arXiv subjects

Daksh Malhotra

Publications and source records attributed to Daksh Malhotra.

5 recordsLinked to original sources

arXiv2026

Surface-condition-mediated supercooling of the A-B transition in confined superfluid helium-3

Chiral $p$-wave superconductors and superfluids are central model systems in the search for topological quantum matter, but in reduced dimensions their properties are inseparable from the boundary conditions imposed by surrounding surfaces. Superfluid $^3$He provides a uniquely clean, well-established spin-triplet $p$-wave condensate in which these boundary conditions can be engineered directly. Here, we use a fourth-sound Helmholtz resonator to study the A--B phase transition of $^3$He confined to a $1.8\,μ$m cavity after preplating the internal surfaces with $\sim4$ atomic layers of $^4$He. In contrast to our previous fourth-sound measurements, which showed no resolvable hysteresis in the transition temperature, the preplated device exhibits a clear separation between cooling and warming transition temperatures together with stochastic run-to-run variations. The hysteresis decreases toward the highest pressures studied, consistent with previous observations of the pressure dependence of $^4$He-mediated quasiparticle boundary scattering. These results demonstrate that surface preparation can qualitatively modify first-order transition kinetics in confined $^3$He, making boundary conditions an experimentally accessible control parameter for metastability and for future studies of reduced-dimension topological superfluid states.

cond-mat.supr-con↗

arXiv2026

Bistable Fourth Sound Resonance in Superfluid $^3$He-B due to Gap Suppression

Superfluidity in $^3$He exhibits many unique properties that are of interest to modern condensed matter research, including multiple superfluid phase transitions, topological defects, and exotic classes of excitations like Majorana and Weyl fermions. Many of the most interesting theoretical proposals, which remain underexplored, are realized in highly confined geometries, where surface effects play a dominant role in the thermodynamic and hydrodynamic properties. We have developed nanofluidic resonators capable of exciting a fourth-sound acoustic mode in thin channels with a highly confined dimension ($750-1800$ nm) that is only $1-2$ orders of magnitude larger than the superfluid coherence length. When a sufficiently large drive force is applied, we observe a non-linear softening of the resonance that we interpret as due to the flow suppression of the superfluid gap. We have developed a model of the device that allows the resonance amplitude to be calibrated into a superfluid velocity, which exhibits critical behavior at particular velocities. We identify one of the observed critical velocities as being the velocity at which the gap component parallel to the flow is suppressed to zero. We compare the calibrated velocity to the prediction of a Ginzburg-Landau model, and find reasonable agreement. This measurement represents an ongoing effort to link the hydrodynamic measurements of these nanofluidic devices to theoretical predictions regarding surface gap suppression and surface-bound states.

cond-mat.other↗

arXiv2026

Topologically-Protected Remnant Vortices in Confined Superfluid $^3$He

Symmetry breaking phase transitions from less to more ordered phases will typically produce topological defects in the ordered phase. Kibble-Zurek theory predicts that for any second-order phase transition, such as the early universe, the density of defects that form should be determined by the scaling law for the system coherence time and the phase transition quench time. We have performed measurements of fourth sound dissipation due to vortex mutual friction in thin channels of superfluid $^3$He where one spatial dimension is smaller than a characteristic length scale predicted by the Kibble-Zurek theory. Our measurements suggest that remnant vortices form after the normal to superfluid second-order phase transition, and that the density of vortices is correlated with the size of the channel, but crucially, is independent of quench time. We propose a modified picture of defect formation, where closely spaced walls prevent the ends of vortex lines from reconnecting into loops. This leads to a mean vortex separation set by the wall spacing, which can result in much higher defect densities than in bulk systems.

cond-mat.mes-hall↗

arXiv2025

Dimensional crossover of superfluid $^{3}$He in a magnetic field

Motivated by recent experiments on superfluid $^3$He in nanoscale-confined geometries, we theoretically investigate the associated phase diagram in a slab geometry and perpendicular magnetic field as the size of confinement is varied. Our analysis is based on minimizing the Ginzburg--Landau free energy for the $3\times 3$ matrix superfluid order parameter for three different boundary conditions. We observe a smooth crossover from the phase diagram of the 3D system to the quasi-2D limit for slab heights of several hundred nanometres and magnetic fields of several kilogauss. We illuminate that, despite the apparent complexity of the underlying equations, many precise numerical and even analytical statements can be made about the phase structure for general values of the coefficients of the free energy functional, which can in turn be used to constrain or measure these parameters. To guide future experimental studies, we compute the phase diagram in dependence of pressure, temperature, slab height, and magnetic field.

cond-mat.mes-hall↗

arXiv2023

A complex dust morphology in the high-luminosity AGN Mrk 876

Recent models for the inner structure of active galactic nuclei (AGN) advocate the presence of a radiatively accelerated, dusty outflow launched from the outer regions of the accretion disk. Here we present the first near-infrared (near-IR) variable (rms) spectrum for the high-luminosity, nearby AGN Mrk 876. We find that it tracks the accretion disk spectrum out to longer wavelengths than the mean spectrum due to a reduced dust emission. The implied outer accretion disk radius is consistent with the infrared results predicted by a contemporaneous optical accretion disk reverberation mapping campaign and much larger than the self-gravity radius. The reduced flux variability of the hot dust could be either due to the presence of a secondary, constant dust component in the mean spectrum or introduced by the destructive superposition of the dust and accretion disk variability signals or some combination of both. Assuming thermal equilibrium for optically thin dust, we derive the luminosity-based dust radius for different grain properties using our measurement of the temperature. We find that in all cases considered the values are significantly larger than the dust response time measured by IR photometric monitoring campaigns, with the least discrepancy present relative to the result for a wavelength-independent dust emissivity law, i.e. a blackbody, which is appropriate for large grain sizes. This result can be well explained by assuming a flared, disk-like structure for the hot dust.

astro-ph.GA↗