2,535 research outputs found
The influence of long-range hopping on ferromagnetism in the Hubbard model
The phase diagram of the Hubbard model in an external magnetic field is
examined by extrapolation of small-cluster exact-diagonalization calculations.
Using a general expression for the hopping matrix elements () the influence of long-range hopping (band asymmetry) on
ferromagnetism in this model is studied. It is found that the long-range
hopping (nonzero ) stabilizes ferromagnetism in an external magnetic field
for . In the opposite limit the fully polarized ferromagnetic
state is generally suppressed with increasing . The critical value of
magnetic field below which the ferromagnetic state becomes unstable is
calculated numerically.Comment: 8 pages, 3 Postscript figures, Late
Optical and magneto-optical response of a doped Mott insulator
We study the optical, Raman, and ac Hall response of the doped Mott insulator within the dynamical mean-field theory (d = infinity ) for strongly correlated electron systems. The occurrence of the isosbectic point in the optical conductivity is shown to be associated with the frequency dependence of the generalized charge susceptibility. We compute the Raman response, which probes the fluctuations of the "stress tensor," and show that the scattering is characterized by appreciable incoherent contributions. The calculated ac Hall constant and Hall angle also exhibit the isosbectic points. These results are also compared with those obtained for a non-FL metal in d = infinity. The role of low-energy coherence (FL) or incoherence (non-FL) in determining the finite frequency response of strongly correlated metals in d = infinity is discussed in detail. As an application of interest, we compute the dielectric figure-of-merit (DFOM), a quantity that is of potential importance for microwave device applications. We demonstrate explicitly that systems near the filling driven Mott transition might be good candidates in this respect, and discuss the influence of real-life factors on the DFOM.64
Fully Frustrated Ising System on a 3D Simple Cubic Lattice: Revisited
Using extensive Monte Carlo simulations, we clarify the critical behaviour of
the 3 dimensional simple cubic Ising Fully Frustrated system. We find two
transition temperatures and two long range ordered phases. Within the present
numerical accuracy, the transition at higher temperature is found to be second
order and we have extracted the standard critical exponent using finite size
scaling method. On the other hand, the transition at lower temperature is found
to be first order. It is argued that entropy plays a major role on determining
the low temperature state.Comment: 14 pages 14 figures iop style include
Temperature-dependent electronic structure and ferromagnetism in the d=oo Hubbard model studied by a modfied perturbation theory
The infinite-dimensional Hubbard model is studied by means of a modified
perturbation theory. The approach reduces to the iterative perturbation theory
for weak coupling. It is exact in the atomic limit and correctly reproduces the
dispersions and the weights of the Hubbard bands in the strong-coupling regime
for arbitrary fillings. Results are presented for the hyper-cubic and an
fcc-type lattice. For the latter we find ferromagnetic solutions. The
filling-dependent Curie temperature is compared with the results of a recent
Quantum Monte Carlo study.Comment: RevTeX, 5 pages, 6 eps figures included, Phys. Rev. B (in press),
Ref. 16 correcte
Cumulant expansion of the periodic Anderson model in infinite dimension
The diagrammatic cumulant expansion for the periodic Anderson model with
infinite Coulomb repulsion () is considered here for an hypercubic
lattice of infinite dimension (). The same type of simplifications
obtained by Metzner for the cumulant expansion of the Hubbard model in the
limit of , are shown to be also valid for the periodic Anderson
model.Comment: 13 pages, 7 figures.ps. To be published in J. Phys. A: Mathematical
and General (1997
Magnetic and lattice polaron in Holstein-t-J model
We investigate the interplay between the formation of lattice and magnetic
polaron in the case of a single hole in the antiferromagnetic background. We
present an exact analytical solution of the Holstein-t-J model in infinite
dimensions. Ground state energy, electron-lattice correlation function, spin
bag dimension as well as spectral properties are calculated. The magnetic and
hole-lattice correlations sustain each other, i.e. the presence of
antiferromagnetic correlations favors the formation of the lattice polaron at
lower value of the electron-phonon coupling while the polaronic effect
contributes to reduce the number of spin defects in the antiferromagnetic
background. The crossover towards a spin-lattice small polaron region of the
phase diagram becomes a discontinuous transition in the adiabatic limit.Comment: revtex, 8 eps figures included NEW version. Appendix with a full
proof include
Superconductivity in the Two-Band Hubbard Model in Infinite Dimensions
We study a two-band Hubbard model in the limit of infinite dimensions, using
a combination of analytical methods and Monte-Carlo techniques. The normal
state is found to display various metal to insulators transitions as a function
of doping and interaction strength. We derive self-consistent equations for the
local Green's functions in the presence of superconducting long-range order,
and extend previous algorithms to this case. We present direct numerical
evidence that in a specific range of parameter space, the normal state is
unstable against a superconducting state characterized by a strongly frequency
dependent order-parameter.Comment: 12 pages (14 figures not included, available upon request), Latex,
LPTENS Preprint 93/1
The Concordance Cosmic Star Formation Rate: Implications from and for the Supernova Neutrino and Gamma Ray Backgrounds
We constrain the Cosmic Star Formation Rate (CSFR) by requiring that massive
stars produce the observed UV, optical, and IR light while at the same time not
overproduce the Diffuse Supernova Neutrino Background as bounded by
Super-Kamiokande. With the massive star component so constrained we then show
that a reasonable choice of stellar Initial Mass Function and other parameters
results in SNIa rates and iron yields in good agreement with data. In this way
we define a `concordance' CSFR that predicts the optical SNII rate and the SNIa
contribution to the MeV Cosmic Gamma-Ray Background. The CSFR constrained to
reproduce these and other proxies of intermediate and massive star formation is
more clearly delineated than if it were measured by any one technique and has
the following testable consequences: (1) SNIa contribute only a small fraction
of the MeV Cosmic Gamma-Ray Background, (2) massive star core-collapse is
nearly always accompanied by a successful optical SNII, and (3) the Diffuse
Supernova Neutrino Background is tantalizingly close to detectability.Comment: Improved discussion. Version accepted for publication in JCA
Transport Properties of the One Dimensional Ferromagnetic Kondo Lattice Model : A Qualitative Approach to Oxide Manganites
The transport properties of the ferromagnetic Kondo lattice model in one
dimension are studied via bosonization methods. The antiferromagnetic
fluctuations, which normally appear because of the RKKY interactions, are
explicitly taken into account as a direct exchange between the ``core'' spins.
It is shown that in the paramagnetic regime with the local antiferromagnetic
fluctuations, the resistivity decays exponentially as the temperature increases
while in the ferromagnetic regime the system is an almost perfect conductor. %A
non-perturbative description of localized spin polarons %in the paramagnetic
region is obtained.
The effect of a weak applied field is discussed to be reduced to the case of
the ferromagnetic state leading to band splitting. The qualitative relevance of
the results for the problem of the Oxide Manganites is emphasized.Comment: 4 pages, REVTe
The Charge Ordered State from Weak to Strong Coupling
We apply the Dynamical Mean Field Theory to the problem of charge ordering.
In the normal state as well as in the Charge Ordered (CO) state the existence
of polarons, i.e. electrons strongly coupled to local lattice deformation, is
associated to the qualitative properties of the Lattice Polarization
Distribution Function (LPDF). At intermediate and strong coupling a CO state
characterized by a certain amount of thermally activated defects arise from the
spatial ordering of preexisting randomly distributed polarons. Properties of
this particular CO state gives a qualitative understanding of the low frequency
behavior of optical conductivity of perovskites.Comment: 4 pages, 3 figures, to be published in J. of Superconductivity
(proceedings Stripes 98
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