There is widespread interest in the search for materials that would allow the fabrication of more efficient thermoelectric devices for cooling and power generation applications. Here, we report a large increase in the thermoelectric power of p-doped antimony bismuth telluride alloys upon pressure tuning under nonhydrostatic compression conditions. Together with measurements of the electrical conductivity and an upper bound estimated for the thermal conductivity under pressure, these results indicate that values of the dimensionless thermoelectric figure of merit, ZT, in excess of 2 have been achieved, substantially larger than the best observed values in bulk materials to date. We suggest an explanation for the observed behavior and strategies for attempting to reproduce it at ambient pressure.

Large Improvement in Thermoelectric Properties in Pressure-Tuned p-Type Sb1.5Bi0.5Te3

In a semimetal, both electrons and holes contribute to the density of states at the Fermi level. The small band overlaps and multiband effects engender novel electronic properties. We show that a moderate hydrostatic pressure effectively suppresses the band gap in the elemental semiconductor black phosphorus. An electronic topological transition takes place at approximately 1.2 GPa, above which black phosphorus evolves into a semimetal state that is characterized by a colossal positive magnetoresistance and a nonlinear field dependence of Hall resistivity. The Shubnikov-de Haas oscillations detected in magnetic field reveal the complex Fermi surface topology of the semimetallic phase. In particular, we find a nontrivial Berry phase in one Fermi surface that emerges in the semimetal state, as evidence of a Dirac-like dispersion. The observed semimetallic behavior greatly enriches the material property of black phosphorus and sets the stage for the exploration of novel electronic states in this material.

Pressure-Induced Electronic Transition in Black Phosphorus.

Topological superconductivity is one of most fascinating properties of topological quantum matters that was theoretically proposed and can support Majorana Fermions at the edge state. Superconductivity was previously realized in a Cu-intercalated Bi2Se3 topological compound or a Bi2Te3 topological compound at high pressure. Here we report the discovery of superconductivity in the topological compound Sb2Te3 when pressure was applied. The crystal structure analysis results reveal that superconductivity at a low-pressure range occurs at the ambient phase. The Hall coefficient measurements indicate the change of p-type carriers at a low-pressure range within the ambient phase, into n-type at higher pressures, showing intimate relation to superconducting transition temperature. The first principle calculations based on experimental measurements of the crystal lattice show that Sb2Te3 retains its Dirac surface states within the low-pressure ambient phase where superconductivity was observed, which indicates a strong relationship between superconductivity and topology nature.

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Superconductivity in Topological Insulator Sb2Te3 Induced by Pressure

A numerical procedure has been used for the prediction of the Seebeck coefficient of a crystalline material based on its electronic band structure with the goal of testing this approach on the simple polymers polythiophene and polyaminosquaraine. The investigation of several representative materials, including the crystalline solids $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Zn}}_{4}{\mathrm{Sb}}_{3}$ and $\mathrm{Au}{\mathrm{In}}_{2}$ and these polymers, under ambient or external pressure conditions, indicates that Seebeck coefficients can be calculated within the rigid-band and constant-relaxation-time approximations. The results are in semiquantitative agreement with experiment and provide a basic understanding of the mechanisms for thermopower. These theoretical results together with previous similar studies show that band-structure calculations can be used to guide the rational design of high-performance thermoelectric materials. We also suggest that appropriate and specially engineered doped low-band-gap polymers may be promising candidate materials for thermoelectric applications.

Theoretical studies on the thermopower of semiconductors and low-band-gap crystalline polymers

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Electrical resistivity, thermoelectric power, and high-pressure x-ray-diffraction measurements are carried out to investigate the anomaly observed earlier in fusion data around 3 GPa in the intermetallic compound ${\mathrm{AuIn}}_{2}.$ While the imaging plate high-pressure angle-dispersive data indicate a structural phase transition beyond 8 GPa, the thermoelectric power shows a peak around 2 GPa, indicating the occurrence of an electronically driven isostructural transition. The first-principles linearized muffin-tin orbital calculations reveal that this transition is brought about by interception of the Fermi level by the energy-band maximum. The Lifshitz nature of this transition is responsible for the anomaly in the high-pressure electrical and fusion data.

A. Jayaraman

Papers

Electronic topological and structural transition in AuIn 2 under pressure