arXiv subjects

Klejdja Xhani

Publications and source records attributed to Klejdja Xhani.

8 recordsLinked to original sources

arXiv2026

Kelvin-Helmholtz instability in annular superfluids across the UFG-BCS crossover

We investigate the Kelvin-Helmholtz instability of counter-rotating annular Fermi superfluids using time-dependent superfluid density functional theory. When the two initially separated superflows merge, a necklace of quantized vortices forms at their interface and subsequently becomes unstable. We extract mode-resolved instability growth rates and study their dependence on relative flow velocity, interaction strength, and temperature across the strongly interacting unitary regime and the weakly attractive BCS regime. In the unitary gas and moderate BCS regimes, the growth rates remain qualitatively consistent with the point-vortex model, though systematically at or above its predictions, with no significant dependence on interaction strength or temperature up to approximately $0.3 T_c$. Deeper in the BCS regime, the dynamics change qualitatively: boundary-generated antivortices proliferate faster than the Kelvin-Helmholtz instability itself, destroying the initial vortex necklace before the instability can fully develop. These results provide a microscopic benchmark for Kelvin-Helmholtz dynamics in fermionic superfluids and help constrain possible explanations of the existing experiment-theory discrepancy.

cond-mat.quant-gas↗

arXiv2026

Reproducible nucleation and control of stable quantum vortex rings in Bose-Einstein condensates

We propose and numerically validate an experimentally feasible on-demand protocol for the nucleation and manipulation of stable quantum vortex rings in trapped Bose-Einstein condensates. The method relies on sweeping a laser-sheet barrier that locally constricts the superflow and triggers vortex-ring formation. By tuning the barrier height and width, and by scanning the barrier velocity, we identify the onset of periodic generation of vortex rings above the critical velocity and achieve direct, deterministic control over the ring nucleation position, radius, and hence propagation speed. After its formation, ad-hoc optical potentials are applied to reshape the vortex ring, creating clean Kelvin-wave excitations. Our results provide a foundation for systematic studies of three-dimensional vortices in atomic superfluids and open the door to tailored vortex dynamics and interactions, enabling controlled access to quantum turbulence.

cond-mat.quant-gas↗

arXiv2025

Impurity-controlled vortex mobility and pair-breaking in fermionic superfluid rings

Using time-dependent density functional theory, we study how density and size of impurities govern dissipation of persistent currents of fermionic superfluid rings in the BCS regime. The critical winding number for vortex emission increases with impurity density, but this enhancement is impurity size-dependent and capped by the pair-breaking threshold. Below this vortex-emission threshold, the winding number remains constant while flow energy dissipates through impurity-enhanced pair-breaking. Above the threshold, vortex-impurity interactions produce distinct mobility regimes-deflected trajectories, individual pinning, collective pinning, and inter-site hopping, controlled by the impurity size and density, which determine the dominant dissipation channel. These findings provide design principles for ultracold-atom experiments and insights into vortex-pinning dynamics in neutron-star crusts and superconductors.

cond-mat.quant-gas↗

arXiv2024

Stability of persistent currents in superfluid fermionic rings

We investigate the stability of persistent currents in superfluid fermionic gases confined to a ring geometry. Our study, conducted at zero temperature using time-dependent density functional theory, cover interaction regimes from strong (unitary Fermi gas) to weak (Bardeen-Cooper-Schrieffer regime) couplings. Stability is tested against the presence of an external defect within the ring. The dissipation mechanism associated with vortex generation is present in all interaction regimes. Vortex emission is accompanied by Cooper pair breaking, which occurs even beyond the vortex core in the weakly interacting regime. The pair-breaking mechanism prevents the imprinting of a persistent current with a winding number above a threshold, which decreases as the system approach the BCS regime. Our study reveals the existence of two types of critical winding numbers above which currents cease to be persistent in Fermi superfluids: one related to the proliferation of quantum vortices and the other to the onset of the pair-breaking mechanism.

cond-mat.quant-gas↗

arXiv2023

Stabilizing persistent currents in an atomtronic Josephson junction necklace

Arrays of Josephson junctions are at the forefront of research on quantum circuitry for quantum computing, simulation and metrology. They provide a testing bed for exploring a variety of fundamental physical effects where macroscopic phase coherence, nonlinearities and dissipative mechanisms compete. Here we realize finite-circulation states in an atomtronic Josephson junction necklace, consisting of a tunable array of tunneling links in a ring-shaped superfluid. We study the stability diagram of the atomic flow by tuning both the circulation and the number of junctions. We predict theoretically and demonstrate experimentally that the atomic circuit withstands higher circulations (corresponding to higher critical currents) by increasing the number of Josephson links. The increased stability contrasts with the trend of the superfluid fraction -- quantified by Leggett's criterion -- which instead decreases with the number of junctions and the corresponding density depletion. Our results demonstrate atomic superfluids in mesoscopic structured ring potentials as excellent candidates for atomtronics applications, with prospects towards the observation of non-trivial macroscopic superpositions of current states.

cond-mat.quant-gas↗

arXiv2023

Decay of persistent currents in annular atomic superfluids

We investigate the role of vortices in the decay of persistent current states of annular atomic superfluids by solving numerically the Gross-Pitaevskii equation, and we directly compare our results with experimental data from Ref. [1]. We theoretically model the optical phase-imprinting technique employed to experimentally excite finite-circulation states in Ref. [1] in the Bose-Einstein condensation regime, accounting for imperfections of the optical gradient imprinting profile. By comparing simulations of this realistic protocol to an ideal imprinting, we show that the introduced density excitations arising from imperfect imprinting are mainly responsible for limiting the maximum reachable winding number $w_\mathrm{max}$ in the superfluid ring. We also investigate the effect of a point-like obstacle with variable potential height $V_0$ onto the decay of circulating supercurrents. For a given obstacle height, a critical circulation $w_c$ exists, such that for an initial circulation $w_0$ larger than $w_c$ the supercurrent decays through the emission of vortices, which cross the superflow and thus induce phase slippage. Higher values of the obstacle height $V_0$ further favour the entrance of vortices, thus leading to lower values of $w_c$. Furthermore, the stronger vortex-defect interaction at higher $V_0$ leads to vortices that propagate closer to the center of the ring condensate. The combination of both these effects leads to an increase of the supercurrent decay rate for increasing $w_0$, in agreement with experimental observations. [1]: G. Del Pace, et al., Phys. Rev. X 12, 041037 (2022)

cond-mat.quant-gas↗

arXiv2023

Dissipation Mechanisms in Fermionic Josephson Junction

We characterize numerically the dominant dynamical regimes in a superfluid ultracold fermionic Josephson junction. Beyond the coherent Josephson plasma regime, we discuss the onset and physical mechanism of dissipation due to the superflow exceeding a characteristic speed, and provide clear evidence distinguishing its physical mechanism across the weakly- and strongly-interacting limits, despite qualitative dynamics of global characteristics being only weakly sensitive to the operating dissipative mechanism. Specifically, dissipation in the strongly interacting regime occurs through the phase-slippage process, caused by the emission and propagation of quantum vortices, and sound waves -- similar to the Bose-Einstein condensation limit. Instead, in the weak interaction limit, the main dissipative channel arises through the pair-breaking mechanism.

cond-mat.quant-gas↗

arXiv2021

Dissipation in a Finite Temperature Atomic Josephson Junction

We numerically demonstrate and characterize the emergence of distinct dynamical regimes of a finite temperature bosonic superfluid in an elongated Josephson junction generated by a thin Gaussian barrier over the entire temperature range where a well-formed condensate can be clearly identified. Although the dissipation arising from the coupling of the superfluid to the dynamical thermal cloud increases with increasing temperature as expected, the importance of this mechanism is found to depend on two physical parameters associated (i) with the initial chemical potential difference, compared to some characteristic value, and (ii) the ratio of the thermal energy to the barrier amplitude. The former determines whether the superfluid Josephson dynamics are dominated by gradually damped plasma-like oscillations (for relatively small initial population imbalances), or whether dissipation at early times is instead dominated by vortex- and sound-induced dissipation (for larger initial imbalances). The latter defines the effect of the thermal cloud on the condensate dynamics, with a reversal of roles, i.e. the condensate being driven by the oscillating thermal cloud, being observed when the thermal particles acquire enough energy to overcome the barrier. Our findings are within current experimental reach in ultracold superfluid junctions.

cond-mat.quant-gas↗