We report theoretical calculations of the normal-state spin diffusion coefficient of 3 He in aerogel, including both elastic and inelastic scattering of 3 He quasiparticles, and compare these results with experimental data for 3 He in 98% porous silica aerogel. This analysis provides a determination of the elastic mean free path within the aerogel. Measurements of the magnetization of the superfluid phase provide a test of the theory of pairbreaking and magnetic response of low-energy excitations in the "dirty" B-phase of 3 He in aerogel. A consistent interpretation of the data for the spin-diffusion coefficient, magnetization and superfluid transition temperature is obtained by including correlation effects in the aerogel density.
Superfluid 3He, with unconventional spin-triplet p-wave pairing, provides a model system for topological superconductors, which have attracted significant interest through potential applications in topologically protected quantum computing. In topological insulators and quantum Hall systems, the surface/edge states, arising from bulk-surface correspondence and the momentum space topology of the band structure, are robust. Here we demonstrate that in topological superfluids and superconductors the surface Andreev bound states, which depend on the momentum space topology of the emergent order parameter, are fragile with respect to the details of surface scattering. We confine superfluid 3He within a cavity of height D comparable to the Cooper pair diameter ξ0. We precisely determine the superfluid transition temperature Tc and the suppression of the superfluid energy gap, for different scattering conditions tuned in situ, and compare to the predictions of quasiclassical theory. We discover that surface magnetic scattering leads to unexpectedly large suppression of Tc, corresponding to an increased density of low energy bound states.
The specific heat of superfluid 3 He, disordered by a silica aerogel, is found to have a sharp discontinuity marking the thermodynamic transition to superfluidity at a temperature reduced from that of bulk 3 He. The magnitude of the discontinuity is also suppressed. This disorder effect can be understood from the Ginzburg-Landau theory which takes into account elastic quasiparticle scattering suppressing both the transition temperature and the amplitude of the order parameter. We infer that the limiting temperature dependence of the specific heat is linear at low temperatures in the disordered superfluid state, consistent with predictions of gapless excitations everywhere on the Fermi surface.PACS numbers: 67.57.Bc, 67.57.Pq Two essential characteristics of superconductors are manifest in the specific heat. First, there is a jump at the transition temperature of magnitude that depends on the strength of the electron pair interactions that lead to superconductivity. Secondly, the temperature dependence at low temperature gives a fingerprint of the energy gap structure. Consequently, the measurement of the specific heat is one of the most fundamental to understanding the nature of superconductivity. In fully gapped superconductors, such as aluminum, the specific heat decreases exponentially with decreasing temperature following an Arrhenius relation[1], thereby demonstrating unambiguously the existence of a full gap in the electron quasiparticle excitation spectrum and providing a direct measurement of its size. Both of these basic thermodynamic behaviors conform to predictions of the Bardeen, Cooper, and Schrieffer theory[2] and can be expected to hold in general for any condensate of fermion pairs into a state that is fully gapped. This is precisely what is expected [3] and observed [4] for the B-phase of superfluid 3 He, a pwave superfluid with an isotropic energy gap, similar to conventional s-wave superconductors.Shortly after the success of BCS theory it was sought to understand how perturbations, such as impurities, modify superconducting behavior. It was found [5] that magnetic scattering of quasiparticles suppresses both the transition temperature and the gap magnitude leading to 'gapless superconductivity' [2]. The same should be true for all superconductors and fermion superfluids even those that do not have s-wave states in which case all forms of elastic scattering will produce these suppression effects [6]. In particular it should be true for superfluid 3 He. In this letter we present systematic measurements of the heat capacity in the B-phase of superfluid 3 He, disordered by impurities, demonstrating suppression of the transition temperature, suppression of the order parameter, and gapless superfluid behavior.Superfluid 3 He is a unique example of a Cooper pair condensation. It was the first unconventional pairing state discovered. By unconventional we mean that there are symmetries of the normal Fermi liquid that are spontaneously broken in the superfluid phases in addition to gauge symmetry, ...
We report theoretical calculations of the thermal conductivity of superfluid 3 He impregnated into high-porosity aerogel and compare these results with available experimental data.
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