1,714 research outputs found
Singularities and Pseudogaps in the Density of States of Peierls Chains
We develop a non-perturbative method to calculate the density of states (DOS)
of the fluctuating gap model describing the low-energy physics of electrons on
a disordered Peierls chain. For real order parameter field we calculate the DOS
at the Fermi energy exactly as a functional of the disorder for a chain of
finite length L. Averaging rho (0) with respect to a Gaussian probability
distribution of the Peierls order parameter, we show that in the thermodynamic
limit the average DOS at the Fermi energy diverges for any finite value of the
correlation length above the Peierls transition. Pseudogap behavior emerges
only if the Peierls order parameter is finite and sufficiently large.Comment: 4 pages, 2 figures; one more reference added; final version to appear
in Phys. Rev. Lett. (Feb. 1999
Exactly solvable toy model for the pseudogap state
We present an exactly solvable toy model which describes the emergence of a
pseudogap in an electronic system due to a fluctuating off-diagonal order
parameter. In one dimension our model reduces to the fluctuating gap model
(FGM) with a gap Delta (x) that is constrained to be of the form Delta (x) = A
e^{i Q x}, where A and Q are random variables. The FGM was introduced by Lee,
Rice and Anderson [Phys. Rev. Lett. {\bf{31}}, 462 (1973)] to study fluctuation
effects in Peierls chains. We show that their perturbative results for the
average density of states are exact for our toy model if we assume a Lorentzian
probability distribution for Q and ignore amplitude fluctuations. More
generally, choosing the probability distributions of A and Q such that the
average of Delta (x) vanishes and its covariance is < Delta (x) Delta^{*}
(x^{prime}) > = Delta_s^2 exp[ {- | x - x^{\prime} | / \xi}], we study the
combined effect of phase and amplitude fluctutations on the low-energy
properties of Peierls chains. We explicitly calculate the average density of
states, the localization length, the average single-particle Green's function,
and the real part of the average conductivity. In our model phase fluctuations
generate delocalized states at the Fermi energy, which give rise to a finite
Drude peak in the conductivity. We also find that the interplay between phase
and amplitude fluctuations leads to a weak logarithmic singulatity in the
single-particle spectral function at the bare quasi-particle energies. In
higher dimensions our model might be relevant to describe the pseudogap state
in the underdoped cuprate superconductors.Comment: 19 pages, 8 figures, submitted to European Physical Journal
Ward identities for the Anderson impurity model: derivation via functional methods and the exact renormalization group
Using functional methods and the exact renormalization group we derive Ward
identities for the Anderson impurity model. In particular, we present a
non-perturbative proof of the Yamada-Yosida identities relating certain
coefficients in the low-energy expansion of the self-energy to thermodynamic
particle number and spin susceptibilities of the impurity. Our proof underlines
the relation of the Yamada-Yosida identities to the U(1) x U(1) symmetry
associated with particle number and spin conservation in a magnetic field.Comment: 8 pages, corrected statements about infintite flatband limi
Influence of the quantum zero-point motion of a vortex on the electronic spectra of s-wave superconductors
We compute the influence of the quantum zero-point motion of a vortex on the
electronic quasiparticle spectra of s-wave superconductors. The vortex is
assumed to be pinned by a harmonic potential, and its coupling to the
quasiparticles is computed in the framework of BCS theory. Near the core of the
vortex, the motion leads to a shift of spectral weight away from the chemical
potential, and thereby reduces the zero bias conductance peak; additional
structure at the frequency of the harmonic trap is also observed.Comment: 14 pages, 7 figures; (v2) added refs; (v3) removed discussion on
d-wave superconductors and moved it to cond-mat/060600
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