arXiv subjects

L. Chioncel

Publications and source records attributed to L. Chioncel.

At least 55 records · Page 3Linked to original sources

arXiv2013

Theoretical investigation of spin-filtering in CrAs / GaAs heterostructures

The electronic structure of bulk fcc GaAs, fcc and tetragonal CrAs, and CrAs/GaAs supercells, computed within LMTO local spin-density functional theory, is used to extract the band alignment (band offset) for the [1,0,0] GaAs/CrAs interface in dependence of the spin orientation. With the lateral lattice constant fixed to the experimental bulk GaAs value, a local energy minimum is found for a tetragonal CrAs unit cell with a slightly ($\approx$ 2%) reduced longitudinal ([1,0,0]) lattice constant. Due to the identified spin-dependent band alignment, half-metallicity of CrAs no longer is a key requirement for spin-filtering. Encouraged by these findings, we study the spin-dependent tunneling current in [1,0,0] GaAs/CrAs/GaAs heterostructures within the non-equilibrium Green's function approach for an effective tight-binding Hamiltonian derived from the LMTO electronic structure. Results indicate that these heterostructures are probable candidates for efficient room-temperature all-semiconductor spin-filtering devices.

cond-mat.mes-hall↗

arXiv2013

De Haas-van Alphen effect and Fermi surface properties of single crystal CrB2

We report the angular dependence of three distinct de Haas-van Alphen (dHvA) frequencies of the torque magnetization in the itinerant antiferromagnet CrB2 at temperatures down to 0.3K and magnetic fields up to 14T. Comparison with the calculated Fermi surface of nonmagnetic CrB2 suggests that two of the observed dHvA oscillations arise from electron-like Fermi surface sheets formed by bands with strong B-px,y character which should be rather insensitive to exchange splitting. The measured effective masses of these Fermi surface sheets display strong enhancements of up to a factor of two over the calculated band masses which we attribute to electron-phonon coupling and electronic correlations. For the temperature and field range studied, we do not observe signatures reminiscent of the heavy d-electron bands expected for antiferromagnetic CrB2. In view that the B-p bands are at the heart of conventional high-temperature superconductivity in the isostructural MgB2, we consider possible implications of our findings for nonmagnetic CrB2 and an interplay of itinerant antiferromagnetism with superconductivity.

cond-mat.str-el↗

arXiv2012

One particle spectral function and analytic continuation for many-body implementation in the EMTO method

We investigate one of the most common analytic continuation techniques in condensed matter physics, namely the Padé approximant. Aspects concerning its implementation in the exact muffin-tin orbitals (EMTO) method are scrutinized with special regard towards making it stable and free of artificial defects. The electronic structure calculations are performed for solid hydrogen, and the performance of the analytical continuation is assessed by monitoring the density of states constructed directly and via the Padé approximation. We discuss the difference between the \textbf{k}-integrated and \textbf{k}-resolved analytical continuations, as well as describing the use of random numbers and pole residues to analyze the approximant. It is found that the analytic properties of the approximant can be controlled by appropriate modifications, making it a robust and reliable tool for electronic structure calculations. At the end, we propose a route to perform analytical continuation for the EMTO + dynamical mean field theory (DMFT) method.

cond-mat.mtrl-sci↗

arXiv2011

Electronic structure and magnetic properties of metallocene multiple-decker sandwich nanowires

We present a study of the electronic and magnetic properties of the multiple-decker sandwich nanowires ($CP-M$) composed of cyclopentadienyl (CP) rings and 3d transition metal atoms (M=Ti to Ni) using first-principles techniques. We demonstrate using Density Functional Theory that structural relaxation play an important role in determining the magnetic ground-state of the system. Notably, the computed magnetic moment is zero in $CP-Mn$, while in $CP-V$ a significant turn-up in magnetic moment is evidenced. Two compounds show a half-metallic ferromagnetic ground state $CP-Fe/Cr$ with a gap within minority/majority spin channel. In order to study the effect of electronic correlations upon the half-metallic ground states in $CP-Cr$, we introduce a simplified three-bands Hubbard model which is solved within the Variational Cluster Approach. We discuss the results as a function of size of the reference cluster and the strength of average Coulomb $U$ and exchange $J$ parameters. Our results demonstrate that for the range of studied parameters $U=2-4eV$ and $J=0.6-1.2eV$ the half-metallic character is not maintained in the presence of local Coulomb interactions.

cond-mat.mtrl-sci↗

arXiv2011

Magnetic Compton profiles of Fe and Ni corrected by dynamical electron correlations

Magnetic Compton profiles (MCPs) of Ni and Fe along [111] direction have been calculated using a combined Density Functional and many-body theory approach. At the level of the local spin density approximation the theoretical MCPs does not describe correctly the experimental results around the zero momentum transfer. In this work we demonstrate that inclusion of electronic correlations as captured by Dynamical Mean Field Theory (DMFT) improves significantly the agreement between the theoretical and the experimental MCPs. In particular, an energy decomposition of Ni MCPs gives indication of spin polarization and intrinsic nature of Ni 6 eV satellite, a genuine many-body feature.

cond-mat.str-el↗

arXiv2011

Absence of halfmetallicity in defect-free Cr, Mn-delta-doped Digital Magnetic Heterostructures

We present results of a combined density functional and many-body calculations for the electronic and magnetic properties of the defect-free digital ferromagnetic heterostructures obtained by doping GaAs with Cr and Mn. While local density approximation/(+U) predicts half-metallicity in these defect-free delta-doped heterostructures, we demonstrate that local many-body correlations captured by Dynamical Mean Field Theory induce within the minority spin channel non-quasiparticle states just above $E_F$. As a consequence of the existence of these many-body states the half-metallic gap is closed and the carriers spin polarization is significantly reduced. Below the Fermi level the minority spin highest valence states are found to localize more on the GaAs layers being independent of the type of electronic correlations considered. Thus, our results confirm the confinement of carriers in these delta-doped heterostructures, having a spin-polarization that follow a different temperature dependence than magnetization. We suggest that polarized hot-electron photoluminescence experiments might bring evidence for the existence of many-body states within the minority spin channel and their finite temperature behavior.

cond-mat.mtrl-sci↗

arXiv2010

Electronic correlations in short period (CrAs)$_n$/(GaAs)$_n$ ferromagnetic heterostructures

We investigate half-metallicity in [001] stacked (CrAs)$_n$/(GaAs)$_n$ heterostructures with $n \leq 3$ by means of a combined many-body and electronic structure calculation. Interface states in the presence of strong electronic correlations are discussed for the case $n=1$. For $n=2,3$ our results indicate that the minority spin half-metallic gap is suppressed by local correlations at finite temperatures, and continuously shrinks upon increasing the heterostructure period. Although around room temperature the magnetization of the heterostructure deviates by only $2%$ from the ideal integer value, finite temperature polarization at $E_F$ is reduced by at least $25%$. Below the Fermi level the minority spin highest valence states are found to localize more on the GaAs layers while lowest conduction states have a many-body origin. Our results, therefore, suggest that in these heterostructures holes and electrons remain separated among different layers.

cond-mat.mtrl-sci↗

arXiv2010

Rare-earth impurities in Co$_2$MnSi: an opportunity to improve Half-Metallicity at finite temperatures

We analyse the effects of doping Holmium impurities into the full-Heusler ferromagnetic alloy Co$_2$MnSi. Experimental results, as well as theoretical calculations within Density Functional Theory in the "Local Density Approximation plus Hubbard U" framework show that the holmium moment is aligned antiparallely to that of the transition metal atoms. According to the electronic structure calculations, substituting Ho on Co sites introduces a finite density of states in the minority spin gap, while substitution on the Mn sites preserves the half-metallic character.

cond-mat.mtrl-sci↗

arXiv2009

Half-metallicity in NiMnSb: a Variational Cluster Approach with ab-initio parameters

Electron correlation effects in the half-metallic ferromagnet NiMnSb are investigated within a combined density functional and many-body approach. Starting from a realistic multi-orbital Hubbard-model including Mn and Ni-d orbitals, the many-body problem is addressed via the Variational Cluster Approach. The density of states obtained in the calculation shows a strong spectral weight transfer towards the Fermi level in the occupied conducting majority spin channel with respect to the uncorrelated case, as well as states with vanishing quasiparticle weight in the minority spin gap. Although the two features produce competing effects, the overall outcome is a strong reduction of the spin polarisation at the Fermi level with respect to the uncorrelated case. This result emphasizes the importance of correlation in this material.

cond-mat.str-el↗

arXiv2009

Titanium Nitride - a correlated metal at the threshold of a Mott transition

We investigate electron correlation effects in stoichiometric Titanium Nitride (TiN) using a combination of electronic structure and many-body calculations. In a first step, the Nth-order muffin tin orbital technique is used to obtain parameters for the low-energy Hamiltonian in the Ti-d(t2g)-band manifold. The Coulomb-interaction U and the Hund's rule exchange parameter J are estimated using a constrained Local-Density-Approximation calculation. Finally, the many-body problem is solved within the framework of the Variational Cluster Approach. Comparison of our calculations with different spectroscopy results stresses the importance of electronic correlation in this material. In particular, our results naturally explain a suppression of the TiN density of states at the Fermi level (pseudogap) in terms of the proximity to a Mott metal-insulator transition.

cond-mat.str-el↗

arXiv2009

Majority-spin non-quasiparticle states in half-metallic ferrimagnet Mn$_2$VAl

The density of non-quasiparticle states in the ferrimagnetic full-Heuslers Mn$_2$VAl alloy is calculated from first principles upon appropriate inclusion of correlations. In contrast to most half-metallic compounds, this material displays an energy gap in the majority-spin spectrum. For this situation, non-quasiparticle states are located below the Fermi level, and should be detectable by spin-polarized photoemission. This opens a new way to study many-body effects in spintronic-related materials.

cond-mat.mtrl-sci↗

arXiv2008

Non-quasiparticle states in Co$_2$MnSi evidenced through magnetic tunnel junction spectroscopy measurements

We investigate the effects of electronic correlations in the full-Heusler Co$_2$MnSi, by combining a theoretical analysis of the spin-resolved density of states with tunneling-conductance spectroscopy measurements using Co$_2$MnSi as electrode. Both experimental and theoretical results confirm the existence of so-called non-quasiparticle states and their crucial contribution to the finite-temperature spin polarisation in this material.

cond-mat.str-el↗

arXiv2007

Half-metallic ferromagnets: From band structure to many-body effects

A review of new developments in theoretical and experimental electronic structure investigations of half-metallic ferromagnets (HMF) is presented. Being semiconductors for one spin projection and metals for another ones, these substances are promising magnetic materials for applications in spintronics (i.e., spin-dependent electronics). Classification of HMF by the peculiarities of their electronic structure and chemical bonding is discussed. Effects of electron-magnon interaction in HMF and their manifestations in magnetic, spectral, thermodynamic, and transport properties are considered. Especial attention is paid to appearance of non-quasiparticle states in the energy gap, which provide an instructive example of essentially many-body features in the electronic structure. State-of-art electronic calculations for correlated $d$-systems is discussed, and results for specific HMF (Heusler alloys, zinc-blende structure compounds, CrO$_{2},$ Fe$_{3}$O$_{4}$) are reviewed.

cond-mat.mtrl-sci↗

arXiv2007

Half-Metallic Ferromagnetism and the spin polarization in CrO$_2$

We present electronic structure calculations in combination with local and non-local many-body correlation effects for the half-metallic ferromagnet CrO$_2$. Finite-temperature Dynamical Mean Field Theory results show the existence of non-quasiparticle states, which were recently observed as almost currentless minority spin states near the Fermi energy in resonant scattering experients. At zero temperatures, Variational Cluster Approach calculations support the half-metallic nature of CrO$_2$ as seen in superconducting point contact spectroscopy. The combination of these two techniques allowed us to qualitatively describe the spin-polarization in CrO$_2$.

cond-mat.str-el↗

arXiv2006

Infuence of correlation effects on the of magneto-optical properties of half-metallic ferromagnet NiMnSb

The magneto-optical spectra of NiMnSb were calculated in the framework of the Local Spin Density Approximation (LSDA) combined with Dynamical Mean-Field Theory (DMFT). Comparing with results based on plain LSDA, an additional account of many-body correlations via DMFT results in a noticably improved agreement of the theoretical Kerr-rotation and ellipticity spectra with corresponding experimental data.

cond-mat.mtrl-sci↗

arXiv2006

Model Hamiltonian parameters for half-metallic ferromagnets NiMnSb and CrO2

Using the recently developed Nth-order muffin-tin-orbital (NMTO) based downfolding technique we revisit the electronic properties of half-metallic ferromagnets, the semi-Heusler NiMnSb and rutile CrO2. The NMTO Wannier orbitals for the Mn-d and Cr-t2g manifolds are constructed and the mechanism of chemical bonding is discussed. The effective hopping Hamiltonian parameters are calculated using a NMTO downfolded basis set. We propose model Hamiltonian parameters with possibly minimal basis sets for both half-metallic ferromagnetic alloys.

cond-mat.str-el↗

arXiv2006

Half-metallic ferromagnetism induced by dynamic electron correlations in VAs

The electronic structure of the VAs compound in the zinc-blende structure is investigated using a combined density-functional and dynamical mean-field theory approach. Contrary to predictions of a ferromagnetic semiconducting ground state obtained by density-functional calculations, dynamical correlations induce a closing of the gap and produce a half-metallic ferromagnetic state. These results emphasize the importance of dynamic correlations in materials suitable for spintronics.

cond-mat.str-el↗

arXiv2006

Electron Correlations and the Minority-Spin Band Gap in Half-Metallic Heusler Alloys

Electron-electron correlations affect the band gap of half-metallic ferromagnets by introducing non-quasiparticle states just above the Fermi level. In contrast to the spin-orbit coupling, a large asymmetric non-quasiparticle spectral weight is present in the minority-spin channel, leading to a peculiar finite-temperature spin depolarization effects. Using recently developed first-principle dynamical mean field theory, we investigate these effects for the half-metallic ferrimagnetic Heusler compound FeMnSb. We discuss depolarization effects in terms of strength of local Coulomb interaction $U$ and temperature in FeMnSb. We propose Ni$_{1-x}$Fe$_{x}$MnSb alloys as a perspective materials to be used in spin-valve structures and for experimental search of non-quasiparticle states in half-metallic materials.

cond-mat.str-el↗