563 research outputs found
Electron-Phonon mechanism for Superconductivity in NaCoO: Valence-Band Suhl-Kondo effect Driven by Shear Phonons
To study the possible mechanism of superconductivity in NaCoO,
we examine the interaction between all the relevant optical phonons (breathing
and shear phonons) and -electrons of Co-ions, and study
the transition temperature for a s-wave superconductivity. The obtained is very low when the -valence-bands are far below the Fermi level.
However, is strongly enhanced when the top of the
-valence-bands is close to the Fermi level (say -50meV), thanks to
interband hopping of Cooper pairs caused by shear phonons. This ``valence-band
Suhl-Kondo mechanism'' due to shear phonons is significant to understand the
superconductivity in NaCoO. By the same mechanism, the kink
structure of the band-dispersion observed by ARPES, which indicates the strong
mass-enhancement () due to optical phonons, is also explained.Comment: 5 pages, 4 figures; v2:Added references, published in J. Phys. Soc.
Jp
Atrial repolarization as observable in the PQ interval
Objective: To study the involvement of atrial repolarization in body surface potentials. Methods: Electrocardiograms of healthy subjects were recorded using a 64-lead system. The data analysis focused on the PQ intervals, while devoting special attention to the low-amplitude signals during the PQ segment: the segment from the end of the P wave till onset QRS. The data were analyzed by inspecting body surface potential maps and the XYZ signals of the vector cardiogram. Results: Standard P wave features exhibited normal values. The local potential extremes were found at positions not sampled by the standard leads. The PQ segment was found to be not iso-electric, the potential distribution being very similar to that at the timing of apex P but for a reversed polarity and a 3-fold lower magnitude. Conclusion: The results demonstrate a significant involvement of atrial repolarization during the PQ interval, with essentially non-concomitant atrial T waves
Evaluation of Ablation Patterns Using a Biophysical Model of Atrial Fibrillation
Atrial fibrillation (AF) is the most common form of cardiac arrhythmia. Surgical/Radiofrequency (RF) ablation is a therapeutic procedure that consists of creating lines of conduction block to interrupt AF. The present study evaluated 13 different ablation patterns by means of a biophysical model of the human atria. In this model, ablation lines were abruptly applied transmurally during simulated sustained AF, and success rate, time to AF termination and average beat-to-beat interval were documented. The gold standard Cox's Maze III procedure was taken as reference. The effectiveness of twelve less invasive patterns was compared to it. In some of these incomplete lines (entailing a gap) were simulated. Finally, the computer simulations were compared to clinical data. The results show that the model reproduces observations made in vivo: (1) the Maze III is the most efficient ablation procedure; (2) less invasive patterns should include lines in both right and left atrium; (3) incomplete ablation lines between the pulmonary veins and the mitral valve annulus lead to uncommon flutter; (4) computer simulations of incomplete lines are consistent with clinical results of non-transumural RF ablation. Biophysical modeling may therefore be considered as a useful tool for understanding the mechanisms underlying AF therapie
Two-Dimensional Nature of Four-Layer Superconductors by Inequivalent Hole Distribution
The magnetization of the four-layer superconductor
CuBa_{2}Ca_{3}Cu_4O_{12-\delta} with T_c\simeq117 K is presented. The
high-field magnetization around T_c(H) follows the exact two-dimensional
scaling function given by Te\v{s}anovi\'{c} and Andreev. This feature is
contrary to the inference that the interlayer coupling becomes strong if the
number of CuO_2 planes in a unit cell increases. Also, the fluctuation-induced
susceptibility in the low-field region was analyzed by using the modified
Lawrence-Doniach model. The effective number of independently fluctuating CuO_2
layers per unit cell, g_{\rm eff}, turned out to be \simeq 2 rather than 4,
which indicated that two among the four CuO_2 layers were in states far from
their optimal doping levels. This result could explain why
CuBa_{2}Ca_{3}Cu_4O_{12-\delta} shows two-dimensional behavior.Comment: 5 pages and 4 figure
Precise Control of Band Filling in NaxCoO2
Electronic properties of the sodium cobaltate NaxCoO2 are systematically
studied through a precise control of band filling. Resistivity, magnetic
susceptibility and specific heat measurements are carried out on a series of
high-quality polycrystalline samples prepared at 200 C with Na content in a
wide range of 0.35 =< x =< 0.70. It is found that dramatic changes in
electronic properties take place at a critical Na concentration x* that lies
between 0.58 and 0.59, which separates a Pauli paramagnetic and a Curie-Weiss
metals. It is suggested that at x* the Fermi level touches the bottom of the
a1g band at the gamma point, leading to a crucial change in the density of
states across x* and the emergence of a small electron pocket around the gamma
point for x > x*.Comment: 4 pages, 5 figures, submitted to J. Phys. Soc. Jp
Electronic States and Superconductivity in Multi-layer High-Tc Cuprates
We study electronic states of multilayer cuprates in the normal phases as
functions of the number of CuO_2 planes and the doping rate. The resonating
valence bond wave function and the Gutzwiller approximation are used for a
two-dimensional multilayer t-t'-t''-J model. We calculate the electron-removal
spectral functions at (\pi,0) in the CuO_2 plane next to the surface to
understand the angle-resolved photoemission spectroscopy (ARPES) spectra. We
find that the trilayer spectrum is narrower than the bilayer spectrum but is
wider than the monolayer spectrum. In the tri- and tetralayer systems, the
outer CuO_2 plane has different superconducting amplitude from the inner CuO_2
plane, while each layer in the bilayer systems has same amplitude. The recent
ARPES and NMR experiments are discussed in the light of the present theory.Comment: 7 pages, 7 figure
Companion stars of Type Ia supernovae and single low-mass white dwarfs
Recent investigations of the WD + MS channel of Type Ia supernovae (SNe Ia)
imply that this channel may be the main contribution to the old population
(>1Gyr) of SNe Ia. In the WD + MS channel, the WD could accrete material from a
main-sequence or a slightly evolved star until it reaches the Chandrasekhar
mass limit. The companions in this channel would survive after SN explosion and
show distinguishing properties. In this Letter, based on SN Ia production
regions of the WD + MS channel and three formation channels of WD + MS systems,
we performed a detailed binary population synthesis study to obtain the
properties of the surviving companions. The properties can be verified by
future observations. We find that the surviving companions of the old SNe Ia
have a low mass, which provides a possible way to explain the formation of the
population of single low-mass WDs (<0.45Msun).Comment: 5 pages, 5 figures, accepted for publication in MNRAS Letter
Role of spin-orbit coupling on the spin triplet pairing in Na_{x}CoO_{2}yH_{2}O I: d-vector under zero magnetic field
The d-vector in possibile spin triplet superconductor Na_{x}CoO_{2}yH_{2}O is
microscopically investigated on the basis of the multi-orbital Hubbard model
including the atomic spin-orbit coupling. As a result of the perturbation
theory, we obtain the stable spin triplet superconductivity where the p-wave
and f-wave states can be stabilized. If we neglect the spin-orbit coupling,
superconducting state has 6-fold (3-fold) degeneracy in the p-wave (f-wave)
state. This degeneracy is lifted by the spin-orbit coupling. We determine the
d-vector within the linearlized Dyson-Gorkov equation. It is shown that the
d-vector is always along the plane when the pairing symmetry is p-wave, while
it depends on the parameters in case of the f-wave state. The lifting of
degeneracy is significant in the p-wave state while it is very small in the
f-wave state. This is because the first order term with respect to the
spin-orbit coupling is effective in the former case, while it is ineffective in
the latter case. The consistency of these results with NMR and \muSR
measurements are discussed.Comment: To appear in J. Phys. Soc. Jpn. 74 (2005) No.
Ferromagnetic and triplet-pairing instabilities controlled by the trigonal distortion of the CoO_6 octahedra in Na_xCoO_2*yH_2O
In the recently discovered Co-oxide superconductor Na_xCoO_2*yH_2O, the
edge-shared CoO6 octahedra are trigonally contracted along the c-axis in the
CoO2-plane. We study how this CoO6 distortion affects the magnetic properties
and superconductivity in this compound by analyzing the multiorbital Hubbard
model using the fluctuation-exchange approximation. It is shown that through
generating the trigonal crystal field, the distortion pushes the Co e'g bands
up and consequently gives rise to the hole-pocket Fermi surfaces, which have
been predicted in the band calculations. As the distortion increases, the hole
pockets are enlarged and the ferromagnetic fluctuation as well as the pairing
instability increases, which is in good agreement with recent NQR results.Comment: 5 pages, 4 figures. Errata: Ref.[10] in published version should be
"Y. Kobayashi, M. Yokoi and M. Sato: J. Phys. Soc. Jpn. 72 (2003) 2161, ibid.
72 (2003) 2453
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