arXiv subjects

A. A. Taskin

Publications and source records attributed to A. A. Taskin.

At least 37 records · Page 2Linked to original sources

arXiv2015

Ferromagnetism in Cr-doped topological insulator TlSbTe2

We have synthesized a new ferromagnetic topological insulator by doping Cr to the ternary topological-insulator material TlSbTe2. Single crystals of Tl_{1-x}Cr_{x}SbTe2 were grown by a melting method and it was found that Cr can be incorporated into the TlSbTe2 matrix only within the solubility limit of about 1%. The Curie temperature θ_c was found to increase with the Cr content but remained relatively low, with the maximum value of about 4 K. The easy axis was identified to be the c-axis and the saturation moment was 2.8 μ_B (Bohr magneton) at 1.8 K. The in-plane resistivity of all the samples studied showed metallic behavior with p-type carriers. Shubnikov-de Hass (SdH) oscillations were observed in samples with the Cr-doping level of up to 0.76%. We also tried to induce ferromagnetism in TlBiTe2 by doping Cr, but no ferromagnetism was observed in Cr-doped TlBiTe2 crystals within the solubility limit of Cr which turned out to be also about 1%.

cond-mat.mtrl-sci↗

arXiv2014

Pb_{5}Bi_{24}Se_{41}: A New Member of the Homologous Series Forming Topological Insulator Heterostructures

We have synthesized Pb_{5}Bi_{24}Se_{41}, which is a new member of the (PbSe)_{5}(Bi2Se3)_{3m} homologous series with m = 4. This series of compounds consist of alternating layers of the topological insulator Bi2Se3 and the ordinary insulator PbSe. Such a naturally-formed heterostructure has recently been elucidated to give rise to peculiar quasi-two-dimensional topological states throughout the bulk, and the discovery of Pb_{5}Bi_{24}Se_{41} expands the tunability of the topological states in this interesting homologous series. The trend in the resistivity anisotropy in this homologous series suggests an important role of hybridization of the topological states in the out-of-plane transport.

cond-mat.mtrl-sci↗

arXiv2014

Top gating of epitaxial (Bi_{1-x}Sb_x)2Te3 topological insulator thin films

The tunability of the chemical potential for a wide range encompassing the Dirac point is important for many future devices based on topological insulators. Here we report a method to fabricate highly efficient top gates on epitaxially grown (Bi_{1-x}Sb_x)2Te3 topological insulator thin films without degrading the film quality. By combining an in situ deposited Al2O3 capping layer and a SiN_x dielectric layer deposited at low temperature, we were able to protect the films from degradation during the fabrication processes. We demonstrate that by using this top gate, the carriers in the top surface can be efficiently tuned from n- to p-type. We also show that magnetotransport properties give evidence for decoupled transport through top and bottom surfaces for the entire range of gate voltage, which is only possible in truly bulk-insulating samples.

cond-mat.mes-hall↗

arXiv2014

Topological Surface Transport in Epitaxial SnTe Thin Films Grown on Bi2Te3

The topological crystalline insulator SnTe has been grown epitaxially on a Bi2Te3 buffer layer by molecular beam epitaxy. In a 30-nm-thick SnTe film, p- and n-type carriers are found to coexist, and Shubnikov--de Haas oscillation data suggest that the n-type carriers are Dirac fermions residing on the SnTe (111) surface. This transport observation of the topological surface state in a p-type topological crystalline insulator became possible due to a downward band bending on the free SnTe surface, which appears to be of intrinsic origin.

cond-mat.mes-hall↗

arXiv2013

Anomalous metallic state above the upper critical field of the conventional three-dimensional superconductor AgSnSe2 with strong intrinsic disorder

We report superconducting properties of AgSnSe2 which is a conventional type-II superconductor in the very dirty limit due to intrinsically strong electron scatterings. While this material is an isotropic three-dimensional (3D) superconductor with a not-so-short coherence length where strong vortex fluctuations are NOT expected, we found that the magnetic-field-induced resistive transition at fixed temperatures becomes increasingly broader toward zero temperature and, surprisingly, that this broadened transition is taking place largely ABOVE the upper critical field determined thermodynamically from the specific heat. This result points to the existence of an anomalous metallic state possibly caused by quantum phase fluctuations in a strongly-disordered 3D superconductor.

cond-mat.supr-con↗

arXiv2013

Checkerboard to Stripe Charge Ordering Transition in TbBaFe2O5

A combined neutron and x-ray diffraction study of TbBaFe2O5 reveals a rare checkerboard to charge ordering transition. TbBaFe2O5 is a mixed valent compound where Fe2+/Fe3+ ions are known to arrange into a stripe charge-ordered state below TV = 291 K, that consists of alternating Fe2+/Fe3+ stripes in the basal plane running along the b direction. Our measurements reveal that the stripe charge-ordering is preceded by a checkerboard charge-ordered phase between TV < T < T* = 308 K. The checkerboard ordering is stabilized by inter-site coulomb interactions which give way to a stripe state stabilized by orbital ordering.

cond-mat.str-el↗

arXiv2012

Fermiology of Strongly Spin-Orbit Coupled Superconductor Sn1-xInxTe and its Implication to Topological Superconductivity

We have performed angle-resolved photoemission spectroscopy of the strongly spin-orbit coupled low-carrier density superconductor Sn1-xInxTe (x = 0.045) to elucidate the electronic states relevant to the possible occurrence of topological superconductivity recently reported for this compound from point-contact spectroscopy. The obtained energy-band structure reveals a small holelike Fermi surface centered at the L point of the bulk Brillouin zone, together with a signature of a topological surface state which indicates that this superconductor is essentially a doped topological crystalline insulator characterized by band inversion and mirror symmetry. A comparison of the electronic states with a band-non-inverted superconductor possessing a similar Fermi surface structure, Pb1-xTlxTe, suggests that the anomalous behavior in the superconducting state of Sn1-xInxTe is likely to be related to the peculiar orbital characteristics of the bulk valence band and/or the presence of a topological surface state.

cond-mat.mes-hall↗

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

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

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↗

arXiv2012

Landau level spectroscopy of surface states in the topological insulator Bi$_{0.91}$Sb$_{0.09}$ 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 $ω\propto \sqrt{H}$. We discuss the ramification of this observation for the surface and bulk properties of topological insulators.

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

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

Berry phase of non-ideal Dirac fermions in topological insulators

A distinguishing feature of Dirac fermions is the Berry phase of $π$ associated with their cyclotron motions. Since this Berry phase can be experimentally assessed by analyzing the Landau-level fan diagram of the Shubnikov-de Haas (SdH) oscillations, such an analysis is widely employed in recent transport studies of topological insulators to elucidate the Dirac nature of the surface states. However, the reported results have usually been unconvincing. Here we show a general scheme for describing the phase factor of the SdH oscillations in realistic surface states of topological insulators, and demonstrate how one could elucidate the Dirac nature in the real experimental data.

cond-mat.mtrl-sci↗

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↗

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↗

arXiv2010

Additional Evidence for the Surface Origin of the Peculiar Angular-Dependent Magnetoresistance Oscillations Discovered in a Topological Insulator Bi_{1-x}Sb_{x}

We present detailed data on the unusual angular-dependent magnetoresistance oscillation phenomenon recently discovered in a topological insulator Bi_{0.91}Sb_{0.09}. Direct comparison of the data taken before and after etching the sample surface gives compelling evidence that this phenomenon is essentially originating from a surface state. The symmetry of the oscillations suggests that it probably comes from the (111) plane, and obviously a new mechanism, such as a coupling between the surface and the bulk states, is responsible for this intriguing phenomenon in topological insulators.

cond-mat.mtrl-sci↗

arXiv2010

Oscillatory angular dependence of the magnetoresistance in a topological insulator Bi_{1-x}Sb_{x}

The angular-dependent magnetoresistance and the Shubnikov-de Haas oscillations are studied in a topological insulator Bi_{0.91}Sb_{0.09}, where the two-dimensional (2D) surface states coexist with a three-dimensional (3D) bulk Fermi surface (FS). Two distinct types of oscillatory phenomena are discovered in the angular-dependence: The one observed at lower fields is shown to originate from the surface state, which resides on the (2\bar{1}\bar{1}) plane, giving a new way to distinguish the 2D surface state from the 3D FS. The other one, which becomes prominent at higher fields, probably comes from the (111) plane and is obviously of unknown origin, pointing to new physics in transport properties of topological insulators.

cond-mat.mtrl-sci↗