404 research outputs found
Metallic ferromagnetism without exchange splitting
In the band theory of ferromagnetism there is a relative shift in the
position of majority and minority spin bands due to the self-consistent field
due to opposite spin electrons. In the simplest realization, the Stoner model,
the majority and minority spin bands are rigidly shifted with respect to each
other. Here we consider models at the opposite extreme, where there is no
overall shift of the energy bands. Instead, upon spin polarization one of the
bands broadens relative to the other. Ferromagnetism is driven by the resulting
gain in kinetic energy. A signature of this class of mechanisms is that a
transfer of spectral weight in optical absorption from high to low frequencies
occurs upon spin polarization. We show that such models arise from generalized
tight binding models that include off-diagonal matrix elements of the Coulomb
interaction. For certain parameter ranges it is also found that reentrant
ferromagnetism occurs. We examine properties of these models at zero and finite
temperatures, and discuss their possible relevance to real materials
Effect of nearest neighbor repulsion on the low frequency phase diagram of a quarter-filled Hubbard-Holstein chain
We have studied the influence of nearest-neighbor (NN) repulsion on the low
frequency phase diagram of a quarter-filled Hubbard-Holstein chain. The NN
repulsion term induces the apparition of two new long range ordered phases (one
CDW for positive and one CDW for
negative ) that did not exist in the V=0 phase diagram. These results
are put into perspective with the newly observed charge ordered phases in
organic conductors and an interpretation of their origin in terms of
electron-molecular vibration coupling is suggested.Comment: 10 pages, 10 figure
Gapless Spin-Fluid Ground State in a Random Quantum Heisenberg Magnet
We examine the spin- quantum Heisenberg magnet with Gaussian-random,
infinite-range exchange interactions. The quantum-disordered phase is accessed
by generalizing to symmetry and studying the large limit. For large
the ground state is a spin-glass, while quantum fluctuations produce a
spin-fluid state for small . The spin-fluid phase is found to be generically
gapless - the average, zero temperature, local dynamic spin-susceptibility
obeys \bar{\chi} (\omega ) \sim \log(1/|\omega|) + i (\pi/2) \mbox{sgn}
(\omega) at low frequencies. This form is identical to the phenomenological
`marginal' spectrum proposed by Varma {\em et. al.\/} for the doped cuprates.Comment: 13 pages, REVTEX, 2 figures available by request from
[email protected]
Thermodynamics of Random Ferromagnetic Antiferromagnetic Spin-1/2 Chains
Using the quantum Monte Carlo Loop algorithm, we calculate the temperature
dependence of the uniform susceptibility, the specific heat, the correlation
length, the generalized staggered susceptibility and magnetization of a
spin-1/2 chain with random antiferromagnetic and ferromagnetic couplings, down
to very low temperatures. Our data show a consistent scaling behavior in all
the quantities and support strongly the conjecture drawn from the approximate
real-space renormalization group treatment.A statistical analysis scheme is
developed which will be useful for the search of scaling behavior in numerical
and experimental data of random spin chains.Comment: 13 pages, 13 figures, RevTe
Doping effect on the anomalous behavior of the Hall effect in electron-doped superconductor NdCeCuO
Transport properties of NdCeCuO single crystal films
are investigated in magnetic fields up to 9T at =(0.4-4.2)K. An analysis
of normal state (at ) Hall coefficient dependence on Ce
doping takes us to a conclusion about the existence both of electron-like and
hole-like contributions to transport in nominally electron-doped system. In
accordance with (x) analysis an anomalous sign reversal of Hall effect
in mixed state at may be ascribed to a flux-flow regime for two
types of carriers with opposite charges.Comment: 14 pages, 5 figure
Robust computations with dynamical systems
In this paper we discuss the computational power of Lipschitz
dynamical systems which are robust to in nitesimal perturbations.
Whereas the study in [1] was done only for not-so-natural systems from
a classical mathematical point of view (discontinuous di erential equation
systems, discontinuous piecewise a ne maps, or perturbed Turing
machines), we prove that the results presented there can be generalized
to Lipschitz and computable dynamical systems.
In other words, we prove that the perturbed reachability problem (i.e. the
reachability problem for systems which are subjected to in nitesimal perturbations)
is co-recursively enumerable for this kind of systems. Using
this result we show that if robustness to in nitesimal perturbations is
also required, the reachability problem becomes decidable. This result
can be interpreted in the following manner: undecidability of veri cation
doesn't hold for Lipschitz, computable and robust systems.
We also show that the perturbed reachability problem is co-r.e. complete
even for C1-systems
RVB Contribution to Superconductivity in
We view as electronically equivalent to (non-staggered) graphite
( layer) that has undergone a zero gap semiconductor to a superconductor
phase transition by a large c-axis (chemical) pressure due to layers.
Further, like the \ppi bonded planar organic molecules, graphite is an old
resonating valence bond (RVB) system. The RVB's are the `preexisting cooper
pairs' in the `parental' zero gap semiconducting (graphite) sheets that
manifests themselves as a superconducting ground state of the transformed
metal. Some consequences are pointed out.Comment: 4 pages, 2 figure, RevTex. Based on a talk given at the Institute
Seminar Week, IMSc, Madras (12-16, Feb. 2001
One particle spectral weight of the three dimensional single band Hubbard model
Dynamic properties of the three-dimensional single-band Hubbard model are
studied using Quantum Monte Carlo combined with the maximum entropy technique.
At half-filling, there is a clear gap in the density of states and well-defined
quasiparticle peaks at the top (bottom) of the lower (upper) Hubbard band. We
find an antiferromagnetically induced weight above the naive Fermi momentum.
Upon hole doping, the chemical potential moves to the top of the lower band
where a robust peak is observed. Results are compared with spin-density-wave
(SDW) mean-field and self consistent Born approximation results, and also with
the infinite dimensional Hubbard model, and experimental photoemission (PES)
for three dimensional transition-metal oxides.Comment: 11 pages, REVTeX, 16 figures included using psfig.sty. Ref.30
correcte
Random Exchange Quantum Heisenberg Chains
The one-dimensional quantum Heisenberg model with random bonds is
studied for and . The specific heat and the zero-field
susceptibility are calculated by using high-temperature series expansions and
quantum transfer matrix method. The susceptibility shows a Curie-like
temperature dependence at low temperatures as well as at high temperatures. The
numerical results for the specific heat suggest that there are anomalously many
low-lying excitations. The qualitative nature of these excitations is discussed
based on the exact diagonalization of finite size systems.Comment: 13 pages, RevTex, 12 figures available on request ([email protected]
Antiferromagnetic and van Hove Scenarios for the Cuprates: Taking the Best of Both Worlds
A theory for the high temperature superconductors is proposed. Holes are
spin-1/2, charge e, quasiparticles strongly dressed by spin fluctuations. Based
on their dispersion, it is claimed that the experimentally observed van Hove
singularities of the cuprates are likely originated by antiferromagnetic (AF)
correlations. From the two carriers problem in the 2D t-J model, an effective
Hamiltonian for holes is defined with %no free parameters. This effective model
has superconductivity in the channel, a critical
temperature at the optimal hole density, ,
and a quasiparticle lifetime linearly dependent with energy. Other experimental
results are also reproduced by the theory.Comment: 12 pages, 4 figures (on request), RevTeX (version 3.0), preprint
NHMF
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