538 research outputs found

### Non-rigid Shell Model and Correlational Mechanism of the Local Pairing

The Hartree-Fock states of the many-electron atomic system can be unstable
with respect to a static or dynamic shift of the electron shells. An
appropriate non-rigid shell model for atomic clusters is developed. It permits
to formulate a convenient approach to the semiempirical description of the
different correlation effects and to reveal some new effects.Comment: 3 pages, 2 ".gif" figures, LaTeX; will be published in Physica C in
Materials of Int.Conf. HTSC-V. (1997, Beijing, China

### Effective R-parity violation from supersymmetry breaking

We present a scenario in which Yukawa-like R-parity violating (RPV) couplings
are naturally suppressed. In our framework, RPV is assumed to originate from
the SUSY breaking mechanism and then transmitted into the SUSY Lagrangian only
through soft SUSY breaking operators in the scalar potential. The RPV
Yukawa-like operators of the superpotential, conventionally parametrized by the
couplings \lambda, \lambda' and \lambda'', are then generated through loops
containing the SUSY scalars, the gauginos and the soft RPV interactions and
are, therefore, manifest as effective operators with a typical strength of
order 10^{-3}.Comment: 4 pages, 2 figures, Revtex4. Main changes with respect to Version 1
are: improved discussions on RGE effects, discussion added on neutrino
masses, a toy model added for the proposed scenario. Conclusions remain
unchanged. As will appear in Phys. Rev.

### Supersymmetry and localization

We study conditions under which an odd symmetry of the integrand leads to
localization of the corresponding integral over a (super)manifold. We also show
that in many cases these conditions guarantee exactness of the stationary phase
approximation of such integrals.Comment: 16 pages, LATE

### Optical sum rule violation, superfluid weight and condensation energy in the cuprates

The model of hole superconductivity predicts that the superfluid weight in
the zero-frequency $\delta$-function in the optical conductivity has an
anomalous contribution from high frequencies, due to lowering of the system's
kinetic energy upon entering the superconducting state. The lowering of kinetic
energy, mainly in-plane in origin, accounts for both the condensation energy of
the superconductor as well as an increased potential energy due to larger
Coulomb repulsion in the paired state. It leads to an apparent violation of the
conductivity sum rule, which in the clean limit we predict to be substantially
larger for in-plane than for c-axis conductivity. However, because cuprates are
in the dirty limit for c-axis transport, the sum rule violation is found to be
greatly enhanced in the c-direction. The model predicts the sum rule violation
to be largest in the underdoped regime and to decrease with doping, more
rapidly in the c-direction that in the plane. So far, experiments have detected
sum rule violation in c-axis transport in several cuprates, as well as a
decrease and disappearance of this violation for increasing doping, but no
violation in-plane. We explore the predictions of the model for a wide range of
parameters, both in the absence and in the presence of disorder, and the
relation with current experimental knowledge.Comment: submitted to Phys.Rev.

### Temperatures of Fragment Kinetic Energy Spectra

Multifragmentation reactions without large compression in the initial state
(proton-induced reactions, reverse-kinematics, projectile fragmentation) are
examined, and it is verified quantitatively that the high temperatures obtained
from fragment kinetic energy spectra and lower temperatures obtained from
observables such as level population or isotope ratios can be understood in a
common framework.Comment: LaTeX, 7 pages, 2 figures available from autho

### Neutrino Oscillations and Collider Test of the R-parity Violating Minimal Supergravity Model

We study the R-parity violating minimal supergravity models accounting for
the observed neutrino masses and mixing, which can be tested in future collider
experiments. The bi-large mixing can be explained by allowing five dominant
tri-linear couplings $\lambda'_{1,2,3}$ and $\lambda_{1,2}$. The desired ratio
of the atmospheric and solar neutrino mass-squared differences can be obtained
in a very limited parameter space where the tree-level contribution is tuned to
be suppressed. In this allowed region, we quantify the correlation between the
three neutrino mixing angles and the tri-linear R-parity violating couplings.
Qualitatively, the relations $| \lambda'_1 | < | \lambda'_2| \sim |
\lambda'_3|$, and $|\lambda_1| \sim |\lambda_2|$ are required by the large
atmospheric neutrino mixing angle $\theta_{23}$ and the small angle
$\theta_{13}$, and the large solar neutrino mixing angle $\theta_{12}$,
respectively. Such a prediction on the couplings can be tested in the next
linear colliders by observing the branching ratios of the lightest
supersymmetric particle (LSP). For the stau or the neutralino LSP, the ratio
$|\lambda_1|^2: |\lambda_2|^2: |\lambda_1|^2 + |\lambda_2|^2$ can be measured
by establishing $Br(e\nu): Br(\mu\nu) : Br(\tau\nu)$ or $Br(\nu e^\pm \tau^\mp
): Br(\nu\mu^\pm\tau^\mp) : Br(\nu\tau^\pm\tau^\mp)$, respectively. The
information on the couplings $\lambda'_i$ can be drawn by measuring $Br(l_i t
\bar{b}) \propto |\lambda'_i|^2$ if the neutralino LSP is heavier than the top
quark.Comment: RevTex, 25 pages, 8 eps figure

### 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
$4k_F$ CDW for positive $U_{eff} = U-2g^2/\omega$ and one $2k_F$ CDW for
negative $U_{eff}$) 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

### Rigorous results on superconducting ground states for attractive extended Hubbard models

We show that the exact ground state for a class of extended Hubbard models
including bond-charge, exchange, and pair-hopping terms, is the Yang
"eta-paired" state for any non-vanishing value of the pair-hopping amplitude,
at least when the on-site Coulomb interaction is attractive enough and the
remaining physical parameters satisfy a single constraint. The ground state is
thus rigorously superconducting. Our result holds on a bipartite lattice in any
dimension, at any band filling, and for arbitrary electron hopping.Comment: 12 page

### Optical Sum Rule anomalies in the High-Tc Cuprates

We provide a brief summary of the observed sum rule anomalies in the
high-T$_c$ cuprate materials. A recent issue has been the impact of a
non-infinite frequency cutoff in the experiment. In the normal state, the
observed anomalously high temperature dependence can be explained as a `cutoff
effect'. The anomalous rise in the optical spectral weight below the
superconducting transition, however, remains as a solid experimental
observation, even with the use of a cutoff frequency.Comment: 4 pages, 2 figures, very brief review of optical sum rule anomal

### Small Fermi energy and phonon anharmonicity in MgB_2 and related compounds

The remarkable anharmonicity of the E_{2g} phonon in MgB_2 has been suggested
in literature to play a primary role in its superconducting pairing. We
investigate, by means of LDA calculations, the microscopic origin of such an
anharmonicity in MgB_2, AlB_2, and in hole-doped graphite. We find that the
anharmonic character of the E_{2g} phonon is essentially driven by the small
Fermi energy of the sigma holes. We present a simple analytic model which
allows us to understand in microscopic terms the role of the small Fermi energy
and of the electronic structure. The relation between anharmonicity and
nonadiabaticity is pointed out and discussed in relation to various materials.Comment: 5 pages, 2 figures replaced with final version, accepted on Physical
Review

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