190 research outputs found
Processing conditions for (Nd, Eu, Gd)-Ba-Cu-O ternary bulk superconductors
Abstract(Nd, Eu, Gd)-Ba-Cu-O ternary bulk superconductors have high potential for practical applications since they exhibit very high critical current densities and thus high field trapping capabilities. (Nd, Eu, Gd)-Ba-Cu-O superconductors are synthesized in a reduced oxygen atmosphere, which requires a control of oxygen partial pressure and needs a special device for hot seeding. In the present study, for simplicity, we employed Ar gas flow into the furnace to control oxygen partial pressure instead of flowing oxygen-controlled gas. Hence, it was necessary to modify the melt processing conditions to produce a single domain. Through the optimization of seeding temperature and cooling rate, we obtained the processing conditions, in which a single domain bulk (Nd, Eu, Gd)-Ba-Cu-O of 20mm diameter could be synthesized
Re-appearance of antiferromagnetic ordering with Zn and Ni substitution in La_{2-x}Sr_xCuO_4
The effects of nonmagnetic Zn and magnetic Ni substitution for Cu site on
magnetism are studied by measurements of uniform magnetic susceptibility for
lightly doped La_{2-x}Sr_xCu_{1-z}M_zO_4 (M=Zn or Ni) polycrystalline samples.
For the parent x=0, Zn doping suppresses the N\'{e}el temperature T_N whereas
Ni doping hardly changes T_N up to z=0.3. For the lightly doped samples with
T_N~0, the Ni doping recovers T_N. For the superconducting samples, the Ni
doping induces the superconductivity-to-antiferromagnetic transition (or
crossover). All the heavily Ni doped samples indicate a spin glass behavior at
\~15 K.Comment: 2 pages including 3 figures, to be published in Physica C (LT23,
Hiroshima 2002
Doping Effect of Nano-Diamond on Superconductivity and Flux Pinning in MgB2
Doping effect of diamond nanoparticles on the superconducting properties of
MgB2 bulk material has been studied. It is found that the superconducting
transition temperature Tc of MgB2 is suppressed by the diamond-doping, however,
the irreversibility field Hirr and the critical current density Jc are
systematically enhanced. Microstructural analysis shows that the diamond-doped
MgB2 superconductor consists of tightly-packed MgB2 nano-grains (~50-100 nm)
with highly-dispersed and uniformly-distributed diamond nanoparticles (~10-20
nm) inside the grains. High density of dislocations and diamond nanoparticles
may take the responsibility for the enhanced flux pinning in the diamond-doped
MgB2.Comment: 16 pages, 6 figure
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
Ni-substituted sites and the effect on Cu electron spin dynamics of YBa2Cu{3-x}NixO{7-\delta}
We report Cu nuclear quadrupole resonance experiment on magnetic impurity
Ni-substituted YBaCuNiO. The distribution of
Ni-substituted sites and its effect on the Cu electron spin dynamics are
investigated. Two samples with the same Ni concentration =0.10 and nearly
the same oxygen content but different 's were prepared: One is an
as-synthesized sample (7-=6.93) in air (), and the
other is a quenched one (7-=6.92) in a reduced oxygen atmosphere
(). The plane-site Cu(2) nuclear spin-lattice
relaxation for the quenched sample was faster than that for the as-synthesized
sample, in contrast to the Cu(1) relaxation that was faster for the
as-synthesized sample. This indicates that the density of plane-site Ni(2) is
higher in the quenched sample, contrary to the chain-site Ni(1) density which
is lower in the quenched sample. From the analysis in terms of the Ni-induced
nuclear spin-lattice relaxation, we suggest that the primary origin of
suppression of is associated with nonmagnetic depairing effect of the
plane-site Ni(2).Comment: 7 pages, 5 figure
Cu Nuclear Quadrupole Resonance Study of the Spin-Peierls Compound Cu1-xMgxGeO3: A Possibility of Precursory Dimerization
We report on a zero-field 63Cu nuclear quadrupole resonance (NQR) study of
nonmagnetic Mg impurity substituted Cu1-xMgxGeO3 (single crystals; the
spin-Peierls transition temperature Tsp~14, 13.5, and 11 K for x=0, 0.0043, and
0.020) in a temperature range from 4.2 K to 250 K. We found that below T*~77 K,
Cu NQR spectra are broadened and nonexponential Cu nuclear spin-lattice
relaxation increases for undoped and more remarkably for Mg-doped samples. The
results indicate that random lattice distortion and impurity-induced spins
appear below T*, which we associate with a precursor of the spin-Peierls
transition. Conventional magnetic critical slowing down does not appear down to
4.2 K below Tsp.Comment: 4 pages, 4 figure
Resonant Spin Excitation in an Overdoped High Temperature Superconductor
An inelastic neutron scattering study of overdoped Bi_2Sr_2CaCu_2O_{8+\delta}
$ (T_c = 83 K) has revealed a resonant spin excitation in the superconducting
state. The mode energy is E_res=38 meV, significantly lower than in optimally
doped Bi_2Sr_2CaCu_2O_{8+\delta} (T_c = 91 K, E_ res =43 meV). This
observation, which indicates a constant ratio E_res /k_B T_c \sim 5.4, helps
resolve a long-standing controversy about the origin of the resonant spin
excitation in high-temperature superconductors.Comment: final version: PRL 86, 1610 (2001
Zn-Neighbor Cu NQR in Zn-Substituted YBa2Cu3O7-d and YBa2Cu4O8
We studied local electronic states near Zn in optimally doped
YBa(CuZn_x)O and underdoped
YBa(CuZn_x)O via satellite signals of plane-site Cu(2)
nuclear quadrupole resonance (NQR) spectra. From the relative intensity of Cu
NQR spectra, the satellite signals are assigned to Zn-neighbor Cu NQR lines.
The Cu nuclear spin-lattice relaxation time of the satellite signal is shorter
than that of the main signal, which indicates that the magnetic correlation is
locally enhanced near Zn both for the underdoped and the optimally doped
systems. The pure YBaCuO is a stoichiometric, homogenous,
underdoped electronic system; nevertheless, the Zn-induced inhomogeneous
magnetic response in the CuO plane is more marked than that of the
optimally doped YBaCuO.Comment: 9 pages including 8 figures, to be published in Phys. Rev.
Anisotropic resistivity and Hall effect in MgB2 single crystals
We report resistivity and the Hall effect measurements in the normal and
superconducting states of MgB2 single crystal. The resistivity has been found
to be anisotropic with slightly temperature dependent resistivity ratio of
about 3.5. The Hall constant, with a magnetic field parallel to the Mg and B
sheets is negative in contrast to the hole-like Hall response with a field
directed along the c-axis indicating presence of both types of charge carriers
and, thus, multi-band electronic structure of MgB2. The Hall effect in the
mixed state shows no sign change anomaly reproducing the Hall effect behavior
in clean limit type-II superconductors.Comment: Minor changed content, 11 pages including 3 figure
- …