Electrodynamics of hidden quasiparticle and plasmons in nodal-squares

The discovery of nodal-line semimetal ZrSiS extends the notion of Dirac fermions from points to lines and loops in the momentum space [1]. Identifying new nodal-line fermions is challenging since often the Dirac nodal-lines are dispersive, submerged within a Fermi sea, and gapped by spin-orbit-coupling. I will first introduce how precise power-law behavior of the optical response function can reveal the hidden nodal-line fermions, using NbAs₂ as an example [2]. The large anisotropy associated with nodal-line structure gives rise to greatly reduced kinetic energy along the line. I will discuss the two fundamental spectroscopic hallmarks of electronic correlations observed in ZrSiSe [3]: strong reduction (1/3) of the free carrier Drude weight and of the Fermi velocity renormalization compared to predictions of density functional theory. Furthermore, in the ZrSiS/Se the nodal-lines form closed squares and I will reveal more exotic quasiparticles hidden in the nodal-squares of ZrSiS/Se through magneto-optical spectroscopy. Finally, another consequence of the nodal-line structure is the large difference in the plasma frequencies for in-plane and out-of-plane responses, leading to a broadband hyperbolic regime covering infrared and visible frequencies. I will show our recent observation of propagating hyperbolic waves in ZrSiSe, enabled by the van Hove singularities in the nodal-squares [4].



[1] L. M. Schoop et al, Nat. Commun. 7, 11696 (2016)

[2] Y. Shao, Z. Sun et al, PNAS 116, 1168 (2019)

[3] Y. Shao et al, Nat. Phys. 16, 6 (2020)

[4] Y. Shao et al, Science Advance 8, eadd6169 (2022)