Topological surface states hybridized with bulk states of Bi-doped PbSb2Te4 revealed in quasiparticle interference

Topological surface states of Bi-doped PbSb2Te4 [Pb(Bi0.20Sb0.80)2Te4] are investigated through analyses of quasiparticle interference (QPI) patterns observed by scanning tunneling microscopy. Interpretation of the experimental QPI patterns in the reciprocal space is achieved by numerical QPI simulations using two types of surface density of states produced by density functional theory calculations or a kp surface state model. We found that the Dirac point (DP) of the surface state appears in the bulk band gap of this material and, with the energy being away from the DP, the isoenergy contour of the surface state is substantially deformed or separated into segments due to hybridization with bulk electronic states. These findings provide a more accurate picture of topological surface states, especially at energies away from the DP, providing valuable insight into the electronic properties of topological insulators.Comment: 7+8 pages, 4+5 figure

Superconductivity in a van der Waals layered quasicrystal

van der Waals (vdW) layered transition-metal chalcogenides are attracting significant attention owing to their fascinating physical properties. This group of materials consists of abundant members with various elements, having a variety of different structures. However, all vdW layered materials studied to date have been limited to crystalline materials, and the physical properties of vdW layered quasicrystals have not yet been reported. Here, we report on the discovery of superconductivity in a vdW layered quasicrystal of Ta1.6Te. The electrical resistivity, magnetic susceptibility, and specific heat of the Ta1.6Te quasicrystal fabricated by reaction sintering, unambiguously validated the occurrence of bulk superconductivity at a transition temperature of ~1 K. This discovery can pioneer new research on assessing the physical properties of vdW layered quasicrystals as well as two-dimensional quasicrystals; moreover, it paves the way toward new frontiers of superconductivity in thermodynamically stable quasicrystals, which has been the predominant challenge facing condensed matter physics since the discovery of quasicrystals almost four decades ago

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

High-Density Well-Aligned Dislocations Introduced by Plastic Deformation in Bi1−xSbx Topological Insulator Single Crystals

Topological insulators (TIs) have a bulk bandgap and gapless edge or surface states that host helically spin-polarized Dirac fermions. Theoretically, it has been predicted that gapless states could also be formed along dislocations in TIs. Recently, conductivity measurements on plastically deformed bismuth antimony (Bi1−xSbx) TIs have revealed excess conductivity owing to dislocation conduction. For further application of them, fundamental study on dislocations in TIs is indispensable. Dislocations controlled based on fundamental studies could potentially be useful not only for experimental investigations of the dislocation properties but also for diverse device applications. In the present study, Bi1−xSbx TI single crystals were fabricated by a zone-melting method. The crystals were plastically deformed at room temperature. The resultant dislocations were observed by transmission electron microscopy (TEM). It was found that high-density dislocations with the Burgers vector satisfying the condition for the formation of gapless states were successfully introduced. The dislocations were mostly of edge type with lengths on the order of more than a few micrometers

Local current conduction due to edge dislocations in deformed GaN studied by scanning spreading resistance microscopy

Local electrical conductivities were measured for plastically deformed n-GaN single crystals by scanning spreading resistance microscopy (SSRM). In the SSRM images, many spots with high conductivity were observed, which can be attributed to introduced edge dislocations whose line direction is along [0 0 0 1] and Burgers vector is b = (a/3)[1 $\bar{2}$ 1 0]. This result is in contrast to the previous studies which showed that grown-in edge dislocations of the same type in GaN films exhibit virtually no conduction. This suggests that the dislocation conduction depends sensitively on the dislocation core structure. Current-voltage spectra indicated a Frenkel-Poole mechanism for the conduction