182 research outputs found
Field-Induced Staggered Magnetic Order in La(2)NiO(4.133)
At low temperature the holes doped into the NiO(2) planes of La(2)NiO(4.133)
by the excess oxygen collect in diagonal stripes that separate narrow
antiferromagnetic domains. The magnetic order drops abruptly to zero at T_m =
110.5 K, but charge order remains with a period of 3a/2. We show that
application of a magnetic field in the regime T > T_m induces staggered
magnetic order of period 3a due to the net magnetic moment of the
high-temperature bond-centered stripes, together with the odd number of Ni
spins across an antiferromagnetic domain.Comment: 4 pages, Revtex, 4 epsf figs included with psfig
Dopant Structural Distortions in High-temperature Superconductors: An Active or a Passive Role?
The parent compounds of high-temperature superconductors, such as YBa2Cu3O6 and La2CuO4, are strongly interacting electron systems, rendering them insulators with Mott-Hubbard gaps of a few electronvolts. Charge carriers (holes) are introduced by chemical doping, causing an insulator-metal (IM) transition and, at low temperatures, superconductivity. the role of dopants is widely seen as limited to the introduction of holes into the CuO2 planes (i.e. occupying electronic states derived from Cu and O 2px,y atomic orbitals). Most theories of high-Tc superconductivity deal with pairing interactions between these planar holes. Local distortions around dopants are poorly understood, because of the experimental difficulty in obtaining such information, particularly at low doping. This has resulted in the neglect, in most theories, of the effect of such distortions on the chemical and electronic structure of high-Tc superconductors. Angular-resolved X-ray absorption fine structure (XAFS) spectroscopy on oriented samples is an ideal technique to elucidate the dopant distortions. Element specificity, together with a large orientation dependence of the XAFS signal in these layered structures, allows the local structure around dopants to be resolved. Results are presented here on (Sr, Ba) and Ni dopants, which substitute at the La and Cu sites, respectively, of insulating La2CuO4. the relevance of the measured local distortions for a complete understanding of the normal and superconducting properties of cuprates is discussed
Enhancement of the upper critical field of Nb3Sn utilizing disorder introduced by ball milling the elements
Nb3Sn was prepared by milling Nb and Sn powder mixtures followed by limited
reactions to restrict disorder recovery. Although disorder reduced the
superconducting critical temperature Tc, the concomitant electron scattering
increased the upper critical field mu0Hc2 to as high as 35 T at 0 K, as
determined by the Werthamer-Helfand-Hohenberg equation. Hc2 was higher for
longer milling times and lower annealing temperatures. Substitution of 2% Ti
for Nb did not appreciably enhance Hc2, suggesting that alloying mitigates the
benefits of disorder. Since alloyed Nb3Sn wires have mu0Hc2(0) approximately 29
T, wires based on heavily milled powders could extend the field range for
applications if they can be made with high current density
Scanning tunneling spectroscopy characterization of the pseudogap and the x = 1/8 anomaly in La2-xSrxCuO4 thin films
Using scanning tunneling spectroscopy we examined the local density of states
of thin c-axis La2-xSrxCuO4 films, over wide doping and temperature ranges. We
found that the pseudogap exists only at doping levels lower than optimal. For x
= 0.12, close to the 'anomalous' x = 1/8 doping level, a zero bias conductance
peak was the dominant spectral feature, instead of the excepted V- shaped
(c-axis tunneling) gap structure. We have established that this surprising
effect cannot be explained by tunneling into (110) facets. Possible origins for
this unique behavior are discussed.Comment: 15 pages, 6 figure
Xafs Study of the Low-Temperature Tetragonal Phase of La₂₋ₓBaₓCuO₄: Disorder, Stripes, and T\u3csub\u3ec\u3c/sub\u3e Suppression at X=0.125
Angular dependent x-ray absorption fine structure (XAFS) measurements of all La, Ba, and Cu sites determined the origin and nature of the intrinsic disorder present in the low temperature tetragonal (LTT) structural ground state of La2-xBaxCuO4 with x=0.125,0.15 at T=10 K. Ba doping induces major local distortions that extend as far as 5 Å from its lattice site. Despite the low Ba content the large distortions significantly affect the XAFS of the majority La and Cu sites. The distribution of the local LTT tilt angle of CuO6 octahedra was directly determined from single and multiple scattering contributions to the XAFS of La and Cu sites. This angle, which regulates the strength of electron-lattice and spin-spin coupling in this material, was found to exhibit large, random fluctuations with rms [〈(θ-〈θ〉)2〉] 1/2≈2〈θ〉, 〈θ〉 being the average tilt angle determined by diffraction techniques. These fluctuations originate in the large, random, static disorder introduced with Ba doping. Their presence has implications for the plausibility of a mobility gap causing Tc suppression at x=0.125 in only the LTT phase, and for the correlation length of the postulated charge and spin stripes
Microstructure and Structural Defects in MgB2 Superconductor
We report a detailed study of the microstructure and defects in sintered
polycrystalline MgB2. Both TEM and x-ray data reveal that MgO is the major
second-phase in our bulk samples. Although MgB2 and MgO have different crystal
symmetries, being P6/mmm and Fm-3m, respectively, their stacking sequence of Mg
and B (or O) and lattice spacings in certain crystallographic orientations are
very similar. The size of MgO varies from 10~500nm, and its mismatch with the
MgB2 matrix can be a source for dislocations. Dislocations in MgB2 often have a
Burgers vector of . 1/3 and 1/3 partial dislocations and their
associated stacking faults were also observed. Since both dislocations and
stacking faults are located in the (001) basal plane, flux pinning anisotropy
is expected. Diffuse scattering analysis suggests that the correlation length
along the c-axis for defect-free basal planes is about 50nm. (001) twist
grain-boundaries, formed by rotations along the c-axis, are major grain
boundaries in MgB2 as a result of the out-of-plane weak bonding between Mg and
B atoms. An excess of Mg was observed in some grain boundaries. High-resolution
nano-probe EELS reveals that there is a difference in near edge structure of
the boron K-edge acquired from grain boundaries and grain interiors. The change
at the edge threshold may be suggestive of variation of the hole concentration
that would significantly alter boundary superconductivity.Comment: 20 pages and 12 figure
Synthesis and characterization of Na03RhO206H2O - a semiconductor with a weak ferromagnetic component
We have prepared the oxyhydrate Na03RhO206H2O by extracting Na+ cations from
NaRhO2 and intercalating water molecules using an aqueous solution of Na2S2O8.
Synchrotron X-ray powder diffraction, thermogravimetric analysis (TGA), and
energy-dispersive x-ray analysis (EDX) reveal that a non-stoichiometric
Na03(H2O)06 network separates layers of edge-sharing RhO6 octahedra containing
Rh3+(4d6, S=0) and Rh4+ (4d5, S=1/2). The resistivities of NaRhO2 and
Na03RhO206H2O (T < 300) reveal insulating and semi-conducting behavior with
activation gaps of 134 meV and 7.8 meV, respectively. Both Na03RhO206H2O and
NaRhO2 show paramagnetism at room temperature, however, the sodium-deficient
sample exhibits simultaneously a weak but experimentally reproducible
ferromagnetic component. Both samples exhibit a temperature-independent Pauli
paramagnetism, for NaRhO2 at T > 50 K and for Na03RhO206H2O at T > 25 K. The
relative magnitudes of the temperature-independent magnetic susceptibilities,
that of the oxide sample being half that of the oxyhydrate, is consistent with
a higher density of thermally accessible electron states at the Fermi level in
the hydrated sample. At low temperatures the magnetic moments rise sharply,
providing evidence of localized and weakl -ordered electronic spins.Comment: 15 fages 5 figures Solid State Communications in prin
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