Showing papers in "Journal of Low Temperature Physics in 1990"

Inhomogeneous liquid 4 He: A density functional approach with a finite-range interaction

Abstract: The properties of liquid helium-4 are investigated in a mean-field approximation using a finite-range effective interaction. The long-range part (i.e.,r>2.556 A) coincides with the standard Lennard-Jones potential and the hard core is phenomenologically renormalized. The model reproduces the static polarizability of bulk liquid, as well as the actual surface energy of helium at zero temperature. The surface width is found to be 5.8 A. A comparison is made with the earlier work of Ebner and Saam.

Low-temperature properties of the heavy-fermion compound CeRu2Si2 at the metamagnetic transition

Abstract: Magnetization measurements performed below 1 K on a single-crystalline sample of the heavy fermion compound CeRu2Si2 are reported. The field variation of the linear term of the specific heat γ derived from these data (via a Maxwell relation) exhibits a large peak at the metamagnetic-like transition at B* (=7.7 atT → 0). Thermal-expansion measurements, show a drastic drop of the temperature where a Fermi liquid behavior is reached whenB →B*. Similar experiments performed on a single-crystalline Ce0.95La0.05Ru2Si2 sample show a broadening of all the anomalies atB* and a reduction of their intensities. These results strongly suggest that pure CeRu2Si2 approaches a magnetic instability atB* forT → 0.

Correlated Single-Electron Tunneling via Mesoscopic Metal Particles: Effects of the Energy Quantization

Abstract: The existing theory of correlated single-electron tunneling in the double normalmetal tunnel junction is extended to the case of an ultrasmall central electrode of the structure. It is shown that the form of theI–V curve of such a system depends on the energy relaxation rate in this electrode. For realistic values of the relaxation rate, the large-scale shape of the dcI–V curve, which is associated with Coulomb correlations, is close to that following from the earlier theory. However, theI–V curve should also exhibit small-scale singularities reflecting the structure of the energy spectrum of the central electrode.

Correlation between magnetic hysteresis and magnetic relaxation in YBaCuO single crystals

Abstract: The dependence of the magnetic momentm obtained from the hysteresis loops on the speed of the magnetic field sweep $$\dot H$$ =dH ext/dt is explained on the basis of Anderson's interpretation of the magnetic flux creep. In addition, a phenomenological model is suggested which predicts a linear dependence ofm on ln ‖ $$\dot H$$ ‖ with the slope ∂m/∂ ln ‖ $$\dot H$$ ‖, numerically equal to the relaxation rate ∂m/∂ ln(t) from the usual magnetic relaxation. Such linear relations betweenm and ln ‖ $$\dot H$$ ‖ were observed experimentally in single crystals of YBaCuO. Preliminary experiments on the complementary time dependent relaxation ofm after a simulated step change ofH ext gave mostly relaxation rates close to the predicted values. The model here presented also enables one to compare the critical state in the superconductor at a field sweep rate $$\dot H$$ with the critical state at some timet eff after a step change ofH ext. The values of $$\dot H$$ analyzed in our experiments actually correspond to the critical state at timest eff between0.04 and4 sec after an imaginary large step change ofH ext.

Elementary excitations and a collective mode in hcp 4 He

Abstract: We have measured the velocity and attenuation of sound at 10, 30, and 50 MHz in solid4He at higher purity, lower density, and lower acoustic power than have been measured previously At 10 MHz the velocity varies asT 4, as would be expected for a very high quality crystal with remarkably low dislocation density The temperature dependence of the attenuation reveals coupling to thermally activated excitations that are consistent with vacancies measured in other work However, in our work the activation energy is ∼07 K so that their concentration is sufficiently high to require Bose statistics At 30 and 50 MHz, an unexpected, additional resonant behavior was observed We show that the resonance is consistent with the existence of a collective mode of the vacancies and with a finite population of ground state vacancies

Shear instabilities of a vortex lattice in layered superconductors

Abstract: We study possible structures of a vortex lattice in highly layered superconductors (ξ c ≈s) with the magnetic field parallel to the layers. When the magnetic field is large and the vortex lattice is dense enough, the shear modulus is exponentially small, and one can expect the vortex lattice to melt in between the layers and a vortex smectic to be formed. For low magnetic fields the vortices are far apart, and a triangular array becomes unstable with respect to the formation of several new types of vortex lattices.

Niobium-titanium nitride thin films for superconducting rf accelerator cavities

Abstract: The development of sputtering-magnetron techniques makes possible the manufacturing of thin-film coated rf cavities for high-energy accelerators of various superconducting alloys or compounds. In this context high-quality (Nb1−xTix)N films have been produced by dc magnetron sputtering and carefully characterized. These films, for x≤0.5, exhibit a critical temperature Tc as high as NbN (Tc=17 K) but show a markedly lower normal state resistivity. The calculated BCS surface impedance of the films is also lower than for NbN. The overall properties of the (Nb1−xTix)N films are compared with those of other superconductors, and the suitability of this material for the production of superconducting sputter-coated copper cavities for particle physics accelerators is demonstrated. The potential interest of (Nb1−xTix)N films for superconducting electronics is also briefly outlined.

Elementary excitations of4He clusters

Abstract: The elementary excitations of4He clusters containingN atoms are investigated by solving the equations of the random phase approximation (RPA) with a phenomenological effective interaction. The calculations were done for 40≦N≦728. The effects of the RPA correlations are explicitly discussed. Important deviations from the predictions of the liquid droplet model (LDM) are found up toN⋍500 for the “breathing” compression mode and up toN⋍100 for surface excitations. Sum rules for compression and surface excitations are also derived and discussed.

The superconducting transition of 2-D Josephson-junction arrays in a small perpendicular magnetic field

Abstract: We have investigated in detail the influence of a small perpendicular field on the observation of the superconducting phase transition of Josephson-tunnel junction arrays with linear and nonlinear resistance measurements. We find that even a small residual field strongly enhances the crossing of single vortices. In the linear resistance these single-vortex processes dominate over the pairunbinding effects just above and below the Kosterlitz-Thouless temperature. Also, through single-vortex processes, the jump in the nonlinear resistance exponent can be washed out in a field, depending on the cell size and the array width.

Superconducting phase diagram of UPt3 studied by thermal expansion and specific heat

Abstract: The specific heat and thermal expansion of UPt3 were measured in the vicinity of the superconducting transition for magnetic fields up to 10 kOe, applied along the hexagonalc axis. Both thermodynamic techniques show clear evidence of two transitions in zero field, which merge at a critical field of approximately 8 kOe. The upper critical field curve determined by the resistivity for the upper transition exhibits a small but abrupt change of slope at a comparable value of the field. The phase diagram that emerges forH∥c is therefore qualitatively similar to that previously found forH⊥c, this time with a critical point (or region) aroundH*=8 kOe,T*=0.36 K. The implications of a nearly isotropic phase diagram are discussed in connection with recent theories.

High-field galvanomagnetic properties of compensated metals under electron-surface and intersheet electron-phonon scattering (tungsten)

Abstract: The field and temperature dependence of the transverse magnetoresistivity and the Hall coefficient for single-crystal tungsten with a resistivity ratio ρ293K/ρ4.2K varying from 780 to 80800 are measured at temperatures between 4.2 K and 50 K in magnetic fields up to 150 kOe. The role of the static skin effect and the temperature breakdown in the galvanomagnetic properties of the compensated metals is evaluated through experiments with tungsten. The experimental data shows that the galvanomagnetic properties of the compensated metals in the high magnetic fields are not dependent only on the Fermi surface geometry. A substantial role in their formation is played by the processes of the electron-surface and the intersheet electron-phonon scatterings.

Transport properties in the superfluid phase of dilute 3 He- 4 He mixtures near T λ

Abstract: A cell for the simultaneous measurements of temperature and3He concentration gradients, induced by a heat currentQ across a fluid helium layer is described. This cell is operated over the temperature range 1.7≤T<2.5 K. Measurement of the anomalous boundary resistance ΔRb in superfluid4He(X(3He)∼2 ppb) near Tλ for heat currents between 8 and 47 µW/cm2 are described. The results for both the weakly divergent and the heat-dependent, more strongly divergent contributions to ΔR b are presented. The observed amplitude of the linear part is found to be larger than predictions by Onuki, by Ginzburg and Sobaynin and by Frank and Dohm, and also larger than recent data by Duncan and Ahlers (DA). These discrepancies are discussed in the light of the effective boundary area in the cell. The nonlinear part of ΔR b is consistent with the data by DA. It is found that the maximum observed ΔR b at the superfluid transition is independent ofQ. Analysis of the conductivity data of mixtures leads to the conjecture that ΔR b will decrease will the addition of3He. Our measurement of both the effective thermal diffusion ratiok * and of the thermal conductivity κeff over the range 10−6

Extended transport measurements on high-purity CeB6

Abstract: Measurements of the electrical resistivity ρ, the thermal conductivityK and the thermoelectric powerQ on a high-purity, single crystal of CeB 6 at low temperature (80 mK-4 K) in a magnetic field (0–7 T) are reported. Special attention is given to the high sensitivity of the transport properties to the magnetic phase diagram. The existence of different ordered phases at moderate applied magnetic fields allows us to demonstrate that transport properties depend strongly on the ground state of heavy fermion compounds. We observe two distinct regions in the power laws describing the transport properties, above and below about 250 mK in a zero applied field. An interesting possibility of explaining the broad maximum observed inQ(T) atB=0 is to ascribe it to a magnon drag effect. Properties of CeB6 are compared to those of otherf instability compounds and to current theories.

Transverse surface impedance of pair-correlated Fermi liquids. Application to normal and superfluid 3 He

Abstract: Hydrodynamic experiments in quantum liquids at low temperatures are sensitively influenced by the interaction of the elementary excitations with the confining walls of the measuring cell. In the case of transverse oscillations of the walls such that the viscous penetration depth of the liquid is small compared to the container dimensions, the dynamical behavior of the quantum liquid is governed by the transverse surface of shear impedance. We calculate the shear impedance for an isotropic, pair-correlated Fermi liquid from a microscopic quasiparticle Landau-Boltzmann equation using variational methods. A general class of elastic quasiparticle-wall scattering laws is considered in deriving the impedance functional. We give exact results for the variational surface impedance in the low-frequency limit and discuss approximate results in context with special wall-scattering laws such as specular, backward and Andreev scattering. Finally we apply our theory to the experimentally accessible, pair-correlated Fermi liquid3He and particularly investigate the influence of pressure, temperature, frequency, and the parameters specifying the wall-scattering process on the shear impedance.

Quantum dynamics of the charge in Josephson tunnel junctions

Abstract: It is shown that (quasi) charge of a Josephson tunnel junction can be considered as a macroscopic quantum degree of freedom. In the adiabatic limit we derive the effective action of the tunnel junction in the quasicharge representation and calculate the quantum decay rate of the charge states.

The surface tension of liquid3He above 200 mK: A density functional approach

Abstract: Using a density-functional approach, we have studied the liquid-gas equilibrium of unpolarized3He for temperatures above 200 mK The free energy density, which depends on the particle density and temperature, has been adjusted to reproduce the liquid density and vapor pressure along the liquid-gas coexistence line, as well as the zero temperature surface tension After achieving a fairly good description of the phase transition, the experimental temperature dependence of the surface tension is well reproduced

Activated flux creep in layered high-temperature superconductors

Abstract: We study thermally activated flux creep in layered superconductors in the case when the magnetic field is parallel to the layers and vortices move across the layers. We assume that the sample is without imperfections, so that the pinning is due to an interaction between vortices and the layered microstructure of the superconductors. The height of the energy barrier that vortices overcome during each activation process is calculated in the case when the vortex lattice period is commensurate with the interlayer spacing. This energy barrier depends on the transport current: it grows at small currents and goes to zero linearly whenj→j c wherej c is the depinning current.

Paraconductivity in an unconventional layered superconductor

Abstract: We study the paraconductivity in an impure, unconventional layered superconductor with an underlying square lattice. The overall magnitude of the paraconductivity is found to be reduced by ordinary impurity scattering. For the case of a one-component order parameter, the paraconductivity has the same form as that of ans-wave, apart from the above mentioned reduction factor, and is independent of the coherence length in the plane. For the case of a two component order parameter, the result consists of two parts, the first being simply twice the one component result, the second nonnegative one arising only when the eigenvalues or the eigenvectors of the gradient term in the free energy cannot be chosen to be independent of the angle in the plane.

Thermal equilibration of fluids near the liquid-vapor critical point: 3 He and 3 He- 4 He mixtures

Abstract: We study the temperature-equilibration process of fluids at constant volume in a thermal conductivity cell, where an initial temperature gradient relaxes to zero. The calculation is performed in the linear approximation for a pure fluid and a binary mixture. Near the critical point of the pure fluid, the adiabatic heating process, which takes place at constant volumeV, causes equilibration to proceed four times faster whenC P /C V ≫1 than for the process at constant pressureP. For the mixtures, the relaxation rate enhancement at constantV compared with constantP is restricted to a temperature region where the coupling between temperature and mass diffusion is small. The predictions are compared with experimental results for3He and for two3He-4He mixtures along their critical isochores. Finally, we discuss the thermal relaxation in the two-phase (liquid-gas) and one-phase (gas) regimes at the critical density, as measured with a conductivity and a calorimetry cell. The contrasting behavior for3He and a3He-4He mixture in these two regimes and under these different constraints is pointed out and discussed.

Temperature and magnetic field dependence of NbN film resistivity: 3D weak localization effects

Abstract: Niobium nitride superconducting films have been deposited by reactive rf magnetron sputtering. Polyepitaxial growth on MgO-coated substrates greatly improves the superconducting properties. The overall film resistivity is found to be widely dependent on the total pressure of the gas mixture. Samples with a negative temperature coefficient of resistance exhibit 3D weak localization effects as characterized by magnetoresistance measurements. The inelastic scattering time is found to vary as T−p with p≃2, in the temperature range Tc-3Tc.

Rough model of a quantum liquid

Abstract: As applied to condensed helium and hydrogen, a theory of quantum effects is suggested, one that is insensitive to the type of statistics of particles—the theory of quantum nondegenerate liquids. Evidently such a theory assumes that the mass of the atom and its polarization are small. For a zero-approximation, a system of hard spheres is chosen, and the attractive forces and the softness of the atom are considered to be small effects. The correlations between thermodynamic and kinetic characteristics of liquid He3 and He4 are obtained. On the basis of the experimental data for He4, and an isotropic law of the corresponding states for quantum systems with a large value of the de Boer parameter, all thermodynamic functions of liquid He3 are defined and tabulated in the regions of temperatures and molar volumes where they have not yet been measured. A rough model of a quantum liquid is suggested, based on the assumption of a dominating contribution by the diffusion excitations to the free energy of a dense condensed medium. The equation of state of the quantum liquid is derived. On the basis of the experimental data for He4, the equation of state of a quantum liquid consisting of hard spheres is compared with the similar equations obtained by a numeric computer simulation. The energy of the ground state of spin-oriented condensed hydrogen isotopes is defined.

Thermelectric power in ultrathin films of A 3 II B 2 V semiconductors in the presence of a quantizing magnetic field

Abstract: An attempt is made to study theoretically the thermoelectric power in ultrathin films ofA 3 II B 2 V semiconductors in the presence of a quantizing magnetic field by formulating a new magneto-dispersion law, within the framework ofk · p formalism incorporating the anisotropies in the band parameters It is found, taking ultrathin films ofn-Cd3P2 as an example, that the same power decreases with increasing surface electron concentration and changes in an oscillatory manner with film thickness and quantizing magnetic field In addition, the well-known results for parabolic energy bands have also obtained from our expressions as special cases

Thermpower of Ce x Y1−x InCu2 and CeInCu y Ag2−y

Abstract: The temperature dependence of the thermoelectric power (TEP) of the solid solutions Ce x Y1−xInCu2 and CeInCu y Ag2−y was measured between 1.2 and 300 K. The substitution of Y for Ce shows the Kondo origin of the giant TEP peak in CeInCu2. Furthermore, the replacement of Cu by Ag allows us to follow both the spin interaction and Kondo terms, which, according to Ref. 1, characterize the TEP of heavy fermion compounds.

Fabry-Perot Optical Resonator at Low Temperatures

Abstract: Results of an experimental study of an optical Fabry-Perot resonant cavity operated between temperatures of 77 and 300 K are presented. In particular, measurements of the cavity finesse and thermal expansion are discussed in terms of new and current applications for such a device. These include energy sensors for Weber-bar type gravitational wave antennas, high-stability laser oscillators, and high-sensitivity thermal-expansion studies of condensed matter.

Shear viscosity of liquid 4 He and 3 He- 4 He mixtures, especially near the superfluid transition

Abstract: High-resolution measurements of ρη are reported for liquid4He and3He-4He mixtures at saturated vapor pressures between 1.2 and 4.2 K with particular emphasis on the superfluid transition. Here ρ is the mass density, η the shear viscosity, and in the superfluid phase both ρ and η are the contributions from the normal component of the fluid (ρ n and η n ). The experiments were performed with a torsional oscillator operating at 151 Hz. The mole fraction X of3He in the mixtures ranged from 0.03 to 0.65. New data for the total density and data for ρ n by various authors led to the calculation of η. For4He, the results for η are compared with published ones, both in the normal and superfluid phases, and also with predictions in the normal phase both over a broad range and close to Tλ. The behavior of ρη and of η in mixtures if presented. The slopedη/dT near Tλ and its change at the superfluid transition are found to decrease with increasing3He concentration. Measurements at one temperature of η versus pressure indicate a decreasing dependence of η on molar volume asX(3He) increases. Comparison of η at Tλ, the minimum of η n in the superfluid phase and the temperature of this minimum is made with previous measurements. Thermal conductivity measurements in the mixtures, carried out simultaneously with those of ρη, revealed no difference in the recorded superfluid transition, contrary to earlier work. In the appendices, we present data from new measurements of the total density for the same mixtures used in viscosity experiments. Furthermore, we discuss the data for ρ n determined for4He and for3He-4He mixtures, and which are used in the analysis of the ρη data.

Kinetic properties of surface electrons over liquid helium under strong electron-electron interaction

Abstract: The mobility μ and lifetime of ground-level electrons τ⊥ are studied experimentally in a two-dimensional electron system at the surface of liquid helium at temperatures of0.4 to1.4 K and charge concentrations of (2.8–12.0) × 108 cm−2. It is shown that for fairly low temperature and high concentration where the frequency of interelectronic collisions is much higher than that of electron-ripplon ones, the so-called complete control condition is realized in the electron system, i.e., when the average drift velocity of electrons and the effective electron temperature can be introduced. This model is found to describe well the kinetic properties of surface electrons in the range of charge concentrations up to 4×108 cm−2, where the one-particle approach is no longer applicable. For the value of parameter Γ=e2(πns)1/2/T=47 that corresponds to a strong electron-electron interaction, the lifetime τ⊥ is found to increase sharply and the short-range order typical of a liquid state appears in the two-dimensional electron layer.

Millikelvin studies of mixed-valence HgSe:Fe

Abstract: Resistivity and magnetic susceptibility measurements of HgSe:Fe have been performed down to 30 mK and in the magnetic field range 50 mOe-3 kOe. Previously observed high conductivities of HgSe:Fe in the mixed valence region of Fe donors have been found to remain unchanged down to the lowest measured temperatures, indicating a minor influence of the resonant scattering and Kondo effect. This points to a weak k-d hybridization and substantiates classification of HgSe:Fe as an inhomogeneous mixed-valence system in which intersite Coulomb interactions impose spatial correlations in positions of charged centers. A resistive and inductive indication of superconductivity has been detected in some samples. The phenomenon is assigned to the presence of Hg-related superconducting inclusions and the associated proximity effect. This interpretation explains the field and temperature dependence of the resistivity and magnetic susceptibility, as well as being consistent with a sensitivity of the effect on deviations from stoichiometry and on annealing in Se vapor.

Viscosity of the 3He-4He dilute phase in the mixing chamber of a dilution refrigerator

Abstract: The viscosity of the dilute phase of a3He-4He solution has been measured using a vibrating wire viscometer situated in the mixing chamber of a dilution refrigerator. The viscosity was extracted from the damping of the resonator for temperatures between 3.7 mK and 100 mK. In the low-temperature Fermi liquid regime ηT2=48×10−9 N sec m−2 K2. For temperatures less than 100 mK, the viscometer is a useful secondary thermometer that is not strongly dependent on the applied magnetic field.

Longitudinal relaxation-time for dilute quantum gases

Abstract: We calculate the longitudinal relaxation timeT 1 for a polarized spin-1/2 Fermi gas, in zero magnetic field, for conditions of temperatureT and densityn such that Boltzmann statistics are valid. Our results show generally thatT 1 is independent of polarization of the gas. At highT, where the thermal wavelength λ is small compared to the scattering lengtha, T 1 is proportionalT 1/2, while at lowT, such that λ is greater thana, T 1 is proportional toT −1/2.T 1 thus has a minimum at some intermediate temperature confirming the numerical results of Shizgal. Physical arguments show that the existence of the minimum does not depend on the presence of an attractive part of the potential. As an example of the expected temperature dependence we calculateT 1 numerically, via the distorted-wave Born approximation, for the case of a gas interacting via a hard core. We also computeT 1 for a spin-1/2 Bose gas, which also shows a minimum.

Critical Rayleigh numbers for cryogenic experiments

Abstract: We give critical Rayleigh numbers, Rc, and the corresponding critical wavevectors, ac, for the onset of Rayleigh-Benard convection for thermal conditions on the horizontal boundaries that model physical experiments, particularly those carried out at low temperatures with liquid helium. We assume that a fluid layer, satisfying the Boussinesq approximation, is bounded above and below by rigid plates with finite, nonzero vertical thicknesses and finite thermal conductivities. The effect of sidewalls on Rc is not likely to be important for many experiments and so is not considered here; specifically, we assume a horizontally infinite layer. At the top of the top plate and the bottom of the bottom plate, we consider boundary conditions for which a linear combination of the convective temperature field and its vertical derivative vanishes. For these boundary conditions, the growth rates of the linear stability problem are necessarily real. We find that Rc only deviates significantly from 1708 and ac only deviates significantly from 3.11, when the thermal conductivity of the fluid is comparable to or larger than that of the boundaries, or when the plates are exceptionally thin. In particular a fixed heat flux applied to highly conducting plates (a configuration frequently used in cryogenic experiments) does not cause Rc to vary much from the standard value, 1708.