Low-temperature nodal-quasiparticle transport in lightly doped YBa_{2}Cu_{3}O_{y} near the edge of the superconducting doping regime

In-plane transport properties of nonsuperconducting YBa_{2}Cu_{3}O_{y} (y = 6.35) are measured using high-quality untwinned single crystals. We find that both the a- and b-axis resistivities show log(1/T) divergence down to 80 mK, and accordingly the thermal conductivity data indicate that the nodal quasiparticles are progressively localized with lowering temperature. Hence, both the charge and heat transport data do not support the existence of a "thermal metal" in nonsuperconducting YBa_{2}Cu_{3}O_{y}, as opposed to a recent report by Sutherland {\it et al.} [Phys. Rev. Lett. {\bf 94}, 147004 (2005)]. Besides, the present data demonstrate that the peculiar log(1/T) resistivity divergence of cuprate is {\it not} a property associated with high-magnetic fields.Comment: 4 pages, 3 figures. Our previous main claim that the pseudogap state of cuprates is inherently insulating was found to be erroneous and has been retracted; the paper now focuses on the log(1/T) resistivity divergence and its implication

Large bulk resistivity and surface quantum oscillations in the topological insulator Bi2Te2Se

Topological insulators are predicted to present novel surface transport phenomena, but their experimental studies have been hindered by a metallic bulk conduction that overwhelms the surface transport. We show that a new topological insulator, Bi2Te2Se, presents a high resistivity exceeding 1 Ohm-cm and a variable-range hopping behavior, and yet presents Shubnikov-de Haas oscillations coming from the surface Dirac fermions. Furthermore, we have been able to clarify both the bulk and surface transport channels, establishing a comprehensive understanding of the transport in this material. Our results demonstrate that Bi2Te2Se is the best material to date for studying the surface quantum transport in a topological insulator.Comment: 4 pages, 3 figure

Landau level spectroscopy of surface states in the topological insulator Bi0.91_{0.91}Sb0.09_{0.09} via magneto-optics

We have performed broad-band zero-field and magneto-infrared spectroscopy of the three dimensional topological insulator Bi$_{0.91}$Sb$_{0.09}$. The zero-field results allow us to measure the value of the direct band gap between the conducting $L_a$ and valence $L_s$ bands. Under applied field in the Faraday geometry (\emph{k} $||$ \emph{H} $||$ C1), we measured the presence of a multitude of Landau level (LL) transitions, all with frequency dependence $\omega \propto \sqrt{H}$. We discuss the ramification of this observation for the surface and bulk properties of topological insulators.Comment: 7 pages, 8 figures, March Meeting 2011 Abstract: J35.0000

Numerical estimation of the β\beta-function in 2D systems with spin-orbit coupling

We report a numerical study of Anderson localization in a 2D system of non-interacting electrons with spin-orbit coupling. We analyze the scaling of the renormalized localization length for the 2D SU(2) model and estimate its $\beta$-function over the full range from the localized to the metallic limits

"Spin-Flop" Transition and Anisotropic Magnetoresistance in Pr_{1.3-x}La_{0.7}Ce_{x}CuO_{4}: Unexpectedly Strong Spin-Charge Coupling in Electron-Doped Cuprates

We use transport and neutron-scattering measurements to show that a magnetic-field-induced transition from noncollinear to collinear spin arrangement in adjacent CuO_{2} planes of lightly electron-doped Pr_{1.3-x}La_{0.7}Ce_{x}CuO_{4} (x=0.01) crystals affects significantly both the in-plane and out-of-plane resistivity. In the high-field collinear state, the magnetoresistance (MR) does not saturate, but exhibits an intriguing four-fold-symmetric angular dependence, oscillating from being positive at B//[100] to being negative at B//[110]. The observed MR of more than 30% at low temperatures induced by a modest modification of the spin structure indicates an unexpectedly strong spin-charge coupling in electron-doped cuprates.Comment: 4 pages, 5 figures, accepted for publication in Phys. Rev. Let