3,486 research outputs found
Finite temperature superfluid density in very underdoped cuprates
The combination of a large superconducting gap, low transition temperature,
and quasi two-dimensionality in strongly underdoped high temperature
superconductors severely constrains the behavior of the ab-plane superfluid
density \rho with temperature T. In particular, we argue that the contribution
of nodal quasiparticles to \rho(T) is essential to account both for the
amplitude of, and the recently observed deviations from, the Uemura scaling. A
relation between T_c and \rho(0) which combines the effects of quasiparticle
excitations at low temperatures and of vortex fluctuations near the critical
temperature is proposed and discussed in light of recent experiments.Comment: 5 RevTex pages, 4 figures (one new); more discussion and comparison
with experiment; version to appear in Phys. Rev.
Scattering theory with finite-gap backgrounds: Transformation operators and characteristic properties of scattering data
We develop direct and inverse scattering theory for Jacobi operators (doubly
infinite second order difference operators) with steplike coefficients which
are asymptotically close to different finite-gap quasi-periodic coefficients on
different sides. We give necessary and sufficient conditions for the scattering
data in the case of perturbations with finite second (or higher) moment.Comment: 23 page
Darboux Transformations, Infinitesimal Symmetries and Conservation Laws for Nonlocal Two-Dimensional Toda Lattice
The technique of Darboux transformation is applied to nonlocal partner of
two-dimensional periodic A_{n-1} Toda lattice. This system is shown to admit a
representation as the compatibility conditions of direct and dual
overdetermined linear systems with quantized spectral parameter. The
generalization of the Darboux transformation technique on linear equations of
such a kind is given. The connections between the solutions of overdetermined
linear systems and their expansions in series at singular points neighborhood
are presented. The solutions of the nonlocal Toda lattice and infinite
hierarchies of the infinitesimal symmetries and conservation laws are obtained.Comment: 12 pages, infinitesimal symmetries and conservation laws are adde
Propagation of cosmic-ray nucleons in the Galaxy
We describe a method for the numerical computation of the propagation of
primary and secondary nucleons, primary electrons, and secondary positrons and
electrons. Fragmentation and energy losses are computed using realistic
distributions for the interstellar gas and radiation fields, and diffusive
reacceleration is also incorporated. The models are adjusted to agree with the
observed cosmic-ray B/C and 10Be/9Be ratios. Models with diffusion and
convection do not account well for the observed energy dependence of B/C, while
models with reacceleration reproduce this easily. The height of the halo
propagation region is determined, using recent 10Be/9Be measurements, as >4 kpc
for diffusion/convection models and 4-12 kpc for reacceleration models. For
convection models we set an upper limit on the velocity gradient of dV/dz < 7
km/s/kpc. The radial distribution of cosmic-ray sources required is broader
than current estimates of the SNR distribution for all halo sizes. Full details
of the numerical method used to solve the cosmic-ray propagation equation are
given.Comment: 15 pages including 23 ps-figures and 3 tables, latex2e, uses
emulateapj.sty (ver. of 11 May 1998, enclosed), apjfonts.sty, timesfonts.sty.
To be published in ApJ 1998, v.509 (December 10 issue). More details can be
found at http://www.gamma.mpe-garching.mpg.de/~aws/aws.html Some references
are correcte
The effect of S-substitution at the O6-guanine site on the structure and dynamics of a DNA oligomer containing a G:T mismatch
The effect of S-substitution on the O6 guanine site of a 13-mer DNA duplex containing a G:T mismatch is studied using molecular dynamics. The structure, dynamic evolution and hydration of the S-substituted duplex are compared with those of a normal duplex, a duplex with Ssubstitution on guanine, but no mismatch and a duplex with just a G:T mismatch. The S-substituted mismatch leads to cell death rather than repair. One suggestion is that the G:T mismatch recognition protein recognises the S-substituted mismatch (GS:T) as G:T. This leads to a cycle of futile repair ending in DNA breakage and cell death. We find that some structural features of the helix are similar for the duplex with the G:T mismatch and that with the S-substituted mismatch, but differ from the normal duplex, notably the helical twist. These differences arise from the change in the hydrogen-bonding pattern of the base pair. However a marked feature of the S-substituted G:T mismatch duplex is a very large opening. This showed considerable variability. It is suggested that this enlarged opening would lend support to an alternative model of cell death in which the mismatch protein attaches to thioguanine and activates downstream damage-response pathways. Attack on the sulphur by reactive oxygen species, also leading to cell death, would also be aided by the large, variable opening
Hamilton Operator and the Semiclassical Limit for Scalar Particles in an Electromagnetic Field
We successively apply the generalized Case-Foldy-Feshbach-Villars (CFFV) and
the Foldy-Wouthuysen (FW) transformation to derive the Hamiltonian for
relativistic scalar particles in an electromagnetic field. In contrast to the
original transformation, the generalized CFFV transformation contains an
arbitrary parameter and can be performed for massless particles, which allows
solving the problem of massless particles in an electromagnetic field. We show
that the form of the Hamiltonian in the FW representation is independent of the
arbitrarily chosen parameter. Compared with the classical Hamiltonian for point
particles, this Hamiltonian contains quantum terms characterizing the
quadrupole coupling of moving particles to the electric field and the electric
and mixed polarizabilities. We obtain the quantum mechanical and semiclassical
equations of motion of massive and massless particles in an electromagnetic
field.Comment: 17 page
Nonlinear saturation of electrostatic waves: mobile ions modify trapping scaling
The amplitude equation for an unstable electrostatic wave in a multi-species
Vlasov plasma has been derived. The dynamics of the mode amplitude is
studied using an expansion in ; in particular, in the limit
, the singularities in the expansion coefficients are
analyzed to predict the asymptotic dependence of the electric field on the
linear growth rate . Generically , as
, but in the limit of infinite ion mass or for
instabilities in reflection-symmetric systems due to real eigenvalues the more
familiar trapping scaling is predicted.Comment: 13 pages (Latex/RevTex), 4 postscript encapsulated figures which are
included using the utility "uufiles". They should be automatically included
with the text when it is downloaded. Figures also available in hard copy from
the authors ([email protected]
Sharks of the order Carcharhiniformes from the British Coniacian, Santonian and Campanian (Upper Cretaceous).
Bulk sampling of phosphate-rich horizons within the British Coniacian to Campanian (Upper Cretaceous) yielded very large samples of shark and ray teeth. All of these samples yielded teeth of diverse members of the Carcharhiniformes, which commonly dominate the fauna. The following species are recorded and described: Pseudoscyliorhinus reussi (Herman, 1977) comb. nov., Crassescyliorhinus germanicus (Herman, 1982) gen. nov., Scyliorhinus elongatus (Davis, 1887), Scyliorhinus brumarivulensis sp. nov., ? Palaeoscyllium sp., Prohaploblepharus riegrafi (Müller, 1989) gen. nov., ? Cretascyliorhinus sp., Scyliorhinidae inc. sedis 1, Scyliorhinidae inc. sedis 2, Pteroscyllium hermani sp. nov., Protoscyliorhinus sp., Leptocharias cretaceus sp. nov., Palaeogaleus havreensis Herman, 1977, Paratriakis subserratus sp. nov., Paratriakis tenuis sp. nov., Paratriakis sp. indet. and ? Loxodon sp. Taxa belonging to the families ?Proscylliidae, Leptochariidae, and Carcharhinidae are described from the Cretaceous for the first time. The evolutionary and palaeoecological implications of these newly recognised faunas are discussed
Spin, gravity, and inertia
The gravitational effects in the relativistic quantum mechanics are
investigated. The exact Foldy-Wouthuysen transformation is constructed for the
Dirac particle coupled to the static spacetime metric. As a direct application,
we analyze the non-relativistic limit of the theory. The new term describing
the specific spin (gravitational moment) interaction effect is recovered in the
Hamiltonian. The comparison of the true gravitational coupling with the purely
inertial case demonstrates that the spin relativistic effects do not violate
the equivalence principle for the Dirac fermions.Comment: Revtex, 12 pages, no figures, accepted in Phys. Rev. Let
- …