89 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
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
Anisotropic Topological Hall Effect with Real and Momentum Space Berry Curvature in the Antiskrymion Hosting Heusler Compound MnPtSn
The topological Hall effect (THE) is one of the key signatures of
topologically non-trivial magnetic spin textures, wherein electrons feel an
additional transverse voltage to the applied current. The magnitude of THE is
often small compared to the anomalous Hall effect. Here, we find a large THE of
0.9 cm that is of the same order of the anomalous Hall effect in the
single crystalline antiskyrmion hosting Heusler compound MnPtSn, a
non-centrosymmetric tetragonal compound. The THE is highly anisotropic and
survives in the whole temperature range where the spin structure is noncoplanar
(<170 K). The THE is zero above the spin reorientation transition temperature
of 170 K, where the magnetization will have a collinear and ferromagnetic
alignment. The large value of the THE entails a significant contribution from
the momentum space Berry curvature along with real space Berry curvature, which
has never been observed earlier
Magnesium Phosphate Cement as Mineral Bone Adhesive
Mineral bone cements were actually not developed for their application as bone-bonding agents, but as bone void fillers. In particular, calcium phosphate cements (CPC) are considered to be unsuitable for that application, particularly under moist conditions. Here, we showed the ex vivo ability of different magnesium phosphate cements (MPC) to adhere on bovine cortical bone substrates. The cements were obtained from a mixture of farringtonite (Mg(PO)) with different amounts of phytic acid (CHOP, inositol hexaphosphate, IP6), whereas cement setting occurred by a chelation reaction between Mg ions and IP6. We were able to show that cements with 25% IP6 and a powder-to-liquid ratio (PLR) of 2.0 g/mL resulted in shear strengths of 0.81 ± 0.12 MPa on bone even after 7 d storage in aqueous conditions. The samples showed a mixed adhesive–cohesive failure with cement residues on the bone surface as indicated by scanning electron microscopy and energy-dispersive X-ray analysis. The presented material demonstrated appropriate bonding characteristics, which could enable a broadening of the mineral bone cements’ application field to bone adhesive
Magnetic and electric properties of double-perovskites and estimation of their Curie temperatures by ab initio calculations
First principles electronic structure calculations have been carried out on
ordered double perovskites Sr_2B'B"O_6 (for B' = Cr or Fe and B" 4d and 5d
transition metal elements) with increasing number of valence electrons at the
B-sites, and on Ba_2MnReO_6 as well as Ba_2FeMoO_6. The Curie temperatures are
estimated ab initio from the electronic structures obtained with the local
spin-density functional approximation, full-potential generalized gradient
approximation and/or the LDA+U method (U - Hubbard parameter). Frozen
spin-spirals are used to model the excited states needed to evaluate the
spherical approximation for the Curie temperatures. In cases, where the induced
moments on the oxygen was found to be large, the determination of the Curie
temperature is improved by additional exchange functions between the oxygen
atoms and between oxygen and B' and B" atoms.
A pronounced systematics can be found among the experimental and/or
calculated Curie temperatures and the total valence electrons of the transition
metal elements.Comment: 8 pages, 11 figures. Submitted to the Physical Review
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