arXiv subjects

R. Podloucky

Publications and source records attributed to R. Podloucky.

17 recordsLinked to original sources

arXiv2019

Stoichiometric and off-stoichiometric full Heusler $\mathbf {Fe_2V_{1-x}W_xAl} $ thermoelectric systems

A series of full-Heusler alloys, $\rm Fe_2V_{1-x}W_xAl$, $0 \leq x \leq 0.2$, was prepared, characterized and relevant physical properties to account for the thermoelectric performance were studied in a wide temperature range. Additionally, off-stoichiometric samples with similar compositions have been included, and a 10~\% improvement of the thermoelectric figure of merit was obtained. The V/W substitution causes i) a change of the main carrier type, from holes to electrons as evidenced from Seebeck and Hall measurements and ii) a substantial reduction of the lattice thermal conductivity due to a creation of lattice disorder by means of a distinct different mass and metallic radius upon the V/W substitution. Moreover $ZT$ values above 0.2 have been obtained. A microscopic understanding of the experimental data observed is revealed from ab-initio calculations of the electronic and phononic structure.

cond-mat.mtrl-sci↗

arXiv2014

Electronic thermal conductivity as derived by density functional theory

Reliable evaluation of the lattice thermal conductivity is of importance for optimizing the figure-of-merit of thermoelectric materials. Traditionally, when deriving the phonon mediated thermal conductivity $κ_{ph} = κ- κ_{el}$ from the measured total thermal conductivity $κ$ the constant Lorenz number $L_0$ of the Wiedemann-Franz law \mbox{$\mathbf{κ_{el}}=T L_0 σ$} is chosen. The present study demonstrates that this procedure is not reliable when the Seebeck coefficient $|S|$ becomes large which is exactly the case for a thermoelectric material of interest. Another approximation using $L_0-S^2$, which seem to work better for medium values of $S^2$ also fails when $S^2$ becomes large, as is the case when the system becomes semiconducting/insulating. For a reliable estimation of $κ_{el}$ it is proposed, that a full first-principles calculations by combining density functional theory with Boltzmann's transport theory has to be made. For the present study such an approach was chosen for investigating the clathrate type-I compound Ba$_8$Au$_{6-x}$Ge$_{40+x}$ for a series of dopings or compositions $x$. For a doping of $0.8$ electrons corresponding to $x=0.27$ the calculated temperature dependent Seebeck coefficient agrees well with recent experiments corroborating the validity of the density functional theory approach.

cond-mat.mtrl-sci↗

arXiv2013

Magnetic structure map for face-centered tetragonal iron: appearance of a new collinear spin structure

For face-centered cubic (fcc) and tetragonal (fct) iron a large number of magnetic configurations as a function of crystal structural parameters were studied by means of density functional theory. The stability of magnetic structures was defined by the magnetic re-orientation energy $Δ{E}^i_\text{reor}$ as the difference of the total energy of configuration $i$ and that of the fcc ferromagnetic state. The Cluster Expansion technique was applied to several volumes deriving $Δ{E}_\text{reor}$ for more than 90.000 collinear spin structures for each volume. A variety of structures with promisingly low $Δ{E}_\text{reor}$ were tetragonally distorted according to a two-dimensional mesh defined by volume per atom and $c/a$ ratio of the distortion. At each of the points on this mesh $ΔE_\text{reor}$ of all collinear structure were compared to results for non-collinear spin spirals (SS) which were calculated for a grid of propagation directions. The lowest $Δ{E}_\text{reor}$ of all investigated spin structures then defined the magnetic structure map spanned by volume per atom and $c/a$ ratio. Three local minima were identified and for each of the minima SS were calculated on a fine grid of propagation vectors. For the volume per atom of 10.6 Å$^3$ and the distortion range $0.94 \le c/a \le 1.01$ we found a new, surprisingly simple collinear spin structure with four atoms per fct unit cell to be the most stable one. This structure was called AFM/NM because it consists of two atoms with anti-ferromagnetically ordered local moments of $\pm 1.8 μ_\text{B}$ and of two nonmagnetic atoms with zero local moment. It seems that the newly detected AFM/NM structure explains a variety of puzzling experimental results.

cond-mat.mtrl-sci↗

arXiv2012

Density functional theory study of phase stability, vibrational and electronic properties of $Mo_3Al_2C$

Based on density functional theory the noncentrosymmetric superconductor Mo_3Al_2C in its well established β-Mn type ($P4_{1}32$) crystal structure is investigated. In particular, its thermodynamical and dynamical stabilities are studied by calculating lattice vibrations within the harmonic approximation. It is found that the fully stoichiometric compound is dynamically unstable. However, compounds with carbon vacancies, i.e., Mo_3Al_2C_{1-x}, can be dynamically stabilized for vacancy concentrations x > 0.09. By means of a simple thermodynamical model we estimate x ~ 0.13-0.14 for Mo_3Al_2C_{1-x} at the experimental preparation temperatures. The influence of the carbon vacancy concentration on the electronic structure is investigated.

cond-mat.mtrl-sci↗

arXiv2012

Optimizing thermoelectric properties of filled MPt$_4$Ge$_{12-x}$Sb$_x$ skutterudites by band engineering

On the basis of density functional theory (DFT) calculations thermoelectric properties are derived for Sb-doped skutterudites MPt$_4$Ge$_{12-x}$Sb$_x$ with M=Ba,La,Th. It is predicted that the originally very small absolute values of Seebeck coefficients $|S|$ of the undoped compounds is increased by factors of 10 or more for suitable dopings. The optimal dopings correspond to a "magic" valence electron number for which all electronic states up to a (pseudo)gap are filled. The theoretical findings are corroborated by measurements of $S$ for LaPt$_4$Ge$_{12-x}$Sb$_x$ skutterudites. DFT derived vibrational rattling-like modes for LaPt$_4$Ge$_{12}$ indicate a small value for the lattice thermal conductivity which in combination with a large value of $S^2$ makes the La-based skutterudites appear as promising thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2012

Strain and structure driven complex magnetic ordering of a CoO overlayer on Ir(100)

We have investigated the magnetic ordering in the ultrathin c(10$\times$2) CoO(111) film supported on Ir(100) on the basis of ab-initio calculations. We find a close relationship between the local structural properties of the oxide film and the induced magnetic order, leading to alternating ferromagnetically and anti-ferromagnetically ordered segments. While the local magnetic order is directly related to the geometric position of the Co atoms, the mismatch between the CoO film and the Ir substrate leads to a complex long-range order of the oxide.

cond-mat.mtrl-sci↗

arXiv2012

First-principles modeling of temperature and concentration dependent solubility in the phase separating Fe$_x$Cu$_{1-x}$ alloy system

We present a novel cluster-expansion (CE) approach for the first-principles modeling of temperature and concentration dependent alloy properties. While the standard CE method includes temperature effects only via the configurational entropy in Monte Carlo simulations, our strategy also covers the first-principles free energies of lattice vibrations. To this end, the effective cluster interactions of the CE have been rendered genuinely temperature dependent, so that they can include the vibrational free energies of the input structures. As a model system we use the phase-separating alloy Fe-Cu with our focus on the Fe-rich side. There, the solubility is derived from Monte Carlo simulations, whose precision had to be increased by averaging multiple CEs. We show that including the vibrational free energy is absolutely vital for the correct first-principles prediction of Cu solubility in the bcc Fe matrix: The solubility tremendously increases and is now in quantitative agreement with experimental findings.

cond-mat.mtrl-sci↗

arXiv2011

Thermoelectric properties of Ba-Cu-Si clathrates

Thermoelectric properties of the type-I clathrates Ba$_8$Cu$_x$Si$_{46-x}$ ($3.6 \leq x \leq 7$, $x$ = nominal Cu content) are investigated both experimentally and theoretically. The polycrystalline samples are prepared either by melting, ball milling and hot pressing or by melt spinning, hand milling and hot pressing techniques. Temperature-dependent electrical resistivity, $ρ(T)$, and the Seebeck coefficient, $S(T)$, measurements reveal metal-like behavior for all samples. For $x = 5$ and 6, density functional theory calculations are performed for deriving the enthalpy of formation and the electronic structure which is exploited for the calculation of Seebeck coefficients and conductivity within Boltzmann's transport theory. For simulating the properties of doped clathrates the rigid band model is applied. On the basis of the density functional theory results the experimentally observed compositional dependence of $ρ(T)$ and $S(T)$ of the whole sample series is analyzed. The highest dimensionless thermoelectric figure of merit $ZT$ of 0.28 is reached for a melt-spun sample at $600^{\circ}$C. The relatively low $ZT$ values in this system are attributed to the too high charge carrier concentrations.

cond-mat.mtrl-sci↗

arXiv2010

Thickness dependent structural and electronic properties of CuO adsorbed on SrTiO3(100): a hybrid density functional theory study

We discuss the structural and electronic properties of tetragonal CuO grown on SrTiO3(100) by means of hybrid density functional theory. Our analysis explains the anomalously large Cu-O vertical distance observed in the experiments (~2.7 A) in terms of a peculiar frustration between two competing local Cu-O environments characterized by different in-plane and out-of-plane bond lengths and Cu electronic populations. The proper inclusion of substrate effects is crucial to understand the tetragonal expansion and to reproduce correctly the measured valence band spectrum for a CuO thickness of 3-3.5 unit cells, in agreement with the experimentally estimated thickness.

cond-mat.mtrl-sci↗

arXiv2010

Unconventional superconductivity in weakly correlated, non-centrosymmetric $\rm{Mo_3Al_2C}$

Electrical resistivity, specific heat and NMR measurements classify non-centrosymmetric $\rm Mo_3Al_2C$ ($β$-Mn type, space group $P4_132$) as a strong-coupled superconductor with $T_c = 9$~K deviating notably from BCS-like behaviour. The absence of a Hebbel-Slichter peak, a power law behaviour of the spin-lattice relaxation rate (from $^{27}$Al NMR), a $T^3$ temperature dependence of the specific heat and a pressure enhanced $T_c$ suggest unconventional superconductivity with a nodal structure of the superconducting gap. Relativistic DFT calculations reveal a splitting of degenerate electronic bands due to the asymmetric spin-orbit coupling, favouring a mix of spin-singlet and spin triplet components in the superconducting condensate, in absence of strong correlations among electrons.

cond-mat.supr-con↗

arXiv2009

Polaronic hole-trapping in doped $\rm BaBiO_3$

The present {\em ab initio} study shows that in BaBiO$_3$, Bi$^{3+}$ sites can trap two holes from the valence band to form Bi$^{5+}$ cations. The trapping is accompanied by large local lattice distortions, therefore the composite particle consisting of the electronic-hole and the local lattice phonon field forms a polaron. Our study clearly shows that even $sp$ elements can trap carriers at lattice sites, if local lattice relaxations are sufficiently large to screen the localised hole. The derived model describes all relevant experimental results, and settles the issue of why hole doped BaBiO$_3$ remains semiconducting upon moderate hole doping.

cond-mat.mtrl-sci↗

arXiv2009

Interplay between magnetic, electronic and vibrational effects in monolayer Mn_3O_4 grown on Pd(100)

The surface stabilized MnO(100)-like monolayer, characterised by a regular c(4x2) distribution of Mn vacancies, is studied by hybrid functionals and discussed in the light of available scanning tunneling microscopy and high-resolution electron energy loss spectroscopy data. We show that the use of hybrid functionals is crucial to account for the intermingled nature of magnetic ineractions, electron localization, structural distortions and surface phonons. The proposed Pd(100) supported Mn3O4 structure is excellently compatible with the experiments previously reported in literature.

cond-mat.mtrl-sci↗

arXiv2008

Two-dimensional manganese oxide nanolayers on Pd(100): Surface phase diagram

Two-dimensional manganese oxide layers have been grown on Pd(100) and have been characterized by scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). The complex surface phase diagram of MnOx on Pd(100) is reported, where nine different novel Mn oxide phases have been detected as a function of the chemical potential of oxygen mO. Three regions of the chemical potential of oxygen can be identified, in which structurally related oxide phases are formed, often in coexistence at the surface. The different regions of mO are reflected in the oxidation states of the respective Mn oxide nanolayers as revealed by the Mn 2p and O 1s XPS binding energies. The MnOx nanolayers form two-dimensional wetting layers and it is speculated that they mediate the epitaxial growth of MnO on Pd(100) by providing structurally graded interfaces.

cond-mat.mtrl-sci↗

arXiv2008

Structural and vibrational properties of two-dimensional $\rm Mn_xO_y$ nanolayers on Pd(100)

Using different experimental techniques combined with density functional based theoretical methods we have explored the formation of interface-stabilized manganese oxide structures grown on Pd(100) at (sub)monolayer coverage. Amongst the multitude of phases experimentally observed we focus our attention on four structures which can be classified into two distinct regimes, characterized by different building blocks. Two oxygen-rich phases are described in terms of MnO(111)-like O-Mn-O trilayers, whereas the other two have a lower oxygen content and are based on a MnO(100)-like monolayer structure. The excellent agreement between calculated and experimental scanning tunneling microscopy images and vibrational electron energy loss spectra allows for a detailed atomic description of the explored models.

cond-mat.mtrl-sci↗

arXiv2007

Superconductivity in novel Ge-based skutterudites: {Sr,Ba}Pt_4Ge_{12}

Combining experiments and ab initio models we report on $\rm SrPt_4Ge_{12}$ and $\rm BaPt_4Ge_{12}$ as members of a novel class of superconducting skutterudites, where Sr or Ba atoms stabilize a framework entirely formed by Ge-atoms. Below $T_c=5.35$ K, and 5.10 K for $\rm BaPt_4Ge_{12}$ and $\rm SrPt_4Ge_{12}$, respectively, electron-phonon coupled superconductivity emerges, ascribed to intrinsic features of the Pt-Ge framework, where Ge-$p$ states dominate the electronic structure at the Fermi energy.

cond-mat.supr-con↗

arXiv2006

Ab initio prediction of half-metallic properties for the ferromagnetic Heusler alloys Co$_2$MSi (M=Ti, V, Cr)

By means of density functional calculations the magnetic and electronic properties and phase stabilities of the Heusler compounds Co$_2$MSi (with M=Ti, V, Cr, Mn, Fe, Co, Ni) were investigated. Based on the calculated results we predict the ferromagnetic phases of the compounds Co$_2$TiSi, Co$_2$VSi and Co$_2$CrSi to be half-metals. Of particular interest is Co$_2$CrSi because of its high density of majority spin states at Fermi energy in combination with a reasonably high estimated Curie temperature of 747K. The compounds Co$_2$TiSi and Co$_2$VSi are thermodynamically stable, whereas Co$_2$CrSi is a metastable phase which might be stabilized by suitable experimental techniques.

cond-mat.mtrl-sci↗

arXiv2004

A new polymorphic material? Structural degeneracy of ZrMn_2

Based on density functional calculations, we propose that ZrMn_2 is a polymorphic material. We predict that at low temperatures the cubic C15, and the hexagonal C14 and C36 structures of the Laves phase compound ZrMn_2 are nearly equally stable within 0.3 kJmol^{-1} or 30 K. This degeneracy occurs when the Mn atoms magnetize spontaneously in a ferromagnetic arrangement forming the states of lowest energy. From the temperature dependent free energies at T approx 160K we predict a transition from the most stable C15 to the C14 structure, which is the experimentally observed structure at elevated temperatures.

cond-mat.mtrl-sci↗