Bernardo Barbiellini

Two-dimensional folding technique for enhancing Fermi surface signatures in the momentum density: Application to Compton scattering data from an Al-3 at. % Li disordered alloy

I. Matsumoto et al. — Wed, 18 Apr 2012 14:44:42 PDT

We present a technique for enhancing Fermi surface (FS) signatures in the two-dimensional (2D) distribution obtained after the 3D momentum density in a crystal is projected along a specific direction in momentum space. These results are usEFul for investigating fermiology via high-resolution Compton scattering and positron annihilation spectroscopies. We focus on the particular case of the (110) projection in a fcc crystal where the standard approach based on the use of the Lock-Crisp-West (LCW) folding theorem fails to give a clear FS image due to the strong overlap with FS images obtained through projection from higher Brillouin zones. We show how these superposed FS images can be disentangled by using a selected set of reciprocal lattice vectors in the folding process. The applicability of our partial folding scheme is illustrated by considering Compton spectra from an Al-3 at. % Li disordered alloy single crystal. For this purpose, high-resolution Compton profiles along nine directions in the (110) plane were measured. Corresponding highly accurate theoretical profiles in Al-3 at. % Li were computed within the local density approximation (LDA)-based Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) first-principles framework. A good level of overall accord between theory and experiment is obtained, some expected discrepancies rEFlecting electron correlation effects notwithstanding, and the partial folding scheme is shown to yield a clear FS image in the (110) plane in Al-3 at. % Li.

Bulk Fermi surface and momentum density in heavily doped La₂₋ₓSrₓCuO₄ using high-resolution Compton scattering and positron annihilation spectroscopies

W. Al-Sawai et al. — Wed, 18 Apr 2012 14:44:40 PDT

We have observed the bulk Fermi surface (FS) in an overdoped (x=0.3) single crystal of La₂₋ₓSrₓCuO₄ by using Compton scattering. A two-dimensional (2D) momentum density reconstruction from measured Compton profiles yields a clear FS signature in the third Brillouin zone along [100]. The quantitative agreement between density functional theory (DFT) calculations and momentum density experiment suggests that Fermi-liquid physics is restored in the overdoped regime. In particular the predicted FS topology is found to be in good accord with the corresponding experimental data. We find similar quantitative agreement between the measured 2D angular correlation of positron annihilation radiation (2D-ACAR) spectra and the DFT-based computations. However, 2D-ACAR does not give such a clear signature of the FS in the extended momentum space in either the theory or the experiment.

Exactly soluble model of resonant energy transfer between molecules

C. King et al. — Wed, 18 Apr 2012 14:44:38 PDT

Förster's theory of resonant energy transfer (FRET) predicts the strength and range of exciton transport between separated molecules. We introduce an exactly soluble model for FRET which reproduces Förster's results as well as incorporating quantum coherence effects. As an application, the model is used to analyze a system composed of quantum dots and the protein bacteriorhodopsin.

Role of oxygen electrons in the metal-insulator transition in the magnetoresistive oxide La2-2xSr1+2xMn2O7 probed by Compton scattering

B. Barbiellini et al. — Wed, 18 Apr 2012 14:44:36 PDT

We have studied the [100]-[110] anisotropy of the Compton profile in the bilayer manganite. Quantitative agreement is found between theory and experiment with respect to the anisotropy in the two metallic phases (i.e., the low temperature ferromagnetic and the colossal magnetoresistant phase under a magnetic field of 7 T). Robust signatures of the metal-insulator transition are identified in the momentum density for the paramagnetic phase above the Curie temperature. We interpret our results as providing direct evidence for the transition from the metalliclike to the admixed ionic-covalent bonding accompanying the magnetic transition. The number of electrons involved in this phase transition is estimated. Our study demonstrates the sensitivity of the Compton scattering technique for identifying the number and type of electrons involved in the metal-insulator transition.

Resonating-valence-bond ground state of lithium nanoclusters

D. Nissenbaum et al. — Wed, 18 Apr 2012 14:44:34 PDT

We have performed diffusion quantum Monte Carlo simulations of Li clusters showing that resonating-valence-bond (RVB) pairing correlations between electrons provide a substantial contribution to the cohesive energy. The RVB effects are identified in terms of electron transfers from s-like to p-like character, constituting a possible explanation for the breakdown of the Fermi-liquid picture observed in recent high-resolution Compton scattering experiments for bulk Li.

Spontaneous magnetization and electron momentum density in three-dimensional quantum dots

R. Saniz et al. — Wed, 18 Apr 2012 14:44:32 PDT

We discuss an exactly solvable model Hamiltonian for describing the interacting electron gas in a quantum dot. Results for a spherical square-well confining potential are presented. The ground state is found to exhibit striking oscillations in spin polarization with dot radius at a fixed electron density. These oscillations are shown to induce characteristic signatures in the momentum density of the electron gas, providing a route for direct experimental observation of the dot magnetization via spectroscopies sensitive to the electron momentum density.

X-ray absorption near-edge spectra of overdoped La₂₋ₓSrₓCuO₄ high-T(c) superconductors

Towfiq Ahmed et al. — Wed, 18 Apr 2012 14:44:30 PDT

We present results for realistic modeling of the x-ray absorption near edge structure (XANES) of the overdoped high-T_c superconductor La₂₋ₓSrₓCuO₄ in the hole doping range x = 0.20 - 0.30. Our computations are based on a real-space Green's function approach in which strong-correlation effects are taken into account in terms of a doping-dependent self-energy. The predicted O K-edge XANES is found to be in good accord with the corresponding experimental results in this overdoped regime. We find that the low energy spectra are dominated by the contribution of O atoms in the cuprate planes, with little contribution from apical O atoms.

Persistence of covalent bonding in liquid silicon probed by inelastic x-ray scattering

J. T. Okada et al. — Wed, 18 Apr 2012 14:44:28 PDT

Metallic liquid silicon at 1787 K is investigated using x-ray Compton scattering. An excellent agreement is found between the measurements and the corresponding Car-Parrinello molecular dynamics simulations. Our results show persistence of covalent bonding in liquid silicon and provide support for the occurrence of theoretically predicted liquid-liquid phase transition in supercooled liquid states. The population of covalent bond pairs in liquid silicon is estimated to be 17% via a maximally localized Wannier function analysis. Compton scattering is shown to be a sensitive probe of bonding effects in the liquid state.

Renormalization of f levels away from the Fermi energy in electron excitation spectroscopies: Density-functional results for Nd2-xCexCuO4

T. Jarlborg et al. — Wed, 18 Apr 2012 14:44:26 PDT

Relaxation energies for photoemission where an occupied electronic state is excited and for inverse photoemission where an empty state is filled are calculated within the density-functional theory with application to Nd₂₋ₓCeₓCuO₄. The associated relaxation energies are obtained by computing differences in total energies between the ground state and an excited state in which one hole or one electron is added into the system. The relaxation energies of f electrons are found to be of the order of several eV’s, indicating that f bands will appear substantially away from the Fermi energy (E_F) in their spectroscopic images, even if these bands lie near E_F. Similar shifts are obtained for the Gd-f states in Gd₂CuO₄. Our analysis explains why it would be difficult to observe f electrons at the E_F even in the absence of strong electronic correlations.

A spin-polarized scheme for obtaining quasi-particle energies within the density functional theory

B. Barbiellini et al. — Wed, 18 Apr 2012 14:44:24 PDT

We discuss an efficient scheme for obtaining spin-polarized quasi-particle excitation energies within the general framework of the density functional theory (DFT). Our approach is to correct the DFT eigenvalues via the electrostatic energy of a majority or minority spin electron resulting from its interaction with the associated exchange and correlation holes by using appropriate spin-resolved pair correlation functions. A version of the method for treating systems with localized orbitals, including the case of partially filled metallic bands, is considered. Illustrative results on Cu are presented.

Electron correlations, spontaneous magnetization and momentum density in quantum dots

A. Bansil et al. — Wed, 18 Apr 2012 14:44:22 PDT

The magnetization of quantum dots is discussed in terms of a relatively simple but exactly solvable model Hamiltonian. The model predicts oscillations in spin polarization as a function of dot radius for a fixed electron density. These oscillations in magnetization are shown to yield distinct signature in the momentum density of the electron gas, suggesting the usefulness of momentum resolved spectroscopies for investigating the magnetization of dot systems. We also present variational quantum Monte Carlo calculations on a square dot containing 12 electrons in order to gain insight into correlation effects on the interactions between like and unlike spins in a quantum dot.

Electronic structure of the metallic ground state of La2−2xSr1+2xMn2O7 for x≈0.59 and comparison with x=0.36,0.38 compounds as revealed by angle-resolved photoemission

Z. Sun et al. — Wed, 18 Apr 2012 14:44:20 PDT

Using angle-resolved photoemission spectroscopy, we present the electronic structure of the metallic ground state of La₂₋₂ₓSr₁₊₂ₓMn₂O₇ (x≈0.59) and interpret the results in terms of first-principles band-structure computations, of which the generalized gradient approximation yields the best agreement with the experimental data. No bilayer-split bands are found in this compound, indicating the near degeneracy of electronic states in the neighboring MnO₂ layers due to its A-type antiferromagnetic structure. The d₃_z²₋_r² states near the zone center were not observed, which is also consistent with its A-type antiferromagnetic structure. Near the Fermi level, a kink in the dispersion reveals an important electron-phonon many-body interaction. The electron-phonon coupling is ~1 near the zone boundary and ~2 near the zone diagonal, showing strong k dependence.

Anomalous electronic correlations in the ground state momentum density of Al₉₇Li₃

J. Kwiatkowska et al. — Wed, 18 Apr 2012 14:44:18 PDT

We report high resolution Compton scattering measurements on an Al₉₇Li₃ disordered alloy single crystal for momentum transfer along the [100], [110] and [111] symmetry directions. The results are interpreted via corresponding KKR-CPA (Korringa-Kohn Rostoker coherent potential approximation) first principles computations. By comparing spectra for Al₉₇Li₃ and Al, we show that the momentum density in the alloy differs significantly from the predictions of the conventional Fermi liquid picture and that the ground state of Al is modified anomalously by the addition of Li.

Fermi-surface study of Ba₁₋ₓKₓBiO₃

S. Sahrakorpi et al. — Wed, 18 Apr 2012 14:44:17 PDT

We present all-electron computations of the three-dimensional (3D) Fermi surfaces (FS’s) in Ba₁₋ₓKₓBiO₃ for a number of different compositions based on the self-consistent Korringa-Kohn-Rostoker coherent-potential-approximation approach for incorporating the effects of Ba/K substitution. By assuming a simple cubic structure throughout the composition range, the evolution of the nesting and other features of the FS of the underlying pristine phase is correlated with the onset of various structural transitions with K doping. A parametrized scheme for obtaining an accurate 3D map of the FS in Ba₁₋ₓKₓBiO₃ for an arbitrary doping level is developed. We remark on the puzzling differences between the phase diagrams of Ba₁₋ₓKₓBiO₃ and BaPbₓBi₁₋ₓO₃ by comparing aspects of their electronic structures and those of the end compounds BaBiO₃, KBiO₃, and BaPbO₃. Our theoretically predicted FS’s in the cubic phase are relevant for analyzing high-resolution Compton scattering and positron-annihilation experiments sensitive to the electron momentum density, and are thus amenable to substantial experimental verification.

Direct observation of localization of the minority-spin-band electrons in magnetite below the Verwey temperature

Hisao Kobayashi et al. — Wed, 18 Apr 2012 14:44:15 PDT

Two-dimensional spin-uncompensated momentum density distributions, $\rho_{\rm s}^{2D}({\bf p})$s, were reconstructed in magnetite at 12K and 300K from several measured directional magnetic Compton profiles. Mechanical de-twinning was used to overcome severe twinning in the single crystal sample below the Verwey transition. The reconstructed $\rho_{\rm s}^{2D}({\bf p})$ in the first Brillouin zone changes from being negative at 300 K to positive at 12 K. This result provides the first clear evidence that electrons with low momenta in the minority spin bands in magnetite are localized below the Verwey transition temperature.

Dyson orbitals, quasi-particle effects and Compton scattering

B. Barbiellini et al. — Wed, 18 Apr 2012 14:44:13 PDT

Dyson orbitals play an important role in understanding quasi-particle effects in the correlated ground state of a many-particle system and are relevant for describing the Compton scattering cross section beyond the frameworks of the impulse approximation (IA) and the independent particle model (IPM). Here we discuss corrections to the Kohn-Sham energies due to quasi-particle effects in terms of Dyson orbitals and obtain a relatively simple local form of the exchange-correlation energy. Illustrative examples are presented to show the usefulness of our scheme.

Proposal to determine the Fermi-surface topology of a doped iron-based superconductor using bulk-sensitive Fourier-transform Compton scattering

Yung Jui Wang et al. — Wed, 18 Apr 2012 14:44:11 PDT

We have carried out first-principles calculations of the Compton scattering spectra to demonstrate that the filling of the hole Fermi surface in LaO₁₋ₓFₓFeAs produces a distinct signature in the Fourier-transformed Compton spectrum when the momentum transfer vector lies along the [100] direction. We thus show how the critical concentration x_c

Decorrelation of samples in quantum Monte Carlo calculations and scaling of autocorrelation time in Li and H₂O clusters

D. Nissenbaum et al. — Wed, 18 Apr 2012 14:44:10 PDT

We have investigated decorrelation of samples in Quantum Monte Carlo (QMC) ground-state energy calculations for large Li and H₂O nanoclusters. Binning data as a way of eliminating statistical correlations, as is the common practice, is found to become increasingly impractical as the system size grows. We demonstrate nevertheless that it is possible to perform accurate energy calculations—without decorrelating samples—by exploiting the scaling of the integrated autocorrelation time τ as a function of the number of electrons in the system.

Electronic coupling of colloidal CdSe nanocrystals monitored by thin-film positron-electron momentum density methods

S. W. H. Eijt et al. — Wed, 18 Apr 2012 14:44:08 PDT

The effect of temperature controlled annealing on the confined valence electron states in CdSe nanocrystal arrays, deposited as thin films, was studied using two-dimensional angular correlation of annihilation radiation (2D-ACAR). A reduction in the intensity by ~35% was observed in a feature of the positron annihilation spectrum upon removal of the pyridine capping molecules above 200 degrees Celsius in a vacuum. This reduction is explained by an increased electronic interaction of the valence orbitals of neighboring nanocrystals, induced by the formation of inorganic interfaces. Partial evaporation of the nanoporous CdSe layer and additional sintering into a polycrystalline thin film was observed at a relatively low temperature of ~486 degrees Celsius.

Compton scattering beyond the impulse approximation

I. G. Kaplan et al. — Wed, 18 Apr 2012 14:44:06 PDT

We treat the non-relativistic Compton scattering process in which an incoming photon scatters from an N-electron many-body state to yield an outgoing photon and a recoil electron, without invoking the commonly used frameworks of either the impulse approximation (IA) or the independent particle model (IPM). An expression for the associated triple differential scattering cross section is obtained in terms of Dyson orbitals, which give the overlap amplitudes between the N-electron initial state and the (N-1) electron singly ionized quantum states of the target. We show how in the high energy transfer regime, one can recover from our general formalism the standard IA based formula for the cross section which involves the ground state electron momentum density (EMD) of the initial state. Our formalism will permit the analysis and interpretation of electronic transitions in correlated electron systems via inelastic x-ray scattering (IXS) spectroscopy beyond the constraints of the IA and the IPM.