146 research outputs found
Optical evidence for the proximity to a spin-density-wave metallic state in NaCoO
We present the optical properties of \na single crystals, measured over a
broad spectral range as a function of temperature (). The capability to
cover the energy range from the far-infrared up to the ultraviolet allows us to
perform reliable Kramers-Kronig transformation, in order to obtain the
absorption spectrum (i.e., the complex optical conductivity). To the complex
optical conductivity we apply the generalized Drude model, extracting the
frequency dependence of the scattering rate () and effective mass
() of the itinerant charge carriers. We find that at low temperatures and for . This suggests that \na is at
the verge of a spin-density-wave metallic phase
Non-Fermi liquid behavior with and without quantum criticality in Ce(1-x)Yb(x)CoIn(5)
One of the greatest challenges to Landau's Fermi liquid theory - the standard
theory of metals - is presented by complex materials with strong electronic
correlations. In these materials, non-Fermi liquid transport and thermodynamic
properties are often explained by the presence of a continuous quantum phase
transition which happens at a quantum critical point (QCP). A QCP can be
revealed by applying pressure, magnetic field, or changing the chemical
composition. In the heavy-fermion compound CeCoIn, the QCP is assumed to
play a decisive role in defining the microscopic structure of both normal and
superconducting states. However, the question of whether QCP must be present in
the material's phase diagram to induce non-Fermi liquid behavior and trigger
superconductivity remains open. Here we show that the full suppression of the
field-induced QCP in CeCoIn by doping with Yb has surprisingly little
impact on both unconventional superconductivity and non-Fermi liquid behavior.
This implies that the non-Fermi liquid metallic behavior could be a new state
of matter in its own right rather then a consequence of the underlying quantum
phase transition.Comment: 7 pages, 5 figure
Effect of a lattice upon an interacting system of electrons: Breakdown of scaling and decay of persistent currents
For an interacting system of N electrons, we study the conditions under which
a lattice model of size L with nearest neighbor hopping t and U/r Coulomb
repulsion has the same ground state as a continuum model. For a fixed value of
N, one gets identical results when the inter-electron spacing to the Bohr
radius ratio r_s < r_s^*. Above r_s^*, the persistent current created by an
enclosed flux begins to decay and r_s ceases to be the scaling parameter. Three
criteria giving similar r_s^* are proposed and checked using square lattices.Comment: 7 pages, 5 postscript figure
The optical conductivity of the quasi one-dimensional conductors: the role of forward scattering by impurities
We calculate the average conductivity sigma (omega) of interacting electrons
in one dimension in the presence of a long-range random potential (forward
scattering disorder). Taking the curvature of the energy dispersion into
account, we show that weak disorder leads to a transport scattering rate that
vanishes as omega^2 for small frequency omega. This implies that the real part
of the conductivity remains finite for omega -> 0, while the imaginary part
diverges. These effects are lost within the usual bosonization approach, which
relies on the linearization of the energy dispersion. We discuss our result in
the light of a recent experiment.Comment: 5 RevTex pages; more careful comparison with experiments and
discussion of interchain hopping added; some references added; to appear in
Phys. Rev.
Interplay of spin density wave and superconductivity with different pairing symmetry
A model study for the coexistence of the spin density wave and
superconductivity is presented. With reference to the recent angle resolved
photo emmission experimental data in high T_c cuprates, presence of the nested
pieces of bands is assumed. The single band Hubbard model, therefore, when
treated within the Hatree-Fock mean field theory leads to a spin density wave
(SDW) ground state. The superconductivity (SC) is assumed to be due to a
generalised attractive potential with a separable form without specifying to
any particular origin. It therefore allows a comparative study of the
coexistence of superconductivity of different order parameter symmetry with the
spin density wave state. We find that the phase diagram, comprising of the
amplitudes of the respective gaps (SC and SDW) Vs. band filling resembles to
that of the high T_c cuprates only when the order parameter of the
superconducting phase has d-wave symmetry. Thermal variation of different order
parameters (e.g, SC and SDW) also show interesting coexistence and reentrance
behaviors that are consistent with experimental observations, specially for the
borocarbides.Comment: 8 pages, 6 figures (postscript attached), Physica C (in press
Zn-doping effect on the magnetotransport properties of Bi_{2}Sr_{2-x}La_{x}CuO_{6+\delta} single crystals
We report the magnetotransport properties of
Bi_{2}Sr_{2-x}La_{x}Cu_{1-z}Zn_{z}O_{6+\delta} (Zn-doped BSLCO) single crystals
with z of up to 2.2%. Besides the typical Zn-doping effects on the in-plane
resistivity and the Hall angle, we demonstrate that the nature of the
low-temperature normal state in the Zn-doped samples is significantly altered
from that in the pristine samples under high magnetic fields. In particular, we
observe nearly-isotropic negative magnetoresistance as well as an increase in
the Hall coefficient at very low temperatures in non-superconducting Zn-doped
samples, which we propose to be caused by the Kondo scattering from the local
moments induced by Zn impurities.Comment: 4 pages, 4 figures, final version (one reference added), published in
Phys. Rev.
Evidence for strong electron-phonon coupling and polarons in the optical response of La_{2-x}Sr_xCuO_4
The normal state optical response of La_{2-x}Sr_xCuO_4 is found to be
consistent with a simple multi-component model, based on free carriers with
strong electron-phonon interaction, localized polaronic states near 0.15 eV and
a mid-infrared band at 0.5 eV. Normal state reflectance and absorbance of
La_{1.83}Sr_{0.17}CuO_4 are investigated and their temperature dependence is
explained. Both, the ac and dc response are recovered and the quasi-linear
behavior of the optical scattering rate up to 3000- 4000 cm^{-1} is found to be
consistent with strong electron-phonon interaction, which also accounts for the
value of T_c. Although not strictly applicable in the superconducting state,
our simple model accounts for the observed penetration depth and the optical
response below T_c can be recovered by introducing a small amount of additional
carriers. Our findings suggest that the optical response of La_{2-x}Sr_xCuO_4
could be explained both, in the normal and superconducting state, by a simple
multi-fluid model with strong electron-phonon interaction if the gap symmetry
and the temperature dependence of the 0.5 eV mid-infrared band are adequately
taken into account.Comment: 22 pages, REVTeX, 12 figures in ps-fil
In-plane optical features of the underdoped La_2CuO_4 based compounds: theoretical multiband analysis
The three-component ab-plane optical conductivity of the high-T_c cuprates is
derived using the gauge invariant response theory, and compared to the data
previously obtained from the optical reflectivity measurements in the La_2CuO_4
based families. The valence electrons are described by the Emery three-band
model with the antiferromagnetic correlations represented by an effective
single-particle potential. In the 0 < delta < 0.3 doping range, it is shown
that the total spectral weight of the three-band model is shared between the
intra- and interband channels nearly in equal proportions. At optimum doping,
the low-frequency conductivity has a (non-Drude) nearly single-component form,
which transforms with decreasing doping into a two-component structure. The
mid-infrared spectral weight is found to be extremely sensitive to the symmetry
of the effective single-particle potential, as well as to the doping level. The
gauge invariant form of the static and elastic Raman vertices is determined,
allowing explicit verification of the effective mass theorem and the related
conductivity sum rules.Comment: 12 pages, 5 figures; Physica C 391 (2003) 25
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