4,761 research outputs found
Effects of correlated disorder on the magnetism of double exchange systems
We study the effects of short-range correlated disorder arising from chemical
dopants or local lattice distortions, on the ferromagnetism of 3d double
exchange systems. For this, we integrate out the carriers and treat the
resulting disordered spin Hamiltonian within local random phase approximation,
whose reliability is shown by direct comparison with Monte Carlo simulations.
We find large scale inhomogeneities in the charge, couplings and spin
densities. Compared with the homogeneous case, we obtain larger Curie
temperatures () and very small spin stiffnesses (). As a result,
large variations of measured in manganites may be explained
by correlated disorder. This work also provides a microscopic model for
Griffiths phases in double exchange systems.Comment: accepted for publication in Phys. Rev. B (rapid comm.
Steps and dips in the ac conductance and noise of mesoscopic structures
The frequency dependence of the equilibrium ac conductance (or the noise
power spectrum) through a mesoscopic structure is shown to exhibit steps and
dips. The steps, at energies related to the resonances of the structure, are
closely related to the partial Friedel phases of these resonances, thus
allowing a direct measurement of these phases (without interferometry). The
dips in the spectrum are related to a destructive interference in the
absorption of energy by transitions between these resonances, in some
similarity with the Fano effect.Comment: 4 pages, 2 figure
D6 Family Symmetry and Cold Dark Matter at LHC
We consider a non-supersymmetric extension of the standard model with a
family symmetry based on D6 Z2 Z2, where one of Z2's is exactly conserved. This
Z2 forbids the tree-level neutrino masses and simultaneously ensures the
stability of cold dark matter candidates. From the assumption that cold dark
matter is fermionic we can single out the D6 singlet right-handed neutrino as
the best cold dark mater candidate. We find that an inert charged Higgs with a
mass between 300 and 750 GeV decays mostly into an electron (or a positron)
with a large missing energy, where the missing energy is carried away by the
cold dark matter candidate. This will be a clean signal at LHC.Comment: 20 pages, 7 figure
The fluctuation-dissipation theorem and the linear Glauber model
We obtain exact expressions for the two-time autocorrelation and response
functions of the -dimensional linear Glauber model. Although this linear
model does not obey detailed balance in dimensions , we show that the
usual form of the fluctuation-dissipation ratio still holds in the stationary
regime. In the transient regime, we show the occurence of aging, with a special
limit of the fluctuation-dissipation ratio, , for a quench at
the critical point.Comment: Accepted for publication (Physical Review E
Multipole Ordering and Fluctuations in f-Electron Systems
We investigate effects of multipole moments in f-electron systems both from
phenomenological and microscopic viewpoints. First, we discuss significant
effects of octupole moment on the magnetic susceptibility in a paramagnetic
phase. It is found that even within mean-field approximation, the magnetic
susceptibility deviates from the Curie-Weiss law due to interactions between
dipole and octupole moments. Next, we proceed to a microscopic theory for
multipole ordering on the basis of a j-j coupling scheme. After brief
explanation of a method to derive multipole interactions from the -electron
model, we discuss several multipole ordered phases depending on lattice
structure. Finally, we show our new development of the microscopic approach to
the evaluation of multipole response functions. We apply fluctuation exchange
approximation to the f-electron model, and evaluate multipole response
functions.Comment: 7 pages, 4 figures, Proceedings of ASR-WYP-200
Cubic Defects: Comparing the Eight-State-System with its Two-Level-Approximation
Substitutional defects in a cubic symmetry (such as a lithium defect in a KCl
host crystal) can be modeled appropriately by an eight-state-system. Usually
this tunneling degree of freedom is approximated by a two-level-system. We
investigate the observable differences between the two models in three
contexts. First we show that the two models predict different relations between
the temperature dependence of specific heat and static susceptibility. Second
we demonstrate that in the presence of external forces (pressure and electric
field) the eight-state-system shows features that cannot be understood within
the framework of the two-level-approximation. In this context we propose an
experiment for measuring the parameter for tunneling along the face diagonal.
Finally we discuss the differences between the models appearing for strongly
coupled pairs. Geometric selection rules and particular forms of asymmetry lead
to clear differences between the two models.Comment: 19 pages, Latex, submitted to J. of Phys., some small supplement
Optical and dc transport properties of a strongly correlated charge density wave system: exact solution in the ordered phase of the spinless Falicov-Kimball model with dynamical mean-field theory
We derive the dynamical mean-field theory equations for transport in an
ordered charge-density-wave phase on a bipartite lattice. The formalism is
applied to the spinless Falicov-Kimball model on a hypercubic lattice at half
filling. We determine the many-body density of states, the dc charge and heat
conductivities, and the optical conductivity. Vertex corrections continue to
vanish within the ordered phase, but the density of states and the transport
coefficients show anomalous behavior due to the rapid development of thermally
activated subgap states. We also examine the optical sum rule and sum rules for
the first three moments of the Green's functions within the ordered phase and
see that the total optical spectral weight in the ordered phase either
decreases or increases depending on the strength of the interactions.Comment: 14 pages, 14 figures, submitted to Phys. Rev.
Unstable particles versus resonances in impurity systems, conductance in quantum wires
We compute the DC conductance for a homogeneous sine-Gordon model and an
impurity system of Luttinger liquid type by means of the thermodynamic Bethe
ansatz and standard potential scattering theory. We demonstrate that unstable
particles and resonances in impurity systems lead to a sharp increase of the
conductance as a function of the temperature, which is characterized by the
Breit-Wigner formula.Comment: 5 pages Latex, 1 figure replaced, version to appear in J. Phys.
Multipole correlations in low-dimensional f-electron systems
By using a density matrix renormalization group method, we investigate the
ground-state properties of a one-dimensional three-orbital Hubbard model on the
basis of a j-j coupling scheme. For , where is a parameter
to control cubic crystalline electric field effect, one orbital is itinerant,
while other two are localized. Due to the competition between itinerant and
localized natures, we obtain orbital ordering pattern which is sensitive to
, leading to a characteristic change of quadrupole state
into an incommensurate structure. At , all the three orbitals are
degenerate, but we observe a peak at in quadrupole
correlation, indicating a ferro-orbital state, and the peak at in
dipole correlation, suggesting an antiferromagnetic state. We
also discuss the effect of octupole on magnetic anisotropy.Comment: 4 pages, 3 figures, Proceedings of ASR-WYP-2005 (September 27-29,
2005, Tokai
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