9 research outputs found
Phase fluctuations and Non-Fermi Liquid Properties of 2D Fermi-system with attraction
The effect of static fluctuations in the phase of the order parameter on the
normal and superconducting properties of a 2D system with attractive
four-fermion interaction has been studied. Analytic expressions for the fermion
Green function, its spectral density and the density of states are derived. The
resultant single-particle Green function clearly demonstrates non-Fermi liquid
behavior. The results show that as the temperature increases through the 2D
critical temperature the width of the quasiparticle peaks broadens
significantly. At the same time one retains the gap in quasiparticle spectrum.
The spectral density for the dynamical fluctuations can also be obtained.
Clearly the dynamical fluctuations fill the gap giving the observed pseudogap
behaviour.Comment: 4 pages, LaTeX; invited paper presented at New^3SC-2, Las Vegas, USA,
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Pseudogap phase formation in the crossover from Bose-Einstein condensation to BCS superconductivity in low dimensional systems
A phase diagram for a 2D metal with variable carrier density has been studied
using the modulus-phase representation for the order parameter in a fully
microscopic treatment. This amounts to splitting the degrees of freedom into
neutral fermion and charged boson degrees of freedom. Although true long range
order is forbidden in two dimensions, long range order for the neutral fermions
is possible since this does not violate any continuous symmetry. The phase
fluctuations associated with the charged degrees of freedom destroy long range
order in the full system as expected. The presence of the neutral order
parameter gives rise to new features in the superconducting condensate
formation in low dimensional systems. The resulting phase diagram contains a
new phase which lies above the superconducting (here
Berezinskii-Kosterlitz-Thouless) phase and below the normal (Fermi-liquid)
phase. We identify this phase with the pseudogap phase observed in underdoped
high- superconducting compounds above their critical temperature. We
also find that the phase diagram persists even in the presence of weak
3-dimensionalisation.Comment: 4 pages, LaTeX; invited paper presented at New^3SC-1, Baton Rouge,
USA, 1998. To be published in Int.J.Mod.Phys.
Gauged Nambu-Jona-Lasinio model with extra dimensions
We investigate phase structure of the D (> 4)-dimensional gauged
Nambu-Jona-Lasinio (NJL) model with extra dimensions
compactified on TeV scale, based on the improved ladder Schwinger-Dyson (SD)
equation in the bulk. We assume that the bulk running gauge coupling in the SD
equation for the SU(N_c) gauge theory with N_f massless flavors is given by the
truncated Kaluza-Klein effective theory and hence has a nontrivial ultraviolet
fixed point (UVFP). We find the critical line in the parameter space of two
couplings, the gauge coupling and the four-fermion coupling, which is similar
to that of the gauged NJL model with fixed (walking) gauge coupling in four
dimensions. It is shown that in the presence of such walking gauge interactions
the four-fermion interactions become ``nontrivial'' even in higher dimensions,
similarly to the four-dimensional gauged NJL model. Such a nontriviality holds
only in the restricted region of the critical line (``nontrivial window'') with
the gauge coupling larger than a non-vanishing value (``marginal triviality
(MT)'' point), in contrast to the four-dimensional case where such a
nontriviality holds for all regions of the critical line except for the pure
NJL point. In the nontrivial window the renormalized effective potential yields
a nontrivial interaction which is conformal invariant. The exisitence of the
nontrivial window implies ``cutoff insensitivity'' of the physics prediction in
spite of the ultraviolet dominance of the dynamics. In the formal limit D -> 4,
the nontrivial window coincides with the known condition of the nontriviality
of the four-dimensional gauged NJL model, .Comment: 34 pages, 6 figures, references added, to appear in Phys.Rev.D. The
title is changed in PR
Dynamical chiral symmetry breaking in gauge theories with extra dimensions
We investigate dynamical chiral symmetry breaking in vector-like gauge
theories in dimensions with () compactified extra dimensions, based on
the gap equation (Schwinger-Dyson equation) and the effective potential for the
bulk gauge theories within the improved ladder approximation. The non-local
gauge fixing method is adopted so as to keep the ladder approximation
consistent with the Ward-Takahashi identities.
Using the one-loop gauge coupling of the truncated KK
effective theory which has a nontrivial ultraviolet fixed point (UV-FP)
for the (dimensionless) bulk gauge coupling , we find that there
exists a critical number of flavors, ( for
for SU(3) gauge theory): For , the dynamical
chiral symmetry breaking takes place not only in the ``strong-coupling phase''
() but also in the ``weak-coupling phase'' ()
when the cutoff is large enough. For , on the other hand,
only the strong-coupling phase is a broken phase and we can formally define a
continuum (infinite cutoff) limit, so that the physics is insensitive to the
cutoff in this case.
We also perform a similar analysis using the one-loop ``effective gauge
coupling''. We find the turns out to be a value similar to
that of the case, notwithstanding the enhancement of the
coupling compared with that of the .Comment: REVTEX4, 38 pages, 18 figures. The abstract is shortened; version to
be published in Phys. Rev.