Strong Orientational Effect of Stretched Aerogel on theHe3Order Parameter

Abstract

Deformation of aerogel strongly modifies the orientation of the order parameter of superfluid 3 He confined in aerogel. We used a radial squeezing of aerogel to keep the orbital angular momentum of the 3 He Cooper pairs in the plane perpendicular to the magnetic field. We did not find strong evidence for a "polar" phase, with a nodal line along the equator of the Fermi surface, predicted to occur at large radial squeezing. Instead we observed 3 He-A with a clear experimental evidence of the destruction of the long-range order by random anisotropy -the Larkin-Imry-Ma effect. In 3 He-B we observed and identified new modes of NMR, which are impossible to obtain in bulk 3 He-B. One of these modes is characterized by a repulsive interaction between magnons, which is suitable for the magnon Bose-Einstein condensation (BEC). The p-wave superfluid 3 He, which is characterized by an 18-dimensional order parameter, is an amazing test system for different aspects of quantum field theory [1]. Confined in a silica aerogel, superfluid 3 He becomes an ideal system for the investigation of the effect of impurities on a long-range order. In a first approximation the aerogel reduces the superfluid transition temperature [2,3]. Here we report the influence of the aerogel anisotropy on the 3 He order parameter orientation. It was demonstrated earlier that a uniaxial squeezing of a cylindrical aerogel sample leads to the orientation of the orbital momentuml along the cylinder axis [4]. Here we report the first results of experiments with superfluid 3 He whose order parameter is deformed by radially squeezing the aerogel, which is equivalent to uniaxially stretching along the axis of a cylindrical sample. The particular interest for this kind of deformation is due to the prediction of a new phase of superfluid 3 He -the "polar" phase, which has a nodal line in the quasiparticle energy gap in the plane perpendicular to the stretching direction [5]. In our experiments no strong evidence of a polar phase has been found. However, a precursor of a polar phase formation -orientation of the orbital vectorl in the plane of the aerogel squeezing in both 3 He-A and 3 He-B -has been observed.This orientational effect allows us to study the influence of the local random anisotropy of aerogel on the U (1)-field of the vectorl, which is kept in the plane of squeezing. We find that instead of a polar phase, the Imry-Ma state of superfluid 3 He-A is formed. The quenched random anisotropy of the aerogel strands destroys the long-range orientational order (LROO) according to the famous Imry-Ma scenario [6] (see [7,8] and references therein). This is the counterpart of the effect of collective pinning in superconductors predicted by Larkin [9], in which weak impurities destroy the longrange translational order of the Abrikosov vortex lattice.The Larkin-Imry-Ma (LIM) effect has been studied experimentally in 3 He-A confined in a non-deformed aerogel [4,10]. The aerogel diminishes the value of Leggett frequency, which leads to decrease of the frequency s...

Impurity Effects of Aerogel in Superfluid3He

“…In the context of identification of the superfluid state we have discussed the NMR frequency shifts and spectra expected for the A and B-phases. In some cases, such as the 99.3% aerogel samples of Baumgardner et al 58) the spectrum is very broad; in other cases 8,14,17,64) much less so. In the first NMR experiments, Sprague et al 8) took data only on warming.…”

“…(20), provided the angular momentum vector is perpendicular to the field, ' ? H. (See the discussion by Elbs et al 17) pure and aerogel superfluids. The splitting ratio, r, was found to be similar for the two cases.…”

“…It was suggested by Vicente et al 10) that the origin of this phase might be associated with anisotropic scattering. Subsequently, some theoretical work 11) and a number of experiments [12][13][14][15][16][17] have investigated 3 He in aerogels having global anisotropy, suggesting new order parameter structures and perhaps new kinds of superfluid phases. These questions and others have been the subject of intense activity.…”

Identification of Superfluid Phases ofHe3in Uniformly Isotropic 98.2% Aerogel

“…However, to date the interpretation of pulsed NMR experiments in aerogel has been complicated by distributions in the frequency shifts owing to spatially non-uniform directions of the angular momentum, called orbital textures, that can be attributed to structural inhomogeneity in aerogel samples on length scales larger than the superfluid coherence length, ξ 0 [5,[15][16][17][18].…”

Mass Coupling and Q −1 of Impurity-Limited Normal 3He in a Torsion Pendulum

“…However, a side effect is that there is a significant dissipative contribution to the empty cell by the losses in the aerogel (Q −1 ). In addition there has been a recrudescence in the interest in uniaxially compressed and stretched aerogel 8,9,10,11,12 , following the theoretical proposal for the appearance of new phases 13 , or at least a displacement of the A-B phase boundary from that in uncompressed aerogel. The present cell was designed to enable these investigations, but in this paper we concentrate on results in the normal state which are of general importance.…”