOnardVon KAErmAEn streets are now well known examples in nonlinear and chaos sciences 1 . In conventional fluids, the flow around an obstacle is characterized by the dimensionless Reynolds number ReD vR= , withv and the fluid velocity and dynamical viscosity, respectively, and R the diameter of the obstacle. When increasing the Reynolds number, laminar flow, stationary vortices, BOnardVon KAErmAEn streets of moving vortices and fully turbulent regimes are successively observed in the wake of the obstacle 1 .

Collective modes and superflow instabilities of strongly correlated Fermi superfluids

We study the superfluid phase of the one-band attractive Hubbard model of fermions as a prototype of a strongly correlated s-wave fermion superfluid on a lattice. We show that the collective mode spectrum of this superfluid exhibits, in addition to the long wavelength sound mode, a sharp roton mode over a wide range of densities and interaction strengths. We compute the sound velocity and the roton gap within a generalized random phase approximation (GRPA) and show that the GRPA results are in good agreement, at strong coupling, with a spin-wave analysis of the appropriate strong-coupling pseudospin model. We also investigate, using this two-pronged approach, the breakdown of superfluidity in the presence of a supercurrent. We find that the superflow can break down at a critical flow momentum via several distinct mechanisms--depairing, Landau instabilities or dynamical instabilities--depending on the dimension, the interaction strength and the fermion density. The most interesting of these instabilities is a charge modulation dynamical instability which is distinct from previously studied dynamical instabilities of Bose superfluids. The charge order associated with this instability can be of two types: (i) a commensurate checkerboard modulation driven by softening of the roton mode at the Brillouin zone corner, or, (ii)more » an incommensurate density modulation arising from superflow-induced finite momentum pairing of Bogoliubov quasiparticles. We elucidate the dynamical phase diagram showing the critical flow momentum of the leading instability over a wide range of fermion densities and interaction strengths and point out implications of our results for experiments on cold atom fermion superfluids in an optical lattice.« less

Generalized Criterion for Stability of Superflow Past an Obstacle

We propose a new criterion of stability or instability of one-directional superflow of Bose condensates. In d-dimensional condensates with steady superflow, spectral function \(\rho(\mathbf {r},\omega)=\sum_{l}|\langle l|\hat{n}(\mathbf{r})|\mathrm{g}\rangle|^{2}\delta (\omega-E_{l}+E_{\mathrm{g}})\) of the local density \(\hat{n}(\mathbf{r})\) at the critical velocity behaves as ρ(r,ω)∝ωβ with β<d at low ω while ρ(r,ω)∝ωd below the critical velocity. We confirm the validity of our criterion within the Gross-Pitaevskii-Bogoliubov theory. In the presence of a penetrable repulsive potential, β is given by d−2 at the critical velocity, where gray-soliton-phonon emission leads to the breakdown of superfluidity. In the case of the Landau instability in spatially uniform systems, on the other hand, β=−1/3,1/3,1 for d=1,2,3, respectively.

Effect of population imbalance on the Berezinskii-Kosterlitz-Thouless phase transition in a superfluid Fermi gas

The Berezinskii-Kosterlitz-Thouless (BKT) mechanism describes the breakdown of superfluidity in a two-dimensional Bose gas or a two-dimensional gas of paired fermions. In the latter case, a population imbalance between the two pairing partners in the Fermi mixture is known to influence pairing characteristics. Here, we investigate the effects of imbalance on the two-dimensional BKT superfluid transition and show that superfluidity is even more sensitive to imbalance than for three-dimensional systems. Finite-temperature phase diagrams are derived using the functional integral formalism in combination with a hydrodynamic action functional for the phase fluctuations. This allows to identify a phase-separation region and tricritical points due to imbalance. In contrast to superfluidity in the three-dimensional case, the effect of imbalance is also pronounced in the strong-coupling regime.

Superfuid-insulator transitions at noninteger filling in optical lattices of fermionic atoms

We determine the superfluid transition temperatures ${T}_{c}$ and the ground states of the attractive Hubbard model and find unusual insulating phases associated with noninteger filling at sufficiently strong pairing attraction $\ensuremath{\mid}U\ensuremath{\mid}$. These states, distinct from the band-and Mott-insulating phases, derive from pair localization; pair hopping at large $\ensuremath{\mid}U\ensuremath{\mid}$ and high densities is impeded by intersite, interpair repulsive interactions. The best way to detect the breakdown of superfluidity is by using fermionic optical lattices, which should reveal these unusual forms of ``bosonic'' order, reflecting ground-state pairing without condensation.

Fragmentation and destruction of the superfluid due to frustration of cold atoms in optical lattices

A one-dimensional optical lattice is considered where a second dimension is encoded in the internal states of the atoms giving effective ladder systems. Frustration is introduced by an additional optical lattice that induces tunnelling of superposed atomic states. The effects of frustration range from the stabilization of the Mott insulator phase with ferromagnetic order, to the breakdown of superfluidity and the formation of a macroscopically fragmented phase.

Excitation Spectra of Bose–Einstein Condensates in Optical Lattice

The excitation spectra of Bose–Einstein condensates are investigated in a three-dimensional optical lattice by the Bogoliubov theory considering the original sinusoidal potential for the optical lattice. When the lattice potential becomes high, the excitations show softening at large wavelengths, suggesting the breakdown of superfluidity. We also study the filling and interaction dependences of the properties of the excitation spectra and one-dimensional lattice case.

Breakdown of superfluidity of an atom laser past an obstacle

The 1D flow of a continuous beam of Bose-Einstein condensed atoms in the presence of an obstacle is studied as a function of the beam velocity and of the type of perturbing potential (representing the interaction of the obstacle with the atoms of the beam). We identify the relevant regimes: stationary/time-dependent and superfluid/dissipative; the absence of drag is used as a criterion for superfluidity. There exists a critical velocity below which the flow is superfluid. For attractive obstacles, we show that this critical velocity can reach the value predicted by Landau's approach. For penetrable obstacles, it is shown that superfluidity is recovered at large beam velocity. Finally, enormous differences in drag occur when switching from repulsive to attractive potential.

Observation of quantum shock waves created with ultra- compressed slow light pulses in a Bose-Einstein condensate.

We have used an extension of our slow light technique to provide a method for inducing small density defects in a Bose-Einstein condensate. These sub- resolution, micrometer-sized defects evolve into large-amplitude sound waves. We present an experimental observation and theoretical investigation of the resulting breakdown of superfluidity, and we observe directly the decay of the narrow density defects into solitons, the onset of the "snake" instability, and the subsequent nucleation of vortices.

Dissipative optical flow in a nonlinear Fabry-Pérot cavity.

We describe a classical nonlinear optical system that displays superfluidity and its breakdown. The system consists of a self-defocusing refractive medium inside a Fabry-Pérot cavity with a cylindrical obstacle. We have numerically solved for the transmitted beam when an incident plane wave strikes the cavity at an oblique angle. The presence of the incident beam pins the steady-state phase of the output, preventing the formation of vortices or time-dependent flow. When the incident beam is switched off, a transient wake of moving optical vortices is produced. This is analogous to the breakdown of superfluidity above a critical velocity.

The breakdown of superfluidity in liquid 4 HE VI. Macroscopic quantum tunnelling by vortices in isotopically pure He II

Measurements are reported of the rate v at which negative ions nucleate quantized vortices in isotopically pure superfluid 4 He for electric fields E , temperatures T and pressures P within the range 10 3 ⩽ E ⩽ 10 6 V m -1 , 75 ⩽ T ⩽ 500 mK, 12 ⩽ P ⩽ 23 bar (= 2.3 MPa). The form of v ( E, T ) differs in unexpected ways from that observed in earlier work at higher P , exhibiting: a pronounced dip in v ( T ) at ca . 0.3 K whose depth and precise position depends on E and P ; an exponential increase in v ( T ) at higher T , with an activation energy considerably smaller than the roton energy gap; and distinct structure in v ( E ). The experimental data are discussed and analysed in terms of the macroscopic quantum tunnelling model proposed by Muirhead et al . ( Phil. Trans. R. Soc. Lond . A 311, 433 (1984)). The relatively small barrier heights of ca . 2—3 K deduced from the data on this basis are construed as confirmation that the initial vortex is a loop rather than an encircling ring. The temperature dependence of v at low pressures is interpreted in terms of a phonon-driven vortex nucleation mechanism, and values for its cross section are extracted from the data. The minima in v ( T ) are ascribed to phonon damping of the tunnelling process, and the kinks observed in some of the low-temperature v ( E ) curves are attributed to tunnelling of the system into the first excited state of the nascent vortex loop.