Vacancies In Solid 4He Research Articles

Superfluidity of vacancies in solid 4He

Under the adiabatic approximation, we present a superfluid state of vacancy motion in solid 4He. The vacancies in solid 4He can sense an attractive effective interaction by exchange of virtual longitudinal acoustic phonons. Such an interaction leads to a Bardeen-Cooper-Schrieffer (BCS) condensation state, in which the vacancies with opposite wave vectors are bound into pairs and single, unpaired bare vacancies are transformed into a new kind of quasiparticle, the dressed vacancy. A dressed vacancy is a vacancy clothed with a cloud of virtual longitudinal-acoustic phonons. The vacancy system undergoes a first-order phase transition from the normal state to the BCS condensation state. The critical temperature may be the transition temperature of the supersolid state. We derive the low-temperature expressions of vacancy heat capacity and vacancy number density in the BCS condensation state. The BCS condensation state of vacancies is the superfluid state of vacancies, which possesses potentially important and novel effects in condensed matter physics.

Possible rectified frictionless transport of localized vacancies in solid 4He

We illustrate the possible frictionless transport induced by localized vacancies of bcc 4He considering the observed excitation (∼1.2 meV) in [T. Markovich, E. Polturak, J. Bossy, E. Farhi, Phys. Rev. Lett. 88, (2002) 195301].

A Microscopic Description of Vacancies in Solid 4He

The changes that vacancies produce in the properties of hcp solid 4He are studied by means of quantum Monte Carlo methods. Our results show that the introduction of vacancies produces significant changes in the behavior of solid 4He, even when the vacancy concentration is very small. We show that there is an onset temperature where the properties of incommensurate 4He change significantly. Below this temperature, we observe the emergence of off-diagonal long range order and a complete spatial delocalization of the vacancies. This temperature is quite close to the temperature where non-classical rotational inertia has been experimentally observed. Finally, we report results on the influence of vacancies in the elastic properties of hcp 4He at zero temperature.

On the Stability of Small Vacancy Clusters in Solid 4He

We study numerically properties of multiple vacancies in solid 4He at zero temperature. Up to four vacancies were introduced into the solid through incommensuration between the number of available lattice sites and the actual number of atoms. Vacancy-vacancy correlation function increases at very short distances indicating effective vacancy attraction between vacancies located on nearby lattice sites. The decay of the pair correlation function at large distances puts an upper bound on the absolute value of the binding energy varying from 4 mK at melting density to 150 mK at the highest considered density \(\rho=32.2~\mbox{nm$^{-3}$}\), and no lower bound; either the four-vacancy clusters are unbound, or are bound too weakly for the temperatures of the supersolid experiments.

Zero-point divacancy concentration in the shadow wave function model for solidH4e

We address the issue of interaction between zero-point vacancies in solid 4He as described within the shadow wave-function model. Applying the reversible-work method and taking into account finite-size effects, we obtain a zero-point monovacancy concentration of (2.03 +- 0.02) 10^{-3}, which is slightly higher than the result due to Reatto et al. for the same model. Utilizing the same methodology, we then consider the divacancy, taking into account both the in-plane as well as out-of-plane configurations with respect to the basal plane. We find no significant anisotropy between both conformation. Furthermore, although there is a small binding tendency, the expected divacancy concentration is only ~4-5 times larger than the value expected in the absence of any clustering propensity, 2.5 10^{-5}. This result suggests that, within the employed model description, no vacancy aggregation leading to phase separation is to be expected in the ground state.

Zero-temperature study of vacancies in solid 4He

We are investigating the properties of vacancies in solid 4He with the exact zero temperature SPIGS method. Our aim is to study the possibility of phase-separation between vacancies and the perfect crystal. We find a significant correlation among vacancies both in two and in three dimensional solid He. In 3D we have considered up to 5 vacancies in a hcp crystal at a density ρ = 0.0293Å-3, close to the melting, and in 2D we have considered up to 6 vacancies in a triangular crystal at ρ = 0.0765Å-2. In all the considered cases the correlation among vacancies seems to display an exponential decay that would suggest the presence of a bound state. The systematically sublinear dependence on the number of vacancies found for the activation energy strengthens the indication of an attractive interaction, even if we have direct evidence that vacancies do not form a single compact cluster.

Path integral calculations of vacancies in solid Helium

We study properties of vacancies in solid 4He using Path Integral Monte Carlo. We find, in agreement with other calculations, that the energy to create a single vacancy is 11.5 K and is monotonic with the number of vacancies. If more then a few percent of the system becomes vacant, we find the system becomes unstable to melting. We show the number of exchanges in the system is increased by vacancies and how the underlying lattice is altered by the presence of a vacancy. We also examine the efficacy of using a tight binding Hamiltonian to describe the vacancy in the crystal, show that vacancies are attractive, and find values for the effective mass and inter-vacancy attraction.

Vacancies in Solid 4He and Bose Einstein Condensation

We find that a finite concentration of vacancies induces Bose Einstein condensation (BEC) of the atoms in solid 4 He at density close to the T=0 K melting where vacancies are delocalized. No BEC is present in the perfect crystal and in the defected solid at higher densities. These results are based on variational Monte Carlo method with shadow wave function which allows for relaxation and delocalization of vacancies. Extension of the theory at finite temperature leads to several possibilities. Depending on presence or not of ground state vacancies and on parameters like energy of formation and effective mass of a vacancy one can have a normal transition with BEC at low temperature or an inverted transition with BEC at high T. 4 He on graphite appears to be a favorable candidate for a state with BEC and spatial order.

Variational study of vacancies in solid4He

We present results of accurate Variational Monte Carlo simulations of a f.c.c.4He crystal with a vacancy atT=0. Calculations are based on the Shadow Wave Function sheme, in which the vacancy can move through the crystal and noa priori equilibrium positions for the atoms are assumed. this is the first microscopic calculation giving a realistic value for the energy of a vacancy, in the range 15–38K depending on the density of the solid. Results on the mobility of the vacancy are presented.

Vacancies in solid4He: Conflicting experimental evidence

We compare the properties of vacancies in4He implied by X-ray, NMR, and acoustic measurements with specific heat, pressure, and melting curve data. Taken together, these different measurements indicate that the model of vacancies as occupying a narrow band of states cannot be correct. Better agreement can be obtained by assuming that vacancies occupy a wide band but inconsistencies remain. An unambiguous determination of the properties of vacancies requires additional experimental and theoretical work.

The role of vacancies in solid 4He

The role of vacancies in solid 4He