Hossein Mirhosseini

Predicting the stability of surface phases of molybdenum selenides

The selenization of molybdenum might become an important step in the production of nanostructures based on the layered compound MoSe2. It is already technologically relevant for the production of thin film chalcopyrite solar cells. However, the control of the process is still very poor, due to the lack of basic knowledge of the surface thermodynamics of the system. Here, we present a theoretical study on the stability of surface adlayers of Se on the Mo(110) surface, predicting surface patterns and their stability range in terms of temperature and selenium partial pressure. Our results, based on density functional theory, show that the attainable Se coverages range from 1/4 to 3/4 of a monolayer for systems in equilibrium with a gas formed of Se molecules. We provide simulated scanning tunneling microscopy images to help the experimental characterization of adsorbed surface patterns.

Tunable Spin Gaps in a Quantum-Confined Geometry

We have studied the interplay of a giant spin-orbit splitting and of quantum confinement in artificial Bi-Ag-Si trilayer structures. Angle-resolved photoelectron spectroscopy reveals the formation of a complex spin-dependent gap structure, which can be tuned by varying the thickness of the Ag buffer layer. This provides a means to tailor the electronic structure at the Fermi energy, with potential applications for silicon-compatible spintronic devices.

Interference of spin states in photoemission from Sb/Ag(111) surface alloys

Using a three-dimensional spin polarimeter we have gathered evidence for the interference of spin states in photoemission from the surface alloy Sb/Ag(111). This system features a small Rashba-type spin splitting of a size comparable to the momentum broadening of the quasiparticles, thus causing an intrinsic overlap between states with orthogonal spinors. Besides a small spin polarization caused by the spin splitting, we observe a large spin polarization component in the plane normal to the quantization axis of the Rashba effect. Strongly suggestive of coherent spin rotation, this effect is largely independent of the photon energy and photon polarization.

Existence of topological nontrivial surface states in strained transition metals: W, Ta, Mo, and Nb

Danny Thonig, Tomáš Rauch, Hossein Mirhosseini, ∗ Jürgen Henk, Ingrid Mertig, 3 Henry Wortelen, Bernd Engelkamp, Anke B. Schmidt, and Markus Donath Department of Physics and Astronomy, Material Theory, University Uppsala, Box 516, 75120 Uppsala, Sweden Institute of Physics, Martin Luther University Halle-Wittenberg, Von-Seckendorff-Platz 1, 06120 Halle (Saale), Germany Max Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle (Saale), Germany Physikalisches Institut, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany (Dated: May 13, 2016)

Incorporation of Li dopant into Cu2ZnSnSe4 photovoltaic absorber: hybrid-functional calculations

We have studied the formation of Li extrinsic defects in CuZnSnSe by first-principles hybrid functional calculations. Li atoms in the Cu site (Li) and Li atoms in the Se site (Li) are the most and the least stable point defect, respectively. The formation energies of two Li interstitial defects with different numbers of nearest neighbors are the same. These interstitial point defects act as a donor but do not create gap states. Formation of the acceptor point defects (Li and Li) is less likely in p-type CuZnSnSe compared with n-type CuZnSnSe. In contrast to Li which does not create gap states, the formation of Li creates two charge transition levels in the middle of the bandgap which might act as recombination centers. (Li–Li) dumbbells are likely to form in p-type CuZnSnSe but the probability of the formation of dumbbells decreases in favor of the formation of two Li point defects when the chemical potential of the electrons increases.

Atomic and electronic structures evolution of the narrow band gap semiconductor Ag2Se under high pressure.

Non-trivial electronic properties of silver telluride and other chalcogenides, such as the presence of a topological insulator state, electronic topological transitions, metallization, and the possible emergence of superconductivity under pressure have attracted attention in recent years. In this work, we studied the electronic properties of silver selenide (Ag2Se). We performed direct current electrical resistivity measurements, in situ Raman spectroscopy, and synchrotron x-ray diffraction accompanied by ab initio calculations to explore pressure-induced changes to the atomic and electronic structure of Ag2Se. The temperature dependence of the electrical resistivity was measured up to 30 GPa in the 4-300 K temperature interval. Resistivity data showed an unusual increase in the thermal energy gap of phase I, which is a semiconductor under ambient conditions. Recently, a similar effect was reported for the 3D topological insulator Bi2Se3. Raman spectroscopy studies revealed lattice instability in phase I indicated by the softening of observed vibrational modes with pressure. Our hybrid functional band structure calculations predicted that phase I of Ag2Se would be a narrow band gap semiconductor, in accordance with experimental results. At a pressure of ~7.5 GPa, Ag2Se underwent a structural transition to phase II with an orthorhombic Pnma structure. The temperature dependence of the resistivity of Ag2Se phase II demonstrated its metallic character. Ag2Se phase III, which is stable above 16.5 GPa, is also metallic according to the resistivity data. No indication of the superconducting transition is found above 4 K in the studied pressure range.

Tuning independently the Fermi energy and spin splitting in Rashba systems: ternary surface alloys on Ag(111).

By detailed first-principles calculations we show that the Fermi energy and the Rashba splitting in disordered ternary surface alloys Bi(x)Pb(y)Sb(1 - x - y)/Ag(111) can be independently tuned by choosing the concentrations x and y of Bi and Pb, respectively. The findings are explained by three fundamental mechanisms, namely the relaxation of the adatoms, the strength of the atomic spin-orbit coupling, and band filling. By mapping the Rashba characteristics, i.e. the splitting k(R) and the Rashba energy E(R), and the Fermi energy of the surface states in the complete range of concentrations, we find that these quantities depend monotonically on x and y, with a very few exceptions. Our results suggest that we should investigate experimentally effects which rely on the Rashba spin-orbit coupling depending on spin-orbit splitting and band filling.

Hybrid functional calculations on the Na and K impurities in substitutional and interstitial positions in Cu2ZnSnSe4

We studied the energetics, atomic and electronic structure of Na and K extrinsic defects in Cu2ZnSnSe4 by ab initio calculations using the HSE06 hybrid functional. Our results show that (i) among all substitutional positions, the Cu-site is the most favorable position for both Na and K. (ii) The tetrahedrally coordinated interstitial site has a lower formation energy than the octahedrally coordinated interstitial site. (iii) Based on the band structure calculations we can conclude, Se-related defects lead to the formation of defect states within the band gap.

Electron correlation beyond the local density approximation: self-interaction correction in gadolinium.

We report on detailed first-principles calculations which focus on the magnetic and structural properties of the (0001) surface of gadolinium. The electronic correlation within the localized 4f states is treated within the self-interaction correction (SIC), thus going beyond the local spin-density approximation. The ferromagnetic ground state is predicted correctly if the SIC is applied; the effect of surface relaxations on Heisenberg exchange parameters and on the Curie temperature are addressed by Monte Carlo calculations. The SIC also has a profound effect on the dispersion of the d surface states, due to hybridization of the 4f states with the 5d valence states. The best agreement with photoemission experiments is obtained within the transition state approximation, which takes into account the orbital relaxation. The Rashba spin-orbit coupling in the d surface states is fully captured by our relativistic multiple scattering approach.

Pressure-induced metallization in layered ReSe2

The evolution of the crystal structure and electrical transport properties of distorted layered transition metal dichalcogenide ReSe2 was studied under high pressure up to ~90 GPa by Raman spectroscopy and electrical resistivity measurements accompanied by ab initio electronic band structure calculations. Raman spectroscopy studies indicate an isostructural phase transition due to layer sliding at ~7 GPa, to the distorted 1T-phase which remains stable up to the highest pressures employed in these experiments. From a direct band gap semiconductor at ambient pressure, ReSe2 undergoes pressure-induced metallization at pressures ~35 GPa, in agreement with the ab initio calculations. Resistivity measurements performed with different loading conditions reveal the possible emergence of superconductivity, which is most likely not an intrinsic property of ReSe2, but is rather conditioned by internal stresses upon compression.