33,088 research outputs found
Holography and the C-Theorem
We review the geometric definition of C-function in the context of field
theories that admit a holographic gravity dual.Comment: Contribution to the proceedings of the TMR 2000 Paris Conferenc
Insulator to Metal Transition Induced by Disorder in a Model for Manganites
The physics of manganites appears to be dominated by phase competition among
ferromagnetic metallic and charge-ordered antiferromagnetic insulating states.
Previous investigations (Burgy {\it et al.}, Phys. Rev. Lett. {\bf 87}, 277202
(2001)) have shown that quenched disorder is important to smear the first-order
transition between those competing states, and induce nanoscale inhomogeneities
that produce the colossal magnetoresistance effect. Recent studies (Motome {\it
et al.} Phys. Rev. Lett. {\bf 91}, 167204 (2003)) have provided further
evidence that disorder is important in the manganite context, unveiling an
unexpected insulator-to-metal transition triggered by disorder in a one-orbital
model with cooperative phonons. In this paper, a qualitative explanation for
this effect is presented. It is argued that the transition occurs for disorder
in the form of local random energies. Acting over an insulating states made out
of a checkerboard arrangement of charge, with ``effective'' site energies
positive and negative, this form of disorder can produce lattice sites with an
effective energy near zero, favorable for the transport of charge. This
explanation is based on Monte Carlo simulations and the study of simplified toy
models, measuring the density-of-states, cluster conductances using the
Landauer formalism, and other observables. The applicability of these ideas to
real manganites is discussed.Comment: 14 pages, 23 figures, submitted to Physical Review
Observational Constraints on Transverse Gravity: a Generalization of Unimodular Gravity
We explore the hypothesis that the set of symmetries enjoyed by the theory
that describes gravity is not the full group of diffeomorphisms Diff(M), as in
General Relativity, but a maximal subgroup of it, TransverseDiff(M), with its
elements having a jacobian equal to unity; at the infinitesimal level, the
parameter describing the coordinate change, xi^mu (x), is transverse, i.e.,
partial_mu(xi^mu)=0. Incidentally, this is the smaller symmetry one needs to
propagate consistently a graviton, which is a great theoretical motivation for
considering these theories. Also, the determinant of the metric, g, behaves as
a "transverse scalar", so that these theories can be seen as a generalization
of the better-known unimodular gravity. We present our results on the
observational constraints on transverse gravity, in close relation with the
claim of equivalence with general scalar-tensor theory. We also comment on the
structure of the divergences of the quantum theory to the one-loop order.Comment: Prepared for the First Mediterranean Conference on Classical and
Quantum Gravity, MCCQG, Kolymbari (Crete, Greece), 14-18 September, 2009;
also, ERE2009: Gravitation in the Large, Bilbao (Spain), 7-11 September, 200
Phase Fluctuations in Strongly Coupled -Wave Superconductors
We present a numerically exact solution for the BCS Hamiltonian at any
temperature, including the degrees of freedom associated with classical phase,
as well as amplitude, fluctuations via a Monte Carlo (MC) integration. This
allows for an investigation over the whole range of couplings: from weak
attraction, as in the well-known BCS limit, to the mainly unexplored
strong-coupling regime of pronounced phase fluctuations. In the latter, for the
first time two characteristic temperatures and , associated with
short- and long-range ordering, respectively, can easily be identified in a
mean-field-motivated Hamiltonian. at the same time corresponds to the
opening of a gap in the excitation spectrum. Besides introducing a novel
procedure to study strongly coupled d-wave superconductors, our results
indicate that classical phase fluctuations are not sufficient to explain the
pseudo-gap features of high-temperature superconductors (HTS).Comment: 5 pages, 3 figure
A Renormalization Group Analysis of the NCG constraints m_{top} = 2\,m_W},
We study the evolution under the renormalization group of the restrictions on
the parameters of the standard model coming from Non-Commutative Geometry,
namely and . We adopt the point of
view that these relations are to be interpreted as {\it tree level} constraints
and, as such, can be implemented in a mass independent renormalization scheme
only at a given energy scale . We show that the physical predictions on
the top and Higgs masses depend weakly on .Comment: 7 pages, FTUAM-94/2, uses harvma
Extraction of silymarin compounds from milk thistle (Silybum marianum) seed using hot, liquid water as the solvent
High-value specialty chemicals are usually obtained from natural products by extracting with generally regarded as safe (GRAS) solvents. Because organic solvents are quite often used, high operating and disposal costs occur. When compared to traditional solvents, water is an interesting alternative because of its low operating and disposal costs. Milk thistle contains compounds (taxifolin, silychristin, silydianin, silybinin A, and silybinin B) that display hepatoxic protection properties. This paper examines the batch extraction of silymarin compounds from milk thistle seed meal in 50°C, 70°C, 85°C and 100°C water as a function of time. For taxifolin, silychristin, silybinin A, and silybinin B, extraction with 100°C water resulted in the highest yields. After 210 min of extraction at 100°C, the yield of taxifolin was 1.2 mg/g of seed while the yields of silychristin, silybinin A, and silybinin B were 5.0, 1.8 and 3.3 mg/g of seed, respectively. The ratios of the extracted compounds, and particularly the ratios at long extraction times, showed that the more polar compounds (taxifolin and silychristin) were preferentially extracted at 85°C, while the less polar silybinin was preferentially extracted at 100°C
Vacuum stability conditions of the economical 3-3-1 model from copositivity
By applying copositivity criterion to the scalar potential of the economical
model, we derive necessary and sufficient bounded-from-below conditions
at tree level. Although these are a large number of intricate inequalities for
the dimensionless parameters of the scalar potential, we present general
enlightening relations in this work. Additionally, we use constraints coming
from the minimization of the scalar potential by means of the orbit space
method, the positivity of the squared masses of the extra scalars, the Higgs
boson mass, the gauge boson mass and its mixing angle with the SM
boson in order to further restrict the parameter space of this model.Comment: 22 pages, 7 figures, added text and references. Matches published
versio
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