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Electronic and Transport Properties of Weyl Semimetals
McCormick, Timothy M

2018, Doctor of Philosophy, Ohio State University, Physics.
Topological Weyl semimetals have attracted substantial recent interest in condensed matter physics. In this thesis, we theoretically explore electronic and transport properties of these novel materials. We also present results of experimental collaborations that support our theoretical calculations. Topological Weyl semimetals (TWS) can be classified as type-I TWS, in which the density of states vanishes at the Weyl nodes, and type-II TWS, in which an electron pocket and a hole pocket meet at a singular point of momentum space, allowing for distinct topological properties. We consider various minimal lattice models for type-II TWS. We present the discovery of a type II topological Weyl semimetal (TWS) state in pure MoTe2, where two sets of WPs (W2$\pm$, W3$\pm$) exist at the touching points of electron and hole pockets and are located at different binding energies above $E_F$. Using ARPES, modeling, DFT and calculations of Berry curvature, we identify the Weyl points and demonstrate that they are connected by different sets of Fermi arcs for each of the two surface terminations.

Weyl semimetals possess low energy excitations which act as monopoles of Berry curvature in momentum space. These emergent monopoles are at the heart of the extensive novel transport properties that Weyl semimetals exhibit. We show how the Nernst effect, combining entropy with charge transport, gives a unique signature for the presence of Dirac bands. The Nernst thermopower of NbP (maximum of 800 $\mu \textrm{V}\cdot \textrm{K}^{-1}$ at 9 T, 109 K) exceeds its conventional thermopower by a hundredfold and is significantly larger than the thermopower of traditional thermoelectric materials. The Nernst effect has a pronounced maximum near $T_M=90 \pm 20 \textrm{K}=\mu_{0}/ \kb$ ($\mu_0$ is chemical potential at $T=0$ K). A self-consistent theory without adjustable parameters shows that this results from electrochemical potential pinning to the Weyl point energy at $T\geq T_M$, driven by charge neutrality and Dirac band symmetry.

We propose that Fermi arcs in Weyl semimetals lead to an anisotropic magnetothermal conductivity, strongly dependent on externally applied magnetic field and resulting from entropy transport driven by circulating electronic currents. The circulating currents result in no net charge transport, but they do result in a net entropy transport. This translates into a magnetothermal conductivity that should be a unique experimental signature for the existence of the arcs. We analytically calculate the Fermi arc-mediated magnetothermal conductivity in the low-field semiclassical limit as well as in the high-field ultra-quantum limit, where only the chiral Landau levels are involved. By numerically including the effects of higher Landau levels, we show how the two limits are linked at intermediate magnetic fields. This work provides the first proposed signature of Fermi arc-mediated thermal transport and sets the stage for utilizing and manipulating the topological Fermi arcs in experimental thermal applications.
Nandini Trivedi (Advisor)
Rolando Valdes Aguilar (Committee Member)
Mohit Randeria (Committee Member)
Yuri Kovchegov (Committee Member)
161 p.

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McCormick, T. (2018). Electronic and Transport Properties of Weyl Semimetals. (Electronic Thesis or Dissertation). Retrieved from https://etd.ohiolink.edu/

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McCormick, Timothy. "Electronic and Transport Properties of Weyl Semimetals." Electronic Thesis or Dissertation. Ohio State University, 2018. OhioLINK Electronic Theses and Dissertations Center. 22 Oct 2018.

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McCormick, Timothy "Electronic and Transport Properties of Weyl Semimetals." Electronic Thesis or Dissertation. Ohio State University, 2018. https://etd.ohiolink.edu/

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