20,574 research outputs found
Chiral corrections to baryon properties with composite pions
A calculational scheme is developed to evaluate chiral corrections to
properties of composite baryons with composite pions. The composite baryons and
pions are bound states derived from a microscopic chiral quark model. The model
is amenable to standard many-body techniques such as the BCS and RPA
formalisms. An effective chiral model involving only hadronic degrees of
freedom is derived from the macroscopic quark model by projection onto hadron
states. Chiral loops are calculated using the effective hadronic Hamiltonian. A
simple microscopic confining interaction is used to illustrate the derivation
of the pion-nucleon form factor and the calculation of pionic self-energy
corrections to the nucleon and Delta(1232) masses.Comment: 29 pages, Revtex, 4 ps figure
Magnetocaloric effect in integrable spin-s chains
We study the magnetocaloric effect for the integrable antiferromagnetic
high-spin chain. We present an exact computation of the Gr\"uneisen parameter,
which is closely related to the magnetocaloric effect, for the quantum spin-s
chain on the thermodynamical limit by means of Bethe ansatz techniques and the
quantum transfer matrix approach. We have also calculated the entropy S and the
isentropes in the (H,T) plane. We have been able to identify the quantum
critical points H_c^{(s)}=2/(s+1/2) looking at the isentropes and/or the
characteristic behaviour of the Gr\"uneisen parameter.Comment: 6 pages, 3 figure
Transport properties of a two impurity system: a theoretical approach
A system of two interacting cobalt atoms, at varying distances, was studied
in a recent scanning tunneling microscope experiment by Bork et. al.[Nature
Phys. 7, 901 (2011)]. We propose a microscopic model that explains, for all
experimentally analyzed interatomic distances, the physics observed in these
experiments. Our proposal is based on the two-impurity Anderson model, with the
inclusion of a two-path geometry for charge transport. This many-body system is
treated in the finite-U slave boson mean-field approximation and the
logarithmic-discretization embedded-cluster approximation. We physically
characterize the different charge transport regimes of this system at various
interatomic distances and show that, as in the experiments, the features
observed in the transport properties depend on the presence of two impurities
but also on the existence of two conducting channels for electron transport. We
interpret the splitting observed in the conductance as the result of the
hybridization of the two Kondo resonances associated with each impurity.Comment: 5 pages, 5 figure
Doped carrier formulation and mean-field theory of the tt't''J model
In the generalized-tJ model the effect of the large local Coulomb repulsion
is accounted for by restricting the Hilbert space to states with at most one
electron per site. In this case the electronic system can be viewed in terms of
holes hopping in a lattice of correlated spins, where holes are the carriers
doped into the half-filled Mott insulator. To explicitly capture the interplay
between the hole dynamics and local spin correlations we derive a new
formulation of the generalized-tJ model where doped carrier operators are used
instead of the original electron operators. This ``doped carrier'' formulation
provides a new starting point to address doped spin systems and we use it to
develop a new, fully fermionic, mean-field description of doped Mott insulators
This mean-field approach reveals a new mechanism for superconductivity, namely
spinon-dopon mixing, and we apply it to the tt't''J model as of interest to
high-temperature superconductors. In particular, we use model parameters
borrowed from band calculations and from fitting ARPES data to obtain a
mean-field phase diagram that reproduces semi-quantitatively that of hole and
electron doped cuprates. The mean-field approach hereby presented accounts for
the local antiferromagnetic and d-wave superconducting correlations which, we
show, provide a rational for the role of t' and t'' in strengthening
superconductivity as expected by experiments and other theoretical approaches.
As we discuss how t, t' and t'' affect the phase diagram, we also comment on
possible scenarios to understand the differences between as-grown and oxygen
reduced electron doped samples.Comment: 17 pages, 2 figures. Homepage http://dao.mit.edu/~wen
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