1,601 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.
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
Kinetic Theory of Flocking: Derivation of Hydrodynamic Equations
It is shown how to explicitly coarse-grain the microscopic dynamics of the
Vicsek model for self-propelled agents. The macroscopic transport equations are
derived by means of an Enskog-type kinetic theory. Expressions for all
transport coefficients at large particle speed are given. The phase transition
from a disordered to a flocking state is studied numerically and analytically.Comment: 4 pages, 1 figur
Thermodynamics of the frustrated - Heisenberg ferromagnet on the body-centered cubic lattice with arbitrary spin
We use the spin-rotation-invariant Green's function method as well as the
high-temperature expansion to discuss the thermodynamic properties of the
frustrated spin- - Heisenberg magnet on the body-centered
cubic lattice. We consider ferromagnetic nearest-neighbor bonds and
antiferromagnetic next-nearest-neighbor bonds and arbitrary spin
. We find that the transition point between the ferromagnetic ground
state and the antiferromagnetic one is nearly independent of the spin ,
i.e., it is very close to the classical transition point . At finite temperatures we focus on the parameter regime
with a ferromagnetic ground-state. We calculate the Curie
temperature and derive an empirical formula describing the
influence of the frustration parameter and spin on . We find
that the Curie temperature monotonically decreases with increasing frustration
, where very close to the -curve exhibits a
fast decay which is well described by a logarithmic term
. To characterize the magnetic ordering
below and above , we calculate the spin-spin correlation functions
, the spontaneous
magnetization, the uniform static susceptibility as well as the
correlation length . Moreover, we discuss the specific heat and the
temperature dependence of the excitation spectrum. As approaching the
transition point some unusual features were found, such as negative
spin-spin correlations at temperatures above even though the ground state
is ferromagnetic or an increase of the spin stiffness with growing temperature.Comment: 19 pages, 10 figures, version as in EPJ
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
Quantum to classical crossover in the 2D easy-plane XXZ model
Ground-state and thermodynamical properties of the spin-1/2 two-dimensional
easy-plane XXZ model are investigated by both a Green's-function approach and
by Lanczos diagonalizations on lattices with up to 36 sites. We calculate the
spatial and temperature dependences of various spin correlation functions, as
well as the wave-vector dependence of the spin susceptibility for all
anisotropy parameters . In the easy--plane ferromagnetic region , the longitudinal correlators of spins at distance change sign
at a finite temperature . This transition, observed in
the 2D case for the first time, can be interpreted as a quantum to classical
crossover.Comment: 4 pages, 6 figures, Contribution to the Ising Centennial Colloquium,
ICM2000, Belo Horizonte, Brazil, August 200
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
Electron-hole pair condensation at the semimetal-semiconductor transition: a BCS-BEC crossover scenario
We act on the suggestion that an excitonic insulator state might
separate---at very low temperatures---a semimetal from a semiconductor and ask
for the nature of these transitions. Based on the analysis of electron-hole
pairing in the extended Falicov-Kimball model, we show that tuning the Coulomb
attraction between both species, a continuous crossover between a BCS-like
transition of Cooper-type pairs and a Bose-Einstein condensation of preformed
tightly-bound excitons might be achieved in a solid-state system. The precursor
of this crossover in the normal state might cause the transport anomalies
observed in several strongly correlated mixed-valence compounds.Comment: 5 pages, 5 figures, substantially revised versio
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