Observation of Anisotropic Properties in Topological Quantum Materials

The discovery of the three-dimensional topological insulator (TI) has enormously impacted our understanding of quantum materials. These novel materials are characterized by the topology rather than some order parameters. The TIs are the materials that exhibit insulating properties in the bulk while possessing the metallic state on the surface. These surface states are protected by time-reversal symmetry; as a result, the electrons featuring surface states are spin-polarized. After the discovery of TIs, other topological semimetallic (TSM) states were discovered, which enhanced the understanding and widened the reach of topological materials. Discoveries of various topological phases such as Dirac, Weyl, nodal line semimetals, etc., provided not only novel quasi-particles in condensed matter physics but also promised the discoveries of new technology based on these topological quantum materials. The next focus of the recent research has been on understanding the interplay among topology, superconductivity, magnetism, geometry, correlation, etc. In this thesis, using angle-resolved photoemission spectroscopy (ARPES), time-resolved ARPES (tr-ARPES), magnetic and transport measurements, in conjunction with first-principles calculations, we studied diverse anisotropies in different topological quantum materials, where anisotropic properties are originated due to the numerous factors including geometry, crystalline symmetry, magnetic orientation, and topology. First, we studied the electronic structures of transition metal dipnictides, which crystallize in the low symmetry space group; found that these crystals show different surface behaviors with different cleaving planes. These materials show high magnetoresistance despite having topologically trivial band structures. Our studies reveal that high magnetoresistance in these materials does not necessarily come from the TSM state. Next, we investigated the anisotropic Dirac cone structure in a tetradymite material, which features the Dirac node arc state in addition to the anisotropic Dirac cone at a high symmetry point away from the zone center. Our topological analysis shows that the material possesses multi-fermionic states, which is rare in topological quantum materials. In our next project, we chose a kagome lattice-based material that could provide an ideal platform to study the interplay among geometry, magnetism, correlation, and topology. We investigated magnetic, transport, and electronic structure, in which we revealed that the material possesses anisotropic Hall resistivities and demonstrates multi-orbital fermiology. Finally, using tr-ARPES we revealed the cooling mechanism of the transient topological bulk state in a nodal line semimetal, and our theoretical analysis corroborates our experimental results that the optical and acoustic phonon relaxation follow the linear decay process

Tunability of the topological nodal-line semimetal phase in ZrSiX-type materials

The discovery of a topological nodal-line (TNL) semimetal phase in ZrSiS has invigorated the study of other members of this family. Here, we present a comparative electronic structure study of ZrSiX (where X = S, Se, Te) using angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. Our ARPES studies show that the overall electronic structure of ZrSiX materials comprises of the diamond-shaped Fermi pocket, the nearly elliptical-shaped Fermi pocket, and a small electron pocket encircling the zone center ($\Gamma$) point, the M point, and the X point of the Brillouin zone, respectively. We also observe a small Fermi surface pocket along the M-$\Gamma$-M direction in ZrSiTe, which is absent in both ZrSiS and ZrSiSe. Furthermore, our theoretical studies show a transition from nodal-line to nodeless gapped phase by tuning the chalcogenide from S to Te in these material systems. Our findings provide direct evidence for the tunability of the TNL phase in ZrSiX material systems by adjusting the spin-orbit coupling (SOC) strength via the X anion.Comment: 7 pages, 4 figure

Unusual magnetic and transport properties in HoMn6_6Sn6_6 kagome magnet

With intricate lattice structures, kagome materials are an excellent platform to study various fascinating topological quantum states. In particular, kagome materials, revealing large responses to external stimuli such as pressure or magnetic field, are subject to special investigation. Here, we study the kagome-net HoMn$_6$Sn$_6$ magnet that undergoes paramagnetic to ferrimagnetic transition (below 376 K) and reveals spin-reorientation transition below 200 K. In this compound, we observe the topological Hall effect and substantial contribution of anomalous Hall effect above 100 K. We unveil the pressure effects on magnetic ordering at a low magnetic field from the pressure tunable magnetization measurement. By utilizing high-resolution angle-resolved photoemission spectroscopy, Dirac-like dispersion at the high-symmetry point K is revealed in the vicinity of the Fermi level, which is well supported by the first-principles calculations, suggesting a possible Chern-gapped Dirac cone in this compound. Our investigation will pave the way to understand the magneto-transport and electronic properties of various rare-earth-based kagome magnets