2,172 research outputs found
Superconductivity at 41 K and its competition with spin-density-wave instability in layered CeOFFeAs
A series of layered CeOFFeAs compounds with x=0 to 0.20 are
synthesized by solid state reaction method. Similar to the LaOFeAs, the pure
CeOFeAs shows a strong resistivity anomaly near 145 K, which was ascribed to
the spin-density-wave instability. F-doping suppresses this instability and
leads to the superconducting ground state. Most surprisingly, the
superconducting transition temperature could reach as high as 41 K. The very
high superconducting transition temperature strongly challenges the classic BCS
theory based on the electron-phonon interaction. The very closeness of the
superconducting phase to the spin-density-wave instability suggests that the
magnetic fluctuations play a key role in the superconducting paring mechanism.
The study also reveals that the Ce 4f electrons form local moments and ordered
antiferromagnetically below 4 K, which could coexist with superconductivity.Comment: 4 pages, 5 figure
Strong-coupling superconductivity in layered nickel-based LaOFNiAs
A series of layered nickel-based LaOFNiAs compounds with x=0 to
0.15 are synthesized by solid state reactions. The pure LaONiAs exhibits bulk
superconductivity with Tc 2.75 K. Partial substitution of oxygen by
fluorine increases the transition temperature to K. The
LaOFNiAs sample shows a sharp superconducting transition and a
sharp specific heat jump at the critical temperature. The magnitude of the
specific heat jump is much larger than that expected from the weak-coupling BCS
theory, indicating that this superconductor is in the strong coupling regime.
Furthermore, the temperature dependence of the specific heat deviates strongly
from the theoretical result for the single-band s- or d-wave superconductor,
but shows some character of a multi-gap system.Comment: 4 pages, 4 figure
Superconducting properties of Fe-based layered superconductor LaOFFeAs
We have employed a new route to synthesize single phase F-doped LaOFeAs
compound and confirmed the superconductivity above 20 K in this Fe-based
system. We show that the new superconductor has a rather high upper critical
field of about 54 T. A clear signature of superconducting gap opening below
T was observed in the far-infrared reflectance spectra, with
23.5-4.2. Furthermore, we show that the new
superconductor has electron-type conducting carrier with a rather low carrier
density.Comment: 4 pages, 5 figures, Phys. Rev. Lett. (accepted
Superconductivity and Phase Diagram in the Iron-based Arsenic-oxides ReFeAsO1-delta (Re = rare earth metal) without F-Doping
Here we report a new class of superconductors prepared by high pressure
synthesis in the quaternary family ReFeAsO1-delta (Re = Sm, Nd, Pr, Ce, La)
without fluorine doping. The onset superconducting critical temperature (Tc) in
these compounds increases with the reduction of Re atom size, and the highest
Tc obtained so far is 55 K in SmFeAsO1-delta. For the NdFeAsO1-delta system
with different oxygen concentration a dome-shaped phase diagram was found
Nernst effect of the new iron-based superconductor LaOFFeAs
We report the first Nernst effect measurement on the new iron-based
superconductor LaOFFeAs . In the normal state, the
Nernst signal is negative and very small. Below a large positive peak
caused by vortex motion is observed. The flux flowing regime is quite large
compared to conventional type-II superconductors. However, a clear deviation of
the Nernst signal from normal state background and an anomalous depression of
off-diagonal thermoelectric current in the normal state between and 50
K are observed. We propose that this anomaly in the normal state Nernst effect
could correlate with the SDW fluctuations.Comment: 8 pages, 4 figures; Latex file changed, references adde
Superconductivity at 53.5 K in GdFeAsO1-delta
Here we report the fabrication and superconductivity of the iron-based
arsenic-oxide GdFeAsO1-delta compound with oxygen-deficiency, which has an
onset resistivity transition temperature at 53.5 K. This material has a same
crystal structure as the newly discovered high-Tc ReFeAsO1-delta family (Re =
rare earth metal) and a further reduced crystal lattice, while the Tc starts to
decrease compared with the SmFeAsO1-delta system
Pressure Effect on the superconducting properties of LaO_{1-x}F_{x}FeAs(x=0.11) superconductor
Diamagnetic susceptibility measurements under high hydrostatic pressure (up
to 1.03 GPa) were carried out on the newly discovered Fe-based superconductor
LaO_{1-x}F_{x}FeAs(x=0.11). The transition temperature T_c, defined as the
point at the maximum slope of superconducting transition, was enhanced almost
linearly by hydrostatic pressure, yielding a dT_c/dP of about 1.2 K/GPa.
Differential diamagnetic susceptibility curves indicate that the underlying
superconducting state is complicated. It is suggested that pressure plays an
important role on pushing low T_c superconducting phase toward the main
(optimal) superconducting phase.Comment: 7 pages, 4 figure
Growth and characterization of A_{1-x}K_xFe_2As_2 (A = Ba, Sr) single crystals with x=0 - 0.4
Single crystals of AKFeAs (A=Ba, Sr) with high quality
have been grown successfully by FeAs self-flux method. The samples have sizes
up to 4 mm with flat and shiny surfaces. The X-ray diffraction patterns suggest
that they have high crystalline quality and c-axis orientation. The
non-superconducting crystals show a spin-density-wave (SDW) instability at
about 173 K and 135 K for Sr-based and Ba-based compound, respectively. After
doping K as the hole dopant into the BaFeAs system, the SDW transition
is smeared, and superconducting samples with the compound of
BaKFeAs (0 0.4) are obtained. The
superconductors characterized by AC susceptibility and resistivity measurements
exhibit very sharp superconducting transition at about 36 K, 32 K, 27 K and 23
K for x= 0.40,0.28,0.25 and 0.23, respectively.Comment: 9 pages, 6 figures, 1 table. This paper together with new data are
modified into a new pape
57Fe Mossbauer spectroscopy and magnetic measurements of oxygen deficient LaFeAsO
We report on the magnetic behavior of oxygen deficient LaFeAsO1-x (x-0.10)
compound, prepared by one-step synthesis, which crystallizes in the tetragonal
(S.G. P4/nmm) structure at room temperature. Resistivity measurements show a
strong anomaly near 150 K, which is ascribed to the spin density wave (SDW)
instability. On the other hand, dc magnetization data shows paramagnetic-like
features down to 5 K, with an effective moment of 0.83 mB/Fe. 57Fe Mossbauer
studies (MS) have been performed at 95 and 200 K. The spectra at both
temperatures are composed of two sub-spectra. At 200 K the major one (88%), is
almost a singlet, and corresponds to those Fe nuclei, which have two oxygen
ions in their close vicinity. The minor one, with a large quadrupole splitting,
corresponds to Fe nuclei, which have vacancies in their immediate neighborhood.
The spectrum at 95 K, exhibits a broadened magnetic split major (84%)
sub-spectrum and a very small magnetic splitting in the minor subspectrum. The
relative intensities of the subspectra facilitate in estimating the actual
amount of oxygen vacancies in the compound to be 7.0(5)%, instead of the
nominal LaFeAsO0.90. These results, when compared with reported 57Fe MS of
non-superconducting LaFeAsO and superconducting LaFeAsO0.9F0.1, confirm that
the studied LaFeAsO0.93 is a superconductivity-magnetism crossover compound of
the newly discovered Fe based superconducting family.Comment: 7 pages text + Figs : Comments/suggestions welcome
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