Properties of crystalline bismuth selenide and its use as a Hall effect magnetometer

Single crystals of n-type Bi2Se3 grown by the Bridgman technique are found to make excellent Hall effect magnetometers. Plots of Hall resistivity sub yx against magnetic field B to 10 tesla are linear to within 1 percent. Furthermore, the slope of the sub yx against B curve varies by about 1 percent in the region 1.1 to 35 K and by less than 20 percent in the region 1.1 to 300 K. Analysis of galvanomagnetic measurements indicate the samples have semimetallic densities of approximately 10 to the 25th power/cu cm, with two band conduction and near carrier compensation. Reflectivity measurements suggest a band gap of approximately 0.08 eV for the samples. The temperature dependence of mobility is also measured. A series of 50 direct immersions into liquid helium and liquid nitrogen demonstrate the reliability of Bi2Se3 magnetometers for cryogenic use

Galvanomagnetic and thermonagnetic effects, in quenched bismuth

An experimental study of the quenching effects on the gailvanomagnetic and thermomagnetic properties of bismuth single crystals is presented. Measurements made include components of the magneto resistivity and magnetothermoelectric power tensors at low and intermediate magnetic fields for samples in both the annealed and quenched conditions, at temperatures between 77 K and room temperature. Marked effects of quenching on each set of tensor components have been observed. After quenching the anisotropy ratio p(_11)/ p(_33) of the zero field resistivity inverts from 0.9446 to 1.030 and the thermomagnetic data show a general reduction in magnetic field dependence. The quenching effects anneal out below room temperature. To allow a detailed quantitative investigation, a formalism has been developed by which the thermomagnetic data and the predictions of both the phenomenological theory and o f the band transport theory can be compared on the same basis as the galvanomagnetic effects. The expressions provide for the first time an analytical explanation for the Umkehr effect and lead to its prediction. The occurrence of Umkehr effect and sign reversal in the thermomagnetic power of bismuth directly follows from the nature o f the Fermi surface. Measurements a t low and intermediate magnetic fields have been analysed in terms of a two-band multi - valley Fermi surface model, using a least-means-square procedure. Reasonable agreement obtains between the model parameters computed from different sources. Magneto resistivity data at low and intermediate fields evidence consistent quenching-induced changes on the model parameters. Quenching results in an increase in carrier densities from 4.4 10(^23)m(^-3) in annealed state to 4.6 10(^23)m(^-3) and 5.3 10(^23)m(^-3) for electrons and holes respectively. Carrier mobilities in the xy-plane are reduced but those along the trigonal (z) axis are increased slightly. The energy separation between the band edges increases markedly. Quenched-in defects have predominantly accepter-like character

Nanometer Structured Epitaxial Films and Foliated Layers Based on Bismuth and Antimony Chalcogenides with Topological Surface States

The thermoelectric and galvanomagnetic properties of nanometer structured epitaxial films and foliated layers based on bismuth and antimony chalcogenides were investigated, and an increase in the figure of merit Z up to 3.85 × 10-3 K-1 was observed in the Bi0.5Sb1.5Te3 films over the temperature range of 180–200 K. It is shown that an increase in the Seebeck coefficient and the change in the slope on temperature, associated with changes in the effective scattering parameter of charge carriers and strong anisotropy of scattering in the films, lead to enhance power factor due to the growth of the effective mass of the density of states. These features are consistent with the results of research of oscillation effects in strong magnetic fields at low temperatures and research of Raman scattering at normal and high pressures in the foliated layers of solid solutions (Bi, Sb)2(Te, Se)3, in which the topological Dirac surface states were observed. The unique properties of topological surface states in the investigated films and layers make topological insulators promising material for innovation nanostructured thermoelectrics

Angle dependence of the orbital magnetoresistance in bismuth

We present an extensive study of angle-dependent transverse magnetoresistance in bismuth, with a magnetic field perpendicular to the applied electric current and rotating in three distinct crystallographic planes. The observed angular oscillations are confronted with the expectations of semi-classic transport theory for a multi-valley system with anisotropic mobility and the agreement allows us to quantify the components of the mobility tensor for both electrons and holes. A quadratic temperature dependence is resolved. As Hartman argued long ago, this indicates that inelastic resistivity in bismuth is dominated by carrier-carrier scattering. At low temperature and high magnetic field, the threefold symmetry of the lattice is suddenly lost. Specifically, a $2\pi/3$ rotation of magnetic field around the trigonal axis modifies the amplitude of the magneto-resistance below a field-dependent temperature. By following the evolution of this anomaly as a function of temperature and magnetic field, we mapped the boundary in the (field, temperature) plane separating two electronic states. In the less-symmetric state, confined to low temperature and high magnetic field, the three Dirac valleys cease to be rotationally invariant. We discuss the possible origins of this spontaneous valley polarization, including a valley-nematic scenario.Comment: 15 pages, 14 figure

Wet-chemical synthesis of enhanced-thermopower Bi1-xSbx nanowire composites for solid-state active cooling of electronics

In 1993, Hicks and Dresselhaus [Thermoelectric figure of merit of a one-dimensional conductor, Phys. Rev. B 47, 16631 (1993).] suggested that Bi nanowires could result in values of the thermoelectric figure of merit zT > 1. The Dresselhaus group also calculated a ternary phase diagram for Bi1-xSbx nanowires as a function of x and wire diameter. This manuscript reports a wet-chemical method to synthesize Bi1-xSbx-silica nanowire composites. Resistivity, Hall electron concentration, electron mobility, Seebeck and Nernst coefficients, and thermal conductivity of composites are measured and compared to bulk polycrystalline Bi1-xSbx samples prepared either by ingot casting or by the same wet chemistry but without nanostructuring. A clear increase of the thermopower in 20-nm Bi94Sb6-silica is reported when compared to bulk samples, and the values are among the highest found in the literature from 300 to 380 K, even though the electron concentration is higher than in the bulk. This suggests that consistent with theory, size quantization is responsible for the thermopower increase

Huge linear magnetoresistance due to open orbits in γ\gamma-PtBi2_2

Some single-crystalline materials present an electrical resistivity which decreases between room temperature and low temperatures at zero magnetic field as in a good metal and switches to a nearly semiconductinglike behavior at low temperatures with the application of a magnetic field. Often, this is accompanied by a huge and nonsaturating linear magnetoresistance which remains difficult to explain. Here we present a systematic study of the magnetoresistance in single-crystal $\gamma$-PtBi$_2$. We observe that the angle between the magnetic field and the crystalline $c$ axis fundamentally changes the magnetoresistance, going from a saturating to a nonsaturating magnetic field dependence. In between, there is one specific angle where the magnetoresistance is perfectly linear with the magnetic field. We show that the linear dependence of the nonsaturating magnetoresistance is due to the formation of open orbits in the Fermi surface of $\gamma$-PtBi$_2$.Comment: 12 pages, 8 figures including Supplementary Materia

Phase diagram of bismuth in the extreme quantum limit

Elemental bismuth provides a rare opportunity to explore the fate of a three-dimensional gas of highly mobile electrons confined to their lowest Landau level. Coulomb interaction, neglected in the band picture, is expected to become significant in this extreme quantum limit with poorly understood consequences. Here, we present a study of the angular-dependent Nernst effect in bismuth, which establishes the existence of ultraquantum field scales on top of its complex single-particle spectrum. Each time a Landau level crosses the Fermi level, the Nernst response sharply peaks. All such peaks are resolved by the experiment and their complex angular-dependence is in very good agreement with the theory. Beyond the quantum limit, we resolve additional Nernst peaks signaling a cascade of additional Landau sub-levels caused by electron interaction