119 research outputs found
Resistance noise in Bi_2Sr_2CaCu_2O
The resistance noise in a Bi_2Sr_2CaCu_2O thin film is found to
increase strongly in the underdoped regime. While the increase of the raw
resistance noise with decreasing temperature appears to roughly track the
previously reported pseudogap temperature for this material, standard noise
analysis rather suggests that the additional noise contribution is driven by
the proximity of the superconductor-insulator transition
First evidence for charge ordering in NaVO from Raman spectroscopy
We argue on the basis of symmetry selection rules and Raman scattering
spectra that NaVO undergoes a charge ordering phase transition at
T=34 K. Such a transition is characterized by the redistribution of the
charges at the phase transition and corresponds to the change of the vanadium
ions, from uniform V to two different V and V states. In
the low temperature phase the V ions are forming a "zig-zag" ladder
structure along the {\bf b}-axis, consistent with the symmetry of the P2/b
space group.Comment: to be published in solid state communication
Pairing in cuprates from high energy electronic states
The in-plane optical conductivity of Bi2Sr2CaCu2O8+d thin films with small
carrier density (underdoped) up to large carrier density (overdoped) is
analyzed with unprecedented accuracy. Integrating the conductivity up to
increasingly higher energies points to the energy scale involved when the
superfluid condensate builds up. In the underdoped sample, states extending up
to 2 eV contribute to the superfluid. This anomalously large energy scale may
be assigned to a change of in-plane kinetic energy at the superconducting
transition, and is compatible with an electronic pairing mechanism.Comment: 11 pages, 3 figure
The in-plane electrodynamics of the superconductivity in Bi2Sr2CaCu2O8+d: energy scales and spectral weight distribution
The in-plane infrared and visible (3 meV-3 eV) reflectivity of
Bi2Sr2CaCu2O8+d (Bi-2212) thin films is measured between 300 K and 10 K for
different doping levels with unprecedented accuracy. The optical conductivity
is derived through an accurate fitting procedure. We study the transfer of
spectral weight from finite energy into the superfluid as the system becomes
superconducting. In the over-doped regime, the superfluid develops at the
expense of states lying below 60 meV, a conventional energy of the order of a
few times the superconducting gap. In the underdoped regime, spectral weight is
removed from up to 2 eV, far beyond any conventional scale. The intraband
spectral weight change between the normal and superconducting state, if
analyzed in terms of a change of kinetic energy is ~1 meV. Compared to the
condensation energy, this figure addresses the issue of a kinetic energy driven
mechanism.Comment: 13 pages with 9 figures include
Optical properties of NaxV2O5
The optical properties of sodium-deficient NaxV2O5 (0.85 < x <1) single
crystals are analyzed in the wide energy range, from 0.012 to 4.5 eV, using
ellipsometry, infrared reflectivity, and Raman scattering techniques. The
material remains insulating up to the maximal achieved hole concentration of
about 15%. In sodium deficient samples the optical absorption peak associated
to the fundamental electronic gap develops at about 0.44 eV. It corresponds to
the transition between vanadium dxy and the impurity band, which forms in the
middle of the pure NaV2O5 gap. Raman spectra measured with incident photon
energy larger then 2 eV show strong resonant behavior, due to the presence of
the hole-doping activated optical transitions, peaked at 2.8 eV.Comment: 7 pages, 4 fugures, to be published in PR
The change of Fermi surface topology in Bi2Sr2CaCu2O8 with doping
We report the observation of a change in Fermi surface topology of
Bi2Sr2CaCu2O8 with doping. By collecting high statistics ARPES data from
moderately and highly overdoped samples and dividing the data by the Fermi
function, we answer a long standing question about the Fermi surface shape of
Bi2Sr2CaCu2O8 close to the (pi,0) point. For moderately overdoped samples
(Tc=80K) we find that both the bonding and antibonding sheets of the Fermi
surface are hole-like. However for a doping level corresponding to Tc=55K we
find that the antibonding sheet becomes electron-like. This change does not
directly affect the critical temperature and therefore the superconductivity.
However, since similar observations of the change of the topology of the Fermi
surface were observed in LSCO and Bi2Sr2Cu2O6, it appears to be a generic
feature of hole-doped superconductors. Because of bilayer splitting, though,
this doping value is considerably lower than that for the single layer
materials, which again argues that it is unrelated to Tc
The nanostructural origin of the ac conductance in dielectric granular metals: the case study of Co_20(ZrO_2)_80
We show which is the nanostructure required in granular Co20(ZrO2)80 thin
films to produce an ac response such as the one that is universally observed in
a very wide variety of dielectric materials. A bimodal size distribution of Co
particles yields randomly competing conductance channels which allow both
thermally assisted tunneling through small particles and capacitive conductance
among larger particles that are further apart. A model consisting on a simple
cubic random resistance-capacitor network describes quantitatively the
experimental results as functions of temperature and frequency, and enables the
determination of the microscopic parameters controlling the ac response of the
samples.Comment: Available online at:
http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=APPLAB000091000005052108000001&idtype=cvips&gifs=ye
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