71 research outputs found
Far-infrared and submillimeter-wave conductivity in electron-doped cuprate La_{2-x}Ce_xCuO_4
We performed far-infrared and submillimeter-wave conductivity experiments in
the electron-doped cuprate La_{2-x}Ce_xCuO_4 with x = 0.081 (underdoped regime,
T_c = 25 K). The onset of the absorption in the superconducting state is
gradual in frequency and is inconsistent with the isotropic s-wave gap.
Instead, a narrow quasiparticle peak is observed at zero frequency and a second
peak at finite frequencies, clear fingerprints of the conductivity in a d-wave
superconductor. A far-infrared conductivity peak can be attributed to 4Delta_0,
or to 2Delta_0 + Delta_spin, where Delta_spin is the resonance frequency of the
spin-fluctuations. The infrared conductivity as well as the suppression of the
quasiparticle scattering rate below T_c are qualitatively similar to the
results in the hole-doped cuprates.Comment: 5 pages, 4 figures include
In-plane polarized collective modes in detwinned YBaCuO observed by spectral ellipsometry
The in-plane dielectric response of detwinned YBaCuO has
been studied by far-infared ellipsometry. A surprisingly lare number of
in-plane polarized modes are observed. Some of them correspond to pure phonon
modes. Others posses a large electronic contribution which strongly increases
in the superconducting state. The free carrier response and the collective
modes exhibit a pronounced a-b anisotropy. We discuss our results in terms of a
CDW state in the 1-d CuO chains and induced charge density fluctuations within
the 2-d CuO planes
Infrared optical properties of the spin-1/2 quantum magnet
We report results on the electrodynamic response of , a
low-dimensional spin-1/2 quantum magnet that shows a spin gap formation for
T= 67 . The Fano-like shape of a few selected infrared active
phonons suggests an interaction between lattice vibrations and a continuum of
low frequency (spin) excitations. The temperature dependence of the phonon mode
parameters extends over a broad temperature range well above ,
indicating the presence of an extended fluctuation regime. In the temperature
interval between 200 and there is a progressive dimensionality
crossover (from two to one), as well as a spectral weight shift from low
towards high frequencies. This allows us to identify a characteristic energy
scale of about 430 , ascribed to a pseudo spin-gap
On the c-axis optical reflectivity of layered cuprate superconductors
Using a conventional BCS -- Fermi liquid model we calculate the c-axis
optical reflectivity of the layered high temperature cuprate superconductors by
obtaining the finite temperature dynamical dielectric function in a microscopic
self-consistent gauge invariant formalism. We get good semi-quantitative
agreement with all the existing experimental data by using the measured normal
state resistivities as the input parameters in obtaining the c-axis
hopping amplitude and the normal state level broadening in our microscopic
calculation.Comment: 10 pages, 6 figures, 1 table gzipped tar fil
Condensation Energy and High Tc Superconductivity
From an analysis of the specific heat of one of the cuprate superconductors
it is shown, that even if a large part of the experimental specific heat
associated with the superconducting phase transition is due to fluctuations,
this part must be counted when one tries to extract the condensation energy
from the data. Previous work by Chakravarty, Kee and Abrahams, where the
fluctuation part was subtracted, has resulted in an incorrect estimation of the
condensation energy.Comment: 4 pages, 5 encapsulated Postscript figures, uses ReVTeX.st
The c axis optical conductivity of layered systems in the superconducting state
In this paper, we discuss the c axis optical conductivity Re [sigma_c(omega)]
in the high T_c superconductors, in the superconducting state. The basic
premise of this work is that electrons travelling along the c axis between
adjacent CuO_2 layers must pass through several intervening layers. In earlier
work we found that, for weak inter-layer coupling, it is preferable for
electrons to travel along the c axis by making a series of interband
transitions rather than to stay within a single (and very narrow) band.
Moreover, we found that many of the properties of the normal state optical
conductivity, including the pseudogap could be explained by interband
transitions. In this work we examine the effect of superconductivity on the
interband conductivity. We find that, while the onset of superconductivity is
clearly evident in the spectrum, there is no clear signature of the symmetry of
the superconducting order parameter.Comment: 6 pages, 4 figure
Experimental Test of the Inter-Layer Pairing Models for High-Tc Superconductivity Using Grazing Incidence Infrared Reflectometry
From measurements of the far-infrared reflectivity at grazing angles of
incidence with p-polarized light we determined the c-axis Josephson plasma
frequencies of the single layer high T_c cuprates Tl_2Ba_2CuO_6 and
La_{2-x}Sr_xCuO_4. We detected a strong plasma resonance at 50 cm^{-1} for
La_{2-x}Sr_xCuO_4 in excellent agreement with previously published results. For
Tl_2Ba_2CuO_6 we were able to determine an upper limit of the unscreened c-axis
Josephson plasma frequency 100 cm^{-1} or a c-axis penetration depth > 15 \mu
m. The small value of stands in contrast to recent a prediction
based on the inter-layer tunneling mechanism of superconductivity.Comment: 4 pages, Phys. Rev. B, in press, Revtex, 4 postscript figure
In-plane optical response of Bi2Sr2CuO6
We report on infrared reflectivity measurements of the -plane response of
superconducting BiSrCuO single crystals. The frequency dependent
conductivity has a maximum near 700 cm at room temperature, which shifts
to lower frequency and merges with a Drude-peak below 100 K. We attribute the
unusual behaviour of the mid-infrared conductivity to low frequency transitions
between electronic bands of mainly BiO character near the point.
The linear temperature dependence of the low-frequency resistivity can be
followed down to approximately 40 K where it saturates.Comment: Revtex, 4 pages, 4 postscript figures, Phys. Rev. B, in pres
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