Bulk Band Gap and Surface State Conduction Observed in Voltage-Tuned Crystals of the Topological Insulator Bi2_2Se3_3

We report a transport study of exfoliated few monolayer crystals of topological insulator Bi$_2$Se$_3$ in an electric field effect (EFE) geometry. By doping the bulk crystals with Ca, we are able to fabricate devices with sufficiently low bulk carrier density to change the sign of the Hall density with the gate voltage $V_g$. We find that the temperature $T$ and magnetic field dependent transport properties in the vicinity of this $V_g$ can be explained by a bulk channel with activation gap of approximately 50 meV and a relatively high mobility metallic channel that dominates at low $T$. The conductance (approximately 2 $\times$ 7$e^2/h$), weak anti-localization, and metallic resistance-temperature profile of the latter lead us to identify it with the protected surface state. The relative smallness of the observed gap implies limitations for EFE topological insulator devices at room temperature.Comment: 4 pages, 4 figures. In new version, panels have been removed from Figures 1, 2, and 4 to improve clarity. Additional data included in Figure 4. Introduction and discussion revised and expande

Thermal Hall Conductivity as a Probe of Gap Structure in Multi-band Superconductors: The Case of Ba1−xKxFe2As2\rm Ba_{1-x}K_xFe_2As_2

The sign and profile of the thermal Hall conductivity $\kappa_{xy}$ gives important insights into the gap structure of multi-band superconductors. With this perspective, we have investigated $\kappa_{xy}$ and the thermal conductivity $\kappa_{xx}$ in $\rm Ba_{1-x}K_xFe_2As_2$ which display large peak anomalies in the superconducting state. The anomalies imply that a large hole-like quasiparticle (qp) population exists below the critical temperature $T_c$. We show that the qp mean-free-path inferred from $\kappa_{xx}$ reproduces the observed anomaly in $\kappa_{xy}$, providing a consistent estimate of a large qp population. Further, we demonstrate that the hole-like signal is consistent with a theoretical scenario where despite potentially large gap variations on the electron pockets, the minimal homogeneous gap of the superconducting phase resides at a hole pocket. Implications for probing the gap structure in the broader class of pnictide superconductors are discussed.Comment: 5 pages, 4 figures. Orientation significantly updated from previous (0811.4668v1) reflecting new theoretical understanding of experimental results and physical implications. Introduction, discussion, and figures updated including additional figure for model calculatio

Superconductivity in CuxBi2Se3 and its implications for pairing in the undoped topological insulator

Bi2Se3 is one of a handful of known topological insulators. Here we show that copper intercalation in the van der Waals gaps between the Bi2Se3 layers, yielding an electron concentration of ~ 2 x 10^20cm-3, results in superconductivity at 3.8 K in CuxBi2Se3 for x between 0.12 and 0.15. This demonstrates that Cooper pairing is possible in Bi2Se3 at accessible temperatures, with implications for study of the physics of topological insulators and potential devices.Comment: 6 pages, 4 figure

Nernst-Ettingshausen effect in two-component electronic liquids

A simple model describing the Nernst-Ettingshausen effect (NEE) in two-component electronic liquids is formulated. The examples considered include graphite, where the normal and Dirac fermions coexist, superconductor in fluctuating regime, with coexisting Cooper pairs and normal electrons, and the inter-stellar plasma of electrons and protons. We give a general expression for the Nernst constant and show that the origin of a giant NEE is in the strong dependence of the chemical potential on temperature in all cases

p-type Bi2Se3 for topological insulator and low temperature thermoelectric applications

The growth and elementary properties of p-type Bi2Se3 single crystals are reported. Based on a hypothesis about the defect chemistry of Bi2Se3, the p-type behavior has been induced through low level substitutions (1 percent or less) of Ca for Bi. Scanning tunneling microscopy is employed to image the defects and establish their charge. Tunneling and angle resolved photoemission spectra show that the Fermi level has been lowered into the valence band by about 400 meV in Bi1.98Ca0.02Se3 relative to the n-type material. p-type single crystals with ab plane Seebeck coefficients of +180 microVK-1 at room temperature are reported. These crystals show a giant anomalous peak in the Seebeck coefficient at low temperatures, reaching +120 microVK-1 at 7 K, giving them a high thermoelectric power factor at low temperatures. In addition to its interesting thermoelectric properties, p-type Bi2Se3 is of substantial interest for studies of technologies and phenomena proposed for topological insulators.Comment: v2: STM characterization of n and p type Bi2Se3 is adde

Low temperature vortex liquid in La2−xSrxCuO4\rm La_{2-x}Sr_xCuO_4

In the cuprates, the lightly-doped region is of major interest because superconductivity, antiferromagnetism, and the pseudogap state \cite{Timusk,Lee,Anderson} come together near a critical doping value $x_c$. These states are deeply influenced by phase fluctuations \cite{Emery} which lead to a vortex-liquid state that surrounds the superconducting region \cite{WangPRB01,WangPRB06}. However, many questions \cite{Doniach,Fisher,FisherLee,Tesanovic,Sachdev} related to the nature of the transition and vortex-liquid state at very low tempera- tures $T$ remain open because the diamagnetic signal is difficult to resolve in this region. Here, we report torque magnetometry results on $\rm La_{2-x}Sr_xCuO_4$ (LSCO) which show that superconductivity is lost at $x_c$ by quantum phase fluctuations. We find that, in a magnetic field $H$, the vortex solid-to-liquid transition occurs at field $H_m$ much lower than the depairing field $H_{c2}$. The vortex liquid exists in the large field interval $H_m \ll H_{c2}$, even in the limit $T\to$0. The resulting phase diagram reveals the large fraction of the $x$-$H$ plane occupied by the quantum vortex liquid.Comment: 6 pages, 4 figures, submitted to Nature Physic

Development of ferromagnetism in the doped topological insulator Bi_(2−x)Mn_xTe_3

The development of ferromagnetism in Mn-doped Bi_2Te_3 is characterized through measurements on a series of single crystals with different Mn content. Scanning tunneling microscopy analysis shows that the Mn substitutes on the Bi sites, forming compounds of the type Bi_(2−x)Mn_xTe_3, and that the Mn substitutions are randomly distributed, not clustered. Mn doping first gives rise to local magnetic moments with Curie-like behavior, but by the compositions Bi_(1.96)Mn_(0.04)Te_3 and Bi_(1.91)Mn_(0.09)Te_3, a second-order ferromagnetic transition is observed, with T_C∼9–12 K. The easy axis of magnetization in the ferromagnetic phase is perpendicular to the Bi2Te3 basal plane. Thermoelectric power and Hall effect measurements show that the Mn-doped Bi_2Te_3 crystals are p-type. Angle-resolved photoemission spectroscopy measurements show that the topological surface states that are present in pristine Bi_2Te_3 are also present at 15 K in ferromagnetic Mn-doped Bi2−xMnxTe3 and that the dispersion relations of the surface states are changed in a subtle fashion

Momentum dependence of superconducting gap, strong-coupling dispersion kink, and tightly bound Cooper pairs in the high-T_c (Sr,Ba)_(1−x)(K,Na)_xFe_2As_2 superconductors

We present a systematic angle-resolved photoemission spectroscopic study of the high-Tc superconductor class (Sr/Ba)_(1−x)K_xFe_2As_2. By utilizing a photon-energy-modulation contrast and scattering geometry we report the Fermi surface and the momentum dependence of the superconducting gap, Δ(k⃗ ). A prominent quasiparticle dispersion kink reflecting strong scattering processes is observed in a binding-energy range of 25–55 meV in the superconducting state, and the coherence length or the extent of the Cooper pair wave function is found to be about 20 Å, which is uncharacteristic of a superconducting phase realized by the BCS-phonon-retardation mechanism. The observed 40±15 meV kink likely reflects contributions from the frustrated spin excitations in a J_1-J_2 magnetic background and scattering from the soft phonons. Results taken collectively provide direct clues to the nature of the pairing potential including an internal phase-shift factor in the superconducting order parameter which leads to a Brillouin zone node in a strong-coupling setting