3,424 research outputs found
Atomic coexistence of superconductivity and incommensurate magnetic order in the Ba(Fe1-xCox)2As2 pnictide
75As NMR and susceptiblity were measured in a Ba(Fe1-xCox)2As2 single crystal
for x=6%. Nuclear Magnetic Resonance (NMR) spectra and relaxation rates allow
to show that all Fe sites experience an incommensurate magnetic ordering below
T=31K. Comparison with undoped compound allows to estimate a typical moment of
0.05 muB. Anisotropy of the NMR widths can be interpreted using a model of
incommensurability with a wavevector (1/2-eps,0,l) with eps of the order of
0.04. Below TC=21.8K, a full volume superconductivity develops as shown by
susceptibility and relaxation rate, and magnetic order remains unaffected,
demonstrating coexistence of both states on each Fe site.Comment: 4 pages, 4 figure
Proposed parametric cooling of bilayer cuprate superconductors by terahertz excitation
We propose and analyze a scheme for parametrically cooling bilayer cuprates
based on the selective driving of a -axis vibrational mode. The scheme
exploits the vibration as a transducer making the Josephson plasma frequencies
time-dependent. We show how modulation at the difference frequency between the
intra- and interbilayer plasmon substantially suppresses interbilayer phase
fluctuations, responsible for switching -axis transport from a
superconducting to resistive state. Our calculations indicate that this may
provide a viable mechanism for stabilizing non-equilibrium superconductivity
even above , provided a finite pair density survives between the bilayers
out of equilibrium.Comment: 4 pages + 7 page supplementa
Active Mass Under Pressure
After a historical introduction to Poisson's equation for Newtonian gravity,
its analog for static gravitational fields in Einstein's theory is reviewed. It
appears that the pressure contribution to the active mass density in Einstein's
theory might also be noticeable at the Newtonian level. A form of its
surprising appearance, first noticed by Richard Chase Tolman, was discussed
half a century ago in the Hamburg Relativity Seminar and is resolved here.Comment: 28 pages, 4 figure
Mn local moments prevent superconductivity in iron-pnictides Ba(Fe 1-x Mn x)2As2
75As nuclear magnetic resonance (NMR) experiments were performed on
Ba(Fe1-xMnx)2As2 (xMn = 2.5%, 5% and 12%) single crystals. The Fe layer
magnetic susceptibility far from Mn atoms is probed by the75As NMR line shift
and is found similar to that of BaFe2As2, implying that Mn does not induce
charge doping. A satellite line associated with the Mn nearest neighbours
(n.n.) of 75As displays a Curie-Weiss shift which demonstrates that Mn carries
a local magnetic moment. This is confirmed by the main line broadening typical
of a RKKY-like Mn-induced staggered spin polarization. The Mn moment is due to
the localization of the additional Mn hole. These findings explain why Mn does
not induce superconductivity in the pnictides contrary to other dopants such as
Co, Ni, Ru or K.Comment: 6 pages, 7 figure
Testing Hawking particle creation by black holes through correlation measurements
Hawking's prediction of thermal radiation by black holes has been shown by
Unruh to be expected also in condensed matter systems. We show here that in a
black hole-like configuration realised in a BEC this particle creation does
indeed take place and can be unambiguously identified via a characteristic
pattern in the density-density correlations. This opens the concrete
possibility of the experimental verification of this effect.Comment: 13 pages, 2 figures. Honorable mention in the 2010 GRF Essay
Competitio
Hierarchy of Conservation Laws of Diffusion--Convection Equations
We introduce notions of equivalence of conservation laws with respect to Lie
symmetry groups for fixed systems of differential equations and with respect to
equivalence groups or sets of admissible transformations for classes of such
systems. We also revise the notion of linear dependence of conservation laws
and define the notion of local dependence of potentials. To construct
conservation laws, we develop and apply the most direct method which is
effective to use in the case of two independent variables. Admitting
possibility of dependence of conserved vectors on a number of potentials, we
generalize the iteration procedure proposed by Bluman and Doran-Wu for finding
nonlocal (potential) conservation laws. As an example, we completely classify
potential conservation laws (including arbitrary order local ones) of
diffusion--convection equations with respect to the equivalence group and
construct an exhaustive list of locally inequivalent potential systems
corresponding to these equations.Comment: 24 page
Black holes and information theory
During the past three decades investigators have unveiled a number of deep
connections between physical information and black holes whose consequences for
ordinary systems go beyond what has been deduced purely from the axioms of
information theory. After a self-contained introduction to black hole
thermodynamics, we review from its vantage point topics such as the information
conundrum that emerges from the ability of incipient black holes to radiate,
the various entropy bounds for non-black hole systems (holographic bound,
universal entropy bound, etc) which are most easily derived from black hole
thermodynamics, Bousso's covariant entropy bound, the holographic principle of
particle physics, and the subject of channel capacity of quantum communication
channels.Comment: RevTeX, 12 pages, 5 figures. To appear in Contemporary Physic
Two hard spheres in a pore: Exact Statistical Mechanics for different shaped cavities
The Partition function of two Hard Spheres in a Hard Wall Pore is studied
appealing to a graph representation. The exact evaluation of the canonical
partition function, and the one-body distribution function, in three different
shaped pores are achieved. The analyzed simple geometries are the cuboidal,
cylindrical and ellipsoidal cavities. Results have been compared with two
previously studied geometries, the spherical pore and the spherical pore with a
hard core. The search of common features in the analytic structure of the
partition functions in terms of their length parameters and their volumes,
surface area, edges length and curvatures is addressed too. A general framework
for the exact thermodynamic analysis of systems with few and many particles in
terms of a set of thermodynamic measures is discussed. We found that an exact
thermodynamic description is feasible based in the adoption of an adequate set
of measures and the search of the free energy dependence on the adopted measure
set. A relation similar to the Laplace equation for the fluid-vapor interface
is obtained which express the equilibrium between magnitudes that in extended
systems are intensive variables. This exact description is applied to study the
thermodynamic behavior of the two Hard Spheres in a Hard Wall Pore for the
analyzed different geometries. We obtain analytically the external work, the
pressure on the wall, the pressure in the homogeneous zone, the wall-fluid
surface tension, the line tension and other similar properties
On the Geometry of Surface Stress
We present a fully general derivation of the Laplace--Young formula and
discuss the interplay between the intrinsic surface geometry and the extrinsic
one ensuing from the immersion of the surface in the ordinary euclidean
three-dimensional space. We prove that the (reversible) work done in a general
surface deformation can be expressed in terms of the surface stress tensor and
the variation of the intrinsic surface metric
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