We report ab initio calculations of the spin splitting of the uppermost valence band (UVB) and the lowermost conduction band (LCB) in bulk and atomically thin GaS, GaSe, GaTe, and InSe. These layered monochalcogenides appear in four major polytypes depending on the stacking order, except for the monoclinic GaTe. Bulk and few-layer ε-and γ -type, and odd-number β-type GaS, GaSe, and InSe crystals are noncentrosymmetric. The spin splittings of the UVB and the LCB near the Γ-point in the Brillouin zone are finite, but still smaller than those in a zinc-blende semiconductor such as GaAs. On the other hand, the spin splitting is zero in centrosymmetric bulk and even-number few-layer β-type GaS, GaSe, and InSe, owing to the constraint of spatial inversion symmetry. By contrast, GaTe exhibits zero spin splitting because it is centrosymmetric down to a single layer. In these monochalcogenide semiconductors, the separation of the non-degenerate conduction and valence bands from adjacent bands results in the suppression of Elliot-Yafet spin relaxation mechanism. Therefore, the electron- and hole-spin relaxation times in these systems with zero or minimal spin splittings are expected to exceed those in GaAs when the D’yakonov-Perel’ spin relaxation mechanism is also suppressed.
We report the complex magnetic phase diagram and electronic structure of Cr 2 (Te 1x W x )O 6 systems. While compounds with different x values possess the same crystal structure, they display different magnetic structures below and above x c = 0.7, where both the transition temperature T N and sublattice magnetization (M s ) reach a minimum. Unlike many known cases where magnetic interactions are controlled either by injection of charge carriers or by structural distortion induced via chemical doping, in the present case it is achieved by tuning the orbital hybridization between Cr 3d and O 2p orbitals through W 5d states. The result is supported by ab-initio electronic structure calculations. Through this concept, we introduce a new approach to tune magnetic and electronic properties via chemical doping.
Cu 3 SbSe 4 is a promising thermoelectric material due to high thermopower (> 400 µV/K) at 300K and higher. Although it has a simple crystal structure derived from zinc blende structure, previous work has shown that the physics of band gap formation is quite subtle due to the importance of active lone pair (5s 2 ) of Sb and the non-local exchange interaction between these and Se 5p electrons. Since for any application of semiconductors understanding the properties of defects is essential, we discuss the results of a systematic study of several point defects in Cu 3 SbSe 4 including vacancies and substitutions for each of the components. First principles calculations using density functional theory show that among variety of possible dopants, p-type doping can be done by substituting Sb with group IV elements including Sn, Ge, Pb and Ti and n-type doping can be done by replacing Cu by Mg, Zn. Doping at the Se site appears to be rather difficult. Electronic structure calculations also suggest that the p-type behavior seen in nominally pure Cu 3 SbSe 4 is most likely due to Cu vacancy rather than Se vacancy.
Electronic structure calculations using local density and generalized gradient(LDA/GGA) approximations for the full Heusler compound Fe2VAl show that it is a pseudo-gap (negative gap) system with very small density of states (DOS) at the Fermi level but rapidly rising DOS away from it, a feature that makes this compound a promising thermoelectric material. Thermopower (S) measurements in nominally pure and n-doped Fe2VAl give indeed large values of S (∼-150 µV/K at 200 K). To improve on the inadequacy of LDA/GGA in handling d-electron systems and to understand the origin of large thermopowers measured, we have carried out electronic structure calculations using GGA+U method with several values of the on-site Coulomb interaction parameter U including the ones calculated using constrained density functional theory (DFT). For the latter, we found Fe2VAl to be a narrow band gap semiconductor with a gap of 0.55 eV. With the calculated band structures, we have studied the carrier concentration and temperature dependence of S using Boltzmann transport equation in constant relaxation time approximation for both the pseudo-gap and gapped cases.Comparison between theory and experiment suggests that neither the pseudo-gap nor the finite gap (0.55 eV) model can explain all the transport properties consistently. Therefore treatment of U beyond simple mean-field approach (done in GGA+U) and/or inclusion of defect induced changes in the host electronic structure might be important in understanding the experiments.
In this paper we discuss the results of ab initio electronic structure calculations for Cu(3)SbSe(4) (Se4) and Cu(3)SbSe(3) (Se3), two narrow bandgap semiconductors of thermoelectric interest. We find that Sb is trivalent in both the compounds, in contrast to a simple nominal valence (ionic) picture which suggests that Sb should be 5 + in Se4. The gap formation in Se4 is quite subtle, with hybridization between Sb 5s and the neighboring Se 4s, 4p orbitals, position of Cu d states, and non-local exchange interaction, each playing significant roles. Thermopower calculations show that Se4 is a better p-type system. Our theoretical results for Se4 agree very well with recent experimental results obtained by Skoug et al (2011 Sci. Adv. Mater. 3 602).
The semimetallic Group V elements display a wealth of correlated electron phenomena due to a small indirect band overlap that leads to relatively small, but equal, numbers of holes and electrons at the Fermi energy with high mobility. Their electronic bonding characteristics produce a unique crystal structure, the rhombohedral A7 structure, which accommodates lone pairs on each site. Here we show via single-crystal and synchrotron x-ray diffraction that SbAs is a compound and the A7 structure can display chemical ordering of Sb and As, which were previously thought to mix randomly. Formation of this compound arises due to differences in electronegativity that are common to IV-VI compounds of average group V such as GeTe, SnS, PbS, and PbTe, and also ordered intra-period compounds such as CuAu and NiPt. High-temperature diffraction studies reveal an order-disorder transition around 550 K in SbAs, which is in stark contrast to IV-VI compounds GeTe and SnTe that become cubic at elevated temperatures but do not disorder. Transport and infrared reflectivity measurements, along with first-principles calculations, confirm that SbAs is a semimetal, albeit with a direct band separation larger than that of Sb or As. Because even subtle substitutions in the semimetals, notably Bi1−xSbx, can open semiconducting energy gaps, a further investigation of the interplay between chemical ordering and electronic structure on the A7 lattice is warranted.
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