Excitonic Instability and Pseudogap Formation in Nodal Line Semimetal ZrSiS

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.

Figure 1

(Top) Band structure of ZrSiS calculated by projecting the DFT Hamiltonian onto WFs obtained within the scheme of maximal localization. Color indicates projection of Bloch states on WFs forming sublattices $A$ and $B$. Original DFT structure is shown for reference. Notation of high-symmetry points of the Brillouin zone is the same as in Ref. [2]. Horizontal dashed line corresponds to the Fermi energy. (Bottom) Real-space representation of WFs being the basis of the TB Hamiltonian used in this Letter to describe many-body effects in ZrSiS. Gray, blue, and yellow balls depict Zr, Si, and S atoms, respectively.

Figure 2

(Top) Schematic illustration of the lattice TB model used in this Letter to describe single-particle states in ZrSiS. (Bottom) Band structure and density of states of the corresponding model.

Figure 3

Renormalized spectral function $A(\mathbf{k},E)=(1/\pi )\mathrm{Im}G(\mathbf{k},E)$ and density of states of ZrSiS calculated near the transition temperature $T_c=135\mathrm{K}$ at which a pseudogap is formed.

Figure 4

Inverse intersublattice susceptibility $1/X_{AB}^0(q\to 0)$ calculated as a function of temperature for different Fermi energies corresponding to different values of hole doping. Intrasublattice Coulomb interaction ($U_{AB}$) estimated within RPA is shown for reference.