Solid Helium Research Articles

A variational path integral molecular dynamics study of a solid helium-4

In the present study, a variational path integral molecular dynamics method developed by the author [Chem. Phys. Lett. 482 (2009) 165] is applied to a solid helium-4 in the ground state. The method is a molecular dynamics algorithm for a variational path integral method which can be used to generate the exact ground state numerically. The solid state is shown to successfully be realized by the method, although a poor trial wavefunction that cannot describe the solid state is used.

Observation of thermomechanical equilibration in the presence of a solidH4econduit

We observe a thermomechanical effect when a chemical-potential difference is created by a temperature difference imposed between two liquid reservoirs connected to each other through Vycor rods in series with solid hcp $^{4}\text{H}\text{e}$. By creating a temperature difference, $\ensuremath{\Delta}T$, between the two reservoirs, we induce a rate-limited growth of a pressure difference between the two reservoirs, $\ensuremath{\Delta}P$. In equilibrium $\ensuremath{\Delta}P$ vs $\ensuremath{\Delta}T$ is in quantitative agreement with the thermomechanical effect in superfluid helium. These observations confirm that below $\ensuremath{\sim}600\text{ }\text{mK}$ a flux-limited flow exists through the solid helium.

Theory of the Phase and Twin Boundaries in Solid Helium and Reversibility of the bcc-hcp Phase Transition

The phase transition between bcc and hcp in solid 4He has martensitic-like features. The phase transition was investigated theoretically in terms of coherent lattice transformations. A theory with three order parameters is developed to take into account the changes in volume and pressure. The order parameters are the relative displacement, the stretching strain, and the compressive strain of the atomic planes. Also we introduce a thermodynamic parameter which characterizes the deviation from phase equilibrium. The parameter describes the fine structure of a 180° boundary and it’s split into two phase boundaries. The twin boundaries are nuclei of the original crystal phase. On this basis we explain the bcc-hcp phase transition asymmetry and pressure change, found in solid 4He experiments, and propose a new experimental scheme to inverse this asymmetry.

Evidence for a High-Temperature Disorder-Induced Mobility in SolidHe4

We have carried out torsional oscillator experiments on solid 4He at temperatures between 1.3 K and 1.9 K. We discovered phenomena similar to those observed at temperatures below 0.2 K, which currently are under debate regarding their interpretation in terms of supersolidity. These phenomena include a partial decoupling of the solid helium mass from the oscillator, a change of the dissipation, and a velocity dependence of the decoupled mass. These were all observed both in the bcc and hcp phases of solid 4He. The onset of this behavior is coincidental with the creation of crystalline disorder but does not depend strongly on the crystalline symmetry or on the temperature.

The Geyser effect in the expansion of solid helium into vacuum

The mechanism behind the intensity oscillations accompanying the flow of solid helium through a micron-sized orifice into vacuum, called the geyser effect, is investigated by measuring the pressure pulses at various locations in the entire flow system. The new results reveal that the source chamber pressure pulses have the same shape as the external detector pulses monitored in the previous experiments [G. Benedek et al., Phys. Rev. Lett. 95, 095301 (2005)]. New experiments in which the external gas reservoir is isolated from the pressure regulator provide direct information on the mechanism of the collapse leading to the geyser pulses. Thus each geyser pulse is triggered by the breakdown of a plug located upstream of the source chamber. The flow of liquid through the orifice determines the shape of the subsequent geyser pulse.

Supersolid Phases in a Realistic Three-Dimensional Spin Model

Supersolid phases, in which a superfluid component coexists with conventional crystalline long range order, have recently attracted a great deal of attention in the context of both solid helium and quantum spin systems. Motivated by recent experiments on 2H-AgNiO2, we study the magnetic phase diagram of a realistic three-dimensional spin model with single-ion anisotropy and competing interactions on a layered triangular lattice, using classical Monte Carlo simulation techniques, complemented by spin-wave calculations. For parameters relevant to experiment, we find a cascade of different phases as a function of magnetic field, including three phases which are supersolids in the sense of Liu and Fisher. One of these phases is continuously connected with the collinear ground state of AgNiO2, and is accessible at relatively low values of magnetic field. The nature of this transition, and its possible observation, are discussed.

Stabilization of HHeF by Complexation: Is it a Really Viable Strategy?

Ab initio calculations at the MP2 and CCSD(T) levels of theory have disclosed the conceivable existence of fluorine-coordinated complexes of HHeF with alkali-metal ions and molecules M(+) (M(+)=Li(+)-Cs(+)), M(+)-OH(2), M(+)-NH(3) (M(+)=Li(+), Na(+)), and MX (M=Li, Na; X=F, Cl, Br). All these ligands L induce a shortening of the H-He distance and a lengthening of the He-F distance accompanied by consistent blue- and redshifts, respectively, of the H-He and He-F stretching modes. These structural effects are qualitatively similar to those predicted for other investigated complexes of the noble gas hydrides HNgY, but are quantitatively more pronounced. For example, the blueshifts of the H-He stretching mode are exceptionally large, ranging between around 750 and 1000 cm(-1). The interactions of HHeF with the ligands investigated herein also enhance the (HHe)(+)F(-) dipole character and produce large complexation energies of around 20-60 kcal mol(-1). Most of the HHeF-L complexes are indeed so stable that the three-body dissociation of HHeF into H+He+F, exothermic by around 25-30 kcal mol(-1), becomes endothermic. This effect is, however, accompanied by a strong decrease in the H-He-F bending barrier. The complexation energies, DeltaE, and the bending barriers, E*, are, in particular, related by the inverse relationship E*(kcal mol(-1))=6.9exp[-0.041DeltaE(kcal mol(-1))]. Therefore the HHeF-L complexes, which are definitely stable with respect to H+He+F+L (DeltaE approximately 25-30 kcal mol(-1)), are predicted to have bending barriers of only 0.5-2 kcal mol(-1). Overall, our calculations cast doubt on the conceivable stabilization of HHeF by complexation.

Amorphous solid helium in porous media

Amorphous solid helium in porous media

Frequency Dependence and Dissipation in the Dynamics of Solid Helium

Torsional oscillator experiments on solid 4He show frequency changes which suggest mass decoupling, but the onset is broad and is accompanied by a dissipation peak. We have measured the elastic shear modulus over a broad frequency range, from 0.5 Hz to 8 kHz, and observe similar behavior-stiffening and a dissipation peak. These features are associated with a dynamical crossover in a thermally activated relaxation process in a disordered system rather than a true phase transition. If there is a transition to a dc response, e.g., to a supersolid state, it must occur below 55 mK.

Is there a true supersolid phase transition ?

New measurements of the rigidity of solid helium show that the emergence of supersolidity is actually a crossover, rather than a true phase transition.

Elastic Properties of Polycrystalline Solid Helium

Recent experiments have shown that the shear modulus of solid helium undergoes a large temperature variation in the range 20 to 200 mK, possibly due to changes in the pinning of dislocations. In this note we report on computer simulations of the elastic properties of polycrystalline solid helium. We calculate how the elastic coefficients of a sample made up of a large number of randomly oriented grains are affected by the changes in the shear modulus c44 of the individual grains.

Dislocation-vibration model for nonclassical rotational inertia

A dislocation model for the nonclassical rotational inertia is proposed. Temperature dependence of the period of the torsional oscillator containing solid helium is derived from the variation in the average pinning length of dislocations in solid helium. The mechanism is that the vibration of dislocations causes the wave number of the shear wave in solid helium to increase. Consequently the apparent moment of inertia of solid helium is increased with increasing temperature.

Superfluidity of a perfect quantum crystal II

In recent years, experimental data were published which point to the possibility of the existence of superfluidity in solid helium. To investigate this phenomenon theoretically we employ a hierarchy of equations for reduced density matrices which describes a quantum system that is in thermodynamic equilibrium below the Bose-Einstein condensation point, the hierarchy being obtained earlier by the author. It is shown that the hierarchy admits solutions relevant to a perfect crystal (immobile) in which there is a frictionless flow of atoms, which testifies to the possibility of superfluidity in ideal solids. The solutions are studied with the help of the bifurcation method and some their peculiarities are found out. Various physical aspects of the problem, among them experimental ones, are discussed as well.

Non-classical response from quench-cooled solid helium confined in porous gold

The non-classical rotational inertia (NCRI) in solid helium was detected by a drop in the resonant period of a torsional oscillator. This non-classical response was interpreted as the first possible evidence of supersolidity. A number of subsequent experiments, however, reported unexpected phenomena within the supersolid context. Experimental and theoretical work have drawn attention to the role of disorder in solid helium to explain the inconsistency. We have investigated the non-classical response of solid 4He confined in porous gold set to torsional oscillation. When solid helium is grown rapidly, nearly 7% of the solid helium appears to be decoupled from the oscillation below about 200 mK. Dissipation appears at temperatures where the decoupling shows maximum variation. In contrast, the decoupling is substantially reduced in slowly grown solid helium. The dynamic response of solid helium was also studied by imposing a sudden increase in the amplitude of oscillation. Extended relaxation in the resonant period shift, suggesting the emergence of the pinning of low-energy excitations, was observed below the onset temperature of the non-classical response. The motion of a dislocation or a glassy solid is restricted in the entangled narrow pores and is not likely responsible for the period shift and long relaxation.

Twin boundaries as nuclei of a new phase in body-centered cubic—hexagonal close-packed phase transitions in solid helium

The reversibility of the coherent phase transition from a body-centered cubic (bcc) phase to a hexagonal close-packed (hep) phase has been considered. Th e topological classification and qualitative description of arising grains (domains) and 60°, 90°, 120°, and 180° twin boundaries have been performed. The thermodynamic parameter of deviation from the phase equilibrium has been introduced. This parameter has made it possible to analytically describe the fine structure of the 180° boundary and its splitting into two interphase boundaries during the reverse phase transition. It has been demonstrated that, in the vicinity of the phase transition, this description is applicable to the 60°, 90°, and 120° boundaries. The twin boundaries have been shown to be nuclei of the initial (parent) phase, for which the structure can be completely restored. On this basis, an explanation has been offered for the origin of the asymmetry of the bcc—hcp phase transition observed in experiments carried out in solid 4He. A scheme has been proposed for a new experiment performed with the aim of reversing the asymmetry of this phase transition.

The role of defects in Supersolid Helium-4

We seek an explanation for the observed non-classical rotational inertia in solid Helium-4. We study the activation energy for a single vacancy in an otherwise perfect crystal, which is huge and leads to the conclusion that there are no thermal vacancies in solid Helium-4 at the experimentally relevant temperatures. We also study grain boundaries and find that they can be mechanically stable when brought into contact with liquid, and can turn superfluid. Contact with recents experiments is made.

Absence of Slow Sound Modes in Supersolid 4He

Torsional oscillator experiments on solid 4He have been interpreted as showing mass decoupling similar to what one observes in a superfluid. Within the context of a two-component model for the supersolid one would expect the appearance of a second, slow acoustic mode. We have searched for this mode using an acoustic resonance technique. We have used porous membranes in bulk solid 4He analogous to a second sound experiment in the superfluid. We also investigated solid helium in Vycor using piezoelectrically driven titanium diaphragms (analogous to a fourth sound experiment in the liquid). Our measurements have shown no indication of an additional sound mode in the kHz range.

Mass Flow through Solid 4H

We continue our solid 4He flow experiments in which we grow solid helium samples at constant temperature in the hcp region of the phase diagram. We exploit the properties of liquid helium in a confined geometry (porous Vycor glass), and induce a mass flow through the solid at pressures higher than the bulk melting pressure. We previously observed flow, but our temperature was limited to T≥350 mK. At T≈400 mK it was found that the flow ceased at P≈27 bar, and no flow was observed for T>550 mK. We have begun to extend our measurements to lower temperatures, and our data show that at lower temperatures we observe flow at higher pressures.

Lattice distortion of hcp solid helium under pressure

The lattice distortion of hcp solid He under pressure is calculated using semiempirical and first-principle approaches. While three-body forces tend to flatten the lattice at all compressions, the effect of pair forces changes from the flattening at small compression to elongation at large one. At large compressions, the lattice distortion due to the triple forces is more than twice as large as those due to pair forces and the lattice is slightly flattened. First-principles results show that over approximately fivefold compressions higher-order, many-body forces become important.

POSSIBLE TRANSPORT INSIDE AN ANNULUS FILLED WITH SOLID HELIUM OF PARTIAL SUPERSOLIDITY

We obtain the possible transport in an annulus filled with solid helium of which parts are of supersolidity near the interface. Our transport results qualitatively resemble those recently reported measurements.