3 research outputs found
Spin-singlet superconductivity with multiple gaps in PrO0.89F0.11FeAs
Since the discovery of high transition-temperature (Tc) superconductivity in
copper oxides two decades ago, continuous efforts have been devoted to
searching for similar phenomenon in other compounds. With the exception of MgB2
(Tc =39 K), however, Tc is generally far lower than desired. Recently,
breakthrough has been made in a new class of oxypnictide compounds. Following
the initial discovery of superconductivity in LaO1-x FxFeAs (Tc =26 K), Tc
onset has been raised to 55 K in ReO1-xFxFeAs (Re: Ce, Pr, Nd, Sm). Meanwhile,
unravelling the nature of the energy associated with the formation of
current-carrying pairs (Cooper pairs), referred to as the superconducting
energy gap, is the first and vital step towards understanding why the
superconductivity occurs at such high temperature and is also important for
finding superconductors with still higher Tc. Here we show that, on the basis
of the nuclear magnetic resonance (NMR) measurements in PrO0.89F0.11FeAs (Tc
=45 K), the Cooper pair is in the spin-singlet state (two spins are
anti-paralleled), with two energy gaps opening below Tc. The results strongly
suggest the existence of nodes (zeros) in the gap. None of superconductors
known to date has such unique gap features, although copper-oxides and MgB2
share part of them.Comment: submitted on May 1
Point-Contact Spectroscopy of Iron-Based Layered Superconductor LaOFFeAs
We present point-contact spectroscopy data for junctions between a normal
metal and the newly discovered F-doped superconductor
LaOFFeAs (F-LaOFeAs). A zero-bias conductance peak was
observed and its shape and magnitude suggests the presence of Andreev bound
states at the surface of F-LaOFeAs, which provides a possible evidence of an
unconventional pairing symmetry with a nodal gap function. The maximum gap
value meV was determined from the measured spectra,
in good agreement with the recent experiments on specific heat and lower
critical field.Comment: 5 pages, 4 figure
Point contact Andreev reflection spectroscopy of NdFeAsO_0.85
The newly discovered oxypnictide family of superconductors show very high
critical temperatures of up to 55K. Whilst there is growing evidence that
suggests a nodal order parameter, point contact Andreev reflection spectroscopy
can provide crucial information such as the gap value and possibly the number
of energy gaps involved. For the oxygen deficient NdFeAsO0.85 with a Tc of
45.5K, we show that there is clearly a gap value at 4.2K that is of the order
of 7meV, consistent with previous studies on oxypnictides with lower Tc.
Additionally, taking the spectra as a function of gold tip contact pressure
reveals important changes in the spectra which may be indicative of more
complex physics underlying this structure.Comment: 11 pages, 3 figures. New references included, extra discussion. This
version is accepted in Superconductor Science and Technolog