1,253 research outputs found
Spin susceptibility and magnetic short-range order in the Hubbard model
The uniform static spin susceptibility in the paraphase of the one-band
Hubbard model is calculated within a theory of magnetic short--range order
(SRO) which extends the four-field slave-boson functional-integral approach by
the trans- formation to an effective Ising model and the self-consistent
incorporation of SRO at the saddle point. This theory describes a transition
from the paraphase without SRO for hole dopings to a
paraphase with anti- ferromagnetic SRO for . In this region the susceptibility consists of interrelated
`itinerant' and `local' parts and increases upon doping. The zero--temperature
susceptibility exhibits a cusp at and reduces to the usual
slave-boson result for larger dopings. Using the realistic value of the
on--site Coulomb repulsion for LSCO, the peak position () as well as the doping dependence reasonably agree with low--temperature
susceptibility experiments showing a maximum at a hole doping of about 25\%.Comment: 4 pages, 1 Postscript figure, revtex-style, accepted for publishing:
Phys. Rev. B, 54, ... (1996
Theory of magnetic short-range order for itinerant electron systems
On the basis of the one--band t-t'-Hubbard model a self-consistent
renormalization theory of magnetic short--range order (SRO) in the paramagnetic
phase is presented combining the four-field slave-boson functional-integral
scheme with the cluster variational method. Contrary to previous SRO approaches
the SRO is incorporated at the saddle-point and pair-approximation levels. A
detailed numerical evaluation of the theory is performed at zero temperature,
where both the hole- and electron-doped cases as well as band-structure effects
are studied. The ground--state phase diagram shows the suppression of magnetic
long-range order in favour of a paramagnetic phase with antiferromagnetic SRO
in a wide doping region. In this phase the uniform static spin susceptibility
increases upon doping up to the transition to the Pauli paraphase. Comparing
the theory with experiments on high--T_c cuprates a good agreement is found.Comment: 33 pages, 4 Postscript figure
Movement Restrictions, Agricultural Trade and Price Transmission between Israel and the West Bank
Imposing military security measures as a consequence of violent conflict may lead to depressing economic effects for all parties involved. One implication is the limited ability to conduct trade, which in turn brings about welfare losses to the economic agents involved and may threat livelihoods and food security. This paper focuses on the consequences of the Israeli-Palestinian conflict, as a prominent example, on bilateral agricultural trade and price dynamics. For this purpose, we consider high-frequency wholesale price data and data on movement restrictions (complete closures) which were imposed by the Israeli Defense Forces in the West Bank between May 2007 and December 2008. In particular, we study the price dynamics of cucumbers and apples, two crops which play an important role for bilateral trade. The spatial and temporal price relationships are assessed using a cointegration framework. Specifically, we use a novel multivariate exogenous regime-switching vector error correction model and employ a recently developed extension of Johansen’s cointegration estimation method. We find the wholesale markets of cucumbers and apples in Hebron and Tel Aviv to be integrated. For both products, the price differentials between both markets quickly adjust to short run deviations from the long-run price equilibria. The regime-dependent model suggests that the movement restrictions effectively cut off both markets from each other temporarily.Agricultural trade, cointegration, Israel, regime-dependent error correction, price transmission, Palestinian territories., International Relations/Trade, Marketing, Political Economy,
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
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.
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
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
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
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