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Register a new userEffect of Bi Substitution on Thermoelectric Properties of SbSe2-based Layered Compounds NdO$_{0.8}$F$_{0.2}$Sb$_{1-x}$Bi$_x$Se$_2$
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Although SbSe2-based layered compounds have been predicted to be high-performance thermoelectric materials and topological materials, most of these compounds obtained experimentally have been insulators so far. Here, we present the effect of Bi substitution on the thermoelectric properties of SbSe2-based layered compounds NdO0.8F0.2Sb1-xBixSe2 (x = 0-0.4). The room temperature electrical resistivity is decreased to 8.0 * 10^-5 ohmm for x = 0.4. The electrical power factor is calculated to be 1.4 * 10^-4 W/mK^2 at 660 K, which is in reasonable agreement with combined Jonker and Ioffe analysis. The room-temperature lattice thermal conductivity of less than 1 W/mK is almost independent of x, in contrast to the point-defect scattering model for conventional alloys. The present work provides an avenue for exploring SbSe2-based insulating and BiSe2-based conducting systems.
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Polycrystalline samples of layered pnictogen diselenide NdO0.8F0.2Sb1-xBixSe2 (x = 0 to 0.8) were successfully synthesized by solid-state reactions. Electrical resistivity in the synthesized samples was systematically decreased with an increase in Bi content x. Crystal structure analysis using synchrotron X-ray diffraction suggests that insulator to metal transition upon Bi doping correlates with anomalous change in c-axis length and/or corrugation in conducting layer. The emergence of superconductivity under high pressure is demonstrated using diamond anvil cell (DAC) with boron-doped diamond electrodes, for x = 0.3 and 0.7 as the representative samples. For Sb-rich one (x = 0.3), we observed a superconducting transition with Tconset = 5.3 K at 50 GPa, which is the first-ever report of the superconductivity in layered SbCh2-based (Ch: chalcogen) compounds. The Tconset of x = 0.3 increased with increasing pressure and reached 7.9 K at 70.8 GPa, followed by the gradual decrease in Tc up to 90 GPa. For Bi-rich one (x = 0.7), a superconducting transition with Tconset = 5.9 K was observed at 43.5 GPa, which is the almost comparable to that of x = 0.3; besides, upper critical field (Hc2) is evaluated to be ~10 T for x = 0.7, which is higher than that of x = 0.3 (Hc2 = 6.7 T at 50 GPa).
The layered Bi-chalcogenide compounds have been drawing much attention as a new layered superconductor family since 2012. Due to the rich variation of crystal structure and constituent elements, the development of new physics and chemistry of the layered Bi-chalcogenide family and its applications as functional materials have been expected. Recently, it was revealed that the layered Bi chalcogenides can show a relatively high thermoelectric performance (ZT = 0.36 in LaOBiSSe at ~650 K). Here, we show the crystal structure variation of the Bi-chalcogenide family and their thermoelectric properties. Finally, the possible strategies for enhancing the thermoelectric performance are discussed on the basis of the experimental and the theoretical facts reviewed here.
The challenge of parasitic bulk doping in Bi-based 3D topological insulator materials is still omnipresent, especially when preparing samples by molecular beam epitaxy (MBE). Here, we present a heterostructure approach for epitaxial BSTS growth. A thin n-type Bi$_2$Se$_3$ (BS) layer is used as an epitaxial and electrostatic seed which drastically improves the crystalline and electronic quality and reproducibility of the sample properties. In heterostructures of BS with p-type (Bi$_{1-x}$Sb$_x$)$_2$(Te$_{1-y}$Se$_y$)$_3$ (BSTS) we demonstrate intrinsic band bending effects to tune the electronic properties solely by adjusting the thickness of the respective layer. The analysis of weak anti-localization features in the magnetoconductance indicates a separation of top and bottom conduction layers with increasing BSTS thickness. By temperature- and gate-dependent transport measurements, we show that the thin BS seed layer can be completely depleted within the heterostructure and demonstrate electrostatic tuning of the bands via a back-gate throughout the whole sample thickness.
We have investigated the optical properties of thin films of topological insulators Bi$_{2}$Te$_{3}$, Bi$_{2}$Se$_{3}$ and their alloys Bi$_2$(Te$_{1-x}$Se$_x$)$_3$ on BaF$_{2}$ substrates by a combination of infrared ellipsometry and reflectivity in the energy range from 0.06 to 6.5 eV. For the onset of interband absorption in Bi$_2$Se$_3$, after the correction for the Burstein-Moss effect, we find the value of direct bandgap of $215pm10$ meV at 10 K. Our data supports the picture that Bi$_2$Se$_3$ has a direct band gap located at the $Gamma$ point in the Brillouin zone and that the valence band reaches up to the Dirac point and has the shape of a downward oriented paraboloid, i.e. without a camel-back structure. In Bi$_2$Te$_3$, the onset of strong direct interband absorption at 10 K is at a similar energy of about 200 meV, with a weaker additional feature at about 170 meV. Our data support the recent GW band structure calculations suggesting that the direct interband transition does not occur at the $Gamma$ point but near the Z-F line of the Brillouin zone. In the Bi$_2$(Te$_{1-x}$Se$_x$)$_3$ alloy, the energy of the onset of direct interband transitions exhibits a maximum near $x=0.3$ (i.e. the composition of Bi$_2$Te$_2$Se), suggesting that the crossover of the direct interband transitions between the two points in the Brillouin zone occurs close to this composition.
We used low-energy, momentum-resolved inelastic electron scattering to study surface collective modes of the three-dimensional topological insulators Bi$_2$Se$_3$ and Bi$_{0.5}$Sb$_{1.5}$Te$_{3-x}$Se$_{x}$. Our goal was to identify the spin plasmon predicted by Raghu and co-workers [S. Raghu, et al., Phys. Rev. Lett. 104, 116401 (2010)]. Instead, we found that the primary collective mode is a surface plasmon arising from the bulk, free carrers in these materials. This excitation dominates the spectral weight in the bosonic function of the surface, $chi (textbf{q},omega)$, at THz energy scales, and is the most likely origin of a quasiparticle dispersion kink observed in previous photoemission experiments. Our study suggests that the spin plasmon may mix with this other surface mode, calling for a more nuanced understanding of optical experiments in which the spin plasmon is reported to play a role.
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