387 research outputs found
Angle-resolved photoemission study of untwinned PrBaCuO: undoped CuO plane and doped CuO chain
We have performed an angle-resolved photoemission study on untwinned
PrBaCuO, which has low resistivity but does not show
superconductivity. We have observed a dispersive feature with a band maximum
around (/2,/2), indicating that this band is derived from the undoped
CuO plane. We have observed another dispersive band exhibiting
one-dimensional character, which we attribute to signals from the doped CuO
chain. The overall band dispersion of the one-dimensional band agrees with the
prediction of model calculation with parameters relevant to cuprates
except that the intensity near the Fermi level is considerably suppressed in
the experiment.Comment: 6 pages, 10 figure
Quasi-particle Lifetimes in a d_{x^2-y^2} Superconductor
We consider the lifetime of quasi-particles in a d-wave superconductor due to
scattering from antiferromagnetic spin-fluctuations, and explicitly separate
the contribution from Umklapp processes which determines the electrical
conductivity. Results for the temperature dependence of the total scattering
rate and the Umklapp scattering rate are compared with relaxation rates
obtained from thermal and microwave conductivity measurements, respectively.Comment: 14 pages, 4 figure
The relationship between posttreatment smile esthetics and the ABO Objective Grading System
ABSTRACT Objective: To evaluate the correlations between the components of the Objective Grading System developed by the American Board of Orthodontics (ABO) and smile esthetics. Materials and Methods: The clinical photographs of 48 orthodontically treated patients were rated by a panel of 25 experienced orthodontists (15 men and 10 women) and 20 parents of orthodontic patients (eight men and 12 women). One of the investigators, a director of the ABO, scored the posttreatment dental casts and panoramic radiographs of all patients according to the guidelines of the Objective Grading System. The relationship between the occlusal outcome and perceived smile attractiveness of the subjects were evaluated by a Pearson product-moment correlation. Logistic regression was used to determine whether the individual component or total combined scores of the ABO Grading System could predict whether a smile would be considered ''attractive'' or ''unattractive'' by the panel of raters. Results: Extremely weak relationships were found among all factors of the ABO Objective Grading System and perceived smile attractiveness (r values ranging from Ϫ0.11 to 0.14; P Ͼ .05). As derived from logistic regression equations, neither total scores nor individual components of the ABO Grading system could predict attractive or unattractive smiles. Conclusions: This study suggests that additional criteria might be incorporated into the assessment of overall orthodontic treatment outcomes, including variables evaluating the smile
Interplane magnetic coupling effects in the multilattice compound Y_2Ba_4Cu_7O_{15}
We investigate the interplane magnetic coupling of the multilattice compound
Y_2Ba_4Cu_7O_{15} by means of a bilayer Hubbard model with inequivalent planes.
We evaluate the spin response, effective interaction and the intra- and
interplane spin-spin relaxation times within the fluctuation exchange
approximation. We show that strong in-plane antiferromagnetic fluctuations are
responsible for a magnetic coupling between the planes, which in turns leads to
a tendency of the fluctuation in the two planes to equalize.
This equalization effect grows whit increasing in-plane antiferromagnetic
fluctuations, i. e., with decreasing temperature and decreasing doping, while
it is completely absent when the in-layer correlation length becomes of the
order of one lattice spacing. Our results provide a good qualitative
description of NMR and NQR experiments in Y_2Ba_4Cu_7O_{15}.Comment: Final version, to appear. in Phys. Rev. B (Rapid Communications),
sched. Jan. 9
Magnetic Coherence as a Universal Feature of Cuprate Superconductors
Recent inelastic neutron scattering (INS) experiments on
LaSrCuO have established the existence of a {\it magnetic
coherence effect}, i.e., strong frequency and momentum dependent changes of the
spin susceptibility, , in the superconducting phase. We show, using the
spin-fermion model for incommensurate antiferromagnetic spin fluctuations, that
the magnetic coherence effect establishes the ability of INS experiments to
probe the electronic spectrum of the cuprates, in that the effect arises from
the interplay of an incommensurate magnetic response, the form of the
underlying Fermi surface, and the opening of the d-wave gap in the fermionic
spectrum. In particular, we find that the magnetic coherence effect observed in
INS experiments on LaSrCuO requires that the Fermi surface be
closed around up to optimal doping. We present several predictions
for the form of the magnetic coherence effect in YBaCuO in
which an incommensurate magnetic response has been observed in the
superconducting state.Comment: 9 pages, 12 figures; extended version of Phys. Rev B, R6483 (2000
Correlative In Situ Multichannel Imaging for Large-Area Monitoring of Morphology Formation in Solution-Processed Perovskite Layers
To scale up production of perovskite photovoltaics, state-of-the-art laboratory recipes and processes must be transferred to large-area coating and drying systems. The development of in situ monitoring methods that provide real-time feedback for process control is pivotal to overcome this challenge. Herein, correlative in situ multichannel imaging (IMI) obtaining reflectance, photoluminescence intensity, and central photoluminescence emission wavelength images on areas larger than 100 cm2 with subsecond temporal resolution using a simple, cost-effective setup is demonstrated. Installed on top of a drying channel with controllable laminar air flow and substrate temperature, IMI is shown to consistently monitor solution film drying, perovskite nucleation, and perovskite crystallization. If the processing parameters differ, IMI reveals characteristic changes in large-area perovskite formation dynamics already before the final annealing step. Moreover, when IMI is used to study >130 blade-coated devices processed at the same parameters, about 90% of low-performing devices contain coating inhomogeneities detected by IMI. The results demonstrate that IMI should be of value for real-time 2D monitoring and feedback control in industrial-scale, high-throughput fabrication such as roll-to-roll printing
Direct Observation and Anisotropy of the Contribution of Gap nodes in the Low Temperature Specific Heat of YBa_2Cu_3O_7
The specific heat due to line nodes in the superconducting gap of YBa2Cu3O7
has been obscured up to now by magnetic terms of extrinsic origin, even for
high quality crystals. We report the specific heat of a new single crystal
grown in a non-corrosive BaZrO3 crucible, for which paramagnetic terms are
reduced to less than one spin-1/2 center for 20'000 Cu atoms. The contribution
of line nodes shows up directly in the difference C(B,T) - C(0,T) at fixed
temperatures (T < 5 K) as a function of the magnetic field parallel to the
c-axis (B<=14 T). These data illustrate the smooth crossover from C propotional
to T^2 at low fields to C propotional to TB^1/2 at high fields, and provide new
values for gap parameters which are quantitatively consistent with tunneling
spectroscopy and thermal conductivity in the framework of dx^2-y^2 pairing
symmetry. Data for B along the nodal and antinodal directions in the ab-plane
are also provided. The in-plane anisotropy predicted in the clean limit is not
observed.Comment: 29 pages(using Revtex style), 14 postscript figures, submitted to
Phys. Rev. B Content of the file changed after replacin
Quasiparticle-quasiparticle Scattering in High Tc Superconductors
The quasiparticle lifetime and the related transport relaxation times are the
fundamental quantities which must be known in order to obtain a description of
the transport properties of the high T_c superconductors. Studies of these
quantities have been undertaken previously for the d-wave, high T_c
superconductors for the case of temperature-independent elastic impurity
scattering. However, much less is known about the temperature-dependent
inelastic scattering. Here we give a detailed description of the
characteristics of the temperature-dependent quasiparticle-quasiparticle
scattering in d-wave superconductors, and find that this process gives a
natural explanation of the rapid variation with temperature of the electrical
transport relaxation rate.Comment: 4 page
Observation of out-of-phase bilayer plasmons in YBa_2Cu_3O_7-delta
The temperature dependence of the c-axis optical conductivity \sigma(\omega)
of optimally and overdoped YBa_2Cu_3O_x (x=6.93 and 7) is reported in the far-
(FIR) and mid-infrared (MIR) range. Below T_c we observe a transfer of spectral
weight from the FIR not only to the condensate at \omega = 0, but also to a new
peak in the MIR. This peak is naturally explained as a transverse out-of-phase
bilayer plasmon by a model for \sigma(\omega) which takes the layered crystal
structure into account. With decreasing doping the plasmon shifts to lower
frequencies and can be identified with the surprising and so far not understood
FIR feature reported in underdoped bilayer cuprates.Comment: 7 pages, 3 eps figures, Revtex, epsfi
Pairing Fluctuation Theory of Superconducting Properties in Underdoped to Overdoped Cuprates
We propose a theoretical description of the superconducting state of under-
to overdoped cuprates, based on the short coherence length of these materials
and the associated strong pairing fluctuations. The calculated and the
zero temperature excitation gap , as a function of hole
concentration , are in semi-quantitative agreement with experiment. Although
the ratio has a strong dependence, different from the
universal BCS value, and deviates significantly from the BCS
prediction, we obtain, quite remarkably, quasi-universal behavior, for the
normalized superfluid density and the Josephson critical
current , as a function of . While experiments on
are consistent with these results, future measurements on
are needed to test this prediction.Comment: 4 pages, 3 figures, REVTeX, submitted to Phys. Rev. Let
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