Showing papers in "Physical Review B in 2001"
TL;DR: In this paper, a self-consistent ab initio technique for modeling quantum transport properties of atomic and molecular scale nanoelectronic devices under external bias potentials was proposed, based on density functional theory using norm conserving nonlocal pseudopotentials to define the atomic core and nonequilibrium Green's functions (NEGF's) to calculate the charge distribution.
Abstract: We report on a self-consistent ab initio technique for modeling quantum transport properties of atomic and molecular scale nanoelectronic devices under external bias potentials. The technique is based on density functional theory using norm conserving nonlocal pseudopotentials to define the atomic core and nonequilibrium Green's functions (NEGF's) to calculate the charge distribution. The modeling of an open device system is reduced to a calculation defined on a finite region of space using a screening approximation. The interaction between the device scattering region and the electrodes is accounted for by self-energies within the NEGF formalism. Our technique overcomes several difficulties of doing first principles modeling of open molecular quantum coherent conductors. We apply this technique to investigate single wall carbon nanotubes in contact with an Al metallic electrode. We have studied the current-voltage characteristics of the nanotube-metal interface from first principles. Our results suggest that there are two transmission eigenvectors contributing to the ballistic conductance of the interface, with a total conductance $G\ensuremath{\approx}{G}_{0}$ where ${G}_{0}{=2e}^{2}/h$ is the conductance quanta. This is about half of the expected value for infinite perfect metallic nanotubes.
2,581 citations
TL;DR: In this article, the dispersion of peak positions and intensities with excitation wavelength is used to understand the nature of resonant Raman scattering in carbon and how to derive the local bonding and disorder from the Raman spectra.
Abstract: The Raman spectra of a wide range of disordered and amorphous carbons have been measured under excitation from 785 to 229 nm. The dispersion of peak positions and intensities with excitation wavelength is used to understand the nature of resonant Raman scattering in carbon and how to derive the local bonding and disorder from the Raman spectra. The spectra show three basic features, the D and G around 1600 and 1350 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for visible excitation and an extra T peak, for UV excitation, at \ensuremath{\sim}1060 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The G peak, due to the stretching motion of ${\mathrm{sp}}^{2}$ pairs, is a good indicator of disorder. It shows dispersion only in amorphous networks, with a dispersion rate proportional to the degree of disorder. Its shift well above 1600 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ under UV excitation indicates the presence of ${\mathrm{sp}}^{2}$ chains. The dispersion of the D peak is strongest in ordered carbons. It shows little dispersion in amorphous carbon, so that in UV excitation it becomes like a density-of-states feature of vibrations of ${\mathrm{sp}}^{2}$ ringlike structures. The intensity ratio $I(D)/I(G)$ falls with increasing UV excitation in all forms of carbon, with a faster decrease in more ordered carbons, so that it is generally small for UV excitation. The T peak, due to ${\mathrm{sp}}^{3}$ vibrations, only appears in UV Raman, lying around 1060 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for H-free carbons and around 980 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in hydrogenated carbons. In hydrogenated carbons, the ${\mathrm{sp}}^{3}{\mathrm{C}\ensuremath{-}\mathrm{H}}_{x}$ stretching modes around 2920 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ can be clearly detected for UV excitation. This assignment is confirmed by deuterium substitution.
2,553 citations
TL;DR: In this paper, first-principles based calculations of the tunneling conductance and magnetoconductance of epitaxial ''mathrm{Fe}(100)|\mathm{MgO''(100), ''mgO''.
Abstract: We present first-principles based calculations of the tunneling conductance and magnetoconductance of epitaxial $\mathrm{Fe}(100)|\mathrm{MgO}(100)|\mathrm{Fe}(100)$ sandwiches. Our results indicate that tunneling is much more interesting and complicated than the simple barrier model used previously. We obtain the following general results: (1) Tunneling conductance depends strongly on the symmetry of the Bloch states in the electrodes and of the evanescent states in the barrier layer. (2) Bloch states of different symmetry decay at different rates within the barrier. The decay rate is determined by the complex energy bands of the same symmetry in the barrier. (3) There may be quantum interference between the decaying states in the barrier. This leads to an oscillatory dependence of the tunneling current on ${k}_{\ensuremath{\Vert}}$ and a damped oscillatory dependence on barrier thickness. (4) Interfacial resonance states can allow particular Bloch states to tunnel efficiently through the barrier. For $\mathrm{Fe}(100)|\mathrm{MgO}(100)|\mathrm{Fe}(100)$ our calculations indicate that quite different tunneling mechanisms dominate the conductance in the two spin channels. In the majority channel the conductance is primarily via Bloch electrons with small transverse momentum. One particular state with ${\ensuremath{\Delta}}_{1}$ symmetry is able to effectively couple from the Fe into the MgO. In the minority channel the conductance is primarily through interface resonance states especially for thinner layers. We predict a large magnetoresistance that increases with barrier thickness.
1,823 citations
TL;DR: In this article, the ability of the embedded-atom method (EAM) and the tight-binding (TB) method to predict reliably energies and stability of nonequilibrium structures by taking Cu as a model material was evaluated.
Abstract: We evaluate the ability of the embedded-atom method ~EAM! potentials and the tight-binding ~TB! method to predict reliably energies and stability of nonequilibrium structures by taking Cu as a model material. Two EAM potentials are used here. One is constructed in this work by using more fitting parameters than usual and including ab initio energies in the fitting database. The other potential was constructed previously using a traditional scheme. Excellent agreement is observed between ab initio, TB, and EAM results for the energies and stability of several nonequilibrium structures of Cu, as well as for energies along deformation paths between different structures. We conclude that not only TB calculations but also EAM potentials can be suitable for simulations in which correct energies and stability of different atomic configurations are essential, at least for Cu. The bcc, simple cubic, and diamond structures of Cu were identified as elastically unstable, while some other structures ~e.g., hcp and 9R! are metastable. As an application of this analysis, nonequilibrium structures of epitaxial Cu films on~001!-oriented fcc or bcc substrates are evaluated using a simple model and atomistic simulations with an EAM potential. In agreement with experimental data, the structure of the film can be either deformed fcc or deformed hcp. The bcc structure cannot be stabilized by epitaxial constraints.
1,803 citations
TL;DR: In this paper, the authors study the intrinsic defect physics of ZnO and find that ZnOs cannot be doped p type via native defects, despite the fact that they are shallow donors.
Abstract: ZnO typifies a class of materials that can be doped via native defects in only one way: either n type or p type. We explain this asymmetry in ZnO via a study of its intrinsic defect physics, including ${\mathrm{Zn}}_{\mathrm{O}},$ ${\mathrm{Zn}}_{i},$ ${\mathrm{V}}_{\mathrm{O}},$ ${\mathrm{O}}_{i},$ and ${V}_{\mathrm{Zn}}$ and n-type impurity dopants, Al and F. We find that ZnO is n type at Zn-rich conditions. This is because (i) the Zn interstitial, ${\mathrm{Zn}}_{i},$ is a shallow donor, supplying electrons; (ii) its formation enthalpy is low for both Zn-rich and O-rich conditions, so this defect is abundant; and (iii) the native defects that could compensate the n-type doping effect of ${\mathrm{Zn}}_{i}$ (interstitial O, ${\mathrm{O}}_{i},$ and Zn vacancy, ${V}_{\mathrm{Zn}}),$ have high formation enthalpies for Zn-rich conditions, so these ``electron killers'' are not abundant. We find that ZnO cannot be doped p type via native defects $({\mathrm{O}}_{i},{V}_{\mathrm{Zn}})$ despite the fact that they are shallow (i.e., supplying holes at room temperature). This is because at both Zn-rich and O-rich conditions, the defects that could compensate p-type doping ${(V}_{\mathrm{O}}{,\mathrm{}\mathrm{Zn}}_{i},{\mathrm{Zn}}_{\mathrm{O}})$ have low formation enthalpies so these ``hole killers'' form readily. Furthermore, we identify electron-hole radiative recombination at the ${V}_{\mathrm{O}}$ center as the source of the green luminescence. In contrast, a large structural relaxation of the same center upon double hole capture leads to slow electron-hole recombination (either radiative or nonradiative) responsible for the slow decay of photoconductivity.
1,724 citations
TL;DR: In this article, the Gibbs free energy was calculated to determine the lowest energy structure of a transition metal oxide surface in thermodynamic equilibrium with an oxygen-rich environment, and it was shown that the commonly assumed stoichiometric termination is only favorable at low oxygen chemical potentials, i.e., low pressures and/or high temperatures.
Abstract: Using density-functional theory we calculate the Gibbs free energy to determine the lowest-energy structure of a ${\mathrm{RuO}}_{2}(110)$ surface in thermodynamic equilibrium with an oxygen-rich environment. The traditionally assumed stoichiometric termination is only found to be favorable at low oxygen chemical potentials, i.e., low pressures and/or high temperatures. At a realistic O pressure, the surface is predicted to contain additional terminal O atoms. Although this O excess defines a so-called polar surface, we show that the prevalent ionic model, that dismisses such terminations on electrostatic grounds, is of little validity for ${\mathrm{RuO}}_{2}(110).$ Together with analogous results obtained previously at the (0001) surface of corundum-structured oxides, these findings on (110) rutile indicate that the stability of nonstoichiometric terminations is a more general phenomenon of transition metal oxide surfaces.
1,612 citations
TL;DR: A theory for the ab initio calculation of all-electron NMR chemical shifts in insulators using pseudopotentials is presented in this article, which is formulated for both finite and infinitely periodic systems and is based on an extension to the projector augmented-wave approach of Bl\"ochl [P. E. Blachl, Phys. Rev. B 50, 17 953 (1994)] and the method of Mauri et al.
Abstract: A theory for the ab initio calculation of all-electron NMR chemical shifts in insulators using pseudopotentials is presented. It is formulated for both finite and infinitely periodic systems and is based on an extension to the projector augmented-wave approach of Bl\"ochl [P. E. Bl\"ochl, Phys. Rev. B 50, 17 953 (1994)] and the method of Mauri et al. [F. Mauri, B. G. Pfrommer, and S. G. Louie, Phys. Rev. Lett. 77, 5300 (1996)]. The theory is successfully validated for molecules by comparison with a selection of quantum chemical results, and in periodic systems by comparison with plane-wave all-electron results for diamond.
1,516 citations
TL;DR: This work presents a method for obtaining well-localized Wannier-like functions (WF's) for energy bands that are attached to or mixed with other bands, and finds WF's which are centered at the tetrahedral-interstitial sites, suggesting an alternative tight-binding parametrization.
Abstract: We present a method for obtaining well-localized Wannier-like functions (WF's) for energy bands that are attached to or mixed with other bands. The present scheme removes the limitation of the usual maximally localized WF's method [N. Marzari and D. Vanderbilt, Phys. Rev. B 56, 12 847 (1997)] that the bands of interest should form an isolated group, separated by gaps from higher and lower bands everywhere in the Brillouin zone. An energy window encompassing N bands of interest is specified by the user, and the algorithm then proceeds to disentangle these from the remaining bands inside the window by filtering out an optimally connected N-dimensional subspace. This is achieved by minimizing a functional that measures the subspace dispersion across the Brillouin zone. The maximally localized WF's for the optimal subspace are then obtained via the algorithm of Marzari and Vanderbilt. The method, which functions as a postprocessing step using the output of conventional electronic-structure codes, is applied to the s and d bands of copper, and to the valence and low-lying conduction bands of silicon. For the low-lying nearly-free-electron bands of copper we find WF's which are centered at the tetrahedral-interstitial sites, suggesting an alternative tight-binding parametrization.
1,453 citations
TL;DR: In this paper, a mean field model of ferromagnetic semiconductors mediated by delocalized or weakly localized holes in zinc-blende and wurzite diluted magnetic semiconductor was presented.
Abstract: A mean-field model of ferromagnetism mediated by delocalized or weakly localized holes in zinc-blende and wurzite diluted magnetic semiconductors is presented. The model takes into account strong spin-orbit and $k\ensuremath{\cdot}p$ couplings in the valence band as well as the influence of strain upon the hole density of states. Possible effects of disorder and carrier-carrier interactions, particularly near the metal-to-insulator transition, are discussed. A quantitative comparison between experimental and theoretical results for (Ga,Mn)As demonstrates that the theory describes the values of the Curie temperatures observed in the studied systems as well as explaining the directions of the easy axes and the magnitudes of the corresponding anisotropy fields as a function of biaxial strain. Furthermore, the model reproduces the unusual sign, magnitude, and temperature dependence of the magnetic circular dichroism in the spectral region of the fundamental absorption edge. Chemical trends and various suggestions concerning design of ferromagnetic semiconductor systems are described.
1,347 citations
TL;DR: In this article, the structure and energy of anatase surfaces were investigated and it was shown that the surface energy of a anatase macroscopic crystal is smaller than that of rutile.
Abstract: We present an ab initio density-functional investigation of the structure and energetics of several stoichiometric $1\ifmmode\times\else\texttimes\fi{}1$ low-index surfaces of anatase, a ${\mathrm{TiO}}_{2}$ polymorph $\ensuremath{\sim}9%$ less dense and $\ensuremath{\sim}1.2$ kcal/mol less stable than rutile. Although our calculations do not reproduce the relative ordering of the two phases that is observed experimentally, the calculated bulk structural and elastic properties of both polymorphs are in excellent agreement with the experiment, suggesting that surface relaxations are correctly described as well. As expected, the surface energies of anatase appear to be related to the presence of undercoordinated Ti atoms: the surfaces with fourfold-coordinated Ti atoms have a larger energy than those with fivefold-coordinated Ti. Furthermore, we find that the average surface energy of a ${\mathrm{TiO}}_{2}$ anatase macroscopic crystal is smaller than that of rutile. Finally, patterns in the relaxation of the surface atoms which are common to different surfaces are analyzed.
1,256 citations
TL;DR: In this article, the tunneling magnetoresistance of an epitaxial Fe/MgO/Fe(001) junction was determined without any approximations from the real-space Kubo formula using tight-binding bands fitted to an ab initio band structure of iron and MgO.
Abstract: Calculation of the tunneling magnetoresistance (TMR) of an epitaxial Fe/MgO/Fe(001) junction is reported. The conductances of the junction in its ferromagnetic and antiferromagnetic configurations are determined without any approximations from the real-space Kubo formula using tight-binding bands fitted to an ab initio band structure of iron and MgO. The calculated optimistic TMR ratio is in excess of 1000% for an MgO barrier of $\ensuremath{\approx}20$ atomic planes and the spin polarization of the tunneling current is positive for all MgO thicknesses. It is also found that spin-dependent tunneling in an Fe/MgO/Fe(001) junction is not entirely determined by states at the $\ensuremath{\Gamma}$ point $({\mathbf{k}}_{\ensuremath{\Vert}}=0)$ even for MgO thicknesses as large as $\ensuremath{\approx}20$ atomic planes. All these results are explained qualitatively in terms of the Fe majority- and minority-spin surface spectral densities and the complex MgO Fermi surface.
TL;DR: The visible Raman spectra of poor quality chemical-vapor-deposited diamond is often used as the signature of nanocrystalline diamond as mentioned in this paper, which is not the case for sp-bonded diamond.
Abstract: The peak near 1150 cm 21 in the visible Raman spectra of poor quality chemical-vapor-deposited diamond is often used as the signature of nanocrystalline diamond. We argue that this peak should not be assigned to nanocrystalline diamond or other sp-bonded phases. Its wave number disperses with excitation energy, its intensity decreases with increasing excitation energy, and it is always accompanied by another peak near 1450 cm, which acts similarly. This behavior is that expected for sp-bonded configurations, with their smaller band gap. The peaks are assigned to transpolyacetylene segments at grain boundaries and surfaces.
TL;DR: In this paper, the performance of basis sets made of numerical atomic orbitals is explored in density-functional calculations of solids and molecules, with the aim of optimizing basis quality while maintaining strict localization of the orbitals, as needed for linear scaling calculations.
Abstract: The performance of basis sets made of numerical atomic orbitals is explored in density-functional calculations of solids and molecules. With the aim of optimizing basis quality while maintaining strict localization of the orbitals, as needed for linear-scaling calculations, several schemes have been tried. The best performance is obtained for the basis sets generated according to a new scheme presented here, a flexibilization of previous proposals. Strict localization is maintained while ensuring the continuity of the basis-function derivative at the cutoff radius. The basis sets are tested versus converged plane-wave calculations on a significant variety of systems, including covalent, ionic, and metallic. Satisfactory convergence is obtained for reasonably small basis sizes, with a clear improvement over previous schemes. The transferability of the obtained basis sets is tested in several cases and it is found to be satisfactory as well.
TL;DR: In this paper, two-dimensional lattices of carbon, boron-nitride, and fluorine-carbon compositions are treated with ab initio methods in order to evaluate and compare their mechanical properties in a uniform fashion.
Abstract: Two-dimensional lattices of carbon, boron-nitride, and fluorine-carbon compositions are treated with ab initio methods in order to evaluate and compare their mechanical properties in a uniform fashion. The demonstrated robustness of continuum elasticity up to very small length-scale allows one to define and compute the in-plane stiffness and flexural rigidity moduli of the representative nanoshells of C, BN, and ${\mathrm{C}}_{x}\mathrm{F}$ $(xl~2).$ While only small deviations from linear elasticity are observed for C and BN, fluorination causes significant spontaneous shell folding. We discover that spontaneous curvature in fluorinated nanotubes shifts the energy minimum from a plane sheet towards the very small diameter tubes of (4,4) and even (3,3) indexes. Moreover, their equilibrium cross sections are distinctly polygonal, due to curvature self-localization, with an equilibrium angle of $71\ifmmode^\circ\else\textdegree\fi{}$ at each fluorine row attachment. Our analysis yields a simple physical model coupling the mechanical strain with chemical transformation energies.
TL;DR: In this article, the authors adapt the spin accumulation model of the perpendicular transport in metallic magnetic multilayers to the issue of spin injection from a ferromagnetic metal (F) into a semiconductor (N) by introducing a spin dependent interface resistance (tunnel junction preferably) at the $F/N$ interfaces.
Abstract: We adapt the spin accumulation model of the perpendicular transport in metallic magnetic multilayers to the issue of spin injection from a ferromagnetic metal (F) into a semiconductor (N). We show that the problem of the conductivity mismatch between F and N can be solved by introducing a spin dependent interface resistance (tunnel junction preferably) at the $F/N$ interfaces. In the case of a $F/N/F$ structure, a significant value of the magnetoresistance can be obtained if the junction resistance at the $F/N$ and $N/F$ interfaces is chosen between two threshold values depending on the resistivity, spin diffusion length and thickness of N. The problem is treated for various geometries (vertical or lateral $F/N/F$ structures).
TL;DR: In this article, a linearized version of the augmented planewave (APW) method is proposed to describe the electronic structure everywhere in a crystal potential, which is called APW+lo linearization.
Abstract: The present thesis concerns method development and applications in the field of first principles electronic structure calculations.Augmented planewaves combine the simple planewaves with exact solutions of the Schrodinger equation for a spherical potential. This combination yields a very good set of basis functions for describing the electronic structure everywhere in a crystal potential. In the present work, developments of the original augmented planewave (APW) method are presented. It is shown that the exact APW eigenvalues can be found using information from the eigenvalues of the APW secular matrix. This provides a more efficient scheme to solve the APW eigenvalue problem, than the traditional evaluation of the secular determinant. Further, a new way of linearizing the APW method is presented and compared to the traditional linearized APW method (LAPW). Using a combination of the original APW basis functions and the so called local orbitals (lo), the APW+lo linearization is found to reproduce the results of the LAPW method, but already at a smaller basis set size. Another advantage of the new linearization is a faster convergence of forces, with respect to the basis set size, as compared to the LAPW method.The applications include studies of the non-collinear magnetic configuration in the fcc-based high-temperature phase of iron, γ-Fe. The system is found to be extremely sensitive to volume changes, as well as to a tetragonal distortion of the cubic unit cell. A continuum of degenerate spin spiral configurations, including the global energy minimum, are found for the undistorted crystal. The in-plane anisotropy of the ideal interface between a ferromagnetic layer of bcc Fe and the semiconducting ZnSe crystal is also investigated. In contrast to the four-fold symmetric arrangement of the atoms at the interface, the in-plane magnetic anisotropy displays a large uniaxiality. The calculated easy axes are in agreement with experiments for both Se and Zn terminated interfaces. In addition, calculations of the hyperfine parameters were performed for Li intercalated battery materials.
TL;DR: In this paper, the combined effects of strain and phonon confinement are seen to explain why the Raman peak near $464{cm} in nanoparticles shifts to progressively lower energies and the lineshape of this feature gets progressively broader and asymmetric as the particle size gets smaller.
Abstract: The combined effects of strain and phonon confinement are seen to explain why the Raman peak near $464{\mathrm{cm}}^{\ensuremath{-}1}$ in ${\mathrm{CeO}}_{2\ensuremath{-}y}$ nanoparticles shifts to progressively lower energies and the lineshape of this feature gets progressively broader and asymmetric (on the low-energy side) as the particle size gets smaller. The increasing lattice constant measured for decreasing particle size explains this Raman shift well. The linewidth change is fairly well explained by the inhomogenous strain broadening associated with the small dispersion in particle size and by phonon confinement. The spectra are also likely to be directly affected by the presence of oxygen vacancies. Comparison of the temperature dependence of the Raman lineshape in the nanoparticles and the bulk shows that phonon coupling is no faster in the nanoparticles, so size-dependent phonon coupling does not contribute to the large nanoparticle peak red shifts and broadening at room temperature. Irreversible thermally induced changes are observed in the Raman peak position of the nanoparticles.
TL;DR: In this paper, the enigmatic pseudogap phase of cuprate superconductors is characterized by a hidden broken symmetry of ${d}_{{x}^{2}\ensuremath{-}{y}^{ 2}}$-type.
Abstract: We propose that the enigmatic pseudogap phase of cuprate superconductors is characterized by a hidden broken symmetry of ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-type. The transition to this state is rounded by disorder, but in the limit that the disorder is made sufficiently small, the pseudogap crossover should reveal itself to be such a transition. The ordered state breaks time-reversal, translational, and rotational symmetries, but it is invariant under the combination of any two. We discuss these ideas in the context of ten specific experimental properties of the cuprates, and make several predictions, including the existence of an as-yet undetected metal-metal transition under the superconducting dome.
TL;DR: In this article, a new scheme for calculating the x-ray absorption near edge structure (XANES) based on the finite-difference method is proposed, which allows completely free potential shape and, in particular, is not constrained to the muffin-tin approximation.
Abstract: A new scheme for calculating the x-ray absorption near edge structure (XANES) based on the finite-difference method is proposed. It allows completely free potential shape and, in particular, is not constrained to the muffin-tin approximation. In our approach, the calculation of the final states is performed in real space. The Schr\"odinger equation is solved in a discrete form on the node points of a three-dimensional grid. The unknowns are the values of the wave functions at the grid points. The validity of the method is shown on two different systems the metallic copper and the carbon monoxide molecule; then, the differences resulting from muffin-tin and non-muffin-tin calculations are shown on different typical molecules.
TL;DR: In this article, an ab initio analysis of electron conduction through a molecular device is presented, where charge transfer from the device electrodes to the molecular region is found to play a crucial role in aligning the lowest unoccupied molecular orbital of the molecular orbital to the Fermi level of the electrodes.
Abstract: We present an ab initio analysis of electron conduction through a ${\mathrm{C}}_{60}$ molecular device. Charge transfer from the device electrodes to the molecular region is found to play a crucial role in aligning the lowest unoccupied molecular orbital of the ${\mathrm{C}}_{60}$ to the Fermi level of the electrodes. This alignment induces a substantial device conductance of $\ensuremath{\sim}2.2\ifmmode\times\else\texttimes\fi{}{(2e}^{2}/h).$ A gate potential can inhibit charge transfer, and introduce a conductance gap near ${E}_{F},$ changing the current-voltage characteristics from metallic to semiconducting, thereby producing a field-effect molecular current switch.
TL;DR: In this article, the Wigner-Poisson (or Hartree) system can be reduced to an effective Schroedinger Poisson (SGP) system, in which the SGP equation contains a new nonlinearity for a zero-temperature one-dimensional electron gas.
Abstract: It is shown that, for a large class of statistical mixtures, the Wigner-Poisson (or Hartree) system can be reduced to an effective Schroedinger-Poisson system, in which the Schroedinger equation contains a new nonlinearity. For the case of a zero-temperature one-dimensional electron gas, this additional nonlinearity is of the form vertical bar {Psi} vertical bar{sup 4}. In the long-wavelength limit, the results obtained from the effective Schroedinger-Poisson system are in agreement with those of the Wigner-Poisson system. The reduced model is further used to describe the stationary states of a quantum electron gas and the two-stream instability.
TL;DR: In this paper, the authors reported a giant magnetoelectric (ME) effect that results from stress-mediated electromagnetic coupling in bilayers and multilayers of nickel ferrite and lead zirconate titanate.
Abstract: Materials capable of field conversion, from magnetic to electric or vice versa, are of fundamental and technological importance. We report a giant magnetoelectric (ME) effect that results from stress-mediated electromagnetic coupling in bilayers and multilayers of nickel ferrite and lead zirconate titanate. Samples with layer thickness 10--200 \ensuremath{\mu}m were synthesized by doctor-blade techniques. The magnetoelectric voltage coefficient ${\ensuremath{\alpha}}_{\mathbf{E}}$ ranges from 460 mV/cm Oe in bilayers to 1500 mV/cm Oe for multilayers. The transverse effect is an order of magnitude stronger than longitudinal ${\ensuremath{\alpha}}_{\mathbf{E}}.$ The ME coefficient is maximum at room temperature and a general increase in ${\ensuremath{\alpha}}_{\mathbf{E}}$ is observed with increasing frequency. Data on the dependence of ${\ensuremath{\alpha}}_{\mathbf{E}}$ on volume fraction of the two phases and bias magnetic field are in excellent agreement with a theoretical model for a perfectly bonded bilayer.
TL;DR: In this paper, it was shown that the 6th-order Devonshire theory cannot support a monoclinic phase, and an extension of the theory to higher orders was proposed.
Abstract: Devonshire theory provides a successful phenomenological description of many cubic perovskite ferroelectrics such as ${\mathrm{BaTiO}}_{3}$ via a sixth-order expansion of the free energy in the polar order parameter. However, the recent discovery of a novel monoclinic ferroelectric phase in the PZT system by Noheda et al. [Appl. Phys. Lett. 74, 2059 (1999)] poses a challenge to this theory. Here, we confirm that the sixth-order Devonshire theory cannot support a monoclinic phase, and consider extensions of the theory to higher orders. We show that an eighth-order theory allows for three kinds of equilibrium phases in which the polarization is confined not to a symmetry axis but to a symmetry plane. One of these phases provides a natural description of the newly observed monoclinic phase. Moreover, the theory makes testable predictions about the nature of the phase boundaries between monoclinic, tetragonal, and rhombohedral phases. A ferroelectric phase of the lowest (triclinic) symmetry type, in which the polarization is not constrained by symmetry, does not emerge until the Devonshire theory is carried to twelfth order. A topological analysis of the critical points of the free-energy surface facilitates the discussion of the phase transition sequences.
TL;DR: In this paper, a large power factor (defined as the Seebeck coefficient and resistivity) was found for single crystals of the Ti O 3d conduction band with a carrier density (n) of 0.1.
Abstract: Thermoelectric properties have been investigated for single crystals of ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{TiO}}_{3}$ for $0l~xl~0.1.$ We have found a large power factor (defined as $\mathrm{PF}{=S}^{2}/\ensuremath{\rho},$ S and $\ensuremath{\rho}$ being the Seebeck coefficient and resistivity, respectively), $28\ensuremath{-}36 \ensuremath{\mu}\mathrm{W}/({\mathrm{K}}^{2} \mathrm{cm}),$ at room temperature in the metallic region with carrier density (n) of $(0.2\ensuremath{-}2)\ifmmode\times\else\texttimes\fi{}{10}^{21} {\mathrm{cm}}^{\ensuremath{-}3}.$ The observed PF is comparable to that of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ with much lower $n (\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}{10}^{19} {\mathrm{cm}}^{\ensuremath{-}3}).$ Such an unexpectedly large PF or S at the metallic carrier density is ascribed to the orbital degeneracy (threefold) of the Ti $3d\ensuremath{-}{t}_{2g}$ conduction band and the relatively large effective mass of conduction electrons as well as to the large energy-dependent scattering rate.
TL;DR: In this article, a dynamic model of tunneling between core and trapped charged states is proposed to explain the universal power-law statistics of the blinking events observed in single CdSe nanocrystal quantum dots (QD's).
Abstract: Statistical studies of fluorescence intermittency in single CdSe nanocrystal quantum dots (QD's) reveal a temperature-independent power-law distribution in the histogram of on and off times---the time periods before the QD turns from emitting to nonemitting (bright to dark) and vice versa. Every QD shows a similar power-law behavior for the off-time distribution regardless of temperature, excitation intensity, surface morphology or size. We propose a dynamic model of tunneling between core and trapped charged states to explain the universal power-law statistics of the blinking events observed. The on-time probability distributions show evidence of both a tunneling mechanism similar to the off-time statistics and a secondary, photoinduced process that leads to a truncation of the power law. The same blinking statistics are also observed for single CdTe nanocrystal QD's.
TL;DR: The second nearest-neighbor modified embedded atom method (MEAM) as discussed by the authors has been applied to all bcc transition metals, Fe, Cr, Mo, W, V, Nb, and Ta.
Abstract: The second nearest-neighbor modified embedded atom method (MEAM) [Phys. Rev. B 62, 8564 (2000)], developed in order to solve problems of the original first nearest-neighbor MEAM on bcc metals, has now been applied to all bcc transition metals, Fe, Cr, Mo, W, V, Nb, and Ta. The potential parameters could be determined empirically by fitting to $(\ensuremath{\partial}B/\ensuremath{\partial}P),$ elastic constants, structural energy differences among bcc, fcc and hcp structures, vacancy-formation energy, and surface energy. Various physical properties of individual elements, including elastic constants, structural properties, point-defect properties, surface properties, and thermal properties were calculated and compared with experiments or high level calculations so that the reliability of the present empirical atomic-potential formalism can be evaluated. It is shown that the present potentials reasonably reproduce nonfitted properties of the bcc transition metals, as well as the fitted properties. The effect of the size of radial cutoff distance on the calculation and the compatibility with the original first nearest-neighbor MEAM that has been successful for fcc, hcp, and other structures are also discussed.
TL;DR: In this paper, the microscopic view of grain-boundary sliding is addressed, and two atomic processes are distinguished in the interfaces during sliding: atomic shuffling and stress-assisted free volume migration.
Abstract: Molecular-dynamics computer simulations of a model Ni nanocrystalline sample with a mean grain size of 12 nm under uniaxial tension is reported. The microscopic view of grain-boundary sliding is addressed. Two atomic processes are distinguished in the interfaces during sliding: atomic shuffling and stress-assisted free-volume migration. The activated accommodation processes under high-stress and room-temperature conditions are grain-boundary and triple-junction migration, and dislocation activity.
TL;DR: In this article, a generalization of the Drude formula based on the impulse response approach and Poisson statistics is proposed, which can mimic the infrared properties of poor metals that display a minimum in the optical conductivity at zero frequency.
Abstract: A simple classical generalization of the Drude formula is derived based on the impulse response approach and Poisson statistics. The new feature is a parameter c, which is a measure of persistence of velocity. With negative values of c, it is possible to mimic the infrared properties of poor metals that display a minimum in the optical conductivity at zero frequency. The electron current in these cases reverses direction before decaying to zero. Specific examples considered are Hg and its amalgams, liquid Te, and the quasicrystal ${\mathrm{Al}}_{63.5}{\mathrm{Cu}}_{24.5}{\mathrm{Fe}}_{12}.$ Discussion is offered on the connection with interband transitions, on the distinction between the electron lifetime and the transport relaxation time, and on other generalizations of the Drude formula.
TL;DR: In this article, high-resolution L-edge x-ray absorption spectroscopy (XAS) was used to investigate the interface of antiferromagnetic oxides with ferromagnetic metals.
Abstract: A chemical and magnetic characterization of ferromagnet/antiferromagnet interfaces is essential to understand the microscopic origins of exchange anisotropy and other magnetic phenomena. We have used high-resolution L-edge x-ray absorption spectroscopy (XAS), which is element specific and sensitive to chemical environment and spin orientation, to investigate the interface of antiferromagnetic oxides with ferromagnetic metals. Clear quantitative evidence of oxidation/reduction reactions at the as-grown metal/oxide interface is presented. In situ-- and ex situ--grown samples of the form oxide $(5--30 \AA{})/\mathrm{metal}$ $(1--10 \AA{}),$ where oxide is either NiO or CoO and metal is either Fe, Co, or Ni, were studied by high-resolution XAS. For all samples, a metal(oxide) layer adjacent to an oxide(metal) layer was partially oxidized(reduced). Quantitative analysis of the spectra showed that one to two atomic layers on either side of the interface were oxidized/reduced. An elemental series of samples showed that the amount of oxidation/reduction was in accord with the difference in oxidation potentials of the adjacent cations, e.g., oxide layers were more strongly reduced by an iron metal layer than by cobalt or nickel metal layers. Annealing to temperatures, typically used to bias devices, was shown to significantly increase the amount of oxidation/reduction. The oxidation behavior of iron was shown to depend on the amount of oxygen available. Our results are believed to provide important information for the improved understanding of exchange anisotropy.
TL;DR: In this article, the complete valence-band structure of the molybdenum dichalcogenides (MoS, MoSe, and MoTe) is presented and discussed in comparison.
Abstract: In this work the complete valence-band structure of the molybdenum dichalcogenides ${\mathrm{MoS}}_{2},$ ${\mathrm{MoSe}}_{2},$ and $\ensuremath{\alpha}\ensuremath{-}{\mathrm{MoTe}}_{2}$ is presented and discussed in comparison. The valence bands have been studied using both angle-resolved photoelectron spectroscopy (ARPES) with synchrotron radiation, as well as ab initio band-structure calculations. The ARPES measurements have been carried out in the constant-final-state (CFS) mode. The results of the calculations show in general very good agreement with the experimentally determined valence-band structures allowing for a clear identification of the observed features. The dispersion of the valence bands as a function of the perpendicular component ${k}_{\ensuremath{\perp}}$ of the wave vector reveals a decreasing three-dimensional character from ${\mathrm{MoS}}_{2}$ to $\ensuremath{\alpha}\ensuremath{-}{\mathrm{MoTe}}_{2}$ which is attributed to an increasing interlayer distance in the three compounds. The effect of this ${k}_{\ensuremath{\perp}}$ dispersion on the determination of the exact dispersion of the individual states as a function of ${k}_{\ensuremath{\Vert}}$ is discussed. By performing ARPES in the CFS mode the ${k}_{\ensuremath{\Vert}}$ component for off-normal emission spectra can be determined. The corresponding ${k}_{\ensuremath{\perp}}$ value is obtained from the symmetry of the spectra along the $\ensuremath{\Gamma}A,$ $KH,$ and $\mathrm{ML}$ lines, respectively.