Interfacial pinning in the superfluid 3He A-B transition in aerogel

Continuous-wave NMR studies of 3He in the presence of 99.3% porosity silica aerogel at 34.0 bars and in a magnetic field of 28.4 mT reveal a first-order phase transition between A-like and B-like superfluid phases on both warming and cooling. NMR spectra show that the phases on warming are the same as the phases on cooling, and the interface between them is found to be strongly pinned, even close to T(c,aero). The observed behavior is consistent with spatial variation of pinning strengths within the aerogel.     

AB interface in superfluid 3He and the Casimir effect

The friction force on the moving interface between two different vacuum states of superfluid ³He is considered at low temperature. Since the dominating mechanism of the friction is the Andreev reflection of the massless “relativistic” fermions, which live on the A-phase side of the interface, the results are similar to that for a perfectly reflecting mirror moving in the quantum vacuum. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 6, 457–462 (25 March 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Spin nutation in the quasi-isotropic A-like superfluid phase of 3He

The order parameter of the quasi-isotropic A-like superfluid phase of ³He has been reduced to a simple form. The frequencies of the spatially homogeneous oscillations of the spin and the spin part of the order parameter of this phase have been obtained taking into account the anisotropy of its magnetic susceptibility. It has been shown that the anisotropy of susceptibility strongly affects the low-frequency oscillation mode, which is similar to the nutation of an asymmetric top. The possibility of observing this mode using the NMR method is discussed.     

Surface Majorana cone of the topological superfluid 3He B phase

Topological superfluids and superconductors have been theoretically proposed, and it is now necessary to experimentally confirm their existence. Superfluid ³He should be the ideal test subject for topological theories because its bulk state is established to be that of a spin-triplet p-wave superfluid. Surface Andreev bound states of superfluid ³He were investigated by transverse acoustic impedance measurements and their linear dispersion was confirmed on a highly specular wall. The superfluid ³He B phase was found to be a topological superfluid showing bulk–edge correspondence and a surface Majorana cone was confirmed on the surface. Possible manifestations of the Majorana nature of the surface states are discussed.     

Phenomenological phase diagram of superfluid 3He in a stretched aerogel

Highly anisotropic “nematically ordered” aerogel induces global uniaxial anisotropy in superfluid ³He. The anisotropy lowers symmetry of 3He in the aerogel from spherical to axial. As a result, instead of one transition temperature in a state with an orbital moment l = 1, there are two, corresponding to projections l _z = 0 and l _z = ±1. This splitting has a pronounced effect on the phase diagram of superfluid ³He and on the structures of the appearing phases. Possible phase diagrams obtained phenomenologically on the basis of Landau expansion of the thermodynamic potential in the vicinity of the transition temperature are presented here. The order parameters corresponding to each phase and their temperature dependences are found.     

Currents in superfluid 3He

In the Ginsburg-Landau region the hydrodynamic variables for superfluid ³He are defined by means of the Lie algebra of the gauge group SO(3) x SO(3) x U(1) for the order parameter. The equilibrium of two different superfluid phases of ³He is considered within the framework of this method. At the surface, dividing the two regions filled with these phases, we consider joint conditions on currents and topological structures.     

Direct measurement of the energy gap of superfluid 3He-B in the low-temperature limit

The zero-temperature limit of the energy gap, Delta(P,T-->0), of superfluid 3He-B has been measured at T/T(c) less, similar0.25, near 0.1 and 4.8 bars, and in zero magnetic field. The energy gap was determined from the 2Delta pair-breaking edge of an acoustic signal obtained by novel, pulsed Fourier-Transform ultrasonic spectroscopy. Our results are independent of the temperature scale and the theoretical model of the gap. The values for Delta(P,T-->0) are lower than predicted by the weak-coupling-plus theory, and the Delta(P approximately 0.1 bars,T-->0) values are lower than predicted by BCS theory. The data indicate that Delta(P,T-->0) of superfluid 3He-B is not well modeled at the lowest pressures.     

Transport properties of the A phase of superfluid 3He at low temperatures

The results of a calculation of transport properties of the A phase of superfluid ³He are reported. The temperature dependence of the normal fluid density, viscosity, thermal conductivity, and nuclear spin relaxation time are given.     

Nonlinear propagation of ultrasound in superfluid3He

After introducing the physics of sound propagation in normal and superfluid³He, nonlinear phenomena are discussed. These bear close resemblance to optical effects, including saturation of the absorption, amplitude dependence of the group velocity, pulse break-up, and pulse compression. Preliminary evidence indicates that above an input power threshold the sound pulses propagate in a solitonlike fashion. A naive sine-Gordon model does not explain the observations.