Effect of Magnetic Boundary Conditions on SuperfluidHe3in Nematic Aerogel

Abstract

We report results of experiments with superfluid 3 He confined in aerogels with parallel strands which lead to anisotropic scattering of 3 He quasiparticles. We vary boundary conditions for the scattering by covering the strands by different numbers of atomic 4 He layers and observe that the superfluid phase diagram and the nature of superfluid phases strongly depend on the coverage. We assume that the main reason of these phenomena is a magnetic channel of the scattering which becomes important at low coverages. Our results show that the magnetic channel also may be important in other Fermi systems with the triplet pairing.Introduction.-In many Fermi systems, e.g. in liquid 3 He, in some cold atomic gases, and unconventional superconductors, a triplet Cooper pairing occurs that results in superfluid (superconducting) states. An ideal object to study an effect of impurities on such states is 3 He: it has a spherical Fermi surface, its superfluid phases (A, B and A 1 ) are well understood, and its superfluid coherence length can be varied by pressure in range of 20-80 nm [1]. Although superfluid 3 He is originally pure, a well defined system of impurities can be introduced by a high porosity aerogel. In such experiments silica aerogels are typically used. The main effect of the aerogel is to scatter 3 He quasiparticles. At temperatures T ∼ 1 mK, where 3 He is superfluid, the scattering occurs only on aerogel strands and boundary conditions may be varied by a small amount of 4 He which covers the strands by a few atomic layers. In pure 3 He the strands are covered by ∼ 2 atomic layers of paramagnetic solid 3 He [2] and the scattering is diffusive but in presence of more than ≈ 2.5 layers of 4 He it is nearly specular at low pressures and becomes purely diffusive above ≈ 25 bar [3][4][5][6][7]. The 4 He coverage also removes the solid 3 He, and spin is conserved during the scattering. In contrast, in pure 3 He spin is not conserved due to a fast exchange between atoms of liquid and solid 3 He that should result in an additional spin-exchange (magnetic) scattering channel. However, experiments with silica aerogels do not show a clear evidence of the magnetic channel. In particular, the observed A-like and B-like phases correspond to A and B phases of bulk 3 He, regardless of presence or absence of 4 He [8][9][10][11][12][13][14]. Moreover, the superfluid transition temperature of 3 He in aerogel (T ca ) is independent on the coverage at high pressures [8,12] but slightly higher in presence of the coverage at lower pressures [15,16] probably due to the change of the scattering specularity. In most of theoretical models of 3 He in aerogel the magnetic channel is neglected except only a few papers where it was shown that the magnetic scattering may affect A-A 1 transition in high magnetic fields [17][18][19] and a heat transport in the normal phase [20].Presumably, the magnetic scattering in 3 He in silica aerogels is masked due to their small global anisotropy.

Superfluid 3He in Squeezed Nematic Aerogel

“…The necessary temperatures were obtained by a nuclear demagnetization cryostat and determined using cw NMR signal from bulk A phase (in gaps between the sample and cell walls) or by a quartz tuning fork calibrated by measurements of the Leggett frequency in bulk 3 He-B. To avoid a paramagnetic signal from surface solid 3 He and stabilize the polar phase [12], the samples were preplated by ∼ 2.5 atomic layers of 4 He.…”

“…(1) belong to a class of Equal Spin Pairing (ESP) states. Previous experiments with 3 He in nematic aerogel were done using samples with strands of AlOOH (Obninsk aerogel) or Al 2 O 3 (nafen) with various porosities and degree of the anisotropy [6,7,8,9,10,11,12]. It was found that on cooling from the normal phase, the superfluid transition of 3 He occurs into the PdA phase (in Obninsk aerogel) or into the polar phase (in nafen).…”

Half-quantum vortices and walls bounded by strings in the polar-distorted phases of topological superfluid 3He

“…Indeed, in confined geometry where the PdA phase is observed immediately below T c , no HQVs are found. 44 In our case the PdA phase is obtained via the second-order phase transition from the polar phase with preformed HQVs. We already know 21 that the maximum tension from the spin-soliton in the polar phase (for µ = π/2) is insufficient to overcome HQV pinning.…”

Effects of 4He Film on Quartz Tuning Forks in 3He at Ultra-low Temperatures

“…In this experiment 4 He is present only to coat the surfaces of nafen to prevent the formation of paramagnetic solid 3 He [24]. The thickness of 4 He on the nafen strands was determined to be approximately 2.5 atomic layers [24]. The surfaces, including the quartz tuning fork, could adsorb more 4 He, thus the 4 He layer was not maximal [24].…”

“…A quartz tuning fork in the bulk 3 He volume (QTF2) is used as a thermometer. In this experiment 4 He is present only to coat the surfaces of nafen to prevent the formation of paramagnetic solid 3 He [24]. The thickness of 4 He on the nafen strands was determined to be approximately 2.5 atomic layers [24].…”