Browsing by Author "Bondarev, Alexey"

When a normal metal is placed in a good electrical contact to a superconductor, Cooper pairs leak across the interface inducing pair correlations into the normal metal. This corresponds to Andreev scattering of electrons into holes at the interface. With long-range phase coherence, this enables charge interconversion effects. A conventional Andreev scattering is a retroreflection where the hole retraces the electron's trajectory in time-reverse. In graphene, however, a specular Andreev reflection (SAR) is possible where the reflected hole, instead, maintains the forward progression of the electron along the interface (similar to light reflection in a mirror). Thus, confining SAR (e.g. by a second boundary) leads to multiple Andreev reflections in a propagating coherent electron-hole superposition, an Andreev mode. In this dissertation, we consider another variation of this: by breaking time-reversal by quantum Hall effect, the trajectory is also forward progressing due to the cyclotron orbits "skipping" along the interface, the chiral Andreev interface mode. We call the Andreev process in such one-way propagating boundary channels, a chiral Andreev reflection (chAR). In this dissertation, chAR is studied in lattice and continuum models of disorder-free graphene on the lowest Hall plateau coupled to a generic superconducting contact.

At first, we show possibility of transport interference of two conjugate Andreev modes upon varying diverse control parameters, and study particularities of distinct graphene-superconductor boundaries in the transport pattern and surface spectrum. This is done in tight-binding as well as more generically via a parametric family of boundary conditions. We then study more concrete models for the interface properties and parameter dependences of the proximity effect in the chAR. Lastly, we study scattering properties of the same in graphene nanostructures with superconductor coupling of varied interface transparency and geometry, which are to be of experimental relevance.

Graphene Fermi velocity makes a strong proximity effect possible despite large sharp potential step from superconductor Fermi wave mismatch unlike in systems of single lattice type. Thus, the strong effect is compatible with metallicity of the superconductor and is only weakly sensitive to graphene parameters. Single-channel chAR is found to be stronger and more robust to reduced transparency interfaces compared to the conventional multimode Andreev retroreflection at the same graphene doping. Intervalley scattering in nanostructures is important and allows for significant electron-hole conversion in weak transparency junctions even despite suppressed interface pairing. At this weak transparency, in nanostructures with two intervalley corners, an interference of chAR with up to full conversion amplitude is shown. The interferometer in the proposed nanostructure should be fairly available in a range of atomistic corners without much finetuning.