4,838 research outputs found
One-step replica symmetry breaking solution for fermionic Ising spin glass in a transverse field
The fermionic Ising spin glass models in a transverse field are investigated
in a Grassmann path integral formalism. The Parisi's scheme of one-step replica
symmetry breaking (RSB) is used within the static ansatz. This formalism has
already been applied in a theory in which (Parisi's block-size parameter)
is taken as a constant (PRB 72, 014447 (2005)). Now, it is extended to consider
as a variational parameter. In this case, the results show that RSB is
present when , in which the system is driven by quantum fluctuations.Comment: 8 pages, 3 figures, accepted for publication as a Brief Report in
Phys. Rev.
Budgetary Separation of Powers in the American States and the Tax Level: A Regression Discontinuity Analysis
A political regime has budgetary separation of powers if the power with the prerogative to raise taxes is not the full residual claimant of a tax increase. In the American states two conditions are needed: the governor must have the line item veto, and the political interests of the legislative majority and the governor must not be perfectly aligned. Political alignment between the executive and the legislative depends on the numbers of seats the governor's party controls in the state legislature; it changes discontinuously as we move from a unifed to a divided government. We use regression discontinuity design to establish a causal relation between a divided government and lower tax rates in states with line item veto. In states with block veto such relation is not present. We estimate the jump in the tax level at the discontinuity semiparametrically.Separation of powers, line item veto, tax level, regression discontinuity, semiparametric.
Role of the transverse field in inverse freezing in the fermionic Ising spin-glass model
We investigate the inverse freezing in the fermionic Ising spin-glass (FISG)
model in a transverse field . The grand canonical potential is
calculated in the static approximation, replica symmetry and one-step replica
symmetry breaking Parisi scheme. It is argued that the average occupation per
site is strongly affected by . As consequence, the boundary phase
is modified and, therefore, the reentrance associated with the inverse freezing
is modified too.Comment: 6 pages, 3 figures, accepted for publication in PR
An AGN Identification for 3EG J2006-2321
We present a multiwavelength analysis of the high-energy gamma-ray source 3EG
J2006-2321. The flux of this source above 100 MeV is shown to be variable on
time scales of days and months. Optical observations and careful examination of
archived radio data indicate that its radio counterpart is PMN J2005-2310, a
flat-spectrum radio quasar with a 5-GHz flux density of 260 mJy. Study of the
optical counterpart indicates a redshift of 0.833 and variable linear
polarization. No X-ray source has been detected near the position of PMN
J2005-2310, but an X-ray upper limit is derived from ROSAT data. This upper
limit provides for a spectral energy distribution with global characteristics
similar to those of known gamma-ray blazars. Taken together, these data
indicate that 3EG J2006-2321, listed as unidentified in the 3rd EGRET Catalog,
is a member of the blazar class of AGN. The 5-GHz radio flux density of this
blazar is the lowest of the 68 EGRET-detected AGN. The fact that EGRET has
detected such a source has implications for unidentified EGRET sources,
particularly those at high latitudes (), many of which may be
blazars.Comment: 22 pages, 6 figures. To appear in ApJ v569 n1, 10 April 200
Pseudogap and the specific heat of high superconductors
The specific heat of a two dimensional repulsive Hubbard model with local
interaction is investigated. We use the two-pole approximation which exhibits
explicitly important correlations that are sources of the pseudogap anomaly.
The interplay between the specific heat and the pseudogap is the main focus of
the present work. Our self consistent numerical results show that above the
occupation , the specific heat starts to decrease due to the
presence of a pseudogap in the density of states. We have also observed a two
peak structure in the specific heat. Such structure is robust with respect to
the Coulomb interaction but it is significantly affected by the occupation
. A detailed study of the two peak structure is carried out in terms of
the renormalized quasi-particle bands. The role of the second nearest neighbor
hopping on the specific heat behavior and on the pseudogap, is extensively
discussed.Comment: 6 pages, 6 figures, accepted for publication in Solid State
Communication
A van Hemmen-Kondo model for disordered strongly correlated electron systems
We present here a theoretical model in order to describe the competition
between the Kondo effect and the spin glass behavior. The spin glass part of
the starting Hamiltonian contains Ising spins with an intersite exchange
interaction given by the local van Hemmen model, while the Kondo effect is
described as usual by the intrasite exchange . We obtain, for large
values, a Kondo phase and, for smaller values, a succession, with
decreasingComment: 14 pages, 4 figures, accepted for publication in Phys. Rev.
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