Session U62: Helium and Exotic Quantum Fluids

Chasing signs of the Kelvin-wave cascade on a single vortex using nanoscale mechanical resonators

It is believed that Kelvin waves propagating on a quantum vortex provide a fundamental channel of dissipation of superfluid turbulence, driven by the Kelvin-wave cascade. In spite of decades of theoretical work, no conclusive experimental evidence on the existence of such cascade has been presented. In our experiments we use nanoscale mechanical resonators operating at about 1MHz, immersed in superfluid ⁴He [1]. These probes are sensitive enough to probe single vortices. A vortex can be attached to the resonator by creating turbulence using a nearby tuning fork. This allows measuring the restoring force and the dissipation the vortex imposes on the resonator as a function of time, until the vortex is disconnected. We compare the observations with the theoretical expectation [2], and discuss possible future experiments.

[1] PRB 100, 020506(R) (2019) [2] PRB 84, 064516 (2011)   

Suppression of triplet superfluidity near magnetically-active surfaces

Recent experiments investigating properties of the chiral A phase of superfluid ³He in thin slabs (P. Heikkinen et al, arXiv:1909.04210) revealed unusually large T_c suppression that exceeds the fully-diffusive scattering limit. This requires re-thinking of the boundary conditions, and inclusion of magnetic degree of freedom to describe scattering of the quasiparticles on surfaces that are covered my magnetically-active solid layer of He-3 atoms. I will review existing models of magnetic scattering to prove that they cannot, in principle, give such suppression of the A phase. I will show that in order to explain the large T_c suppression, the internal quantum degrees of freedom of the solid spins have to be taken into account.
  

Emergence of a New Superfluid Phase in Quasi-2D 3He

³He is a fermionic superfluid that exhibits exotic properties at ultra-low temperatures due to its unconventional p-wave Cooper pairing. The orbital and spin degrees of freedom of ³He Cooper pairs allows for a rich array of possible superfluid phases, which are determined by geometry as well as the thermodynamic properties of the fluid. Much of the current theoretical and experimental interest in ³He has focused on quasi-2D slabs where new phases are predicted to emerge. Our lab has created nanofluidic mechanical resonators capable of both confining ³He and detecting phase transitions over a wide range of pressures and temperatures. In doing so we have observed a new phase transition which is stabilized by confinement and thought to be due to the breaking of translational symmetry within the superfluid slab.
  

Memory Effect and the Metastability of the A to B Transition in Superfluid 3He.

We experimentally study the metastability of the A phase of superfluid ³He, as either temperature (T) or pressure (P) is varied. We find significant supercooling, with the A-B transition occurring well below the temperature of the thermodynamic phase boundary in bulk. The observed transition in two chambers (separated by a 1.1 μm tall channel) shadows the thermodynamic A-B line in the T-P plane but extends below the polycritical point and terminates in the B phase (and not at T_c). We find significant path dependence (memory effect) in the metastability: When we sweep pressure at fixed temperature, the region of metastable A is larger than when we sweep temperature at fixed pressure. We find U-shaped depressurizing-cooling-pressurizing paths in the T-P plane which enable the metastable A phase to exist well below the polycritical point at 21.2 bar that cannot be reached by a temperature sweep. While cooling at constant pressure, we observe A-B transitions down to 20.85 bar, wheras while depressurizing, A-B transitions are seen down to 19.5 bar. Ongoing experiments in a 50 G field show unexpected behavior. The measurements highlight the role of surfaces in nucleation and the memory effect.   

Mott Insulator to Superfluid Quantum Phase Transition for Helium on Strained Graphene

An exciting development in the field of correlated systems is the possibility of realizing two-dimensional (2D) superfluidity, particularly by adsorbing helium on novel 2D quantum materials, such as graphene. How superfluidity emerges in this system, and whether it could exist in the first, second, etc. layer, has been a topic of considerable controversy. We argue theoretically that under certain favorable external conditions where uniaxial stress is applied to graphene, 2D anisotropic superfluidity can form in the first layer. This result is based on preliminary large-scale ab initio quantum Monte Carlo simulations combined with a mapping of the problem to an effective Bose-Hubbard model. We show that a critical ratio of the onsite repulsion to hopping strength (U/t) can be achieved via strain, allowing for a quantum phase transition to a superfluid state below a critical value. Our analysis supports, for the first time, the existence of an unconventional first layer 2D anisotropic superfluid, possibly also exhibiting supersolid correlations reflecting the underlying graphene lattice structure.   

Modelling a Neutron Star in Superfluid Helium

We discuss a lab experiment intended to model neutron star behavior. Superfluid helium is encased in a container that can rotate freely. The container is spun up, then monitored as it gradually slows down. Pinned vortices inside which release and repin can produce rotational glitches mimicking those of pulsars. We discuss the experimental design and the expected measurement sensitivity. The design is based on similar work from 40 years ago. We revisit the experiment now because of advances in the speed of computers and videocameras, as well as in the theoretical modelling of neutron stars.   

Few-body precursor of the Higgs mode in a superfluid Fermi gas - from theory to experiments.

We demonstrate the presence of an undamped few-body precursor of the Higgs mode in an ultracold trapped Fermi gas.
Here, the lowest excitation mode frequency has non-monotonical dependence on the interaction strength, attaining a minimum in the crossover region.
In the many-body limit, the Higgs mode appears as a zero-energy excitation mode at the quantum phase transition point between the superfluid and the normal phase.
In the few-body limit, we observe an energy minimum that deepens with increasing particle number, reflecting the fact that we see a few-body Higgs mode analog which develops towards a zero-energy excitation.
This hallmark of the Higgs mode is persistent also for weakly anharmonic and anisotropic traps, and it can readily be observed in a new generation of microtraps where the Higgs mode is selectively excited by modulating the interaction strength via Feshbach resonances.
Using variations of the Hartree-Fock Bogoliubov method combined with exact diagonalization, we track the development of the few-body mode into the regime of N~100 particles.   

A doping-dependent switch from one- to two-component electron-hole superfluidity with high transition temperatures in coupled TMD monolayers.

Electron-hole (e-h) superfluidity in a Transition Metal Dichalcogenide heterostructure consisting of coupled MoSe₂-WSe₂ monolayers, is investigated using mean-field approach. We include self-consistent screening of the electron-hole interaction and the multibands effects arising from the strong spin-orbit band splitting [1].

The different magnitude of the valence and conduction band splitting results in a large energy misalignment of the electron and hole bands [2]. This misalignment is completely different if the doping of the monolayers is interchanged between MoSe₂(e)/WSe₂(h) and MoSe₂(h)/WSe₂(e), and depending on the choice of doping, the superfluidity may or may not be tuneable by density from one- to two-components. We find that only MoSe2(h)/WSe2(e) can have both one-component and two-component superfluidity.

The electron-hole pairing is much stronger than in graphene[3] due to the large effective masses so superfluidity is predicted for densities up to 10¹³ cm^-2. The transition temperatures are high, up to 100 K, which is consistent with a very recent experiment on the identical system[4].

[1] S. Conti et al, arXiv:1909.03411 (2019)
[2] K. Kosmider et al, Phys. Rev. B 88, 245436 (2013)
[3] S. Conti et al, Phys. Rev. B 99, 144517 (2019)
[4] Z. Wang et al Nature 574, 76 (2019)   

Anomalous phase fluctuations of a superfluid flowing in a random potential

The phase structures of driven quantum many-body systems have attracted considerable interest owing to recent experimental progress in ultracold atomic gases. A fundamental issue in this subject is to clarify how and when the long-range order of a nonequilibrium steady state is destroyed by random perturbations. In thermal equilibrium, the Mermin-Wagner theorem can be invoked to address the stability of the ordered phase against thermal fluctuations. However, in nonequilibrium situations the universal mechanisms responsible for the breakdown of the long-range order are poorly understood. In this study, we investigate the stability of the off-diagonal long-range order of a superfluid flowing in a weak random potential [1]. Within the classical field theory, we show that for an arbitrarily small flow velocity the off-diagonal long-range order is destroyed in one and two dimensions. We argue that the superfluid flowing in a random potential can be identified with the corresponding uniform system at thermal equilibrium with an effective temperature, where the long-range order is prohibited in one and two dimensions by the Mermin-Wagner theorem.

[1] Taiki Haga and Masahito Ueda, arXiv:1909.11997.   

Disordered Creutz lattice: The role of flat bands in superfluid behavior

Physical systems displaying flat or quasi-flat bands have been intensively investigated due to the enhanced effects that interactions may bring. This is for example, believed to be behind the manifestation of superconductivity in the twisted bilayer graphene, where by adjusting the twisting angles, one can induce the formation of (quasi-) flat bands close to the Fermi energy. On a typically non-interacting picture, a flat band would correspond to an infinite effective mass for the carriers, halting transport. In the case one possess attractive interactions, it has been recently shown that superfluid transport is robust and the associated superfluid weight bounded by topological invariants the bands may possess. Further, when comparing the robustness of the superfluid properties to the presence of quenched disorder, the constrast between topological and non-topological systems becomes extremely clear. In this presentation, we will show results for the disordered Hubbard model with attractive interactions in the presence of flat or dispersive bands, checking the influence topological properties may possess in stabilizing the superfluid behavior in this interacting system.   

The Dynamical Structure Factor of a Fermionic Super-solid on an Optical Lattice

We perform a Quantum Monte Carlo simulation and study of a cold atomic Fermi gas on a 2D optical lattice. This system is modeled with a Hubbard hamiltonian with on-site attractive interaction. At half-filling, when on average one fermion occupies each lattice site, with equal numbers of fermions spin up and spin down, the system displays an interesting supersolid phase: a superfluid with a checkerboard density modulation. Combining unbiased Auxiliary-Field Monte Carlo simulations with state-of-art analytic continuation techniques, we compute the density dynamical structure factor S(q,ω) of the system and the density response function χ(q), in order to characterize this supersolid phase. These results shed light on this interesting physical regime, where s-wave pairing superfluidity coexists with a non-uniform local density.   

Density-wave steady-state phase of dissipative ultracold fermions with nearest-neighbor interactions

In the current work, we investigate the effect of local dissipation on the presence of density-wave ordering in spinful fermions with both local and nearest-neighbor interactions, as described by the extended Hubbard model [1]. For this purpose we use the recently developed Lindblad dynamical mean-field theory (L-DMFT) [2,3], which allows to study directly steady-state properties of strongly correlated fermionic systems. To take into account nearest-neighbor interactions we perform a mean-field decoupling. We find the density-wave order to be robust against decoherence effects up to a critical point where the system becomes homogeneous with no spatial ordering. These results will be relevant for future cold-atom experiments using fermions with nonlocal interactions arising from the dressing by highly excited Rydberg states, which have finite lifetimes due to spontaneous emission processes.
[1] J. Panas et al., Phys. Rev. B 99, 115125 (2019)
[2] E. Arrigoni et al., Phys. Rev. Lett. 110, 086403 (2013)
[3] I. Titvinidze et al., Phys. Rev. B 92, 245125 (2015)   

Squeezed Photons Reconsidered

Using an exact one-photon thermodynamic Green’s Function determined by a matrix method for the Liouville operator, the relationship between squeezed photons and the amplitude of the vector potential is determined without the use of the usual coherent states or the squeezing operator. The Hamiltonian for each photon mode is determined for all levels of uncertainty to be given by an extended simple harmonic oscillator Hamiltonian. The matrix method used and the simplest approximation for the generation of squeezed photons is presented.
  

Anomalous hydrodynamics in condensed matter systems

Fluid dynamics is a powerful tool in studying both quantum and classical many body systems. I will review recent progress in using fluid dynamics to describe two-dimensional isotropic fluids with broken parity. The viscous stress tensor for these fluids can possess an anomalous part known as odd or Hall viscosity. This peculiar viscosity does not lead to any dissipation in the fluid. Examples of fluids with odd viscosity include rotating superfluids, plasmas in magnetic fields, quantum Hall fluids, and chiral active fluids. I will describe some manifestations of the odd viscosity. In particular, I will focus on surface waves propagating along the boundaries of such fluids.   

Collective modes for a Fermi Liquid under Harmonic Trap

Based on the Boltzmann transport equation of quasi-particles, we examine the low energy excitations of a normal Fermi liquid conned in a two dimensional harmonic trap. In the non-interacting limit, the excitation spectrum is discrete and the highly degenerate eigenfrequencies locate exactly at integer multiples of the corresponding trap frequency. When the interaction is switched on, the degeneracy will be broken and every degenerate frequency will extend to be a continuous band but with some discrete modes out of the band. The continuum and discrete modes in the spectrum can be interpreted as particle-hole excitations and collective modes (“zero sound”) respectively. For a repulsive interaction, the lowest collective mode frequencies are higher than those of the corresponding particle-hole excitations and can be detected by the experiments on cold atom systems.