906 research outputs found
59-Co and 75-As NMR Investigation of Electron-Doped High Tc Superconductor BaFe(1.8)Co(0.2)As(2) (Tc = 22K)
We report an NMR investigation of the superconductivity in BaFe(2)As(2)
induced by Co doping (Tc=22K). We demonstrate that Co atoms form an alloy with
Fe atoms and donate carriers without creating localized moments. Our finding
strongly suggests that the underlying physics of iron-pnictide superconductors
is quite different from the widely accepted physical picture of high Tc
cuprates as doped Mott insulators. We also show a crossover of electronic
properties into a low temperature pseudo-gap phase with a pseudo-gap Delta
560K, where chi(spin) constant and resisitivty T. The NMR Knight shift below Tc
decreases for both along the c-axis and ab-plane, and is consistent with the
singlet pairing scenario.Comment: Accepted for publication in J. Phys. Soc. Jpn. (4 pages
Enhancement of electronic anomalies in iron-substituted La_2-x_Sr_x_Cu_1-y_Fe_y_O_4_ around x=0.22
We have measured the temperature dependences of Rho and Chi for
Fe-substituted La_2-x_Sr_x_Cu_1-y_Fe_y_O_4_ in the overdoped regime, in order
to investigate Fe-substitution effects on electronic properties around x=0.22.
From the Rho measurements, it has been found around x=0.22 that the values of
Rho are large at room temperature and that Rho exhibits a pronounced upturn at
low temperatures. Moreover, from the Rho and Chi measurements, it has been
found that T_c_ is anomalously depressed around x=0.22. These results indicate
that the electronic anomalies around x=0.22 are enhanced by Fe substitution,
which might be related to the development of stripe correlations by Fe
substitution.Comment: 7 pages, 3 figure
Magnetic ordering in Sr2RuO4 induced by nonmagnetic impurities
We report unusual effects of nonmagnetic impurities on the spin-triplet
superconductor Sr2RuO4. The substitution of nonmagnetic Ti4+ for Ru4+ induces
localized-moment magnetism characterized by unexpected Ising anisotropy with
the easy axis along the interlayer c direction. Furthermore, for x(Ti) > 0.03
magnetic ordering occurs in the metallic state with the remnant magnetization
along the c-axis. We argue that the localized moments are induced in the Ru4+
and/or oxygen ions surrounding Ti4+ and that the ordering is due to their
interaction mediated by itinerant Ru-4d electrons with strong spin
fluctuations.Comment: 5 pages, 4figure
Disorder and transport in cuprates: weak localization and magnetic contributions
We report resistivity measurements in underdoped YBaCuOand overdoped TlBaCuO single crystals in which the
concentration of defects in the CuO planes is controlled by electron
irradiation. Low upturns of the resistivity are observed in both cases for
large defect content. In the Tl compound the decrease of conductivity scales as
expected from weak localization theory. On the contrary in
YBaCuO the much larger low T contribution to the
resistivity is proportional to the defect content and might then be associated
to a Kondo like spin flip scattering term. This would be consistent with the
results on the magnetic properties induced by spinless defects.Comment: latex rullier1.tex, 5 files, 4 pages [SPEC-S01/003], submitted to
Phys. Rev. Let
Charge Localization from Local Destruction of Antiferromagnetic Correlation in Zn-doped YBa2Cu3O7-d
The in-plane normal-state resistivity of Zn-doped YBa2Cu3O7-d single crystals
is measured down to low temperatures by suppressing superconductivity with
magnetic fields up to 18 T. Substitution of Cu with Zn in the CuO2 planes is
found to induce carrier localization at low temperatures in "clean" samples
with kF l > 5, where the mean free path l is larger than the electron wave
length and thus localization is not normally expected. The destruction of the
local antiferromagnetic correlation among Cu spins by Zn is discussed to be the
possible origin of this unusual charge localization.Comment: 4 pages of LaTeX (revtex and epsf) including 4 postscript figure
Spin Dynamics near the Superconductor-to-Insulator Transition in Impurity-Doped YBa2Cu4O8
We studied low-frequency spin dynamics near the impurity-induced
superconductor-to-insulator transition for underdoped high-Tc superconductor
YBa2(Cu1-xMx)4O8 (M=Ni, Zn) using the Cu nuclear quadrupole resonance (NQR)
spin-echo technique. We observed remarkable suppression of the normal-state
pseudo spin-gap and a loss of Cu NQR spectrum intensity at low temperatures
around the critical impurity concentration.Comment: 6 pages, 4 figures. To be published in J. Phys. Soc. Jpn. Vol.70,
No.7 (2001
Unusual T_c variation with hole concentration in Bi_2Sr_{2-x}La_xCuO_{6+\delta}
We have investigated the variation with the hole concentration in
the La-doped Bi 2201 system, BiSrLaCuO. It is
found that the Bi 2201 system does not follow the systematics in and
observed in other high- cuprate superconductors (HTSC's). The vs
characteristics are quite similar to what observed in Zn-doped HTSC's. An
exceptionally large residual resistivity component in the inplane resistivity
indicates that strong potential scatterers of charge carriers reside in CuO
planes and are responsible for the unusual variation with , as in the
Zn-doped systems. However, contrary to the Zn-doped HTSC's, the strong scatter
in the Bi 2201 system is possibly a vacancy in the Cu site.Comment: RevTeX, 3 figures, to be published in the Physical Review
Universal Heat Conduction in YBa_2Cu_3O_6.9
The thermal conductivity of YBa_2Cu_3O_6.9 was measured at low temperatures
in untwinned single crystals with concentrations of Zn impurities from 0 to 3%
of Cu. A linear term kappa_0/T = 0.19 mW/K^2.cm is clearly resolved as T -> 0,
and found to be virtually independent of Zn concentration. The existence of
this residual normal fluid strongly validates the basic theory of transport in
unconventional superconductors. Moreover, the observed universal behavior is in
quantitative agreement with calculations for a gap function of d-wave symmetry.Comment: Latex file, 4 pages, 3 EPS figures, to appear in Physical Review
Letter
Finite temperature transport at the superconductor-insulator transition in disordered systems
I argue that the incoherent, zero-frequency limit of the universal crossover
function in the temperature-dependent conductivity at the
superconductor-insulator transition in disordered systems may be understood as
an analytic function of dimensionality of system d, with a simple pole at d=1.
Combining the exact result for the crossover function in d=1 with the recursion
relations in d=1+\epsilon, the leading term in the Laurent series in the small
parameter \epsilon for this quantity is computed for the systems of disordered
bosons with short-range and Coulomb interactions. The universal,
low-temperature, dc critical conductivity for the dirty boson system with
Coulomb interaction in d=2 is estimated to be 0.69 (2e)^2 /h, in relatively
good agreement with many experiments on thin films. The next order correction
is likely to somewhat increase the result, possibly bringing it closer to the
self-dual value.Comment: 9 pages, LaTex, no figure
Effects of In-Plane Impurity Substitution in Sr2RuO4
We report comparative substitution effects of nonmagnetic Ti^(4+) and
magnetic Ir^(4+) impurities for Ru^(4+) in the spin-triplet superconductor
Sr2RuO4. We found that both impurities suppress the superconductivity
completely at a concentration of approximately 0.15%, reflecting the high
sensitivity to translational symmetry breaking in Sr2RuO4. In addition, a rapid
enhancement of residual resistivity is in quantitative agreement with
unitarity-limit scattering. Our result suggests that both nonmagnetic and
magnetic impurities in Sr2RuO4 act as strong potential scatterers, similar to
the nonmagnetic Zn^(2+) impurity in the high-Tc cuprates.Comment: 4 pages, 2 figures. submitted to Journal of the Physical Society of
Japa
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