arXiv subjects

A. J. Freeman

Publications and source records attributed to A. J. Freeman.

At least 37 records · Page 2Linked to original sources

arXiv2002

Canted ferromagnetism in RuSr$_2$GdCu$_2$O$_8$

First principles calculations using the full-potential linearized augmented plane wave (FLAPW) method including intra-atomic noncollinear magnetism have been performed to determine the magnetic structures of RuSr$_{2}$GdCu$_{2}$O$_{8}$. The magnetism clearly arises from the RuO$_{6}$ octahedra where the moments on neighboring Ru sites order antiferromagnetically but cant perpendicular to the AFM axis - and so induce a weak ferromagnetism. The projected Ru moments along the AFM and FM axes result in magnetic moments of 1.16 and 0.99 $μ_{B}$ respectively. The results are consistent with the possible coexistence of canted ferromagnetism and superconductivity in the RuSr$_{2}$GdCu$_{2}$O$_{8}$ - inferred from experiments.

cond-mat.supr-con↗

arXiv2002

Accurate first principles detailed balance determination of Auger recombination and impact ionization rates in semiconductors

The technologically important problem of predicting Auger recombination lifetimes in semiconductors is addressed by means of a fully first--principles formalism. The calculations employ highly precise energy bands and wave functions provided by the full--potential linearized augmented plane wave (FLAPW) code based on the screened exchange local density approximation. The minority carrier Auger lifetime is determined by two closely related approaches: \emph{i}) a direct evaluation of the Auger rates within Fermi's Golden Rule, and \emph{ii}) an indirect evaluation, based on a detailed balance formulation combining Auger recombination and its inverse process, impact ionization, in a unified framework. Calculated carrier lifetimes determined with the direct and indirect methods show excellent consistency \emph{i}) between them for $n$-doped GaAs and \emph{ii}%) with measured values for GaAs and InGaAs. This demonstrates the validity and accuracy of the computational formalism for the Auger lifetime and indicates a new sensitive tool for possible use in materials performance optimization.

cond-mat.mtrl-sci↗

arXiv2002

Large anisotropy in the optical conductivity of YNi2B2C

The optical properties of YNi$_2$B$_2$C are studied by using the first-principles full-potential linearized augmented plane wave (FLAPW) method within the local density approximation. Anisotropic behavior is obtained in the optical conductivity, even though the electronic structure shows 3D character. A large peak in $σ_z$ is obtained at 2.4 eV. The anisotropic optical properties are analyzed in terms of interband transitions between energy levels and found that the Ni site plays an important role. The electronic energy loss spectroscopy (EELS) spectra are also calculated to help elucidate the anisotropic properties in this system.

cond-mat.supr-con↗

arXiv2002

Orbital Ordering in Paramagnetic LaMnO3 and KCuF3

{\it Ab-initio} studies of the stability of orbital ordering, its coupling to magnetic structure and its possible origins (electron-phonon and/or electron-electron interactions) are reported for two perovskite systems, LaMnO$_3$ and KCuF$_3$. We present a new Average Spin State (ASS) calculational scheme that allowed us to treat a paramagnetic state. Using this scheme, we succesfully described the experimental magnetic/orbital phase diagram of both LaMnO$_3$ and KCuF$_3$ in crystal structures when the Jahn-Teller distortions are neglected. Hence, we conclude that the orbital ordering in both compounds is purely electronic in origin.

cond-mat.str-el↗

arXiv2001

Electric field gradients in s-, p- and d-metal diborides and the effect of pressure on the band structure and T$_c$ in MgB$_2$

Results of FLMTO-GGA (full-potential linear muffin-tin orbital -- generalized gradient approximation) calculations of the band structure and boron electric field gradients (EFG) for the new medium-T$_c$ superconductor (MTSC), MgB$_2$, and related diborides MB$_2$, M=Be, Al, Sc, Ti, V, Cr, Mo and Ta are reported. The boron EFG variations are found to be related to specific features of their band structure and particularly to the M-B hybridization. The strong charge anisotropy at the B site in MgB$_2$ is completely defined by the valence electrons - a property which sets MgB$_2$ apart from other diborides. The boron EFG in MgB$_2$ is weakly dependent of applied pressure: the B p electron anisotropy increases with pressure, but it is partly compensated by the increase of core charge assymetry. The concentration of holes in bonding $σ$ bands is found to decrease slightly from 0.067 to 0.062 holes/B under a pressure of 10 GPa. Despite a small decrease of N(E$_F$), the Hopfield parameter increases with pressure and we believe that the main reason for the reduction under pressure of the superconducting transition temperature, T$_c$, is the strong pressure dependence of phonon frequencies, which is sufficient to compensate the electronic effects.

cond-mat.supr-con↗

arXiv2001

Impact ionization in GaAs: a screened exchange density functional approach

Results are presented of a fully ab-initio calculation of impact ionization rates in GaAs within the density functional theory framework, using a screened-exchange formalism and the highly precise all-electron full-potential linearized augmented plane wave (FLAPW) method. The calculated impact ionization rates show a marked orientation dependence in {\bf k} space, indicating the strong restrictions imposed by the conservation of energy and momentum. This anisotropy diminishes as the impacting electron energy increases. A Keldysh type fit performed on the energy-dependent rate shows a rather soft edge and a threshold energy greater than the direct band gap. The consistency with available Monte Carlo and empirical pseudopotential calculations shows the reliability of our approach and paves the way to ab-initio calculations of pair production rates in new and more complex materials.

cond-mat.mtrl-sci↗

arXiv2001

Role of structural relaxations, chemical substitutions and polarization fields on the potential line-up in [0001] wurtzite GaN/Al systems

First-principles full-potential linearized augmented plane wave (FLAPW) calculations are performed to clarify the role of the interface geometry on piezoelectric fields and on potential line-ups at the [0001]-wurtzite and [111]-zincblende GaN/Al junctions. The electric fields (polarity and magnitude) are found to be strongly affected by atomic relaxations in the interface region. A procedure is tested to evaluate the Schottky barrier in the presence of electric fields and used to show that their effect is quite small (a few tenths of an eV). These calculations assess the rectifying behaviour of the GaN/Al contact, giving very good agreement with experimental values for the barrier. Stimulated by the complexity of the problem, we disentangle chemical and structural effects on the relevant properties (such as the potential discontinuity and electric fields) by studying auxiliary unrelaxed nitride/metal systems. Focusing on simple electronegativity arguments, we outline the leading mechanisms that result in the final values of the electric fields and Schottky barriers in these ideal interfaces. Finally, the transitivity rule in the presence of two inequivalent junctions is proved to give reliable results.

cond-mat.mtrl-sci↗

arXiv2001

Electronic structure of superconducting MgB2 and related binary and ternary borides

First principles FLMTO-GGA electronic structure calculations of the new medium-$T_C$ superconductor (MTSC) $MgB_2$ and related diborides indicate that superconductivity in these compounds is related to the the existence of $p_{x,y}$-band holes at the $Γ$ point. Based on these calculations, we explain the absence of medium-$T_C$ superconductivity for $BeB_2$, $AlB_2$ $ScB_2$ and $YB_2$. The simulation of a number of $MgB_2$-based ternary systems using a supercell approach demonstrates that (i) the electron doping of $MgB_2$ (i.e., $MgB_{2-y}X_y$ with X=Be, C, N, O) and the creation of isoelectronic defects in the boron sublattice (nonstoichiometric $MgB_{y<2}$) are not favorable for superconductivity, and (ii) a possible way of searching for similar MTSC should be via hole doping of $MgB_2$ (i.e., $Mg_{1-x}M_xB_2$ with M=Be, Ca, Li, Na, Cu, Zn) or $CaB_2$ or via creating layered superstructures of the $MgB_2/CaB_2$ type. A recent report of superconductivity in Cu doped $MgB_2$ supports this view.

cond-mat.supr-con↗

arXiv2001

Energetics and bonding properties of the Ni/$β$-SiC (001) interface: an ab-initio study

We investigate the adsorption of a Ni monolayer on the $β$-SiC(001) surface by means of highly precise first-principles all-electron FLAPW calculations. Total energy calculations for the Si- and C-terminated surfaces reveal high Ni-SiC adsorption energies, with respect to other metals, confirmining the strong reactivity and the stability of the transition metal/SiC interface. These high binding energies, about 7.3-7.4 eV, are shown to be related to strong $p$-$d$ hybridization, common to both surface terminations and different adsorption sites and despite the large mismatch, can stabilize overlayer growth. A detailed analysis of the bonding mechanism, hybridization of the surface states, charge transfer and surface core level shifts reveals the strong covalent character of the bonding. After a proper accounting of the Madelung term, the core level shift is shown to follow the charge transfer trend.

cond-mat.mtrl-sci↗

arXiv2001

Coordination and chemical effects on the structural, electronic and magnetic properties in Mn pnictides

Simple structures of MnX binary compounds, namely hexagonal NiAs and zincblende, are studied as a function of the anion (X = Sb, As, P) by means of the all-electron FLAPW method within local spin density and generalized gradient approximations. An accurate analysis of the structural, electronic and magnetic properties reveals that the cubic structure greatly favours the magnetic alignment in these compounds leading to high magnetic moments and nearly half-metallic behaviour for MnSb and MnAs. The effect of the anion chemical species is related to both its size and the possible hybridization with the Mn $d$ states; both contributions are seen to hinder the magnitude of the magnetic moment for small and light anions. Our results are in very good agreement with experiment - where available - and show that the generalized gradient approximation is essential to correctly recover both the equilibrium volume and magnetic moment.

cond-mat.mtrl-sci↗

arXiv2000

Magnetic and electronic structures of superconducting RuSr$_2$GdCu$_2$O$_8$

The coexistence of ferromagnetism and superconductivity in RuSr$_2$GdCu$_2$O$_8$ was reported both from experiments (by Tallon et. al.) and first-principles calculations (by Pickett et. al.). Here we report that our first-principles full-potential linearized augmented plane wave (FLAPW) calculations, employing the precise crystal structure with structural distortions (i.e., RuO$_6$ rotations) determined by neutron diffraction, demonstrate that antiferromagnetic ordering of the Ru moments is energetically favored over the previously proposed ferromagnetic ordering. Our results are consistent with recently performed magnetic neutron diffraction experiments (Lynn et. al). Ru $t_{2g}$ states, which are responsible for the magnetism, have only a very small interaction with Cu $e_g$ states, which results in a small exchange splitting of these states. The Fermi surface, characterized by strongly hybridized $dpσ$ orbitals, has nesting features similar to those in the two-dimensional high $T_c$ cuprate superconductors.

cond-mat.supr-con↗

arXiv2000

New features of dislocation structures arising from lattice discreteness

New aspects of a relation between lattice and dislocation structures are examined within a physically transparent theoretical scheme. Predicted features originating from the lattice discreteness include: (i) multiple core dislocation structures and (ii) their dependence on the position of the dislocation axis. These effects, which in principle can be observed directly and may also manifest themselves in dislocation motion or/and transformation (cross-slip) characteristics, are very general and present in any crystal in which they may be more or less pronounced depending on the material.

cond-mat.mtrl-sci↗

arXiv2000

Metal induced gap states and Schottky barrier heights at non-reactive GaN/noble metal interfaces

We present ab-initio local density FLAPW calculations on non-reactive N-terminated [001] ordered GaN/Ag and GaN/Au interfaces and compare the results (such as metal induced gap states and Schottky barrier heights) with those obtained for GaN/Al, in order to understand the dependence of the relevant electronic properties on the deposited metal. Our results show that the density of gap states is appreciable only in the first semiconductor layer close to the interface. The decay length of the gap states in the semiconductor side is about 2.0 $\pm$ 0.1 Å$\:$ and is independent of the deposited metal, therefore being to a good extent a bulk property of GaN. Our calculated values of the Schottky barrier heights are $Φ_{B_p}(GaN/Ag)$ = 0.87 eV and $Φ_{B_p}(GaN/Au)$ = 1.08 eV; both values are smaller than the GaN/Al value ($Φ_{B_p}(GaN/Al)$ = 1.51 eV) and this quite large spread of values excludes the possibility of a Fermi level pinning within the GaN band gap. Because of the low screening in GaN, the potential barrier at the junction is strongly affected by the structural arrangement of the first metal layer at the interface. This leads to quite large variations of the Schottky barrier height as a function of the metal, in contrast with the behavior of GaAs/metal interfaces.

cond-mat.mtrl-sci↗

arXiv1999

Magnetoelastic mechanism of spin-reorientation transitions at step-edges

The symmetry-induced magnetic anisotropy due to monoatomic steps at strained Ni films is determined using results of first - principles relativistic full-potential linearized augmented plane wave (FLAPW) calculations and an analogy with the Néel model. We show that there is a magnetoelastic anisotropy contribution to the uniaxial magnetic anisotropy energy in the vicinal plane of a stepped surface. In addition to the known spin-direction reorientation transition at a flat Ni/Cu(001) surface, we propose a spin-direction reorientation transition in the vicinal plane for a stepped Ni/Cu surface due to the magnetoelastic anisotropy. We show that with an increase of Ni film thickness, the magnetization in the vicinal plane turns perpendicular to the step edge at a critical thickness calculated to be in the range of 16-24 Ni layers for the Ni/Cu(1,1,13) stepped surface.

cond-mat.mtrl-sci↗

arXiv1998

Generalized stacking fault energetics and dislocation properties: compact vs. spread unit dislocation structures in TiAl and CuAu

We present a general scheme for analyzing the structure and mobility of dislocations based on solutions of the Peierls-Nabarro model with a two component displacement field and restoring forces determined from the ab-initio generalized stacking fault energetics (ie., the so-called $γ$-surface). The approach is used to investigate dislocations in L1$_{0}$ TiAl and CuAu; predicted differences in the unit dislocation properties are explicitly related with features of the $γ$-surface geometry. A unified description of compact, spread and split dislocation cores is provided with an important characteristic "dissociation path" revealed by this highly tractable scheme.

cond-mat↗

arXiv1998

Influence of the exchange reaction on the electronic structure of GaN/Al junctions

Ab-initio calculations have been used to study the influence of the interface morphology and, notably, of the exchange reaction on the electronic properties of Al/GaN (100) interfaces. In particular, the effects of interface structure (i.e. interfacial bond lengths, semiconductor surface polarity, and reacted intralayers) on the SBH at the Al/GaN (001) junction are specifically addressed. The electronic structure of the following atomic configurations have been investigated theoretically: (i) an abrupt, relaxed GaN/Al interface; (ii) an interface which has undergone one monolayer of exchange reaction; and interfaces with a monolayer-thick interlayer of (iii) AlN and (iv) Ga$_{0.5}$Al$_{0.5}$N. Intermixed interfaces are found to pin the interface Fermi level at a position not significantly different from that of an abrupt interface. Our calculations also show that the interface band line--up is not strongly dependent on the interface morphology changes studied. The p-type SBH is reduced by less than 0.1 eV if the GaN surface is Ga-terminated compared to the N-terminated one. Moreover, we show that both an ultrathin Ga$_x$Al$_{1-x}$N ($x$ = 0, 0.5) intralayer and a Ga$\leftrightarrow$Al atomic swap at the interface does not significantly affect the Schottky barrier height.

cond-mat.mtrl-sci↗

arXiv1997

Electric fields and valence band offsets at strained [111] heterojunctions

[111] ordered common atom strained layer superlattices (in particular the common anion GaSb/InSb system and the common cation InAs/InSb system) are investigated using the ab initio full potential linearized augmented plane wave (FLAPW) method. We have focused our attention on the potential line-up at the two sides of the homopolar isovalent heterojunctions considered, and in particular on its dependence on the strain conditions and on the strain induced electric fields. We propose a procedure to locate the interface plane where the band alignment could be evaluated; furthermore, we suggest that the polarization charges, due to piezoelectric effects, are approximately confined to a narrow region close to the interface and do not affect the potential discontinuity. We find that the interface contribution to the valence band offset is substantially unaffected by strain conditions, whereas the total band line-up is highly tunable, as a function of the strain conditions. Finally, we compare our results with those obtained for [001] heterojunctions.

cond-mat.mtrl-sci↗

arXiv1997

Influence of growth direction and strain conditions on the band line-up at GaSb/InSb and InAs/InSb interfaces

First-principles full potential linearized augmented plane wave (FLAPW) calculations have been performed for lattice-mismatched common-atom III-V interfaces. In particular, we have examined the effects of epitaxial strain and ordering direction on the valence band offset in [001] and [111] GaSb/InSb and InAs/InSb superlattices, and found that the valence band maximum is always higher at the InSb side of the heterojunction, except for the common-anion system grown on an InSb substrate. The comparison between equivalent structures having the same substrate lattice constant, but different growth axis, shows that for comparable strain conditions, the ordering direction slightly influences the band line-up, due to small differences of the charge readjustment at the [001] and [111] interfaces. On the other hand, strain is shown to strongly affect the VBO; in particular, as the pseudomorphic growth conditions are varied, the bulk contribution to the band line-up changes markedly, whereas the interface term is almost constant. On the whole, our calculations yield a band line-up that decreases linearly as the substrate lattice constant is increased, showing its high tunability as a function of different pseudomorphic growth conditions. Finally, the band line-up at the lattice matched InAs/GaSb interface determined using the transitivity rule gave perfect agreement between predicted and experimental results.

cond-mat.mtrl-sci↗