Excitonic Instability and Pseudogap Formation in Nodal Line Semimetal ZrSiS

Abstract

Electron correlation effects are studied in ZrSiS using a combination of first-principles and model approaches. We show that basic electronic properties of ZrSiS can be described within a two-dimensional lattice model of two nested square lattices. A high degree of electron-hole symmetry characteristic for ZrSiS is one of the key features of this model. Having determined model parameters from first-principles calculations, we then explicitly take electron-electron interactions into account and show that, at moderately low temperatures, ZrSiS exhibits excitonic instability, leading to the formation of a pseudogap in the electronic spectrum. The results can be understood in terms of Coulomb-interaction-assisted pairing of electrons and holes reminiscent of that of an excitonic insulator. Our finding allows us to provide a physical interpretation of the unusual mass enhancement of charge carriers in ZrSiS recently observed experimentally.

Excitonic instability and unconventional pairing in the nodal-line materials ZrSiS and ZrSiSe

“…In this case we find a critical scale consistent with the ordering temperature of the ladder or RPA approximation of Ref. 10. As it was pointed out in Refs.…”

“…To that end, we use the effective twoorbital Hubbard model on the bilayer square lattice with the set of parameters which approximately reproduces the low-energy effective band structure found in the DFT calculations of Ref. 10. We confirm the previously found excitonic instability and provide additional characterization of it as an onsite spin exciton.…”

“…Ab-initio estimates by the approach used in Ref. 10 V 1 ∼ 0.8 eV. While currently we cannot provide detailed fRG calculations on the complex 3D band structure, we identified a mechanism that may be relevant to keep the exciton energy scale at an observable level.…”

“…First, we use the full set of parameters suggested in Ref. 10. This produces an instability with the wavevector dependence of the effective interaction shown in the left panel of Fig.…”

“…where we have the electron creation (annihilation) operators c † is (c is ), and s =↑, ↓ is the spin projection. The sums implicitly include summation over the repreated index s. For ZrSiS the nearest-neighbor hoppings between the sites are t AA ≈ −t BB ≈ 0.74 eV and ∆ = 0.835 eV is an onsite energy shift between the layers [10]. We neglect more remote hopping amplitudes and also the interlayer hopping t AB ∼ 0.1 eV, which was found to be significantly smaller than the nearest-neighbor hopping terms [10].…”