arXiv subjects

Balázs Szalai

Publications and source records attributed to Balázs Szalai.

3 recordsLinked to original sources

arXiv2026

Dynamical backaction in nanoscale superfluid electromechanics

Nanofluidic acoustic resonators employing superfluid $^4$He can be used to study quantized vortices from collective behavior in two-dimensional superfluid turbulence down to few individual vortices created by rotation. In order to improve sensitivity to the level needed for probing individual quantized vortices, readout mechanisms employing optomechanical or optomechanics-inspired approaches seem to be promising in this regard. In this work, we develop an electromechanical system, which couples a 4$^\mathrm{th}$ sound acoustic resonance to a superconducting LC tank circuit in a sideband-resolved regime. Using this system, we demonstrate electromechanically induced transparency (EMIT), optical spring effect and optomechanical sideband damping and amplification. Furthermore, by rotating the cryostat, we demonstrate sensitivity to quantized vortices, which can become trapped and released in avalanche-like process inside the nanofluidic volume.

cond-mat.other↗

arXiv2026

Kosterlitz-Thouless transition in uniformly confined $^4$He

This study investigates the Kosterlitz-Thouless (KT) transition in superfluid $^4$He confined within uniform nanochannels. While the universal jump in superfluid density is a well-established phenomenon, predicting the absolute transition temperature ($T_{KT}$) based on film geometry has remained a long-standing challenge, often relying on empirical fits. Using on-chip nanofluidic Helmholtz resonators with channel heights of 10, 15, and 20 nm, we probe the transition using 4th sound resonant modes.We demonstrate that the observed shift in the transition temperature relative to the bulk lambda point ($T_λ$) is accurately accounted for by including two-dimensional thermal excitations, specifically 2D rotons. By incorporating these roton-like excitations into the static KT theory, we can predict absolute transition temperatures that align with our experimental measurements and historical data without invoking traditional coherence length scaling arguments. Furthermore, we show that the dynamical extension of the KT theory (AHNS) fully describes the dissipation peaks observed near the transition without requiring ad-hoc free vortex contributions. These results provide compelling evidence that roton excitations, rather than correlation length scaling, govern the finite-size behaviour of confined superfluid $^4$He

cond-mat.quant-gas↗

arXiv2025

Spherically symmetric counterflow turbulence in open geometry

We report preliminary results on spherical thermal counterflow generated by a small central heater in an open geometry, an open bath of superfluid He~II, as closed-cell experiments could have introduced artifacts such as overheating and boundary-induced flows. In order to eliminate them, we measure second sound attenuation in a plane-parallel resonator. Our results are at variance with the previous experiments in closed spherical cavity that showed plateau in the steady-state vortex line density and its inverse time decay, neither of which is observed presently. We find that in open geometry the vortex line density $L$ increases steadily with counterflow velocity $v_\mathrm{ns}$, displaying a crossover between $L \propto v_\mathrm{ns}^2$ typical for counterflow and $L \propto v_\mathrm{ns}^{3/2}$, characteristic for the quasi-classical scaling.

physics.flu-dyn↗