Fermion Loops, Linear Magnetoresistance, Linear In Temperature Resistance, and Bad Metals

Bad metals including the high $T_c$ superconductors display an exotic resistance that is linear in both temperature and magnetic field. This hallmark of strong correlations is poorly understood. We show that Fourier transforming the magnetoconductance with respect to magnetic field obtains a curve describing the area distribution of loops traced by electrons and holes within the sample. Analysis of this area distribution reveals that linear resistance is caused by scattering and quantum interference, but with more large loops than occur in ordinary 2-D and 3-D materials where scattering destroys quantum coherence and limits loop size. This limit on quantum coherence is absent in linear resistance materials, resulting in larger loops limited only by thermal decoherence. Linear resistance signals that quantum coherence is maintained in the presence of scattering

Bulk effects on topological conduction on the surface of 3-D topological insulators

The surface states of a topological insulator in a fine-tuned magnetic field are ideal candidates for realizing a topological metal which is protected against disorder. Its signatures are (1) a conductance plateau in long wires in a finely tuned longitudinal magnetic field and (2) a conductivity which always increases with sample size, and both are independent of disorder strength. We numerically study how these experimental transport signatures are affected by bulk physics in the interior of the topological insulator sample. We show that both signatures of the topological metal are robust against bulk effects. However the bulk does substantially accelerate the metal's decay in a magnetic field and alter its response to surface disorder. When the disorder strength is tuned to resonance with the bulk band the conductivity follows the predictions of scaling theory, indicating that conduction is diffusive. At other disorder strengths the bulk reduces the effects of surface disorder and scaling theory is systematically violated, signaling that conduction is not fully diffusive. These effects will change the magnitude of the surface conductivity and the magnetoconductivity

Hole Spin Helix: Anomalous Spin Diffusion in Anisotropic Strained Hole Quantum Wells

We obtain the spin-orbit interaction and spin-charge coupled transport equations of a two-dimensional heavy hole gas under the influence of strain and anisotropy. We show that a simple two-band Hamiltonian can be used to describe the holes. In addition to the well-known cubic hole spin-orbit interaction, anisotropy causes a Dresselhaus-like term, and strain causes a Rashba term. We discover that strain can cause a shifting symmetry of the Fermi surfaces for spin up and down holes. We predict an enhanced spin lifetime associated with a spin helix standing wave similar to the Persistent Spin Helix which exists in the two-dimensional electron gas with equal Rashba and Dresselhaus spin-orbit interactions. These results may be useful both for spin-based experimental determination of the Luttinger parameters of the valence band Hamiltonian and for creating long-lived spin excitations