989 research outputs found
Effects of carbon incorporation on doping state of YBa2Cu3Oy
Effects of carbon incorporation on the doping state of YBa2Cu3Oy (Y-123) were
investigated. Quantitative carbon analysis revealed that carbon could be
introduced into Y-123 from both the precursor and the sintering gas. Nearly
carbon-free (< 200 ppm) samples were prepared from a vacuum-treated precursor
by sintered at 900 ˚C and cooling with 20 ˚C /min in flowing oxygen
gas. The lower Tc (= 88 K) and higher oxygen content (y = 6.98) strongly
suggested the overdoping state, which was supported by the temperature
dependence of resisitivity and thermoelectric power. The nuclear quadrapole
resonance spectra and the Raman scattering spectra indicated that there was
almost no oxygen defect in the Cu-O chain in these samples. On the other hand,
in the same cooling condition, the samples sintered in air stayed at optimal
doping level with Tc = 93 K, and the intentionally carbon-doped sample was in
the underdoping state. It is revealed that about 60% of incorporated carbon was
substituted for Cu at the chain site in the form of CO32+, and the rest remains
at the grain boundary as carbonate impurities. Such incorporation affected the
oxygen absorption process in Y-123. It turned out that the oxygen content in
Y-123 cannot be controlled only by the annealing temperature and the oxygen
partial pressure but also by the incorporated carbon concentration.Comment: 16pages, 9figure
Anomalous radon emanation linked to preseismic electromagnetic phenomena
International audienceAnomalous emanation of radon (222Rn) was observed preceding large earthquakes and is considered to be linked to preseismic electromagnetic phenomena (e.g. great changes of atmospheric electric field and ionospheric disturbances). Here we analyze atmospheric radon concentration and estimate changes of electrical conditions in atmosphere due to preseismic radon anomaly. The increase of radon emanation obeys crustal damage evolution, following a power-law of time-to-earthquake. Moreover, the radon emanation decreases the atmospheric electric field by 40%, besides influencing the maximum strength of atmospheric electric field by 104?105 V/m enough to trigger ionospheric disturbances. These changes are within the ranges observed or explaining electromagnetic phenomena associated with large earthquakes
Theory of Electron Differentiation, Flat Dispersion and Pseudogap Phenomena
Aspects of electron critical differentiation are clarified in the proximity
of the Mott insulator. The flattening of the quasiparticle dispersion appears
around momenta and on square lattices and determines the
criticality of the metal-insulator transition with the suppressed coherence in
that momentum region of quasiparticles. Such coherence suppression at the same
time causes an instability to the superconducting state if a proper incoherent
process is retained. The d-wave pairing interaction is generated from such
retained processes without disturbance from the coherent single-particle
excitations. Pseudogap phenomena widely observed in the underdoped cuprates are
then naturally understood from the mode-mode coupling of d-wave
superconducting(dSC) fluctuations with antiferromagnetic ones. When we assume
the existence of a strong d-wave pairing force repulsively competing with
antiferromagnetic(AFM) fluctuations under the formation of flat and damped
single-particle dispersion, we reproduce basic properties of the pseudogap seen
in the magnetic resonance, neutron scattering, angle resolved photoemission and
tunneling measurements in the cuprates.Comment: 9 pages including 2 figures, to appear in J. Phys. Chem. Solid
Nuclear spin-spin coupling in La_{2-x}Sr_{x}CuO_{4} studied by stimulated echo decay
We have performed copper NQR experiments in high temperature superconductors
YBa_{2}Cu_{4}O_{8}, YBa_{2}Cu_{3}O_{7}, and La_{2-x}Sr_{x}CuO_{4} (x=0.12 and
0.15), using the stimulated echo technique. The stimulated echo intensity is
analyzed by a model that includes the spin-lattice relaxation process (T_ {1
}-process) and the fluctuating local field due to nuclear spin-spin coupling.
The model gives quantitative account of the experimental results in Y-based
compounds using the known values of 1/T_{1} and 1/T_{2G}, the gaussian decay
rate of the spin echo intensity. The same model applied to LSCO enables us to
extract the value of T_{2G}. Our results indicate that T_{1}T/T_{2G} is
independent of temperature, implying that the dynamic exponent is one in
La_{2-x}Sr_{x}CuO_{4}.Comment: 14 pages, 11 fugures, The bibliography field is correcte
Spontaneous deformation of the Fermi surface due to strong correlation in the two-dimensional t-J model
Fermi surface of the two-dimensional t-J model is studied using the
variational Monte Carlo method. We study the Gutzwiller projected d-wave
superconducting state with an additional variational parameter t'_v
corresponding to the next-nearest neighbor hopping term. It is found that the
finite t'_v<0 gives the lowest variational energy in the wide range of
hole-doping rates. The obtained momentum distribution function shows that the
Fermi surface deforms spontaneously. It is also shown that the van Hove
singularity is always located very close to the Fermi energy. Using the
Gutzwiller approximation, we show that this spontaneous deformation is due to
the Gutzwiller projection operator or the strong correlation.Comment: 4 pages, 3 eps figures, revte
A Universal Intrinsic Scale of Hole Concentration for High-Tc Cuprates
We have measured thermoelectric power (TEP) as a function of hole
concentration per CuO2 layer, Ppl, in Y1-xCaxBa2Cu3O6 (Ppl = x/2) with no
oxygen in the Cu-O chain layer. The room-temperature TEP as a function of Ppl,
S290(Ppl), of Y1-xCaxBa2Cu3O6 behaves identically to that of La2-zSrzCuO4 (Ppl
= z). We argue that S290(Ppl) represents a measure of the intrinsic equilibrium
electronic states of doped holes and, therefore, can be used as a common scale
for the carrier concentrations of layered cuprates. We shows that the Ppl
determined by this new universal scale is consistent with both hole
concentration microscopically determined by NQR and the hole concentration
macroscopically determined by the Cu valency. We find two characteristic
scaling temperatures, TS* and TS2*, in the TEP vs. temperature curves that
change systematically with doping. Based on the universal scale, we uncover a
universal phase diagram in which almost all the experimentally determined
pseudogap temperatures as a function of Ppl fall on two common curves; upper
pseudogap temperature defined by the TS* versus Ppl curve and lower pseudogap
temperature defined by the TS2* versus Ppl curve. We find that while pseudogaps
are intrinsic properties of doped holes of a single CuO2 layer for all high-Tc
cuprates, Tc depends on the number of layers, therefore the inter-layer
coupling, in each individual system.Comment: 11 pages, 9 figures, accepted for publication in Physical Review
Phase Inhomogeneity of the Itinerant Ferromagnet MnSi at High Pressures
The pressure induced quantum phase transition of the weakly itinerant
ferromagnet MnSi is studied using zero-field NMR spectroscopy and
relaxation. Below , the intensity of the signal and the
nuclear spin-lattice relaxation is independent of pressure, even though the
amplitude of the magnetization drops by 20% from the ambient pressure
amplitude. For , the decreasing intensity within the experimentally
detectable bandwidth signals the onset of an inhomogeneous phase that persists
to the highest pressure measured, , which is well beyond the
known critical pressure . Implications for the non-Fermi Liquid
behavior observed for are discussed.Comment: 4 pages, 4 figure
Spin dynamics and antiferromagnetic order in PrBa2Cu4O8 studied by Cu nuclear respnance
Results of the nuclear resonance experiments for the planar Cu sites in
PrBa2Cu4O8 are presented. The NMR spectrum at 1.5 K in zero magnetic field
revealed an internal field of 6.1 T, providing evidence for an
antiferromagnetic order of the planar Cu spins. This confirms that the CuO2
planes are insulating, therefore, the metallic conduction in this material is
entirely due to the one-dimensional zigzag Cu2O2 chains. The results of the
spin-lattice relaxation rates measured by zero field NQR above 245 K in the
paramagnetic state are explained by the theory for a Heisenberg model on a
square lattice.Comment: 4 pages, 2 figure
Field-induced instability of the quantum-spin-liquid ground state in the triangular-lattice compound NaYbO
Polycrystalline samples of NaYbO are investigated by bulk magnetization
and specific-heat measurements, as well as by nuclear magnetic resonance (NMR)
and electron spin resonance (ESR) as local probes. No signatures of long-range
magnetic order are found down to 0.3~K, evidencing a highly frustrated
spin-liquid-like ground state in zero field. Above 2\,T, signatures of magnetic
order are observed in thermodynamic measurements, suggesting the possibility of
a field-induced quantum phase transition. The Na NMR relaxation rates
reveal the absence of magnetic order and persistent fluctuations down to 0.3~K
at very low fields and confirm the bulk magnetic order above 2~T. The -
phase diagram is obtained and discussed along with the existing theoretical
concepts for layered spin- triangular-lattice antiferromagnetsComment: 4 figure
Novel critical exponent of magnetization curves near the ferromagnetic quantum phase transitions of Sr1-xAxRuO3 (A = Ca, La0.5Na0.5, and La)
We report a novel critical exponent delta=3/2 of magnetization curves
M=H^{1/delta} near the ferromagnetic quantum phase transitions of Sr1-xAxRuO3
(A = Ca, La0.5Na0.5, and La), which the mean field theory of the
Ginzburg-Landau-Wilson type fails to reproduce. The effect of dirty
ferromagnetic spin fluctuations might be a key.Comment: 4 pages, 5 figure
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