32 research outputs found
Structural Evolution of One-dimensional Spin Ladder Compounds Sr14-xCaxCu24O41 with Ca doping and Related Hole Redistribution Evidence
Incommensurate crystal structures of spin ladder series Sr14-xCaxCu24O41
(x=3, 7, 11, 12.2) were characterized by powder neutron scattering method and
refined using the superspace group Xmmm(00{\gamma})ss0 (equivalent to
superspace group Fmmm(0,0,1+{\gamma})ss0); X stands for non-standard centering
(0,0,0,0), (0,1/2,1/2,1/2), (1/2,1/2,0,0), (1/2,0,1/2,1/2)) with a modulated
structure model. The Ca doping effects on the lattice parameters, atomic
displacement, Cu-O distances, Cu-O bond angles and Cu bond valence sum were
characterized. The refined results show that the CuO4 planar units in both
chain and ladder sublattices become closer to square shape with an increase of
Ca doping. The Cu bond valence sum calculation provided new evidence for the
charge transfer from the chains to ladders (approximately 0.16 holes per Cu
from x=0 to 12.2). The charge transfer was attributed to two different
mechanisms: (a) the Cu-O bond distance shrinkage on the ladder; (b) increase of
the interaction between two sublattices, resulting in Cu-O bonding between the
chains and ladders. The low temperature structural refinement resulted in the
similar conclusion, with a slight charge backflow to the chains.Comment: 29 pages, 16 figures, submitted to physics review b, accepte
Electron doping evolution of the magnetic excitations in BaFe2-xNixAs2
We use inelastic neutron scattering (INS) spectroscopy to study the magnetic
excitations spectra throughout the Brioullion zone in electron-doped iron
pnictide superconductors BaFeNiAs with .
While the sample is near optimal superconductivity with K
and has coexisting static incommensurate magnetic order, the
samples are electron-overdoped with reduced of 14 K and 8 K,
respectively, and have no static antiferromagnetic (AF) order. In previous INS
work on undoped () and electron optimally doped () samples, the
effect of electron-doping was found to modify spin waves in the parent compound
BaFeAs below 100 meV and induce a neutron spin resonance at the
commensurate AF ordering wave vector that couples with superconductivity. While
the new data collected on the sample confirms the overall features of
the earlier work, our careful temperature dependent study of the resonance
reveals that the resonance suddenly changes its -width below similar
to that of the optimally hole-doped iron pnictides
BaKFeAs. In addition, we establish the dispersion of
the resonance and find it to change from commensurate to transversely
incommensurate with increasing energy. Upon further electron-doping to
overdoped iron pnictides with and 0.18, the resonance becomes weaker
and transversely incommensurate at all energies, while spin excitations above
100 meV are still not much affected. Our absolute spin excitation
intensity measurements throughout the Brillouin zone for
confirm the notion that the low-energy spin excitation coupling with itinerant
electron is important for superconductivity in these materials, even though the
high-energy spin excitations are weakly doping dependent.Comment: 16 pages, 16 figure
Doping evolution of antiferromagnetism and transport properties in the non-superconducting BaFe2-2xNixCrxAs2
We report elastic neutron scattering and transport measurements on the Ni and
Cr equivalently doped iron pnictide BaFeNiCrAs.
Compared with the electron-doped BaFeNiAs, the long-range
antiferromagnetic (AF) order in BaFeNiCrAs is
gradually suppressed with vanishing ordered moment and N\'{e}el temperature
near without the appearance of superconductivity. A detailed analysis
on the transport properties of BaFeNiAs and
BaFeNiCrAs suggests that the non-Fermi-liquid
behavior associated with the linear resistivity as a function of temperature
may not correspond to the disappearance of the static AF order. From the
temperature dependence of the resistivity in overdoped compounds without static
AF order, we find that the transport properties are actually affected by Cr
impurity scattering, which may induce a metal-to-insulator crossover in highly
doped BaFeNiCrAs.Comment: 10 pages, 12 figure
Spin gap evolution upon Ca doping in the spin ladder series by inelastic neutron scattering
The spin gap evolution upon Ca doping in Sr14-xCaxCu24O41 was systematically
investigated using inelastic neutron scattering. We discover that the
singlet-triplet spin gap excitation survives in this series with x up to 13,
indicating the singlet dimer ground state in these compounds. This observation
corrects the previous speculation that the spin gap collapses at x~13 by the
NMR technique. The strong intensity modulation along QH in x=0 gradually
evolves into a Q-independent feature in x>11. This could be attributed to the
localized Cu moment magnetism developing into an itinerant magnetism with
increasing x. It is a surprise that the spin gap persists in the normal state
of this spin ladder system with metallic behaviour, which evidences the
possibility of magnetically-mediated carrier pairing mechanism in a two-leg
spin ladder lattice.Comment: 17 pages, 6 figure
Micro-fabrication process for small transport devices of layered manganite
Devices have been fabricated based on the bilayer manganite La1.4Sr1.6Mn2O7, which in the bulk state orders magnetically below 90 K, at which point both in-plane and c-axis bulk resistivity decrease by 2-3 orders of magnitude. We provide an optimized procedure to fabricate devices to electrical transport in- and out of plane. Fabricated mesoscopic devices have dimensions comparable to a typical magnetic domain, allowing us to study structures going from a single domain to several domains