8,149 research outputs found
Structural and magnetic phase diagram of CeFeAsO1-xFx and its relationship to high-temperature superconductivity
We use neutron scattering to study the structural and magnetic phase
transitions in the iron pnictides CeFeAsO1-xFx as the system is tuned from a
semimetal to a high-transition-temperature (high-Tc) superconductor through
Fluorine (F) doping x. In the undoped state, CeFeAsO develops a structural
lattice distortion followed by a stripe like commensurate antiferromagnetic
order with decreasing temperature. With increasing Fluorine doping, the
structural phase transition decreases gradually while the antiferromagnetic
order is suppressed before the appearance of superconductivity, resulting an
electronic phase diagram remarkably similar to that of the high-Tc copper
oxides. Comparison of the structural evolution of CeFeAsO1-xFx with other
Fe-based superconductors reveals that the effective electronic band width
decreases systematically for materials with higher Tc. The results suggest that
electron correlation effects are important for the mechanism of high-Tc
superconductivity in these Fe pnictides.Comment: 19 pages, 5 figure
Limits on the Superconducting Order Parameter in NdFeAsOF from Scanning SQUID Microscopy
Identifying the symmetry of the superconducting order parameter in the
recently-discovered ferro-oxypnictide family of superconductors,
RFeAsOF, where is a rare earth, is a high priority. Many of
the proposed order parameters have internal phase shifts, like the d-wave
order found in the cuprates, which would result in direction-dependent phase
shifts in tunnelling. In dense polycrystalline samples, these phase shifts in
turn would result in spontaneous orbital currents and magnetization in the
superconducting state. We perform scanning SQUID microscopy on a dense
polycrystalline sample of \NdFeAsOF with K and find
no such spontaneous currents, ruling out many of the proposed order parameters.Comment: 10 pages, 5 figures; to appear in JPS
Integer and half-integer flux-quantum transitions in a niobium/iron-pnictide loop
The recent discovery of iron-based superconductors challenges the existing
paradigm of high-temperature superconductivity. Owing to their unusual
multi-orbital band structure, magnetism, and electron correlation, theories
propose a unique sign reversed s-wave pairing state, with the order parameter
changing sign between the electron and hole Fermi pockets. However, because of
the complex Fermi surface topology and material related issues, the predicted
sign reversal remains unconfirmed. Here we report a novel phase-sensitive
technique for probing unconventional pairing symmetry in the polycrystalline
iron-pnictides. Through the observation of both integer and half-integer
flux-quantum transitions in composite niobium/iron-pnictide loops, we provide
the first phase-sensitive evidence of the sign change of the order parameter in
NdFeAsO0.88F0.12, lending strong support for microscopic models predicting
unconventional s-wave pairing symmetry. These findings have important
implications on the mechanism of pnictide superconductivity, and lay the
groundwork for future studies of new physics arising from the exotic order in
the FeAs-based superconductors.Comment: 23 pages, including 4 figures and supplementary informatio
Spin susceptibility, phase diagram, and quantum criticality in the electron-doped high Tc Superconductor Ba[Fe(1-x)Co(x)]2As2
We report a systematic investigation of Ba[Fe(1-x)Co(x)]2As2 based on
transport and 75-As NMR measurements, and establish the electronic phase
diagram. We demonstrate that doping progressively suppresses the uniform spin
susceptibility and low frequency spin fluctuations. The optimum superconducting
phase emerges at x_c~0.08 when the tendency toward spin ordering completely
diminishes. Our findings point toward the presence of a quantum critical point
near x_c between the SDW (spin density wave) and superconducting phases.Comment: 5 Figure
Systematic Study on Fluorine-doping Dependence of Superconducting and Normal State Properties in LaFePO1-xFx
We have investigated the fluorine-doping dependence of lattice constants,
transports and specific heat for polycrystalline LaFePO1-xFx. F doping slightly
and monotonically decreases the in-plane lattice parameter. In the normal
state, electrical resistivity at low temperature is proportional to the square
of temperature and the electronic specific heat coefficient has large value,
indicating the existence of moderate electron-electron correlation in this
system. Hall coefficient has large magnitude, and shows large temperature
dependence, indicating the low carrier density and multiple carriers in this
system. Temperature dependence of the upper critical field suggests that the
system is a two gap superconductor. The F-doping dependence of these properties
in this system are very weak, while in the FeAs system (LaFeAsO), the F doping
induces the large changes in electronic properties. This difference is probably
due to the different F-doping dependence of the lattice in these two systems.
It has been revealed that a pure effect of electron doping on electronic
properties is very weak in this Fe pnictide compound.Comment: 8 pages, 5 figures, accepted for publication in J. Phys. Soc. Jp
Lattice and Magnetic structures of PrFeAsO, PrFeAsO0.85F0.15 and PrFeAsO0.85
We use powder neutron diffraction to study the spin and lattice structures of
polycrystalline samples of nonsuperconducting PrFeAsO and superconducting
PrFeAsO0.85F0.15 and PrFeAsO0.85. We find that PrFeAsO exhibits an abrupt
structural phase transitions at 153 K, followed by static long range
antiferromagnetic order at 127 K. Both the structural distortion and magnetic
order are identical to other rare-earth oxypnictides. Electron-doping the
system with either Fluorine or oxygen deficiency suppresses the structural
distortion and static long range antiferromagnetic order, therefore placing
these materials into the same class of FeAs-based superconductors.Comment: 14 pages, 3 figures, 1 tabl
Field-induced quantum fluctuations in the heavy fermion superconductor CeCu2Ge2
Quantum-mechanical fluctuations in strongly correlated electron systems cause
unconventional phenomena such as non-Fermi liquid behavior, and arguably high
temperature superconductivity. Here we report the discovery of a field-tuned
quantum critical phenomenon in stoichiometric CeCu2Ge2, a spin density wave
ordered heavy fermion metal that exhibits unconventional superconductivity
under ~ 10 GPa of applied pressure. Our finding of the associated quantum
critical spin fluctuations of the antiferromagnetic spin density wave order,
dominating the local fluctuations due to single-site Kondo effect, provide new
information about the underlying mechanism that can be important in
understanding superconductivity in this novel compound.Comment: Heavy Fermion, Quantum Critical Phenomeno
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