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Several recent experiments find evidence of superfluidity of 3 He in 98%-porous aerogel. The primary effect of the aerogel is that it scatters the quasiparticles of 3 He. We find that many experimental findings are quantitatively understood by a relatively simple model that takes into account strong inhomogeneity of the scattering on a length scale of 100 nm. PACS: 74.25.Fy, 74.70.Tx, 72.15.Eb The discovery of unconventional paring states in hightemperature superconductors has generated a lot of interest in impurity scattering in these materials. In particular, the inhomogeneity of the scattering has been considered recently [1]. However, both the experimental and theoretical studies are difficult because of the complicated structure of these substances. Recently, a new possibility was opened for studying impurity effects on unconventional pairing states: superfluid 3 He in very porous aerogel. This system has many advantages. For example, the pure state of superfluid 3 He is absolutely pure in experiments, and it is theoretically very well understood. A crucial parameter, the coherence length ξ 0 , can easily be varied within a factor 5 by varying the pressure. The torsional oscillator experiments [2,3] and NMR experiments [4] measure directly such basic quantities as the superfluid density, the pairing amplitude and the spin susceptibility.In this letter we give theoretical explanations for some of the experimental observations on superfluid 3 He in aerogel. As a first attempt we study a model, where the aerogel is assumed as a homogeneous scatterer of the quasiparticles of 3 He. This model gives predictions with a correct tendency, but it is insufficient quantitatively. A "slab model" gives a clue that the inhomogeneity of the scattering is crucial for understanding the discrepancy. Based on that we construct a relatively simple model of inhomogeneous scattering that quantitatively can explain both the transition temperature and the pairing amplitude, and predicts an inhomogeneity length scale of 100 nm. We also consider the upper limit for anisotropic scattering set by the NMR measurements.In the experiments the aerogel fills only 2% of the total volume (V = 0.02), and its surface to volume ratio is A = 260, 000 cm −1 [5]. Assuming naively that the material consists of a network of one-dimensional strands, we can from these numbers alone estimate the strand diameter 4V /A = 3 nm. The distance between strands is √ 4πV /A = 20 nm. The mean free path for straight line trajectories is estimated as ℓ = 4/A = 150 nm. This is also the mean free path for quasiparticles of 3 He when the aerogel is filled with 3 He at millikelvin temperatures. Quasiclassical theory.-Because the volume fraction of the aerogel strands (including an inert layer of 3 He atoms on the strands) is small, we neglect all effects that are linear in the volume fraction. In particular, we assume that the density, the Landau Fermi-liquid parameters, the coupling constant of the pairing interaction, and the dipole-dipole interaction constant are...

The Josephson coupling of two isotropic s-wave superconductors through a small, magnetically active junction is studied. This is done as a function of junction transparency and of the degree of spin-mixing occurring in the barrier. In the tunneling limit, the critical current shows an anomalous 1/T temperature dependence at low temperatures and for certain magnetic realizations of the junction. The behavior of the Josephson current is governed by Andreev bound states appearing within the superconducting gap and the position of these states in energy is tunable with the magnetic properties of the barrier. This study is done using the equilibrium part of the quasiclassical Zaitsev-Millis-Rainer-Sauls boundary condition for spin-active interfaces and a general solution of the boundary condition is found. This solution is a generalization of the one recently presented by Eschrig [M. Eschrig, Phys. Rev B 61, 9061 (2000)] for spin-conserving interfaces and allows an effective treatment of the problem of a superconductor in proximity to a magnetically active material.Comment: 8 pages + 3 eps figure

The superconducting gap function of Sr2RuO4 was investigated by means of quasiparticle reflection and transmission at the normal conductor-superconductor interface of Sr2RuO4-Pt point contacts. We found two distinctly different types of dV /dI vs V spectra either with a double-minimum structure or with a zero-bias conductance anomaly. Both types of spectra are expected in the limit of high and low transparency, respectively, of the interface barrier between a normal metal and a spin-triplet superconductor. Together with the temperature dependence of the spectra this result strongly supports a spin-triplet superconducting order parameter for Sr2RuO4. 74.70.Dd, 71.20.Lp, 73.40.Jn, 74.80.Fp