arXiv subjects

Bartlomiej Wiendlocha

Publications and source records attributed to Bartlomiej Wiendlocha.

25 records · Page 2Linked to original sources

arXiv2016

Resonant levels, vacancies and doping in Bi2Te3, Bi2Te2Se and Bi2Se3 tetradymites

Electronic structures of the tetradymites, Bi$_2$Te$_3$, Bi$_2$Te$_2$Se and Bi$_2$Se$_3$, containing various dopants and vacancies, are studied using the first principles calculations methods. We focus on the possibility of formation of the resonant levels (RL), confirming the formation of the RL by Sn in Bi$_2$Te$_3$, and predicting similar behavior of Sn in Bi$_2$Te$_2$Se and Bi$_2$Se$_3$. Vacancies, which are likely present on the chalcogen atoms in the real samples of Bi$_2$Te$_2$Se and Bi$_2$Se$_3$, are also studied and their charged donor and resonant behavior is discussed. Doping of the vacancy-containing materials with regular acceptors, like Ca or Mg, is shown to compensate the donor effect of vacancies, and $n-p$ crossover, while increasing the dopant concentration, is observed. We verify, that RL on Sn is not disturbed by the chalcogen vacancies in Bi$_2$Te$_2$Se and Bi$_2$Se$_3$, and for the Sn-doped materials with Se or Te vacancies, double-doping, instead of heavy doping with Sn, is suggested as an effective way of reaching the resonant level. This should help to avoid the smearing of the RL, which was a possible reason for an earlier unsuccessful experimental observation of the influence of the RL on thermoelectric properties of Sn doped Bi$_2$Te$_2$Se. Finally we show, that Al and Ga are possible new resonant impurities in the tetradymites, hoping that it will stimulate further experimental studies.

cond-mat.mtrl-sci↗

arXiv2015

P-type doping of elemental bismuth with indium, gallium and tin: a novel doping mechanism in solids

A new doping mechanism is described, whereby a doping impurity does not simply transfer charge to the bands of a host semiconductor or semimetal, but rearranges the core energy levels deep in the valence band of the host. This, in turn, leads to a redistribution of all electrons in the host, and, if designed properly, changes the location of the Fermi level EF and the density of conducting charge carriers near EF. The principle is proven experimentally in elemental Bi, whereby group III elements In and Ga dope Bi p-type, in spite of the fact that all three atoms are trivalent in the solid state. Electronic structure calculations show the formation of a hyperdeep defect state (HDS) and its effect on the EF in Bi doped with In (Bi:In) and Ga (Bi:Ga). The HDS at -5 to -6 eV below the EF of elemental Bi hybridizes with the Bi 6p electrons, and deprives the Bi valence band of two electrons per impurity atom. This then perturbs the electron count in the solid and lowers the EF. The charge on the impurity atoms is unchanged. In principle, this doping action does not result in the appearance of ionized impurities that scatter conduction electrons and holes in conventional doping. Experimentally, Shubnikov - de Haas and Hall Effect measurements show that adding In to Bi results in an increase in the density of holes. Thermoelectric, galvanomagnetic and thermomagnetic data are given for single-crystal and polycrystalline samples. In-doping leads to an enhancement of the thermoelectric figure of merit, which suggests that the new doping mechanism also gives a route to develop better thermoelectric materials. The same mechanism is at work for Sn-doped Bi (Bi:Sn), although here ionized impurity scattering is not avoided.

cond-mat.mtrl-sci↗

arXiv2014

Localization and magnetism of the resonant impurity states in Ti doped PbTe

The problem of localization of the resonant impurity states is discussed for an illustrative example of Titanium doped Lead Telluride. Electronic structure of PbTe:Ti is studied using first principles methods, densities of states and Bloch spectral functions are analyzed. We show that Ti creates resonant states in the conduction band of PbTe, however, spectral functions of the system strongly suggest localization of these states and show poor hybridization with PbTe electronic structure. The contrast between results presented here and previously reported spectral functions for PbTe:Tl correlate very well with the different effect of those impurities on thermopower (S) of PbTe, which is large increase is S for PbTe:Tl and almost no effect on S for PbTe:Ti. Moreover, magnetic properties of the system are studied and formation of magnetic moments on Ti atoms is found, both for ordered (ferromagnetic) and disordered (paramagnetic-like) phases, showing that PbTe:Ti can be a magnetic semiconductor.

cond-mat.mtrl-sci↗

arXiv2014

Effect of the tetragonal distortion on the electronic structure, phonons and superconductivity in the Mo3Sb7 superconductor

Effect of tetragonal distortion on the electronic structure, dynamical properties and superconductivity in Mo$_3$Sb$_7$ is analyzed using first principles electronic structure and phonon calculations. Rigid muffin tin approximation (RMTA) and McMillan formulas are used to calculate the electron-phonon coupling constant $λ$ and superconducting critical temperature. Our results show, that tetragonal distortion has small, but beneficial effect on superconductivity, slightly increasing $λ$, and the conclusion that the electron-phonon mechanism is responsible for the superconductivity in Mo$_3$Sb$_7$ is supported. The spin-polarized calculations for the ordered (ferromagnetic or antiferromagnetic), as well as disordered (disordered local moment) magnetic states yielded non-magnetic ground state. We point out that due to its experimentally observed magnetic properties the tetragonal Mo$_3$Sb$_7$ might be treated as noncentrosymmetric superconductor, which could have influence for the pairing symmetry. In this context the relativistic band structure is calculated and spin-orbit interaction effects are discussed.

cond-mat.supr-con↗

arXiv2013

Fermi surface and electron dispersions of PbTe doped with resonant Tl impurity from KKR-CPA calculations

We present results of detailed study on the electron dispersions and Fermi surface of lead telluride doped with $2\%$ of thallium, which is resonant impurity in PbTe. Using the KKR--CPA method, Bloch spectral functions (BSFs), which replace the dispersion relations in alloys, are calculated, and BSFs intensity maps over the Brillouin zone (alloy Fermi surface cross-sections) are presented. It is shown, that close to the valence band edge, Tl does not create an isolated impurity band, but due to its resonant character, strongly disturbs the host electronic bands, leading to disappearance of sharp and well defined electronic energy bands. Consequences of this effect on the transport properties are discussed and new qualitative explanation of the improvement in thermoelectric properties of PbTe:Tl is suggested.

cond-mat.mtrl-sci↗

arXiv2011

Electronic structure, magnetism and spin fluctuations in the superconducting weak ferromagnet Y4Co3

Results of the first principles study on the electronic structure and magnetism of the superconducting weak ferromagnet Y$_4$Co$_3$, are presented. Using the full potential Korringa-Kohn-Rostoker (FP-KKR) method, densities of states, dispersion curves and magnetic moments were calculated for quasi-ordered structural model of the compound in the framework of the local density approximation. Spin-polarized KKR calculations confirm that weak ferromagnetic properties of Y$_4$Co$_3$ can be attributed to only one cobalt atom located on (2b) site in the unit cell, while other twenty Co and Y atoms acts as a diamagnetic environment. Moreover, the magnetic Co atoms form a quasi-one-dimensional chains along $z$ direction. The magnitude of Co(2b) magnetic moment ($0.55~μ_B$) markedly overestimates the experimental value (0.23~$μ_B$), which suggests the importance of spin fluctuations in this system. Calculated distribution of spin magnetization in the unit cell provides a background for discussion of the coexistence of ferromagnetism and superconductivity in Y$_4$Co$_3$. Finally, the effect of pressure on magnetism is discussed and compared with experimental data, also supporting weak ferromagnetic behaviors in the system.

cond-mat.supr-con↗

arXiv2008

Superconductivity of Mo3Sb7 from first principles

Superconductivity in Mo3Sb7 is analyzed using the combined electronic structure and phonon calculations, and the electron--phonon coupling constant λ_{ph}=0.54 is determined from first principles. This value explains the experimental value of the superconducting critical temperature T_c=2.2 K. The possible influence of spin fluctuations and spin gap on the superconductivity in Mo3Sb7 is discussed, and electron--paramagnon interaction is found to be weak.

cond-mat.supr-con↗