20 research outputs found
Low energy magnetic excitations of the Mn_{12}-acetate spin cluster observed by neutron scattering
We performed high resolution diffraction and inelastic neutron scattering
measurements of Mn_{12}-acetate. Using a very high energy resolution, we could
separate the energy levels corresponding to the splitting of the lowest S
multiplet. Data were analyzed within a single spin model (S=10 ground state),
using a spin Hamiltonian with parameters up to 4^{th} order.
The non regular spacing of the transition energies unambiguously shows the
presence of high order terms in the anisotropy (D= -0.457(2) cm^{-1}, B_4^0 =
-2.33(4) 10^{-5}cm^{-1}).
The relative intensity of the lowest energy peaks is very sensitive to the
small transverse term, supposed to be mainly responsible for quantum tunneling.
This allows an accurate determination of this term in zero magnetic field
(B_4^4 = \pm 3.0(5) 10^{-5} cm^{-1}). The neutron results are discussed in view
of recent experiments and theories.Comment: 4 pages ? 3 figures, submitted to Physical Review Lette
Theory for Dynamical Short Range Order and Fermi Surface Volume in Strongly Correlated Systems
Using the fluctuation exchange approximation of the one band Hubbard model,
we discuss the origin of the changing Fermi surface volume in underdoped
cuprate systems due to the transfer of occupied states from the Fermi surface
to its shadow, resulting from the strong dynamical antiferromagnetic short
range correlations. The momentum and temperature dependence of the quasi
particle scattering rate shows unusual deviations from the conventional Fermi
liquid like behavior. Their consequences for the changing Fermi surface volume
are discussed. Here, we investigate in detail which scattering processes
might be responsible for a violation of the Luttinger theorem. Finally, we
discuss the formation of hole pockets near half filling.Comment: 5 pages, Revtex, 4 postscript figure
The Hubbard model within the equations of motion approach
The Hubbard model has a special role in Condensed Matter Theory as it is
considered as the simplest Hamiltonian model one can write in order to describe
anomalous physical properties of some class of real materials. Unfortunately,
this model is not exactly solved except for some limits and therefore one
should resort to analytical methods, like the Equations of Motion Approach, or
to numerical techniques in order to attain a description of its relevant
features in the whole range of physical parameters (interaction, filling and
temperature). In this manuscript, the Composite Operator Method, which exploits
the above mentioned analytical technique, is presented and systematically
applied in order to get information about the behavior of all relevant
properties of the model (local, thermodynamic, single- and two- particle ones)
in comparison with many other analytical techniques, the above cited known
limits and numerical simulations. Within this approach, the Hubbard model is
shown to be also capable to describe some anomalous behaviors of the cuprate
superconductors.Comment: 232 pages, more than 300 figures, more than 500 reference
Defects in correlated metals and superconductors
In materials with strong local Coulomb interactions, simple defects such as
atomic substitutions strongly affect both macroscopic and local properties of
the system. A nonmagnetic impurity, for instance, is seen to induce magnetism
nearby. Even without disorder, models of such correlated systems are generally
not soluble in 2 or 3 dimensions, and so few exact results are known for the
properties of such impurities. Nevertheless, some simple physical ideas have
emerged from experiments and approximate theories. Here, we first review what
we can learn about this problem from 1D antiferromagnetically correlated
systems. We then discuss experiments on the high Tc cuprate normal state which
probe the effect of impurities on local charge and spin degrees of freedom, and
compare with theories of single impurities in correlated hosts, as well as
phenomenological effective Kondo descriptions. Subsequently, we review theories
of impurities in d-wave superconductors including residual quasiparticle
interactions, and compare with experiments in the superconducting state. We
argue that existing data exhibit a remarkable similarity to impurity-induced
magnetism in the 1D case, implying the importance of electronic correlations
for the understanding of these phenomena, and suggesting that impurities may
provide excellent probes of the still poorly understood ground state of the
cuprates.Comment: 66 pages, 48 figures, review articl
Recent Developments in Chiral Perturbation Theory
I review recent developments in chiral perturbation theory (CHPT) which is
the effective field theory of the standard model below the chiral symmetry
breaking scale. The effective chiral Lagrangian formulated in terms of the
pseudoscalar Goldstone bosons () is briefly discussed. It
is shown how one can gain insight into the ratios of the light quark masses and
to what extent these statements are model--independent. A few selected topics
concerning the dynamics and interactions of the Goldstone bosons are
considered. These are and scattering, some non--leptonic kaon
decays and the problem of strong pionic final state interactions. CHPT also
allows to make precise statements about the temperature dependence of QCD Green
functions and the finite size effects related to the propagation of the
(almost) massless pseudoscalar mesons. A central topic is the inclusion of
matter fields, baryon CHPT. The relativistic and the heavy fermion formulation
of coupling the baryons to the Goldstone fields are discussed. As applications,
photo--nucleon processes, the --term and non--leptonic hyperon
decays are presented. Implications of the spontaneously broken chiral symmetry
on the nuclear forces and meson exchange currents are also described. Finally,
the use of effective field theory methods in the strongly coupled Higgs sector
and in the calculation of oblique electroweak corrections is touched upon.Comment: TeX, 110 pages, 15 figures available upon request, BUTP-93/0