Simultaneous enhancement in electrical conductivity and Seebeck coefficient by single-to double-valley transition in a Dirac-like band

SnTe possesses a single-to double-valley transition in the conduction band minimum when a compressive strain is applied.Through a tight-binding analysis,it is shown that the variation of the band structure is attributed to the strain-induced delocalization of both the Sn-5s orbitals and Te-5p orbitals with different angular momenta.This effect can largely increase the electron density of states near the band edge and thus keep the Fermi level of the compressed SnTe closer to it,where the electrons have lower scattering rates.The strain-induced double valleys lead to simultaneous increases in the electrical conductivity and the Seebeck coefficient and thereby nearly four times the enhancement of the power factor at the doping concentration of 5×10^(19) cm^(–3).This work suggests a feasible concept that can be employed to promote the power factor of a Dirac semiconductor via manipulating the valley degeneracy in the conduction band minimum.

Origins of three-dimensional charge and two-dimensional phonon transports in Pnma phase PbSnSe_(2)thermoelectric crystal

Recently,PbSnSe_(2)alloy was found to exhibit a large hysteresis effect on transport properties,demonstrating its significant potential for thermoelectric applications.Using ab initio approaches,we studied the carrier transport properties of PbSnSe_(2)crystal,which is a special case of the alloy with the shortest-range order.A peak power factor of 134.2μW cm^(-1)K^(-2)was found along the crossplane direction in the n-type PbSnSe_(2)at a doping concentration of 7×10^(20)cm^(-3)at 700 K.This high power factor originates from delocalized p electrons between intra-plane Pb-Se pairs and between cross-plane Sn-Se pairs that can build up transport channels for conducting electrons,leading to a high electrical conductivity of 5.9×10^(5)S m^(-1).Introducing Pb atoms into Pnma phase SnSe can decrease the phonon group velocities and enhance the phonon-phonon scatterings,leading to a low thermal conductivity of 0.53 W m^(-1)K^(-1)at 700 K along the cross-plane direction.The calculated peak ZT of~3 along the cross-plane direction at an n-type doping concentration of around 5×10^(19)cm^(-3),which represents a theoretical upper limit for an idealized PbSnSe_(2)crystal.This work interprets the origins of three-dimensional charge and two-dimensional phonon transport behavior in PbSnSe_(2)and demonstrates that such crystals are promising high-performance thermoelectric semiconductors.