Magnetism in tetragonal manganese-rich Heusler compounds

A comprehensive study of the total energy of manganese-rich Heusler compounds using density functional theory is presented. Starting from a large set of cubic parent systems, the response to tetragonal distortions is studied in detail. We single out the systems that remain cubic from those that most likely become tetragonal. The driving force of the tetragonal distortion and its effect on the magnetic properties, especially where they deviate from the Slater--Pauling rule, as well as the trends in the Curie temperatures, are highlighted. By means of partial densities of states, the electronic structural changes reveal the microscopic origin of the observed trends. We focus our attention on the magnetocrystalline anisotropy and find astonishingly high values for tetragonal Heusler compounds containing heavy transition metals accompanied by low magnetic moments, which indicates that these materials are promising candidates for spin-transfer torque magnetization-switching applications

Stability of Weyl points in magnetic half-metallic Heusler compounds

We employ {\it ab-initio} fully-relativistic electronic structure calculations to study the stability of the Weyl points in the momentum space within the class of the half-metallic ferromagnetic full Heusler materials, by focusing on Co$_2$TiAl as a well-established prototype compound. Here we show that both the number of the Weyl points together with their $k$-space coordinates can be controlled by the orientation of the magnetization. This alternative degree of freedom, which is absent in other topological materials (e.g. in Weyl semimetals), introduces novel functionalities, specific for the class of half-metallic ferromagnets. Of special interest are Weyl points which are preserved irrespectively of any arbitrary rotation of the magnetization axis

Topological phase transitions in bulk

We consider the analogy between the topological phase transition which occurs as a function of spatial coordinate on a surface of a non-trivial insulator, and the one which occurs in the bulk due to the change of internal parameters (such as crystal field and spin-orbit coupling). In both cases the system exhibits a Dirac cone, which is the universal manifestation of topological phase transition, independently on the type of driving parameters. In particular, this leads to a simple way of determining the topological class based solely on the bulk information even for the systems with translational symmetry broken by atomic disorder or by strong electron correlations. Here we demonstrate this on example of the zinc-blende related semiconductors by means of the {\it ab-initio} fully-relativistic band structure calculations involving the coherent potential approximation (CPA) technique.Comment: Phys. Status Solidi RRL, DOI 10.1002/pssr.2012064xx (2012), submitte

Topological insulators and thermoelectric materials

Topological insulators (TIs) are a new quantum state of matter which have gapless surface states inside the bulk energy gap. Starting with the discovery of two dimensional TIs, the HgTe-based quantum wells, many new topological materials have been theoretically predicted and experimentally observed. Currently known TI materials can possibly be classified into two families, the HgTe family and the Bi2Se family. The signatures found in the electronic structure of a TI also cause these materials to be excellent thermoelectric materials. On the other hand, excellent thermoelectric materials can be also topologically trivial. Here we present a short introduction to topological insulators and thermoelectrics, and give examples of compound classes were both good thermoelectric properties and topological insulators can be found.Comment: Phys. Status Solidi RRL, accepte

Electronic Structure, Localization and Spin-State Transition in Cu-substituted FeSe: Fe1−x_{1-x}Cux_xSe

We report density functional studies of the Fe$_{1-x}$Cu$_x$Se alloy done using supercell and coherent potential approximation methods. Magnetic behavior was investigated using the disordered local moment approach. We find that Cu occurs in a nominal $d^{10}$ configuration and is highly disruptive to the electronic structure of the Fe sheets. This would be consistent with a metal insulator transition due to Anderson localization. We further find a strong cross over from a weak moment itinerant system to a local moment magnet at $x \approx 0.12$. We associate this with the experimentally observed jump near this concentration. Our results are consistent with the characterization of this concentration dependent jump as a transition to a spin-glass

Large zero-field cooled exchange-bias in bulk Mn2PtGa

We report a large exchange-bias (EB) effect after zero-field cooling the new tetragonal Heusler compound Mn2PtGa from the paramagnetic state. The first-principle calculation and the magnetic measurements reveal that Mn2PtGa orders ferrimagnetically with some ferromagnetic (FM) inclusions. We show that ferrimagnetic (FI) ordering is essential to isothermally induce the exchange anisotropy needed for the zero-field cooled (ZFC) EB during the virgin magnetization process. The complex magnetic behavior at low temperatures is characterized by the coexistence of a field induced irreversible magnetic behavior and a spin-glass-like phase. The field induced irreversibility originates from an unusual first-order FI to antiferromagnetic transition, whereas, the spin-glass like state forms due to the existence of anti-site disorder intrinsic to the material.Comment: 5 pages, 4 figures, supplementary material included in a separate file; accepted for publication in PR

Radiocarbon in Vegetation of coastal Zone of Finnish Bay (Russia)

AbstractRadiocarbon is a radioactive isotope of carbon, which is forming in the nature constantly by interaction of cosmic fast neutrons and nitrogen nuclei at the low atmospheric layers. Another source of radiocarbon in the environment is pollution in processes of Nuclear Power Plant exploitation. The expanding construction of nuclear industrial plants and nuclear power stations on the shores of the Baltic Sea is creating a real possibility for the introduction of radioactive wastes into environment of Finnish Bay basin. The activity of radiocarbon in the plant of coastal zone was determined by a system of Sample Oxidizer 307 and low-level liquid scintillation system Quantulus 1220 (Wallace, Turku, Finland) The Radiocarbon analysis is a sensitive tool for the registration of pollution. The variations of radiocarbon concentration in the one-years grasses can be used as an indicator of carbon dioxide pollution of the urban environment. The variation of radiocarbon in tree-rings reflects the local and global radioactive contaminations

Electron correlations in Co2_2Mn1−x_{1-x}Fex_xSi Heusler compounds

This study presents the effect of local electronic correlations on the Heusler compounds Co$_2$Mn$_{1-x}$Fe$_x$Si as a function of the concentration $x$. The analysis has been performed by means of first-principles band-structure calculations based on the local approximation to spin-density functional theory (LSDA). Correlation effects are treated in terms of the Dynamical Mean-Field Theory (DMFT) and the LSDA+U approach. The formalism is implemented within the Korringa-Kohn-Rostoker (KKR) Green's function method. In good agreement with the available experimental data the magnetic and spectroscopic properties of the compound are explained in terms of strong electronic correlations. In addition the correlation effects have been analysed separately with respect to their static or dynamical origin. To achieve a quantitative description of the electronic structure of Co$_2$Mn$_{1-x}$Fe$_x$Si both static and dynamic correlations must be treated on equal footing.Comment: 12 pages, 5 figure

Application of Many-Body Perturbation Theory to the Description of Correlated Metals

An efficient computational LSDA+DMFT toolbox for the description of correlated materials has been established. The method developed in this work provides an appropriate description of 3d-transition metal correlated bulk systems, concerning their ground-state properties (magnetic moments, total energies) as well as the high- and low-energy spectroscopies (valence-band angular-resolved photoemission, Fano-effect, optical and magneto-optical properties). The incorporation of the perturbational impurity solvers within the spin-polarized relativistic Korringa-Kohn-Rostoker (SPR-KKR) Green’s function method gives rise to a fully self-consistent procedure with respect both to the DFT (charge) and the DMFT (localized dynamical self-energy) self-consistency requirements. Thus, the solution of the many-electron problem can be achieved with a high precision. In turn this opens a possibility to investigate very delicate properties, as the orbital magnetic moments of 3d-transition metals. To develop a relatively fast and accurate approach for the low-energy spectroscopies, the DMFT was implemented within the wave function formalism in the framework of the Linearized Muffin-Tin Orbitals method (LMTO). Calculations are performed in a one-shot run, that does not allow to get the charge-self-consistent solution. In such a way all effects of the localized correlations are encapsulated in the Green’s function constructed as a resolvent to the LMTO one-particle Hamiltonian and accounting for the corresponding self-energy via the Dyson equation. The LMTO+DMFT scheme gives in comparison to a plain LSDA a significantly improved description of the magneto-optics in the 3d-transition metals, half-metallic Heusler ferromagnet NiMnSb, as well as for the heavy-fermion US compound