Effective quasiclassical models for odd-frequency superconductivity: energy-symmetry, preserved spectral weight, and Meissner response

TL;DR

This paper develops criteria for physically reasonable quasiclassical models of odd-frequency superconductivity, showing that different models can predict diverse spectral and magnetic behaviors despite satisfying fundamental conservation laws.

Contribution

The authors establish necessary criteria for effective quasiclassical Green functions in odd-frequency superconductors and demonstrate that models satisfying these can still predict varied physical phenomena.

Findings

Models can have conserved spectral weight but differ in density of states.

Effective models predict both conventional and unconventional Meissner responses.

Careful consideration of model properties is crucial for physically accurate predictions.

Abstract

The odd-frequency superconducting state appears generally in hybrid structures consisting of conventional superconductors and other materials, and features electrons that form temporally non-local Cooper pairs. The quasiclassical theory of superconductivity has been extensively used to model such systems, finding in many cases excellent agreement with experimental measurements. Therefore, it is of interest to study effective models of odd-frequency superconductivity to predict new phenomena associated with this form of unconventional pairing. We establish necessary criteria that the quasiclassical Green functions in odd-frequency superconducting systems in the dirty limit must satisfy in order to be physically reasonable, including conservation of spectral weight. We show that it is possible to write down effective models which satisfy all the abovementioned criteria, but which still predict different behavior when it comes to the density of states and the magnetic response of the superconductor. For instance, an effective model for the odd-frequency anomalous Green function that gives a conserved spectral weight can yield either a peaked or gapped density of states at the Fermi energy, and exhibit conventional, zero, or unconventional Meissner response. This finding demonstrates the importance of carefully considering the properties of effective models describing odd-frequency superconductivity in order to obtain physically reasonable results.