New chiral phases of superfluid 3He stabilized by anisotropic silica aerogel

Abstract

He, that anisotropic disorder, engineered from highly porous silica aerogel, stabilizes a chiral superfluid state that otherwise would not exist. Furthermore, we find that the chiral axis of this state can be uniquely oriented with the application of a magnetic field perpendicular to the aerogel anisotropy axis. At sufficiently low temperature we observe a sharp transition from a uniformly oriented chiral state to a disordered structure consistent with locally ordered domains, contrary to expectations for a superfluid glass phase 6 . Superconducting states with non-zero orbital angular momentum, L = 0, are characterized by a competitive, but essential, relationship with magnetism, strong normal-state anisotropy or both [1][2][3]5 . Moreover, these states are strongly suppressed by disorder, an important consideration for applications 7 and a signature of their unconventional behaviour 3,8,9 . Although liquid 3 He in its normal phase is perfectly isotropic, it becomes a p-wave superfluid at low temperatures with non-zero orbital and spin angular momenta, L = S = 1 (ref. 10). One of its two superfluid phases in zero magnetic field is anisotropic with chiral symmetry, where the handedness results from the orbital motion of the bound 3 He pairs about an axis . This chiral superfluid, called the A phase or axial state, is stable at high pressure near the normal-to-superfluid transition, Fig. 1a-c, whereas the majority of the phase diagram is the non-chiral B phase, with isotropic physical properties. The stability of the A phase is attributed to strong-coupling effects arising from collisions between 3 He quasiparticles 10 . However, in the presence of isotropic disorder these strong-coupling effects are reduced and the stable chiral phase disappears 11,12 , Fig. 1a. Here we show that anisotropic disorder can reverse this process and stabilize an anisotropic phase over the entire phase diagram, Fig. 1c.For many years it was thought to be impossible to introduce disorder into liquid 3 He because it is intrinsically chemically and isotopically pure at low temperatures. Then it was discovered 13,14 that 3 He imbibed in ∼ 98% porosity silica aerogel, Fig. 1d, is a superfluid with a transition temperature that is sharply defined 12 , but reduced from that of pure 3 He. To test predictions that isotropic disorder favours isotropic states and anisotropic disorder favours anisotropic states 15 , we have grown a 97.5% porosity anisotropic aerogel with growth-induced radial compression 16 , effectively stretching it along its cylinder axis by 14.3%. Experiments using uncharacterized stretched aerogels have been previously reported 17,18 and are in disagreement with the work presented here. Silica aerogels, as in Fig. 1d, are formed by silica particles ≈ 3 nm in diameter, precipitated from a tetramethylorthosilicate solution, and aggregated in a diffusion-limited process. After supercritical drying we obtain a Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, USA. *e-mail: w-halperin@northw...

Enhanced self-diffusion of adsorbed methanol in silica aerogel

“…Consistent with expectations based on other characterization methods 13 the RSA appears to be anisotropic and recent reports indicate that this type of anisotropy stabilizes an anisotropic superfluid state. 4,29 Additionally, a second anisotropic superfluid state emerges at even lower temperatures where the theory indicates that this is the manifestation of a biaxial state. 30 However, the requirements of this theory are for the elastic mean-free-path anisotropy for 3 He quasiparticle scattering in the aerogel to be λ || < λ ⊥ in contrast to our findings for the elastic mean-free-path anisotropy for scattering of methanol molecules.…”

“…Our application of the methanol diffusion experiment has significance for recent discoveries in superfluid 3 He in anisotropic aerogel where the anisotropy is responsible for stabilizing new quantum states. 4 …”

Symmetry-Protected Topological Superfluids and Superconductors —From the Basics to3He—

“…The self-charge conjugation relation (51) for Majorana fermions is satisfied only when the antiunitary particle-hole (or charge conjugation) operator C is…”

“…[45][46][47][48][49] New phase diagrams have also been proposed for aerogels. 50,51) (iv) The surface density of states peculiar to gapless quasiparticle states has already been observed in specific heat measurements and high-precision spectroscopy based on transverse acoustics under well-controlled surface conditions. [52][53][54][55][56][57][58] Motivated by puzzling issues on the intrinsic angular momentum paradox, investigations on the nontrivial momentum space topology were first initiated in 3 He by Stone et al 59,60) and Volovik,61,62) independently.…”

Anisotropic Phases of SuperfluidHe3in Compressed Aerogel

“…They are just two of many p-wave states each distinguished by unique broken symmetries and these are the only stable states in pure 3 He in low magnetic field [1]. However, new phases in this manifold have been reported for superfluid confined within an anisotropic external framework; in one case chiral states within positively strained 98% porous, silica aerogel [2] and in another, equal spin pairing states within nematically oriented alumina hydrate aerogels [3]. Additionally, for a negatively strained aerogel, obtained by compression, we found that the relative thermodynamic stability between the A and B-phases can be reversed resulting in an unusual tricritical point between A, B, and normal phases with a critical magnetic field of H c ≈ 100 mT [4].…”