Observation of atomiclike electronic excitations in pure 3He and 4He clusters studied by fluorescence excitation spectroscopy

The structure of the electronically excited states of 3He and 4He clusters is investigated using fluorescence excitation spectroscopy. Distinct bands are observed energetically close to atomic 1s-ns, nd, np transitions and attributed to perturbed excited He atomiclike states with different principle and orbital quantum numbers. The line shifts and widths of the bands of 3He and 4He clusters of the same size are different and correlate with the average particle density inside the clusters calculated using the density functional method.     

Bubble formation and decay in 3He and 4He clusters

The energy transfer in 3He and 4He clusters electronically excited by monochromatic synchrotron radiation is investigated by luminescence spectroscopy. Depending on the cluster size and the isotopic constitution, either sharp, broadened, or shifted emission bands of single He molecules are observed. The spectral features show that He molecules emit light either within a bubble inside the cluster or in the vacuum after desorption from the cluster. From the luminescence intensity, the cluster diameter, and the radiative lifetime, an average velocity of approximately 7 m/s of bubbles in 4He clusters could be deduced. In the nonsuperfluid 3He clusters this velocity was observed to be significantly lower.     

Correlation effects in the heat capacity of separating 3He-4He solid solutions

We propose to describe the impurity heat capacity of solid ³He-⁴He mixtures both below and above the phase separation temperature T ^s by an extension of the Bethe-Guggenheim approximation for the lattice gas model. It is shown that at T > T _s, the temperature behavior of the heat capacity is completely defined by correlation effects in the impurity subsystem. The developed theory enables us to explain from the common stand-point the experimental data by Edwards, McWilliams, and Daunt for all concentrations of ₄He and make some conclusions about the structure of second phase nuclei.     

Microscopic theory of3He—4He solutions

The ring diagram contribution to the grand partition function of a mixture of fermions and bosons is obtained from a rigrous formula which extends the single component formula of Montroll and Ward. Together with the contributions from some other relevant graphs, it is applied to a dilute solution of³He with⁴He. For a small mass difference parameter, the ground state energy is shown to be expressed as a linear function of³He concentration. The coefficients of this energy expression are obtained explicitly for soft and hard-sphere potential models. From the energy, the differential volume coefficient is obtained also.This work was supported by ONR under Contract No. N00014-79-C-0451     

Phase-separation kinetics of 3He-4He solid solutions at various degrees of supersaturation

The phase-separation kinetics of ³He-⁴He solid solutions at various degrees of supersaturation Δx are studied using precise pressure measurements. It is found that the characteristic phase-separation times do not depend on the degree of supersaturation but are determined by the final temperature to which the crystal is cooled. The behavior of the effective diffusion coefficient, which governs the corresponding mass transfer, cannot be described within the impuriton model of the mobility of ³He atoms in a ⁴He crystal. Fiz. Tverd. Tela (St. Petersburg) 41, 2062–2064 (November 1999)     

Stability of small mixed clusters of 4He and 3He atoms

The existence of small helium clusters containing a variable number of 4He and 3He atoms is studied within a variational Monte Carlo calculation employing the Aziz HFD-B(HE) pair interaction. The clusters 4He(2) support one and two 3He atoms; however, the system with three 3He atoms is metastable, and the next bound system requires at least 18 fermions. All clusters obtained by adding 3He atoms to the trimer 4He(3) and the tetramer 4He(4) are bound, but the clusters 4He(3)-3He(3,4,5) and 4He(3,4)-3He(9) are metastable. All remaining clusters with three or more bosons and any number of fermions are stable.     

Observation of unusual mass transport in solid hcp (4)He

Solid (4)He has been created off the melting curve by growth at a nearly constant mass via the "blocked capillary" technique and growth from the (4)He superfluid at constant temperature. The experimental apparatus allows injection of (4)He atoms from superfluid directly into the solid. Evidence for the superfluidlike transport of mass through a sample cell filled with hcp solid (4)He off the melting curve is found. This mass flux depends on temperature and pressure.     

Self-cooling of weak solutions of 3He in 4He during adiabatic melting

A new method of pressure regularization in the measuring cell is proposed for experiments on self-cooling of weak solutions. Preliminary results are presented for solutions with concentrations of 2% and 7%. Cooling by a factor of 1.5–2 was observed in the starting-temperature range 40–140 mK. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 3, 263–267 (10 February 1997)     

Binding of a 3He impurity to a screw dislocation in solid 4He

Using first-principles simulations for the probability density of finding a 3He atom in the vicinity of the screw dislocation in solid 4He, we determine the binding energy to the dislocation nucleus E(B)=0.8+/-0.1 K and the density of localized states at larger distances. The specific heat due to 3He features a peak similar to the one observed in recent experiments, and our model can also account for the observed increase in shear modulus at low temperature. We further discuss the role of 3He in the picture of superfluid defects.     

Bose-Einstein condensation in solid 4He

We present neutron scattering measurements of the atomic momentum distribution n(k) in solid helium under a pressure p=41 bar (molar volume Vm=20.01+/-0.02 cm3/mol) and at temperatures between 80 and 500 mK. The aim is to determine whether there is Bose-Einstein condensation (BEC) below the critical temperature, Tc=200 mK, where a superfluid density has been observed. Assuming BEC appears as a macroscopic occupation of the k=0 state below Tc, we find a condensate fraction of n0=(-0.10+/-1.20)% at T=80 mK and n0=(0.08+/-0.78)% at T=120 mK, consistent with zero. The shape of n(k) also does not change on crossing Tc within measurement precision.