Topological insulators are known to their metallic surface states, a result of strong-spin-orbital coupling, that show unique surface transport phenomenon. But these surface transports are buried in presence of metallic bulk conduction. We synthesized very high quality Bi2Te2Se single crystals by modified Bridgman method, that possess high bulk resistivity of >20 Ωcm below 20 K, whereas the bulk is mostly inactive and surface transport dominates. Temperature dependence resistivity follows the activation law like a gap semiconductor in temperature range 20-300 K. We designed a special measurement geometry, which aims to extract the surface transport from the bulk. This special geometry is applied to measure the resistance and found that Bi2Te2Se single crystal exhibits a cross over from bulk to surface conduction at 20 K. Simultaneously, the material also shows strong evidence of weak anti-localization in magneto-transport due to the protection against scattering by conducting surface states. This novel simple geometry is an easy route to find the evidence of surface transport in topological insulators, which are the promising materials for future spintronic applications.Topological insulators (TIs) have recently attracted significant attention since they are materialized into a new state of quantum matters, and they possess topologically protected metallic states on their edges or surfaces.1,2 These conducting SSs originate from the inversion of bulk bands due to the results of strong-spin-orbital coupling. Therefore, these novel metallic states are a subject of intensive investigations because of not only its fundamental novelty but also its high potential applications for spintronics devices 3 and quantum computations. 4In 3D TIs, the topological surface states have been successfully investigated by surface-sensitive techniques as angle-resolved photoemission spectroscopy 5,6 and scanning tunneling microscopy 7,8 . However, the evidences of transport through the surface sates remain a challenge due to the presence of the parallel bulk conducting channels that usually dominate the transport properties. 9-11Various strategies to improve surface conductivity, e.g., studying thin films or nano-flacks 6,12-16 , gating 15,17,18 , and doping 10,11,19 have been employed. Among the all known TIs, the ternary tetradymite Bi 2 Te 2 Se (BTS) has largely been investigated material that gives excellent performance of surface in temperature dependence resistivity 6,[12][13][14][15][16]20 . It shows large bulk resistivity due to the nearly perfect crystalline structure, and it has been predicted nearly perfect Dirac cones in terms of less entanglement of bulk and surface states 6,16,19,20 . The quantum oscillations have also been observed in BTS 20-22 and doped BTS 10,19 bulk samples, and they show surfaceddominated transport contributes up to 70% of the total conductance 10,20 . Besides showing this metallic behavior, excellent performance of the surface states of TIs appears in magneto-conductance in effect of weak antiloca...

The surface band bending tunes considerably the surface band structures and transport properties in topological insulators. We present a direct measurement of the band bending on the Bi2Se3 by using the bulk sensitive angular-resolved hard x-ray photospectroscopy (HAXPES). We tracked the depth dependence of the energy shift of Bi and Se core states. We estimate that the band bending extends up to about 20 nm into the bulk with an amplitude of 0.23-0.26 eV, consistent with profiles previously deduced from the binding energies of surface states in this material.PACS numbers: 79.60. Bm, 03.65.Vf Topological insulators (TIs), a new quantum state, are characterized by robust metallic surface states inside the bulk energy gap, which are due to the topology of bulk band structures.1-4 A large amount of efforts were devoted to observe the topological surface states of many TI materials 5 (and references therein) by surface-sensitive experiments. Specially, Bi 2 Se 3 is one of the most extensively studied TI materials because of its simple Diractype surface states and large bulk gap. 6-8Surface band bending (SBB) effects of Bi 2 Se 3 has been commonly observed in angle-resolved photoemission spectra 8,9 (ARPES) and transport measurements 10,11 . The SBB is usually caused by surface degrading in ambient environment and surface doping 12-15 , with a downshift of the surface Dirac point, indicating an electrondoped surface 16 . SBB induces a quantum confinement effect 17 and modifies the surface and bulk bands dramatically. A clear feature of SBB in Bi 2 Se 3 is a pair of Rashba-splitting bands above the Dirac cone. In transport experiments, SBB is also supposed to affect the measurement in a considerable way by directly tuning the bulk and surface charge carrier densities.11,18-20 So far, this surface band bending has only been deduced 13,14 from Rashba-splitting of the conduction bands measured by ARPES, that is mainly sensitive to several surface atomic layers, although SBB is predicted to extends in an order of 10 nm distance from the surface into the bulk. A direct measurement of SBB from the surface into the bulk region is yet to be performed.In this Letter, we reported the direct observation of SBB on the Bi 2 Se 3 surface by HArd X-ray PhotoElectron Spesctroscopy (HAXPES), a bulk sensitive method. The hard x-ray excitation (∼8keV) produces photoelectrons with high kinetic energy and consequently high inelastic mean free path (λ) resulting in an enhanced probing depth. HAXPES has been successfully utilized in the study of Heusler TIs. 21,22 The SBB can be directly measured in photoelectron spectroscopy by controlling the escape depth in the photoemission process to track the depth dependence of core level energies. Such controlling can be achieved by changing the photon energy and consequently the inelastic mean free path, as demonstrated by Himpsel et. al 23 for low photon energy regime. The precision of this approach however depends on the energy distribution and the determination of the Fermi edges for differen...

Optically transparent conducting materials are essential in modern technology. These materials are used as electrodes in displays, photovoltaic cells, and touchscreens; they are also used in energyconserving windows to reflect the infrared spectrum. The most ubiquitous transparent conducting material is tin-doped indium oxide (ITO), a wide-gap oxide whose conductivity is ascribed to n-type chemical doping. Recently, it has been shown that ionic liquid gating can induce a reversible, nonvolatile metallic phase in initially insulating films of WO 3 . Here, we use hard X-ray photoelectron spectroscopy and spectroscopic ellipsometry to show that the metallic phase produced by the electrolyte gating does not result from a significant change in the bandgap but rather originates from new in-gap states. These states produce strong absorption below ∼1 eV, outside the visible spectrum, consistent with the formation of a narrow electronic conduction band. Thus WO 3 is metallic but remains colorless, unlike other methods to realize tunable electrical conductivity in this material. Core-level photoemission spectra show that the gating reversibly modifies the atomic coordination of W and O atoms without a substantial change of the stoichiometry; we propose a simple model relating these structural changes to the modifications in the electronic structure. Thus we show that ionic liquid gating can tune the conductivity over orders of magnitude while maintaining transparency in the visible range, suggesting the use of ionic liquid gating for many applications.electrolyte gating | metal-insulator transition | transparent conducting oxide | TCO T ungsten trioxide (WO 3 ) is a d 0 transition metal oxide with an energy band gap of about 3 eV. WO 3 is a transparent insulator but has been shown to become metallic and even superconducting when doped with significant amounts of electropositive elements such as Rb (1), K (2) or Cs (3), and H (4). The optical transmittance of WO 3 can also be manipulated by the electrochemical insertion of small cations, such as H + or Li + , which makes WO 3 extremely desirable for smart window applications (5-7). Both fundamental studies and potential applications of WO 3 require the control of charge carriers; in addition to chemical doping, this control can also be achieved by the growth of oxygen deficient structures (4, 5, 8-10). Here we investigate an alternative method for controlling the electronic properties via ionic liquid gating. Previous work on VO 2 thin films has shown that liquid electrolyte gating produces structural modifications and leads to the suppression of the metal-insulator transition (11,12). Recent gating experiments on epitaxial WO 3 thin films indicate changes in the out-of-plane lattice parameter, concomitant with the metallization throughout the film volume (13). In the work presented here, we correlate the structural changes with modification of electronic energy bands, which result in a transparent conducting oxide, thus yielding a much more complete understanding of such ...

The development of new phases of matter at oxide interfaces and surfaces by extrinsic electric fields is of considerable significance both scientifically and technologically. Vanadium dioxide (VO2), a strongly correlated material, exhibits a temperature-driven metal-to-insulator transition, which is accompanied by a structural transformation from rutile (high-temperature metallic phase) to monoclinic (low-temperature insulator phase). Recently, it was discovered that a low-temperature conducting state emerges in VO2 thin films upon gating with a liquid electrolyte. Using photoemission spectroscopy measurements of the core and valence band states of electrolyte-gated VO2 thin films, we show that electronic features in the gate-induced conducting phase are distinct from those of the temperature-induced rutile metallic phase. Moreover, polarization-dependent measurements reveal that the V 3d orbital ordering, which is characteristic of the monoclinic insulating phase, is partially preserved in the gate-induced metallic phase, whereas the thermally induced metallic phase displays no such orbital ordering. Angle-dependent measurements show that the electronic structure of the gate-induced metallic phase persists to a depth of at least ∼40 Å, the escape depth of the high-energy photoexcited electrons used here. The distinct electronic structures of the gate-induced and thermally induced metallic phases in VO2 thin films reflect the distinct mechanisms by which these states originate. The electronic characteristics of the gate-induced metallic state are consistent with the formation of oxygen vacancies from electrolyte gating.

Phase transitions and magnetic properties of shape-memory materials can be tailored by tuning the size of the constituent materials, such as nanoparticles. However, owing to the lack of suitable synthetic methods for size-controlled Heusler nanoparticles, there is no report on the size dependence of their properties and functionalities. In this contribution, we present the first chemical synthesis of size-selected Co-Ni-Ga Heusler nanoparticles. We also report the structure and magnetic properties of the biphasic Co-Ni-Ga nanoparticles with sizes in the range of 30-84 nm, prepared by a SBA-15 nanoporous silicatemplated approach. The particle sizes could be readily tuned by controlling the loading and concentration of the precursors. The fractions and crystallite sizes of each phase of the Co-Ni-Ga nanoparticles are closely related to their particle size. Enhanced magnetization and decreased coercivity are observed with increasing particle size. The Curie temperature (T (c)) of the Co-Ni-Ga nanoparticles also depends on their size. The 84 nm-sized particles exhibit the highest T (c) (ae 1,174 K) among all known Heusler compounds. The very high Curie temperatures of the Co-Ni-Ga nanoparticles render them promising candidates for application in high-temperature shape memory alloy-based devices