Session G37: Emergent Phenomena in Correlated and Topological Square nets

Electronic signatures of quasiparticles in high-Tc iron-chalcogenide superconductors under pressure

Iron-chalcogenide superconductors of the FeSe family provide an exciting platform to investigate the high-temperature superconductivity that often emerges from intertwined electronic nematic and spin-density wave orders. Tuning parameters, such as applied pressure and chemical pressure [1,2], can be used to explore the phase diagram and establish the characteristics of the normal quasiparticles. In this talk, I will present quantum oscillations studies under high applied hydrostatic pressure in single crystals of FeSe_1-xS_x using magnetotransport and tunnel diode oscillator experiments up to 45T [3,4,5]. I will discuss the evolution of the Fermi surface and the quasiparticle effective masses in the vicinity of the nematic end point as well as inside the high-pressure phase where superconductivity is enhanced. I will compare phase diagrams of different systems and discuss the role of Cu impurity scattering [6,7]. These studies emphasize the robustness of the high-T_c superconducting phase, the role of electronic correlations as well as the characteristics of the anomalous normal electronic phases.   

Materials search algorithms for square net Dirac materials

Square net materials can exhibit Dirac nodal lines, as exemplified by ZrSiS, whose striking linear dispersion spans 2eV. Since the discovery of ZrSiS, there has been a search to find more square net Dirac materials. To this end, we derived a symmetry classification of square nets, which reveals that the glide symmetry in ZrSiS is not necessary to protect a Dirac crossing. We use our classification to derive a materials search algorithm, predicting new square net materials in both symmorphic and non-symmorphic space groups. We further derive a geometric tolerance factor to refine the materials search. Finally we discuss correlated square net materials.   

Ultrafast dynamics of vortex strings in a charge density wave

Excitation of materials with ultrafast light pulses can induce novel states of matter not accessible in thermodynamic equilibrium. Visualizing the pathways of transformation into those transient states can inspire rational ways to control materials’ functionality. Fast non-adiabatic transients between two structures typically leads to proliferation of topological defects, which govern the return to equilibrium as well as the metastability of competing orders in systems with intertwined orders. Yet, the dynamics of these defects in ultrafast phase transitions have been elusive.

We use ultrafast x-ray diffraction at an x-ray free electron laser (XFEL) to capture the dynamics of the charge density waves (CDW) of SmTe₃ and LaTe₃ after strong photoexcitation above the CDW gap. Exploiting the unique momentum- and time-resolution of the XFEL we obtain direct experimental evidence of the creation and evolution of topological defects of this incommensurate CDW. The scaling properties of the structure factor S(k, t) show unequivocally that the scattering originates from vortex strings characteristic of the topological defects of a complex order parameter in three dimensions, which correspond to dislocations of the CDW. While most features of the scattering can be explained by a simple Ginzburg-Landau model with a complex order parameter, certain scaling properties suggest additional conserved quantities. These observations and analysis show the unique capabilities of ultrafast x-ray scattering as an incisive tool to characterize nonequilibrium states.

    

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)