1,679 research outputs found
Dynamic spin susceptibility of superconducting cuprates: A microscopic theory of the magnetic resonance mode
A microscopic theory of the dynamic spin susceptibility (DSS) in the
superconducting state within the t-J model is presented. It is based on an
exact representation for the DSS obtained by applying the Mori-type projection
technique for the relaxation function in terms of Hubbard operators. The static
spin susceptibility is evaluated by a sum-rule-conserving generalized
mean-field approximation, while the self-energy is calculated in the
mode-coupling approximation. The spectrum of spin excitations is studied in the
underdoped and optimally doped regions. The DSS reveals a resonance mode (RM)
at the antiferromagnetic wave vector Q = \pi(1,1) at low temperatures due to a
strong suppression of the damping of spin excitations. This is explained by an
involvement of spin excitations in the decay process besides the particle-hole
continuum usually considered in random-phase-type approximations. The spin gap
in the spin-excitation spectrum at Q plays a dominant role in limiting the
decay in comparison with the superconducting gap which results in the
observation of the RM even above in the underdoped region. A good
agreement with inelastic neutron-scattering experiments on the RM in YBCO
compounds is found.Comment: 15 pages, 20 figures, references adde
Superconductivity of strongly correlated electrons on the honeycomb lattice
A microscopic theory of the electronic spectrum and of superconductivity
within the t-J model on the honeycomb lattice is developed. We derive the
equations for the normal and anomalous Green functions in terms of the Hubbard
operators by applying the projection technique. Superconducting pairing of d +
id'-type mediated by the antiferromagnetic exchange is found. The
superconducting Tc as a function of hole doping exhibits a two-peak structure
related to the van Hove singularities of the density of states for the two-band
t-J model. At half-filling and for large enough values of the exchange
coupling, gapless superconductivity may occur. For small doping the coexistence
of antiferromagnetic order and superconductivity is suggested. It is shown that
the s-wave pairing is prohibited, since it violates the constraint of
no-double-occupancy.Comment: 10 pages, 3 figures, to be published in Eur. Phys. J.
Transport coefficients of multi-particle collision algorithms with velocity-dependent collision rules
Detailed calculations of the transport coefficients of a recently introduced
particle-based model for fluid dynamics with a non-ideal equation of state are
presented. Excluded volume interactions are modeled by means of biased
stochastic multiparticle collisions which depend on the local velocities and
densities. Momentum and energy are exactly conserved locally. A general scheme
to derive transport coefficients for such biased, velocity dependent collision
rules is developed. Analytic expressions for the self-diffusion coefficient and
the shear viscosity are obtained, and very good agreement is found with
numerical results at small and large mean free paths. The viscosity turns out
to be proportional to the square root of temperature, as in a real gas. In
addition, the theoretical framework is applied to a two-component version of
the model, and expressions for the viscosity and the difference in diffusion of
the two species are given.Comment: 31 pages, 8 figures, accepted by J. Phys. Cond. Matte
Spin excitations and thermodynamics of the t-J model on the honeycomb lattice
We present a spin-rotation-invariant Green-function theory for the dynamic
spin susceptibility in the spin-1/2 antiferromagnetic t-J Heisenberg model on
the honeycomb lattice. Employing a generalized mean-field approximation for
arbitrary temperatures and hole dopings, the electronic spectrum of
excitations, the spin-excitation spectrum and thermodynamic quantities
(two-spin correlation functions, staggered magnetization, magnetic
susceptibility, correlation length) are calculated by solving a coupled system
of self-consistency equations for the correlation functions. The temperature
and doping dependence of the magnetic (uniform static) susceptibility is
ascribed to antiferromagnetic short-range order. Our results on the doping
dependencies of the magnetization and susceptibility are analyzed in comparison
with previous results for the t_J model on the square lattice.Comment: 9 pages, 7 figures, submitted to European Physical Journal B. arXiv
admin note: text overlap with arXiv:1703.0839
Mesoscopic model for the fluctuating hydrodynamics of binary and ternary mixtures
A recently introduced particle-based model for fluid dynamics with continuous
velocities is generalized to model immiscible binary mixtures. Excluded volume
interactions between the two components are modeled by stochastic multiparticle
collisions which depend on the local velocities and densities. Momentum and
energy are conserved locally, and entropically driven phase separation occurs
for high collision rates. An explicit expression for the equation of state is
derived, and the concentration dependence of the bulk free energy is shown to
be the same as that of the Widom-Rowlinson model. Analytic results for the
phase diagram are in excellent agreement with simulation data. Results for the
line tension obtained from the analysis of the capillary wave spectrum of a
droplet agree with measurements based on the Laplace's equation. The
introduction of "amphiphilic" dimers makes it possible to model the phase
behavior and dynamics of ternary surfactant mixtures.Comment: 7 pages including 6 figure
Resummed Green-Kubo relations for a fluctuating fluid-particle model
A recently introduced stochastic model for fluid flow can be made Galilean
invariant by introducing a random shift of the computational grid before
collisions. This grid shifting procedure accelerates momentum transfer between
cells and leads to a collisional contribution to transport coefficients. By
resumming the Green-Kubo relations derived in a previous paper, it is shown
that this collisional contribution to the transport coefficients can be
determined exactly. The resummed Green-Kubo relations also show that there are
no mixed kinetic-collisional contributions to the transport coefficients. The
leading correlation corrections to the transport coefficients are discussed,
and explicit expressions for the transport coefficients are presented and
compared with simulation data.Comment: 4 pages including 4 figures, submitted to PRE Rapid Com
Using Language Knowledge to Comprehend Academic Discourse
The purpose of this study was to investigate the effects of using the Text Pattern Intervention in secondary reading classes to improve the reading comprehension of struggling learners. The study took place in two parts, which included (a) a design-based research sequence with three phases (i.e., preliminary, prototype, and pilot) and (b) a validation study. During the preliminary and prototype phase, the research was used to inform the development of an intervention. The effects of the intervention were further studied during the pilot, and the validation phase involved a comparison-group study of 49 students across three separate high schools. Three teachers delivered instruction in the intervention to their experimental class. The comparison classes received instruction as usual. Results indicated statistically significant differences between groups in favor of Text Pattern Intervention use on a content area passage measure. Social validity measures indicated a high degree of student and teacher satisfaction with the intervention
Comment on ``Solidification of a Supercooled Liquid in a Narrow Channel''
Comment on PRL v. 86, p. 5084 (2001) [cond-mat/0101016]. We point out that
the authors' simulations are consistent with the known theory of steady-state
solutions in this system
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