3,764 research outputs found
Anharmonicity and self-energy effects of the E2g phonon in MgB2
We present a Raman scattering study of the E2g phonon anharmonicity and of
superconductivity induced self-energy effects in MgB2 single crystals. We show
that anharmonic two phonon decay is mainly responsible for the unusually large
linewidth of the E2g mode. We observe ~ 2.5 % hardening of the E2g phonon
frequency upon cooling into the superconducting state and estimate the
electron-phonon coupling strength associated with this renormalization.Comment: 4 pages, 3 figures, accepted to PR
Geometry of fully coordinated, two-dimensional percolation
We study the geometry of the critical clusters in fully coordinated
percolation on the square lattice. By Monte Carlo simulations (static
exponents) and normal mode analysis (dynamic exponents), we find that this
problem is in the same universality class with ordinary percolation statically
but not so dynamically. We show that there are large differences in the number
and distribution of the interior sites between the two problems which may
account for the different dynamic nature.Comment: ReVTeX, 5 pages, 6 figure
Brainstem Cholinergic Modulation of Muscle Tone in Infant Rats
In week-old rats, lesions of the dorsolateral pontine tegmentum (DLPT) and nucleus pontis oralis (PnO) have opposing effects on nuchal muscle tone. Specifically, pups with DLPT lesions exhibit prolonged bouts of nuchal muscle atonia (indicative of sleep) and pups with PnO lesions exhibit prolonged bouts of high nuchal muscle tone (indicative of wakefulness). Here we test the hypothesis that nuchal muscle tone is modulated, at least in part, by cholinergically mediated interactions between these two regions. First, in unanesthetized pups, we found that chemical infusion of the cholinergic agonist carbachol (22 mM, 0.1 µL) within the DLPT produced high muscle tone. Next, chemical lesions of the nucleus pontis oralis (PnO) were used to produce a chronic state of high nuchal muscle tone, at which time the cholinergic antagonist scopolamine (10 mM, 0.1 µL) was infused into the DLPT. Scopolamine effectively decreased nuchal muscle tone, thus suggesting that lesions of the PnO increase muscle tone via cholinergic activation of the DLPT. Using 2-deoxyglucose (2-DG) autoradiography, metabolic activation throughout the DLPT was observed after PnO lesions. Finally, consistent with the hypothesis that PnO inactivation produces high muscle tone, infusion of the sodium channel blocker, lidocaine (2%), into the PnO of unanesthetized pups produced rapid increases in muscle tone. We conclude that, even early in infancy, the DLPT is critically involved in the regulation of muscle tone and behavioral state and that its activity is modulated by a cholinergic mechanism that is directly or indirectly controlled by the PnO
The Development of Sleep-Wake Rhythms and the Search for Elemental Circuits in the Infant Brain
Despite the predominance of sleep in early infancy, developmental science has yet to play a major role in shaping concepts and theories about sleep and its associated ultradian and circadian rhythms. Here we argue that developmental analyses help us to elucidate the relative contributions of the brainstem and forebrain to sleep-wake control and to dissect the neural components of sleep-wake rhythms. Developmental analysis also makes it clear that sleep-wake processes in infants are the foundation for those of adults. For example, the infant brainstem alone contains a fundamental sleep-wake circuit that is sufficient to produce transitions among wakefulness, quiet sleep, and active sleep. In addition, consistent with the requirements of a flip-flop model of sleep-wake processes, this brainstem circuit supports rapid transitions between states. Later in development, strengthening bidirectional interactions between the brainstem and forebrain contribute to the consolidation of sleep and wake bouts, the elaboration of sleep homeostatic processes, and the emergence of diurnal or nocturnal circadian rhythms. The developmental perspective promoted here critically constrains theories of sleep-wake control and provides a needed framework for the creation of fully realized computational models. Finally, with a better understanding of how this system is constructed developmentally, we will gain insight into the processes that govern its disintegration due to aging and disease
Constant effective mass across the phase diagram of high-T cuprates
We investigate the hole dynamics in two prototypical high temperature
superconducting systems: LaSrCuO and YBaCuO using a combination of DC transport and infrared spectroscopy. By
exploring the effective spectral weight obtained with optics in conjunction
with DC Hall results we find that the transition to the Mott insulating state
in these systems is of the "vanishing carrier number" type since we observe no
substantial enhancement of the mass as one proceeds to undoped phases. Further,
the effective mass remains constant across the entire underdoped regime of the
phase diagram. We discuss the implications of these results for the
understanding of both transport phenomena and pairing mechanism in high-T
systems.Comment: 5 pages, 2 figure
Raman scattering from the CaC6 superconductor in the presence of disorder
Polarized Raman scattering has been performed on CaC6 superconductor. We
identify two of the three Raman active Eg phonon modes at 440 and 1508 cm-1
expected for the R-3m space group of CaC6. These first order scattering modes
appear along with the D and G bands around 1300 cm-1 and 1600 cm-1 that are
similar in origin to the corresponding bands in plain graphite. The intensities
of the D and G bands in CaC6 correlate with degree of disorder. The D band
arises from the double resonant Raman scattering process; its frequency shifts
as a function of excitation energy with approximately 35 cm-1/eV. The double
resonant Raman scattering probes phonon excitations with finite wave vector q.
We estimate the characteristic spacing of structural defects to be on the scale
of about 100 Angstrom by comparing the intensity of the D band and the 1508
cm-1 Eg mode in CaC6 to calibrated intensity ratio of analogous bands in
disordered graphites. A sharp superconducting coherence peak at 24 cm-1 is
observed below Tc.Comment: 10 pages, 5 figure
Observation of Leggett's collective mode in a multi-band MgB2 superconductor
We report observation of Leggett's collective mode in a multi-band MgB2
superconductor with T_c=39K arising from the fluctuations in the relative phase
between two superconducting condensates. The novel mode is observed by Raman
spectroscopy at 9.4 meV in the fully symmetric scattering channel. The observed
mode frequency is consistent with theoretical considerations based on the first
principle computations.Comment: Accepted for PR
Observation of a 500meV Collective Mode in LaSrCuO and NdCuO
Utilizing resonant inelastic x-ray scattering, we report a previously
unobserved mode in the excitation spectrum of LaSrCuO at 500
meV. The mode is peaked around the (,0) point in reciprocal space and is
observed to soften, and broaden, away from this point. Samples with x=0, 0.01,
0.05, and 0.17 were studied. The new mode is found to be rapidly suppressed
with increasing Sr content and is absent at =0.17, where it is replaced by a
continuum of excitations. The peak is only observed when the incident x-ray
polarization is normal to the CuO planes and is also present in NdCuO.
We suggest possible explanations for this excitation.Comment: 5 pages, 5 figure
Nonmonotonic d_{x^2-y^2} Superconducting Order Parameter in Nd_{2-x}Ce_xCuO_4
Low energy polarized electronic Raman scattering of the electron doped
superconductor Nd_1.85Ce_0.15CuO_4 (T_c=22 K) has revealed a nonmonotonic
d_{x^2-y^2} superconducting order parameter. It has a maximum gap of 4.4 k_BT_c
at Fermi surface intersections with antiferromagnetic Brillouin zone (the ``hot
spots'') and a smaller gap of 3.3 k_BT_c at fermionic Brillouin zone
boundaries. The gap enhancement in the vicinity of the ``hot spots'' emphasizes
role of antiferromagnetic fluctuations and similarity in the origin of
superconductivity for electron- and hole-doped cuprates.Comment: 4 pages, 4 figure
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