2,909 research outputs found
Hypothesis of two-dimensional stripe arrangement and its implications for the superconductivity in high-Tc cuprates
The hypothesis that holes doped into high-Tc cuprate superconductors organize
themselves in two-dimensional (2D) array of diagonal stripes is discussed, and,
on the basis of this hypothesis, a new microscopic model of superconductivity
is proposed and solved. The model describes two kinds of hole states localized
either inside the stripes or in the antiferromagnetic domains between the
stripes. The characteristic energy difference between these two kinds of states
is identified with the pseudogap. The superconducting (SC) order parameter
predicted by the model has two components, whose phases exhibit a complex
dependence on the the center-of-mass coordinate. The model predictions for the
tunneling characteristics and for the dependence of the critical temperature on
the superfluid density show good quantitative agreement with a number of
experiments. The model, in particular, predicts that the SC peaks in the
tunneling spectra are asymmetric, only when the ratio of the SC gap to the
critical temperature is greater than 4. It is also proposed that, at least in
some high-Tc cuprates, there exist two different superconducting states
corresponding to the same doping concentration and the same critical
temperature. Finally, the checkerboard pattern in the local density of states
observed by scanning tunneling microscopy in Bi-2212 is interpreted as coming
from the states localized around the centers of stripe elements forming the 2D
superstructure.Comment: Text close to the published version. This version is 10 per cent
shorter than the previous one. All revisions are mino
Dynamical Reduction of Discrete Systems Based on the Renormalization Group Method
The renormalization group (RG) method is extended for global asymptotic
analysis of discrete systems. We show that the RG equation in the discretized
form leads to difference equations corresponding to the Stuart-Landau or
Ginzburg-Landau equations. We propose a discretization scheme which leads to a
faithful discretization of the reduced dynamics of the original differential
equations.Comment: LaTEX. 12pages. 1 figure include
Neutron Scattering Study of Temperature-Concentration Phase Diagram of (Cu1-xMgx)GeO3
In doped CuGeO3 systems, such as (Cu1-xZnx)GeO3 and Cu(Ge1-xSix)O3, the
spin-Peierls (SP) ordering (T<Tsp) coexists with the antiferromagnetic (AF)
phase (T<TN<Tsp). Tsp decreases while TN increases with increasing x in low
doping region. For higher x, however, the SP state disappears and only the AF
state remains. These features are common for all the doped CuGeO3 systems so
far studied, indicating the existence of universal T-x phase diagram. Recently,
Masuda et al. carried out comprehensive magnetic susceptibility (chi)
measurements of (Cu1-xMgx)GeO3, in which doping concentration can be controlled
significantly better than the Zn doped systems. They found that TN suddenly
jumps from 3.43 to 3.98K at the critical concentration xc sim 0.023 and that a
drop in chi corresponding to the SP ordering also disappears at x>xc. They thus
concluded that there is a compositional phase boundary between two distinct
magnetic phases. To clarify the nature of two phases, we performed
neutron-scattering measurements on (Cu1-xMgx)GeO3 single crystals with various
x. Analysis of the data at fixed temperature points as a function of doping
concentration has revealed sudden changes of order parameters at the critical
concentration xc=0.027 +- 0.001. At finite temperatures below TN, the drastic
increase of the AF moment takes place at xc. The spin-Peierls order parameter
delta associated with lattice dimerization shows a precipitous decrease at all
temperature below Tsp. However, it goes to zero above xc only at the low
temperature limit.Comment: 9 pages, 9 figure
Complex Analysis of a Piece of Toda Lattice
We study a small piece of two dimensional Toda lattice as a complex dynamical
system. In particular the Julia set, which appears when the piece is deformed,
is shown analytically how it disappears as the system approaches to the
integrable limit.Comment: 17 pages, LaTe
Hirota equation as an example of integrable symplectic map
The hamiltonian formalism is developed for the sine-Gordon model on the
space-time light-like lattice, first introduced by Hirota. The evolution
operator is explicitely constructed in the quantum variant of the model, the
integrability of the corresponding classical finite-dimensional system is
established.Comment: 10 pages, LaTe
Bi-differential calculi and integrable models
The existence of an infinite set of conserved currents in completely
integrable classical models, including chiral and Toda models as well as the KP
and self-dual Yang-Mills equations, is traced back to a simple construction of
an infinite chain of closed (respectively, covariantly constant) 1-forms in a
(gauged) bi-differential calculus. The latter consists of a differential
algebra on which two differential maps act. In a gauged bi-differential
calculus these maps are extended to flat covariant derivatives.Comment: 24 pages, 2 figures, uses amssymb.sty and diagrams.sty, substantial
extensions of examples (relative to first version
Bicomplexes and Integrable Models
We associate bicomplexes with several integrable models in such a way that
conserved currents are obtained by a simple iterative construction. Gauge
transformations and dressings are discussed in this framework and several
examples are presented, including the nonlinear Schrodinger and sine-Gordon
equations, and some discrete models.Comment: 17 pages, LaTeX, uses amssymb.sty and diagrams.st
Elementary excitations, exchange interaction and spin-Peierls transition in CuGeO
The microscopic description of the spin-Peierls transition in pure and doped
CuGeO_3 is developed taking into account realistic details of crystal
structure. It it shown that the presence of side-groups (here Ge) strongly
influences superexchange along Cu-O-Cu path, making it antiferromagnetic.
Nearest-neighbour and next-nearest neighbour exchange constants and
are calculated. Si doping effectively segments the CuO_2-chains
leading to or even slightly ferromagnetic. Strong
sensitivity of the exchange constants to Cu-O-Cu and (Cu-O-Cu)-Ge angles may be
responsible for the spin-Peierls transition itself (``bond-bending mechanism''
of the transition). The nature of excitations in the isolated and coupled
spin-Peierls chains is studied and it is shown that topological excitations
(solitons) play crucial role. Such solitons appear in particular in doped
systems (Cu_{1-x}Zn_xGeO_3, CuGe_{1-x}Si_xO_3) which can explain the
phase diagram.Comment: 7 pages, revtex, 7 Postscript figure
Excerpts from the paper: Research Status and Recommendation from the Alaska Workshop on Gravity Waves and Turbulence in the Middle Atmosphere, part 1.3A
Internal gravity waves are disturbances whose intrinsic frequencies k(c - u) are smaller than the Brunt-Vaisala frequency (N). Their importance arises because: they are the major components of the total flow and temperature variability fields of the mesosphere (i.e., shears and lapse rates) and hence constitute the likely sources of turbulence; and they are associated with fluxes of momentum that communicate stresses over large distances. For example, gravity waves exert a drag on the flow in the upper mesosphere. However, in order for gravity waves to exert a net drag on the atmosphere, they must be attenuated. There are two general types of processes that seek to attenuate gravity waves: dissipation and saturation. Dissipation is any process that is effective independent of the wave amplitude, while saturation occurs when certain wave amplitude conditions are met. Radiative damping is an example of dissipation, while convective overturning is an example of saturation. The two processes are not mutually exclusive
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