Superfluid Transition Of 4He Research Articles

Thermodynamical description and vortex formation at the superfluid transition in 4He

Thermodynamical description and vortex formation at the superfluid transition in 4He

Field-induced phase coexistence in an artificial spin ice

Artificial spin-ice systems are magnetic metamaterials consisting of nanomagnet arrays that can be designed to study exotic magnetic states not found in natural materials. Typically, these arrays are modelled as interacting binary macrospins that can only be in an up or down state and are described by the Ising model. These materials have demonstrated ordering transitions, but only via a spontaneous symmetry-breaking mechanism. We have designed and studied a quadrupole artificial spin-ice system consisting of interacting plaquettes of coupled single-domain nanomagnets that can be interpreted as a composite, ternary variable. After annealing this system in an external magnetic field, we observe both a ferroquadrupolar and an antiferroquadrupolar phase, with an apparent first-order phase boundary and a coexistence regime. The phase diagram of this material is reminiscent of a model used to describe phase coexistence in the superfluid transition of 4He with 3He impurities. These results illustrate how composite magnetic objects realize exotic statistical physics models beyond the Ising model. Artificial spin-ice materials are usually described by spins that are either up or down. Here, a new type of spin ice is fabricated where the spins can be in one of three states with different coexisting phases separated by a first-order transition.

Critical Point Coupling and Proximity Effects inHe4at the Superfluid Transition

We report measurements of the superfluid fraction ρ(s)/ρ and specific heat c(p) near the superfluid transition of 4He when confined in an array of (2 μm)3 boxes at a separation of S=2 μm and coupled through a 32.5 nm film. We find that c(p) is strongly enhanced when compared with data where coupling is not present. An analysis of this excess signal shows that it is proportional to the finite-size correlation length in the boxes ξ(t,L), and it is measurable as far as S/ξ∼30-50. We obtain ξ(0,L) and the scaling function (within a constant) for ξ(t,L) in an L3 box geometry. Furthermore, we find that ρ(s)/ρ of the film persists a full decade closer to the bulk transition temperature T(λ) than a film uninfluenced by proximity effects. This excess in ρ(s)/ρ is measurable even when S/ξ>100, which cannot be understood on the basis of mean field theory.

Scaling and universality of the thermal conductivity of liquid 4He near the superfluid transition and in restricted geometries

We present measurements of the thermal conductivity λ( t, P, L) = l/ ρ( t, P, L) near the superfluid transition of 4He at saturated vapor pressure and confined in cylindrical geometries with radii L = 0.5 and 1.0 μm ( t ≡ T/ T λ ( P) − 1). For L = 1.0 μm measurements at six pressures P are presented. At and above T λ the data are consistent with a universal scaling function F( X) = ( L/ ξ o ) x/ ν ( ρ/ ρ 0), X = ( L/ ξ o ) 1/ ν t valid for all P ( ρ 0 and x are the pressure-dependent amplitude and effective exponent of the bulk resistivity ρ( t, P, ∞) = ρ 0 t x and ξ = ξ 0 t − ν is the correlation length). Indications of breakdown of scaling and universality are observed below T λ .

Scaling, Dimensionality Crossover, Surface and Edge Specific Heats in Confined4He

We have studied the superfluid transition of 4He in situations where the growth of the correlation length is limited by a uniform spatial confinement. Under these conditions the critical behavior attained in the thermodynamic limit is greatly modified. One expects that data for similar confinement should scale with the correlation length. This is found to be true for the specific heat in planar confinement, 2D crossover, except in the region of the specific heat maximum and on the superfluid side. The modifications due to confinement depend on the details of the geometry. Studies of the specific heat with 1D and 0D crossover show the important role played by the lower dimension. Far from the transition the details of the confining geometry reveal contributions to the specific heat which can be attributed to surfaces and edges. Comparison of experimental resugts with theoretical calculations show agreement in some areas and disagreement in others.

Specific Heat of4He Confined in Channels of 1 µm Square Cross-Section

We report measurements of the specific heat near the superfluid transition of 4He confined in uniform channels of 1 µm square cross-section. This system undergoes a crossover from three dimensional behavior (3D) to 1D as the transition is approached. This resugts in a substantial rounding of the specific heat maximum as well as a shift to colder temperatures relative to the bulk system. We compare these data to previous measurements where crossovers from 3D to 2D and 0D were studied with the smallest confining dimension being the same (1 µm) in each case. We also compare these resugts in the context of finite-size scaling to previous studies where crossover to 1D was measured in cylindrical geometries. We identify regions where surface and edge effects dominate the specific heat, and compare these amplitudes to theory, where available. The realization of the confining geometry in this work is achieved with a combination of silicon lithography and direct wafer bonding.

Decoherence under a heat flux near the superfluid transition in 4He

Measurements of heat transport at the transition from perfect thermal super-conductivity to nonlinear heat diffusion in pure 4 He provide a very sensitive probe of matter wave coherence. Superfluid heat transport is proportional to the product of the superfluid density and the superfluid velocity, which are both directly related to the superfluid order parameter. From dynamic scaling theory, the correlation length near the superfluid transition provides a measure of the length over which phase fluctuations of the order parameter persist. Our measurements suggest that both the hydrostatic pressure variation within the liquid helium column, together with the heat flux Q, limit the otherwise divergent correlation length near the superfluid transition. Future measurements planned for the microgravity laboratory will provide the first extensive experimental test of a renormalized, field theoretic description of heat transport near the superfluid transition. It will also provide a conclusive experimental study of the influence of hydrostatic pressure effects and dynamical effects on the correlation length. A new class of microgravity experiments is proposed that will permit measurements to within 10 pK of the superfluid transition temperature, allowing an entirely new class of ultra-accurate scientific investigations to be performed.

Breakdown of Fourier's Law near the Superfluid Transition in4He

Breakdown of Fourier's Law near the Superfluid Transition in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow /><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msup></mml:mrow><mml:mi mathvariant="normal">He</mml:mi></mml:math>

Thermal conductivity measurements and self-organized criticality very close to the superfluid transition in 4He

First Page

Opportunities for High-Resolution Measurements and Microgravity Research near the Superfluid Transition of Liquid 4He

Six years ago an experiment to determine the specific heat of liquid helium near the superfluid transition in a microgravity environment opened up a new era of very-high resolution measurements. Recently this led to the establishment of the Fundamental Physics Discipline by NASA for the support of microgravity research in low-temperature/condensed-matter physics, laser cooling and atomic physics, and gravitation and relativity. This paper describes very-high resolution and microgravity research in one particular sub-field of low-temperature physics, namely the superfluid transition of 4 He, in order to illustrate the diverse research opportunities which exist within the Discipline. The effect of gravity on this system will be illustrated. Projects carried out already and expected to be undertaken in the near future on critical phenomena, surface effects, and non-equilibrium phenomena, will be discussed. These include measurements of the specific heat and the thermal conductivity, both in bulk samples and finite geometries, of the singularity of the boundary resistance between helium and a solid surface, as well as of non-equilibrium effects due to finite heat currents.

Gravitational Effects on Nonlinear Heat Transport Near the Superfluid Transition in 4He

We have recently observed nonlinear heat transport within 30 nK of the superfluid transition temperature using heat flux, Q, in the range 0.1 < Q < 2 erg/(s cm2). While Haussmann and Dohm (HD) accurately predict the initial departure of the thermal conductivity, κ, from the linear response region, κ is greater than expected very close to Tλ. We anticipate that the nature of the thermal conductivity's nonlinearity may depend upon Earth's gravity in the low heat flux limit (Q < 0.5 erg/(s cm2)). Comparison of our data to similar data to be taken in a microgravity laboratory will provide an experimental determination of the effect of gravity on nonlinear heat transport near the superfluid transition. The microgravity measurements will also permit the first experimental test of theories that do not consider gravitational effects, such as those by HD.

Superfluid transition in4He in gravity and heat flow

When4He in a normal fluid state near the superfluid transition is cooled from the bottom below the transition, a superfluid region appears at the bottom and slowly expands in the upward direction. We performed simulations for this case in one dimension fixing the temperature Tb at the bottom and the heat flux at the top Q. We find densely distributed phase slip centers rapidly oscillating in time in the expanding superfluid region. Their role is to produce a temperature gradient compensating the transition temperature gradient (dTλ(p)/dp) pg in gravity and to produce a constant negative reduced temperature T-Tλ(p) with small fluctuations superposed. The superfluid velocity multiplied by the correlation length is found to be violently fluctuating around ħ/√3m.

Thermal resistance below the superfluid transition of4He in a heat current

We report on the thermal resistivity of4He nearT λ in heat currents 20≲Q≲300 μW/cm 2. The temperature at the bottom of a layer was measured as a function of that at the top and compared with integrations of models for the conductivity λ(Q,t). The data reveal a new dissipative region belowTλ(Q) with a small but finite resistivity and a sharp onset which can not be explained by the usual “mutual friction” due to vortices.

4He very nearT λ heated from above

We discuss the feasibility and likely benefits of studying the superfluid transition of4He very nearT λ in the presence of gravity, and of a finite heat currentQ applied from the top of the sample and extracted from below. Part of or the entire sample can exist in a homogeneous critical state represented by a uniform reduced temperaturet=[T(z)−T λ(z)]/T λ (z is the vertical coordinate) without inducing convection in the HeI layer. Precision measurements of the thermal conductivity and boundary resistance very nearT λ, including inside the nonlinear region, should be possible; but only along a single path in theQ−t plane.

Absence of superfluidity in4He adsorbed in zeolite

We present the results of a torsion-oscillator experiment which has searched for the superfluid transition in4He adsorbed in chabazite. A novel non-destructive dehydration technique has been used to make the 10 A pores of the crystal accessible to4He. No superfluid transition has been observed to within our resolution for4He coverages ranging from 50% of the full pore to complete saturation at temperatures above 130 mK.

Frequency dependence of the superfluid onset in4He films on hydrogen

The superfluid transition in4He films adsorbed on solid hydrogen of 5 layers was studied by using two torsional oscillators for frequencies of 640 and 2250 Hz. We observed the frequency dependences of the onset temperature and the dissipation peak temperatureT p. The coverage dependence of the difference ΔT p between the two frequencies indicates that an intrinsic parameterb 2/3 l d of a superfluid vortex becomes a minimum at a coverage withT p=1.15 K, whereb is a coefficient of the correlation length andl d an effective diffuse-length of a superfluid vortex.

Vortex Core Size in Submonolayer Superfluid 4He Films.

The superfluid transition of4He films adsorbed on 500 A diameter slip-cast alumina powder is measured with a torsion oscillator technique for films with transition temperatures between 50 and 700 mK. The transitions are found to be controlled by the universal Kosterlitz- Thouless critical line, but a strong increase in the broadening of the transition is observed for the thinnest films. Analysis in terms of vortex pair excitations shows that this broadening results from a vortex core size that increases rapidly as the film is thinned, scaling roughly with the interparticle spacing of the superfluid submonolayer. Third sound modes are identified in the dissipation, in addition to a broad peak from the vortex unbinding. This system provides evidence of a three-dimensional superfluid transition mediated by vortex excitations.

A Phenomenological Model on the Superfluid Transition of 4He in Porous Media11The project supported by National Basic Research Project “Nonlinear Science” and by the Education Committee of the State Council through the Foundation of Doctoral Training

The superfluid transition of 4He in porous media is discussed. The eigenstates of the Hamiltonian may be localized whether the disorder is spatially correlated or not. Based on this argument we proposed a phenomenological model to describe the effects of localization on phase transitions. The system may be divided into many isolated blocks because of the localization. We calculated the superfluid density and the free energy density for the superfluid transition according to this model. These thermodynamical quantities are described by power laws in a certain temperature region but the hyperscaling relation is violated as one can expect. The comparison of the result of this model with the experiment on the superfluid transition of 4He in gels is given.

The superfluid transition of 4He in the presence of a heat current: renormalization-group theory

A review is given on the effect of a stationary heat current Q on the superfluid transition of 4He in the linear and nonlinear response regime. Results on various static and dynamic quantities as derived by the renormalization- group theory are discussed. This includes the temperature profile, the thermal bulk and boundary resistance, the interface between normal-liquid and superfluid 4He, the depression of T λ, the superfluid density, second sound and the specific heat at finite Q.

Two low temperature applications of path integrals: superfluidity in 4He and orientational ordering in solid H 2, D 2

Computational path integrals, because of the intuitive picture they provide and their natural inclusion of temperature, are being increasingly used to simulate low temperature systems. We discuss two such applications. (1) The superfluid transition of 4He in three dimensions, where path integrals combined with finite-size scaling techniques can give transition temperatures accurate to within 1 or 2%. (2) Orientational ordering in solid 2, D 2 where, using a fixed node approximation, the ordering temperature for ortho H 2 can be reproduced to within about 15% in contrast to mean-field-type calculations which predict transition temperatures too high by factors of two to three.