56,580 research outputs found
Theory of Raman scattering from Leggett's collective mode in a multiband superconductor: Application to MgB
In 1966 Leggett used a two-band superconductor to show that a new collective
mode could exist at low temperatures, corresponding to a counter-flow of the
superconducting condensates in each band. Here, the theory of electronic Raman
scattering in a superconductor by Klein and Dierker (1984) is extended to a
multiband superconductor. Raman scattering creates particle/hole pairs. In the
relevant \ symmetry, the attraction that produces pairing necessarily
couples excitations of superconducting pairs to these p/h excitations. In the
Appendix it is shown that for zero wave vector transfer this coupling
modifies the Raman response and makes the long-range Coulomb correction null.
The 2-band result is applied to MgB where this coupling activates
Leggett's collective mode. His simple limiting case is obtained when the
interband attractive potential is decreased to a value well below that given by
LDA theory. The peak from Leggett's mode is studied as the potential is
increased through the theoretical value: With realistic MgB\ parameters,
the peak broadens through decay into the continuum above the smaller (
band) superconducting gap. Finite effects are also taken into account,
yielding a Raman peak that agrees well in energy with the experimental result
by Blumberg \textit{et el.} (2007). This approach is also applied to the ,
2-band model of the Fe-pnictides considered by Chubukov \textit{et al.}(2009).Comment: 10 pages, 3 figures. To appear in Physical Review
Geography and intra-national home bias : U.S. domestic trade in 1949 and 2007
This paper examines home bias in U.S. domestic trade in 1949 and 2007. We use a unique
data set of 1949 carload waybill statistics produced by the Interstate Commerce Commission,
and 2007 Commodity Flow Survey data. The results show that home bias was considerably
smaller in 1949 than in 2007 and that home bias in 1949 was even negative for several
commodities. We argue that the difference between the geographical distribution of the
manufacturing activities in 1949 and that of 2007 is an important factor explaining the
differences in the magnitudes of home-bias estimates in those years
Classical mappings of the symplectic model and their application to the theory of large-amplitude collective motion
We study the algebra Sp(n,R) of the symplectic model, in particular for the
cases n=1,2,3, in a new way. Starting from the Poisson-bracket realization we
derive a set of partial differential equations for the generators as functions
of classical canonical variables. We obtain a solution to these equations that
represents the classical limit of a boson mapping of the algebra. The
relationship to the collective dynamics is formulated as a theorem that
associates the mapping with an exact solution of the time-dependent Hartree
approximation. This solution determines a decoupled classical symplectic
manifold, thus satisfying the criteria that define an exactly solvable model in
the theory of large amplitude collective motion. The models thus obtained also
provide a test of methods for constructing an approximately decoupled manifold
in fully realistic cases. We show that an algorithm developed in one of our
earlier works reproduces the main results of the theorem.Comment: 23 pages, LaTeX using REVTeX 3.
A Polynomial-time Bicriteria Approximation Scheme for Planar Bisection
Given an undirected graph with edge costs and node weights, the minimum
bisection problem asks for a partition of the nodes into two parts of equal
weight such that the sum of edge costs between the parts is minimized. We give
a polynomial time bicriteria approximation scheme for bisection on planar
graphs.
Specifically, let be the total weight of all nodes in a planar graph .
For any constant , our algorithm outputs a bipartition of the
nodes such that each part weighs at most and the total cost
of edges crossing the partition is at most times the total
cost of the optimal bisection. The previously best known approximation for
planar minimum bisection, even with unit node weights, was . Our
algorithm actually solves a more general problem where the input may include a
target weight for the smaller side of the bipartition.Comment: To appear in STOC 201
Making sense of the manufacturing belt : determinants of U.S. industrial location, 1880-1920
This paper investigates the ability of the new economic geography to explain the persistence of the manufacturing belt in the United States around the turn of the 20th century using a model which subsumes both market-potential and factor-endowment arguments. The results show that market potential was central to the existence of the manufacturing belt, that it mattered more than factor endowments, and that its impact
came through interactions both with scale economies and with linkage effects. Natural advantage played a role in industrial location but only through agricultural inputs which were important for a small subset of manufacturing
NoSEBrEaK - Attacking Honeynets
It is usually assumed that Honeynets are hard to detect and that attempts to
detect or disable them can be unconditionally monitored. We scrutinize this
assumption and demonstrate a method how a host in a honeynet can be completely
controlled by an attacker without any substantial logging taking place
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