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We describe systematic heat-transport measurements of a 7.5% ${}^{3}{\mathrm{H}\mathrm{e}\ensuremath{-}}^{4}\mathrm{He}$ solid mixture coexisting with the liquid at temperatures near 1 K. We observed that under a temperature gradient, macroscopic liquid bubbles appear and move through the solid. Visually, this phenomenon resembles boiling. However, in our case the bubbles are formed and subsequently move from the cold side towards the hot side of the experimental cell. The bubbles move with a speed that depends linearly on the temperature gradient. We present a model explaining the motion of the bubbles as resulting from the dynamics of superfluid mixtures. A possible mechanism of bubble formation is discussed.

We microscopically evaluate the excitation spectrum of the 3 He impurity in liquid 4 He at T50 and compare it with the experimental curve at equilibrium density. The adopted correlated basis perturbative scheme includes up to two independent phonons, intermediate correlated states, and the correlation operator is built up with two- and three-body correlation functions. The experimental spectrum is well described by the theory along all the available momentum range. A marked deviation from the simple Landau-Pomeranchuck quadratic behavior is found and the momentum-dependent effective mass of the impurity increases by ;50% at q ;1.7 A 21 with respect to its q50 value. No signature of rotonlike structures is found. @S0163-1829~98!05233-3#

We have measured the contact angle, \ensuremath{\Theta}, of $^{3}\mathrm{\ensuremath{-}}^{4}$He mixtures on Cs and its temperature dependence together with the lower wetting transition temperatures of the reentrant behavior. We have obtained the difference in interface energies, ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{Cs},\mathit{v}}$-${\mathrm{\ensuremath{\sigma}}}_{\mathrm{Cs},\mathit{l}}$, as a function of temperature. The results can be explained on the basis of a two-dimensional layer of $^{3}\mathrm{He}$ at the Cs-He interface as well as on the free surface. The bound state energy of the $^{3}\mathrm{He}$ at the interface is -(2.2\ifmmode\pm\else\textpm\fi{}0.1) K relative to $^{3}\mathrm{He}$ in liquid $^{4}\mathrm{He}$ and the effective mass is (1.55\ifmmode\pm\else\textpm\fi{}0.1) ${\mathit{m}}_{3}$. Both the energy and effective mass are near the calculated values. \textcopyright{} 1996 The American Physical Society.

The dielectric constant and losses of good quality ceramic samples of Nb-doped (1 wt % ${\mathrm{Nb}}_{2}$${\mathrm{O}}_{5}$) ${\mathrm{PbZr}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ti}}_{\mathit{x}}$${\mathrm{O}}_{3}$ about the ferroelectric-paraelectric transition for 0.03\ensuremath{\le}x\ensuremath{\le}0.09 have been measured with the aim of investigating the behavior of the thermal hysteresis with composition and the possible existence of tricritical points in this mixed system. The trend of the data, analyzed in terms of a generalized effective-field theory (${\mathit{E}}_{\mathrm{eff}}$=E+\ensuremath{\beta}P+\ensuremath{\gamma}${\mathit{P}}^{3}$+...) indicates that two tricritical points, at x\ensuremath{\simeq}0.26 and x\ensuremath{\simeq}0.51, are implied by the observed behavior at lower x.

An attempt is made to describe the dynamical structure factors of $^{3}\mathrm{\ensuremath{-}}^{4}$He mixtures recently measured by F\aa{}k et al. [Phys. Rev. B 41, 8732 (1990)] in the polarization potential approach, which is formulated to include the structure factor ${\mathit{S}}_{34}$. Good qualitative agreement of calculation and data is achieved. It turns out that virtual phonon-roton excitations provide significant strength to the particle-hole peak of the structure function and that virtual quasiparticle-quasihole excitations contribute significantly to the phonon-roton peak.

We theoretically describe the absorption spectrum of atomic impurities in isotopic mixtures of liquid helium within a zero-temperature density functional approach. Two situations are considered. In the first one, the absorption spectrum of Na atoms attached to $^{4}\mathrm{He}$${}_{1000}$-$^{3}\mathrm{He}$${}_{{N}_{3}}$ droplets with ${N}_{3}$ values from 100 to 3000 is presented as a case study of an impurity that does not dissolve into helium droplets. In the second one, the absorption spectrum of Mg atoms in liquid $^{3}\mathrm{He}$-$^{4}\mathrm{He}$ mixtures is presented as a case study of an impurity dissolved into liquid helium. We have found that the absorption spectrum of the impurity is rather insensitive to the isotopic composition because the line shift is mostly affected by the total He density around the impurity, not by its actual composition. For bulk liquid mixtures, results are presented as a function of pressure at selected values of the $^{3}\mathrm{He}$ concentration. The results for isotopically pure $^{3}\mathrm{He}$ and $^{4}\mathrm{He}$ liquids doped with Mg are compared with available experimental data.

We performed $^{31}\mathrm{P}$ NMR measurements of the temperature-dependent spin-lattice relaxation time in several mixed single crystals of ${\mathrm{Rb}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$(${\mathrm{ND}}_{4}$${)}_{\mathit{x}}$${\mathrm{D}}_{2}$${\mathrm{PO}}_{4}$ [DRADP] having different ammonium concentration (x=0.22, 0.44, 0.78) as well as in three pure single crystals (${\mathrm{ND}}_{4}$)${\mathrm{D}}_{2}$${\mathrm{PO}}_{4}$ [DADP], (${\mathrm{NH}}_{4}$)${\mathrm{H}}_{2}$${\mathrm{PO}}_{4}$ [ADP], and ${\mathrm{RbH}}_{2}$${\mathrm{PO}}_{4}$ [RDP]. The $^{31}\mathrm{P}$ NMR spin-lattice relaxation-time measurements in mixed crystals show that the phosphorus nuclei are strongly influenced by the gradual slowing down of the motions of protons in the nearby O-D...O acid bonds in the structural spin-glass states. In addition to a common ${\mathit{T}}_{1}$ minimum observed in all ammonium-containing systems, a second ${\mathit{T}}_{1}$ minimum is observed in the temperature region of the glass transition in mixed spin-glass systems but not in pure ADP, DADP, or RDP. We attribute this lower temperature ${\mathit{T}}_{1}$ minimum to the extreme slowing down of the O-D...O intrabond motion, which is unique to the glass system. In addition, the correlation times and activation energies for the ${\mathit{T}}_{1}$ minima in all samples were determined. A comparison between the pure and mixed systems reveals that the mixed system ${\mathit{T}}_{1}$ minimum must be fit to a distribution of correlation functions but that each pure system requires only a single correlation function. Furthermore, measurements on systems having x=0.78 and 0.22 show that $^{31}\mathrm{P}$ NMR can be used to determine the threshold concentrations that characterize the glass phase.

The solubility of $^{3}\mathrm{He}$ in $^{4}\mathrm{He}$ has been measured under pressure up to 20 bars in the temperature range between 1 and 250 mK. At temperatures below 60 mK, the solubility varies as ${\mathit{T}}^{2}$ at all pressures. The coefficient of the ${\mathit{T}}^{2}$ term at 0 bar, \ensuremath{\beta}, is 8.29\ifmmode\pm\else\textpm\fi{}0.03 ${\mathrm{K}}^{\mathrm{\ensuremath{-}}2}$, lower than the values of 9--11 ${\mathrm{K}}^{\mathrm{\ensuremath{-}}2}$ reported previously. The value of \ensuremath{\beta} rapidly decreases down to 3.92\ifmmode\pm\else\textpm\fi{}0.03 ${\mathrm{K}}^{\mathrm{\ensuremath{-}}2}$ at 15 bars with increasing pressure. The interaction potential between $^{3}\mathrm{He}$ quasiparticles is obtained by analyzing the solubility data on the basis of Ebner's formula. The effective mass derived from the potential has a weak and positive dependence on the $^{3}\mathrm{He}$ concentration at all pressures.

The Hugoniot of shock-compressed liquid helium was calculated using the activity expansion dense plasma program. The predicted maximum compression is at $100\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and $58\phantom{\rule{0.2em}{0ex}}000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, at a density nearly six-fold that of the initial state. Comparisons are made with recent path-integral Monte Carlo simulations, which predict a smaller maximum compression of 5.24-fold, near $360\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and $150\phantom{\rule{0.2em}{0ex}}000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

We discuss the structural properties and optical response of a small Na cluster inside rare gas (RG) matrices of Ne, Ar, or Kr atoms. The mixed systems are described with a hierarchical model, treating the cluster at a quantum mechanical level and the matrix atoms classically in terms of their positions and polarizations. We pay special attention to the differences caused by the different matrix types. These differences can be explained by the interplay of core repulsion and dipole attraction in the interaction between the cluster electrons and the RG atoms., 9 pages, 3 figures, 1 table accepted in Phys. Rev. B

We have developed a method for measuring the transmission coefficient of a sound propagating through the interface in phase-separated $^{3}\mathrm{He}\ensuremath{-}^{4}\mathrm{He}$ liquid mixtures. The method and the results are described with discussions by examining the phase-conversion process of $^{3}\mathrm{He}$ quasiparticles driven to flow across the interface. From the data, we have determined the kinetic growth coefficient of the interface, $\ensuremath{\xi}(T,P,\ensuremath{\omega})$, as a function of temperature, pressure, and frequency. The temperature range of the present investigation is about $2--100\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$ at the pressure mainly around $1\phantom{\rule{0.3em}{0ex}}\text{bar}$ with sound frequency 9.64, 14.4, and $32.4\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$. The main specific features observed for the kinetic growth coefficient are, as follows: (i) there is a maximum at some temperature ${T}_{m}(\ensuremath{\omega})$ depending on the frequency, (ii) above ${T}_{m}(\ensuremath{\omega})$, $\ensuremath{\xi}$ decreases with the increase of temperature as $\ensuremath{\propto}{\ensuremath{\omega}}^{5∕2}{T\phantom{\rule{0.2em}{0ex}}}^{\ensuremath{-}3}$, and (iii) below ${T}_{m}(\ensuremath{\omega})$, $\ensuremath{\xi}$ becomes frequency independent and diminishes as a cube of temperature, ${T}^{3}$.

We investigate the spreading of ${}^{4}\mathrm{He}$ droplets on alkali-metal surfaces at zero temperature, within the frame of finite range density-functional theory. The equilibrium configurations of several ${}^{4}{\mathrm{He}}_{N}$ clusters and their asymptotic trend with increasing particle number N, which can be traced to the wetting behavior of the quantum fluid, are examined for nanoscopic droplets. We discuss the size effects inferring that the asymptotic properties of large droplets correspond to those of the prewetting film.

We calculate the phase diagram of ${}^{3}\mathrm{He}{\ensuremath{-}}^{4}\mathrm{He}$ mixtures in two dimensions as a function of density and ${}^{3}\mathrm{He}$ concentration. A number of features distinguish this system significantly from its three-dimensional counterpart. Specifically, we find that the ${}^{3}\mathrm{He}$ phase consists, at low ${}^{3}\mathrm{He}$ concentrations and positive pressures, of loosely bound dimers. The dimerized liquid phase separates at a ${}^{3}\mathrm{He}$ concentration of 2--3 % because the chemical potential of the ${}^{3}\mathrm{He}$ component in the mixture exceeds the chemical potential in the pure phase. Atomic ${}^{3}\mathrm{He}{\ensuremath{-}}^{4}\mathrm{He}$ mixtures can be found only in a metastable state in a concentration regime above 3%, where the mixture is stable against infinitesimal concentration fluctuations. The softening of concentration-fluctuation modes with decreasing ${}^{3}\mathrm{He}$ concentration is accompanied by a divergence of the ${}^{3}\mathrm{He}$ hydrodynamic effective mass, and the magnetic susceptibility vanishes. We verify, wherever possible, that our results are consistent with simulation data and exact estimates.

The high-momentum dynamic structure function of liquid 3He-4He mixtures has been studied introducing final state effects. Corrections to the impulse approximation have been included using a generalized Gersch-Rodriguez theory that properly takes into account the Fermi statistics of 3He atoms. The microscopic inputs, as the momentum distributions and the two-body density matrices, correspond to a variational (fermi)-hypernetted chain calculation. The agreement with experimental data obtained at $q=23.1$ \AA$^{-1}$ is not completely satisfactory, the comparison being difficult due to inconsistencies present in the scattering measurements. The significant differences between the experimental determinations of the 4He condensate fraction and the 3He kinetic energy, and the theoretical results, still remain unsolved.

Brillouin scattering was used to study the dynamic properties of the mixed crystals (KBr${)}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$(KCN${)}_{\mathrm{x}}$. By analyzing the frequencies and linewidths of the ${T}_{2g}$-symmetry phonon, we were able to determine the cyanide quadrupolar relaxation rate as a function of temperature in the paraelastic regime. We found that this relaxation frequency is proportional to exp(-1/T), rather than proportional to the square root of temperature (\ensuremath{\surd}T ), as assumed in Michel's random-field theory. Modifying this theory by incorporating the observed temperature dependence of the relaxation rate improves the agreement with experimental data but is still not adequate. The hierarchically constrained relaxation model of Palmer et al., on the other hand, does provide a phenomenological explanation for our data. In addition, we have determined the value of the random-field strength in these mixed systems.

The variational density-matrix theory for Bose fluids is suitably generalized for treating binary boson mixtures at nonzero temperatures. We demonstrate that binary isotopic mixtures of arbitrary composition which are unstable at zero temperature may exist at sufficiently elevated temperatures. We explore the phase diagrams of some specialized two-component systems and of fluid $^{4}\mathrm{\ensuremath{-}}^{6}$He mixtures on a semiquantitative level. For $^{4}\mathrm{\ensuremath{-}}^{6}$He we study the phase-separation region and determine the line of instabilities at two differing total densities where the concentration mode becomes soft. We also present numerical results on the first sound mode and on the optimized radial distribution functions of the liquid $^{4}\mathrm{\ensuremath{-}}^{6}$He mixtures.

We present third-sound velocity measurements in layered mixture films. The analysis of our data, with use of independent superfluid mass measurements by torsional-oscillator techniques, constitutes a direct check of the validity of the hydrodynamic relations in layered films, and provides some information on the interfacial region between the superfluid and normal phases.

A simple model has been developed to study the phonons in mixed crystals showing either one-mode or two-mode behavior. A nonrandomness parameter $\ensuremath{\beta}$ is introduced to represent the phenomenon of two-mode behavior in mixed systems of the type ${A}_{1\ensuremath{-}x}{B}_{x}C$. When $\ensuremath{\beta}$ is reduced to zero, the model gives one-mode behavior. Though the method developed is general in its application to any structure and any concentration $x$, the calculations have been done for ${\mathrm{K}}_{0.5}$${\mathrm{Rb}}_{0.5}$I, a mixed alkali-halide system, the one which shows two-mode behavior in its class for all concentrations $x$ in the range $0lxl1$. The theoretical results are in good agreement with the experimental ones.

Measurements of the shear viscosity \ensuremath{\eta} and thermal conductivity \ensuremath{\kappa} in four $^{3}\mathrm{\ensuremath{-}}^{4}$He mixtures having $^{3}\mathrm{He}$ concentrations X\ensuremath{\le}0.09 indicate different \ensuremath{\lambda}-transition temperatures for \ensuremath{\eta} and \ensuremath{\kappa} at the same X. Absent in additional data on pure $^{4}\mathrm{He}$, this difference in transition temperatures increases with $^{3}\mathrm{He}$ concentration. We tentatively attribute the effect to an unexpectedly large enhancement of the $^{4}\mathrm{He}$ concentration near the walls of either the viscometer or the thermal conductivity cell.