Topological insulators are predicted to present interesting surface transport phenomena but their experimental studies have been hindered by a metallic bulk conduction that overwhelms the surface transport. We show that the topological insulator Bi 2 Te 2 Se presents a high resistivity exceeding 1 ⍀ cm and a variable-range hopping behavior, and yet presents Shubnikov-de Haas oscillations coming from the topological surface state. 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 Bi 2 Te 2 Se is, to our knowledge, the best material to date for studying the surface quantum transport in a topological insulator.
We show that in the new topological-insulator compound Bi(1.5)Sb(0.5)Te(1.7)Se(1.3) one can achieve a surfaced-dominated transport where the surface channel contributes up to 70% of the total conductance. Furthermore, it was found that in this material the transport properties sharply reflect the time dependence of the surface chemical potential, presenting a sign change in the Hall coefficient with time. We demonstrate that such an evolution makes us observe both Dirac holes and electrons on the surface, which allows us to reconstruct the surface band dispersion across the Dirac point.
To optimize the bulk-insulating behavior in the topological insulator materials having the tetradymite structure, we have synthesized and characterized single-crystal samples of Bi2−xSbxTe3−ySey (BSTS) solid solution at various compositions. We have elucidated that there are a series of "intrinsic" compositions where the acceptors and donors compensate each other and present a maximally bulk-insulating behavior. At such compositions, the resistivity can become as large as several Ωcm at low temperature and one can infer the role of the surface-transport channel in the non-linear Hall effect. In particular, the composition of Bi1.5Sb0.5Te1.7Se1.3 achieves the lowest bulk carrier density and appears to be best suited for surface transport studies.
The massless Dirac fermions residing on the surface of three-dimensional topological insulators are protected from backscattering and cannot be localized by disorder, but such protection can be lifted in ultrathin films when the three-dimensionality is lost. By measuring the Shubnikov-de Haas oscillations in a series of high-quality Bi2Se3 thin films, we revealed a systematic evolution of the surface conductance as a function of thickness and found a striking manifestation of the topological protection: The metallic surface transport abruptly diminishes below the critical thickness of ~6 nm, at which an energy gap opens in the surface state and the Dirac fermions become massive. At the same time, the weak antilocalization behavior is found to weaken in the gapped phase due to the loss of π Berry phase.
Single crystals of the layered perovskite GdBaCo2O5+x (GBCO) have been grown by the floatingzone method, and their transport, magnetic, and structural properties have been studied in detail over a wide range of oxygen contents, 0 ≤ x ≤ 0.77. The obtained data are used to establish a rich phase diagram centered at the "parent" compound GdBaCo2O5.5 -an insulator with Co ions in the 3+ state. An attractive feature of GdBaCo2O5+x is that it allows a precise and continuous doping of CoO2 planes with either electrons or holes, spanning a wide range from the charge-ordered insulator at 50% electron doping (x = 0) to the undoped band insulator (x = 0.5), and further towards the heavily hole-doped metallic state. This continuous doping is clearly manifested in the behavior of thermoelectric power which exhibits a spectacular divergence with approaching x=0.5, where it reaches large absolute values (±800 µV/K) and abruptly changes its sign. At low temperatures, the homogeneous distribution of doped carriers in GBCO becomes unstable, as is often the case with strongly correlated systems, and both the magnetic and transport properties point to an intriguing nanoscopic phase separation into two insulating phases (for electron-doped region) or an insulating and a metallic phases (for hole-doped region). We also find that throughout the composition range the magnetic behavior in GBCO is governed by a delicate balance between ferromagnetic (FM) and antiferromagnetic (AF) interactions, which can be easily affected by temperature, doping, or magnetic field, bringing about FM-AF transitions and a giant magnetoresistance (MR) phenomenon. What distinguishes GBCO from the colossal-MR manganites is an exceptionally strong uniaxial anisotropy of the Co spins, which dramatically simplifies the possible spin arrangements. This spin anisotropy together with the possibility of continuous ambipolar doping turn GdBaCo2O5+x into a model system for studying the competing magnetic interactions, nanoscopic phase separation and accompanying magnetoresistance phenomena.
The oxygen-exchange behavior has been studied in half-doped manganese and cobalt perovskite oxides. We have found that the oxygen diffusivity in Gd0.5Ba0.5MnO 3−δ can be enhanced by orders of magnitude by inducing crystallographic ordering among lanthanide and alkali-earth ions in the A-site sublattice. Transformation of a simple cubic perovskite, with randomly occupied A-sites, into a layered crystal GdBaMn2O5+x (or isostructural GdBaCo2O5+x for cobalt oxide) with alternating lanthanide and alkali-earth planes reduces the oxygen bonding strength and provides disorder-free channels for ion motion, pointing to an efficient way to design new ionic conductors.PACS numbers: 66.30. Hs, 82.47.Ed Oxygen ion conductors -solids exhibiting very fast oxygen diffusion -constitute the basis for such emerging technologies as the membrane oxygen separation or the solid-oxide fuel cell (SOFC) power generation.1,2 . These technologies offer enormous economical and ecological benefits provided high performance materials can be developed: The scientific challenge is to design materials demonstrating high oxygen diffusivity at low enough temperature.In general, a crystal must meet two fundamental requirements to be a good oxygen-ion conductor: (i) it must contain a lot of vacancies in the oxygen sublattice, and (ii) the energy barrier for oxygen migration from one site to another must be fairly small, typically less than ∼ 1 eV. Only a few types of oxides, and perovskites ABO 3 (A is a rare-earth or an alkali-earth element and B is typically a transition metal) among them, have been found to possess these features. 1,2,3,4,5,6,7,8 Doped perovskites, which possess a high electronic conductivity in addition to the ionic one, are considered for using as electrodes in SOFC and as oxygen-selective membranes; for example, strontium-doped lanthanum manganese oxide, La 1−y Sr y MnO 3−δ , is a standard cathode material for SOFC applications operating at temperatures around 1000• C. 6 Recently, serious efforts are made to reduce the operation temperature of SOFC, and for the operation at 700 -800• C, strontium-doped lanthanum cobalt oxide, La 1−y Sr y CoO 3−δ , is considered to be the most promising cathode material.7,8 The performance of perovskite oxides has been already optimized as much as possible mostly by means of various ion substitutions in both A and B sublattices, 6,7,8,9 but they still fail to operate at low enough temperatures ∼ 500• C required for successful commercialization of the fuel cell technology.In this Letter, we show that the oxygen-ion diffusion in a doped perovskite can be enhanced by orders of magnitude if a simple cubic crystal [schematically shown in Fig.1(a)] transforms into a layered compound with ordered lanthanide and alkali-earth ions [ Fig. 1(b)], which reduces the oxygen bonding strength and provides disorder-free channels for ion motion.Recently, the A-site-ordered manganese and cobalt perovskite oxides have been synthesized by several , which demonstrate the doubling of the unit cell along the c axis ...
The existence of topological superconductors preserving time-reversal symmetry was recently predicted, and they are expected to provide a solid-state realization of itinerant massless Majorana fermions and a route to topological quantum computation. Their first likely example, Cu(x)Bi(2)Se(3), was discovered last year, but the search for new materials has so far been hindered by the lack of a guiding principle. Here, we report point-contact spectroscopy experiments suggesting that the low-carrier-density superconductor Sn(1-x)In(x)Te is accompanied by surface Andreev bound states which, with the help of theoretical analysis, would give evidence for odd-parity pairing and topological superconductivity. The present and previous finding of possible topological superconductivity in Sn(1-x)In(x)Te and Cu(x)Bi(2)Se(3) suggests that odd-parity pairing favored by strong spin-orbit coupling is likely to be a common underlying mechanism for materializing topological superconductivity.
We observed pronounced angular-dependent magnetoresistance (MR) oscillations in a high-quality Bi2Se3 single crystal with the carrier density of 5×10 18 cm −3 , which is a topological insulator with residual bulk carriers. We show that the observed angular-dependent oscillations can be well simulated by using the parameters obtained from the Shubnikov-de Haas oscillations, which clarifies that the oscillations are solely due to the bulk Fermi surface. By completely elucidating the bulk oscillations, this result paves the way for distinguishing the two-dimensional surface state in angulardependent MR studies in Bi2Se3 with much lower carrier density. Besides, the present result provides a compelling demonstration of how the Landau quantization of an anisotropic three-dimensional Fermi surface can give rise to pronounced angular-dependent MR oscillations.
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