arXiv subjects

Satoshi Sasaki

Publications and source records attributed to Satoshi Sasaki.

51 records · Page 3Linked to original sources

arXiv2012

Odd-Parity Pairing and Topological Superconductivity in a Strongly Spin-Orbit Coupled Semiconductor

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 concrete example, CuxBi2Se3, was discovered last year, but the search for new materials has so far been hindered by the lack of guiding principle. Here, we report point-contact spectroscopy experiments showing that the low-carrier-density superconductor Sn_{1-x}In_{x}Te is accompanied with surface Andreev bound states which, with the help of theoretical analysis, give evidence for odd-parity pairing and topological superconductivity. The present and previous finding of topological superconductivity in Sn_{1-x}In_{x}Te and CuxBi2Se3 demonstrates that odd-parity pairing favored by strong spin-orbit coupling is a common underlying mechanism for materializing topological superconductivity.

cond-mat.supr-con↗

arXiv2012

Ambipolar transport in bulk crystals of a topological insulator by gating with ionic liquid

We report that the ionic-liquid gating of bulk single crystals of a topological insulator can control the type of the surface carriers and even results in ambipolar transport. This was made possible by the use of a highly bulk-insulating BiSbTeSe2 system where the chemical potential is located close to both the surface Dirac point and the middle of the bulk band gap. Thanks to the use of ionic liquid, the control of the surface chemical potential by gating was possible on the whole surface of a bulk three-dimensional sample, opening new experimental opportunities for topological insulators. In addition, our data suggest the existence of a nearly reversible electrochemical reaction that causes bulk carrier doping into the crystal during the ionic-liquid gating process.

cond-mat.mes-hall↗

arXiv2012

Manifestation of Topological Protection in Transport Properties of Epitaxial Bi2Se3 Thin Films

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 πBerry phase.

cond-mat.mes-hall↗

arXiv2012

Observation of Hysteretic Transport Due to Dynamic Nuclear Spin Polarization in a GaAs Lateral Double Quantum Dot

We report a new transport feature in a GaAs lateral double quantum dot that emerges only for magnetic field sweeps and shows hysteresis due to dynamic nuclear spin polarization (DNP). This DNP signal appears in the Coulomb blockade regime by virtue of the finite inter-dot tunnel coupling and originates from the crossing between ground levels of the spin triplet and singlet extensively used for nuclear spin manipulations in pulsed gate experiments. The unexpectedly large signal intensity is suggestive of unbalanced DNP between the two dots, which opens up the possibility of controlling electron and nuclear spin states via DC transport.

cond-mat.mes-hall↗

arXiv2012

Fermi level tuning and a large activation gap achieved in the topological insulator Bi_{2}Te_{2}Se by Sn doping

We report the effect of Sn doping on the transport properties of the topological insulator Bi_{2}Te_{2}Se studied in a series of Bi_{2-x}Sn_{x}Te_{2}Se crystals with 0 \leq x \leq 0.02. The undoped stoichiometric compound (x = 0) shows an n-type metallic behavior with its Fermi level pinned to the conduction band. In the doped compound, it is found that Sn acts as an acceptor and leads to a downshift of the Fermi level. For x \geq 0.004, the Fermi level is lowered into the bulk forbidden gap and the crystals present a resistivity considerably larger than 1 Ohmcm at low temperatures. In those crystals, the high-temperature transport properties are essentially governed by thermally-activated carriers whose activation energy is 95-125 meV, which probably signifies the formation of a Sn-related impurity band. In addition, the surface conductance directly obtained from the Shubnikov-de Haas oscillations indicates that a surface-dominated transport can be achieved in samples with several um thickness.

cond-mat.mtrl-sci↗

arXiv2011

Spin-orbital Kondo effect in a parallel double quantum dot

Transport properties of the two-orbital Kondo effect involving both spin and orbital (pseudospin) degrees of freedom were examined in a parallel double quantum dot with a sufficient interdot Coulomb interaction and negligibly small interdot tunneling. The Kondo effect was observed at the interdot Coulomb blockade region with degeneracies of both spin and orbital degrees of freedom. When the orbital degeneracy is lifted by applying a finite detuning, the Kondo resonance exhibits a triple-peak structure, indicating that both spin and orbital contributions are involved.

cond-mat.mes-hall↗

arXiv2011

Optimizing the Bi_(2-x)Sb_(x)Te_(3-y)Se_(y) solid solutions to approach the intrinsic topological insulator regime

To optimize the bulk-insulating behavior in the topological insulator materials having the tetradymite structure, we have synthesized and characterized single-crystal samples of Bi_(2-x)Sb_(x)Te_(3-y)Se_(y) (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 Ohmcm 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.

cond-mat.mtrl-sci↗

arXiv2011

Topological Superconductivity in CuxBi2Se3

A topological superconductor (TSC) is characterized by the topologically-protected gapless surface state that is essentially an Andreev bound state consisting of Majorana fermions. While a TSC has not yet been discovered, the doped topological insulator CuxBi2Se3, which superconducts below ~3 K, has been predicted to possess a topological superconducting state. We report that the point-contact spectra on the cleaved surface of superconducting CuxBi2Se3 present a zero-bias conductance peak (ZBCP) which signifies unconventional superconductivity. Theoretical considerations of all possible superconducting states help us conclude that this ZBCP is due to Majorana Fermions and gives evidence for a topological superconductivity in CuxBi2Se3. In addition, we found an unusual pseudogap that develops below ~20 K and coexists with the topological superconducting state.

cond-mat.supr-con↗

arXiv2011

Observations of two-dimensional quantum oscillations and ambipolar transport in the topological insulator Bi2Se3 achieved by Cd doping

We present a defect-engineering strategy to optimize the transport properties of the topological insulator Bi2Se3 to show a high bulk resistivity and clear quantum oscillations. Starting with a p-type Bi2Se3 obtained by combining Cd doping and a Se-rich crystal-growth condition, we were able to observe a p-to-n-type conversion upon gradually increasing the Se vacancies by post annealing. With the optimal annealing condition where a high level of compensation is achieved, the resistivity exceeds 0.5 Ohmcm at 1.8 K and we observed two-dimensional Shubnikov-de Haas oscillations composed of multiple frequencies in magnetic fields below 14 T.

cond-mat.mtrl-sci↗

arXiv2011

Electrochemical synthesis and superconducting phase diagram of Cu_xBi2Se3

The superconducting Cu_xBi_2Se_3 is an electron-doped topological insulator and is a prime candidate of the topological superconductor which still awaits discovery. The electrochemical intercalation technique for synthesizing Cu_xBi2Se3 offers good control of restricting Cu into the van-der-Waals gap and yields samples with shielding fractions of up to ~50%. We report essential details of this synthesis technique and present the established superconducting phase diagram of T_c vs x, along with a diagram of the shielding fraction vs x. Intriguingly, those diagrams suggest that there is a tendency to spontaneously form small islands of optimum superconductor in this material.

cond-mat.supr-con↗

arXiv2011

Observation of Dirac Holes and Electrons 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.

cond-mat.mtrl-sci↗

arXiv2011

Bulk superconducting phase with a full energy gap in the doped topological insulator Cu_xBi_2Se_3

The superconductivity recently found in the doped topological insulator Cu_xBi_2Se_3 offers a great opportunity to search for a topological superconductor. We have successfully prepared a single-crystal sample with a large shielding fraction and measured the specific-heat anomaly associated with the superconductivity. The temperature dependence of the specific heat suggests a fully-gapped, strong-coupling superconducting state, but the BCS theory is not in full agreement with the data, which hints at a possible unconventional pairing in Cu_xBi_2Se_3. Also, the evaluated effective mass of 2.6m_e (m_e is the free electron mass) points to a large mass enhancement in this material.

cond-mat.supr-con↗

arXiv2010

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.

cond-mat.mtrl-sci↗

arXiv2001

Excitation of Resonators by Electron Beams

Elements of a vector theory of open resonators and experiments on excitation of a fundamental mode with transverse and longitudinal polarization in such resonators are discussed.

physics.class-ph↗

arXiv2000

Excitation of resonators by electron beams

In this paper the main consequences of the vector theory of excitation of resonators by particle beams are presented. Some features of excitation of broadband radiation in longitudinal modes of the enclosed and open resonators are discussed.

physics.class-ph↗