24 research outputs found
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
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
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
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
Topological Insulators in Ternary Compounds with a Honeycomb Lattice
One of the most exciting subjects in solid state physics is a single layer of
graphite which exhibits a variety of unconventional novel properties. The key
feature of its electronic structure are linear dispersive bands which cross in
a single point at the Fermi energy. This so-called Dirac cone is closely
related to the surface states of the recently discovered topological
insulators. The ternary compounds, such as LiAuSe and KHgSb with a honeycomb
structure of their Au-Se and Hg-Sb layers feature band inversion very similar
to HgTe which is a strong precondition for existence of the topological surface
states. In contrast to graphene with two Dirac cones at K and K' points, these
materials exhibit the surface states formed by only a single Dirac cone at the
\Gamma -point together with the small direct band gap opened by a strong
spin-orbit coupling (SOC) in the bulk. These materials are centro-symmetric,
therefore, it is possible to determine the parity of their wave functions, and
hence, their topological character. Surprisingly, the compound KHgSb with the
strong SOC is topologically trivial, whereas LiAuSe is found to be a
topological non-trivial insulator.Comment: 4 pages + 1 page supplementa
Electron correlations in CoMnFeSi Heusler compounds
This study presents the effect of local electronic correlations on the
Heusler compounds CoMnFeSi as a function of the concentration
. 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
CoMnFeSi both static and dynamic correlations must be treated
on equal footing.Comment: 12 pages, 5 figure
Design of compensated ferrimagnetic Heusler alloys for giant tunable exchange bias
The discovery of materials with improved functionality can be accelerated by
rational material design. Heusler compounds with tunable magnetic sublattices
allow to implement this concept to achieve novel magnetic properties. Here, we
have designed a family of Heusler alloys with a compensated ferrimagnetic
state. In the vicinity of the compensation composition in Mn-Pt-Ga, a giant
exchange bias (EB) of more than 3 T and a similarly large coercivity are
established. The large exchange anisotropy originates from the exchange
interaction between the compensated host and ferrimagnetic clusters that arise
from intrinsic anti-site disorder. We demonstrate the applicability of our
design concept on a second material, Mn-Fe-Ga, with a magnetic transition above
room temperature, exemplifying the universality of the concept and the
feasibility of room-temperature applications. Our study points to a new
direction for novel magneto-electronic devices. At the same time it suggests a
new route for realizing rare-earth free exchange-biased hard magnets, where the
second quadrant magnetization can be stabilized by the exchange bias.Comment: Four figure