arXiv subjects

Volker Eyert

Publications and source records attributed to Volker Eyert.

At least 19 recordsLinked to original sources

arXiv2022

Structural study and evaluation of thermoelectric properties of single-phase isocubanite (CuFe2S3) synthesized via an ultra-fast efficient microwave radiation technique

The current state-of-the-art thermoelectric materials are generally composed of expensive, scarce, and toxic elements. In this respect, copper-based sulfide compounds have emerged as viable alternatives. Herein, we report for the first time the successful synthesis of single-phase cubic isocubanite CuFe2S3 using mechanical-alloying combined with microwave-assisted synthesis. The isocubanite phase synthesized via this ultra-fast out-of-equilibrium process exhibits a maximum thermoelectric figure of merit, zTmax ~ 0.14 at 673 K. Besides the thermoelectric properties, insights about the structure of isocubanite, based on the refinement of X-ray diffraction data and first principles calculations, are also investigated in detail. It confirms that the Cu-Fe cations in synthetic isocubanite overwhelmingly occupy the 4d sites of the cubic structure in an inherently disordered fashion.

cond-mat.mtrl-sci↗

arXiv2018

Electronic structure and magnetic ordering of NiN and Ni$_2$N from first principles

The results of first-principles electronic structure calculations for the nitrogen-rich nickel nitrides $ {\rm NiN} $ and $ {\rm Ni_2N} $ are presented. The calculations are based on density functional theory and used the generalized gradient approximation (GGA) as well as the GGA$ +U $ approach. The latter turned out to be crucial for a correct description of the crystal phase stability and magnetic instabilities of both compounds. While for $ {\rm NiN} $ GGA calculations predict a non-magnetic ground state with the zincblende structure, GGA$ +U $ calculations result in a half-metallic ferromagnet with the rocksalt structure in line with indications from the neighboring transition-metal nitrides making $ {\rm NiN} $ a possible candidate for spin-filter devices. For $ {\rm Ni_2N} $ GGA calculations likewise lead to a non-magnetic behavior, which is contrasted with a ferrimagnetic ordering obtained from the GGA$ +U $ approach. This ground state results from complex three-dimensional exchange interaction via $ σ$-type and $ π$-type overlap of the Ni $ 3d $ orbitals with the N $ 2p $ orbitals and may explain the reported sensitivity of the magnetic ordering to details of the crystal structure. For both nitrides, experimental data are called for to confirm our predictions.

cond-mat.str-el↗

arXiv2017

Unconventional aspects of electronic transport in delafossite oxides

The electronic transport properties of the delafossite oxides ABO$_2$ are usually understood in terms of two well separated entities, namely, the triangular A$^+$ and (BO$_2$)$^-$ layers. Here we review several cases among this extensive family of materials where the transport depends on the interlayer coupling and displays unconventional properties. We review the doped thermoelectrics based on CuRhO$_2$ and CuCrO$_2$, which show a high-temperature recovery of Fermi-liquid transport exponents, as well as the highly anisotropic metals PdCoO$_2$, PtCoO$_2$ and PdCrO$_2$ where the sheer simplicity of the Fermi surface leads to unconventional transport. We present some of the theoretical tools that have been used to investigate these transport properties and review what can and cannot be learned from the extensive set of electronic structure calculations that have been performed.

cond-mat.mtrl-sci↗

arXiv2017

Pressure-Temperature Phase Diagram of Vanadium Dioxide

The complexity of strongly correlated electron physics in vanadium dioxide is exemplified as its rich phase diagrams of all kinds, which in turn shed light on the mechanisms behind its various phase transitions. In this work, we map out the hydrostatic pressure - temperature phase diagram of vanadium dioxide nanobeams by independently varying pressure and temperature with a diamond anvil cell. In addition to the well-known insulating M1 (monoclinic) and metallic R (tetragonal) phases, the diagram identifies the existence at high pressures of the insulating M1' (monoclinic, more conductive than M1) phase, and two metallic phases of X (monoclinic) and O (orthorhombic, at high temperature only). Systematic optical and electrical measurements combined with density functional calculations allow us to delineate their phase boundaries as well as reveal some basic features of the transitions.

cond-mat.mtrl-sci↗

arXiv2014

Electronic structure and crystal phase stability of palladium hydrides

The results of electronic structure calculations for a variety of palladium hydrides are presented. The calculations are based on density functional theory and used different local and semilocal approximations. The thermodynamic stability of all structures as well as the electronic and chemical bonding properties are addressed. For the monohydride, taking into account the zero-point energy is important to identify the octahedral Pd-H arrangement with its larger voids and, hence, softer hydrogen vibrational modes as favorable over the tetrahedral arrangement as found in the zincblende and wurtzite structures. Stabilization of the rocksalt structure is due to strong bonding of the 4d and 1s orbitals, which form a characteristic split-off band separated from the main d-band group. Increased filling of the formerly pure d states of the metal causes strong reduction of the density of states at the Fermi energy, which undermines possible long-range ferromagnetic order otherwise favored by strong magnetovolume effects. For the dihydride, octahedral Pd-H arrangement as realized e.g. in the pyrite structure turns out to be unstable against tetrahedral arrangement as found in the fluorite structure. Yet, from both heat of formation and chemical bonding considerations the dihydride turns out to be less favorable than the monohydride. Finally, the vacancy ordered defect phase Pd3H4 follows the general trend of favouring the octahedral arrangement of the rocksalt structure for Pd:H ratios less or equal to one.

cond-mat.mtrl-sci↗

arXiv2011

Semiconducting (Half-Metallic) Ferromagnetism in Mn(Fe) Substituted Pt and Pd Nitrides

Using first principles calculations as based on density functional theory, we propose a class of so far unexplored diluted ferromagnetic semiconductors and half-metals. Here, we study the electronic properties of recently synthesized $ 4d $ and $ 5d $ transition metal dinitrides. In particular, we address Mn- and Fe-substitution in PtN$_2$ and PdN$_2$. Structural relaxation shows that the resulting ordered compounds, Pt$_{0.75}$(Mn,Fe)$_{0.25}$N$_2$ and Pd$_{0.75}$(Mn,Fe)$_{0.25}$N$_2$, maintain the cubic crystal symmetry of the parent compounds. On substitution, all compounds exhibit long-range ferromagnetic order. While both Pt$_{0.75}$Mn$_{0.25}$N$_2$ and Pd$_{0.75}$Mn$_{0.25}$N$_2$ are semiconducting, Fe-substitution causes half-metallic behavior for both parent materials.

cond-mat.mtrl-sci↗

arXiv2009

Influence of the vacuum interface on the charge distribution in V2O3 thin films

The electronic structure of V2O3 thin films is studied by means of the augmented spherical wave method as based on density functional theory and the local density approximation. We establish that effects of charge redistribution, induced by the vacuum interface, in such films are restricted to a very narrow surface layer of approx. 15 Angstroem thickness. As a consequence, charge redistribution can be ruled out as a source of the extraordinary thickness-dependence of the metal-insulator transition observed in V2O3 thin films of 100-1000 Angstroem thickness.

cond-mat.str-el↗

arXiv2009

On the strong impact of doping in the triangular antiferromagnet CuCrO2

Electronic band structure calculations using the augmented spherical wave method have been performed for CuCrO2. For this antiferromagnetic (T_N = 24 K) semiconductor crystallizing in the delafossite structure, it is found that the valence band maximum is mainly due to the t_2g orbitals of Cr^3+ and that spin polarization is predicted with 3 mu_B per Cr^3+. The structural characterizations of CuCr1-xMgxO2 reveal a very limited range of Mg^2+ substitution for Cr^3+ in this series. As soon as x = 0.02, a maximum of 1% Cr ions substituted by Mg site is measured in the sample. This result is also consistent with the detection of Mg spinel impurities from X-ray diffraction for x = 0.01. This explains the saturation of the Mg^2+ effect upon the electrical resistivity and thermoelectric power observed for x > 0.01. Such a very weak solubility limit could also be responsible for the discrepancies found in the literature. Furthermore, the measurements made under magnetic field (magnetic susceptibility, electrical resistivity and Seebeck coefficient) support that the Cr^4+ "holes", created by the Mg^2+ substitution, in the matrix of high spin Cr^3+ (S = 3/2) are responsible for the transport properties of these compounds.

cond-mat.str-el↗

arXiv2009

Electronic structure and thermoelectric properties of CuRh(1-x)MgxO2

Electronic structure calculations using the augmented spherical wave method have been performed for CuRhO2. For this semiconductor crystallizing in the delafossite structure, it is found that the valence band maximum is mainly due to the 4d t2g orbitals of Rh^{3+}. The structural characterizations of CuRh(1-x)MgxO2 show a broad range of Mg^{2+} substitution for Rh^{3+} in this series, up to about 12%. Measurements of the resistivity and thermopower of the doped systems show a Fermi liquid-like behavior for temperatures up to about 1000K, resulting in a large weakly temperature dependent power factor. The thermopower is discussed both within the Boltzmann equation approach as based on the electronic structure calculations and the temperature independent correlation functions ratio approximation as based on the Kubo formalism.

cond-mat.str-el↗

arXiv2009

Fermi surface of MoO2 studied by angle-resolved photoemission spectroscopy, de Haas-van Alphen measurements, and electronic structure calculations

A comprehensive study of the electronic properties of monoclinic MoO2 from both an experimental and a theoretical point of view is presented. We focus on the investigation of the Fermi body and the band structure using angle resolved photoemission spectroscopy, de Haas-van Alphen measurements, and electronic structure calculations. For the latter, the new full-potential augmented spherical wave (ASW) method has been applied. Very good agreement between the experimental and theoretical results is found. In particular, all Fermi surface sheets are correctly identified by all three approaches. Previous controversies concerning additional hole-like surfaces centered around the Z- and B-point could be resolved; these surfaces were an artefact of the atomic-sphere approximation used in the old calculations. Our results underline the importance of electronic structure calculations for the understanding of MoO2 and the neighbouring rutile-type early transition-metal dioxides. This includes the low-temperature insulating phases of VO2 and NbO2, which have crystal structures very similar to that of molybdenum dioxide and display the well-known prominent metal-insulator transitions.

cond-mat.str-el↗

arXiv2008

Substrate effects on V2O3 thin films

We apply density functional theory and the augmented spherical wave method to analyze the electronic structure of V2O3 in the vicinity of an interface to Al2O3. The interface is modeled by a heterostructure setup of alternating vanadate and aluminate slabs. We focus on the possible modifications of the V2O3 electronic states in this geometry, induced by the presence of the aluminate layers. In particular, we find that the tendency of the V 3d states to localize is enhanced and may even cause a metal-insulator transition.

cond-mat.str-el↗

arXiv2008

Magnetic ordering in the static intermediate-valent cerium compound Ce2RuZn4

The low-temperature behavior of Ce2RuZn4 has been investigated. Specific heat and magnetic susceptibility data reveal an antiferromagnetic transition at a Neel temperature of 2 K. Ce2RuZn4 is a static intermediate-valent compound with two crystallographically independent cerium atoms. The magnetic data clearly show that only one cerium site is magnetic (Ce^3+), while the second one carries no magnetic moment. The experimental data are interpreted with the help of first principles electronic structure calculations using density functional theory and the augmented spherical wave method. The calculations reveal the occurrence of two different cerium sites, which are characterized by strongly localized magneticmoments and strong Ce-Ru bonding, respectively.

cond-mat.str-el↗

arXiv2008

Long-range magnetic order and spin-lattice coupling in the delafossite CuFeO2

The electronic and magnetic properties of the delafossite CuFeO2 are investigated by means of electronic structure calculations. They are performed using density functional theory in the generalized gradient approximation as well as the new full-potential augmented spherical wave method. The calculations reveal three different spin states at the iron sites. Taking into account the correct crystal structure, we find long-range antiferromagnetic ordering in agreement with experiment. Contrasting previous work, our calculations show that non-local exchange interactions lead to a semiconducting ground state.

cond-mat.str-el↗

arXiv2008

On the metallic conductivity of the delafossites PdCoO2 and PtCoO2

The origin of the quasi two-dimensional behavior of PdCoO2 and PtCoO2 is investigated by means of electronic structure calculations. They are performed using density functional theory in the generalized gradient approximation as well as the new full-potential augmented spherical wave method. We show that the electric conductivity is carried almost exclusively by the in-plane Pd (Pt) d orbitals. In contrast, the insulating CoO2 sandwich layers of octahedrally coordinated Co atoms may be regarded as charge carrier reservoirs. This leads to a weak electronic coupling of the Pd (Pt) layers. The obtained nearly cylindrical Fermi surface causes the strong anisotropy of the electric conductivity.

cond-mat.mtrl-sci↗

arXiv2005

Orbital Ordering and Spin-Ladder Formation in La2RuO5

The semiconductor-semiconductor transition of La2RuO5 is studied by means of augmented spherical wave (ASW) electronic structure calculations as based on density functional theory and the local density approximation. This transition has lately been reported to lead to orbital ordering and a quenching of the local spin magnetic moment. Our results hint towards an orbital ordering scenario which, markedly different from the previously proposed scheme, preserves the local S = 1 moment at the Ru sites in the low-temperature phase. The unusual magnetic behaviour is interpreted by the formation of spin-ladders, which result from the structural changes occurring at the transition and are characterized by antiferromagnetic coupling along the rungs.

cond-mat.str-el↗

arXiv2005

Orbital ordering in the two-dimensional ferromagnetic semiconductor Rb_2CrCl_4

We present the results of electronic structure calculations for the two-dimensional ferromagnet Rb_2CrCl_4. They are obtained by the augmented spherical wave method as based on density functional theory and the local density approximation. In agreement with experimental data Rb_2CrCl_4 is found to be semiconducting and displays long-range ferromagnetic order of the localized Cr 3d moments. The magnetic properties are almost independent of the structural modifications arising from the Jahn-Teller instability, which leads from the parent body-centered tetragonal K_2NiF_4 structure to a side-centered orthorhombic lattice. In contrast, our calculations give evidence for a strong response of the optical band gap to the corresponding structural changes.

cond-mat.str-el↗

arXiv2004

The Origin of Magnetic Interactions in Ca3Co2O6

We investigate the microscopic origin of the ferromagnetic and antiferromagnetic spin exchange couplings in the quasi one-dimensional cobalt compound Ca3Co2O6. In particular, we establish a local model which stabilizes a ferromagnetic alignment of the S=2 spins on the cobalt sites with trigonal prismatic symmetry, for a sufficiently strong Hund's rule coupling on the cobalt ions. The exchange is mediated through a S=0 cobalt ion at the octahedral sites of the chain structure. We present a strong coupling evaluation of the Heisenberg coupling between the S=2 Co spins on a separate chain. The chains are coupled antiferromagnetically through super-superexchange via short O-O bonds.

cond-mat.str-el↗

arXiv2004

The vanadium Magneli phases VnO(2n-1)

To compare the metal-insulator transitions (MITs) of VO2 and V2O3 we analyze the relations between the structural and electronic properties of the vanadium Magneli phases. These materials set up the homologous series VnO(2n-1) (3 <= n <= 9) and have crystal structures comprising typical dioxide-like and sesquioxide-like regions. As the MITs of the vanadium Magneli phases are accompanied by structural transformations, we are able to discuss the effects of characteristic changes in the local atomic environments. The systematic investigation of the transport properties is based on a new and unifying description of the crystal structures of the Magneli phases including VO2 and V2O3. Our results lead to a comprehensive understanding of the MITs in the Magneli class and shed new light on the role of particular electronic states for the MIT of V2O3.

cond-mat.str-el↗