Dr Priya Sharma

NMR experiments on liquid 3He infused into uniaxially anisotropic silica aerogels show the stabilisation of two equal-spin-pairing chiral phases on cooling from the normal phase. The alignment of the chiral axis relative to the anisotropy axis for these phases is predicted to depend upon temperature. A chiral A-like phase is also stabilized when 3He is confined to a slab of thickness 𝐷∼𝜉, the superfluid coherence length. For both types of confinement, scattering of quasiparticles by the random potential —aerogel or surface— is pair breaking and generates a sub-gap density of quasiparticle states. The random field also conspires with the chiral order parameter to generate skew scattering of quasiparticles in the plane normal to the chiral axis. This scattering mechanism leads to anomalous thermal Hall transport for nonequilibrium quasiparticles driven by a thermal gradient. We report theoretical results for the anomalous thermal Hall conductivity for theoretical models for chiral phases of 3He in both anisotropic aerogel and slabs. The anomalous thermal Hall effect (ATHE) provides an important tool to identify signatures of broken time-reversal and mirror symmetries and topology in chiral superconductors/superfluids.

We present a microscopic calculation of the inverse Faraday effect in metals. We derive a static local magnetic moment induced on the application of high-frequency light, using the Eilenberger formulation of quasiclassical theory. We include the effect of disorder and formulate a theory applicable across the entire temperature range, in the absence of external applied fields. For light-induced electric fields of amplitude '100 kV/cm, the induced fields are large '0.1 T for metallic Nb. The predictions of our theory agree with recent experimental and 134439 (2018)]. An extension of this approach to superconductors would open a new route of inducing orbital magnetic field and potentially vortices in superconductors.

In the Fermi liquid metallic state, a static local magnetic moment is induced on the application of a circularly polarized electromagnetic wave, via the inverse Faraday effect (IFE). The direction of this moment is along the direction of propagation of light, and the magnitude of the moment depends on the frequency of light, the temperature and various material parameters characteristic of the metal. I propose an analogous effect in the Fermi liquid state of He-3. A static circulating current is induced when liquid He-3 is driven by a circularly polarized transverse acoustic wave. For liquid He-3 filled into aerogel, the coupled system supports a low-attenuation transverse sound mode. I estimate the magnitude of induced circulating currents for this system and find that these are within the range of experimental measurement in the low-attenuation regime. The axis of circulation is along the direction of propagation of the acoustic wave. I propose this analogue of the inverse Faraday effect as a scheme to experimentally demonstrate the propagation of transverse sound in He-3-aerogel.

Superconducting circuits can exhibit quantized energy levels and long coherence times. Harnessing the anharmonicity offered by Josephson junctions, such circuits have been successfully employed as qubits, quantum-limited amplifiers and sensors. Here, we consider superfluidity as the charge-neutral analogue of superconductivity. Both dissipationless mass flow and Josephson tunneling have been demonstrated in superfluid helium. We propose a quantum device, consisting of a superfluid weak link and a mechanical element. The superfluid motion in this device is quantized. The resulting discrete energy levels are resolvable at millikelvin temperatures essential to maintaining the superfluid state. Appropriate device engineering can yield the necessary nonlinearity to realize qubit functionality. Hence, this device can potentially operate as a charge-neutral, superfluid quantum bit with micron-sized dimensions and millisecond-scale coherence time. We show that this quantum regime is within reach for a range of device designs.

We report theoretical results for heat transport by quasiparticle excitations in superfluid He infused into silica aerogel engineered with uniaxial anisotropy. For this system, two distinct equal spin pairing (ESP) superfluid phases have been reported based on NMR spectroscopy. Theoretical analysis predicts the first ESP state to be the chiral A phase with chiral axis aligned along the strain axis, and the lower temperature phase to be a polar-distorted chiral phase with random transverse fluctuations in the orientation of the chiral axis. We report calculations of heat transport for the high temperature chiral phase, including an anomalous (zero field) thermal Hall current originating from branch conversion scattering of Bogoliubov quasiparticles by the chiral order parameter induced by potential scattering by the silica aerogel. Observation of an anomalous thermal Hall current would provide a direct signature of the underlying chirality and topology of the superfluid phase of He in “stretched” silica aerogels.

Superfluid He-3, with unconventional spin-triplet p-wave pairing, provides a model system for topological superconductors, which have attracted significant interest through potential applications in topologically protected quantum computing. In topological insulators and quantum Hall systems, the surface/edge states, arising from bulk-surface correspondence and the momentum space topology of the band structure, are robust. Here we demonstrate that in topological superfluids and superconductors the surface Andreev bound states, which depend on the momentum space topology of the emergent order parameter, are fragile with respect to the details of surface scattering. We confine superfluid He-3 within a cavity of height D comparable to the Cooper pair diameter xi(0). We precisely determine the superfluid transition temperature T-c and the suppression of the superfluid energy gap, for different scattering conditions tuned in situ, and compare to the predictions of quasiclassical theory. We discover that surface magnetic scattering leads to unexpectedly large suppression of T-c, corresponding to an increased density of low energy bound states.

Quasiclassical equations with manifest gauge invariance are discussed in the context of unconventional singlet superconducting states in the static limit. Deviations of the quasiclassical propagator from its equilibrium solutions in the presence of magnetic fields and Hall terms are analysed in terms of a "small" parameter and a formulation developed to first order in "small". A modified quasiclassical propagator is defined to this order that is a solution of a new gauge-invariant Eilenberger-like equation with a normalization condition. A Riccati parametrization with manifest gauge invariance is proposed. Riccati equations are derived to leading order in "small" that are directly applicable to superconducting systems in the presence of magnetic fields.