308 research outputs found
A first-principles DFT+GW study of spin-filter and spin-gapless semiconducting Heusler compounds
Among Heusler compounds, the ones being magnetic semiconductors (also known
as spin-filter materials) are widely studied as they offer novel
functionalities in spintronic/magnetoelectronic devices. The spin-gapless
semiconductors are a special case. They possess a zero or almost-zero energy
gap in one of the two spin channels. We employ the approximation, which
allows an elaborate treatment of the electronic correlations, to simulate the
electronic band structure of these materials. Our results suggest that in most
cases the use of self energy instead of the usual density functionals is
important to accurately determine the electronic properties of magnetic
semiconductors.Comment: Final version as publishe
Role of defects and disorder in the half-metallic full-Heusler compounds
Half-metallic ferromagnets and especially the full-Heusler alloys containing
Co are at the center of scientific research due to their potential applications
in spintronics. For realistic devices it is important to control accurately the
creation of defects in these alloys. We review some of our late results on the
role of defects and impurities in these compounds. More precisely we present
results for the following cases (i) doping and disorder in CoCr(Mn)Al(Si)
alloys, (ii) half-metallic ferrimagnetism appeared due to the creation of
Cr(Mn) antisites in these alloys, (iii) Co-doping in MnVAl(Si) alloys
leading to half-metallic antiferromagnetism, and finally (iv) the occurrence of
vacancies in the full-Heusler alloys containing Co and Mn. These results are
susceptible of encouraging further theoretical and experimental research in the
properties of these compounds.Comment: Chapter intended for a book with contributions of the invited
speakers of the International Conference on Nanoscale Magnetism 2007. Revised
version contains new figure
Quasiparticle band structure of the almost-gapless transition-metal-based Heusler semiconductors
Transition-metal-based Heusler semiconductors are promising materials for a
variety of applications ranging from spintronics to thermoelectricity.
Employing the approximation within the framework of the FLAPW method, we
study the quasi-particle band structure of a number of such compounds being
almost gapless semiconductors. We find that in contrast to the
\textit{sp}-electron based semiconductors such as Si and GaAs, in these systems
the many-body corrections have a minimal effect on the electronic band
structure and the energy band gap increases by less than 0.2~eV, which makes
the starting point density functional theory (DFT) a good approximation for the
description of electronic and optical properties of these materials.
Furthermore, the band gap can be tuned either by the variation of the lattice
parameter or by the substitution of the \emph{sp}-chemical element
Broken-Bond Rule for the Surface Energies of Noble Metals
Using two different full-potential ab-initio techniques we introduce a
simple, universal rule based on the number of broken first-neighbor bonds to
determine the surface energies of the three noble metals Cu, Ag and Au. When a
bond is broken, the rearrangement of the electronic charge for these metals
does not lead to a change of the remaining bonds. Thus the energy needed to
break a bond is independent of the surface orientation. This novel finding can
lead to the development of simple models to describe the energetics of a
surface like step and kink formation, crystal growth, alloy formation,
equilibrium shape of mesoscopic crystallites and surface faceting.Comment: 4 pages, 2 figure
Introduction to half-metallic Heusler alloys: Electronic Structure and Magnetic Properties
Intermetallic Heusler alloys are amongst the most attractive half-metallic
systems due to the high Curie temperatures and the structural similarity to the
binary semiconductors. In this review we present an overview of the basic
electronic and magnetic properties of both Heusler families: the so-called
half-Heusler alloys like NiMnSb and the the full-Heusler alloys like
CoMnGe. \textit{Ab-initio} results suggest that both the electronic and
magnetic properties in these compounds are intrinsically related to the
appearance of the minority-spin gap. The total spin magnetic moment
scales linearly with the number of the valence electrons , such that
for the full-Heusler and for the half-Heusler alloys,
thus opening the way to engineer new half-metallic alloys with the desired
magnetic properties.Comment: 28 pages, submitted for a special issue of 'Journal of Physics D:
Applied Physics' on Heusler alloy
Brillouin light scattering study of CoCrFeAl and CoFeAl Heusler compounds
The thermal magnonic spectra of CoCrFeAl (CCFA) and
CoFeAl were investigated using Brillouin light scattering spectroscopy
(BLS). For CCFA, the exchange constant A (exchange stiffness D) is found to be
0.48 erg/cm (203 meV A), while for CoFeAl the corresponding values
of 1.55 erg/cm (370 meV A) were found. The observed asymmetry in the
BLS spectra between the Stokes and anti-Stokes frequencies was assigned to an
interplay between the asymmetrical profiles of hybridized Damon-Esbach and
perpendicular standing spin-wave modes, combined with the optical sensitivity
of the BLS signal to the upper side of the CCFA or CoFeAl film
Role of the van Hove Singularity in the Quantum Criticality of the Hubbard Model
A quantum critical point (QCP), separating the non-Fermi liquid region from
the Fermi liquid, exists in the phase diagram of the 2D Hubbard model
[Vidhyadhiraja et. al, Phys. Rev. Lett. 102, 206407 (2009)]. Due to the
vanishing of the critical temperature associated with a phase separation
transition, the QCP is characterized by a vanishing quasiparticle weight. Near
the QCP, the pairing is enhanced since the real part of the bare d-wave p-p
susceptibility exhibits algebraic divergence with decreasing temperature,
replacing the logarithmic divergence found in a Fermi liquid [Yang et. al,
Phys. Rev. Lett. 106, 047004 (2011)]. In this paper we explore the
single-particle and transport properties near the QCP. We focus mainly on a van
Hove singularity (vHS) coming from the relatively flat dispersion that crosses
the Fermi level near the quantum critical filling. The flat part of the
dispersion orthogonal to the antinodal direction remains pinned near the Fermi
level for a range of doping that increases when we include a negative
next-near-neighbor hopping t' in the model. For comparison, we calculate the
bare d-wave pairing susceptibility for non-interacting models with the usual
two-dimensional tight binding dispersion and a hypothetical quartic dispersion.
We find that neither model yields a vHS that completely describes the critical
algebraic behavior of the bare d-wave pairing susceptibility. The resistivity,
thermal conductivity, thermopower, and the Wiedemann-Franz Law are examined in
the Fermi liquid, marginal Fermi liquid, and pseudo-gap doping regions. A
negative next-near-neighbor hopping t' increases the doping region with
marginal Fermi liquid character. Both T and negative t' are relevant variables
for the QCP, and both the transport and the motion of the vHS with filling
suggest that they are qualitatively similar in their effect.Comment: 15 pages, 17 figure
Tunable Multifunctional Topological Insulators in Ternary Heusler Compounds
Recently the Quantum Spin Hall effect (QSH) was theoretically predicted and
experimentally realized in a quantum wells based on binary semiconductor
HgTe[1-3]. QSH state and topological insulators are the new states of quantum
matter interesting both for fundamental condensed matter physics and material
science[1-11]. Many of Heusler compounds with C1b structure are ternary
semiconductors which are structurally and electronically related to the binary
semiconductors. The diversity of Heusler materials opens wide possibilities for
tuning the band gap and setting the desired band inversion by choosing
compounds with appropriate hybridization strength (by lattice parameter) and
the magnitude of spin-orbit coupling (by the atomic charge). Based on the
first-principle calculations we demonstrate that around fifty Heusler compounds
show the band inversion similar to HgTe. The topological state in these
zero-gap semiconductors can be created by applying strain or by designing an
appropriate quantum well structure, similar to the case of HgTe. Many of these
ternary zero-gap semiconductors (LnAuPb, LnPdBi, LnPtSb and LnPtBi) contain the
rare earth element Ln which can realize additional properties ranging from
superconductivity (e. g. LaPtBi[12]) to magnetism (e. g. GdPtBi[13]) and
heavy-fermion behavior (e. g. YbPtBi[14]). These properties can open new
research directions in realizing the quantized anomalous Hall effect and
topological superconductors.Comment: 20 pages, 5 figure
Higher order effective low-energy theories
Three well-known perturbative approaches to deriving low-energy effective
theories, the degenerate Brillouin-Wigner perturbation theory (projection
method), the canonical transformation, and the resolvent methods are compared.
We use the Hubbard model as an example to show how, to fourth order in hopping
t, all methods lead to the same effective theory, namely the t-J model with
ring exchange and various correlated hoppings. We emphasize subtle technical
difficulties that make such a derivation less trivial to carry out for orders
higher than second. We also show that in higher orders, different approaches
can lead to seemingly different forms for the low-energy Hamiltonian. All of
these forms are equivalent since they are connected by an additional unitary
transformation whose generator is given explicitly. The importance of
transforming the operators is emphasized and the equivalence of their
transformed structure within the different approaches is also demonstrated.Comment: 14 pages, no figure
Half-metallic Antiferromagnet BaCrFeAs2
First-principles calculations and a tight-binding analysis predict that the
iron-pnictide BaCrFeAs2 is a promising candidate for half-metallic material
with fully-compensated magnetization. The transition-metal ions Cr and Fe
prefer the three-dimensional intervening lattice, which yields the
antiferromagnetic order of spin orientations. Due to the difference between Cr
and Fe in the electronegativity, a band gap is opened at the Fermi level in the
spin channel in which Fe provides the majority carriers. The selective
hybridization between 3d orbitals of Cr and As:4p states due to the peculiar
lattice structure of the iron-pnictide is shown to be crucial for the novel
properties.Comment: added reference
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