3,087 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
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
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
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
Benchmark calculations for elastic fermion-dimer scattering
We present continuum and lattice calculations for elastic scattering between
a fermion and a bound dimer in the shallow binding limit. For the continuum
calculation we use the Skorniakov-Ter-Martirosian (STM) integral equation to
determine the scattering length and effective range parameter to high
precision. For the lattice calculation we use the finite-volume method of
L\"uscher. We take into account topological finite-volume corrections to the
dimer binding energy which depend on the momentum of the dimer. After
subtracting these effects, we find from the lattice calculation kappa a_fd =
1.174(9) and kappa r_fd = -0.029(13). These results agree well with the
continuum values kappa a_fd = 1.17907(1) and kappa r_fd = -0.0383(3) obtained
from the STM equation. We discuss applications to cold atomic Fermi gases,
deuteron-neutron scattering in the spin-quartet channel, and lattice
calculations of scattering for nuclei and hadronic molecules at finite volume.Comment: 16 pages, 5 figure
Convective-core overshooting and the final fate of massive stars
Massive stars can explode in powerful supernovae (SNe) forming neutron stars
but they may also collapse directly into black holes (BHs). Understanding and
predicting their final fate is increasingly important, e.g, in the context of
gravitational-wave astronomy. The interior mixing of stars in general and
convective boundary mixing remain some of the largest uncertainties in their
evolution. Here, we investigate the influence of convective boundary mixing on
the pre-SN structure and explosion properties of massive stars. Using the 1D
stellar evolution code Mesa, we model single, non-rotating stars of solar
metallicity with initial masses of and convective core
step-overshooting of . Stars are evolved until the onset
of iron core collapse, and the pre-SN models are exploded using a parametric,
semi-analytic SN code. We use the compactness parameter to describe the
interior structure of stars at core collapse. Larger convective core
overshooting shifts the location of the compactness peak by
to higher . As the luminosity of the
pre-SN progenitor is determined by , we predict BH formation for
progenitors with luminosities and
. The luminosity range of BH formation agrees
well with the observed luminosity of the red supergiant star N6946BH1 that
disappeared without a bright SN and likely collapsed into a BH. While some of
our models in the luminosity range indeed
collapse to form BHs, this does not fully explain the lack of observed SN~IIP
progenitors at these luminosities, ie the missing red-supergiant problem.
Convective core overshooting affects the BH masses, the pre-SN location of
stars in the Hertzsprung-Russell diagram, the plateau luminosity and duration
of SN~IIP lightcurves.[Abridged]Comment: Accepted for publication in Astronomy & Astrophysics: 23 pages, 14
figure
Concepts and Their Dynamics: A Quantum-Theoretic Modeling of Human Thought
We analyze different aspects of our quantum modeling approach of human
concepts, and more specifically focus on the quantum effects of contextuality,
interference, entanglement and emergence, illustrating how each of them makes
its appearance in specific situations of the dynamics of human concepts and
their combinations. We point out the relation of our approach, which is based
on an ontology of a concept as an entity in a state changing under influence of
a context, with the main traditional concept theories, i.e. prototype theory,
exemplar theory and theory theory. We ponder about the question why quantum
theory performs so well in its modeling of human concepts, and shed light on
this question by analyzing the role of complex amplitudes, showing how they
allow to describe interference in the statistics of measurement outcomes, while
in the traditional theories statistics of outcomes originates in classical
probability weights, without the possibility of interference. The relevance of
complex numbers, the appearance of entanglement, and the role of Fock space in
explaining contextual emergence, all as unique features of the quantum
modeling, are explicitly revealed in this paper by analyzing human concepts and
their dynamics.Comment: 31 pages, 5 figure
A BPS Interpretation of Shape Invariance
We show that shape invariance appears when a quantum mechanical model is
invariant under a centrally extended superalgebra endowed with an additional
symmetry generator, which we dub the shift operator. The familiar mathematical
and physical results of shape invariance then arise from the BPS structure
associated with this shift operator. The shift operator also ensures that there
is a one-to-one correspondence between the energy levels of such a model and
the energies of the BPS-saturating states. These findings thus provide a more
comprehensive algebraic setting for understanding shape invariance.Comment: 15 pages, 2 figures, LaTe
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