BCS superconducting transitions in lattice fermions

TL;DR

This paper develops a unified BCS-based framework for understanding superconductivity in lattice fermions, highlighting how different density of states structures influence the transition temperature and related thermodynamic properties.

Contribution

It introduces a modeling approach for the density of states in lattice fermions, deriving asymptotic forms of $T_c$ and revealing universal relationships affected by DOS singularities.

Findings

Delta-functional DOS leads to highest $T_c$ in weak coupling

Universal ratios depend on DOS singularity parameters

Multi-band effects can induce effective DOS singularities

Abstract

We develop a general description of the superconductivity of lattice fermions based on the BCS theory. We propose a modeling of the density of states (DOS) of lattice fermions, where divergent and semi-metallic structures are described by asymptotic expansions around the Fermi energy. This modeling leads to a unified representation of the transition temperature $T_c$ at half filling, which reproduces asymptotic forms of $T_c$ derived in several lattices, such as the square, honeycomb, and Lieb lattices, for the weakly interacting limit. The derived asymptotic forms of $T_c$ are categorized into four types, which is attributed to the different responses of the degenerate fermions depending on the DOS structures. The DOS with a delta-functional singularity induces the highest $T_c$ in the weakly interacting region, where $T_c$ is linearly proportional to the pairing interaction $U$. Three kinds of universal ratios defined in the BCS theory no longer reduce to constants independent of the system parameters but can be parameterized with a certain variable that characterizes the singular structures of the DOS. We find universal relationship among thermodynamic quantities that holds for all parameter regions. Further, we numerically demonstrate that in multi-band systems the correlation effects can induce an effective delta-functional singularity. This phenomenon generally appears in multi-energy (or multi-gap) systems and may provide a plausible guideline for material designs of high-$T_c$ superconductor.