Experimental verification of band convergence in Sr and Na codoped PbTe

Scanning tunneling microscopy and transport measurements have been performed to investigate the electronic structure and its temperature dependence in heavily Sr and Na codoped PbTe, which is recognized as one of the most promising thermoelectric materials. Our main findings are as follows: (i) Below T=4.5 K, all carriers are distributed in the first valence band at the L point (L band), which forms tube-shaped Fermi surfaces with concave curvature. With Sr and Na doping, the dispersion of the L band changes, and the band gap increases from 200 meV to 300 meV. (ii) At T=4.5 K, the Fermi energy is located ~100 meV below the edge of the L band for the Sr/Na codoped PbTe. The second valence band at the Sigma point (Sigma band) is lower than the L band by 150 meV, which is significantly smaller than that of pristine PbTe (200 meV). The decrease in the band offset, leading to band convergence, provides a desirable condition for thermoelectric materials.(iii) With increasing temperature, the carrier distribution to the Sigma band starts at T=100 K and we estimate that about 50 percent of the total carriers are redistributed in the Sigma band at T=300 K.Our work demonstrates that scanning tunneling microscopy and angular dependent magnetoresistance measurements are particularly powerful tools to determine the electronic structure and carrier distribution. We believe that they will provide a bird's eye view of the doping strategy towards realizing high-efficiency thermoelectric materials.Comment: 36+12 pages, 4+9 figures, including Supplementary Material

Fermi surface of the organic superconductor (MDT-ST)(I3)0.417 reconstructed by incommensurate potential

The effect of incommensurate potential on Fermi surface (FS), which loses any translational symmetry in the energy bands, is investigated in the organic superconductor (MDT-ST) (I3) 0.417, where MDT-ST is 5H -2-(1,3-dithiol-2- ylidene)-1,3-diselena-4,6-dithiapentalene. The observed three fundamental Shubnikov–de Haas oscillations are explained by the reconstruction with the three times of the incommensurate anion periodicity. This unambiguously verifies the selection rule of the reconstructing vectors in incommensurate crystals. The angular magnetoresistance oscillations show that the FS has a p -type staggered corrugation with two nodes in the interlayer transfer integral

Charge transfer degree and superconductivity of the incommensurate organic superconductor (MDT-TSF)(I3)0.422

The influence of a small change of carrier number on the superconducting transition temperature Tc in the incommensurate organic superconductors is investigated for (MDT-TSF) (I3) 0.422 (MDT-TSF: methylenedithio- tetraselenafulvalene, Tc =4.9 K) in comparison with (MDT-TSF) (AuI2) 0.436 (Tc =4.5 K). Careful estimation of the degree of charge transfer by means of the Raman spectra as well as the Shubnikov–de Haas (SdH) oscillations indicates that the charge-transfer degree 0.422 for the I3 salt is obviously smaller than 0.436 in the AuI2 salt. According to the band calculation, the former salt has smaller density of states. However, the former salt exhibits higher Tc than the latter compounds, and this disagrees with the naive prediction of the BCS theory. The former salt shows considerably large effective cyclotron mass extracted from the SdH oscillations. These observations demonstrate that the strength of the many-body effect is the major factor that determines Tc in these organic superconductors