arXiv subjects
Publications and source records attributed to J. A. Sauls.
We report theoretical calculations of the spectrum of third sound modes for a cylindrically symmetric film of superfluid $^3$He, and compare these results with experimental data for the mode frequencies and amplitude spectrum of surface waves of superfluid $^3$He films.
We present a theory for quasiparticle heat transport through superconducting weak links. The thermal conductance depends on the phase difference ($ϕ$) of the superconducting leads. Branch conversion processes, low-energy Andreev bound states near the contact and the suppression of the local density of states near the gap edge are related to phase-sensitive transport processes. Theoretical results for the influence of junction transparency, temperature and disorder, on the phase modulation of the conductance are reported. For high-transmission weak links, $D\to 1$, the formation of an Andreev bound state at $ε_{\text{\tiny b}}=Δ\cos(ϕ/2)$ leads to suppression of the density of states for the continuum excitations that transport heat, and thus, to a reduction in the conductance for $ϕ\simeqπ$. For low-transmission ($D\ll 1$) barriers resonant scattering at energies $ε\simeq(1+D/2)Δ$ leads to an increase in the thermal conductance as $T$ drops below $T_c$ (for phase differences near $ϕ=π$).
The pairing correlations in superfluid He-3 are strongly modified by quasiparticle scattering off a surface or an interface. We present theoretical results and predictions for the order parameter, the quasiparticle excitation spectrum and the free energy for thin films of superfluid He-3. Both specular and diffuse scattering by a substrate are considered, while the free surface is assumed to be a perfectly reflecting specular boundary. The results are based on self-consistent calculations of the order parameter and quasiparticle excitation spectrum at zero pressure. We obtain new results for the phase diagram, free energy, entropy and specific heat of thin films of superfluid He-3.
We present a comprehensive study of heat transport through small superconducting point contacts. The heat current for a temperature biased weak link is computed as a function of temperature and barrier transparency of the junction. The transport of thermal energy is controlled by the quasiparticle transmission probability for the point contact that couples the superconducting leads. We derive this transmission probability and results for the heat current starting from nonequilibrium transport equations and interface boundary conditions for the Keldysh propagators in quasiclassical approximation. We discuss the thermal conductance for both clean and dirty superconducting leads, as well as aspects of the nonlinear current response. We show that the transmission probability for continuum quasiparticle states is both energy- and phase-dependent, and controlled by an interface Andreev bound state below the continuum. For high transparency barriers the formation of a low-energy bound state, when the phase is tuned to $ϕ=π$, leads to a reduction of the heat current relative to that for $ϕ=0$. For low-transparency barriers, a shallow Andreev bound state just below the continuum edge is connected with resonant transmission of quasiparticles for energies just above the gap edge, and leads to enhanced heat conductance as the temperature is lowered below the superconducting transition.
When liquid 3He is impregnated into silica aerogel a solid-like layer of 3He atoms coats the silica structure. The surface 3He is in fast exchange with the liquid on NMR timescales. The exchange coupling of liquid 3He quasiparticles with the localized 3He spins modifies the scattering of 3He quasiparticles by the aerogel structure. In a magnetic field the polarization of the solid spins gives rise to a splitting of the scattering cross-section of for `up' vs. `down' spin quasiparticles, relative to the polarization of the solid 3He. We discuss this effect, as well as the effects of non-magnetic scattering, in the context of a possible splitting of the superfluid transition for $\uparrow\uparrow$ vs. $\downarrow\downarrow$ Cooper pairs for superfluid 3He in aerogel, analogous to the A_1-A_2 splitting in bulk 3He. Comparison with the existing measurements of T_c for B< 5 kG, which show no evidence of an A_1-A_2 splitting, suggests a liquid-solid exchange coupling of order J = 0.1 mK. Measurements at higher fields, B > 20 kG, should saturate the polarization of the solid 3He and reveal the A_1-A_2 splitting.
We present theoretical calculations and predictions for the shot noise in voltage biased junctions of $d_{x^2-y^2}$ superconductors and normal metal counter-electrodes. In the clean limit for the d-wave superconductor the shot noise vanishes at zero voltage because of resonant Andreev reflection by zero-energy surface bound states. We examine the sensitivity of this resonance to impurity scattering. We report theoretical results for the magnetic field dependence of the shot noise, as well the fingerprints of subdominant $s$- and $d_{xy}$ pairing channels.
The ab-plane NIS-tunnelling conductance in d-wave superconductors shows a zero-bias conductance peak which is predicted to split in a magnetic field. In a pure d-wave superconductor the splitting is linear for fields small on the scale of the thermodynamic critical field. The field dependence is shown to be nonlinear, even at low fields, in the vicinity of a surface phase transition into a local time-reversal symmetry breaking state. The field evolution of the conductance is sensitive to temperature, doping, and the symmetry of the sub-dominant pairing channel.
We report theoretical calculations of the thermal conductivity of $^3$He impregnated into high-porosity aerogel and compare these results with available experimental data.
The equilibrium superfluid phase of 3He impregnated into high-porosity silica aerogels appears to be a non-equal-spin-pairing state in zero field at all pressures, which is generally assumed to be the Balian-Werthamer (BW) phase modified by the depairing effects of the aerogel structure. The nuclear magnetic susceptibility played a key role in identifying the B-phase of pure 3He with the BW state. We report theoretical calculations of the nuclear magnetic susceptibility for the BW model of superfluid 3He in aerogel within the framework of the Fermi-liquid theory of superfluid 3He. Scattering of quasiparticles by the aerogel, in addition to Fermi-liquid exchange corrections, leads to substantial changes in the susceptibility of the BW phase. The increase in the magnetic susceptibility of 3He-aerogel compared to pure 3He-B is related to the polarizability of the gapless excitations and the impurity-induced local field. The limited data that is available is in rough agreement with theoretical predictions. Future measurements could prove important for a more definitive identification of the ordered phase, as well as for refining the theoretical model for the effects of disorder and scattering on the properties of superfluid 3He.
We investigate the equilibrium and nonequilibrium properties of the core region of vortices in layered superconductors. We discuss the electronic structure of singly and doubly quantized vortices for both s-wave and d-wave pairing symmetry. We consider the intermediate clean regime, where the vortex-core bound states are broadened into resonances with a width comparable to or larger than the quantized energy level spacing, and calculate the response of a vortex core to an {\em a.c.} electromagnetic field for vortices that are pinned to a metallic defect. We concentrate on the case where the vortex motion is nonstationary and can be treated by linear response theory. The response of the order parameter, impurity self energy, induced fields and currents are obtained by a self-consistent calculation of the distribution functions and the excitation spectrum. We then obtain the dynamical conductivity, spatially resolved in the region of the core, for external frequencies in the range, $0.1Δ< \hbarω\lsim 3Δ$. We also calculate the dynamically induced charge distribution in the vicinity of the core. This charge density is related to the nonequilibrium response of the bound states and collective mode, and dominates the electromagnetic response of the vortex core.
This paper summarizes the results of a comprehensive analysis of the thermodynamic and transport data for the superconducting phases of UPt_3. Calculations of the transverse sound attenuation as a function of temperature, frequency, polarization, and disorder are presented for the leading models of the superconducting order parameter. Measurements of the specific heat, thermal conductivity, and transverse sound attenuation place strong constraints on the orbital symmetry of the superconducting order parameter. We show that the superconducting A and B phases are in excellent agreement with pairing states belonging to the odd-parity E_{2u} orbital representation.
We discuss possible magnetic structures in UPt$_3$ based on our analysis of elastic neutron-scattering experiments in high magnetic fields at temperatures $T<T_N$. The existing experimental data can be explained by a single-{\bf q} antiferromagnetic structure with three independent domains. For modest in-plane spin-orbit interactions, the Zeeman coupling between the antiferromagnetic order parameter and the magnetic field induces a rotation of the magnetic moments, but not an adjustment of the propagation vector of the magnetic order. A triple-{\bf q} magnetic structure is also consistent with neutron experiments, but in general leads to a non-uniform magnetization in the crystal. New experiments could decide between these structures.
These lecture notes provide an introduction to non-equilibrium superfluid 3He, with applications to high-frequency excitations, including aspects of symmetry breaking in 3He and its effects on collective mode dynamics. In 1957 Landau predicted that liquid 3He would support propagating shear waves at low temperatures, i.e. a transverse sound mode. Such waves have recently been observed at low temperatures in the superfluid B-phase of liquid 3He. These observations provide a beautiful example of the effect of spontaneous symmetry breaking in condensed matter. I discuss the theory of transverse wave propagation in 3He and the recent detection of these waves by magneto-acoustic rotation of the 3He polarization in a magnetic field.
In superfluid 3He-B the off-resonant coupling of the J=2-, M=+/- 1 order parameter collective modes to transverse current excitations stabilizes propagating transverse waves with low damping for frequencies above that of the J=2- modes. Right- (RCP) and left circularly polarized (LCP) transverse modes are degenerate in zero field; however, a magnetic field with H || q lifts this degeneracy giving rise to the acoustic analog of circular birefringence and an acoustic Faraday effect for linearly polarized transverse sound waves. We present theoretical results for the temperature, pressure and field dependence of the Faraday rotation angle, and compare the theory with recent measurements. The analysis provides a direct measurement of the Lande' g-factor for the J=2- modes, and new information on the magnitude of f-wave pairing correlations in 3He-B.
We show that an isolated impurity in a spin singlet d_{x2-y2} superconductor generates a d_{xy} order parameter with locally broken time-reversal symmetry. The origin of this effect is a coupling between the d_{x2-y2} and the d_{xy} order parameter induced by spin-orbit scattering off the impurity. The signature of locally broken time-reversal symmetry is an induced orbital charge current near the impurity, which generates a localized magnetic field in the vicinity of the impurity. We present a microscopic theory for the impurity induced d_{xy} component, discuss its spatial structure as well as the pattern of induced current and local magnetic field near the localized impurity spin.
We report new theoretical results and analysis for the transport properties of superconducting UPt3 based on the leading models for the pairing symmetry. We use Fermi surface data and the measured inelastic scattering rate to show that the low-temperature thermal conductivity and transverse sound attenuation in the A and B phase of UPt3 are in excellent agreement with pairing states belonging to the two-dimensional orbital E2u representation.
We report the discovery of the acoustic Faraday effect in superfluid 3He-B. The observation of this effect provides the first direct evidence for propagating transverse acoustic waves in liquid 3He, a mode first predicted by Landau in 1957. The Faraday rotation is large and observable because of spontaneously broken spin-orbit symmetry in 3He-B. We compare the experimental observations with a simulation of the transverse acoustic impedance that includes the field-induced circular birefringence of transverse waves.
The electronic spin susceptibility of YBa_2Cu_3O_{6.95} has been measured with high precision up to 24 Tesla with 17O nuclear magnetic resonance. Its temperature dependence can be accounted for by superconducting fluctuations that result in a smooth crossover from the normal to the vortex liquid state. A magnetic field-temperature phase diagram for this crossover has been established having strong upward curvature.