Electronic Response Function Research Articles

Quasielastic electron scattering in a derivative coupling model with relativistic random phase approximation

We apply the derivative coupling model with ZM and ZM3 parameters to investigate the longitudinal response function in quasielastic electron scattering in the relativistic random phase approximation. The non-spectral method is chosen to describe the nucleon Green's function in a finite nucleus. Some remarks have been made in conclusion.

Effect of dimensionality and spin-orbit coupling on charge-density-wave transition in 2H-TaSe2

A first-principles investigation of the charge-density-wave (CDW) instability in bulk and single-layer 2H-TaSe${}_{2}$ is presented, focusing on the origin of the CDW instability, the role of the interlayer interactions, and the effect of spin-orbit coupling. While interlayer interactions and spin-orbit coupling have a nontrivial effect on the electronic structure and Fermi surface, the CDW instability is predicted to remain robust, with little or no change in the ordering wave vector. This is in contrast to the closely related 2H-NbSe${}_{2}$ material, where the CDW wave vector depends on dimensionality. The results are analyzed in terms of the interplay between the momentum dependence of the electron-phonon coupling and that of the electronic response function.

Theory of High Temperature Superconductivity in Cuprates and Other Doped Polar Insulators

In the last two decades, there have been tremendous attempts to build an adequate theory of high-temperature superconductivity. Most studies used some model Hamiltonians with input parameters not directly related to the material. The dielectric response function of electrons in strongly correlated high-temperature superconductors is a priori unknown. Hence, one has to start with the generic Hamiltonian including unscreened Coulomb and Frohlich electron–phonon interactions operating on the same scale since any ad-hoc assumption on their range and relative magnitude might fail. Using such a generic Hamiltonian, the analytical theory of high-temperature superconductivity in doped polar insulators predicting the critical temperature in excess of a hundred Kelvin without any adjustable parameters has been built. The many-particle electron system is described by an analytically solvable polaronic t–Jp Hamiltonian with reduced hopping integral, t, allowed double on-site occupancy, large phonon-induced antiferromagnetic exchange, Jp>t, and a high-temperature superconducting state of small superlight bipolarons protected from clustering. Here, major steps of the theory are outlined suggesting that the true origin of high-temperature superconductivity is found in a proper combination of strong electron–electron correlations with a significant finite-range (Frohlich) EPI, and that the theory is fully compatible with the key experiments.

Vibrational contributions to cubic response functions from vibrational configuration interaction response theory

In continuation of our recent paper on vibrational quadratic response functions for vibrational configuration interaction wave functions, we present in this paper a derivation and implementation of the pure vibrational cubic response function for vibrational configuration interaction wave functions. In addition, we present combined electronic and vibrational cubic response functions derived from sum-over-states expressions in the Born-Oppenheimer framework and a discussion of complicating issues. The implementation enables analytic calculation of the pure vibrational cubic response function via response theory, which constitutes a part of the vibronic cubic response function.

Simulation of Optical Properties of Mn-Doped ZnO Based on Density Functional Theory

The electronic structures and optical linear response functions of ZnO are calculated. The relationships between electronic structures and optical properties are investigated by using first-principles based upon the density functional theory. The dielectric functions , reflection spectra ,refractive index and of ZnO dominated by electron inter-band transitions are analyzed interms of the precisely calculated band structure and Conductivity density of state. Furthermore, we analyzed the change of electron structure, bonding and optical properties after doping in comparison with the experimental results. The theoretical results offering theoretical data for the design and application of optoelectronic materials of ZnO. Meanwhile , the calculated results also enable more precise monitoring and controlling during the growth of ZnO material .

Lattice softening effects at the Mott critical point of Cr-doped V2O3

We have performed sound velocity measurements in (V${}_{1\ensuremath{-}x}$Cr${}_{x}$)${}_{2}$O${}_{3}$ in the vicinity of the critical point of the first-order Mott transition line. The pressure sweeps at constant temperature reveal a large dip in the ${c}_{33}$ compression modulus; this dip sharpens as the critical point is approached. We do not observe signs of criticality on the shear modulus ${c}_{44}$, which is consistent with a transition governed by a scalar order parameter, in accordance with the dynamic mean field theory (DMFT) description of the transition. However, the amplitude of the effect is an order of magnitude smaller than the one obtained from DMFT calculations for a single-band Hubbard model. We analyze our results using a simple model with the electronic response function obtained from the scaling relations for the conductivity.

Automated calculation of anharmonic vibrational contributions to first hyperpolarizabilities: Quadratic response functions from vibrational configuration interaction wave functions

Quadratic response functions are derived and implemented for a vibrational configuration interaction state. Combined electronic and vibrational quadratic response functions are derived using Born-Oppenheimer vibronic product wave functions. Computational tractable expressions are derived for determining the total quadratic response contribution as a sum of contributions involving both electronic and vibrational linear and quadratic response functions. In the general frequency-dependent case this includes a new and more troublesome type of electronic linear response function. Pilot calculations for the FH, H(2)O, CH(2)O, and pyrrole molecules demonstrate the importance of vibrational contributions for accurate comparison to experiment and that the vibrational contributions in some cases can be very large. The calculation of transition properties between vibrational states is combined with sum-over-states expressions for analysis purposes. On the basis of this some simple analysis methods are suggested. Also, a preliminary study of the effect of finite lifetimes on quadratic response functions is presented.

Local-field-correction effects on the electron response functions and on the electrical conductivity in a hydrogen plasma

In a one-component plasma constituted by electrons with a uniform positive background, the correlations are studied in the framework of the Singwi, Tosi, Land, and Sjölander (STLS) model. The local-field corrections (LFCs) are calculated with electron density ranging from 10;{19}to10;{26}cm;{-3} and with a temperature of 10;{4}K . Then, a dielectric formalism is used to deduce the potential energy of a positive test charge (a proton) imbedded in the electron medium. A significant departure of this potential from the random phase approximation (RPA) one has been found. In particular, as the density increases, the discrepancy between the two potentials grows up to reach a maximum correlated to the maximum of the electron coupling parameter. In addition, it is found that in both high and low density limits, the STLS and RPA approaches yield similar results. On the other hand, the effect of the LFC on the electrical conductivity is also estimated in hydrogen plasmas.

Energy loss spectra of lithium under pressure

According to the nearly free-electron-like approximation, electrons in alkaline metals are expected to be delocalized under pressure. However, recent theoretical and experimental results strikingly indicate the opposite behavior. For example, under pressure electronic band structure flattens anisotropically and increases the covalent character of alkaline metals, which induces the observed complexities in these systems. Specially considering that even at ambient pressure the observed anomalous features in the loss spectra of lithium cannot be explained within the nearly free-electron-like model, these additional anomalies under pressure are also expected to strongly modify its dynamic electronic response function. In this paper, we present ab initio calculations of the energy-loss spectra of lithium up to 40 GPa. Besides analyzing the dispersion of the new type of plasmons arising in lithium under pressure, the role of band structure versus crystal local field effects and exchange correlation has also been investigated.

Fermi surface deformation in lithium under high pressure

Since recent both theoretical and experimental results have proved that the simple behaviour light alkaline metals present at equilibrium breaks when high pressures are applied, they have become an important object of study in Condensed Matter Physics. In this work, we present an analysis of the pressure-induced Fermi surface (FS) deformation in lithium with increasing pressure. The spheroidal FS at equilibrium becomes highly anisotropic with increasing pressure, so that at around 8 GPa touches the FS explaining the observed bcc→fcc transition via a Hume–Rothery effect, and at around 30 GPa develops increasing cooper-like necks and an extended nesting which could be the origin of the complex phase transitions produced via a phonon instability. On the other hand, these distortions in the FS is expected to enhance the electronic susceptibility response function and increase the electron–phonon interaction with pressure, which might also allow a better understanding of the observed superconducting transition in lithium at around the same pressure range.

Complexity and Fermi surface deformation in compressed lithium

Recently reported structural complexity and enhanced temperature superconducting transitions in lithium under pressure have increased the interest in light alkalies, otherwise considered as simple and well-known systems under normal conditions. Here we present an analysis of the pressure-induced Fermi surface deformation in lithium and its relation to the observed complexity. According to our calculations, the Fermi surface becomes increasingly anisotropic with pressure and at $8\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ contacts the Brillouin zone boundary inducing a Hume-Rothery mechanism explaining the bcc-fcc transition. Around $30\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ increasing cooper-like necks and an extended nesting are observed in the Fermi surface in the fcc phase, enhancing the electronic susceptibility response function and inducing a strong phonon softening. This softening, besides preluding the transition to complex structures and providing a better understanding of the observed superconductivity, is expected to induce other yet unexplored anomalies in compressed lithium.

Electronic structure and response functions in random alloys: an application of blockrecursion and Green matrices

We present here a generalization of the recursion method of Haydock et al (1972J. Phys. C: Solid State Phys. 5 2845) for the calculation of Green matrices (in angularmomentum space). Earlier approaches concentrated on the diagonal elements, since thefocus was on spectral densities. However, calculations of configuration averaged responsefunctions or neutron scattering cross-sections require the entire Green matrices andself-energy matrices obtained from them. This necessitated the generalization of therecursion method presented here, with examples.

Electronic Raman response in anisotropic metals

Using a generalized response theory we derive the electronic Raman response function for metals with anisotropic relaxation rates. The calculations account for the long-range Coulomb interaction and treat the collision operator within a charge conserving relaxation time approximation. We extend earlier treatments to finite wave numbers $(\ensuremath{\mid}\mathbf{q}\ensuremath{\mid}⪡{k}_{\mathrm{F}})$ and incorporate inelastic electron-electron scattering besides elastic impurity scattering. Moreover we generalize the Lindhard density response function to the Raman case. Numerical results for the quasiparticle scattering rate and the Raman response function for cuprate superconductors are presented.

Adaptive linear prediction of radiation belt electrons using the Kalman filter

Prior studies have examined the time‐stationary (and quasi‐stationary) dynamic response of relativistic electrons in the Earth's outer radiation belt to changes in solar wind bulk speed using linear prediction filters [Baker et al., 1990; Vassiliadis et al., 2002]. For this study, we have implemented an adaptive system identification scheme, based on the Kalman filter with process noise, to determine optimal time‐dependent electron response functions. The nonlinear dynamic response of the radiation belts can then be tracked in time by recursively updating the optimal linear filter coefficients as new observations become available. We demonstrate a significant improvement in zero‐time‐lag electron log‐flux “predictions” relative to models that are based on time‐stationary linear prediction filters, while incurring only a slight increase in computational complexity. We conclude by discussing modifications necessary for an operational specification and forecast model, including the assimilation of real‐time data, more sophisticated model structures, and a more practical gridded description of the radiation belt state.

Relations among several nuclear and electronic density functional reactivity indexes

An expansion of the energy functional in terms of the total number of electrons and the normal coordinates within the canonical ensemble is presented. A comparison of this expansion with the expansion of the energy in terms of the total number of electrons and the external potential leads to new relations among common density functional reactivity descriptors. The formulas obtained provide explicit links between important quantities related to the chemical reactivity of a system. In particular, the relation between the nuclear and the electronic Fukui functions is recovered. The connection between the derivatives of the electronic energy and the nuclear repulsion energy with respect to the external potential offers a proof for the “Quantum Chemical le Chatelier Principle.” Finally, the nuclear linear response function is defined and the relation of this function with the electronic linear response function is given.

Delta-isobar relativistic meson exchange currents in quasielastic electron scattering

We study the role of the Δ-isobar current on the response functions for high energy inclusive quasielastic electron scattering from nuclei. We consider a general Lagrangian which is compatible with contact invariance and perform a fully relativistic calculation in first-order perturbation theory for one-particle emission. The dependence of the responses upon off-shell parametrizations is analyzed and found to be mild. A discussion of scaling behaviour and a comparison with various non-relativistic approaches are also presented.

Optoelectronic Simulation of the PMS 260X Optical Array Probe and Application to Drizzle in a Marine Stratocumulus

Abstract Measurement of drizzle drop sizes and concentrations are often made using optical probes with linear arrays. Their accuracy is affected by both diffraction of light and response times of the electronics connected to the sensor photodiodes. In this study the behavior of the Particle Measuring Systems, Inc. (PMS), 260X probe (a 62-photodiode system) is studied by calculating the two-dimensional Fresnel diffraction patterns for opaque disks and averaging this optical signal onto areas corresponding to the sizes of the photodiodes. The two-dimensional optical field is then transformed to a one-dimensional optical time-varying field; this simulates the passage of a drop past the photodiode array when the probe is mounted outside an aircraft. The optical field is convolved with an electronic response function to simulate the variation of the electronic signal as a drop passes through the diode array. For aircraft speeds less than 120 m s−1, one component of the probe sample volume, the depth of field, ...

Longitudinal response functions for quasielastic electron scattering in relativistic non-linear models

The longitudinal response functions for quasielastic electron scattering on 12C, 40Ca and 56Fe have been calculated in relativistic non-linear models taking into account RPA correlations. For these calculations, a covariant, consistent, calculation of the nuclear matter linear response has been performed. The effect of the non-linear terms on the longitudinal response has been discussed.

Relativistic pionic effects in quasielastic electron scattering

The impact of relativistic pionic correlations and meson-exchange currents on the response functions for electromagnetic quasielastic electron scattering from nuclei is studied in detail. Results in first-order perturbation theory are obtained for one-particle emission electronuclear reactions within the context of the relativistic Fermi gas model. Improving upon previous analyses where nonrelativistic reductions of the currents were performed, here a fully relativistic analysis in which both forces and currents are treated consistently is presented. Lorentz covariance is shown to play a crucial role in enforcing the gauge invariance of the theory. Effects stemming uniquely from relativity in the pionic correlations are identified and, in particular, a comprehensive study of the self-energy contributions and of the currents associated with the pion is presented. First- and second-kind scaling for high momentum transfer is investigated.

Ab initio fermi surface calculation for charge-density wave instability in transition metal oxide bronzes.

The electronic structure of the charge density wave (CDW) bronze (PO2)(4)(WO3)(2m), m = 4, is determined using ab initio density functional theory. The calculation shows that the Fermi surface (FS) consists in the superposition of three one-dimensional FS's associated with three types of chains. The q dependence of the electronic response function calculated from the electronic structure quantitatively accounts for the anisotropy of the fluctuations probed by x-ray diffuse scattering. The results validate the hidden nesting mechanism proposed for the CDW transitions in this series of bronzes.