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...

The 17 O(p,γ ) 18 F and 17 O(p,α) 14 N reactions have a profound influence on hydrogen-burning nucleosynthesis in a number of stellar sites, including red giants, asymptotic giant branch (AGB) stars, massive stars, and classical novae. Previously evaluated thermonuclear rates for both reactions carry large uncertainties. We investigated the proton-capture reaction on 17 O in the bombarding energy range of E lab p = 180-540 keV. We observed a previously undiscovered resonance at E lab R = 193.2 ± 0.9 keV. The resonance strength amounts to (ωγ ) pγ = (1.2 ± 0.2) × 10 −6 eV. With this value, the uncertainties of the 17 O(p,γ ) 18 F reaction rates are reduced by orders of magnitude in the peak temperature range of classical novae (T = 0.1-0.4 GK). We also report on a reevaluation of the 17 O(p,γ ) 18 F reaction rates at lower temperatures that are pertinent to red giants, AGB stars, or massive stars. The present work establishes the 17 O(p,γ ) 18 F reaction rates over a temperature range of T = 0.01-1.5 GK with statistical uncertainties of 10-50%. The new recommended reaction rates deviate from the previously accepted values by an order of magnitude around T ≈ 0.2 GK and by factors of 2-3 at T < 0.1 GK.

a b s t r a c tWe discuss two methods by which high porosity silica aerogels can be engineered to exhibit global anisotropy. First, anisotropy can be introduced with axial strain (i.e. axial compression). In addition, intrinsic anisotropy can result during growth and drying stages and, suitably controlled, it can be correlated with preferential radial shrinkage in cylindrical samples. We have performed small angle X-ray scattering (SAXS) to characterize these two types of anisotropy. We show that global anisotropy originating from either strain or shrinkage leads to optical birefringence and that optical cross-polarization studies are a useful characterization of the uniformity of the imposed global anisotropy.

Superfluid3 He confined to high porosity silica aerogel is the paradigm system for understanding impurity effects in unconventional superconductors. However, a crucial first step has been elusive; exact identification of the microscopic states of the superfluid in the presence of quenched disorder. Using a new class of highly uniform aerogel materials, we report pulsed nuclear magnetic resonance experiments that demonstrate definitively that the two observed superfluid states in aerogel are impure versions of the isotropic and axial p-wave states. The theoretically predicted destruction of long-range orbital order (Larkin-Imry Ma effect) in the impure axial state is not observed.PACS numbers: 67.30. Hm, 67.30.Er, 67.30.Hj, 74.20.Rp The discovery of effects of quenched disorder on superfluid 3 He using high porosity silica aerogel [1, 2] has created an opportunity for systematic study of the role of impurities on unconventional pairing. Although the two observed superfluid phases in aerogel have characteristics similar to those of pure 3 He (where the A-phase is the axial state and the B-phase is the isotropic state), the identification of the states is lacking. Theory indicates that the presence of elastic quasiparticle scattering reduces strong coupling [3], which is known to be responsible for the axial state in pure 3 He. This should favor the isotropic state in aerogel, consistent with susceptibility and acoustic experiments [4][5][6]. However, a metastable phase is observed at high pressure on cooling, stabilized by magnetic field [6], and its identity is more in question. In this regard we note that without strong coupling, the planar and axial states are degenerate [7]. There are predictions that local or global anisotropy in the scattering rates favor various possible anisotropic states, e.g. axial [3], polar [8], or possibly a family of robust states [9]. Furthermore, random local disorder is predicted to lead to an orbitally disordered superfluid glass [10,11] or Larkin-Imry-Ma (LIM) state [12,13]. To resolve this problem, we have grown a new type of highly homogeneous aerogel and developed methods for its characterization [14]. In this Letter we present results on 3 He in a 98.2% porosity uniformly-isotropic aerogel sample in which we precisely determine the order parameter structure and identify the microscopic states of the superfluid phases to be the impurity suppressed axial and isotropic states. Additionally, we find no evidence for the existence of the predicted LIM superfluid glass.Pulsed NMR is a powerful technique for identifying the superfluid states of 3 He, where the frequency shifts of the spectrum are directly related to the amplitude of the order parameter, ∆, and the dependence of the shift on tip angle is a fingerprint of the microscopic state [7]. 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 b...

The discovery of superfluid 3 He in high porosity silica aerogels, and subsequent experimental and theoretical work, have led to a better general understanding of quasiparticle scattering from impurities in unconventional pairing systems. It is immensely helpful for understanding impurity effects in the case of superfluid 3 He that the structure of its order parameter is well-established. An overview of impurity effects is presented with emphasis on those experiments which have a quantitative interpretation in terms of theoretical models for homogeneous and inhomogeneous scattering. The latter can account successfully for most experimental results.

Clean two-dimensional electron systems in GaAs/AlGaAs heterostructures exhibit anisotropic collective phases, the quantum Hall nematics, at high Landau level occupancy and low temperatures.An as yet unknown native symmetry-breaking potential consistently orients these phases relative to the crystalline axes of the host material. Here we report an extensive set of measurements examining the role of the structural symmetries of the heterostructure in determining the orientation of the nematics. In single quantum well samples we find that neither the local symmetry of the confinement potential nor the distance between the electron system and the sample surface dictates the orientation of the nematic. In remarkable contrast, for two-dimensional electrons confined at a single heterointerface between GaAs and AlGaAs, the nematic orientation depends on the depth of the two-dimensional electron system beneath the sample surface.

High resolution measurements of the specific heat of liquid 3He in the presence of a silver surface have been performed at temperatures near the superfluid transition in the pressure range of 1-29 bar. The surface contribution to the heat capacity is identified with Andreev bound states of quasiparticles that have a range of half a coherence length.