Superconducting electron and hole lenses

TL;DR

This paper demonstrates how a superconducting region can act as a lens for electron and hole waves via crossed Andreev reflection, enabling controllable focusing and entanglement of electrons in solid-state systems.

Contribution

It introduces a novel electron-hole lensing mechanism using superconducting regions and barriers, distinct from graphene systems, with controllable focusing properties.

Findings

Electron and hole focusing occurs inside the normal lead due to CAR.

Focusing behavior is robust against thermal excitations.

Barrier strength controls electron or hole density at focus points.

Abstract

We show how a superconducting region (S) sandwiched between two normal leads (N), in the presence of barriers, can act as a lens for propagating electron and hole waves by virtue of the so- called crossed Andreev reflection (CAR). The CAR process which is equivalent to the Cooper pair splitting into the two N electrodes provides a unique possibility of constructing entangled electrons in solid state systems. When electrons are locally injected from an N lead, due to the CAR and normal reflection of quasiparticles by the insulating barriers at the interfaces, sequences of electron and hole focuses are established inside another N electrode. This behavior originates from the change of momentum during electron-hole conversion beside the successive normal reflections of electrons and holes due to the barriers. The focusing phenomena studied here is fundamentally different from the electron focusing in other systems like graphene pn junctions. In particular due to the electron-hole symmetry of superconducting state, the focusing of electrons and holes are robust against thermal excitations. Furthermore the effect of superconducting layer width, the injection point position, and barriers strength is investigated on the focusing behavior of the junction. Very intriguingly, it is shown that by varying the barriers strength, one can separately control the density of electrons or holes at the focuses.