2,977 research outputs found
Mass enhancement, correlations, and strong coupling superconductivity in the beta-pyrochlore KOs2O6
To assess electron correlation and electron-phonon coupling in the recently
discovered beta-pyrochlores KOs2O6 and RbOs2O6, we have performed specific heat
measurements in magnetic fields up to 14 T. We present data from high quality
single crystalline KOs2O6, showing that KOs2O6 is a strong coupling
superconductor with a coupling parameter lambda_ep \approx 1.0 to 1.6 (RbOs2O6:
lambda_ep \approx 1). The estimated Sommerfeld coefficient of KOs2O6, gamma=76
to 110 mJ/(mol K^2), is twice that of RbOs2O6 [gamma=44 mJ/(mol K^2)]. Using
strong-coupling corrections, we extract useful thermodynamic parameters of
KOs2O6. Quantifying lambda_ep allows us to determine the mass enhancement over
the calculated band electronic density of states. A significant contribution in
addition to the electron-phonon term of lambda_c=1.7 to 4.3 is deduced. In an
effort to understand the origin of the enhancement mechanism, we also
investigate an unusual energetically low-lying phonon. There are three phonon
modes per RbOs2O6, suggestive of the phonon source being the rattling motion of
the alkali ion. This dynamic instability of the alkali ions causes large
scattering of the charge carriers which shows up in an unusual temperature
dependence of the electrical resistivity.Comment: Accepted for publication in PR
Average Relaxations of Extremal Problems
In this paper extremal problems that include averaging operation in constraints and objective are considered. The relaxation caused by a replacement of a problem without averaging with a problem that includes averaging is formally defined and investigated. Canonical form for nolinear programming problem with averaging is constructed and its conditions for optimality are derived. It is shown how optimality conditions for optimal control problems with various types of objectives and constraints can be derived using its averaged relaxation.averaging; constraint relaxation; nonlinear programming; optimal control problem; optimality conditions
Optimal Dispatch in Electricity Markets
The problem of calculating the optimal dispatch and prices in a single-period electricity auction in a wholesale electricity market is considered here. The novel necessary and sufficient conditions of optimality for this problem are derived and computational algorithms for solving these conditions are constructed.optimal dispatch; electricity market; nonlinear programming; non-convex problems; dynamic programming
Rational Theories of 2D Gravity from the Two-Matrix Model
The correspondence claimed by M. Douglas, between the multicritical regimes
of the two-matrix model and 2D gravity coupled to (p,q) rational matter field,
is worked out explicitly. We found the minimal (p,q) multicritical potentials
U(X) and V(Y) which are polynomials of degree p and q, correspondingly. The
loop averages W(X) and \tilde W(Y) are shown to satisfy the Heisenberg
relations {W,X} =1 and {\tilde W,Y}=1 and essentially coincide with the
canonical momenta P and Q. The operators X and Y create the two kinds of
boundaries in the (p,q) model related by the duality (p,q) - (q,p). Finally, we
present a closed expression for the two two-loop correlators and interpret its
scaling limit.Comment: 24 pages, preprint CERN-TH.6834/9
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