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.