389 research outputs found
BaFe2Se2O as an Iron-Based Mott Insulator with Antiferromagnetic Order
A new compound with a quasi-two-dimensional array of FeSe3O tetrahedra and an
orthorombic structure, namely BaFe2Se2O, has been successfully fabricated.
Experimental results show that this compound is an insulator and has an
antiferromagnetic (AF) transition at 240 K. Band structure calculation reveals
the narrowing of Fe 3d bands near the Fermi energy, which leads to the
localization of magnetism and the Mott insulating behavior. The large distances
between the Fe atoms perhaps are responsible for the characters. Linear
response calculation further indicates a strong in-plane AF exchange , this
can account for the enhanced magnetic susceptibility (which has a maximum at
about 450 K) above the Neel temperature.Comment: submitted to PRL on 2 May 2012, resubmitted to PRB on 31 May 2012,
and accepted by PRB on 5 July 201
Vortex images on Ba{1-x}KxFe2As2 observed directly by the magnetic force microscopy
The vortex states on optimally doped Ba0.6K0.4Fe2As2 and underdoped
Ba0.77K0.23Fe2As2 single crystals are imaged by magnetic force microscopy at
various magnetic fields below 100 Oe. Local triangular vortex clusters are
observed in optimally doped samples. The vortices are more ordered than those
in Ba(Fe{1-x}Co{x})2As2, and the calculated pinning force per unit length is
about 1 order of magnitude weaker than that in optimally Co-doped 122 at the
same magnetic field, indicating that the Co doping at the Fe sites induces
stronger pinning. The proportion of six-neighbored vortices to the total amount
increases quickly with increasing magnetic field, and the estimated value
reaches 100% at several tesla. Vortex chains are also found in some local
regions, which enhance the pinning force as well as the critical current
density. Lines of vortex chains are observed in underdoped samples, and they
may have originated from the strong pinning near the twin boundaries arising
from the structural transition.Comment: 7 pages, 8 figure
Transition of stoichiometricSr2VO3FeAs to a superconducting state at 37.2 K
The superconductor Sr4V2O6Fe2As2 with transition temperature at 37.2 K has
been fabricated. It has a layered structure with the space group of p4/nmm, and
with the lattice constants a = 3.9296Aand c = 15.6732A. The observed large
diamagnetization signal and zero-resistance demonstrated the bulk
superconductivity. The broadening of resistive transition was measured under
different magnetic fields leading to the discovery of a rather high upper
critical field. The results also suggest a large vortex liquid region which
reflects high anisotropy of the system. The Hall effect measurements revealed
dominantly electron-like charge carriers in this material. The
superconductivity in the present system may be induced by oxygen deficiency or
the multiple valence states of vanadium.Comment: 5 pages, 4 figure
Superconductivity at 15.6 K in Calcium-doped Tb_{1-x}Ca_xFeAsO: the structure requirement for achieving superconductivity in the hole-doped 1111 phase
Superconductivity at about 15.6 K was achieved in Tb_{1-x}Ca_xFeAsO by
partially substituting Tb^{3+} with Ca^{2+} in the nominal doping region x =
0.40 \sim 0.50. A detailed investigation was carried out in a typical sample
with doping level of x = 0.44. The upper critical field of this sample was
estimated to be 77 Tesla from the magnetic field dependent resistivity data.
The domination of hole-like charge carriers in the low-temperature region was
confirmed by Hall effect measurements. The comparison between the calcium-doped
sample Pr_{1-x}Ca_xFeAsO (non-superconductive) and the Strontium-doped sample
Pr_{1-x}Sr_xFeAsO (superconductive) suggests that a lager ion radius of the
doped alkaline-earth element compared with that of the rare-earth element may
be a necessary requirement for achieving superconductivity in the hole-doped
1111 phase.Comment: 7 pages, 7 figure
Doping effect of Cu and Ni impurities on the Fe-based superconductor Ba0.6K0.4Fe2As2
Copper and Nickel impurities have been doped into the iron pnictide
superconductor Ba0.6K0.4Fe2As2. Resistivity measurements reveal that Cu and Ni
impurities suppress superconducting transition temperature T_c with rates of
\Delta T_c/Cu-1% = -3.5 K and \Delta T_c/Ni-1% = -2.9 K respectively.
Temperature dependence of Hall coefficient R_H of these two series of samples
show that both Cu-doping and Ni-doping can introduce electrons into
Ba0.6K0.4Fe2As2. With more doping, the sign of R_H gradually changes from
positive to negative, while the changing rate of Cu-doped samples is much
faster than that of Ni-doped ones. Combining with the results of
first-principles calculations published previously and the non-monotonic
evolution of the Hall coefficient in the low temperature region, we argue that
when more Cu impurities were introduced into Ba0.6K0.4Fe2As2, the removal of
Fermi spectral weight in the hole-like Fermi surfaces is much stronger than
that in the electron-like Fermi surfaces, which is equivalent to significant
electron doping effect. DC magnetization and the lattice constants analysis
reveal that static magnetic moments and notable lattice compression have been
formed in Cu-doped samples. It seems that the superconductivity can be
suppressed by the impurities disregard whether they are magnetic or nonmagnetic
in nature. This gives strong support to a pairing gap with a sign reversal,
like S^\pm. However, the relatively slow suppression rates of T_c show the
robustness of superconductivity of Ba0.6K0.4Fe2As2 against impurities, implying
that multi-pairing channels may exist in the system.Comment: 7 pages, 7 figure
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