8,164 research outputs found
On the Ricci tensor in type II B string theory
Let be a metric connection with totally skew-symmetric torsion \T
on a Riemannian manifold. Given a spinor field and a dilaton function
, the basic equations in type II B string theory are \bdm \nabla \Psi =
0, \quad \delta(\T) = a \cdot \big(d \Phi \haken \T \big), \quad \T \cdot \Psi
= b \cdot d \Phi \cdot \Psi + \mu \cdot \Psi . \edm We derive some relations
between the length ||\T||^2 of the torsion form, the scalar curvature of
, the dilaton function and the parameters . The main
results deal with the divergence of the Ricci tensor \Ric^{\nabla} of the
connection. In particular, if the supersymmetry is non-trivial and if
the conditions \bdm (d \Phi \haken \T) \haken \T = 0, \quad \delta^{\nabla}(d
\T) \cdot \Psi = 0 \edm hold, then the energy-momentum tensor is
divergence-free. We show that the latter condition is satisfied in many
examples constructed out of special geometries. A special case is . Then
the divergence of the energy-momentum tensor vanishes if and only if one
condition \delta^{\nabla}(d \T) \cdot \Psi = 0 holds. Strong models (d \T =
0) have this property, but there are examples with \delta^{\nabla}(d \T) \neq
0 and \delta^{\nabla}(d \T) \cdot \Psi = 0.Comment: 9 pages, Latex2
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
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
Edge singularities in high-energy spectra of gapped one-dimensional magnets in strong magnetic fields
We use the dynamical density matrix renormalization group technique to show
that the high-energy part of the spectrum of a S=1 Haldane chain, placed in a
strong external magnetic field exceeding the Haldane gap , contains
edge singularities, similar to those known to exist in the low-energy spectral
response. It is demonstrated that in the frequency range
the longitudinal (with respect to the applied field) dynamical structure factor
is dominated by the power-law singularity
. We study
the behavior of the high-energy edge exponent and the edge
as functions of the magnetic field. The existence of edge
singularities at high energies is directly related to the Tomonaga-Luttinger
liquid character of the ground state at and is expected to be a
general feature of one-dimensional gapped spin systems in high magnetic fields.Comment: (v2) error in Eq.(11) correcte
Killing spinors in supergravity with 4-fluxes
We study the spinorial Killing equation of supergravity involving a torsion
3-form \T as well as a flux 4-form \F. In dimension seven, we construct
explicit families of compact solutions out of 3-Sasakian geometries, nearly
parallel \G_2-geometries and on the homogeneous Aloff-Wallach space. The
constraint \F \cdot \Psi = 0 defines a non empty subfamily of solutions. We
investigate the constraint \T \cdot \Psi = 0, too, and show that it singles
out a very special choice of numerical parameters in the Killing equation,
which can also be justified geometrically
The G_2 sphere over a 4-manifold
We present a construction of a canonical G_2 structure on the unit sphere
tangent bundle S_M of any given orientable Riemannian 4-manifold M. Such
structure is never geometric or 1-flat, but seems full of other possibilities.
We start by the study of the most basic properties of our construction. The
structure is co-calibrated if, and only if, M is an Einstein manifold. The
fibres are always associative. In fact, the associated 3-form results from a
linear combination of three other volume 3-forms, one of which is the volume of
the fibres. We also give new examples of co-calibrated structures on well known
spaces. We hope this contributes both to the knowledge of special geometries
and to the study of 4-manifolds.Comment: 13 page
Many-body effects on the Rashba-type spin splitting in bulk bismuth tellurohalides
We report on many-body corrections to one-electron energy spectra of bulk
bismuth tellurohalides---materials that exhibit a giant Rashba-type spin
splitting of the band-gap edge states. We show that the corrections obtained in
the one-shot approximation noticeably modify the spin-orbit-induced spin
splitting evaluated within density functional theory. We demonstrate that
taking into account many-body effects is crucial to interpret the available
experimental data.Comment: 6 pages, 1 figur
Exact diagonalization solver for the extended dynamical mean-field theory
We present an efficient exact diagonalization scheme for the extended
dynamical mean-field theory and apply it to the extended Hubbard model on the
square lattice with nonlocal charge-charge interactions. Our solver reproduces
the phase diagram of this approximation with good accuracy. Details on the
numerical treatment of the large Hilbert space of the auxiliary
Holstein-Anderson impurity problem are provided. Benchmarks with a numerically
exact strong-coupling continuous-time quantum-Monte Carlo solver show better
convergence behavior of the exact diagonalization in the deep insulator.
Special attention is given to possible effects due to the discretization of the
bosonic bath. We discuss the quality of real axis spectra and address the
question of screening in the Mott insulator within extended dynamical
mean-field theory.Comment: 12 pages, 8 figure
- …