Showing papers in "Physical Review B in 1981"
TL;DR: In this paper, the self-interaction correction (SIC) of any density functional for the ground-state energy is discussed. But the exact density functional is strictly selfinteraction-free (i.e., orbitals demonstrably do not selfinteract), but many approximations to it, including the local spin-density (LSD) approximation for exchange and correlation, are not.
Abstract: The exact density functional for the ground-state energy is strictly self-interaction-free (i.e., orbitals demonstrably do not self-interact), but many approximations to it, including the local-spin-density (LSD) approximation for exchange and correlation, are not. We present two related methods for the self-interaction correction (SIC) of any density functional for the energy; correction of the self-consistent one-electron potenial follows naturally from the variational principle. Both methods are sanctioned by the Hohenberg-Kohn theorem. Although the first method introduces an orbital-dependent single-particle potential, the second involves a local potential as in the Kohn-Sham scheme. We apply the first method to LSD and show that it properly conserves the number content of the exchange-correlation hole, while substantially improving the description of its shape. We apply this method to a number of physical problems, where the uncorrected LSD approach produces systematic errors. We find systematic improvements, qualitative as well as quantitative, from this simple correction. Benefits of SIC in atomic calculations include (i) improved values for the total energy and for the separate exchange and correlation pieces of it, (ii) accurate binding energies of negative ions, which are wrongly unstable in LSD, (iii) more accurate electron densities, (iv) orbital eigenvalues that closely approximate physical removal energies, including relaxation, and (v) correct longrange behavior of the potential and density. It appears that SIC can also remedy the LSD underestimate of the band gaps in insulators (as shown by numerical calculations for the rare-gas solids and CuCl), and the LSD overestimate of the cohesive energies of transition metals. The LSD spin splitting in atomic Ni and $s\ensuremath{-}d$ interconfigurational energies of transition elements are almost unchanged by SIC. We also discuss the admissibility of fractional occupation numbers, and present a parametrization of the electron-gas correlation energy at any density, based on the recent results of Ceperley and Alder.
16,027 citations
TL;DR: In this paper, it was shown that the quantization of the Hall conductivity of two-dimensional metals is a consequence of gauge invariance and the existence of a mobility gap, and an estimate of the error based on thermal activation of carriers to the mobility edge is suggested.
Abstract: It is shown that the quantization of the Hall conductivity of two-dimensional metals which has been observed recently by Klitzing, Dorda, and Pepper and by Tsui and Gossard is a consequence of gauge invariance and the existence of a mobility gap. Edge effects are shown to have no influence on the accuracy of quantization. An estimate of the error based on thermal activation of carriers to the mobility edge is suggested.
1,905 citations
TL;DR: In this article, the authors generalized the linearized-augmented-plane-wave (LAPW) method for thin films by removing the remaining shape approximation to the potential inside the atomic spheres, and implemented a new technique for solving Poisson's equation for a general charge density and potential is described and implemented in the film LAPW method.
Abstract: The linearized-augmented-plane-wave (LAPW) method for thin films is generalized by removing the remaining shape approximation to the potential inside the atomic spheres. A new technique for solving Poisson's equation for a general charge density and potential is described and implemented in the film LAPW method. In the resulting full-potential LAPW method (FLAPW), all contributions to the potential are completely taken into account in the Hamiltonian matrix elements. The accuracy of the method---already well known for clean metal surfaces---is demonstrated for the case of a nearly free (noninteracting) ${\mathrm{O}}_{2}$ molecule which is a severe test case of the method because of its large anisotropic charge distribution. Detailed comparisons show that the accuracy of the FLAPW results for ${\mathrm{O}}_{2}$ exceeds that of existing state-of-the-art local-density linear-combination-of-atomic-orbitals (LCAO)-type calculations, and that taking the full potential LAPW results as a reference, the LCAO basis can be improved by adding off-site functions. Thus the full-potential LAPW is a unified method which is ideally suited to test not only molecular adsorption on surfaces, but also the components of the same system separately, i.e., the extreme limits of the molecule and the clean surface.
1,459 citations
TL;DR: In this paper, the authors proposed a heuristic approach for the analysis of the conductivity of a one-dimensional (ID) chain in which Landauer's expression was used.
Abstract: The dc conductance 1 of a finite system with static disorder is related to its transmission matrix t by the simple relation 1 = (e /2nt) Tr(t t). This relation is derived from the Kubo formula and is valid for any number of scattering channels with or without time-reversal symmetry. Differences between various definitions of the conductance of a finite system are discussed. Some time ago Landauer' proposed that the dc conductance I' of noninteracting (spinless) electrons in a disordered medium in strictly one dimension is given by I' = (e'/2vrh) ~ r ~'/~ r (' where t and r are the transmission and reflection amplitudes. A relation of this kind, especially if it can be generalized to higher dimensions, is of great interest for at least two reasons. First, the cost of numerical computation may be greatly reduced compared with the conventional use of the Kubo formula. '~ Second, such a relation emphasizes the fundamental role of the conductance which is assumed to be the only relevant variable in a recent scaling theory treatment of the localization problem. " This point of view was discussed in a recent analysis of the conductivity of a one-dimensional (ID) chain9 in which Landauer's expression was used. The argument given by Landauer is a heuristic one and not easily generalized to higher dimensions.
1,234 citations
TL;DR: In this paper, a simple model of disordered systems, called the random energy model, is introduced and solved, and the model exhibits a phase transition and the low-temperature phase is completely frozen.
Abstract: A simple model of disordered systems---the random-energy model---is introduced and solved. This model is the limit of a family of disordered models, when the correlations between the energy levels become negligible. The model exhibits a phase transition and the low-temperature phase is completely frozen. The corrections to the thermodynamic limit are discussed in detail. The magnetic properties are studied, and a constant susceptibility is found at low temperature. The phase diagram in the presence of ferromagnetic pair interactions is described. Many results are qualitatively the same as those of the Sherrington-Kirkpatrick model. The problem of using the replica method is analyzed. Lastly, this random-energy model provides lower bounds for the ground-state energy of a large class of spin-glass models.
1,230 citations
TL;DR: In this paper, the band structure of GaAs-GaAlAs and InAsGaSb superlattices is calculated by matching propagating or evanescent envelope functions at the boundary of consecutive layers.
Abstract: The band structure of GaAs-GaAlAs and InAs-GaSb superlattices is calculated by matching propagating or evanescent envelope functions at the boundary of consecutive layers. For GaAs-GaAlAs materials, the envelope functions are the solutions of an effective Hamiltonian in which both band edges and effective masses are position dependent. The effective-mass jumps modify the boundary conditions which are imposed to the eigenstates of the effective-mass Hamiltonian. In InAs-GaSb superlattices, the dispersion relations, although quite similar to those obtained in GaAs-GaAlAs materials, reflect the genuine symmetry mismatch of InAs (electrons) and GaSb (light-holes) levels. The evolution of the InAs-GaSb band structure with increasing periodicity is calculated and found to be in excellent agreement with previous LCAO results. The dispersion relations of heavy-hole bands are obtained.
1,016 citations
TL;DR: A variational calculation of hydrogenic impurity states in a quantum well has been performed in this article, where the binding energy of donor (acceptor) levels is calculated as a function of layer thickness and of the impurity position.
Abstract: A variational calculation of hydrogenic impurity states in a quantum well has been performed. The binding energy of donor (acceptor) levels is calculated as a function of layer thickness and of the impurity position. It is found that the ground impurity state degeneracy with respect to the impurity position is lifted, leading to the formation of some sort of an "impurity band." The density of states of this impurity band exhibits one or two peaks energetically located at the "band" extrema. This one-dimensional feature can be evidenced in the optical absorption associated with valence subband\ensuremath{\rightarrow}donor transitions, whereas acceptor\ensuremath{\rightarrow}conduction processes are almost featureless. In the case of conduction\ensuremath{\rightarrow}acceptor luminescence a smooth curve is obtained for degenerate electronic distribution, whereas nondegenerate electron\ensuremath{\rightarrow}trapped hole recombination spectra should again exhibit a double peak.
778 citations
623 citations
TL;DR: In this article, it was shown that all interatomic potentials of the classical type (Morse, Lennard-Jones, etc.,) yield, by their very nature, an expansion of the interlayer separation between the topmost surface layers, and that Friedel's tight-binding model for transition metals yields contraction for the (100, (110), and (111) surfaces of a face-centered-cubic transition metal, a result in agreement with experiment.
Abstract: It is shown that all interatomic potentials of the classical type---Morse, Lennard-Jones, etc.,---yield, by their very nature, an expansion of the interlayer separation between the topmost surface layers. No such prediction can be made a priori for the oscillatory-type potentials. Using a simple procedure to compute the relaxations, we also show that Friedel's tight-binding model for transition metals yields contraction for the (100), (110), and (111) surfaces of a face-centered-cubic transition metal, a result in agreement with experiment.
601 citations
TL;DR: In this article, the authors used Raman scattering and x-ray diffraction measurements to correlate finite-size effects on the Raman spectra of nonpolar vibrational modes in BN.
Abstract: Raman scattering and x-ray diffraction measurements are used to correlate finite-size effects on the Raman spectra of nonpolar vibrational modes in BN. The BN microcrystalline samples used exhibited domain sizes varying from 4.4 to 78.5 nm in the plane and 1.5 and 47.5 nm perpendicular to the planes. The Raman measurements indicated that the high-frequency ${E}_{2g}$ mode shifted to higher frequency and broadened as the crystallite size decreased. A formulation of the Raman cross section for scattering from nonpolar microcrystals is presented. The development includes evaluation of the susceptibility correlation function over a limited spatial extent. The results indicate that spectral changes are related to the phonon dispersion, and the wave-vector uncertainty is accounted for. The formulation is applied to describe the BN light scattering results, and good agreement was obtained to describe the observed shift in frequency.
520 citations
TL;DR: In this article, a theory for the properties of vibrational excitations in molecules adsorbed on a metal surface was developed, where the coherent potential approximation (CPA) was used in the treatment of the vibrational interaction between the molecules.
Abstract: We have developed a theory for the properties of vibrational excitations in molecules adsorbed on a metal surface. The coherent potential approximation (CPA) is used in the treatment of the vibrational interaction between the molecules. We show, by interpreting infrared spectra of substitutionally disordered systems consisting of isotopic mixtures of CO on Cu(100), that the molecules interact mainly through their dipole fields. We also show that in interpreting the integrated absorptance in infrared spectroscopy or the relative loss intensity in electron-energy-loss spectroscopy it is necessary to take into account the screening due to the electronic polarizability of the adsorbed molecules. A simplified version of the CPA result is used for a discussion of the absorption spectra of partial monolayers of one isotope. With the assumption that the CO molecules are randomly distributed, comparison between theory and experiment indicates that the dipole-dipole interaction alone is responsible for the coverage-dependent frequency shift for CO adsorbed on a transition metal [Ru(001)], whereas there is an almost equally large counteracting chemical shift on a noble metal [Cu(100)]. The meaning and origin of the dynamical dipole moment of adsorbed CO molecules are discussed. We find that the increase of the dynamical dipole moment (by a factor 2-3) upon adsorption probably is due to charge oscillations between CO $2{\ensuremath{\pi}}^{*}$ molecular orbitals and the metal. Finally, we outline how the theory developed here can be applied to a fundamental step in photosynthesis.
TL;DR: In this paper, a fitting fitting formula for the dielectric screening function of the degenerate electron liquids at metallic and lower densities was proposed, which accurately reproduces the recent Monte Carlo results as well as those of the microscopic calculations and satisfies the selfconsistency conditions in the compressibility sum rule and the short-range correlation.
Abstract: We propose a fitting formula for the dielectric screening function of the degenerate electron liquids at metallic and lower densities which accurately reproduces the recent Monte Carlo results as well as those of the microscopic calculations and which satisfies the self-consistency conditions in the compressibility sum rule and the short-range correlation.
TL;DR: In this paper, the authors used macroscopic diffusion equations for heat and mass transport, cast in a finite-difference form to allow for the temperature and spatial dependences of the thermal conductivity, absorption coefficient, reflectivity, and other quantities.
Abstract: Pulses of radiation from ruby and Nd:YAG $Q$-switched lasers have been used recently to anneal the lattice damage caused by ion implantation of semiconductors. Other similar applications include the laser-induced diffusion of thin dopant films deposited on the surface of samples, recrystallization of doped amorphous films deposited on single-crystal substrates, and the removal of precipitates present after conventional high-temperature dopant diffusion. All of these processes can be understood in terms of models and calculations based on macroscopic diffusion equations for heat and mass transport, cast in a finite-difference form to allow for the temperature and spatial dependences of the thermal conductivity, absorption coefficient, reflectivity, and other quantities. Results of calculations on silicon with the models show that the near-surface region of a sample can melt and stay molten for times of the order of 100 nsec during which dopant diffusion in the liquid state and nonequilibrium segregation during ultrarapid recrystallization are sufficient to explain the major features of the experimental results. In this paper, a description of the model used in our heat-transport calculations is given. Results of the modeling are illustrated by a variety of calculations which should be of particular interest to experimentalists working with pulsed-laser annealing. These results include, e.g., the effects of pulse duration, shape, and energy density, the effects of assumptions made about the latent heat of amorphous silicon, the effects of substrate heating, the role played by the absorption coefficient in determining melt-front penetration, and the duration of surface melting.
TL;DR: Deep-level optical spectroscopy (DLOS) as discussed by the authors uses photostimulated capacitance transients measurements after electrical, thermal, or optical excitation of the sample, i.e., a diode.
Abstract: An experimental method which we call deep-level optical spectroscopy (DLOS) is described. It is based on photostimulated capacitance transients measurements after electrical, thermal, or optical excitation of the sample, i.e., a diode. This technique provides the spectral distribution of both ${\ensuremath{\sigma}}_{n}^{0}(\mathrm{hv})$ and ${\ensuremath{\sigma}}_{p}^{0}(\mathrm{hv})$, the optical cross sections for the transitions between a deep-level and the conduction and valence bands. Besides its sensitivity, DLOS is selective in the double sense that ${\ensuremath{\sigma}}_{n}^{0}(\mathrm{hv})$ and ${\ensuremath{\sigma}}_{p}^{0}(\mathrm{hv})$ are unambiguously separated, and that the signals due to different traps can be resolved from one another. As a result, the ${\ensuremath{\sigma}}^{0}(\mathrm{hv})$ spectra are measured from their threshold up to the energy gap of the semiconductor, over a generally large temperature range. In addition, the straightforward coupling of DLOS with deep-level transient spectroscopy allows a clear identification of the optical spectra with known levels and the simultaneous determination of both thermal and optical properties for each defect. This experimental method has been used to analyze the most commonly observed deep levels in GaAs. For the well known "O" level, a comprehensive analysis of the results obtained through other techniques (as reported in the literature) is given, to compare with our DLOS data. The spectral shape of all ${\ensuremath{\sigma}}_{n}^{0}(\mathrm{hv})$ curves appears to be strongly related to the density-of-states distribution in the conduction band, i.e., transitions towards $\ensuremath{\Gamma}$, $L$, and $X$ minima of this band are generally well resolved; this is a unique feature of DLOS. A simple theoretical model is proposed to take advantage of these newly available experimental data and to explain the sharpness of the ${\ensuremath{\sigma}}_{p}^{0}(\mathrm{hv})$ curves, as compared with, e.g., Lucovsky's model. Phonon coupling is taken into account. A good fit of the DLOS results is obtained with a small number of adjustable parameters: the deep-level envelope wave function extent (in the $\ensuremath{\delta}$-potential approximation), the relative transition probabilities to the various conduction-band minima, and the Franck-Condon parameter. The values thus obtained for these physical parameters are discussed, and finally, all results concerning each trap are summarized on a configuration coordinate diagram.
TL;DR: In this article, the integrated near-field intensity efficiency of metallic spheres immersed in water has been calculated for dipolar and multipolar surface-plasmon modes of Ag, Cu, and Au spheres of varying radii.
Abstract: Enhancement of the Raman scattering and fluorescence emission on noble metals (Ag, Cu, and Au) is believed to be caused in part by large local fields at the incident wavelength on the surface of metallic microstructures, such as colloidal suspensions and surface roughness on electrodes and thin films. For metallic spheres immersed in water, calculations are made of the integrated near-field intensity efficiency (${Q}_{\mathrm{NF}}$) and that part associated only with the radial field component as a function of incident wavelength (200- 1200 nm) and sphere radius (0- 300 nm) which exceeds the usual Rayleigh limit and extends well into the Lorenz-Mie region. The calculated wavelength and radius dependencies of ${\mathrm{Q}}_{\mathrm{NF}}$ are compared with those for the better-known efficiencies: extinction (${Q}_{\mathrm{E}}$), far-field scattering (${Q}_{\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{sca}}$), and absorption (${Q}_{\mathrm{abs}}$). The peak values of these efficiencies have been evaluated when the incident wavelength is in resonance with dipolar and multipolar surface-plasmon modes of Ag, Cu, and Au spheres of varying radii immersed in water.
TL;DR: In this article, the SERS by molecules adsorbed at the surface of colloidal particles has led to analysis of classical electromagnetic field effects which contribute to enhancement of the Raman signals.
Abstract: The observation of SERS by molecules adsorbed at the surface of colloidal particles has led to analysis of classical electromagnetic field effects which contribute to enhancement of the Raman signals. Such effects alone can give rise to large enhancements which are sensitive to particle size, shape and optical constants. The theory is formulated in such a way that specific changes in Raman polarizability which occur upon adsorption can be incorporated, when these are known.
TL;DR: In this paper, the relation between ionic coordinations and the distribution of electric field gradients (EFG) is investigated, and an analytic approximation for the distribution function is derived for amorphous Gd-Ni alloys.
Abstract: The observation of nuclear quadrupole interactions in amorphous solids provides a unique possibility of obtaining information about the angular distribution of local ionic coordinations, complementary to the information about radial distributions deduced from x-ray and neutron diffraction and from extended x-ray absorption fine structure measurements. In the present paper the relation between ionic coordinations and the distribution of electric field gradients (EFG) is investigated. It is shown that the distribution function $P({V}_{\mathrm{zz}},\ensuremath{\eta})$ of the splitting parameters ${V}_{\mathrm{zz}}$ (the electric field gradient) and $\ensuremath{\eta}$ (the asymmetry parameter) in general yields zero probability both for ${V}_{\mathrm{zz}}=0$ and for $\ensuremath{\eta}=0$. For solids which are isotropic on the average, the distribution function of the components ${V}_{\mathrm{ik}}$ of the EFG tensor depends only on two variables, the invariant functions of the tensor components [$\mathrm{Det}({V}_{\mathrm{ik}}) \mathrm{and} \ensuremath{\Sigma}{V}_{\mathrm{ik}}^{2}$]. Expressions for these quantities in terms of the radial coordinates of the ions causing the EFG and of the bond angles between pairs of ions are given. For amorphous solids with random ionic coordination an analytic approximation for the distribution function $P({V}_{\mathrm{zz}},\ensuremath{\eta})$ is derived. This function is strongly dominated by the distribution of ions in the first coordination shell. The results are applied to the analysis of M\"ossbauer spectra of $^{155}\mathrm{Gd}$ in amorphous Gd-Ni alloys.
TL;DR: In this article, the light and heavy-hole two-dimensional exciton term values are determined directly from the excitation spectra of GaAs-${\mathrm{Al}}_{x}{\mathrm-Ga}}_{1\ensuremath{-}x}\mathrm {As}$ heterostructures with GaAs well widths from 42 to 145 \AA{}.
Abstract: The light- and heavy-hole two-dimensional exciton term values are determined directly from the excitation spectra of GaAs-${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ heterostructures with GaAs well widths $L$ from 42 to 145 \AA{}. these values are in excellent agreement with a theoretical model which contains no adjustable parameters. This theory also gives integrated strengths for the excitons which agree with experiment, and ground-state binding energies as a function of $L$.
TL;DR: In this paper, an exact formula for the short-circuit photovoltaic current in a pure crystal in terms of its Bloch states and energy bands was derived, and the authors showed that the anomalous bulk PV effect can occur even in pure non-pyroelectric piezoelectric crystals, e.g., Te and GaP.
Abstract: A theory is presented for the intrinisic anomalous bulk photovoltaic effect observed in noncentrosymmetric crystals, e.g., BaTi${\mathrm{O}}_{3}$. An exact formula is derived for the calculation of the short-circuit photovoltaic current in a pure crystal in terms of its Bloch states and energy bands. Unlike a conventional field or diffusion current, the photovoltaic current is essentially determined by the change of wave functions upon photoexcitation of an electron from the valence to the conduction band. Our theory also reveals that the bulk photovoltaic effect can occur even in pure nonpyroelectric piezoelectric crystals, e.g., Te and GaP, which have no polar axis and therefore no a priori direction for the photovoltaic current.
TL;DR: In this paper, it was shown that helium atoms in a metal lattice are able to cluster with each other, producing vacancies and nearby self-interstitial defects, and that the binding energy of each additional helium atom to these clusters increases with helium concentration.
Abstract: Atomistic calculations are presented which demonstrate that helium atoms in a metal lattice are able to cluster with each other, producing vacancies and nearby self-interstitial defects. Even a small number of helium atoms is found to be sufficient to create these large distortions. As few as five interstitial helium atoms can spontaneously produce a lattice vacancy and nearby self-interstitial. An eight-helium-atom cluster gives rise to two such defects, and 16 helium atoms to more than five self-interstitial vacancy pairs. It was noted that the self-interstitials prefer to agglomerate on the same "side" of the helium cluster rather than to spread themselves out uniformly. The binding energy of each additional helium atom to these clusters increases with helium concentration and the trap is apparently unsaturable. A rate theory using these atomistic binding energies has been used to calculate the kinetics of helium-bubble nucleation and growth. The results are consistent with measurements of the properties of helium resulting from tritium decay.
TL;DR: A three-dimensional solid with an equilibrium concentration of unbound dislocation loops displays a resistance to torsion not present in isotropic liquids as discussed by the authors, and there is residual bond-angle order analogous to that found in the two-dimensional hexatic phase.
Abstract: A three-dimensional solid with an equilibrium concentration of unbound dislocation loops displays a resistance to torsion not present in isotropic liquids. There is residual bond-angle order analogous to that found in the two-dimensional hexatic phase. A bulk phase with bond orientational order may be observable in supercooled liquids. Such a phase would display an angular modulation in monodomain x-ray-diffraction patterns, and would give rise to an intrinsic asymmetry in the limits of supercooling and superheating. Bond-angle order may also be present in glasses. A dislocation-loop mechanism for the smectic-$A$\char22{}to-nematic transition implies anisotropic scaling and fixes the ratio of the transverse and longitudinal correlation length exponents.
TL;DR: This material is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form.
Abstract: Powered by TCPDF (www.tcpdf.org) This material is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user. Puska, Martti; Nieminen, R. M.; Manninen, M.
TL;DR: In this paper, the authors performed a Monte Carlo simulation of high-field transport in GaAs including a realistic band structure to study the band-structure dependence of electron transport and impact ionization.
Abstract: We have performed a Monte Carlo simulation of high-field transport in GaAs including a realistic band structure to study the band-structure dependence of electron transport and impact ionization. The band structure has been calculated using the empirical pseudopotential method. Unlike previous theories of impact ionization, our method is capable of calculating various parameters, such as mean free path, from first principles. The calculated electron mean free path, drift velocity, and impact ionization rate are in reasonable agreement with the experimental data in spite of several simplifications of the model. Within statistical uncertainty we do not observe any orientation dependence of the ionization rate in contradiction to the interpretation of recently reported experimental results. We also find that the contribution of ballistic electrons to impact ionization is negligibly small. Based on the results of the calculation, a general discussion of impact ionization is given.
TL;DR: In this article, the authors examined the electronic excitation spectrum of a bipolaronic crystal and showed that in the low bipolaron density limit the spectrum is superfluidlike, so that bipolarons might be superconducting.
Abstract: It is shown that in narrow-band crystals with sufficiently strong electron-lattice interaction a new energy band occurs. The tunneling motion of localized electron pairs (bipolarons), which is responsible for this band, is caused by virtual transitions of bipolarons to the polaron state. The electronic excitation spectrum of a bipolaronic crystal is examined. It is shown that in the low bipolaron-density limit the excitation spectrum is superfluidlike, so that bipolarons might be superconducting. In case of high density of bipolarons and their strong repulsion, a charge-density wave is predicted.
TL;DR: In this article, current susceptibility measurements on powdered samples of the spin-glass were performed at various temperatures above and below the freezing temperature, and the critical exponent was determined and a distribution of blocking temperatures was determined from the experimental data.
Abstract: Alternating current susceptibility measurements on powdered samples of the spin-glass $\mathrm{Cu}\mathrm{Mn}$ (Mn concentrations: $0.23l~cl~6.3$ at.%) showed relatively broad maxima as well as sharp cusps in ${\ensuremath{\chi}}^{\ensuremath{'}}$ as a function of temperature depending on the method of preparing the sample. In contrast to the broadly peaked ${\ensuremath{\chi}}^{\ensuremath{'}}(T)$ curves, the sharply cusped ones were more affected by small, external, static fields $H$. This field dependence was measured at various temperatures above and below ${T}_{f}$, the freezing temperature. At ${T}_{f}$ the behavior of ${\ensuremath{\chi}}^{\ensuremath{'}}(H)$ was analyzed and the critical exponent $\ensuremath{\delta}$ determined. The susceptibility data for the lower concentration alloys were independent of the measurement frequency ($1 \mathrm{Hz}l\ensuremath{
u}l10 \mathrm{kHz}$) within the absolute experimental accuracy of about 1%. However, in the concentration regime of \ensuremath{\approx} 1 at.% Mn and above, the sharply cusped, so-called "quenched" samples exhibited a small frequency dependence in ${\ensuremath{\chi}}^{\ensuremath{'}}(T)$ near ${T}_{f}$. The relative shift in freezing temperature $\ensuremath{\Delta}\frac{{T}_{f}}{{T}_{f}}$ per decade of frequency was found to be 0.0050 independent of concentration from 1 to 6.3 at.% Mn. Below ${T}_{f}$ the various ${\ensuremath{\chi}}^{\ensuremath{'}}(\ensuremath{
u})$ curves seemed to converge towards a single, nonzero ${\ensuremath{\chi}}^{\ensuremath{'}}$ value as $T\ensuremath{\rightarrow}0$ K. Above ${T}_{f}$ and below about 50 K, the susceptibility obeyed a simple Curie-like law, whereas in the higher temperature region from 100 to 150 K a Curie-Weiss-like behavior was observed with a small, positive paramagnetic Curie-Weiss temperature. To analyze the behavior of ${\ensuremath{\chi}}^{\ensuremath{'}}(T)$ near ${T}_{f}$ and at the lowest temperatures of measurement 0.4 K, the superparamagnetic blocking model of Wohlfarth was used. A distribution of blocking temperatures was thus determined from the experimental ${\ensuremath{\chi}}^{\ensuremath{'}}$ data. This distribution function shows a rather sharp step at ${T}_{f}$, indicating that there are cooperative effects in the freezing of a spin-glass.
TL;DR: In this article, the authors present a new approach to the spin dynamics of the one-dimensional isotropic Heisenberg antiferromagnetic (HB AF) which does not involve the many-body techniques usually employed.
Abstract: Spin-dynamical calculations on one-dimensional systems have relied heavily on classical ($s=\ensuremath{\infty}$) theories, despite abundant evidence that quantum effects can be extremely important at low temperatures. We present a new approach to the spin dynamics of the one-dimensional isotropic $s=\frac{1}{2}$ Heisenberg antiferromagnetic (HB AF) which does not involve the many-body techniques usually employed. It is based on analytic Bethe ansatz calculations of excitation energies and densities of states combined with finite-chain calculations of matrix elements. An important feature of our method is the use of rigorous selection rules and the introduction of new selection rules, which are valid for macroscopic systems in a magnetic field. We show that in zero field the dynamical two-spin correlation function ${S}_{\ensuremath{\mu}\ensuremath{\mu}}(q,\ensuremath{\omega})$ at $T=0$ is governed by a two-parameter continuum of spin-wave-type excitations. In nonzero field, the longitudinal component ${S}_{\mathrm{zz}}(q,\ensuremath{\omega})$ and the transverse components ${S}_{\mathrm{xx}}(q,\ensuremath{\omega})\ensuremath{\equiv}{S}_{\mathrm{yy}}(q,\ensuremath{\omega})$ behave quite differently because they are dominated by different continua of excitations. The former is characterized by a lowest excitation branch with a zero-frequency mode moving from the zone boundary ($q=\ensuremath{\pi}$) towards the zone center ($q=0$) as the field increases, whereas the latter is characterized by a lowest branch with a zero frequency mode moving from $q=0 \mathrm{to} \ensuremath{\pi}$ with increasing field. The first part of our work features an approximate analytic expression for ${S}_{\ensuremath{\mu}\ensuremath{\mu}}(q,\ensuremath{\omega})$ at zero temperature and in zero field. Although our expression is not rigorous, exact sum rules are violated only by a small amount, and good agreement exists with the few known exact results. Our studies are extended to nonzero temperatures by placing major reliance on exact finite-chain calculations. Our work was stimulated by recent neutron scattering experiments and is oriented towards experimental comparisons. Our result for the $s=\frac{1}{2}$ integrated intensity is in much better agreement with neutron scattering data on Cu${\mathrm{Cl}}_{2}$\ifmmode\cdot\else\textperiodcentered\fi{}2N(${\mathrm{C}}_{5}$${\mathrm{D}}_{5}$) (CPC) than the corresponding semiclassical result. Moreover, the spectral-weight distribution in ${S}_{\ensuremath{\mu}\ensuremath{\mu}}(q,\ensuremath{\omega})$ shows increasing asymmetry as $q\ensuremath{\rightarrow}\ensuremath{\pi}$, a quantum effect, again in agreement with more recent neutron scattering data. The second part of our work deals with the effects of an applied magnetic field. We extend the analytic work of Ishimura and Shiba to obtain expressions for the energies and densities of states of the various excitation continua. It is shown that these continua are expected to give rise to multiple structures in the scattering intensity. Our results appear to be in quantitative agreement with preliminary results of a neutron study in CPC in a field of 70 kOe, revealing anomalous scattering intensity peaks. Our results repeatedly demonstrate the inadequacy of classical spin-wave theory for this problem. They call for additional experimental studies on quasi-one-dimensional antiferromagnets to examine other unusual and interesting phenomena predicted by our approach.
TL;DR: The intrinsic electronic surface-state structure in the region of the bulk band gap for the (110), (100), and (001) surfaces of Ti${\mathrm{O}}_{2}$ (rutile) has been determined by fracturing single-crystal samples in ultrahigh vacuum and measuring their ultraviolet photoemission spectra as discussed by the authors.
Abstract: The intrinsic electronic surface-state structure in the region of the bulk band gap for the (110), (100), and (001) surfaces of Ti${\mathrm{O}}_{2}$ (rutile) has been determined by fracturing single-crystal samples in ultrahigh vacuum and measuring their ultraviolet photoemission spectra. None of the faces exhibits an appreciable density of surface states in the bulk band gap, in disagreement with recent calculations for the Ti${\mathrm{O}}_{2}$(001) surface by Kasowski and Tait. The atomic geometry of both perfect and defect surfaces is examined, and the incomplete screening of pairs of surface cations at defect sites is suggested to give rise to occupied band-gap surface states rather than the coordinative unsaturation of surface cations. Hydrogen-exposure experiments indicate that Ti${\mathrm{O}}_{2}$ surfaces may not interact as strongly with hydrogen as has been suggested.
TL;DR: In this paper, the authors present a very simple scheme for calculating the Green's function of a semi-infinite surface system described within a localized orbital basis by generating a series of matching conditions.
Abstract: We present a very simple scheme for calculating the Green's function of a semi-infinite surface system described within a localized orbital basis. By generating a series of matching conditions for the Green's function we can calculate its matrix elements much faster than any method currently available. We present the formalism for a specific class of systems and include a simple example to illustrate the use of the technique.
TL;DR: In this paper, the tunneling density of states of a $p$-wave super-conductor in a magnetic field was calculated, and particular features which allow experimental identification of the superconductors were discussed.
Abstract: Tunneling spectra of hypothetical $p$-wave and conventional $s$-wave superconductors are expected to differ significantly. There is a bulk effect which arises due to the differences in the structure of the Cooper pairs, and a surface effect, which dominates, and is associated with the interface of the tunnel junction. We calculate the tunneling density of states of a $p$-wave super-conductor in a magnetic field and discuss particular features which allow experimental identification of $p$-wave superconductors.