159 research outputs found

### Quasiparticle Self-Consistent GW Theory

In past decades the scientific community has been looking for a reliable
first-principles method to predict the electronic structure of solids with high
accuracy. Here we present an approach which we call the quasiparticle
self-consistent GW approximation (QpscGW). It is based on a kind of
self-consistent perturbation theory, where the self-consistency is constructed
to minimize the perturbation. We apply it to selections from different classes
of materials, including alkali metals, semiconductors, wide band gap
insulators, transition metals, transition metal oxides, magnetic insulators,
and rare earth compounds. Apart some mild exceptions, the properties are very
well described, particularly in weakly correlated cases. Self-consistency
dramatically improves agreement with experiment, and is sometimes essential.
Discrepancies with experiment are systematic, and can be explained in terms of
approximations made.Comment: 12 pages, 3 figure

### Heusler-based synthetic antiferrimagnets

Antiferromagnet spintronic devices eliminate or mitigate long-range dipolar fields, thereby promising ultrafast operation. For spin transport electronics, one of the most successful strategies is the creation of metallic synthetic antiferromagnets, which, to date, have largely been formed from transition metals and their alloys. Here, we show that synthetic antiferrimagnetic sandwiches can be formed using exchange coupling spacer layers composed of atomically ordered RuAl layers and ultrathin, perpendicularly magnetized, tetragonal ferrimagnetic Heusler layers. Chemically ordered RuAl layers can both be grown on top of a Heusler layer and allow for the growth of ordered Heusler layers deposited on top of it that are as thin as one unit cell. The RuAl spacer layer gives rise to a thickness-dependent oscillatory interlayer coupling with an oscillation period of ~1.1 nm. The observation of ultrathin ordered synthetic antiferrimagnets substantially expands the family of synthetic antiferromagnets and magnetic compounds for spintronic technologies

### Bound States in Time-Dependent Quantum Transport: Oscillations and Memory Effects in Current and Density

The presence of bound states in a nanoscale electronic system attached to two
biased, macroscopic electrodes is shown to give rise to persistent,
non-decaying, localized current oscillations which can be much larger than the
steady part of the current. The amplitude of these oscillations depends on the
entire history of the applied potential. The bound-state contribution to the
{\em static} density is history-dependent as well. Moreover, the time-dependent
formulation leads to a natural definition of the bound-state occupations out of
equilibrium.Comment: 4 pages, 3 figure

### Summing Divergent Perturbative Series in a Strong Coupling Limit. The Gell-Mann - Low Function of the \phi^4 Theory

An algorithm is proposed for determining asymptotics of the sum of a
perturbative series in the strong coupling limit using given values of the
expansion coefficients. Operation of the algorithm is illustrated by test
examples, method for estimating errors is developed, and an optimization
procedure is described. Application of the algorithm to the $\phi^4$ theory
gives a behavior $\beta(g)\approx 7.4 g^{0.96}$ at large $g$ for its Gell-Mann
-- Low function. The fact that the exponent is close to unity can be
interpreted as a manifestation of the logarithmic branching of the type
$\beta(g)\sim g (\ln g)^{-\gamma}$ (with $\gamma\approx 0.14$), which is
confirmed by independent evidence. In any case, the $\phi^4$ theory is
internally consistent. The procedure of summing perturbartive series with
arbitrary values of expansion parameter is discussed.Comment: 23 pages, PD

### High orders of perturbation theory: are renormalons significant?

According to Lipatov, the high orders of perturbation theory are determined
by saddle-point configurations (instantons) of the corresponding functional
integrals. According to t'Hooft, some individual large diagrams, renormalons,
are also significant and they are not contained in the Lipatov contribution.
The history of the conception of renormalons is presented, and the arguments in
favor of and against their significance are discussed. The analytic properties
of the Borel transforms of functional integrals, Green functions, vertex parts,
and scaling functions are investigated in the case of \phi^4 theory. Their
analyticity in a complex plane with a cut from the first instanton singularity
to infinity (the Le Guillou - Zinn-Justin hypothesis) is proved. It rules out
the existence of the renormalon singularities pointed out by t'Hooft and
demonstrates the nonconstructiveness of the conception of renormalons as a
whole. The results can be interpreted as an indication of the internal
consistency of \phi^4 theory.Comment: 28 pages, 8 figures include

- …