arXiv subjects

P. E. Mijnarends

Publications and source records attributed to P. E. Mijnarends.

16 recordsLinked to original sources

arXiv2013

Surfaces of colloidal PbSe nanocrystals probed by thin-film positron annihilation spectroscopy

Positron annihilation lifetime spectroscopy (PALS) and positron-electron momentum density (PEMD) studies on multilayers of PbSe nanocrystals (NCs), supported by transmission electron microscopy (TEM), show that positrons are strongly trapped at NC surfaces, where they provide insight into the surface composition and electronic structure of PbSe NCs. Our analysis indicates abundant annihilation of positrons with Se electrons at the NC surfaces and with O electrons of the oleic ligands bound to Pb ad-atoms at the NC surfaces, which demonstrates that positrons can be used as a sensitive probe to investigate the surface physics and chemistry of nanocrystals inside multilayers. Ab-initio electronic structure calculations provide detailed insight in the valence and semi-core electron contributions to the positron-electron momentum density of PbSe. Both lifetime and PEMD are found to correlate with changes in the particle morphology characteristic of partial ligand removal.

cond-mat.mes-hall↗

arXiv2012

Lindhard and RPA susceptibility computations in extended momentum space in electron doped cuprates

We present an approximation for efficient calculation of the Lindhard susceptibility $χ^{L}(q,ω)$ in a periodic system through the use of simple products of real space functions and the fast Fourier transform (FFT). The method is illustrated by providing $χ^{L}(q,ω)$ results for the electron doped cuprate Nd$_{2-x}$Ce$_{x}$CuO$_{4}$ extended over several Brillouin zones. These results are relevant for interpreting inelastic X-ray scattering spectra from cuprates.

cond-mat.str-el↗

arXiv2012

Bulk Fermi surface and momentum density in heavily doped La$_{2-x}$Sr$_x$CuO$_4$ using high resolution Compton scattering and positron annihilation spectroscopies

We have observed the bulk Fermi surface (FS) in an overdoped ($x$=0.3) single crystal of La$_{2-x}$Sr$_x$CuO$_4$ 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.

cond-mat.supr-con↗

arXiv2010

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

We have carried out first-principles calculations of the Compton scattering spectra to demonstrate that the filling of the hole Fermi surface in LaO$_{1-x}$F$_{x}$FeAs 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 $x_c$, where hole Fermi surface pieces are filled up and the superconductivity mediated by antiferromagnetic spin fluctuations is expected to be suppressed, can be obtained in a bulk-sensitive manner.

cond-mat.str-el↗

arXiv2010

High resolution Compton scattering as a Probe of the Fermi surface in the Iron-based superconductor $LaO_{1-x}F_xFeAs$

We have carried out first principles all-electron calculations of the (001)-projected 2D electron momentum density and the directional Compton profiles along the [100], [001] and [110] directions in the Fe-based superconductor LaOFeAs within the framework of the local density approximation. We identify Fermi surface features in the 2D electron momentum density and the directional Compton profiles, and discuss issues related to the observation of these features via Compton scattering experiments.

cond-mat.str-el↗

arXiv2009

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

Two-dimensional spin-uncompensated momentum density distributions, $ρ_{\rm s}^{2D}({\bf p})$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 $ρ_{\rm s}^{2D}({\bf p})$ 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.

cond-mat.str-el↗

arXiv2009

Role of Oxygen Electrons in the Metal-Insulator Transition in the Magnetoresistive Oxide La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ Probed by Compton Scattering

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 metallic-like to the admixed ionic-covalent bonding accompanying the magnetic transition. The number of electrons involved in this phase transition is estimated from the area enclosed by the Compton profile anisotropy differences. Our study demonstrates the sensitivity of the Compton scattering technique for identifying the number and type of electrons involved in the metal-insulator transition.

cond-mat.mtrl-sci↗

arXiv2009

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

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.

cond-mat.mtrl-sci↗

arXiv2007

Positron studies of surfaces, structure and electronic properties of nanocrystals

A brief review is given of recent positron studies of metal and semiconductor nanocrystals. The prospects offered by positron annihilation as a sensitive method to access nanocrystal (NC) properties are described and compared with other experimental methods. The tunability of the electronic structure of nanocrystals underlies their great potential for application in many areas. Owing to their large surface-to-volume ratio, the surfaces and interfaces of NCs play a crucial role in determining their properties. Here we focus on positron 2D angular correlation of annihilation radiation (2D-ACAR) and (two-detector) Doppler studies for investigating surfaces and electronic properties of CdSe NCs.

cond-mat.mtrl-sci↗

arXiv2007

Spin moment over 10-300 K and delocalization of magnetic electrons above the Verwey transition in magnetite

In order to probe the magnetic ground state, we have carried out temperature dependent magnetic Compton scattering experiments on an oriented single crystal of magnetite (Fe$_3$O$_4$), together with the corresponding first-principles band theory computations to gain insight into the measurements. An accurate value of the magnetic moment $μ_S$ associated with unpaired spins is obtained directly over the temperature range of 10-300K. $μ_S$ is found to be non-integral and to display an anomalous behavior with the direction of the external magnetic field near the Verwey transition. These results reveal how the magnetic properties enter the Verwey energy scale via spin-orbit coupling and the geometrical frustration of the spinel structure, even though the Curie temperature of magnetite is in excess of 800 K. The anisotropy of the magnetic Compton profiles increases through the Verwey temperature $T_v$ and indicates that magnetic electrons in the ground state of magnetite become delocalized on Fe B-sites above $T_v$.

cond-mat.str-el↗

arXiv2007

Magnetic momentum density, Fermi surface and directional magnetic Compton profiles in LaSr$_{2}$Mn$_{2}$O$_{7}$ and La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$

We have carried out first principles, all-electron computations of the magnetic momentum density $ρ_{mag}({\mathbf p})$ and magnetic Compton profiles (MCPs) for momentum transfer along the [100], [001], and [110] directions in LaSr$_2$Mn$_2$O$_7$ and La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ within the local spin density approximation (LSDA) based band theory framework. Parallel measurements of these three MCPs from a single crystal of La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ at 5 K in a magnetic field of 7 T are also reported. $ρ_{mag}({\mathbf p})$ is shown to contain distinct peaks arising from the occupied majority-spin $t_{2g}$ electrons and to display images of the Fermi surface (FS) in the first and higher Brillouin zones (BZs). The overall shape of the MCPs, $J_{mag}(p_z)$, obtained by integrating $ρ_{mag}({\mathbf p})$ over $p_x$ and $p_y$, is found to be dominated by the majority-spin $t_{2g}$ states. The FS-related fine structure in the MCPs is however substantial only in the [100] MCP, which contains features arising from the large majority-spin hole sheets. The overall shapes and widths of the experimental MCPs along all three directions investigated are in reasonably good accord with theoretical predictions, although some discrepancies indicating inadequacy of the LSDA in treating the magnetic states can be identified. We discuss details of the FS-related signatures in the first and higher BZs in the [100] MCP and show that high resolution magnetic Compton scattering experiments with a momentum resolution of 0.1 a.u. FWHM (full-width-at-half-maximum) or better will be necessary to observe this fine structure. We comment also on the feasibility of using positron annihilation spectroscopy in this connection.

cond-mat.str-el↗

arXiv2005

Extracting d-orbital occupancy from magnetic Compton scattering in bilayer manganites

We consider the shape of the magnetic Compton profile (MCP), $J_{mag}(p_z)$, in La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ for momentum transfer $p_z$ along the [110] direction and the associated reciprocal form factor $B(r)$ defined by taking the one-dimensional Fourier transform of $J_{mag}(p_z)$. $B(r)$ is shown to contain a prominent dip at $r\approx$ 1 $Å$, where the minimum value $B_{min}$ of $B(r)$ can be related to the occupancies of the $e_g$ orbitals of $d_{x^2-y^2}$ and $d_{3z^2-r^2}$ symmetry in the system. We illustrate our procedure in detail by analyzing the measured MCP at 5K and the MCP computed within the framework of the local spin density approximation (LSDA) and comment on the differences between the measured and computed $e_g$ occupancies as a reflection of the limitations of the LSDA in treating electron correlation effects.

cond-mat.str-el↗

arXiv2004

Temperature dependent orbital degree of freedom in a bilayer manganite by magnetic Compton scattering

We have measured temperature-dependent magnetic Compton profiles (MCPs) from a single crystal of La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$. The MCPs, which involved the scattering of circularly polarized x-rays, are in general related to the momentum density of all the unpaired spins in the system. Nevertheless, we show that when the x-ray scattering vector lies along the [110] direction, the number of magnetic electrons of a specific symmetry, i.e. $d$-electrons of $x^2-y^2$ symmetry, yield a distinct signature in the MCP, allowing us to monitor substantial changes in the occupancy of the $d_{x^2-y^2}$ states over the investigated temperature range of 5-200K. This study indicates that magnetic Compton scattering can provide a powerful window on the properties of specific magnetic electrons in complex materials.

cond-mat.str-el↗

arXiv2001

A Novel 2D Folding Technique for Enhancing Fermi Surface Signatures in the Momentum Density: Application to Compton Scattering Data from an Al-3at%Li Disordered Alloy

We present a novel technique for enhancing Fermi surface (FS) signatures in the 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 an 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-3at%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-3at%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%Li.

cond-mat.mtrl-sci↗

arXiv2000

A High-Resolution Compton Scattering Study of the Electron Momentum Density in Al

We report high-resolution Compton profiles (CP's) of Al along the three principal symmetry directions at a photon energy of 59.38 keV, together with corresponding highly accurate theoretical profiles obtained within the local-density approximation (LDA) based band-theory framework. A good accord between theory and experiment is found with respect to the overall shapes of the CP's, their first and second derivatives, as well as the anisotropies in the CP's defined as differences between pairs of various CP's. There are however discrepancies in that, in comparison to the LDA predictions, the measured profiles are lower at low momenta, show a Fermi cutoff which is broader, and display a tail which is higher at momenta above the Fermi momentum. A number of simple model calculations are carried out in order to gain insight into the nature of the underlying 3D momentum density in Al, and the role of the Fermi surface in inducing fine structure in the CP's. The present results when compared with those on Li show clearly that the size of discrepancies between theoretical and experimental CP's is markedly smaller in Al than in Li. This indicates that, with increasing electron density, the conventional picture of the electron gas becomes more representative of the momentum density and that shortcomings of the LDA framework in describing the electron correlation effects become less important.

cond-mat.mtrl-sci↗

arXiv1999

Electronic structure and magnetism of Fe$_{3-x}$V$_{x}$X (X = Si, Ga and Al) alloys by the KKR-CPA method

We present first principles charge- and spin-selfconsistent electronic structure computations on the Heusler-type disordered alloys Fe$_{3-x}$V$_{x}$X for three different metalloids X=(Si, Ga and Al). In these calculations we use the methodology based on the Korringa-Kohn- Rostoker formalism and the coherent-potential approximation (KKR-CPA), generalized to treat disorder in multi-component complex alloys.

cond-mat.mtrl-sci↗