The widespread use of thermoelectric technology is constrained by a relatively low conversion efficiency of the bulk alloys, which is evaluated in terms of a dimensionless figure of merit ( zT ). The zT of bulk alloys can be improved by reducing lattice thermal conductivity through grain boundary and point-defect scattering, which target low- and high-frequency phonons. Dense dislocation arrays formed at low-energy grain boundaries by liquid-phase compaction in Bi 0.5 Sb 1.5 Te 3 (bismuth antimony telluride) effectively scatter midfrequency phonons, leading to a substantially lower lattice thermal conductivity. Full-spectrum phonon scattering with minimal charge-carrier scattering dramatically improved the zT to 1.86 ± 0.15 at 320 kelvin (K). Further, a thermoelectric cooler confirmed the performance with a maximum temperature difference of 81 K, which is much higher than current commercial Peltier cooling devices.

http://science.sciencemag.org/content/sci/348/6230/109.full.pdf

Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics

Topological insulators represent a new quantum state of matter which is characterized by peculiar edge or surface states that show up due to a topological character of the bulk wave functions. This review presents a pedagogical account on topological insulator materials with an emphasis on basic theory and materials properties. After presenting a historical perspective and basic theories of topological insulators, it discusses all the topological insulator materials discovered as of May 2013, with some illustrative descriptions of the developments in materials discoveries in which the author was involved. A summary is given for possible ways to confirm the topological nature in a candidate material. Various synthesis techniques as well as the defect chemistry that are important for realizing bulk-insulating samples are discussed. Characteristic properties of topological insulators are discussed with an emphasis on transport properties. In particular, the Dirac fermion physics and the resulting peculiar quantum oscillation patterns are discussed in detail. It is emphasized that proper analyses of quantum oscillations make it possible to unambiguously identify surface Dirac fermions through transport measurements. The prospects of topological insulator materials for elucidating novel quantum phenomena that await discovery conclude the review.

Topological Insulator Materials

Bismuth telluride based thermoelectric materials have been commercialized for a wide range of applications in power generation and refrigeration. However, the poor machinability and susceptibility to brittle fracturing of commercial ingots often impose significant limitations on the manufacturing process and durability of thermoelectric devices. In this study, melt spinning combined with a plasma-activated sintering (MS-PAS) method is employed for commercial p-type zone-melted (ZM) ingots of Bi_0.5Sb_1.5Te_3. This fast synthesis approach achieves hierarchical structures and in-situ nanoscale precipitates, resulting in the simultaneous improvement of the thermoelectric performance and the mechanical properties. Benefitting from a strong suppression of the lattice thermal conductivity, a peak ZT of 1.22 is achieved at 340 K in MS-PAS synthesized structures, representing about a 40% enhancement over that of ZM ingots. Moreover, MS-PAS specimens with hierarchical structures exhibit superior machinability and mechanical properties with an almost 30% enhancement in their fracture toughness, combined with an eightfold and a factor of six increase in the compressive and flexural strength, respectively. Accompanied by an excellent thermal stability up to 200 °C for the MS-PAS synthesized samples, the MS-PAS technique demonstrates great potential for mass production and large-scale applications of Bi_2Te_3 related thermoelectrics.

Mechanically robust BiSbTe alloys with superior thermoelectric performance: A case study of stable hierarchical nanostructured thermoelectric materials

Topological superconductors represent a newly predicted phase of matter that is topologically distinct from conventional superconducting condensates of Cooper pairs. As a manifestation of their topological character, topological superconductors support solid-state realizations of Majorana fermions at their boundaries. The recently discovered superconductor Cu(x)Bi(2)Se(3) has been theoretically proposed as an odd-parity superconductor in the time-reversal-invariant topological superconductor class, and point-contact spectroscopy measurements have reported the observation of zero-bias conductance peaks corresponding to Majorana states in this material. Here we report scanning tunneling microscopy measurements of the superconducting energy gap in Cu(x)Bi(2)Se(3) as a function of spatial position and applied magnetic field. The tunneling spectrum shows that the density of states at the Fermi level is fully gapped without any in-gap states. The spectrum is well described by the Bardeen-Cooper-Schrieffer theory with a momentum independent order parameter, which suggests that Cu(x)Bi(2)Se(3) is a classical s-wave superconductor contrary to previous expectations and measurements.

/pdf/experimental-evidence-for-s-wave-pairing-symmetry-in-46i2jmlhq9.pdf

Experimental evidence for s-wave pairing symmetry in superconducting Cu(x)Bi2Se3 single crystals using a scanning tunneling microscope

http://absimage.aps.org/image/MAR15/MWS_MAR15-2014-000628.pdf

Mechanical robust BiSbTe alloys with superior thermoelectric performance:A case study of stable hierarchical nanostructured thermoelectric materials

Lattice constant of Bi2Se3 and Sb2Te3 crystals is determined by x-ray powder diffraction measurement in a wide temperature range. Linear thermal expansion coefficients (α) of the crystals are extracted, and considerable anisotropy between α|| and α⊥ is observed. The low temperature values of α can be fit well by the Debye model, while an anomalous behavior at above 150 K is evidenced and explained. Gruneisen parameters of the materials are also estimated at room temperature.

/pdf/thermal-expansion-coefficients-of-bi2se3-and-sb2te3-crystals-4c8n2bzzq1.pdf

Thermal expansion coefficients of Bi2Se3 and Sb2Te3 crystals from 10 K to 270 K

Inelastic light scattering spectra of Bi2Se3 and Sb2Te3 single crystals have been measured over the temperature range from 5 K to 300 K. The temperature dependence of dominant A1g2 phonons shows similar behavior in both materials. The temperature dependence of the peak position and linewidth is analyzed considering the anharmonic decay of optical phonons and the material thermal expansion. This work suggests that Raman spectroscopy can be used for thermometry in Bi2Se3- and Sb2Te3-based devices in a wide temperature range.

/pdf/temperature-dependence-of-raman-active-optical-phonons-in-1m6ax0plzt.pdf

Temperature dependence of Raman-active optical phonons in Bi2Se3 and Sb2Te3

Lattice constant of Bi$_2$Se$_3$ and Sb$_2$Te$_3$ crystals is determined by X-ray powder diffraction measurement in a wide temperature range. Linear thermal expansion coefficients ($\alpha$) of the crystals are extracted, and considerable anisotropy between $\alpha_\parallel$ and $\alpha_\perp$ is observed. The low temperature values of $\alpha$ can be fit well by the Debye model, while an anomalous behavior at above 150 K is evidenced and explained. Gruneisen parameters of the materials are also estimated at room temperature.

Thermal Expansion Coefficients of Bi$_2$Se$_3$ and Sb$_2$Te$_3$ Crystals from 10 K to 270 K

Inelastic light scattering spectra of Bi_2Se_3 and Sb_2Te_3 single crystals have been measured over the temperature range from 5 K to 300 K. The temperature dependence of dominant A^{2}_{1g} phonons shows similar behavior in both materials. The temperature dependence of the peak position and linewidth is analyzed considering the anharmonic decay of optical phonons and the material thermal expansion. This work suggests that Raman spectroscopy can be used for thermometry in Bi_2Se_3- and Sb_2Te_3-based devices in a wide temperature range.

Temperature dependence of Raman-active optical phonons in Bi_2Se_3 and Sb_2Te_3

We perform a point-contact Andreev reflection spectroscopic study of the topological superconducting material, Cu0.25Bi2Se3, in the ballistic regime using a normal-metal gold tip. We observe distinct point-contact spectra on the superconducting and non-superconducting regions of the crystal surface: the former shows a marked zero-bias conductance peak, indicative of unconventional superconductivity, while the latter exhibits a pseudogap-like feature. In both cases the measured differential conductance spectra exhibit a large linear background, preventing direct quantitative comparison with theory. We attribute this background to inelastic scattering at the tip-sample interface, and compare the background-subtracted spectra with a single-band p-wave model.

Xunchi Chen

Papers

Thermal expansion coefficients of Bi2Se3 and Sb2Te3 crystals from 10 K to 270 K

Temperature dependence of Raman-active optical phonons in Bi2Se3 and Sb2Te3

Thermal Expansion Coefficients of Bi$_2$Se$_3$ and Sb$_2$Te$_3$ Crystals from 10 K to 270 K

Temperature dependence of Raman-active optical phonons in Bi_2Se_3 and Sb_2Te_3

Point-contact Andreev reflection spectroscopy of candidate topological superconductor Cu0.25Bi2Se3