Electronic structure basis for the titanic magnetoresistance in WTe2_2

The electronic structure basis of the extremely large magnetoresistance in layered non-magnetic tungsten ditelluride has been investigated by angle-resolved photoelectron spectroscopy. Hole and electron pockets of approximately the same size were found at the Fermi level, suggesting that carrier compensation should be considered the primary source of the effect. The material exhibits a highly anisotropic, quasi one-dimensional Fermi surface from which the pronounced anisotropy of the magnetoresistance follows. A change in the Fermi surface with temperature was found and a high-density-of-states band that may take over conduction at higher temperatures and cause the observed turn-on behavior of the magnetoresistance in WTe$_2$ was identified

The Ground State of the Pseudogap in Cuprate Superconductors

We present studies of the electronic structure of La2-xBaxCuO4, a system where the superconductivity is strongly suppressed as static spin and charge orders or "stripes" develop near the doping level of x=1/8. Using angle-resolved photoemission and scanning tunneling microscopy, we detect an energy gap at the Fermi surface with magnitude consistent with d-wave symmetry and with linear density of states, vanishing only at four nodal points, even when superconductivity disappears at x=1/8. Thus, the non-superconducting, "striped" state at x=1/8 is consistent with a phase incoherent d-wave superconductor whose Cooper pairs form spin/charge ordered structures instead of becoming superconducting.Comment: This is the author's version of the wor

Gapped Surface States in a Strong-Topological-Semimetal

A three-dimensional strong-topological-insulator or -semimetal hosts topological surface states which are often said to be gapless so long as time-reversal symmetry is preserved. This narrative can be mistaken when surface state degeneracies occur away from time-reversal-invariant momenta. The mirror-invariance of the system then becomes essential in protecting the existence of a surface Fermi surface. Here we show that such a case exists in the strong-topological-semimetal Bi$_4$Se$_3$. Angle-resolved photoemission spectroscopy and \textit{ab initio} calculations reveal partial gapping of surface bands on the Bi$_2$Se$_3$-termination of Bi$_4$Se$_3$(111), where an 85 meV gap along $\bar{\Gamma}\bar{K}$ closes to zero toward the mirror-invariant $\bar{\Gamma}\bar{M}$ azimuth. The gap opening is attributed to an interband spin-orbit interaction that mixes states of opposite spin-helicity.Comment: 5 pages, 3 figure

Measurement of an Exceptionally Weak Electron-Phonon Coupling on the Surface of the Topological Insulator Bi2_2Se3_3 Using Angle-Resolved Photoemission Spectroscopy

Gapless surface states on topological insulators are protected from elastic scattering on non-magnetic impurities which makes them promising candidates for low-power electronic applications. However, for wide-spread applications, these states should have to remain coherent at ambient temperatures. Here, we studied temperature dependence of the electronic structure and the scattering rates on the surface of a model topological insulator, Bi$_2$Se$_3$, by high resolution angle-resolved photoemission spectroscopy. We found an extremely weak broadening of the topological surface state with temperature and no anomalies in the state's dispersion, indicating exceptionally weak electron-phonon coupling. Our results demonstrate that the topological surface state is protected not only from elastic scattering on impurities, but also from scattering on low-energy phonons, suggesting that topological insulators could serve as a basis for room temperature electronic devices.Comment: published version, 5 pages, 4 figure