Abstract

A new theoretical framework for classifying superconducting nodes---key observational markers of how the constituent electrons pair up---can offer deeper understanding into unconventional superconductivity.

Highlights

  • While it is often difficult to determine the symmetry property of Cooper pairs [1–26], superconducting nodes—geometry of gapless regions in the Bogoliubov quasiparticle spectrum— are key ingredients to identify pairing symmetries

  • The interplay between superconducting nodes and topology has been actively investigated, and intensive research in the past decade has revealed various intriguing nodes out of the scope of the pioneering work to classify superconducting order parameters based on the point groups

  • While most previous studies are based on the homotopy theory, our theory is on the basis of the symmetry-based analysis of band topology, which enables systematic diagnoses of nodes in all nonmagnetic and magnetic space groups

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Summary

INTRODUCTION

While it is often difficult to determine the symmetry property of Cooper pairs (called pairing symmetry in this work) [1–26], superconducting nodes—geometry of gapless regions in the Bogoliubov quasiparticle spectrum— are key ingredients to identify pairing symmetries. Since the order parameters are described by basis functions of the irreducible representations in these theories, the intersection between Fermi surfaces and regions where the basis functions vanish is understood as superconducting nodes. A comprehensive theory to classify and predict superconducting nodes for arbitrary symmetry classes has long been awaited To achieve this goal, we need to answer the following two questions:. The other one is that our framework leads to an efficient algorithm to detect and diagnose nodes in realistic materials, requiring only pairing symmetry and information of irreducible representations of Bloch wave functions at high-symmetry momenta.

OVERVIEW OF THIS STUDY
Emergent Altland-Zirnbauer classes and zero-dimensional topological invariants
Diagnosis of nodal structures based on compatibility conditions
Gapless point classifications on lines
Unification of compatibility conditions and gapless point classifications
Applications to materials
BdG Hamiltonian and symmetry representations
Cell decomposition
Emergent Altland-Zirnbauer classes
Compatibility conditions
CLASSIFICATION OF GAPLESS POINTS ON 1-CELL
Cornfeld-Chapman method for 2D systems
Character decomposition formulas
C CT D DT AI AIC CI
Example
P210 with B pairing
P4 with 1E pairing
Pmc2110 with A2 pairing
UNIFICATION OF COMPATIBILITY CONDITIONS AND GAPLESS POINT CLASSIFICATIONS
Revisiting compatibility conditions and the first differential
Classifications of nodes on 1-cell
Examples
APPLICATIONS TO MATERIALS
Efficient algorithm for detection of nodal structures
Material example
FURTHER EXTENSION TO NODES AT GENERIC POINTS
VIII. CONCLUSION AND OUTLOOK

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