22,468 research outputs found
Rational material design of mixed-valent high T superconductors
We design, from first principles calculations, a novel family of thallium
halide-based compounds as candidates for new high temperature superconductors,
whose superconductivity is mediated by the recently proposed mechanism of
non-local correlation-enhanced strong electron-phonon coupling. Two prototype
compounds namely CsTlF and CsTlCl are studied with various hole doping
levels and volumes. The critical superconducting temperature T are
predicted to be about 30 K and 20 K with 0.35/f.u. hole doping and
require only modest pressures (10 and 2 GPa), respectively. Our
procedure of designing this class of superconductors is quite general and can
be used to search for other "other high temperature superconductors".Comment: 6- ages, EPL 101, 27002 (2013
Correlation-enhanced electron-phonon coupling: Applications of GW and screened hybrid functional to bismuthates, chloronitrides, and other high Tc superconductors
We show that the electron-phonon coupling (EPC) in many materials can be
significantly underestimated by the standard density functional theory (DFT) in
the local density approximation (LDA) due to large non-local correlation
effects. We present a simple yet efficient methodology to evaluate the
realistic EPC going beyond LDA by using more advanced and accurate GW and
screened hybrid functional DFT approaches. The corrections we propose explain
the extraordinarily high superconducting temperatures that are observed in two
distinct classes of compounds-the bismuthates and the transition metal
chloronitrides, thus solving a thirty-year-old puzzle. Our work calls for the
critically reevaluation of the EPC of certain phonon modes in many other
materials such as cuprates and iron-based superconductors. The proposed
methodology can be used to design new correlation-enhanced high temperature
superconductors and other functional materials involving electron-phonon
interaction.Comment: Substantilly extended version of the previous manuscript, 19 pages,
10 figures, accepted for publication in Phys. Rev.
The Role of Crystal Symmetry in the Magnetic Instabilities of -YbAlB and -YbAlB
Density functional theory methods are applied to investigate the properties
of the new superconductor -YbAlB and its polymorph
-YbAlB. We utilize the generalized gradient approximation + Hubbard
U (GGA+U) approach with spin-orbit(SO) coupling to approximate the effects of
the strong correlations due to the open shell of Yb. We examine closely
the differences in crystal bonding and symmetry of -YbAlB and
-YbAlB. The in-plane bonding structure amongst the dominant
itinerant electrons in the boron sheets is shown to differ significantly. Our
calculations indicate that, in both polymorphs, the localized 4 electrons
hybridize strongly with the conduction sea when compared to the related
materials YbRhSi and YbB. Comparing -YbAlB to the
electronic structure of related crystal structures indicates a key role of the
7-member boron coordination of the Yb ion in -YbAlB in producing its
enhanced Kondo scale and superconductivity. The Kondo scale is shown to depend
strongly on the angle between the B neighbors and the Yb ion, relative to the
plane, which relates some of the physical behavior to structural
characteristics.Comment: 9 pages, 9 figures, 2 table
A numerical approach to optimal dividend policies with capital injections and transaction costs
postprin
Optimal debt ratio and dividend payment strategies with reinsurance
This paper derives the optimal debt ratio and dividend payment strategies for an insurance company. Taking into account the impact of reinsurance policies and claims from the credit derivatives, the surplus process is stochastic that is jointly determined by the reinsurance strategies, debt levels, and unanticipated shocks. The objective is to maximize the total expected discounted utility of dividend payment until financial ruin. Using dynamic programming principle, the value function is the solution of a second-order nonlinear Hamilton–Jacobi–Bellman equation. The subsolution–supersolution method is used to verify the existence of classical solutions of the Hamilton–Jacobi–Bellman equation. The explicit solution of the value function is derived and the corresponding optimal debt ratio and dividend payment strategies are obtained in some special cases. An example is provided to illustrate the methodologies and some interesting economic insights.postprin
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