360 research outputs found
Magnetic order in lightly doped cuprates: Coherent vs. incoherent hole quasiparticles and non-magnetic impurities
We investigate magnetic properties of lightly doped antiferromagnetic Mott
insulators in the presence of non-magnetic impurities. Within the framework of
the t-J model we calculate the doping dependence of the antiferromagnetic order
parameter using the self-consistent diagrammatic techniques. We show that in
the presence of non-magnetic impurities the antiferromagnetic order is more
robust against hole doping in comparison with the impurity-free host, implying
that magnetic order can re-appear upon Zn doping into lightly hole-doped
cuprates. We argue that this is primarily due to the loss of coherence and
reduced mobility of the hole quasiparticles caused by impurity scattering.
These results are consistent with experimental data on Zn-doped LaSrCuO.Comment: 11 pages, 7 figs, (v2) final version as publishe
Cu Nuclear Quadrupole Resonance Study of Site-Disorder and Chemical Pressure Effects on Y(Ba1-xSrx)2Cu4O8
We report a zero-field Cu nuclear quadrupole resonance (NQR) study on the
effects of nonmagnetic Sr substitution for high-Tc superconductors,
Y(Ba1-xSrx)2Cu4O8 (Tc=82-80 K for x=0-0.4), using a spin-echo technique. The
site-disordering and chemical pressure effects associated with doping Sr were
observed in the broadened, shifted Cu NQR spectra. Nevertheless, the site
disorder did not significantly affect the homogeneity of Cu electron spin
dynamics, in contrast to the in-plane impurity. The peak shift of Cu NQR
spectrum due to Sr was different between the chain- and the plane-Cu sites,
more remarkably than those under a hydrostatic physical pressure, suggesting
anisotropic or nonuniform local structural strains. The small decrease of Tc
due to Sr can be traced back to either a cancellation effect on Tc between the
disorder and the pressure, or an anisotropic or nonuniform chemical pressure
effect on Tc.Comment: 4 pages, 5 figure
Muon-spin-relaxation and magnetic-susceptibility studies of effects of the magnetic impurity Ni on the Cu-spin dynamics and superconductivity in La_2-x_Sr_x_Cu_1-y_Ni_y_O_4_ with x = 0.13
Effects of the magnetic impurity Ni on the Cu-spin dynamics and
superconductivity have been studied in La_2-x_Sr_x_Cu_1-y_Ni_y_O_4_ with x =
0.13 changing y finely up to 0.10. Compared with the case of the nonmagnetic
impurity Zn, it has been found from the muon-spin-relaxation measurements that
a large amount of Ni is required to stabilize a magnetic order of Cu spins.
However, the evolution toward the stabilization of the magnetic order with
increasing impurity concentration is qualitatively similar to each other. The
area of the non-superconducting and slowly fluctuating or static region of Cu
spins around Ni has been found to be smaller than that around Zn, suggesting
that the pinning of rather long-ranged dynamical spin correlation such as the
so-called dynamical stripe by Ni is weaker than that by Zn. This may be the
reason why Zn destroys the superconductivity in the hole-doped high-T_c_
cuprates more markedly than Ni.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev.
Comparative study of macroscopic quantum tunneling in Bi_2Sr_2CaCu_2O_y intrinsic Josephson junctions with different device structures
We investigated macroscopic quantum tunneling (MQT) of
BiSrCaCuO intrinsic Josephson junctions (IJJs) with two device
structures. One is a nanometer-thick small mesa structure with only two or
three IJJs and the other is a stack of a few hundreds of IJJs on a narrow
bridge structure. Experimental results of switching current distribution for
the first switching events from zero-voltage state showed a good agreement with
the conventional theory for a single Josephson junction, indicating that a
crossover temperature from thermal activation to MQT regime for the former
device structure was as high as that for the latter device structure. Together
with the observation of multiphoton transitions between quantized energy levels
in MQT regime, these results strongly suggest that the observed MQT behavior is
intrinsic to a single IJJ in high- cuprates, independent of device
structures. The switching current distribution for the second switching events
from the first resistive state, which were carefully distinguished from the
first switchings, was also compared between two device structures. In spite of
the difference in the heat transfer environment, the second switching events
for both devices were found to show a similar temperature-independent behavior
up to a much higher temperature than the crossover temperature for the first
switching. We argue that it cannot be explained in terms of the self-heating
owing to dissipative currents after the first switching. As possible
candidates, the MQT process for the second switching and the effective increase
of electronic temperature due to quasiparticle injection are discussed.Comment: 10pages, 7figures, submitted to Phys. Rev.
Induced representations and Mackey theory
This is an exposition on Mackey theory for induced representations of finite group
AC loss reduction of TFA-MOD coated conductors in long length by laser scribing technique
AbstractTFA-MOD process is expected to be promising for future applications since it can produce high performance YBCO coated conductors with low cost. Applying YBCO coated conductors to the power electric devices such as transformer, cable, motors, reduction of AC loss for long wire is necessary. Multifilamentation, which is one of the effective approaches for AC loss reduction, has been developed by the scribing process. YBCO coated conductors produced by our standard TFA-MOD process delaminated into two parts by the laser scribing. The delamination was clarified to occur within the superconducting layer caused by the defects such as pores in the superconducting layer. In order to reduce the defects in the superconducting layer, we modify the heat treatment profile performed on the decomposed precursor films by applying the interim annealing(550-600°C) before crystallization heat treatment(740-770°C). The interim annealed samples had much less and smaller pores than the standard processed ones. The peel strength measured by transverse tensile test was as high as the PLD derived coated conductors which was successfully scribed into five filaments resulting in 1/5 AC loss. A 50m long YBCO coated conductor with the characteristics of 398A/cmwidth was obtained and cut into 5mm width, followed by the laser scribing process into five filaments. The multifilamentation process was successfully performed without delamination throughout the wire. The hysteresis loss was down to 1/N (N: number of filaments), as we aimed. The IC properties of the filaments were 29±4A, indicating the wire was uniformly fabricated
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