964 research outputs found
Ferromagnetism in Correlated Electron Systems: Generalization of Nagaoka's Theorem
Nagaoka's theorem on ferromagnetism in the Hubbard model with one electron
less than half filling is generalized to the case where all possible
nearest-neighbor Coulomb interactions (the density-density interaction ,
bond-charge interaction , exchange interaction , and hopping of double
occupancies ) are included. It is shown that for ferromagnetic exchange
coupling () ground states with maximum spin are stable already at finite
Hubbard interaction . For non-bipartite lattices this requires a hopping
amplitude . For vanishing one obtains as in
Nagaoka's theorem. This shows that the exchange interaction is important
for stabilizing ferromagnetism at finite . Only in the special case
the ferromagnetic state is stable even for , provided the lattice allows
the hole to move around loops.Comment: 13 pages, uuencoded postscript, includes 1 table and 2 figure
Manned Mars landing missions using electric propulsion
Manned Mars landing missions using electric propulsion - evaluation of various mission profile
Superconductivity from correlated hopping
We consider a chain described by a next-nearest-neighbor hopping combined
with a nearest-neighbor spin flip. In two dimensions this three-body term
arises from a mapping of the three-band Hubbard model for CuO planes to a
generalized model and for large O-O hopping favors resonance-valence-bond
superconductivity of predominantly -wave symmetry. Solving the ground state
and low-energy excitations by analytical and numerical methods we find that the
chain is a Luther-Emery liquid with correlation exponent , where is the particle density.Comment: 10 pages, RevTeX 3.0 + 2 PostScript figs. Accepted for publication in
Phys.Rev.
Exact ground-state correlation functions of the one-dimensional strongly correlated electron models with the resonating-valence-bond ground state
We investigate the one-dimensional strongly correlated electron models which
have the resonating-valence-bond state as the exact ground state. The
correlation functions are evaluated exactly using the transfer matrix method
for the geometric representations of the valence-bond states. In this method,
we only treat matrices with small dimensions. This enables us to give
analytical results. It is shown that the correlation functions decay
exponentially with distance. The result suggests that there is a finite
excitation gap, and that the ground state is insulating. Since the
corresponding non-interacting systems may be insulating or metallic, we can say
that the gap originates from strong correlation. The persistent currents of the
present models are also investigated and found to be exactly vanishing.Comment: 59 pages, REVTeX 3.0, Figures are available on reques
Resistivity studies under hydrostatic pressure on a low-resistance variant of the quasi-2D organic superconductor kappa-(BEDT-TTF)2Cu[N(CN)2]Br: quest for intrinsic scattering contributions
Resistivity measurements have been performed on a low (LR)- and high
(HR)-resistance variant of the kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br superconductor.
While the HR sample was synthesized following the standard procedure, the LR
crystal is a result of a somewhat modified synthesis route. According to their
residual resistivities and residual resistivity ratios, the LR crystal is of
distinctly superior quality. He-gas pressure was used to study the effect of
hydrostatic pressure on the different transport regimes for both variants. The
main results of these comparative investigations are (i) a significant part of
the inelastic-scattering contribution, which causes the anomalous rho(T)
maximum in standard HR crystals around 90 K, is sample dependent, i.e.
extrinsic in nature, (ii) the abrupt change in rho(T) at T* approx. 40 K from a
strongly temperature-dependent behavior at T > T* to an only weakly T-dependent
rho(T) at T < T* is unaffected by this scattering contribution and thus marks
an independent property, most likely a second-order phase transition, (iii)
both variants reveal a rho(T) proportional to AT^2 dependence at low
temperatures, i.e. for T_c < T < T_0, although with strongly sample-dependent
coefficients A and upper bounds for the T^2 behavior measured by T_0. The
latter result is inconsistent with the T^2 dependence originating from coherent
Fermi-liquid excitations.Comment: 8 pages, 6 figure
Propagation of a hole on a Neel background
We analyze the motion of a single hole on a N\'eel background, neglecting
spin fluctuations. Brinkman and Rice studied this problem on a cubic lattice,
introducing the retraceable-path approximation for the hole Green's function,
exact in a one-dimensional lattice. Metzner et al. showed that the
approximationalso becomes exact in the infinite-dimensional limit. We introduce
a new approach to this problem by resumming the Nagaoka expansion of the
propagator in terms of non-retraceable skeleton-paths dressed by
retraceable-path insertions. This resummation opens the way to an almost
quantitative solution of the problemin all dimensions and, in particular sheds
new light on the question of the position of the band-edges. We studied the
motion of the hole on a double chain and a square lattice, for which deviations
from the retraceable-path approximation are expected to be most pronounced. The
density of states is mostly adequately accounted for by the
retra\-ce\-able-path approximation. Our band-edge determination points towards
an absence of band tails extending to the Nagaoka energy in the spectrums of
the double chain and the square lattice. We also evaluated the spectral density
and the self-energy, exhibiting k-dependence due to finite dimensionality. We
find good agreement with recent numerical results obtained by Sorella et al.
with the Lanczos spectra decoding method. The method we employ enables us to
identify the hole paths which are responsible for the various features present
in the density of states and the spectral density.Comment: 26 pages,Revte
Improved stability regions for ground states of the extended Hubbard model
The ground state phase diagram of the extended Hubbard model containing
nearest and next-to-nearest neighbor interactions is investigated in the
thermodynamic limit using an exact method. It is found that taking into account
local correlations and adding next-to-nearest neighbor interactions both have
significant effects on the position of the phase boundaries. Improved stability
domains for the -pairing state and for the fully saturated ferromagnetic
state at half filling have been constructed. The results show that these states
are the ground states for model Hamiltonians with realistic values of the
interaction parameters.Comment: 21 pages (10 figures are included) Revtex, revised version. To be
published in Phys. Rev. B. E-mail: [email protected]
Bond-charge Interaction in the extended Hubbard chain
We study the effects of bond-charge interaction (or correlated hopping) on
the properties of the extended ({\it i.e.,} with both on-site () and
nearest-neighbor () repulsions) Hubbard model in one dimension at
half-filling. Energy gaps and correlation functions are calculated by Lanczos
diagonalization on finite systems. We find that, irrespective of the sign of
the bond-charge interaction, , the charge--density-wave (CDW) state is more
robust than the spin--density-wave (SDW) state. A small bond-charge interaction
term is enough to make the differences between the CDW and SDW correlation
functions much less dramatic than when . For and fixed (
is the uncorrelated hopping integral), there is an intermediate phase between a
charge ordered phase and a phase corresponding to singly-occupied sites, the
nature of which we clarify: it is characterized by a succession of critical
points, each of which corresponding to a different density of doubly-occupied
sites. We also find an unusual slowly decaying staggered spin-density
correlation function, which is suggestive of some degree of ordering. No
enhancement of pairing correlations was found for any in the range
examined.Comment: 10 pages, 7 PostScript figures, RevTeX 3; to appear in Phys Rev
Superconductivity in the Hubbard model with correlated hopping: Slave-boson study
The slave boson mean-field studies of the ground state of the Hubbard model
with correlated hopping were performed. The approach qualitatively recovers the
exact results for the case of the hopping integral t equal to the correlated
hopping integral X. The phase diagram for the strongly correlated state with
only singly occupied sites, the weakly correlated state, where single and
double occupation is allowed, and for the superconducting state, was determined
for any values of X and any electron concentration n. At the half-filled band
(n=1) a direct transition from the superconductor to the Mott insulator was
found. In the region of strong correlations the superconducting solution is
stable for n close to 1, in contrast to the case of weak correlations, in which
superconductivity occurs at n close to 0 and n close to 2. We found also that
strong correlations change characteristics of the superconducting phase, e.g.
the gap in the excitation spectrum has a nonexponential dependence close to the
point of the phase transition.Comment: 13 pages, 24 Postscript figures (in 12 files
Metallic ferromagnetism: Progress in our understanding of an old strong-coupling problem
Metallic ferromagnetism is in general an intermediate to strong coupling
phenomenon. Since there do not exist systematic analytic methods to investigate
such types of problems, the microscopic origin of metallic ferromagnetism is
still not sufficiently understood. However, during the last two or three years
remarkable progress was made in this field: It is now certain that even in the
one-band Hubbard model metallic ferromagnetism is stable in dimensions
2, and on regular lattices and at intermediate values of the
interaction and density . In this paper the basic questions and recent
insights regarding the microscopic conditions favoring metallic ferromagnetism
in this model are reviewed. These findings are contrasted with the results for
the orbitally degenerate case.Comment: 16 pages, 13 figures, latex using vieweg.sty (enclosed); typos
corrected; to appear in "Advances in Solid State Physics", Vol. 3
- …