Surface fluctuations of normal and superfluid 3He probed by Wigner solid dynamics

Electron scattering from surface fluctuations on normal and superfluid 3He has been measured by its effect on the linewidth of the low-wave-vector transverse magnetophonon mode of the electron crystal (the Wigner solid) floating on the helium surface. The relaxation rate becomes anomalously low below 70 mK, and reaches a plateau at about 3 times less than its expected value before dropping further at the superfluid transition. The absence of such anomalous behavior on 4He suggests that the effect is specific to liquid 3He.     

Decay of pure quantum turbulence in superfluid 3He-B

We describe measurements of the decay of pure superfluid turbulence in superfluid 3He-B, in the low temperature regime where the normal fluid density is negligible. We follow the decay of the turbulence generated by a vibrating grid as detected by vibrating wire resonators. Despite the absence of any classical normal fluid dissipation processes, the decay is consistent with turbulence having the classical Kolmogorov energy spectrum and is remarkably similar to that measured in superfluid 4He at relatively high temperatures. Further, our results strongly suggest that the decay is governed by the superfluid circulation quantum rather than kinematic viscosity.     

二分量玻色-爱因斯坦凝聚中的二超流体模型

4He超流体在一定温度下可用二流体模型描述,包括常规流体和超流体两种成分.用这种二流体模型来描述二分量玻色爱因斯坦凝聚时叫做二超流体模型,是从耦合Gross-Pitaevskii方程出发推导得到的.二超流体模型与4He超流体中的二流体模型非常相近.在特定条件下,二超流体中的两个波模行为非常接近4He超流体中二流体模型中...     

Electron spin pumping of Rb atoms on He nanodroplets via nondestructive optical excitation

We measured laser-induced-fluorescence (LIF) and beam-depletion (BD) spectra of rubidium atoms (5S-5P transition) on the surface of superfluid helium nanodroplets (M-He_{N} with M=Rb). It is known that when M is a lighter alkali atom electronic excitation always leads to detachment of the excited atom (M;{*}). The dissociation energy, few tens cm;{-1}, comes either as photon excess energy or from the barrierless formation of a M;{*}-He exciplex. We observe that this picture does not hold when M=Rb and the photon excess energy is small: we are able to excite atoms without detaching them from the droplet, thanks to a barrier preventing formation of the exciplex. This system is ideally suited for optical spin pumping in a He nanodroplet, whose achievement we explicitly demonstrate in a pump-probe magnetic circular dichroism experiment.     

Decay of counterflow turbulence in superfluid <Superscript>4</Superscript>He

We summarize recent experiments on thermal counterflow turbulence in superfluid ⁴He, emphasizing the observation of turbulence in the normal fluid and its effect on the decay process when the heat flux is turned off. We argue that what is observed as turbulence in the normal fluid is a novel form of coupled turbulence in the superfluid and normal components, with injection of energy on the scales of both the (large) channel width and the (small) spacing between quantized vortices. Although an understanding of this coupled turbulence remains challenging, a theory of its decay is developed which accounts for the experimental observations.     

Surface Majorana cone of the topological superfluid 3He B phase

Topological superfluids and superconductors have been theoretically proposed, and it is now necessary to experimentally confirm their existence. Superfluid ³He should be the ideal test subject for topological theories because its bulk state is established to be that of a spin-triplet p-wave superfluid. Surface Andreev bound states of superfluid ³He were investigated by transverse acoustic impedance measurements and their linear dispersion was confirmed on a highly specular wall. The superfluid ³He B phase was found to be a topological superfluid showing bulk–edge correspondence and a surface Majorana cone was confirmed on the surface. Possible manifestations of the Majorana nature of the surface states are discussed.     

Surface Andreev bound states of superfluid (3)He and Majorana fermions

Superfluid (3)He is an intensively investigated and well characterized p-wave superfluid. In the bulk Balian-Werthamer state, which is commonly called the (3)He B phase, the superfluid gap is opened isotropically but near a flat boundary such as a wall of a container it can harbor interesting quasi-particle states inside the gap. These states are called surface Andreev bound states, and have not been experimentally explored in detail. Transverse acoustic impedance measurement has revealed their existence and provided spectroscopic details of the dispersion of the bound states. Recent theoretical arguments claim that the surface Andreev bound states of the superfluid (3)He B phase can be recognized as the edge states of the topological superfluid and be regarded as a Majorana fermion, a fancy particle which has not been confirmed in elementary particle physics. In this review, we present up-to-date knowledge on the surface Andreev bound states of the (3)He B phase revealed by acoustic spectroscopy and the possible realization of a Majorana fermion, along with related studies on this topic.     

High-Tc spin superfluidity in antiferromagnets

We report the observation of the unusual behavior of induction decay signals in antiferromagnetic monocrystals with Suhl-Nakamura interactions. The signals show the formation of the Bose-Einstein condensation (BEC) of magnons and the existence of spin supercurrent, in complete analogy with the spin superfluidity in the superfluid (3)He and the atomic BEC of quantum gases. In the experiments described here, the temperature of the magnon BEC is a thousand times larger than in the superfluid (3)He. It opens a possibility to apply the spin supercurrent for various magnetic spintronics applications.     

Condensate-free superfluid induced by the frustrated proximity effect

Since the discovery of superfluidity in 4He and Landau's phenomenological theory, the relationship between Bose condensation and superfluidity has been intensely debated. 4He is known by now to be both superfluid and condensed at low temperature, and more generally, in dimension D≥2, all superfluid bosonic models realized in experiments are condensed in their ground state, the most recent example being provided by ultracold bosonic atoms trapped in an optical lattice. In this Letter, it is shown that a 2D gas of bosons which is not condensed at T=0 can be achieved by populating a layer through a frustrated proximity effect from a superfluid reservoir. This condensate-free bosonic fluid is further shown to be a superfluid with incommensurate correlations.     

Electrons on superfluid He

Low-frequency transport properties of the Wigner solid on normal and superfluid ³He have been studied. It is known that the Wigner solid deforms the free surface of liquid He to form a dimple lattice. The low-frequency resistivity is successfully interpreted by the ballistic and specular scattering of ³He quasiparticles from the dimple lattice. The resistivity decreases abruptly below the superfluid transition temperature. For superfluid ³He-A, the l-texture, presumably perpendicularly oriented to the free surface, has an essential influence on the resistivity.     

Superfluidity of 4He in one and three dimensions realized in nanopores

Superfluidity in one and three dimensions has been studied for 4He fluid films adsorbed in nanopores which are straight channels and three-dimensionally connected pores, respectively. We observed the superfluid in one and three dimensions where thermal phonon wavelengths are much longer than the channel diameter and the period of the pore connection, respectively, and found that the superfluid onset depends on the pore connection. In the straight channels, the observed superfluid density disappears at a temperature far below the heat capacity anomaly of the Ginzburg-Landau transition, while in the pores connected in three dimension, the adsorbed 4He films show an evident three-dimensional transition where the superfluid onset occurs at the heat capacity peak.     

Bose-Einstein condensation, phase coherence, and two-fluid behavior in 4He

It is shown that the assumption that the many particle wave functions describing superfluid 4He have some phase coherence properties of correlated basis functions provides a new explanation of the link between Bose-Einstein condensation and two fluid behavior. This assumption also implies that the condensate fraction is proportional to the superfluid fraction, in agreement with experiment, and provides a simple quantitative explanation of the anomalous reduction in spatial order and liquid density, observed as the temperature is lowered in superfluid 4He.     

Transport properties of the A phase of superfluid 3He at low temperatures

The results of a calculation of transport properties of the A phase of superfluid ³He are reported. The temperature dependence of the normal fluid density, viscosity, thermal conductivity, and nuclear spin relaxation time are given.     

Hydrodynamics of superfluid helium in a single nanohole

The flow of liquid helium through a single nanohole with radius smaller than 25 nm was studied. Mass flow was induced by applying a pressure difference of up to 1.4 bar across a 50 nm thick Si(3)N(4) membrane and was measured directly by means of mass spectrometry. In liquid He I, we experimentally show that the fluid is not clamped by the short pipe with diameter-to-length ratio D/L?1, despite the small diameter of the nanohole. This viscous flow is quantitatively understood by making use of a model of flow in short pipes. In liquid He II, a two-fluid model for mass flow is used to extract the superfluid velocity in the nanohole for different pressure heads at temperatures close to the superfluid transition. These velocities compare well to existing data for the critical superflow of liquid helium in other confined systems.     

Nonlinear NMR in a superfluid B phase of <Superscript>3</Superscript>He in aerogel

The properties of liquid ³He in a low-density aerogel preliminarily covered with a few monolayers of ⁴He were studied by pulsed and nonlinear CW NMR techniques. It was found that an NMR frequency shift from the Larmor value exhibits a sharp increase at a magnetization tilting angle exceeding 104°. Nonlinear CW NMR signals related to the formation of a macroscopic region featuring homogeneous precession of the magnetization (homogeneous precession domain) were observed. The experimental results confirm that the low-temperature superfluid ³He phase in the aerogel is analogous to the B-phase in bulk ³He and indicate that the spin supercurrents play an important role in the spin dynamics of superfluid ³He in aerogel.     

NMR on superfluid<Superscript>3</Superscript>He in aerogel

The transition to superfluidity of³He in high porosity (98.2%) acrogel has been observed by nuclear magnetic resonance techniques. The onset of the transition at all pressures above 13 bar is marked by a sharp increase in NMR frequency similar to that observed in bulk³He-A. This suggests that the aerogel/superfluid phase is highly homogeneous although both the transition temperature, T_c, and the amplitude of the order parameter are substantially suppressed. The acrogel strands are ≈ 50Å in diameter, much smaller than the superfluid coherence length. Consequently, we have attempted to interpret our observations as an impurity scattering problem. Based on our measurements of the magnetic field dependence of T_c it appears that both magnetic and potential scattering play important roles where the magnetic scattering can be associated with solid³He on the aerogel surface. This is determined by isotopic exchange with⁴He, a process which appears to stabilize a new superfluid state similar to the bulk B-phase.     

Energy spectra of quantum turbulence in He II and 3He-B: A unified view

Quantum turbulence in superfluid He II and in ³He-B that can be regarded as nearly isothermal, isotropic, and homogeneous is discussed within the two-fluid model. A general form of the 3D energy spectrum is proposed: at large length scales, where normal and superfluid eddies are locked together by the mutual friction force, the energy spectrum is essentially classical and includes an inertial range of a Kolmogorov K62 form. With increasing wavenumber k, the normal fluid part of the spectrum terminates due to finite viscosity, while the superfluid part of the spectral energy density changes towards k ⁻³ and then back into Kolmogorov-like k ^−5/3 again. Agreement with computer simulations and experiments is claimed if account is taken of the turbulent box size and of the energy decay rate. The text was submitted by the author in English.     

Critical velocities and two mechanisms of transition in superfluid liquid He

Two mechanisms of transition of the superfluid liquid ⁴He to quantum turbulence regimes are proposed for the case when the influence of the normal fluid on superfluid flow is suppressed by introducing superleaks at the ends of the capillary. Using dimensional analysis it is found that in the roton mechanism the critical velocity depends on channel size as vc∝d−1/4, matching the experiments. For the second, super-flow mechanism, the analysis of independent parameters relevant for this phenomena leads to critical velocity depending on d as vc∝d−1. It is shown that turbulence for super-flow mechanism arises when a “quantum Reynolds number” exceeds some critical value which is about 10³ for 1D geometry. The dimensional analysis of the equation for critical velocity of superfluid flow without superleaks at the ends of the capillary is also presented.     

Nonlinear Wigner solid transport on the free surface of normal and superfluid 3He

We present measurements and a theoretical concept of the nonlinear transport in the two-dimensional Wigner solid on the free surface of normal and superfluid 3He. We have observed the transition from Ohmic to nonlinear transport over a wide temperature range, from 200 microK to 200 mK at low electron velocities. The field-velocity (E-nu) characteristics show a nonmonotonic structure with a particular local maxima. We attribute this nonlinearity to the velocity-induced deformation of the periodic surface displacement due to the extremely strong damping in Fermi-liquid 3He. The calculated E-nu curves and thresholds are in good agreement with experiment.