arXiv subjects

Ján Minár

Publications and source records attributed to Ján Minár.

43 records · Page 3Linked to original sources

arXiv2016

Vectorial spin-polarization detection in multichannel spin-resolved photoemission spectroscopy using an Ir(001) imaging spin filter

We report on spin- and angular resolved photoemission spectroscopy using a high-resolution imaging spin filter based on a large Ir(001) crystal enhancing the effective figure of merit for spin detection by a factor of over $10^3$ compared to standard single channel detectors. Furthermore, we review the spin filter preparation, and its lifetime. The spin filter efficiency is mapped on a broad range of scattering energies and azimuthal angles. Large spin filter efficiencies are observed for the spin component perpendicular as well as parallel to the scattering plane depending on the azimuthal orientation of the spin filter crystal. A spin rotator capable of manipulating the spin direction prior to detection complements the measurement of three observables, thus allowing for a derivation of all three components of the spin polarization vector in multichannel spin polarimetry. The experimental results nicely agree with spin-polarized low energy electron diffraction calculations based on a fully relativistic multiple scattering method in the framework of spin-polarized density functional theory.

physics.ins-det↗

arXiv2016

Study of the hyperfine fields in the BaFe$_2$As$_2$ family and its relation to the magnetic moment

The hyperfine field $B_\text{hf}$ and the magnetic properties of the BaFe$_2$As$_2$ family are studied using the fully relativistic Dirac formalism for different types of substitution. The study covers electron doped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$, hole doped (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ and also isovalently doped Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ and BaFe$_2$(As$_{1-x}$P$_x$)$_2$ for a wide range of the concentration $x$. For the substituted compounds the hyperfine fields show a very strong dependence on the dopant type and its concentration $x$. Relativistic contributions were found to have a significantly stronger impact for the iron pnictides when compared to bulk Fe. As an important finding, we demonstrate that it is not sensible to relate the hyperfine field $B_\text{hf}$ to the average magnetic moment $μ$ of the compound, as it was done in earlier literature.

cond-mat.supr-con↗

arXiv2016

In-plane anisotropic magnetoresistance in antiferromagnetic Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ and Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$

Using the Kubo-Greenwood formalism the resistivity anisotropy for electron doped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, hole doped (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ and isovalently doped Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ in their antiferromagnetic state has been calculated in order to clarify the origin of this important phenomenon. The results show good agreement with experiment for all cases without considering impurity states extending over several unit cells or temperature induced spin fluctuations. From this it is concluded that the resistivity anisotropy at low temperatures is primarily caused by an in-plane anisotropic magnetoresistance. Accounting for the band dispersion with respect to $k_z$ is however mandatory to explain the results, showing the importance of the three-dimensional character of the electronic structure for the iron pnictides. Furthermore, it is shown that the counterintuitive sign of the resistivity anisotropy is no fundamental property but just a peculiarity of the anisotropic band structure.

cond-mat.supr-con↗

arXiv2016

Theoretical study on the anisotropic electronic structure of antiferromagnetic BaFe$_2$As$_2$ and Co-doped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ seen by angle-resolved photoemission

By means of one-step model calculations the strong in-plane anisotropy seen in angle-resolved photoemission of the well-known iron pnictide prototype compounds BaFe$_2$As$_2$ and Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ in their low-temperature antiferromagnetic phases is investigated. The fully-relativistic calculations are based on the Korringa-Kohn-Rostoker-Green function approach combined with the coherent potential approximation alloy theory to account for the disorder induced by Co substitution on Fe sites in a reliable way. The results of the calculations can be compared directly to experimental spectra of detwinned single crystals. One finds very good agreement with experiment and can reveal all features of the electronic structure contributing to the in-plane anisotropy. In particular the local density approximation can capture most of the correlation effects for the investigated system without the need for more advanced techniques. In addition, the evolution of the anisotropy for increasing Co concentration $x$ in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ can be tracked almost continuously. The results are also used to discuss surface effects and it is possible to identify clear signatures to conclude about different types of surface termination.

cond-mat.supr-con↗

arXiv2015

Multiferroic heterostructures for spin filter application - an ab initio study

Novel imaging spin-filter techniques, which are based on low energy electron diffraction, are currently of high scientific interest. To improve the spin-detection efficiency a variety of new materials have been introduced in recent years. A new class of promising spin-filter materials are represented by multiferroic systems, as both magnetic and electric ordering exist in these materials. We have investigated Fe/BaTiO3(001), which defines a prominent candidate due to its moderate spontaneous polarization, for spin filter applications calculating diffraction patterns for spin polarized electrons incident on the Fe surface. Motivated by the fact that spin polarized low energy electron diffraction is a powerful method for the determination of the properties of surfaces we investigated the influence of switching the BaTiO3 polarization on the exchange and spin orbit scattering as well as on reflectivity and figure of merit. This system obviously offers the possibility to realize a multiferroic spin filter and manipulating the spin-orbit and exchange scattering by an external electric field. The calculations have been done for a large range of kinetic energies and polar angles of the diffracted electrons considering different numbers of Fe monolayers.

physics.comp-ph↗

arXiv2014

Theoretical investigation of the electronic and magnetic properties of the orthorhombic phase of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

We present a comprehensive study on the low-temperature orthorhombic phase of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ based on the Korringa-Kohn-Rostoker-Green function approach. Using this bandstructure method in combination with the coherent potential approximation alloy theory we are able to investigate the evolution of the magnetic and electronic properties of this prototype iron pnictide for arbitrary concentrations $x$, while dealing with the chemical disorder without uncontrolled simplifications by using solely a rigid band shift or the virtual crystal approximation. We discuss the development of the site resolved magnetic moments for the experimentally observed stripe antiferromagnetic order together with the strong electronic anisotropy of the Fermi surface and compare it with angle-resolved photoemission spectroscopy measurements of detwinned crystals. We furthermore calculate magnetic exchange coupling parameters $J_{ij}$ and use them for Monte-Carlo simulations on the basis of the classical Heisenberg model to get an insight on the temperature dependence of the magnetic ordering on the cobalt concentration.

cond-mat.supr-con↗

arXiv2012

Impact of correlations and finite temperatures on the anomalous Hall conductivity of 3d-transition-metals

Employing the linear response Kubo formalism as implemented in a fully relativistic multiple-scattering Korringa-Kohn-Rostoker Green function method a systematic first-principles study based on density-functional theory (DFT) of the anomalous Hall conductivity (AHC) of the 3$d$-transition-metals Fe, Co and Ni is presented. To account for the temperature dependence of the AHC an alloy-analogy for a set of thermal lattice displacements acting as a scattering mechanism is used which is subsequently solved using the coherent potential approximation. Further, impurity scattering has been considered to elucidate the importance of an additional possible contribution to the AHC that might be present in experiment. The impact of correlations beyond the local spin-density approximation to the exchange-correlation functional in DFT is studied within the LSDA+$U$ approach. It is shown that both, the inclusion of correlations and thermal lattice vibrations, is needed to give a material-specific description of the AHC in transition-metals.

cond-mat.mtrl-sci↗