First-principles calculations were employed to analyze the possible slip systems in Mo5SiB2. A striking result was obtained that the three most favorable slip systems, <100>(001), <110>(001) and [001]{010}, have close stacking fault energies, and the preference among them cannot be established. This finding explains a large variety of experimentally observed slip systems in Mo5SiB2. The dislocations associated with these slip systems may dissociate into partials joined with stacking faults and separated by the large splitting width of 5–6 nm.

The electric-field-induced modification in the Curie temperature of prototypical transition-metal thin films with the perpendicular magnetic easy axis, a freestanding Fe(001) monolayer and a Co monolayer on Pt(111), is investigated by first-principles calculations of spin-spiral structures in an external electric field (E field). An applied E field is found to modify the magnon (spin-spiral formation) energy; the change arises from the E-field-induced screening charge density in the spin-spiral states due to p-d hybridizations. The Heisenberg exchange parameters obtained from the magnon energy suggest an E-field-induced modification of the Curie temperature, which is demonstrated via Monte Carlo simulations that take the magnetocrystalline anisotropy into account.

While noncentrosymmetric MoS2 monolayer is known to exhibit efficient second-harmonic generation (SHG), there is currently no agreement on its absolute nonlinear susceptibility x((2)), varying over three orders of magnitude according to recent experiments. In order to resolve this conflicting issue, we have studied the nonlinear optical properties of MoS2 monolayer grown by chemical vapor deposition. The polycrystalline nature of the monolayer was directly probed by the SHG polarization dependence across the grain boundaries using femtosecond pulses. Broadband wavelength-dependent SHG response (lambda = 1.1-2.0 mu m) using picosecond pulses was studied by comparing the relative SHG counts of MoS2 to quartz and incorporating the structural and optical characteristics of the monolayer. Significant nonlinear optical dispersion gives rise to x((2)) similar to 430 pm/V at 580 nm, where SHG is neither affected by any excitonic absorption/resonance nor by fundamental absorption. We also show that x((2)) must be derived from a thin bulk (sheet) optical nonlinearity and that the previous measurements are in fact all consistent, together with our measurements and first-principle calculations.

Magneto-transport properties in Fe thin films in an external electric field ( E -field) were investigated by means of the first-principles full-potential linearized augmented plane-wave method. Results for an Fe monolayer predict a reduction (enhancement) in the in-plane dc electric (intrinsic Hall) conductivity due to a change in the band structure around the Fermi energy when the E -field is introduced. In addition, a magnetization reorientation from the out-of-plane direction to the in-plane direction leads to an abrupt change in the conductivity. Modification of the ac electric conductivity by an E -field is also presented.

The excited states, T-4(2g) and E-2(g), of a Cr impurity in Al2O3 were treated by constraint density functional theory by imposing a density matrix constraint (constraint field) to control the electron occupation numbers of the d orbitals. The calculated excitation energies, directly calculated from the self- consistent total energies of the (4)A(2g) ground states and the various excited states, correctly reproduce the experimental ordering. In addition, we find that there is no stationary solution for the excited T-4(2g) state corresponding to the crystal-field transition state in the usual Kohn-Sham equation, i.e., with no constraint field. By contrast, the excited E-2(g) state of the spin-flip transition state is a (meta-) stable stationary solution, and may be responsible for the long radiative decay lifetime observed in experiments on ruby.

Magnetism and multiplets for metal-phthalocyanine (MPc) molecules with transition-metals (M) of Mn and Co were investigated based on the constraint density functional theory calculations by imposing density matrix constraint on the d-orbital occupation numbers. For the MnPc, the ground state is found to be the 4Eg state with the perpendicular magnetic anisotropy with respect to the molecular plane, while for the CoPc, the ground state is the 2A1g state with a planar magnetic anisotropy.

The magnetocrystalline anisotropy (MA) of Fe-based transition-metal thin films, consisting of only magnetic 3d elements, was systematically investigated from full-potential linearized augmented plane-wave calculations. The results predict that giant MA with a perpendicular magnetic easy axis (PMA) can be achieved by tuning the atomic-layer alignments in an Fe-Ni thin film. This giant PMA arises from the spin-orbit coupling interaction between occupied and unoccupied Ni dx2-y2,xy bands crossing the Fermi level. A promising 3d transition-metal thin film for the MgO-based magnetic tunnel junctions with the giant PMA was, thus, demonstrated.

The importance of interfaces in determining the physical properties of technologically important materials — addressed in the Guest Editors' introduction to the articles in this issue — has stimulated theoretical efforts to determine from first principles a detailed understanding of their chemical, electronic, and mechanical properties. Fortunately, this has now been made possible as a result of the dramatic advances in condensed matter theory made in the last decade, driven in large part by new and sophisticated experiments on high-purity materials that have been well and carefully characterized. Particularly in electronic structure, these advances may be attributable directly to the close collaboration of theoretical and experimental researchers. Indeed, the new found ability to apply fundamental theoretical concepts to real materials (rather than to simple model systems) made possible by using the continued rapid development of computer power, has served to fill the increasingly urgent demand of experimentalists for theoretical interpretation of their data. Also, in some cases, these computational efforts can be used to provide data that would be currently impossible or impractical to obtain experimentally. This development has been an essential element in the phenomenal growth in this area of materials science.More specifically, the advent of accurate self-consistent (local) spin density functional (LSDF) calculations for surfaces, interfaces, and multilayers means that theory is no longer limited to simple, parameter-dependent models. These complex systems are of growing interest because the reduced symmetry, lower coordination number, and availability and role of highly localized surface and interface states offers the possibility of inducing new and exotic phenomena and promotes the possibility of new device applications.

First-principles calculations of electric-field-driven superconductivity at the hydrogenated diamond (110) surface are presented. While the hydrogens on the surface effectively maintain the intrinsic sp(3) covalent nature of diamond, the hole carriers induced by an external negative electric field (E-field) lead to a metallic surface region. Importantly, the concentration of hole carriers, confined within a few carbon layers of thickness similar to 5-10 angstrom below the surface, exceeds 10(21) cm(-3), which is larger than the critical hole density responsible for superconductivity in the boron-doped diamond, while the calculated electron-phonon coupling constants are comparable in magnitude, suggesting the possibility of superconductivity with enhanced critical field.

The structures of αand β-Ag3VO4 were studied via singlecrystal X-ray diffraction (XRD). The transition from α-phase to β-phase was found to occur at 110 °C. Single-crystal XRD revealed that the integrity of the single crystals was maintained as Ag3VO4 reversibly transitioned between α-Ag3VO4 and β-Ag3VO4. The optical and electrical properties of polycrystalline α-Ag3VO4 were studied by diffuse reflectance spectroscopy and impedance spectroscopy. In order to assess the optical and electrical properties of β-Ag3VO4, in situ measurements were performed above the phase-transition temperature. Thin films of α-Ag3VO4 were prepared by combinatorial sputtering and pulsed laser deposition (PLD). The crystallographic, optical, and electrical conductivity properties of the α-Ag3VO4 thin films were compared with the bulk properties.

Multiplets in a ligand field are treated within total-energy density-functional calculations by imposing density-matrix constraints on the d-orbital occupation numbers consistent with the local site and state symmetries. We demonstrate the utility of this approach for the case of isolated Fe phthalocyanine (FePc) molecules with overall D-4h symmetry: We find three stationary states of E-3(g), (3)A(2g), and B-3(2g) symmetries of the Fe2+ ion, and total-energy calculations clearly demonstrate that the ground state is (3)A(2g).By contrast, a columnar stacking of the FePc molecules (alpha-FePc) is found to change the ground state to E-3(g) due to hybridization between adjacent molecules.

We have grown Bi1-x Sbx alloy thin films on CdTe(111)B over a wide range of Sb concentrations (0≤x≤0.183) using MBE. We have observed several differences with the bulk system. The 3.5 and 5.1% Sb alloys show semiconducting behavior, and the Sb concentration with the maximum bandgap is shifted to a lower Sb concentration, from 15% in bulk to 9%. The power factor S2/ρ (where S is thermoelectric power(TEP) and ρ electrical resistivity) peaks at a significantly higher temperature (250K) than previously reported for the bulk alloy (80K). The magnetotransport properties of Bi1-x,Sbx thin films (x = 0, 0.09, and 0.16) and Bi/CdTe superlattices have been determined by applying the Quantitative Mobility Spectrum Analysis (QMSA) and multicarrier fitting to the magneticfield- dependent resistivities and Hall coefficients, using algorithms which account for the strong anisotropy of the mobilities. The calculated S values are in good agreement with experimental results. The structural stability of bulk Bi is studied using the local density linear muffin-tin orbital method. It is shown that the internal displacement changes the Bi electronic structure from a metal to a semimetal, in qualitative agreement with a Jones-Peierls-type transition. The total energy is calculated to have a double well dependence on the internal displacement, and to provide a stabilization of the trigonal phase. We show that an increase of the trigonal shear angle leads to a semimetal-semiconductor transition in Bi.

Stimulated by recent experimental observations of room temperature ferromagnetism of MnxCd1−xGeP2, we investigate the structural, electronic and magnetic properties of these systems as a function of Mn concentration by means of first-principles density-functional-theory-based codes. Moreover, we investigate the effect of the anion substitution (P vs As) in Mn-rich chalcopyirites. Our calculations indicate that the antiferromagnetic alignment is the most stable ordering for all the systems studied, at variance with that experimentally reported. Moreover, we find that there is a slight reduction of the total magnetic moment per Mn atom from ∼5 µB in all the Cd-rich P-based chalcopyrites to ∼4 µB in the Mn-rich MnGeP2 and MnGeAs2 systems.

Using the all-electron full potential linearized augmented plane wave (FLAPW) total energy method, the influence of P impurity atoms on the cohesion of the Fe Σ3[1$\overline 1$10](111) grain boundary is studied through direct comparison of phosphorus/iron interactions in the grain boundary and free surface environments. The calculated nearest P–Fe distance in P/Fe(111) is 2.14 Å—amounting to a 5% contraction compared to that (2.26 Å) measured for the Fe3P compound and assumed for the P–Fe grain boundary. The polar-covalent P–Fe chemical bonding, which is a strong function of the P–Fe interatomic distance, is thus stronger on the Fe(111) surface, while P reduces the spin polarization of the surrounding Fe atoms more efficiently in the grain boundary environment. These effects are examined in terms of the relative segregation energies affecting the work of boundary fracture.

The electric-field-induced switching of magnetocrystalline anisotropy (MCA) between in-plane and out-of-plane orientations is investigated by first-principles calculations for the prototypical Fe on MgO(001) system. Our results predict that an ideal abrupt Fe/MgO interface gives rise to a large out-of-plane MCA due to weak Fe-O hybridization at the interface, but the MCA switching by an applied electric field is found to be difficult to achieve. Instead, the existence of an interfacial FeO layer plays a key role in demonstrating the MCA switching that accompanies an electric-field-induced displacement of Fe atoms on the interfacial FeO layer.

In a search for promising ferromagnets exhibiting a giant magnetocrystalline anisotropy (MCA) change by an external electric field, the MCA in the Fe-Co(001) alloy monolayers is determined by means of full-potential linearized augmented plane-wave method. A large MCA change by the electric field is found to appear in the Fe0.75Co0.25 monolayer, and a reasonable origin from a band structural change in the minority-spin d states is obtained, where a position of Fermi level relative to the d band level is a key factor for designing the giant MCA change.

Experimental magneto-optical Kerr effect spectra of Mn-implanted Ge samples are interpreted using an ab-initio theoretical model based on density functional theory. The presence of Mn in the Ge matrix is accounted for considering either Mn precipitation as Mn5Ge3 nanoparticles or as perfect dilution. The Mn concentration profile in the sample is also considered in the theoretical model. The computational results are in good qualitative agreement if Mn5Ge3 nanocluster formation is assumed. The effect of partial etching of the implanted region is also studied by comparing our experimental and theoretical MOKE spectra data.

Based on all-electron density-functional theory calculations using the generalized gradient approximation, we demonstrate the continuous tunability of the band gap and strength of the built-in electric field in GaN/AlN (0001) superlattices by control of the thickness of both the well (GaN) and barrier (AlN) regions. The effects of strain for these quantities are also studied. Calculations taking into account the self-interaction correction exhibit the same dependence on thickness. The calculated electric field strength values are in good agreement with recent experiments. Spontaneous polarization dominates the contribution to the electric field and the strain-induced piezoelectric polarization is estimated to contribute only about 5-10%.