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Using first principles calculations, we studied the effect of Si substitution in the hexagonal Fe3Ge. We find the low temperature magnetic anisotropy in this system to be planar and originating mostly from the spin-orbit coupling in Fe-d states. Reduction of the unitcell volume reduces the magnitude of in-plane magnetic anisotropy, eventually turning it positive which reorients the magnetic moments to the axial direction. Substituting Ge with the smaller Si ions also increases the anisotropy, potentially enhancing the region of stability of the axial magnetization, which is beneficial for magnetic applications such as permanent magnets. Our experimental measurements on samples of Fe3Ge1–xSix confirm these predictions and show that substitution of about 6% of the Ge with Si increases by approximately 35K the temperature range over which anisotropy is uniaxial. [ABSTRACT FROM AUTHOR]

With the help of computer simulations we have studied the crystallization kinetics of amorphous silicon in solid phase epitaxial (SPE) and random nucleation growth processes. Our simulations employing classical molecular dynamics and first principles methods suggest qualitatively similar behavior in both processes. Pressure is found to reduce the difference in molar volumes and coordination numbers between the amorphous and crystalline phases, which in turn lowers the energy barrier of crystallization. The activation energy for the SPE growth of four coordinated diamond phase is found to reach a minimum (a maximum in growth rates) close to 10 GPa when its density becomes equal to that of the amorphous phase. The crystallization temperatures of successive high pressure phases of silicon are found to decrease, offering a possible explanation for the pressure induced crystallization reported in this material. [ABSTRACT FROM AUTHOR]

High pressure x-ray diffraction studies on inverse-perovskite BaLiF3 show that this compound is structurally stable up to ∼50 GPa. The bulk modulus of BaLiF3 is determined to be 75.9 GPa which is in close agreement with that determined from a semi-empirical formulation. Our ab initio calculations show that among the three alkaline earth fluoro perovskites (ALiF3, A = Ba, Ca, and Sr) which crystallize in the inverse-perovskite structure, BaLiF3 is the least brittle at ambient conditions and also that the degree of brittleness decreases at high pressures. The behavior of the elastic constants at high pressure accompanied by a reduction in the bandgap indicates a decrease in the directional nature of the bonding. [ABSTRACT FROM AUTHOR]

We have investigated the high pressure behavior of amorphous silicon (a-Si) using x-ray diffraction and Raman scattering techniques. Our experiments show that a-Si undergoes a polyamorphous transition from the low density amorphous to the high density amorphous phase, followed by pressure induced crystallization to the primitive hexagonal (ph) phase. On the release path, the sequence of observed phase transitions depends on whether the pressure is reduced slowly or rapidly. Using the results of our first principles calculations, pressure induced preferential crystallization to the ph phase is explained in terms of a thermodynamic model based on phenomenological random nucleation and the growth process. [ABSTRACT FROM AUTHOR]

The origin of Rashba spin splitting in the two-dimensional electron gas at the (001) surface of SrTiO3 is studied using first-principles calculations and tight-binding model. Calculations of oxygen vacancies under virtual crystal approximation reveal a two-dimensional electron-gas subband structure similar to polar materials, consistent with observations on SrTiO3. Our studies also confirm that k dependence of the spin splitting is predominantly cubic in the surface Ti-t2g states, even though structural relaxations diminish the effect in dxy bands. A tight-binding model, explicitly including Ti-d and O-p states as well as next-nearest-neighbor interactions, is derived to understand the first-principles results. Effective Rashba Hamiltonians for the surface bands are derived using quasidegenerate perturbation theory and scenarios in which linear k contribution may be suppressed are discussed. However, the cubic terms in the Hamiltonian are found to be different from the model derived using k • p theory, leading to different pseudospin symmetry in the Brillouin zone. [ABSTRACT FROM AUTHOR]

We present a systematic method for developing a five-band Hamiltonian for the metal d orbitals that can be used to study the effect of electric and magnetic fields on multilayer MX2 (M = Mo,W and X = S,Se) systems. On a hexagonal lattice of d orbitals, the broken inversion symmetry of the monolayers is incorporated via fictitious s orbitals at the chalcogenide sites. A tight-binding Hamiltonian is constructed and then downfolded to get effective d-orbital overlap parameters using quasidegenerate perturbation theory. The steps to incorporate the effects of multiple layers, external electric and magnetic fields, are also detailed. We find that an electric field produces a linear-k Rashba splitting around the Γ point, while a magnetic field removes the valley pseudospin degeneracy at the ±K points. Our model provides a simple tool to understand the recent experiments on electric and magnetic control of valley pseudospin in monolayer dichalcogendies. [ABSTRACT FROM AUTHOR]

Theoretical calculations are performed to understand the electronic structure and magnetic properties of CuFe2Ge2. The band structure reveals large electron density N(EF) at the Fermi level suggesting a strong itinerant character of magnetism. The Fermi surface is dominated by two dimensional sheet like structures, with potentially strong nesting between them. The magnetic ground state appears to be ferromagnetic along a and antiferromagnetic in other directions. These results show that CuFe2Ge2 is an antiferromagnetic metal, with similarities to the Fe-based superconductors; such as magnetism with substantial itinerant character and coupling between magnetic order and electrons at the Fermi energy. [ABSTRACT FROM AUTHOR]

Theoretical calculations are performed to understand the electronic structure and magnetic properties of CuFe2Ge2. The band structure reveals large electron density N(EF) at the Fermi level suggesting a strong itinerant character of magnetism. The Fermi surface is dominated by two dimensional sheet like structures, with potentially strong nesting between them. The magnetic ground state appears to be ferromagnetic along a and antiferromagnetic in other directions. These results show that CuFe2Ge2 is an antiferromagnetic metal, with similarities to the Fe-based superconductors; such as magnetism with substantial itinerant character and coupling between magnetic order and electrons at the Fermi energy. [ABSTRACT FROM AUTHOR]

Using density-functional methods, we study the electronic structure of Ba3 CuSb2O9, a candidate material for the quantum spin liquid behavior. We study both the triangular lattice as well as the recently proposed hexagonal lattice structures with flipped Cu-Sb dumbbells. The band structure near the Fermi energy is described very well by a tight-binding Hamiltonian involving the Cu (eg) orbitals, confirming their central role in the physics of the problem. A minimal tight-binding Hamiltonian for the triangular structure is presented. The Cu (d9) ions (a single eg hole in the band structure) present in the compound are expected to be Jahn-Teller centers, while the nature of the Jahn-Teller distortions in this material is still under debate. Solving a simple model by exact diagonalization, we show that electronic correlation effects in general enhance the tendency towards a Jahn-Teller distortion by reducing the kinetic energy due to correlation effects. Our density-functional calculations do indeed show a significant Jahn-Teller distortion of the CuO6 octahedra when we include the correlation effects within the Coulomb-corrected GGA+U method, so that the Jahn-Teller effect is correlation driven. We argue for the presence of a random static Jahn-Teller distortion in the hexagonal structure rather than a dynamical one because of the broken octahedral symmetry around the CuO6 octahedra and the potential fluctuations inherently present in the system caused by a significant disorder, which is believed to be present, in particular, due to the flipped Cu-Sb dumbbells. [ABSTRACT FROM AUTHOR]

The nature of orbital ordering and type of the Jahn-Teller (JT) distortion in the doped manganite La0.5Sr1.5MnO4 has been a topic of long standing debate, with both experiments and theory supporting opposite views. With the help of ab initio density functional and exact diagonalization studies we have investigated the energetics of the cooperative JT distortion and orbital ordering in this system. The density functional calculations yield a x2-1/y2-1 (rod-type) orbital order as well as a Jahn-Teller distortion where the octahedral axes on the plane are elongated along the chain direction and compressed perpendicular to it in the ab plane (type I). The essential physics of the problem is captured by a single zigzag chain model, which was studied with a tight-binding Hamiltonian and exact diagonalization on a finite chain, which help unravel the reason for the rod-type orbital order. The main conclusions from the latter studies are that: (i) The Coulomb correlation effects are relatively unimportant due to the low electron density in the eg states, (ii) the kinetic energy term (hopping along the ferromagnetic zigzag chain) favors the rod-type orbital order, and (iii) a significant noncubic crystal field, proposed earlier and found here from our DFT calculations, helps stabilize the rod-type orbital order even further. [ABSTRACT FROM AUTHOR]