Low-energy quasiparticle states at superconductor-CDW interfaces

TL;DR

This paper theoretically investigates low-energy quasiparticle bound states at superconductor-charge density wave interfaces, revealing their dependence on interface orientation and order parameter magnitude, and their impact on Josephson junction properties.

Contribution

It introduces a theoretical model for quasiparticle bound states at superconductor-CDW interfaces, highlighting effects of order parameter size and interface orientation on bound state energies.

Findings

Zero-energy bound states occur for small order parameters regardless of interface orientation.

Dispersionless zero-energy states are limited to (110) interfaces at larger order parameters.

Andreev bound states influence the low-temperature behavior of Josephson critical currents.

Abstract

Quasiparticle bound states are found theoretically on transparent interfaces of d-wave superconductors (dSC) with charge density wave solids (CDW), as well as s-wave superconductors (sSC) with d-density waves (DDW). These bound states represent a combined effect of Andreev reflection from the superconducting side and an unconventional quasiparticle Q-reflection from the density wave solid. If the order parameter for a density wave state is much less than the Fermi energy, bound states with almost zero energy take place for an arbitrary orientation of symmetric interfaces. For larger values of the order parameter, dispersionless zero-energy states are found only on (110) interfaces. Two dispersive energy branches of subgap quasiparticle states are obtained for (100) symmetric interfaces. Andreev low-energy bound states, taking place in junctions with CDW or DDW interlayers, result in anomalous junction properties, in particular, the low-temperature behavior of the Josephson critical current.