4,624 research outputs found
Compressibility of the Two-Dimensional infinite-U Hubbard Model
We study the interactions between the coherent quasiparticles and the
incoherent Mott-Hubbard excitations and their effects on the low energy
properties in the Hubbard model. Within the framework of a
systematic large-N expansion, these effects first occur in the next to leading
order in 1/N. We calculate the scattering phase shift and the free energy, and
determine the quasiparticle weight Z, mass renormalization, and the
compressibility. It is found that the compressibility is strongly renormalized
and diverges at a critical doping . We discuss the nature
of this zero-temperature phase transition and its connection to phase
separation and superconductivity.Comment: 4 pages, 3 eps figures, final version to appear in Phys. Rev. Let
Exact Solution of a Electron System Combining Two Different t-J Models
A new strongly correlated electron model is presented. This is formed by two
types of sites: one where double occupancy is forbidden, as in the t-J model,
and the other where double occupancy is allowed but vacancy is not allowed, as
an inverse t-J model. The Hamiltonian shows nearest and next-to-nearest
neighbour interactions and it is solved by means of a modified algebraic nested
Bethe Ansatz. The number of sites where vacancy is not allowed, may be treated
as a new parameter if the model is looked at as a t-J model with impurities.
The ground and excited states are described in the thermodynamic limit.Comment: Some corrections and references added. To be published in J. Phys.
Report of the Terrestrial Bodies Science Working Group. Volume 4: The moon
A rationale for furture exploration of the moon is given. Topics discussed include the objectives of the lunar polar orbiter mission, the mission profile, and general characteristics of the spacraft to be used
Linked Cluster Expansion Around Mean-Field Theories of Interacting Electrons
A general expansion scheme based on the concept of linked cluster expansion
from the theory of classical spin systems is constructed for models of
interacting electrons. It is shown that with a suitable variational formulation
of mean-field theories at weak (Hartree-Fock) and strong (Hubbard-III) coupling
the expansion represents a universal and comprehensive tool for systematic
improvements of static mean-field theories. As an example of the general
formalism we investigate in detail an analytically tractable series of ring
diagrams that correctly capture dynamical fluctuations at weak coupling. We
introduce renormalizations of the diagrammatic expansion at various levels and
show how the resultant theories are related to other approximations of similar
origin. We demonstrate that only fully self-consistent approximations produce
global and thermodynamically consistent extensions of static mean field
theories. A fully self-consistent theory for the ring diagrams is reached by
summing the so-called noncrossing diagrams.Comment: 17 pages, REVTEX, 13 uuencoded postscript figures in 2 separate file
All Coronal Loops are the Same: Evidence to the Contrary
The 1998 April 20 spectral line data from the Coronal Diagnostics
Spectrometer (CDS) on the {\it Solar and Heliospheric Observatory} (\SOHO)
shows a coronal loop on the solar limb. Our original analysis of these data
showed that the plasma was multi-thermal, both along the length of the loop and
along the line of sight. However, more recent results by other authors indicate
that background subtraction might change these conclusions, so we consider the
effect of background subtraction on our analysis. We show Emission Measure (EM)
Loci plots of three representative pixels: loop apex, upper leg, and lower leg.
Comparisons of the original and background-subtracted intensities show that the
EM Loci are more tightly clustered after background subtraction, but that the
plasma is still not well represented by an isothermal model. Our results taken
together with those of other authors indicate that a variety of temperature
structures may be present within loops.Comment: Accepted for publication in ApJ Letter
Effects of Umklapp Scattering on Electronic States in One Dimension
The effects of Umklapp scattering on electronic states are studied in one
spatial dimension at absolute zero. The model is basically the Hubbard model,
where parameters characterizing the normal () and Umklapp () scattering
are treated independently. The density of states is calculated in the t-matrix
approximation by taking only the forward and Umklapp scattering into account.
It is found that the Umklapp scattering causes the global splitting of the
density of states. In the presence of sufficiently strong Umklapp scattering, a
pole in the t-matrix appears in the upper half plane, signalling an instability
towards the 'pairing' ordered state ( is the reciprocal lattice
vector), whose consequences are studied in the mean field approximation. It
turns out that this ordered state coexists with spin-density-wave state and
also brings about Cooper-pairs. A phase diagram is determined in the plane of
and electron filling .Comment: 22 pages, LaTeX, 17 figures included, uses jpsj.st
On the derivation of the t-J model: electron spectrum and exchange interactions in narrow energy bands
A derivation of the t-J model of a highly-correlated solid is given starting
from the general many-electron Hamiltonian with account of the
non-orthogonality of atomic wave functions. Asymmetry of the Hubbard subbands
(i.e. of ``electron'' and ``hole''cases) for a nearly half-filled bare band is
demonstrated. The non-orthogonality corrections are shown to lead to occurrence
of indirect antiferromagnetic exchange interaction even in the limit of the
infinite on-site Coulomb repulsion. Consequences of this treatment for the
magnetism formation in narrow energy bands are discussed. Peculiarities of the
case of ``frustrated'' lattices, which contain triangles of nearest neighbors,
are considered.Comment: 4 pages, RevTe
Strong-Coupling Expansion for the Hubbard Model
A strong-coupling expansion for models of correlated electrons in any
dimension is presented. The method is applied to the Hubbard model in
dimensions and compared with numerical results in . Third order expansion
of the Green function suffices to exhibit both the Mott metal-insulator
transition and a low-temperature regime where antiferromagnetic correlations
are strong. It is predicted that some of the weak photoemission signals
observed in one-dimensional systems such as should become stronger as
temperature increases away from the spin-charge separated state.Comment: 4 pages, RevTex, 3 epsf figures include
Spectral functions for strongly correlated 5f-electrons
We calculate the spectral functions of model systems describing 5f-compounds
adopting Cluster Perturbation Theory. The method allows for an accurate
treatment of the short-range correlations. The calculated excitation spectra
exhibit coherent 5f bands coexisting with features associated with local
intra-atomic transitions. The findings provide a microscopic basis for partial
localization. Results are presented for linear chains.Comment: 10 Page
Assisted hopping and interaction effects in impurity models
We study, using Numerical Renormalization Group methods, the generalization
of the Anderson impurity model where the hopping depends on the filling of the
impurity. We show that the model, for sufficiently large values of the assisted
hopping term, shows a regime where local pairing correlations are enhanced.
These correlations involve pairs fluctuating between on site and nearest
neighbor positions
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