arXiv subjects

Luka Pavešič

Publications and source records attributed to Luka Pavešič.

4 recordsLinked to original sources

arXiv2024

Strong-coupling theory of quantum dot Josephson junctions: role of the residual quasiparticle

We consider an interacting quantum dot strongly coupled to two superconducting leads in a Josephson junction geometry. By defining symmetry-adapted superpositions of states from the leads, we formulate an effective Hamiltonian for the strong-hybridisation regime with a single orbital directly coupled to the dot and three additional indirectly coupled orbitals. This minimal basis set allows to account for the quasiparticles in the vicinity of the dot as well as those further away in the leads, and to describe how their role evolves as a function of coupling strength and phase bias $ϕ$. This formulation also reveals the changing nature of the spin-doublet state for the experimentally relevant coupling strengths. The binding of a nearly decoupled quasiparticle in the vicinity of the QD explains the "doublet chimney" in the phase diagram for $ϕ\sim π$, in contrast to $ϕ\sim 0$ where the residual quasiparticle escapes to infinity and plays no active role.

cond-mat.mes-hall↗

arXiv2023

Richardson model with complex level structure and spin-orbit coupling for hybrid superconducting islands: stepwise suppression of pairing and magnetic pinning

An epitaxial semiconductor-superconductor nanowire is a superconducting system with a complex level structure originating from hybridization: in addition to a dense set of higher-energy states derived predominantly from the metallic superconducting shell above the bulk gap $Δ$, there is a smaller number of lower-energy proximitized states from the semiconducting core that define the induced gap $Δ^*$. Nanostructures built from such nanowires can furthermore incorporate quantum dots in order to obtain localized spins for storing and manipulating quantum information. We discuss the magnetic field dependence in three devices with different combinations of embedded quantum dots and superconducting islands. For strong fields, they show pinning of excitation energies to a uniform spacing, even if for weak fields they have non-universal properties with different behaviors for even and odd number of confined electrons. We propose a quantum impurity model for hybrid devices that incorporates all relevant physical effects. We show that the model accounts for the key observations and permits unambiguous interpretation in terms of many-particle states. In particular, we study the replicas of the Yu-Shiba-Rusinov states in the hybrid gap, their collapse and oscillation around zero bias with increasing field, and the strong smoothing effect of the spin-orbit coupling (SOC) on these oscillations. We propose that the SOC-induced mixing of many-body states is a generic mechanism and that magnetic pinning is likely to be a ubiquitous feature in hybrid nanowires.

cond-mat.mes-hall↗

arXiv2023

Entanglement between quasiparticles in superconducting islands mediated by a single spin

Condensed matter is composed of a small set of identical units, yet it shows an immense range of behaviour. Recently, an array of cold atoms was used to generate long-range quantum entanglement, a property of topological matter. Another approach to strong non-local correlations employs the macroscopic coherence of superconductors. Impurity spins in superconductors are thought to be unamenable to the formation of long-range spin entanglement because each spin tends to be screened by binding to a quasiparticle from the superconductor to form a local singlet. Here we demonstrate that it is possible to attach a second quasiparticle to the spin, overscreening it into a doublet state carrying ferromagnetic correlations between two quasiparticles over a micrometer distance. To demonstrate this effect, which is strongest for equal binding, we symmetrically couple the spin of a quantum dot to two ultrasmall superconducting islands. The overscreened state requires sufficiently large Coulomb repulsion and exchange binding to become well defined. We predict that this state will carry long-range correlations for an alternating chain of quantum dots and superconducting islands, opening a new route to controllable large-scale entanglement in the solid state.

cond-mat.mes-hall↗

arXiv2022

Excitations in a superconducting Coulombic energy gap

Cooper pairing and Coulomb repulsion are antagonists, producing distinct energy gaps in superconductors and Mott insulators. When a superconductor exchanges unpaired electrons with a quantum dot, its gap is populated by a pair of electron-hole symmetric Yu-Shiba-Rusinov excitations between doublet and singlet many-body states. The fate of these excitations in the presence of a strong Coulomb repulsion in the superconductor is unknown, but of importance in applications such as topological superconducting qubits and multi-channel impurity models. Here we couple a quantum dot to a superconducting island with a tunable Coulomb repulsion. We show that a strong Coulomb repulsion changes the singlet many-body state into a two-body state. It also breaks the electron-hole energy symmetry of the excitations, which thereby lose their Yu-Shiba-Rusinov character.

cond-mat.mes-hall↗