5,748 research outputs found
Spontaneous breaking of the Fermi surface symmetry in the t-J model: a numerical study
We present a variational Monte Carlo (VMC) study of spontaneous Fermi surface
symmetry breaking in the t-J model. We find that the variational energy of a
Gutzwiller projected Fermi sea is lowered by allowing for a finite asymmetry
between the x- and the y-directions. However, the best variational state
remains a pure superconducting state with d-wave symmetry, as long as the
underlying lattice is isotropic. Our VMC results are in good overall agreement
with slave boson mean field theory (SBMFT) and renormalized mean field theory
(RMFT), although apparent discrepancies do show up in the half-filled limit,
revealing some limitations of mean field theories. VMC and complementary RMFT
calculations also confirm the SBMFT predictions that many-body interactions can
enhance any anisotropy in the underlying crystal lattice. Thus, our results may
be of consequence for the description of strongly correlated superconductors
with an anisotropic lattice structure.Comment: 6 pages, 7 figures; final versio
The Unpredictability of the Most Energetic Solar Events
Observations over the past two solar cycles show a highly irregular pattern
of occurrence for major solar flares, gamma-ray events, and solar energetic
particle (SEP) fluences. Such phenomena do not appear to follow the direct
indices of solar magnetic activity, such as the sunspot number. I show that
this results from the non-Poisson occurrence for the most energetic events.
This Letter also points out a particularly striking example of this
irregularity in a comparison between the declining phases of the recent two
solar cycles (1993-1995 and 2004-2006, respectively) and traces it through the
radiated energies of the flares, the associated SEP fluences, and the sunspot
areas. These factors suggest that processes in the solar interior involved with
the supply of magnetic flux up to the surface of the Sun have strong
correlations in space and time, leading to a complex occurrence pattern that is
presently unpredictable on timescales longer than active region lifetimes
(weeks) and not correlated well with the solar cycle itself.Comment: 4 page
Spontaneous R-Symmetry Breaking in O'Raifeartaigh Models
We study the question of whether spontaneous U(1)_R breaking can occur in
O'Raifeartaigh-type models of spontaneous supersymmetry breaking. We show that
in order for it to occur, there must be a field in the theory with R-charge
different from 0 or 2. We construct the simplest O'Raifeartaigh model with this
property, and we find that for a wide range of parameters, it has a meta-stable
vacuum where U(1)_R is spontaneously broken. This suggests that spontaneous
U(1)_R breaking actually occurs in generic O'Raifeartaigh models.Comment: 19 pages; v2: reference added, minor changes; v3: important typo
fixe
Supersymmetry Breaking, R-Symmetry Breaking and Metastable Vacua
Models of spontaneous supersymmetry breaking generically have an R-symmetry,
which is problematic for obtaining gaugino masses and avoiding light R-axions.
The situation is improved in models of metastable supersymmetry breaking, which
generically have only an approximate R-symmetry. Based on this we argue, with
mild assumptions, that metastable supersymmetry breaking is inevitable. We also
illustrate various general issues regarding spontaneous and explicit R-symmetry
breaking, using simple toy models of supersymmetry breaking.Comment: 23 page
Exact Black Hole Degeneracies and the Topological String
Motivated by the recent conjecture of Ooguri, Strominger and Vafa, we compute
the semi-canonical partition function of BPS black holes in N=4 and N=8 string
theories, to all orders in perturbation theory. Not only are the black hole
partition functions surprisingly simple; they capture the full topological
string amplitudes, as expected from the OSV conjecture. The agreement is not
perfect, however, as there are differences between the black hole and
topological string partition functions even at the perturbative level. We
propose a minimal modification of the OSV conjecture, in which these
differences are understood as a nontrivial measure factor for the topological
string.Comment: 24 page
Batalin-Vilkovisky Integrals in Finite Dimensions
The Batalin-Vilkovisky method (BV) is the most powerful method to analyze
functional integrals with (infinite-dimensional) gauge symmetries presently
known. It has been invented to fix gauges associated with symmetries that do
not close off-shell. Homological Perturbation Theory is introduced and used to
develop the integration theory behind BV and to describe the BV quantization of
a Lagrangian system with symmetries. Localization (illustrated in terms of
Duistermaat-Heckman localization) as well as anomalous symmetries are discussed
in the framework of BV.Comment: 35 page
Staggered-vorticity correlations in a lightly doped t-J model: a variational approach
We report staggered vorticity correlations of current in the d-wave
variational wave function for the lightly-doped t-J model. Such correlations
are explained from the SU(2) symmetry relating d-wave and staggered-flux
mean-field phases. The correlation functions computed by the variational Monte
Carlo method suggest that pairs are formed of holes circulating in opposite
directions.Comment: ReVTeX, 4 pages, 3 figure
Limits on Phase Separation for Two-Dimensional Strongly Correlated Electrons
From calculations of the high temperature series for the free energy of the
two-dimensional t-J model we construct series for ratios of the free energy per
hole. The ratios can be extrapolated very accurately to low temperatures and
used to investigate phase separation. Our results confirm that phase separation
occurs only for J/t greater than 1.2. Also, the phase transition into the phase
separated state has Tc of approximately 0.25J for large J/t.Comment: 4 pages, 6 figure
Field and intensity correlations in amplifying random media
We study local and nonlocal correlations of light transmitted through active
random media. The conventional approach results in divergence of ensemble
averaged correlation functions due to existence of lasing realizations. We
introduce conditional average for correlation functions by omitting the
divergent realizations. Our numerical simulation reveals that amplification
does not affect local spatial correlation. The nonlocal intensity correlations
are strongly magnified due to selective enhancement of the contributions from
long propagation paths. We also show that by increasing gain, the average mode
linewidth can be made smaller than the average mode spacing. This implies that
light transport through a diffusive random system with gain could exhibit some
similarities to that through a localized passive system, owing to dominant
influence of the resonant modes with narrow width.Comment: 5 pages, 4 figure
Backlund Transformations, D-Branes, and Fluxes in Minimal Type 0 Strings
We study the Type 0A string theory in the (2,4k) superconformal minimal model
backgrounds, focusing on the fully non-perturbative string equations which
define the partition function of the model. The equations admit a parameter,
Gamma, which in the spacetime interpretation controls the number of background
D-branes, or R-R flux units, depending upon which weak coupling regime is
taken. We study the properties of the string equations (often focusing on the
(2,4) model in particular) and their physical solutions. The solutions are the
potential for an associated Schrodinger problem whose wavefunction is that of
an extended D-brane probe. We perform a numerical study of the spectrum of this
system for varying Gamma and establish that when Gamma is a positive integer
the equations' solutions have special properties consistent with the spacetime
interpretation. We also show that a natural solution-generating transformation
(that changes Gamma by an integer) is the Backlund transformation of the KdV
hierarchy specialized to (scale invariant) solitons at zero velocity. Our
results suggest that the localized D-branes of the minimal string theories are
directly related to the solitons of the KdV hierarchy. Further, we observe an
interesting transition when Gamma=-1.Comment: 17 pages, 3 figure
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