Papers by Keyword: Spin-Orbit Coupling

Abstract: Presence of Bismuth (Bi) leads to topologically nontrivial band structure in many materials, especially in topological insulators. Traditionally Bi is known to be a semimetal but, quite surprisingly, in a recent experiment bulk Bi has been found to be a superconductor below 0.53 mK at ambient pressure. In order to have a closer look at the electronic properties of bulk Bi in the wake of this unexpected experimental evidence of superconducting phase, we have performed density-functional-theory (DFT) based first principle calculations using plane-wave basis set and with suitable ionic pseudopotentials. We have computed the band structure, density of states and Fermi surfaces for two different type of exchange-correlation (XC) functionals, namely Perdew-Zunger (PZ) and Perdew-Burke-Ernzerhof (PBE) type. Each of these XC functional has been considered without and with spin orbit (SO) interaction. After carefully examining the energy-convergence with respect to plane wave basis set and k-points in each case, the band structure has been calculated along the path Γ-L-T-Γ. Without SO coupling, electron pocket is found near ‘L’ and exactly at ‘Г’ and hole pocket is at ‘T’ for PZ type XC functional, while in the case of PBE-type electron pocket is found exactly at ‘L’ but the hole pocket to be near to ‘T’. With SO coupling, in PZ-type, electron pocket remains at same position, but hole pocket appears only at ‘Г’ point. Finally, when SO coupling is taken into account along with PBE-type XC functional electrons and holes are found at ‘L’ and at ‘T’ respectively. Furthermore, in this case we also observe an increase in the number of holes at ‘T’.

Abstract: Transition metal dichalcogenides (TMDs) display unique properties in their monolayer structures, namely a direct band-gap transition, which becomes a promising candidate for optoelectronics applications. Among them, WS₂ exhibits strong spin-orbit interaction which splits the excitonic peaks as observed in the experimental data up to ~400 meV. Unlike the other TMDs, the first excitonic peak A is very sharp for WS₂, while the secondary peak B is broader with smaller relative intensity. In this paper, we perform first-principles calculations on the electronic band structure and solve the Bethe-Salpeter equation for the complex dielectric function of monolayer WS₂ to study the effects of spin-orbit coupling on its excitonic structures. To resolve the excitonic peaks, in particular the B peak, we implement the double-grid method. We discuss the effects of electron-hole interaction on the absorption spectrum by comparing it with that calculated at the independent-particle level.

Abstract: Polarimetry enables to measure the state of polarization (SoP) of a light beam, which is essential in many disciplines. Typical polarimeters use bulky and expensive optical elements such as half-wave plates and grid polarizers. Plasmonic nanostructures may help to transform such bulky components into subwavelength metallic elements showing similar performance. Based on the concept of spin-orbit coupling, here we demonstrate a nanophotonic polarimeter that measures the Stokes parameters of a light beam over an ultrabroad bandwidth in a less than a square wavelength active region. Furthermore, the presented approach is applicable to any wavelength regime and technological platform, paving the way for the miniaturization of polarimeters.

Abstract: Transport and spin relaxation characteristics of the conduction electrons in silicon samples doped with bismuth in the 1.1·10¹³ - 7.7·10¹⁵ cm^-3 concentration range were studied by the Hall and electron spin resonance spectroscopy. Hall effect measurements in the temperature range 10-80 K showed a deviation from the linear dependence of the Hall resistance in the magnetic field, which is a manifestation of the anomalous Hall effect. The magnetoresistance investigation shows that with current increasing magnetoresistance may change its sign from positive to negative, which is most clearly seen when the bismuth concentration goes up to 7.7·10¹⁵ cm^-3. The conduction electron spin relaxation rate dramatically increases in silicon samples with sufficiently low concentration of bismuth ~ 2·10¹⁴ cm^-3. All these results can be explained in terms of the concept of spin-dependent and spin flip scattering induced by heavy bismuth impurity centers.

Abstract: We report a detailed study of electron longitudinal and transverse spin relaxation times for Li donors in monoisotopic ²⁸Si over the temperature range 4–20 K using continuous wave and pulsed electron paramagnetic resonance. Comparison of the obtained spin-lattice relaxation times for the states of the isolated donor center and lithium complex LiO showed that due to the presence of orbital degeneracy, relaxation is faster for single lithium than for the LiO complexes with the nondegenerate ground state. For the isolated lithium center in silicon the relaxation is well described by Blume-Orbach process, with the parameters of the spin-orbit coupling ~ 1·10^-6 meV compare to Orbach process for LiO complex with spin-orbit coupling parameter ~ 1.5·10^-2 meV.

Abstract: Band inversion mechanisms of half Heusler compouds XMBi (X=Sc, Y, Lu, La, M=Pd, Pt) is investigated under different conditions, based on first-principles calculations with generalized gradient approximation approach. The results shown that Spin-orbit coupling, however, is not the leading cause but an account can add further fuel to the band inversion. The present finding provides an effective scheme to search for topologically nontrivial materials.

Abstract: In this work, the electronic structure and dielectric function of chalcopyrite CuInSe₂ are presented. The results are based on the full-potential linearized augmented plane wave (FPLAPW) method using the generalized gradient approximation (GGA) plus an onsite Coulomb interaction U of the Cu d states. The dielectric constant, absorption coefficient and refractive index are explored by means of optical response. The spin-orbit coupling effect is considered for the calculations of electronic structure and optical properties. We find that the results based on our calculation method have good agreement compared with experimental and other earlier simulations results.

Abstract: We have studied the influence of spin-orbit coupling (SOC) on the electronic structures and half-metallicity for the V₂ReZ (Z=Al, Ga, In) compounds. It was found that the SOC has a slight influence on the whole configuration of the electronic structure and the degenerated states were split into several singlet states at the high-symmetry points. The e_g and t_1u states near the Fermi level are not sensitive to the SOC. The t_2g states composed of the side of half-metallic gap are sensitive to the SOC. The dispersivity of t_2g states was strongly reduced, which lead to an increase of the valence electron effective mass and the width of half-metallic gap. The SOC can slightly increase the spin splitting of Re and V(B) atoms. The Z atom has an influence on the intensity of SOC to act on half-metallic gap.

Abstract: In this paper we have analyzed the possibility of enhancing spin-polarization performance of conventional nonmagnetic semiconductor heterostructures which rely on the resonant tunneling mechanism. Both the bulk inversion asymmetry (BIA) and the structural inversion asymmetry (SIA) effects are taken into account in the presented model. The aim is to engineer nanostructures with maximal degree of spin separation in the electron tunneling current, which might be useful in studying various spin-related phenomena in semiconductor materials. Spin-polarization status of the current, in the devices under consideration, should be controllable by moderate emitter-collector voltages. Additionally, the spin orbit-interactions affect the dwell times of electrons in spin-up and spin-down states, therefore the prospects of spin-filtering in the time domain may be considered as well.