Weak-Coupling Theory of Pair Density-Wave Instabilities in Transition Metal Dichalcogenides

TL;DR

This paper develops a weak-coupling theory for pair density wave (PDW) instabilities in transition metal dichalcogenides, revealing conditions under which unconventional superconducting phases can emerge due to Fermi surface topology and interactions.

Contribution

It introduces a weak-coupling analysis of PDW instabilities in transition metal dichalcogenides, highlighting the role of Fermi surface pockets and effective interactions in forming unconventional pairing states.

Findings

PDW instability is robust against small Fermi surface distortions.

Effective attractive interactions in PDW channels arise from repulsive electronic interactions.

Unusual phases, including odd-frequency charge-2e superconductors, can emerge in the ground state.

Abstract

The possibility of realizing pair density wave (PDW) phases, in which Cooper pairs have a finite momentum, presents an interesting challenge that has been studied in a wide variety of systems. In conventional superconductors, this is only possible when external fields lift the spin degeneracy of the Fermi surface, leading to pair formation at an incommensurate momentum. Here, we study a second possibility, potentially relevant to transition metal dichalcogenides, in which the Fermi surface consists of a pair of pockets centered at the $\pm K$ points of the Brillouin zone as well as a central pocket at the $\Gamma$ point. In the limit where these three pockets are identical, the pairing susceptibility has a logarithmic divergence at the non-zero wave-vectors $\pm \mathbf{K}$, allowing for a weak-coupling analysis of the PDW instability. We find that repulsive electronic interactions combine to yield effective attractive interactions in the singlet and triplet PDW channels, as long as the $\Gamma$ pocket is present. Because these PDW channels decouple from the uniform superconducting channel, they can become the leading unconventional pairing instability of the system. Upon solving the linearized gap equations, we find that the PDW instability is robust against small trigonal warping of the $\pm K$ pockets and small detuning between the $\Gamma$ and $\pm K$ pockets, which affect the PDW transition in a similar way as the Zeeman magnetic field affects the uniform superconducting transition. We also derive the Ginzburg-Landau free energy for the PDW gaps with momenta $\pm \mathbf{K}$, analyzing the conditions for and consequences of the emergence of FF-type and LO-type PDW ground states. Our classification of the induced orders in each ground state reveals unusual phases, including an odd-frequency charge-$2e$ superconductor in the LO-type PDW.