arXiv subjects

S. Kaprzyk

Publications and source records attributed to S. Kaprzyk.

At least 19 recordsLinked to original sources

arXiv2016

Identifying a descriptor for d-orbital delocalization in cathodes of Li batteries based on x-ray Compton scattering

We discuss how x-ray Compton scattering spectra can be used for investigating the evolution of electronic states in cathode materials of Li batteries under the lithiation/delithiation process. In particular, our analysis of the Compton spectra taken from polycrystalline LixCoO2 samples shows that the spectra are dominated by the contribution of the O-2p redox orbital. We identify a distinct signature of d-orbital delocalization, which is tied directly to the conductivity of the material, providing a descriptor based on Compton spectra for monitoring the lithiation range with improved conductivity and kinetics for electrochemical operation. Our study demonstrates that Compton scattering spectroscopy can provide a window for probing complex electronic mechanisms underlying the charging and discharging processes in Li-battery materials.

cond-mat.mtrl-sci↗

arXiv2016

Superconductivity of Ta34Nb33Hf8Zr14Ti11 high entropy alloy from first principles calculations

The Korringa-Kohn-Rostoker method with the coherent potental approximation (KKR-CPA) is applied to study the first superconducting high entropy alloy (HEA) Ta34Nb33Hf8Zr14Ti11 (discovered in 2014 with Tc = 7.3 K), focusing on estimations of the electron-phonon coupling constant λ. Electronic part of λhas been calculated using the rigid muffin-tin approximation (RMTA), while the phonon part has been approximated using average atomic mass and experimental Debye temperature. The estimated λ= 1.16 is close to the value determined from specific heat measurements, λ= 0.98, and suggests rather strong electron-phonon coupling in this material.

cond-mat.supr-con↗

arXiv2016

Non-destructive measurement of in-operando lithium concentration in batteries via x-ray Compton scattering

Non-destructive determination of lithium distribution in a working battery is key for addressing both efficiency and safety issues. Although various techniques have been developed to map the lithium distribution in electrodes, these methods are mostly applicable to test cells. Here we propose the use of high-energy x-ray Compton scattering spectroscopy to measure the local lithium concentration in closed electrochemical cells. A combination of experimental measurements and parallel first-principles computations is used to show that the shape parameter S of the Compton profile is linearly proportional to lithium concentration and thus provides a viable descriptor for this important quantity. The merits and applicability of our method are demonstrated with illustrative examples of LixMn2O4 cathodes and a working commercial lithium coin battery CR2032.

cond-mat.mtrl-sci↗

arXiv2015

Electronic structure and thermoelectric properties of n- and p-type SnSe from first principles calculations

We present results of electronic band structure, Fermi surface and electron transport properties calculations in orthorhombic $n$- and $p$-type SnSe, applying Korringa-Kohn-Rostoker method and Boltzmann transport approach. The analysis accounted for temperature effect on crystallographic parameters in $Pnma$ structure as well as the phase transition to $CmCm$ structure at $T_c\sim 807 $K. Remarkable modifications of conduction and valence bands were notified upon varying crystallographic parameters within the structure before $T_c$, while the phase transition mostly leads to jump in the band gap value. The diagonal components of kinetic parameter tensors (velocity, effective mass) and resulting transport quantity tensors (electrical conductivity $σ$, thermopower $S$ and power factor PF) were computed in wide range of temperature ($15-900 $K) and, hole ($p-$type) and electron ($n-$type) concentration ($10^{17}-10^{21}$ cm$^{-3}$). SnSe is shown to have strong anisotropy of the electron transport properties for both types of charge conductivity, as expected for the layered structure. In general, $p$-type effective masses are larger than $n$-type ones. Interestingly, $p$-type SnSe has strongly non-parabolic dispersion relations, with the 'pudding-mold'-like shape of the highest valence band. The analysis of $σ$, $S$ and PF tensors indicates, that the inter-layer electron transport is beneficial for thermoelectric performance in $n$-type SnSe, while this direction is blocked in $p$-type SnSe, where in-plane transport is preferred. Our results predict, that $n$-type SnSe is potentially even better thermoelectric material than $p$-type one. Theoretical results are compared with single crystal $p$-SnSe measurements, and good agreement is found.

cond-mat.mtrl-sci↗

arXiv2015

Extracting the Redox Orbitals in Li Battery Materials with High-Resolution X-Ray Compton Scattering Spectroscopy

We present an incisive spectroscopic technique for directly probing redox orbitals based on bulk electron momentum density measurements via high-resolution x-ray Compton scattering. Application of our method to spinel LixMn2O4, a lithium ion battery cathode material, is discussed. The orbital involved in the lithium insertion and extraction process is shown to mainly be the oxygen 2p orbital. Moreover, the manganese 3d states are shown to experience spatial delocalization involving 0.16 electrons per Mn site during the battery operation. Our analysis provides a clear understanding of the fundamental redox process involved in the working of a lithium ion battery.

cond-mat.mtrl-sci↗

arXiv2014

Importance of relativistic effects in electronic structure and thermopower calculations for Mg2Si, Mg2Ge and Mg2Sn

We present a theoretical study of the influence of the relativistic effects on electronic band structure and thermopower of Mg2X(X= Si, Ge, Sn) semiconductors. The full potential Korringa-Kohn-Rostoker (KKR) method is used, and the detailed comparison between the fully relativistic and semi-relativistic electronic structure features is done. We show that the spin-orbit (S-O) interaction splits the valence band structure at Gamma point in good agreement with the experimental data, and this effect strongly depends on X atom. The S-O modifications of the topology of the Gamma-centered hole-like Fermi surface pockets lead to a change in electron transport properties, which are investigated using the Boltzmann approach. In addition, the simple and efficient method is presented for the calculation of density of states effective mass m*, and then used to examine the impact of relativistic effects on m*. It is found that S-O coupling of the valence bands reduces effective mass and therefore significantly lowers the thermopower, primarily in Mg2Sn, but also in Mg2Ge. A detrimental influence of the S-O interaction on thermoelectric performance of p-type Mg2X is analyzed in function of temperature (10-900 K) and carrier concentration (10^18-10^22 cm-3). Interestingly, similar calculations in n-type Mg2X, show negligible effect of the S-O interaction on lowest conduction bands and consequently also on the Seebeck coefficient.

cond-mat.mtrl-sci↗

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↗

arXiv2008

Magnetocaloric properties of Fe_{2-x}T_xP (T = Ru and Rh) from electronic structure calculations and magnetisation measurements

An analysis of the magnetocaloric properties of the pure and substituted Fe2P compounds is made based on KKR-CPA electronic structure calculations and magnetisation M(H,T) measurements. The computed electronic densities of states and magnetic moments are used to calculate both the values of the electronic and magnetic entropies, which fairly agree with the experimental findings. To enlighten the magnetic properties above Curie temperature, the paramagnetic state behaviours are simulated using the disordered local moments (DLM) concept. The KKR-CPA computations show, that in Fe2P, the Fe magnetic moment of the (3f) site disappears in the DLM state, while the moment of the (3g) site is only little lowered, comparison made with the low temperature ferromagnetic state.

cond-mat.mtrl-sci↗

arXiv2007

Electronic structure of the noncentrosymmetric superconductor Mg10Ir19B16

Electronic structure of a novel superconducting noncentrosymmetric compound Mg10Ir19B16 was calculated using the Korringa-Kohn-Rostoker method. Electronic part of the electron-phonon coupling constant, McMillan-Hopfield parameters, were calculated using the rigid-muffin-tin approximation (RMTA). The magnitude of the electron-phonon coupling constant, analysing atomic contributions, is discussed. Our results show, that superconductivity in Mg10Ir19B16 is presumably mediated by electron-phonon interaction.

cond-mat.supr-con↗

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↗

arXiv2006

Theoretical search for superconductivity in Sc3XB perovskites and weak ferromagnetism in Sc3X (X = Tl, In, Ga, Al)

A possibility for a new family of intermetallic perovskite superconductors Sc3XB, with X = Tl, In, Ga and Al, is presented as a result of KKR electronic structure and pseudopotential phonon calculations. The large values of computed McMillan--Hopfield parameters on scandium suggest appearance of superconductivity in Sc3XB compounds. On the other hand, the possibility of weak itinerant ferromagnetic behavior of Sc3X systems is indicated by the small magnetic moment on Sc atoms in two cases of X =~ l and In. Also the electronic structure and resulting superconducting parameters for more realistic case of boron--deficient systems Sc3XB_x are computed using KKR--CPA method, by replacing boron atom with a vacancy. The comparison of the calculated McMillan--Hopfield parameters of the Sc3XB series with corresponding values in MgCNi3 and YRh3B superconductors is given, finding the favorable trends for superconductivity.

cond-mat.supr-con↗

arXiv2006

Anomalous electronic correlations in ground state momentum density of Al$_{97}$Li$_3$

We report high resolution Compton scattering measurements on an Al$_{97}$Li$_3$ 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 Al$_{97}$Li$_3$ 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.

cond-mat.mtrl-sci↗

arXiv2005

Competition of ferromagnetism and superconductivity in Sc3InB

We present results of electronic structure calculations for intermetallic perovskite Sc3InB with Full--Potential KKR-LDA method. Sc3InB is very promising candidate for a new superconductor (related to 8K MgCNi3) and can be regarded as a boron-inserted cubic Sc3In, which is a high--pressure allotropic form of the hexagonal weak ferromagnet Sc3In. We predict that cubic Sc3In can be also magnetic, whereas Sc3InB having large DOS in the vicinity of EF exhibits non-magnetic ground state. Estimation of the electron--phonon coupling for Sc3InB gives $λ\simeq 1$. Furthermore, the effect of vacancy in Sc3InB{1-x} and antisite disorder in Sc3(In-B) on critical parameters is also discussed in view of KKR--CPA method. All theoretical results support possibility of the superconductivity onset in Sc3InB. Preliminary experimental measurements established the transition temperature close to 4.5 K, with a very abrupt change in susceptibility and a correlated drop of the resistivity when cooling down.

cond-mat.supr-con↗