Two-dimensional 3He Research Articles

The Two-Dimensional World of 3He in 3He-4He Mixture Films

For thin films of 4 He at low temperatures the addition of sub-monolayer numbers of 3 He atoms results in the 3 He residing in a ground state at the free surface of the 4 He film. For low 3 He coverage the 3 He are a nearly ideal two-dimensional Fermi system. With increasing numbers of 3 He atoms the interactions among the 3 He increase. The results from a number of experiments that reveal the remarkable properties of this rich, interacting, two-dimensional 3 He system are described. These results include the magnetization, the energetics of the 3 He atoms in quantum states, the diffusion of the 3 He along the 4 He film, and the 3 He effective mass. The techniques employed include NMR, specific heat, quartz crystal microbalance and third sound. A clearer picture of the properties of the 3 He is now emerging, and preliminary numbers for the 3 He coverage dependence of the Fermi Liquid parameters are presented. Prospects for the observation of superfluidity of the 3 He in this system are discussed.

Heat Capacity and Susceptibility of Two-Dimensional 3He Solid Within the Heisenberg Model

The validity of a two-dimensional Heisenberg model for the interpretation of the experimentally observed heat capacity and magnetic susceptibility of the second 3He layer solid adsorbed on graphite has been discussed. The proposed theoretical method provides an analytical description of thermodynamic functions in the whole temperature range. To make the theory fit to the experimental data two independent adjustable parameters: the exchange constant J and the number of “magnetically active” spins n2 have been used. It has been shown that in the ferromagnetic regime the pure Heisenberg behaviour of 2D solid 3He occurs and the proposed theory is in excellent agreement with the experimental data for all the temperatures.

LOCALIZATION-FERMI LIQUID TRANSITION OF 3HE ADSORBED ON 4HE-COATED HECTORITE

To create a phase diagram of two-dimensional (2D)3He fluids, we measured heat capacity of the3He adatoms down to 20mK as a function of the density. The3He adatoms are deposited on hectorite precoated with4He in the amount of 24.70μmol/m2, just above the onset density for the4He super-fluid. Isotherms of the heat capacity steeply increase with the3He density up to nc=2.4μmol/m2. Above nc, the heat capacity shows properties of the 2D Fermi liquids. We pointed out that the density dependence of the heat capacity is similar to those of the insulator-metal (I-M) transitions of some electron systems, e.g. Si:P. Assuming the transition from a localized state to the Fermi liquid at the density nc, the diameter of a localization area aH* was obtained as about 3.8A using the Mott relation that the mean distance equals 2.2 aH*at the transition. Effective mass and 2D spin fluctuations of the Fermi liquids are enhanced with the density by increasing correlations in the 2D liquids.

Excitations in Two-Dimensional 4He

We present the study of the ground state and of the low-energy excited states of liquid4He in two dimensions (2D) within the Variational Monte Carlo method. As trial wave function a shadow wave function is used. The energies of the maxon and of the roton are close together at the equilibrium density ρeq due to the low value of ρeq in 2D but at higher densities a well developed roton minimum is present. The short range backflow effects are stronger in 2D than in 3D and close to freezing the roton energy is about 5 K. This value is close to the lowest branch of excitations found from inelastic neutron scattering from thin4He film on graphite and interpreted as due to layer rotons. Some strong similarities between the 2D and 3D case are present: the maxon energy close to freezing is about twice the roton energy, the relative strength of the single roton peak in the dynamical structure factor is essentially density independent and very close in value to the 3D case, there is a similar shift between the wave vectors of the roton and of the position of the main maximum of the static structure factor.

Structure and superfluidity of He4 films on plated graphite

The results of an experimental study using torsional oscillators of the superfluidity of 4He films adsorbed on hydrogen plated graphite are reported. The evolution of superfluidity with the growth of the film shows considerable structure arising from the atomic layering of the film. There is evidence that the superfluidity of a single fluid layer is strongly suppressed, possibly due to the influence of the periodic potential arising from the underlying solid layer. The behavior of two fluid layers is quite distinct, but shows similarities to that of thicker films on heterogeneous substrates; we suggest that the usual theory of the superfluidity of two-dimensional 4He should be extended to account for superfluid onset temperatures in such a film.

Fermi liquid phenomenology combined with scaling theory of the metal-insulator transition

Continous transitions between a Mott insulator and a correlated metal are studied with the scaling theory. The validity of hyperscaling analysis is examined. As an example of the criticality, two-dimensional 3He systems near the Mott transition observed experimentally are analyzed in favor of the mass singularity in the form m ∗ α δ −1 where δ is the density of 3He measured from the Mott insulator. Several consequences of the scaling theory are derived when the metallic state is described by the Femi liquid. The breakdown of the Femi liquid towards the transition point is figured out in this framework.

Many-body structure of quantum vortices in thin 4He films

We propose a microscopic theory of quantum vortices in two-dimensional 4He, applicable to thin superfluid 4He films. The theory describes the vortex core and reproduces the classical logarithmic behavior at large distances. The single vortex is treated as a quasiparticle. We calculate the energy needed to create a single vortex, the vortex inertial mass, the microscopic interaction between vortices and the binding energy of the vortex-antivortex pair as a function of density at zero temperature. At 4He densities less than 0.037 Å −2 the binding energy of the vortex-antivortex pair becomes negative, indicating an instability for the creation of a novel state.

Longitudinal NMR relaxation of degenerate two-dimensional 3He in 4He films.

The NMR spin-lattice relaxation time ${\mathit{T}}_{1}$ of a degenerate submonolayer coverage of $^{3}\mathrm{He}$ in a thin adsorbed $^{4}\mathrm{He}$ film has been measured. As a function of $^{4}\mathrm{He}$ coverage, ${\mathit{T}}_{1}$ exhibits a maximum and undergoes a change in temperature dependence at a coverage near that of the superfluid transition in the underlying $^{4}\mathrm{He}$ film. Two relaxation mechanisms contribute to the observed ${\mathit{T}}_{1}$.

Quantized transport coefficient accompanying the phase modulation of order parameters in two-dimensional superfluids

A conjecture is made of the quantization of particle transport occurring in two-dimensional superfluid 3He. This is a consequence of the modulation of the phase functions associated with order parameters (or gap functions) and a direct analogy with the quantum Hall effect which is regarded as a topological quantization.

Some new observations at the superfluid transition in 3He–4He films

We have studied quasi two-dimensional 3He–4He mixtures by means of a high-Q torsional oscillator. In contrast to the case of pure 4He films, two superfluid phase transitions have been observed in mixture films where the 4He coverage is near one active layer. For a somewhat larger coverage of 4He, the addition of 3He causes the superfluid transition to shift, at first, to higher temperatures rather than lower ones as is usually observed.

Phase transition of two-dimensional 3He from a dilute to a dense phase.

We have measured the heat capacity of $^{3}\mathrm{He}$ in films of $^{4}\mathrm{He}$ as thin as 10 A\r{} formed on a Nuclepore filter substrate. At low temperatures, where the $^{3}\mathrm{He}$ is in the lowest state as far as motion perpendicular to the film surface, we find that the $^{3}\mathrm{He}$ undergoes a transition from a dilute phase to a dense phase. We have observed this transition for films of $^{4}\mathrm{He}$ of 12.3- and 10-A\r{} thickness and for coverages of $^{3}\mathrm{He}$ below about 0.2 atomic layers. For thicker $^{4}\mathrm{He}$ films, and higher $^{3}\mathrm{He}$ coverages, the $^{3}\mathrm{He}$ remains homogeneously spread out over the surface of the $^{4}\mathrm{He}$. The striking characteristic of the transition is the sudden onset of linear temperature dependence with a slope which is proportional to the amount of $^{3}\mathrm{He}$ in the calorimeter. This is consistent with the formation of islands of a dense two-dimensional phase which grows in extent proportionately to the amount of $^{3}\mathrm{He}$. Two puzzling aspects of our results are the lack of a significant heat-capacity jump at the transition and, based on a linear extrapolation of the data to zero temperature, a substantial amount of missing entropy.

The orientational effects in the two-dimensional superfluid 3He

The dipolar and magnetic orientational effects are investigated and the stable phases are found for the two-dimensional superfluid 3He film. Both ABM and BW phases are of the ESP (equal spin pairing) type and exhibit the anisotropy in the spin space.

Phase Transitions in Two-Dimensional SuperfluidHe3

A discussion of phase transitions in two-dimensional 3He is presented. A new type of phase transition is proposed, in which islands of reversed l→ spontaneously nucleate, leading to an Ising-like transition. It is shown that such transitions may need to be taken into account, possibly competing with the well-know Kosterlitz-Thouless transition. Consequences of both types of transition are discussed.