693 research outputs found
NMR and NQR study of pressure-induced superconductivity and the origin of critical-temperature enhancement in the spin-ladder cuprate SrCaCuO
Pressure-induced superconductivity was studied for a spin-ladder cuprate
SrCaCuO using nuclear magnetic resonance (NMR) under
pressures up to the optimal pressure 3.8 GPa. Pressure application leads to a
transitional change from a spin-gapped state to a Fermi-liquid state at
temperatures higher than . The relaxation rate shows
activated-type behavior at an onset pressure, whereas Korringa-like behavior
becomes predominant at the optimal pressure, suggesting that an increase in the
density of states (DOS) at the Fermi energy leads to enhancement of .
Nuclear quadrupole resonance (NQR) spectra suggest that pressure application
causes transfer of holes from the chain to the ladder sites. The transfer of
holes increases DOS below the optimal pressure. A dome-shaped versus
pressure curve arises from naive balance between the transfer of holes and
broadening of the band width
Dynamic fluctuations in the superconductivity of NbN films from microwave conductivity measurements
We have measured the frequency and temperature dependences of complex ac
conductivity, \sigma(\omega)=\sigma_1(\omega)-i\sigma_2(\omega), of NbN films
in zero magnetic field between 0.1 to 10 GHz using a microwave broadband
technique. In the vicinity of superconducting critical temperature, Tc, both
\sigma_1(\omega) and \sigma_2(\omega) showed a rapid increase in the low
frequency limit owing to the fluctuation effect of superconductivity. For the
films thinner than 300 nm, frequency and temperature dependences of fluctuation
conductivity, \sigma(\omega,T), were successfully scaled onto one scaling
function, which was consistent with the Aslamazov and Larkin model for two
dimensional (2D) cases. For thicker films, \sigma(\omega,T) data could not be
scaled, but indicated that the dimensional crossover from three dimensions (3D)
to 2D occurred as the temperature approached Tc from above. This provides a
good reference of ac fluctuation conductivity for more exotic superconductors
of current interest.Comment: 8 pages, 7 Figures, 1 Table, Accepted for publication in PR
Experimental observation of the crystallization of a paired holon state
A new excitation is observed at 201 meV in the doped-hole ladder cuprate
SrCuO, using ultraviolet resonance Raman scattering with
incident light at 3.7 eV polarized along the direction of the rungs. The
excitation is found to be of charge nature, with a temperature independent
excitation energy, and can be understood via an intra-ladder pair-breaking
process. The intensity tracks closely the order parameter of the charge density
wave in the ladder (CDW), but persists above the CDW transition
temperature (), indicating a strong local pairing above .
The 201 meV excitation vanishes in LaCaCuO,
and LaCaCuO which are samples with no holes in the
ladders. Our results suggest that the doped holes in the ladder are composite
bosons consisting of paired holons that order below .Comment: Accepted for publication in Physical Review Letters (4 figures
Macroscopic Quantum Tunneling of a Fluxon in a Long Josephson Junction
Macroscopic quantum tunneling (MQT) for a single fluxon moving along a long
Josephson junction is studied theoretically. To introduce a fluxon-pinning
force, we consider inhomogeneities made by modifying thickness of an insulating
layer locally. Two different situations are studied: one is the quantum
tunneling from a metastable state caused by a single inhomogeneity, and the
other is the quantum tunneling in a two-state system made by two
inhomogeneities. In the quantum tunneling from a metastable state, the decay
rate is estimated within the WKB approximation. Dissipation effects on a fluxon
dynamics are taken into account by the Caldeira-Leggett theory. We propose a
device to observe quantum tunneling of a fluxon experimentally. Required
experimental resolutions to observe MQT of a fluxon seem attainable within the
presently available micro-fabrication technique. For the two-state system, we
study quantum resonance between two stable states, i.e., macroscopic quantum
coherence (MQC). From the estimate for dissipation coefficients due to
quasiparticle tunneling, the observation of MQC appears to be possible within
the Caldeira-Leggett theory.Comment: 30 pages LaTeX including 11 PS figures, using jpsj.sty. To be
published on J. Phys. Soc. Jpn. Overestimates for damping amplitude is
correcte
Direct optical excitation of two and three magnons in α-Fe₂O₃
Direct excitation of two and three magnons is observed in midinfrared absorption and Raman
scattering spectra of α-Fe₂O₃ crystals. These polarization characteristics and the spectra themselves
are shown to be understood from group-theoretical point of view. The microscopic mechanism
of three-magnon excitation is proposed in addition to that of well-known two-magnon excitation
process
Effect of a magnetic field on the spin- and charge-density wave order in La1.45Nd0.4Sr0.15CuO4
The spin-density wave (SDW) and charge-density wave (CDW) order in
superconducting La1.45Nd0.4Sr0.15CuO4 were studied under an applied magnetic
field using neutron and X-ray diffraction techniques. In zero field,
incommensurate (IC) SDW order appears below ~ 40 K, which is characterized by
neutron diffraction peaks at (1/2 +/- 0.134, 1/2 +/- 0.134, 0). The intensity
of these IC peaks increases rapidly below T_Nd ~ 8 K due to an ordering of the
Nd^3+ spins. The application of a 1 T magnetic field parallel to the c-axis
markedly diminishes the intensity below T_Nd, while only a slight decrease in
intensity is observed at higher temperatures for fields up to 7 T. Our
interpretation is that the c-axis field suppresses the parasitic Nd^3+ spin
order at the incommensurate wave vector without disturbing the stripe order of
Cu^2+ spins. Consistent with this picture, the CDW order, which appears below
60 K, shows no change for magnetic fields up to 4 T. These results stand in
contrast to the significant field-induced enhancement of the SDW order observed
in superconducting La2-xSrxCuO4 with x ~ 0.12 and stage-4 La2CuO4+y. The
differences can be understood in terms of the relative volume fraction
exhibiting stripe order in zero field, and the collective results are
consistent with the idea that suppression of superconductivity by vortices
nucleates local patches of stripe order.Comment: 7 pages, 5 figure
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