Zhang, Qingyun; Schwingenschlögl, Udo(Physical Review B, American Physical Society (APS), 2018-04-16)[Article]
Using first-principles calculations, we investigate the electronic properties of the two-dimensional GaX/MX2 (M = Mo, W; X = S, Se, Te) heterostructures. Orbital hybridization between GaX and MX2 is found to result in Rashba splitting at the valence-band edge around the Γ point, which grows for increasing strength of the spin-orbit coupling in the p orbitals of the chalcogenide atoms. The location of the valence-band maximum in the Brillouin zone can be tuned by strain and application of an out-of-plane electric field. The coexistence of Rashba splitting (in-plane spin direction) and band splitting at the K and K′ valleys (out-of-plane spin direction) makes GaX/MX2 heterostructures interesting for spintronics and valleytronics. They are promising candidates for two-dimensional spin-field-effect transistors and spin-valley Hall effect devices. Our findings shed light on the spin-valley coupling in van der Waals heterostructures.
Ghosh, Sumit; Manchon, Aurelien(Physical Review B, American Physical Society (APS), 2018-04-02)[Article]
Current-driven spin-orbit torques are investigated in a heterostructure composed of a ferromagnet deposited on top of a three-dimensional topological insulator using the linear response formalism. We develop a tight-binding model of the heterostructure adopting a minimal interfacial hybridization scheme that promotes induced magnetic exchange on the topological surface states, as well as induced Rashba-like spin-orbit coupling in the ferromagnet. Therefore our model accounts for the spin Hall effect from bulk states together with inverse spin galvanic and magnetoelectric effects at the interface on equal footing. By varying the transport energy across the band structure, we uncover a crossover from surface-dominated to bulk-dominated transport regimes. We show that the spin density profile and the nature of the spin-orbit torques differ substantially in both regimes. Our results, which compare favorably with experimental observations, demonstrate that the large dampinglike torque reported recently is more likely attributed to the Berry curvature of interfacial states, while spin Hall torque remains small even in the bulk-dominated regime.
Guo, Zaibing; Mi, W. B.; Aboljadayel, Razan; Zhang, Bei; Zhang, Qiang; Gonzalez Barba, Priscila; Manchon, Aurelien; Zhang, Xixiang(Physical Review B, American Physical Society (APS), 2012-09-27)[Article]
In this paper, we report the results of surface and interface scattering on anomalous Hall effect in Co/Pd multilayers with perpendicular magnetic anisotropy. The surface scattering effect has been extracted from the total anomalous Hall effect. By scaling surface scattering contribution with ρAHs∼ργss, the exponent γ has been found to decrease with the increase of surface scattering resistivity, which could account for the thickness-dependent anomalous Hall effect. Interface diffusion induced by rapid thermal annealing modifies not only the magnetization and longitudinal resistivity but also the anomalous Hall effect; a large exponent γ ∼ 5.7 has been attributed to interface scattering-dominated anomalous Hall effect.
Cheng, Yingchun; Han, Yu; Schwingenschlögl, Udo; Wang, H. T.; Zhang, Xixiang; Zhu, Yihan; Zhu, Zhiyong(Physical Review B, American Physical Society (APS), 2012-02-17)[Article]
The edge structure and width of graphene nanoribbons (GNRs) are crucial factors for the electronic properties. A combination of experiment and first-principles calculations allows us to determine the mechanism of the hexagon-hexagon to pentagon-heptagon transformation. GNRs thinner than 2 nm have been fabricated by bombardment of graphene with high-energetic Au clusters. The edges of the GNRs are modified in situ by electron irradiation. Tensile strain along the edge decreases the transformation energy barrier. Antiferromagnetism and a direct band gap are found for a zigzag GNR, while a fully reconstructed GNR shows an indirect band gap. A GNR reconstructed on only one edge exhibits ferromagnetism. We propose that strain is an effective method to tune the edge and, therefore, the electronic structure of thin GNRs for graphene-based electronics.
Zhu, Zhiyong; Cheng, Yingchun; Schwingenschlögl, Udo(Physical Review B, American Physical Society (APS), 2011-10-14)[Article]
Fully relativistic first-principles calculations based on density functional theory are performed to study the spin-orbit-induced spin splitting in monolayer systems of the transition-metal dichalcogenides MoS2, MoSe2, WS2, and WSe2. All these systems are identified as direct-band-gap semiconductors. Giant spin splittings of 148–456 meV result from missing inversion symmetry. Full out-of-plane spin polarization is due to the two-dimensional nature of the electron motion and the potential gradient asymmetry. By suppression of the Dyakonov-Perel spin relaxation, spin lifetimes are expected to be very long. Because of the giant spin splittings, the studied materials have great potential in spintronics applications.
Cheng, Yingchun; Huang, Gaoshan; Schwingenschlögl, Udo; Zhu, Zhiyong(Physical Review B, American Physical Society (APS), 2011-03-28)[Article]
We present a detailed analysis of the effects of uniaxial and biaxial strain on the frequencies of the G mode of monolayer graphene, using first principles calculations. Our results allow us to explain discrepancies in the experimentally determined values of the Grüneisen parameter. The direction and strength of the applied strain, Poisson's ratio of the substrate, and the intrinsic strain in different experimental setups turn out to be important. A reliable determination of the Grüneisen parameter is a prerequisite of strain engineering.
López-Moreno, S.; Muñoz, A.; Romero, A. H.; Schwingenschlögl, Udo(Physical Review B, American Physical Society (APS), 2010-01-25)[Article]
We have performed density-functional theory calculations, including the spin-orbit correction, to investigate atomic gold chains on Ge(001). A set of 26 possible configurations of the Au/Ge(001) system with c(4×2) and c(8×2) symmetries is studied. Our data show that the c(4×2) order results in the lowest energy, which is not in direct agreement with recent experiments. Using total-energy calculations, we are able to explain these differences. We address the electronic band structure and apply the Tersoff-Hamann approach to correlate our data to scanning-tunneling microscopy (STM). We obtain two highly competitive structures of the atomic Au chains for which we report simulated STM images in order to clarify the composition of the experimental Au/Ge(001) surface.
Nazir, Safdar; Schwingenschlögl, Udo; Singh, Nirpendra(Physical Review B, American Physical Society (APS), 2011-03-29)[Article]
The electronic and optical properties of the KTaO3/SrTiO3 heterointerface are analyzed by the full-potential linearized augmented plane-wave approach of density functional theory. Optimization of the atomic positions points at subordinate changes in the crystal structure and chemical bonding near the interface, which is due to a minimal lattice mismatch. The creation of metallic interface states thus is not affected by structural relaxation but can be explained by charge transfer between transition metal and oxygen atoms. It is to be expected that a charge transfer is likewise important for related interfaces such as LaAlO3/SrTiO3. The KTaO3/SrTiO3 system is ideal for disentangling the complex behavior of metallic interface states, since almost no structural relaxation takes place.
Zhu, Zhiyong; Cheng, Yingchun; Schwingenschlögl, Udo(Physical Review B, American Physical Society (APS), 2012-06-27)[Article]
All-electron ab initio calculations are used to study the microscopic origin of the charge density wave (CDW) in 1T-TiSe2. A purely electronic picture is ruled out as a possible scenario, indicating that the CDW transition in the present system is merely a structural phase transition. The CDW instability is the result of a symmetry lowering by electron correlations occurring with electron localization. Suppression of the CDW in pressurized and in Cu-intercalated 1T-TiSe2 is explained by a delocalization of the electrons, which weakens the correlations and counteracts the symmetry lowering.
Zhang, Qingyun; Cheng, Yingchun; Gan, Liyong; Schwingenschlögl, Udo(Physical Review B, American Physical Society (APS), 2013-12-30)[Article]
Using first-principles calculations, we study the electronic structure of monolayer MoS2 under uniaxial strain. We show that the energy valleys drift far off the corners of the Brillouin zone (K points), about 12 times the amount observed in graphene. Therefore, it is essential to take this effect into consideration for a correct identification of the band gap. The system remains a direct band gap semiconductor up to 4% uniaxial strain, while the size of the band gap decreases from 1.73 to 1.54 eV. We also demonstrate that the splitting of the valence bands due to inversion symmetry breaking and spin-orbit coupling is not sensitive to strain.
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