M. Efthymiou-Tsironi | Searcharxiv

arXiv2025

Voltage-tuned anomalous-metal to metal transition in hybrid Josephson junction arrays

We report voltage-tuned phase transitions in arrays of hybrid semiconductor-superconductor islands arranged in a square lattice. A double-layer electrostatic gate geometry enables independent tuning of inter-island coupling and proximity-induced superconductivity. This design enables access to the superconductor-insulator, superconductor-metal, and metal-insulator transitions in a single device, revealing critical points and emergent intermediate phases. We find that the superconductor-insulator transition is interrupted by an anomalous metallic phase with saturating low-temperature resistivity. Across gate voltages, this regime extends over three orders of magnitude in resistivity and can be continuously tuned into the conventional metallic phase. The signature of the anomalous metallic phase is suppressed by magnetic frustration.

cond-mat.mes-hall↗

arXiv2022

Polariton Bose-Einstein condensate from a Bound State in the Continuum

Optical bound states in the continuum (BIC) are peculiar topological states that, when realized in a planar photonic crystal lattice, are symmetry-protected from radiating in the far field despite lying within the light cone, i.e., in the energy-momentum dispersion region for which radiation can propagate out of the lattice plane. These BICs possess an invariant topological charge given by the winding number of the polarization vectors, similarly to vortices in quantum fluids, such as superfluid helium and atomic Bose-Einstein condensates. In spite of several reports of optical BICs in patterned dielectric slabs with evidence of lasing, their potential as topologically protected states with theoretically infinite lifetime has not been fully exploited, yet. Here we show Bose-Einstein condensation of polaritons, hybrid light-matter excitations, occuring in a BIC thanks to its peculiar non-radiative nature. The combination of the ultra-long BIC lifetime and the tight confinement of the waveguide geometry allow to achieve an extremely low threshold density for condensation, which is not reached in the dispersion minimum but at a saddle point in reciprocal space. By bridging bosonic condensation and symmetry-protected radiation eigenmodes, we unveil new ways of imparting topological properties onto macroscopic quantum states with unexplored dispersion features. Such an observation may open a route towards energy-efficient polariton condensation in cost-effective integrated devices, ultimately suited for the development of hybrid light-matter optical circuits

physics.optics↗