1,296 research outputs found
Kondo lattice model: Unitary transformations, spin dynamics, strongly correlated charged modes, and vacuum instability
Using unitary transformations, we express the Kondo lattice Hamiltonian in
terms of fermionic operators that annihilate the ground state of the
interacting system and that represent the best possible approximations to the
actual charged excitations. In this way, we obtain an effective Hamiltonian
which, for small couplings, consists in a kinetic term for conduction electrons
and holes, an RKKY-like term, and a renormalized Kondo interaction. The
physical picture of the system implied by this formalism is that of a vacuum
state consisting in a background of RKKY-induced spin correlations, where two
kinds of elementary modes can be excited: Soft neutral modes associated with
deformations of the spin liquid, which lead to very large low-temperature
values of the heat capacity and magnetic susceptibility, and charged modes
corresponding to the excitation of electrons and holes in the system. Using the
translational and spin rotational symmetries, we construct a simple ansatz to
determine the charged excitations neglecting the effects of the spin
correlations. Apart from the `normal', uncorrelated states, we find strongly
correlated charged modes involving soft electrons (or holes) and spin
fluctuations, which strongly renormalize the low-energy charged spectrum, and
whose energy becomes negative beyond a critical coupling, signaling a vacuum
instability and a transition to a new phase.Comment: 35 pages, revtex 3.
The Effective Potential of the N=0* Yang-Mills Theory
We study the \N=4 SYM theory with SU(N) gauge group in the large N limit,
deformed by giving equal mass to the four adjoint fermions. With this
modification, a potential is dynamically generated for the six scalars in the
theory, \phi^i. We show that the resulting theory is stable (perturbatively in
the 't Hooft coupling), and that there are some indications that =0 is
the vacuum of the theory. Using the AdS/CFT correspondence, we compare the
results to the corresponding supergravity computation, i.e. brane probing a
deformed AdS_5 x S^5 background, and we find qualitative agreement.Comment: 12 pages, 2 figures, version to appear in JHE
Testing Lorentz invariance violations in the tritium beta-decay anomaly
We consider a Lorentz non-invariant dispersion relation for the neutrino,
which would produce unexpected effects with neutrinos of few eV, exactly where
the tritium beta-decay anomaly is found. We use this anomaly to put bounds on
the violation of Lorentz invariance. We discuss other consequences of this
non-invariant dispersion relation in neutrino experiments and high-energy
cosmic-ray physics.Comment: 11 pages, 2 figures, elsart style. Some references added. Final
version to appear in Physics Letters
Chern-Simons like term generation in an extended model of QED under external conditions
The possibility of a Chern-Simons like term generation in an extended model
of QED, in which a Lorentz and CPT non-covariant interaction term for fermions
is present, has been investigated at finite temperature and in the presence of
a background color magnetic field. To this end, the photon polarization
operator in an external constant axial-vector field has been considered.
One-loop contributions to its antisymmetric component due to fermions in the
linear order of the axial-vector field have been obtained. Moreover, the first
nontrivial correction to the induced CS term due to the presence of a weak
constant homogeneous color magnetic field has been derived.Comment: RevTex, 10 pages with 3 figure
Fermions on the Electroweak String
We construct a simple class of exact solutions of the electroweak theory
including the naked --string and fermion fields. It consists in the
--string configuration (), the {\it time} and components
of the neutral gauge bosons () and a fermion condensate
(lepton or quark) zero mode. The --string is not altered (no feed back from
the rest of fields on the --string) while fermion condensates are zero modes
of the Dirac equation in the presence of the --string background (no feed
back from the {\it time} and components of the neutral gauge bosons on the
fermion fields). For the case of the --vortex --string the number of zero
modes found for charged leptons and quarks is (according to previous results by
Jackiw and Rossi) equal to , while for (massless) neutrinos is .
The presence of fermion fields in its core make the obtained configuration a
superconducting string, but their presence (as well as that of
) does not enhance the stability of the --string.Comment: 12 text pages (Latex) and 8 postscript figures in a uuencoded fil
Failure of Gauge Invariance in the Nonperturbative Formulation of Massless Lorentz-Violating QED
We consider a Lorentz-violating modification to the fermionic Lagrangian of
QED that is known to produce a finite Chern-Simons term at leading order. We
compute the second order correction to the one-loop photon self-energy in the
massless case using an exact propagator and a nonperturbative formulation of
the theory. This nonperturbative theory assigns a definite value to the
coefficient of the induced Chern-Simons term; however, we find that the theory
fails to preserve gauge invariance at higher orders. We conclude that the
specific nonperturbative value of the Chern-Simons coefficient has no special
significance.Comment: 8 pages, very minor change
Renormalized Wick expansion for a modified PQCD
The renormalization scheme for the Wick expansion of a modified version of
the perturbative QCD introduced in previous works is discussed. Massless QCD is
considered, by implementing the usual multiplicative scaling of the gluon and
quark wave functions and vertices. However, also massive quark and gluon
counter-terms are allowed in this mass less theory since the condensates are
expected to generate masses. A natural set of expansion parameters of the
physical quantities is introduced: the coupling itself and to masses and
associated to quarks and gluons respectively. This procedure allows to
implement a dimensional transmutation effect through these new mass scales. A
general expression for the new generating functional in terms of the mass
parameters and is obtained in terms of integrals over arbitrary but
constant gluon or quark fields in each case. Further, the one loop potential,
is evaluated in more detail in the case when only the quark condensate is
retained. This lowest order result again indicates the dynamical generation of
quark condensates in the vacuum.Comment: 13 pages, one figur
Bosonization of the Low Energy Excitations of Fermi Liquids
We bosonize the low energy excitations of Fermi Liquids in any number of
dimensions in the limit of long wavelengths. The bosons are coherent
superposition of electron-hole pairs and are related with the displacement of
the Fermi Surface in some arbitrary direction. A coherent-state path integral
for the bosonized theory is derived and it is shown to represent histories of
the shape of the Fermi Surface. The Landau equation for the sound waves is
shown to be exact in the semiclassical approximation for the bosons.Comment: 10 pages, RevteX, P-93-03-027 (UIUC
Long-lived oscillons from asymmetric bubbles
The possibility that extremely long-lived, time-dependent, and localized
field configurations (``oscillons'') arise during the collapse of asymmetrical
bubbles in 2+1 dimensional phi^4 models is investigated. It is found that
oscillons can develop from a large spectrum of elliptically deformed bubbles.
Moreover, we provide numerical evidence that such oscillons are: a) circularly
symmetric; and b) linearly stable against small arbitrary radial and angular
perturbations. The latter is based on a dynamical approach designed to
investigate the stability of nonintegrable time-dependent configurations that
is capable of probing slowly-growing instabilities not seen through the usual
``spectral'' method.Comment: RevTeX 4, 9 pages, 11 figures. Revised version with a new approach to
stability. Accepted to Phys. Rev.
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