4He films on graphite studied by neutron scattering

The properties of ⁴He films adsorbed on graphite have been studied by neutron scattering. In particular excitations of the commensurate phase of the monolayer are discussed. The first two adsorbed layers are solid and the next ones stay liquid. At the boundaries of the superfluid film excitations could be studied. Also the phonons, maxon and rotons of the film are investigated. An explanation of the lower density of the very thin films compared to bulk ⁴He is given.Presented by H. J. Lauter.     

Fluctuations in small volumes of liquid 3He

We calculate fluctuation contributions to the thermodynamic properties in a small volume of liquid ³He. Functional integration techniques are employed to derive the fluctuation free-energy functional for temperatures just above the superfluid transition temperature and for zero temperature. Due to finite-size effects, the critical regime is estimated to be large enough to warrant an experimental observation of fluctuations. At zero temperature we find two kinds of quasicollective gapless excitations, which correspond to fluctuations of phase and rotations of the order parameter in spin space, respectively. These excitations result in a periodic time dependence of the pair correlation function. Finally, the experimental observability of the effects considered are contemplated.     

A New Anomaly in Solid 4He

A new anomaly in the acoustic properties of solid⁴He has been observed when as little as a few parts per million of³He impurities are added. We show that the specific properties of the anomaly, can be explained by a continuous (second order) phase transition from a Bose condensed state above a critical temperature to a normal state below it. In this model the Bose particles which condense are long lived, thermally activated excitations.     

GRID and Multiphonon States

The development of the GRID technique for determining nuclear level lifetimes of excited low-spin states populated in thermal neutron capture reactions has resulted in the ability to perform detailed studies of proposed multiphonon excitations for the first time. This paper discusses the experimental evidence for multiphonon excitations determined using the GRID technique. In deformed nuclei several good examples of γγK^π = 4⁺ excitations have been established, whereas the experimental evidence gathered on K^π= 0⁺ bands is contradictory, and any interpretations will likely involve the mixing of several different configurations. In vibrational nuclei the GRID technique has helped to establish the existence of multiple quadrupole phonon excitations in ¹¹⁴Cd, and an almost complete set of quadrupole-octupole coupled states in ¹⁴⁴Nd.     

Very low temperature properties of liquid helium-3

Heat capacity and phase-boundary-line data on liquid³He in the few millikelvin temperature range obtained by Wheatley and his co-workers are analyzed within the framework of thermodynamics. The data favor the thermally anomalous disordered high temperature liquid phase to become of normal thermal behavior in its ordered B phase. The latter exhibits entropy decrease on isothermal compression, or its isobaric volume expansion coefficient is positive. At temperatures substantially below the phase-boundary temperatures, the ordered liquid might revert smoothly into a modification of anomalous thermal behavior, i.e., with entropy increase on isothermal compression. This alternation in the thermal behavior of the B phase, on experimental confirmation, could become helpful for a determination of the nature of its dominant thermal excitations in the indicated two temperature ranges, a situation reminiscent of the one existing in liquid⁴He II. Currently available magnetic susceptibility data raise the possibility, at very low temperatures, of further cooling liquid³He-B on adiabatic magnetization.     

Magnetic Susceptibility of the Balian–Werthamer Phase of 3He in Aerogel

The equilibrium superfluid phase of ³He impregnated into high-porosity (98%) silica aerogels appears to be a non-equal-spin-pairing state in zero field at all pressures, which is generally assumed to be the Balian–Werthamer (BW) phase modified by the depairing effects of the aerogel structure. The nuclear magnetic susceptibility played a key role in identifying the B-phase of pure ³He with the BW state. We report theoretical calculations of the nuclear magnetic susceptibility for the BW model of superfluid ³He in aerogel within the framework of the Fermi-liquid theory of superfluid ³He. Scattering of quasiparticles by the aerogel, in addition to Fermi-liquid exchange corrections, leads to substantial changes in the susceptibility of the BW phase. The increase in the magnetic susceptibility of ³He-aerogel compared to pure³He-B is related to the polarizability of the gapless excitations and the impurity-induced local field. The limited data that is available is in rough agreement with theoretical predictions. Future measurements could prove important for a more definitive identification of the ordered phase, as well as for refining the theoretical model for the effects of disorder and scattering on the properties of superfluid ³He.     

A Simple Model for a Superfluid Stirling Refrigerator at High Operating Temperature

The superfluid Stirling refrigerator (SSR) is a Stirling machine that uses the ideal gas behavior of the ³ He Component of a ³ He- ⁴ He mixture to provide cooling at temperatures below 2 K. The design of SSR's to date has used the Schmidt model to predict the performance of a given machine. Unfortunately, the Schmidt model does not account for the phonon and roton excitations present in ⁴ He component of the mixture. These excitations significantly change the performance of the SSR. An analytical model of the SSR that accounts for the phonon and roton excitations is presented. High temperature SSR experimental data is also presented and found to compare well with the model.     

Temperature and Concentration Relaxation in Phase-separated Superfluid 3He–4He Mixtures

The kinetics of the temperature and concentration variations in the superfluid ³He–⁴He mixtures with initial concentration of 9.8% ³He, and heated from below, was studied experimentally under the pressure of 0.38 bar over a temperature range of 150–400 mK. It is found that in contrast to homogeneous liquids, the temperature and concentration relaxation in phase-separated mixtures can be described by a superposition of two exponential processes in which the time constants of temperature and concentration variations coincide. If the initial mixture was homogeneous and phase separation was triggered by a heat flow, the temperature and concentration vary non-monotonically and exhibit anomalous features at the moment of phase separation. In this case the phase transition starts in the metastable superfluid, formed out of a quite supersaturated mixture where the nucleation of the new phase may be caused by quantized vortices. The results are analyzed in terms of two possible mechanisms of relaxation–the acoustic mechanism with the second sound velocity and the diffusive one connected with dissipative flows of impurity and thermal excitations. It is shown that the measured relaxation times agree with a prediction of the theory.      

Excitations and Stability of 2D Liquid 4He

The excitation modes of two-dimensional liquid ⁴He are approached from two angles: Firstly, the phonon-roton spectrum is calculated and the related transition currents examined to gain insight into the detailed microscopic structure of the excitations and to look for possible evidence of the proposed spontaneous formation of vortex-antivortex pairs at low densities. The roton excitation is interpreted as a resonance effect in which the wavelength of the long-range density fluctuation matches favorably with the short-range oscillations caused by two-particle correlations. Contrary to the 3D case, no backflow rolls are observed in the two-body current at high momenta. The calculations reproduce reasonably the density dependence of the spectrum, and the saturation of the high-momentum part following from the decay processes of excitations is satisfactorily predicted. The stability of the liquid ground state is then studied by searching for soft modes. It is found that at densities near the expected liquid-solid phase transition the energy of the liquid can be lowered by changing the symmetry of the pair distribution function from spherical to non-spherical. This two-body structure indicates the point-group symmetry of the emerging solid phase to be hexagonal.     

Frequency and Size Dependences of Superfluidity in?Low-Dimensional 4He Fluids

We have studied superfluidity of ⁴He fluids in two- and one-dimensional states. In the 2D state of the ⁴He films on flat substrate, superfluidity was observed in the normal fluid state above the 2D Kosterlitz-Thouless temperature at high measurement frequencies. The superfluidity in 2D also depends on the system size, e.g. pore diameter of porous glasses and grain size of powder. The 1D state was realized for ⁴He fluid nanotubes formed in 1D nanopores 1.8?C4.7 nm in diameter and about 300 nm in length. Superfluidity in the 1D state was observed by the torsional oscillator experiment. The results are qualitatively well reproduced by the Monte Carlo calculation for a classical XY spin system modeled on the present ⁴He nanotubes. Although the 1D state is in the normal fluid state at any finite low temperatures due to the 1D phonon fluctuation, the superfluid frequency shift ??f of the oscillator can be observed. Above a temperature, ??f decreases due to another kind of 1D thermal fluctuation of which excitations destroy the phase coherence in the nanotubes. The excitations depend on the tube length as well as the tube diameter.     

Theoretical Analysis of Neutron and X-ray Scattering Data on 3He

X-ray scattering experiments on bulk liquid ³He (Albergamo et al. in Phys. Rev. Lett. 99:205301, 2007; Schmets and Montfrooij in Phys. Rev. Lett. 100:239601, 2008; Albergamo et?al. in Phys. Rev. Lett. 100:239602, 2008) have indicated the possibility of the existence of a sharp collective mode at large momentum transfers. We address this issue within a manifestly microscopic theory of excitations in a Fermi fluid that can be understood as proper generalization of the time-honored theory of Jackson, Feenberg, and Campbell (Jackson in Phys. Rev. A 8:1529, 1973; Feenberg in Theory of Quantum Fluids, 1969; Chang and Campbell in Phys. Rev. B 13:3779, 1976) of excitations in ⁴He. We show that both neutron and X-ray data can be well explained within a theory where the high momentum excitations lie in fact inside the particle-hole continuum. ??Pair fluctuations?? contribute a sharpening of the mode compared to the random phase approximation (RPA). When the theoretical results are convoluted with the experimental resolution, the agreement between theory and X-ray data is quite good.     

Quantum molecular tunneling of NH4 + ions in mixed ammonium-alkali halide systems

The rotational excitations of NH₄ ⁺ ions in dilute solution in metal alkali halide lattices have been studied at 4K by inelastic neutron scattering technique. It is found that the ions exist in matrix isolation at very low ammonium ion concentration,c (c → 0), and it transforms to orientational glass phase on increase ofc. It is shown that the onset of the orientational glass phase is moderated through the strain field generated by the substituted impurity in the lattice. The variation of the intensity profiles with increase ofc have been successfully explained by a phenomenological model based on gaussian distributed heights.     

Observation of Zero-Sound at Atomic Wave-Vectors in a Monolayer of Liquid 3He

The elementary excitations of a strongly interacting two-dimensional Fermi liquid have been investigated by inelastic neutron scattering in an experimental model system: a monolayer of liquid ³He adsorbed on graphite preplated by a monolayer of solid ⁴He. We observed for the first time the particle-hole excitations characterizing the Fermi liquid state of two-dimensional liquid ³He, and we were also able to identify the highly interesting zero-sound collective mode above a particle-hole band. Contrarily to bulk ³He, at low wave-vectors this mode lies very close to the particle-hole band. At intermediate wave-vectors, the collective mode enters the particle-hole band, where it is strongly broadened by Landau damping. At high wave-vectors, where the Landau theory is not applicable, the zero-sound collective mode reappears beyond the particle-hole band as a well defined excitation, with a dispersion relation quite similar to that of superfluid ⁴He. This spectacular effect is observed for the first time in a Fermi liquid (including plasmon excitations in electronic systems).     

Nonlinear excitations in a film of a mixture of 3He and superfluid 4He

Using a combination of the superfluid ⁴He film model of Rutledge et al. and the layered model of Guyer and Miller we have investigated the nonlinear excitations in a film of a mixture of ³He and superfluid ⁴He. We have shown that such a system allows one-dimensional soliton excitations. These solitons, however, are not stable with respect to transverse perturbations. In that case the system allows two-dimensional lump solutions.The paper was presented at Stat. Phys. 18, IUPAP International Conference on Statistical Physics held at Berlin, August 1992.     

Scattering of 3He and 4He Atoms from 4He Clusters

We develop a microscopic theory of elastic and inelastic scattering and sticking of ³ He and ⁴ He atoms impinging on superfluid ⁴ He clusters. A number of essential aspects must be observed in a physically meaningful and reliable theory of such scattering processes. These are all connected with multiparticle processes, particularly the possibility of energy loss. These processes are (a) the coupling to low-lying excitations which is manifested in a finite imaginary part of the self energy, (b) the interaction with roton–like excitations and the broadening of the roton minimum due to finite–size effects, and (c) the discreteness of low–lying excitations in helium clusters.     

The AB Interface in Superfluid 3He as a Simulated Cosmological Brane

We present measurements of the transport of superfluid ³He quasiparticle excitations in the ballistic limit at temperatures well below T _c , and an interpretation of unexpected results as an experimental simulation of cosmological processes. Using a variable magnetic field profile we stabilize a layer of A phase across a cylinder of B phase, creating both an AB and a BA interface. These highly ordered interfaces may provide an ideal laboratory analogy for the branes and anti-branes of current cosmology. It has been suggested that brane interaction and annihilation are involved in inflation in the early Universe and leave behind topological defects such as cosmic strings. In our experiments we have annihilated our AB/BA branes by ramping down the magnetic field to remove the A phase layer. We then find that the quasiparticles face an extra impedance owing to defects left behind in the B phase texture. This is the first definitive observation of such a phenomenon.     

The surface tension of4He: Low-temperature behavior

The low-temperature surface tension of⁴He is calculated in the excitation approximation based upon a fundamental microscopic theory. The low-temperature behavior is shown to be due entirely to the effects of the coexistence curve. The role of the excitations in surface tension calculations is clarified, and it is found that only bulk excitations contribute to the surface tension and that the excitations can be ignored up to 2.2 K with an error of about 1%. Consequently, the introduction of surface excitations must be interpreted as an attempt to calculate the effects of the coexistence curve on the surface tension.