ScholarlyCommons

Title

Three Dimensional Dirac Semimetals

Author

Saad Zaheer, University of PennsylvaniaFollow

Date of Award

2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Physics & Astronomy

First Advisor

Eugene J. Mele

Abstract

We extend the physics of graphene to three dimensional systems by showing that Dirac points can exist on the Fermi surface of realistic materials in three dimensions. Many of the exotic electronic properties of graphene can be ascribed to the pseudorelativistic behavior of its charge carriers due to two dimensional Dirac points on the Fermi surface. We show that certain nonsymmorphic spacegroups exhibit Dirac points among the irreducible representations of the appropriate little group at high symmetry points on the surface of the Brillouin zone. We provide a list of all Brillouin zone momenta in the 230 spacegroups that can host Dirac points.

We describe microscopic considerations necessary to design materials in one of the candidate spacegroups such that the Dirac point appears at the Fermi energy without any additional non-Dirac-like Fermi pockets. We use density functional theory based methods to propose six new Dirac semimetals: BiO₂ and SbO₂ in the β-cristobalite lattice (spacegroup 227), and BiCaSiO₄, BiMgSiO₄, BiAlInO₄, and BiZnSiO₄ in the distorted spinels lattice (spacegroup 74). Additionally we derive effective Dirac Hamiltonians given group representative operators as well as tight binding models incorporating spin-orbit coupling.

Finally we study the Fermi surface of zincblende (spacegroup 216) HgTe which is effectively point-like at Γ in the Brillouin zone and exhibits accidental degeneracies along a threefold rotation axis. Whereas compressive strain gaps the band structure into a topological insulator, tensile strain shifts the accidental degeneracies away from Γ and enlarges the Fermi surface. States on the Fermi surface exhibit nontrivial spin texture marked by winding of spins around the threefold rotation axis and by spin vortices indicating a change in the winding number. This is confirmed by microscopic calculations performed in tensile strained HgTe and Hg_0.5Zn_0.5 Te as well as k.p theory.

We conclude with a summary of recent work on the physics of Dirac semimetals especially after the observation of the topological Dirac semimetals Cd₃As₂ and Na₃Bi and outline topics for future research. Symmetry protected Dirac semimetals, on the other hand, have yet to be observed experimentally.

Recommended Citation

Zaheer, Saad, "Three Dimensional Dirac Semimetals" (2014). Publicly Accessible Penn Dissertations. 1514.
https://repository.upenn.edu/edissertations/1514