UC Berkeley

This dissertation presents the results of several research projects on condensed matter systems that fall within the broad scope of quantum materials. These are compounds which possess emergent phenomena that would not be expected from conventional solid state theory. These include the topological semimetals Cd3As2, TaAs, and ZrTe5, as well as the frustrated antiferromagnet Fe1/3NbS2. In Cd3As2, a Dirac semimetal, focused ion beam microstructured devices were found to exhibit a new type of coherent electron orbit. This “Weyl orbit” involves the Fermi arc surface states. Although these states are a necessary consequence of the topological nature of Cd3As2, they had not previously been observed in electronic transport. In the Weyl semimetal TaAs, the same focused ion beam microstructuring techniques were found to induce superconductivity on the device surface due to the differential sputtering of tantalum and arsenic. Instead, mechanical polishing techniques were used to thin devices in order to observe signatures of surface-driven transport, likely stemming from Fermi arc states as well.

In ZrTe5, magnetization and magnetic torque measurements found a paramagnetic to diamagnetic crossover at the quantum limit magnetic field. This is the result of charge carriers entering a chiral, zeroth Landau level pinned at zero energy. This is a direct consequence of a topological band crossing, and therefore points to ZrTe5 as a Dirac semimetal. A possible transition out of this topological phase was also observed as a function of temperature.

Finally, at sufficiently low temperatures and high current densities, Fe1/3NbS2 was found to be a switchable antiferromagnet. A spin transfer torque produced by an applied current was found to rotate the antiferromagnetic order. The rotation of these moments is reflected in the anisotropic magnetoresistance, changing the resistance of the device. In this manner, microstructured devices of Fe1/3NbS2 form an antiferromagnetic memory bit with electronic write-in and read-out. The low current densities involved and tunability of the device response point to Fe1/3NbS2 as a promising platform for antiferromagnetic spintronics.