360 research outputs found
Tunnel junctions of unconventional superconductors
The phenomenology of Josephson tunnel junctions between unconventional
superconductors is developed further. In contrast to s-wave superconductors,
for d-wave superconductors the direction dependence of the tunnel matrix
elements that describe the barrier is relevant. We find the full I-V
characteristics and comment on the thermodynamical properties of these
junctions. They depend sensitively on the relative orientation of the
superconductors. The I-V characteristics differ from the normal s-wave RSJ-like
behavior.Comment: 4 pages, revtex, 4 (encapsulated postscript) figures (figures
replaced
The Equation of State of Neutron-Star Matter in Strong Magnetic Fields
We study the effects of very strong magnetic fields on the equation of state
(EOS) in multicomponent, interacting matter by developing a covariant
description for the inclusion of the anomalous magnetic moments of nucleons.
For the description of neutron star matter, we employ a field-theoretical
approach which permits the study of several models which differ in their
behavior at high density. Effects of Landau quantization in ultra-strong
magnetic fields ( Gauss) lead to a reduction in the electron
chemical potential and a substantial increase in the proton fraction. We find
the generic result for Gauss that the softening of the EOS caused
by Landau quantization is overwhelmed by stiffening due to the incorporation of
the anomalous magnetic moments of the nucleons. In addition, the neutrons
become completely spin polarized. The inclusion of ultra-strong magnetic fields
leads to a dramatic increase in the proton fraction, with consequences for the
direct Urca process and neutron star cooling. The magnetization of the matter
never appears to become very large, as the value of never deviates from
unity by more than a few percent. Our findings have implications for the
structure of neutron stars in the presence of large frozen-in magnetic fields.Comment: 40 pages, 7 figures, accepted for publication in Ap
Atomistic study on the pressure dependence of the melting point of NdFe12
We investigated, using molecular dynamics, how pressure affects the melting point of the recently theorised and epitaxially grown structure NdFe12. We modified Morse potentials using experimental constants and a genetic algorithm code, before running two-phase solid-liquid coexistence simulations of NdFe12 at various temperatures and pressures. The refitting of the Morse potentials allowed us to significantly improve the accuracy in predicting the melting temperature of the constituent elements
Derivative-Coupling Models and the Nuclear-Matter Equation of State
The equation of state of saturated nuclear matter is derived using two
different derivative-coupling Lagrangians. We show that both descriptions are
equivalent and can be obtained from the sigma-omega model through an
appropriate rescaling of the coupling constants. We introduce generalized forms
of this rescaling to study the correlations amongst observables in infinite
nuclear matter, in particular, the compressibility and the effective nucleon
mass.Comment: 16 pages, 6 figures, 36 kbytes. To appear in Zeit. f. Phys. A
(Hadrons and Nuclei
Disorder-induced magnetic memory: Experiments and theories
Beautiful theories of magnetic hysteresis based on random microscopic
disorder have been developed over the past ten years. Our goal was to directly
compare these theories with precise experiments. We first developed and then
applied coherent x-ray speckle metrology to a series of thin multilayer
perpendicular magnetic materials. To directly observe the effects of disorder,
we deliberately introduced increasing degrees of disorder into our films. We
used coherent x-rays to generate highly speckled magnetic scattering patterns.
The apparently random arrangement of the speckles is due to the exact
configuration of the magnetic domains in the sample. In effect, each speckle
pattern acts as a unique fingerprint for the magnetic domain configuration.
Small changes in the domain structure change the speckles, and comparison of
the different speckle patterns provides a quantitative determination of how
much the domain structure has changed. How is the magnetic domain configuration
at one point on the major hysteresis loop related to the configurations at the
same point on the loop during subsequent cycles? The microscopic return-point
memory(RPM) is partial and imperfect in the disordered samples, and completely
absent when the disorder was not present. We found the complementary-point
memory(CPM) is also partial and imperfect in the disordered samples and
completely absent when the disorder was not present. We found that the RPM is
always a little larger than the CPM. We also studied the correlations between
the domains within a single ascending or descending loop. We developed new
theoretical models that do fit our experiments.Comment: 26 pages, 25 figures, Accepted by Physical Review B 01/25/0
Peierls transition in the presence of finite-frequency phonons in the one-dimensional extended Peierls-Hubbard model at half-filling
We report quantum Monte Carlo (stochastic series expansion) results for the
transition from a Mott insulator to a dimerized Peierls insulating state in a
half-filled, 1D extended Hubbard model coupled to optical bond phonons. Using
electron-electron (e-e) interaction parameters corresponding approximately to
polyacetylene, we show that the Mott-Peierls transition occurs at a finite
value of the electron-phonon (e-ph) coupling. We discuss several different
criteria for detecting the transition and show that they give consistent
results. We calculate the critical e-ph coupling as a function of the bare
phonon frequency and also investigate the sensitivity of the critical coupling
to the strength of the e-e interaction. In the limit of strong e-e couplings,
we map the model to a spin-Peierls chain and compare the phase boundary with
previous results for the spin-Peierls transition. We point out effects of a
nonlinear spin-phonon coupling neglected in the mapping to the spin-Peierls
model.Comment: 7 pages, 5 figure
Mean field theory of the Mott-Anderson transition
We present a theory for disordered interacting electrons that can describe
both the Mott and the Anderson transition in the respective limits of zero
disorder and zero interaction. We use it to investigate the T=0 Mott-Anderson
transition at a fixed electron density, as a the disorder strength is
increased. Surprisingly, we find two critical values of disorder W_{nfl} and
W_c. For W > W_{nfl}, the system enters a ``Griffiths'' phase, displaying
metallic non-Fermi liquid behavior. At even stronger disorder, W=W_c > W_{nfl}
the system undergoes a metal insulator transition, characterized by the linear
vanishing of both the typical density of states and the typical quasiparticle
weight.Comment: 4 pages, 2 figures, REVTEX, eps
Probability distribution of substituted Titanium in RT12 (R = Nd, Sm, T = Fe, Co) structures
This is the author accepted manuscript. The final version is available from IEEE via the DOI in this recordWe investigated the atomic fill site probability distributions across supercell structures of RT12-xTi (R=Nd, Sm, T=Fe, Co). We use a combined molecular dynamics and Boltzmann distribution approach to extrapolate the probability distributions for Ti substitution from lower to higher temperatures with an equilibrium condition to assess how temperature affects the predictability of the structures fill path. It was found that the Nd and Sm based Fe systems have the highest filling probability path at lower temperatures but the cohesive energy change due to Ti substitution in Sm and Nd based crystals indicates that a more stable system could be achieved with a combination Co and Fe in the transition metal site.Engineering and Physical Sciences Research Council (EPSRC)Vienna Science and Technology FundRoyal SocietyToyota Motor Corporatio
Role of the Coulomb and the vector-isovector potentials in the isospin asymmetry of nuclear pseudospin
We investigate the role of the Coulomb and the vector-isovector
potentials in the asymmetry of the neutron and proton pseudospin splittings in
nuclei. To this end, we solve the Dirac equation for the nucleons using central
vector and scalar potentials with Woods-Saxon shape and and dependent
Coulomb and potentials added to the vector potential. We study the
effect of these potentials on the energy splittings of proton and neutron
pseudospin partners along a Sn isotopic chain. We use an energy decomposition
proposed in a previous work to assess the effect of a pseudospin-orbit
potential on those splittings. We conclude that the effect of the Coulomb
potential is quite small and the potential gives the main contribution
to the observed isospin asymmetry of the pseudospin splittings. This isospin
asymmetry results from a cancellation of the various energy terms and cannot be
attributed only to the pseudospin-orbit term, confirming the dynamical
character of this symmetry pointed out in previous works.Comment: 9 pages, 11 figures, uses revtex4; title was changed and several
small corrections were made throughout the tex
Prospects of Detecting Baryon and Quark Superfluidity from Cooling Neutron Stars
Baryon and quark superfluidity in the cooling of neutron stars are
investigated. Observations could constrain combinations of the neutron or
Lambda-hyperon pairing gaps and the star's mass. However, in a hybrid star with
a mixed phase of hadrons and quarks, quark gaps larger than a few tenths of an
MeV render quark matter virtually invisible for cooling. If the quark gap is
smaller, quark superfluidity could be important, but its effects will be nearly
impossible to distinguish from those of other baryonic constituents.Comment: 4 pages, 3 ps figures, uses RevTex(aps,prl). Submitted to Phys. Rev.
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