Solid Helium Research Articles

Optical Observations of Disorder in Solid Helium 4

It has been proposed that the supersolidity of helium 4 is associated with disorder in the solid samples. We report optical observations of this disorder in samples grown by different methods. We show that, when grown by the “blocked capillary method” as in most experiments showing anomalous phenomena, solid helium 4 can be polycrystalline with grain sizes down to the micrometer range, much smaller than the 0.1 mm found in some other experiments.We analyze the properties of grain boundaries, and in particular the existence of liquid channels at the contact of grain boundaries with walls. Our analysis includes measurements of the contact angle of the liquid-solid interface with these walls, which exhibits a large hysteresis. Furthermore, we predict that similar channels should exist at the junction between grain boundaries.

Neutron scattering study of the excitation spectrum of solid helium at ultra-low temperatures

There has been a resurgence of interest in the properties of solid helium due to the recent discovery of non-classical rotational inertia (NCRI) in solid 4He by Chan and coworkers below 200 mK which they have interpreted as a transition to a ‘supersolid’ phase. We have carried out a series of elastic and inelastic neutron scattering measurements on single crystals of hcp 4He at temperatures down to 60 mK. While we have found no direct evidence of any change in the excitation spectrum at low temperatures, we have found that the excitation spectrum of solid 4He shows several interesting features, including extra branches in addition to the phonon branches. We interpret these extra branches as single particle excitations due to propagating vacancy waves, which map on to the famous ‘roton minimum’ long known in the excitation spectrum of superfluid liquid 4He. The results show that in fact solid 4He shares several features in common with the superfluid.

Optical Observations of Disorder in Solid Helium 4

Optical Observations of Disorder in Solid Helium 4

First-Principles Studies of the Metallization and the Equation of State of Solid Helium

The insulator-to-metal transition in hcp solid helium at high pressure is studied with first-principles simulations. Diffusion quantum Monte Carlo (DMC) calculations predict that the band gap closes at a density of 21.3 g/cm;{3} and a pressure of 25.7 terapascals, which is 20% higher in density and 40% higher in pressure than predicted by density functional calculations based on the generalized gradient approximation (GGA). The metallization density derived from GW calculations is found to be in very close agreement with DMC predictions. The zero point motion of the nuclei had no effect on the metallization density within the accuracy of the calculation. Finally, fit functions for the equation of state are presented, and the magnitude of the electronic correlation effects left out of the GGA approximation are discussed.

How could a solid be superfluid?

Experiments indicate that, as in a superfluid, mass can flow through solid helium-4 without viscous resistance. Recent calculations shed light on how this may happen thanks to defects in the crystal lattice.

Observation of a superfluid phase in solid helium

Evidence of a superfluid liquid phase present in polycrystalline helium at a temperature of 0.2 K and a pressure of 51 bar has been obtained by means of inelastic neutron scattering. The superfluid component is absent at a temperature of 0.6 K and the same pressure. Thus, a “solid helium-superfluid helium” phase transition has been discovered. The sample of solid helium in a porous medium (silica aerogel) has been prepared with the use of a capillary blocking technique. The shape of the structure factor of the superfluid phase indicates the presence of clusters or the effects of a restricted geometry. The results may be used to explain the nonclassical rotational inertia phenomenon in solid helium (often referred to as supersolidity, Nature, 2004).

He4Adsorbed on the Outer Surface of Carbon Nanotube Bundles

The results of diffusion Monte Carlo calculations on the behavior of 4He adsorbed on the external surface of a bundle of carbon nanotubes are presented. The corrugation effects are found to be very important, making the outside part of the bundles a quite inhomogeneous substrate. No stable solid helium monolayer at high density was found. Instead, helium atoms are promoted to a second quasi-one-dimensional phase on top of the liquid first layer. On increasing the helium intake, a two layer structure is formed in which the helium directly in contact with the carbon surface solidifies.

Compressibility of solid helium

The compressibility of solid helium(3He and 4He) in the hcp and fcc phases has been studied by the path-integralMonte Carlo method. Simulations were carried out in both canonical(NVT) andisothermal–isobaric (NPT) ensembles at temperatures between 10 and 300 K, showing consistent resultsin both ensembles. For pressures between 4 and 10 GPa, the bulk modulusB is found to decrease by about 10%, when the temperature increases from thelow-temperature limit to the melting temperature. The isotopic effect on the bulkmodulus of helium crystals has been quantified for a wide range of parameters.At 25 K and pressures on the order of 1 GPa, the relative difference between3He and 4He amounts to about 2%. The thermal expansion has also been quantified from results obtained in bothNPT and NVT simulations.

Thermal history of solidH4eunder oscillation

We have studied the thermal history of the resonant frequency of a torsional oscillator containing solid $^{4}\text{H}\text{e}$. We find that the magnitude of the frequency shift that occurs below $\ensuremath{\sim}100\text{ }\text{mK}$ is multivalued in the low-temperature limit, depending strongly on how the state is prepared. This result can be qualitatively explained in terms of the motion and pinning of quantized vortices within the sample. Several aspects of the data are also consistent with the response of dislocation lines to oscillating stress fields imposed on the solid. However, studies of solid helium in porous media, several control experiments, and the magnitude of the frequency shift found in this and other experiments all indicate that the most appropriate interpretation of the torsional oscillator results is the existence of a supersolid $^{4}\text{H}\text{e}$ phase.

Nearly frictionless transport of glassy solid helium at low temperature regime

Glassy matter, as subjected to high shear rates, exhibit shear-thinning: i.e., the viscosity diminishes with increasing shear rate. One possible outcome for almost vanishing viscosity is the nearly frictionless transport in micropores subjected to some surface conditions. Here we show that, by treating the glass as a shear-thinning matter and using the transition rate dependent model together with the boundary perturbation method, we can observe a sudden change of the shearing characteristics (which directly relates to the resistance and the time or rate dependent shear modulus) at low temperature regime (ranged from 30 to 50 mK) for corresponding physical activation energy. If the solid helium could be treated as glassy matter at very low temperature regime then our results resemble those recently reported by Day and Beamish [J. Day, J. Beamish, Nature 450 (2007) 853].

Pressure driven transport of glassy solid helium in cylindrical nanopores at very low temperature regime

AbstractBy using the glassy helium model and boundary perturbation method, we can obtain the velocity fields (as well as the flow rate up to the second order) at very low temperature regime inside the wavy‐rough cylindrical nanopores. Our results show that the velocities of solid helium reported before by many groups could be reproduced by carefully selecting relevant physical parameters.

Pressure driven transport of glassy solid helium in cylindrical nanopores at very low temperature regime

By using the glassy helium model and boundary perturbation method, we can obtain the velocity fields (as well as the flow rate up to the second order) at very low temperature regime inside the wavy-rough cylindrical nanopores. Our results show that the velocities of solid helium reported before by many groups could be reproduced by carefully selecting relevant physical parameters.

Squeeze play makes solid helium flow

Squeeze play makes solid helium flow

Squeeze Play Makes Solid Helium Flow

AMERICAN PHYSICAL SOCIETY MEETINGPreliminary data reported at the American Physical Society meeting provide the most direct evidence yet that ultracold, highly pressurized solid helium can flow like the thinnest possible liquid.

Self-assembly of neon into a quantum gel with crystalline structure in superfluidHe4: Prediction from density functional theory

Doping superfluid $^{4}\mathrm{He}$ with a low concentration of neon atoms is predicted to lead to the self-assembly of ``quantum gel'' with crystalline structure, where each atom is isolated by a mixture of liquid and solid helium. The formation of this structure is shown to originate from the repulsive or attractive interactions between the solvent shells surrounding the neon atoms. The lattice geometry for neon atoms is predicted to be very sensitive to many-body effects, but by assuming a simple cubic symmetry, an upper limit estimate for neon atom density can be established (7% of bulk neon). Based on the energy estimates, this quantum gel structure is not related to the previously observed neon impurity helium solid. Experimental aspects of the quantum gel preparation are also briefly discussed.

Formation of a glass phase in solid He4. Contribution to the pressure in the supersolid region

High-precision barometry is used to perform measurements of the temperature dependence of the pressure of helium crystals, located in a constant volume, in the temperature range 50–800mK on crystals of different quality ranging from freshly prepared to carefully annealed. It is found that an additional contribution to the pressure, proportional to T2, appears in on annealed samples. This contribution becomes dominant at T<300mK, i.e. the region where anomalous behavior of solid helium, ascribed to the super solid effect, was previously observed. Such behavior of the pressure is characteristic for the glass phase, and this phase was practically absent in carefully annealed samples. The observed effect shows that in solid helium containing a high concentration of nonequilibrium defects a glass phase forms easily on cooling. Anomalously high pressure relaxation in low-quality samples was also detected in the experiments at temperatures close to the melting temperature. The possible reasons for this effect are discussed.

Bose–Einstein condensation in a decorated lattice: an application to the problem of supersolid He

The Bose–Einstein condensation of vacancies in a three-dimensional decorated lattice is considered. The model describes possible scenario of superfluidity of solid helium, caused by the presence of zero-point vacancies in a dislocation network. It is shown that the temperature of Bose–Einstein condensation decreases under increase of the length of the network segments, and the law of decrease depends essentially on the properties of the vertices of the network. If the vertices correspond to barriers with a small transparency, the critical temperature varies inversely as the square of the length of the segment. On the contrary, if the vertices correspond to traps for the vacancies (it is energetically preferable for the vacancies to be localized at the vertices), an exponential lowering of the temperature of transition takes place. The highest temperature of Bose–Einstein condensation is reached in the intermediate case of vertices with large transparency, but in the absence of tendency of localization at them. In the latter case the critical temperature is inversely as the length of the segment.

Supersolidity

The observation of nonclassical rotational inertia (NCRI) by the torsional oscillator in 2004 gave rise to a renaissance in the study of solid helium-4. Recent theoretical and experimental studies found evidence that disorder in the solid plays a key role in enabling superfluidity. A recent experiment found a marked increase in the shear modulus that shares the same temperature and helium-3 impurity concentration dependence as that of NCRI. This correlation indicates that the onset of superfluidity requires the pinning and stiffening of the dislocation network by helium-3.

Stages of Homogeneous Nucleation in Solid Isotopic Helium Mixtures

We have made pressure and NMR measurements during the evolution of phase separation in solid helium isotopic mixtures. Our observations indicate clearly all three stages of the homogeneous nucleation-growth process: (1) creation of nucleation sites; (2) growth of the new-phase component at these nucleation sites; and (3) coarsening: the dissolution of subcritical droplets with the consequent further late-stage growth of the supercritical droplets. The time exponent for the coarsening, a=1/3, is consistent with the conserved order parameter Lifshitz-Slezov evaporation-condensation mechanism.

Surface instability and isotopic impurities in quantum solids

In this paper, we employ a self-consistent harmonic approximation to investigate surface melting and local melting close to quantum impurities in quantum solids. We show that surface melting can occur at temperatures much lower than the critical temperature ${T}_{c}$ of the solid phase instability in the bulk. Similar effects are driven by the presence of an isotope substitution. In this latter case, we show that stronger local lattice fluctuations, induced by a lighter isotope atom, can induce local melting of the host bulk phase. Experimental consequences and the possible relevance in solid helium are discussed.