20,601 research outputs found
Low-energy moments of non-diagonal quark current correlators at four loops
We compute the leading four physical terms in the low-energy expansions of
heavy-light quark current correlators at four-loop order. As a by-product we
reproduce the corresponding top-induced non-singlet correction to the
electroweak rho parameter.Comment: 13 pages, no figures. Extended discussion and added reference
Five-Loop Static Contribution to the Gravitational Interaction Potential of Two Point Masses
We compute the static contribution to the gravitational interaction potential
of two point masses in the velocity-independent five-loop (and 5th
post-Newtonian) approximation to the harmonic coordinates effective action in a
direct calculation. The computation is performed using effective field methods
based on Feynman diagrams in momentum-space in space
dimensions. We also reproduce the previous results including the 4th
post-Newtonian order.Comment: 15 pages, 4 figure
Effective three-particle interactions in low-energy models for multiband systems
We discuss different approximations for effective low-energy interactions in
multi-band models for weakly correlated electrons. In the study of Fermi
surface instabilities of the conduction band(s), the standard approximation
consists only keeping those terms in the bare interactions that couple only to
the conduction band(s), while corrections due to virtual excitations into bands
away from the Fermi surface are typically neglected. Here, using a functional
renormalization group approach, we present an improved truncation for the
treatment of the effective interactions in the conduction band that keeps track
of the generated three-particle interactions (six-point term) and hence allows
one to include important aspects of these virtual interband excitations. Within
a simplified two-patch treatment of the conduction band, we demonstrate that
these corrections can have a rather strong effect in parts of the phase diagram
by changing the critical scales for various orderings and the phase boundaries.Comment: revised version, 16 pages, 13 figure
Evolution of the neutron resonances in AFe2Se2
Recent experiments on the alkali-intercalated iron selenides have raised
questions about the symmetry of the superconducting phase. Random phase
approximation calculations of the leading pairing eigenstate for a tight-
binding 5-orbital Hubbard-Hund model of AFe2Se2 find that a d-wave (B1g) state
evolves into an extended s{\pm} (A1g) state as the system is hole-doped.
However, over a range of doping these two states are nearly degenerate. Here,
we calculate the imaginary part of the magnetic spin susceptibility
\chi"(q,{\omega}) for these gaps and discuss how the evolution of neutron
scattering resonances can distinguish between them
Biaxial order parameter in the homologous series of orthogonal bent-core smectic liquid crystals
The fundamental parameter of the uniaxial liquid crystalline state that governs nearly all of its physical properties is the primary orientational order parameter (S) for the long axes of molecules with respect to the director. The biaxial liquid crystals (LCs) possess biaxial order parameters depending on the phase symmetry of the system. In this paper we show that in the first approximation a biaxial orthogonal smectic phase can be described by two primary order parameters: S for the long axes and C for the ordering of the short axes of molecules. The temperature dependencies of S and C are obtained by the Haller's extrapolation technique through measurements of the optical birefringence and biaxiality on a nontilted polar antiferroelectric (Sm-APA) phase of a homologous series of LCs built from the bent-core achiral molecules. For such a biaxial smectic phase both S and C, particularly the temperature dependency of the latter, are being experimentally determined. Results show that S in the orthogonal smectic phase composed of bent cores is higher than in Sm-A calamatic LCs and C is also significantly large
Origin of Gap Anisotropy in Spin Fluctuation Models of the Fe-pnictides
We discuss the large gap anisotropy found for the A1g (s-wave) state in RPA
spin-fluctuation and functional renormalization group calculations and show how
the simple arguments leading to isotropic sign-switched s-wave states in these
systems need to be supplemented by a consideration of pair scattering within
Fermi surface sheets and between the individual electron sheets as well. In
addition, accounting for the orbital makeup of the states on the Fermi surface
is found to be crucial.Comment: 6 pages, 7 figure
Isotropic-nematic phase equilibria of polydisperse hard rods: The effect of fat tails in the length distribution
We study the phase behaviour of hard rods with length polydispersity, treated
within a simplified version of the Onsager model. We give a detailed
description of the unusual phase behaviour of the system when the rod length
distribution has a "fat" (e.g. log-normal) tail up to some finite cutoff. The
relatively large number of long rods in the system strongly influences the
phase behaviour: the isotropic cloud curve, which defines the where a nematic
phase first occurs as density is increased, exhibits a kink; at this point the
properties of the coexisting nematic shadow phase change discontinuously. A
narrow three-phase isotropic-nematic-nematic coexistence region exists near the
kink in the cloud curve, even though the length distribution is unimodal. A
theoretical derivation of the isotropic cloud curve and nematic shadow curve,
in the limit of large cutoff, is also given. The two curves are shown to
collapse onto each other in the limit. The coexisting isotropic and nematic
phases are essentially identical, the only difference being that the nematic
contains a larger number of the longest rods; the longer rods are also the only
ones that show any significant nematic ordering. Numerical results for finite
but large cutoff support the theoretical predictions for the asymptotic scaling
of all quantities with the cutoff length.Comment: 21 pages, 13 figure
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