The surface-to-volume ratio: a key parameter in the thermoelectric transport of topological insulator Bi2Se3 nanowires.

We systematically investigated the role of topological surface states on thermoelectric transport by varying the surface-to-volume ratio (s/v) of Bi2Se3 nanowires. The thermoelectric coefficients of Bi2Se3 nanowires were significantly influenced by the topological surface states with increasing the s/v. The Seebeck coefficient of Bi2Se3 nanowires decreased with increasing the s/v, while the electrical conductivity increased with increasing the s/v. This implies that the influence of metallic surface states become dominant in thermoelectric transport in thin nanowires, and the s/v is a key parameter which determines the total thermoelectric properties. Our measurements were corroborated by using a two-channel Boltzmann transport model.

Local Magnetic Suppression of Topological Surface States in Bi2Te3 Nanowires.

Locally induced, magnetic order on the surface of a topological insulator nanowire could enable room-temperature topological quantum devices. Here we report on the realization of selective magnetic control over topological surface states on a single facet of a rectangular Bi2Te3 nanowire via a magnetic insulating Fe3O4 substrate. Low-temperature magnetotransport studies provide evidence for local time-reversal symmetry breaking and for enhanced gapping of the interfacial 1D energy spectrum by perpendicular magnetic-field components, leaving the remaining nanowire facets unaffected. Our results open up great opportunities for development of dissipation-less electronics and spintronics.

Kinetics of the charge ordering in magnetite below the Verwey temperature.

In this work the kinetics of the charge ordering in magnetite (Fe3O4) below the Verwey transition temperature TV is investigated in time and energy domain. After excitation by a one-second voltage pulse to destruct the charge-ordered state below TV, an alternating current (AC) is used to perturb its recovery process. Upon warming up to above a temperature T(r)(< TV) the charge order recovers despite the ac perturbation, because the temperature-dependent relaxation time becomes shorter than the polarity change of the ac. From the frequency dependence of T(r)(f), an activation energy of ΔE = 126 meV is extracted. Below Tr the real part of the ac conductance Greal follows the relation Greal ∼ f(α) with α = 0.98 ± 0.18, suggesting that the charge reordering is driven by correlated hopping transport. The relaxation time τ = 1/f(T(r)) of the charge-ordered state is determined for temperatures between 70 and 98 K and is extrapolated to τ(TV) = 1.6 ms, continuously slowing down its dynamics as the temperature is decreased.

The influence of a Te-depleted surface on the thermoelectric transport properties of Bi2Te3 nanowires.

We report on thermoelectric transport measurements along the basal plane of several individual, single-crystalline Bi2Te3 nanowires (NWs) with different cross-sectional areas, grown by a vapor-liquid-solid method. Lithographically defined microdevices allowed us to determine the Seebeck coefficient S, electrical conductivity σ, and thermal conductivity κ of individual NWs. The NWs studied show near intrinsic transport properties with low electrical conductivities of around σ = (3.2 ± 0.9) × 104 Ω⁻¹ m⁻¹ at room temperature. We observe a transition of the Seebeck coefficient from positive to negative values (S = +133 µVK⁻¹ to S = -87 µVK⁻¹) with increasing surface-to-volume ratio at room temperature, which can be explained by the presence of an approximately 5 nm thick Te-depleted layer at the surface of the NWs. The thermal conductivities of our NWs are in the range of κ = (1.4 ± 0.4) Wm⁻¹ K⁻¹ at room temperature, which is lower than literature values for bulk Bi2Te3. We attribute this suppression in thermal conductivity to enhanced scattering of phonons at the surface of the NWs. Despite their reduced thermal conductivities, the NWs investigated only show a moderate figure of merit between 0.02 and 0.18 due to their near intrinsic transport properties.

Thermoelectric characterization of bismuth telluride nanowires, synthesized via catalytic growth and post-annealing.

Bi(2) Te(3) nanowires are of significant interest for two fields: nanostructured thermoelectrics and topological insulators. The vapor-liquid-solid method is employed in combination with annealing in a Te atmosphere, to obtain single-crystalline Bi(2) Te(3) nanowires with reproducible electronic transport properties (electrical conductivity and Seebeck coefficient) that are close to those of intrinsic bulk Bi(2) Te(3) .