Emergence of metallic surface states and negative differential conductance in thin β\beta-FeSi2_2 films on Si(001)

The electronic properties of the surface of $\beta$-FeSi$_2$ have been debated for a long while. We studied the surface states of $\beta$-FeSi$_2$ films grown on Si(001) substrates using scanning tunnelling microscopy (STM) and spectroscopy (STS), with the aid of density functional theory (DFT) calculations. STM simulations using the surface model proposed by Romanyuk et al. [Phys. Rev. B 90, 155305 (2014)] reproduce the detailed features of experimental STM images. The result of STS showed metallic surface states in accordance with theoretical predictions. The Fermi level was pinned by a surface state that appeared in the bulk band gap of the $\beta$-FeSi$_2$ film, irrespective of the polarity of the substrate. We also observed negative differential conductance at $\sim$0.45 eV above the Fermi level in STS measurements performed at 4.5 K, reflecting the presence of an energy gap in the unoccupied surface states of $\beta$-FeSi$_2$.Comment: 16 pages, 5 figure

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

Emergence of metallic surface states and negative differential conductance in thinβ-FeSi2films on Si(001)

The electronic properties of the surface of $\beta$-FeSi$_2$ have been debated for a long \textcolor{blue}. We studied the surface states of $\beta$-FeSi$_2$ films grown on Si(001) substrates using scanning tunnelling microscopy (STM) and spectroscopy (STS), with the aid of density functional theory (DFT) calculations. STM simulations using the surface model proposed by Romanyuk \textit{et al.} [Phys. Rev. B 90, 155305 (2014)] reproduce the detailed features of experimental STM images. The result of STS showed metallic surface states in accordance with theoretical predictions. The Fermi level was pinned \textcolor{blue}{by a} surface state that appeared in the bulk band gap of the $\beta$-FeSi$_2$ film, irrespective of \textcolor{blue}{the polarity of the} substrate. We also observed negative differential conductance at $\sim$0.45 eV above the Fermi level in STS measurements performed at 4.5 K, reflecting the presence of an energy gap in the unoccupied surface states of $\beta$-FeSi$_2$