30 research outputs found
Topotactic synthesis of a new BiS2-based superconductor Bi2(O,F)S2
A new BiS2-based superconductor Bi2(O,F)S2 was discovered. This is a layered
compound consisting of alternate stacking structure of rock-salt-type BiS2
superconducting layer and fluorite-type Bi(O,F) blocking layer. Bi2(O,F)S2 was
obtained as the main phase by topotactic fluorination of undoped Bi2OS2 using
XeF2, which is the first topotactic synthesis of an electron-doped
superconductor via reductive fluorination. With increasing F-content, a- and
c-axis length increased and decreased, respectively, and Tc increased up to 5.1
K.Comment: 9 pages, 4 figure
New iron-based arsenide oxides (Fe2As2)(Sr4M2O6)(M = Sc, Cr)
We have discovered new layered oxyarsenides (Fe2As2)(Sr4M2O6) (M = Sc, Cr:
M-22426). These materials are isostructural with (Fe2P2)(Sr4Sc2O6), which was
found in our previous study. The new compounds are tetragonal with a space
group of P4/nmm and consist of the anti-fluorite type FeAs layer and
perovskite-type blocking layer. The lattice constants are a = 4.050 A, c =
15.809 A for M = Sc and a = 3.918 A, c = 15.683 A for M = Cr. These compounds
have long interlayer Fe-Fe distances corresponding to the c-axis length, the
15.8 A in Sc-22426 is the longest in the iron-based oxypnictide systems.
Chemical flexibility of the perovskite block in this system was probed by
chromium containing (Fe2As2)(Sr4Cr2O6). Different trends were found in bond
angle and bond length of the new oxypnictides compared to the reported systems,
such as REFePnO. Absence of superconductivity in these compounds is considered
to be due to insufficient carrier concentration as in the case of undoped
REFeAsO.Comment: 7 pages, 5 figures, 1 tabl
Strongly connected ex-situ MgB2 polycrystalline bulks fabricated by solid-state self-sintering
We have investigated the microstructure, normal-state electrical
connectivity, and critical current density of ex-situ MgB2 polycrystalline
bulks prepared by systematically varying the sintering conditions under low
pressure. Samples heated at a high temperature of ~900{\deg}C for a long period
showed an increased packing factor, a larger intergrain contact area, and a
significantly enhanced electrical connectivity, all of which indicate
solid-state self-sintering of MgB2. Sintered ex-situ MgB2 bulks from a
laboratory-made ball-milled powder exhibited a greatly enhanced connectivity of
28%, which is the highest connectivity of pressureless ex-situ MgB2 bulks,
wires, and tapes. Surprisingly, grain growth did not occur during long-duration
(~100 h) sintering in the sintered ex-situ MgB2 bulks. This is in marked
contrast to in-situ processed MgB2 samples for which significant grain growth
occurred during heat treatment at ~900{\deg}C, producing grains that are
several tens of times larger than the initial boron grains. Consequently, the
critical current density as a function of the external magnetic field at 20 K
progressively improved with sintering due to the relatively small grain size
and good intergrain connectivity. We thus conclude that solid-state
self-sintering is an effective approach for producing strongly connected, dense
ex-situ MgB2 polycrystals without grain growth.Comment: 21 pages, 9 figures, to be published in Supercond. Sci. Techno
Superconductivity Above 40 K Observed in a New Iron Arsenide Oxide (Fe2As2)(Ca4(Mg,Ti)3Oy)
A new layered iron arsenide oxide (Fe2As2)(Ca4(Mg,Ti)3Oy) was discovered. Its
crystal structure is tetragonal with a space group of I4/mmm consisted of the
anti-fluorite type FeAs layer and blocking layer of triple perovskite cells and
is identical with (Fe2As2)(Sr4(Sc,Ti)3O8) discovered in our previous study. The
lattice constants of (Fe2As2)(Ca4(Mg,Ti)3Oy) are a = 3.877 A and c = 33.37 A.
This compound exhibited bulk superconductivity up to 43 K in susceptibility
measurement without intentional carrier doping. A resistivity drop was observed
at ~47 K and zero resistance was achieved at 42 K. These values correspond to
the second highest Tc among the layered iron-based superconductors after
REFeAsO system
New Series of Nickel-Based Pnictide Oxide Superconductors (Ni2Pn2)(Sr4Sc2O6) (Pn = P, As)
We have discovered new nickel-based pnictide oxide superconductors,
(Ni2Pn2)(Sr4Sc2O6) (Pn = P, As). These compounds have a tetragonal unit cell
with a space group of P4/nmm and they consist of alternate stacking of
anti-fluorite Ni2Pn2 layers and K2NiF4-type Sr4Sc2O6 blocking layers. Lattice
parameters were a = 4.044 A and c = 15.23 A for (Ni2P2)(Sr4Sc2O6) and a = 4.078
A and c = 15.41 A for (Ni2As2)(Sr4Sc2O6), indicating their thicker blocking
layers than that of LaNiPO (c ~ 8.1 A). Both (Ni2P2)(Sr4Sc2O6) and
(Ni2As2)(Sr4Sc2O6) exhibited superconductivity with zero resistivity at 3.3 K
and 2.7 K, respectively. The perfect diamagnetism observed in both compounds
guaranteed their bulk superconductivity.Comment: 4 pages, 3 figures, 1 tabl
Co-co-doping effect for (Ca,RE)FeAs2 sintered bulk
Superconducting properties of Co-co-doped (Ca,RE)FeAs2 ((Ca,RE)112: RE = La,
Pr) were investigated. Co-co-doping increased Tc of (Ca,Pr)112 while
Mn-co-doping suppressed superconductivity of (Ca,RE)112. Co-co-doped (Ca,La)112
showed large diamagnetic screening and sharper superconducting transition than
Co-free (Ca,La)112. Tczero observed in resistivity measurements increased from
14 K to 30 K by Co-co-doping, while Tconset was not increased. The critical
current density (Jc) of Co-co-doped (Ca,La)112 were approximately 2.1 x 104
Acm-2 and 3.2 x 103 Acm-2 at 2 K and 25 K, respectively, near zero field. These
relatively high Jcs and large diamagnetic screening observed in susceptibility
measurement as for polycrystalline bulks suggest bulk superconductivity of
Co-co-doped (Ca,RE)112 compounds
First Homologous Series of Iron Pnictide Oxide Superconductors (Fe2As2)(Can+1(Sc,Ti)nOy) [n = 3,4,5] with Extremely Thick Blocking Layers
We have discovered first homologous series of iron pnictide oxide
superconductors (Fe2As2)(Can+1(Sc,Ti)nOy) [n = 3,4,5]. These compounds have
extremely thick blocking layers up to quintuple perovskite oxide layers
sandwiched by the Fe2As2 layers. These samples exhibited bulk superconductivity
with relatively high Tc up to 42 K. The relationship between Tc and the
iron-plane interlayer distance suggested that superconductivity due to the mono
Fe2As2 layer is substantially 40 K-class.Comment: 11pages, 5figure
Effects of post-annealing and cobalt co-doping on superconducting properties of (Ca,Pr)Fe2As2 single crystals
In order to clarify the origin of anomalous superconductivity in
(Ca,RE)Fe2As2 system, Pr doped and Pr,Co co-doped CaFe2As2 single crystals were
grown by the FeAs flux method. These samples showed two-step superconducting
transition with Tc1 = 25~42 K, and Tc2 < 16 K, suggesting that (Ca,RE)Fe2As2
system has two superconducting components. Post-annealing performed for these
crystals in evacuated quartz ampoules at various temperatures revealed that
post-annealing at ~400{\deg}C increased the c-axis length for all samples. This
indicates that as-grown crystals have a certain level of strain, which is
released by post-annealing at ~400{\deg}C. Superconducting properties also
changed dramatically by post-annealing. After annealing at 400{\deg}C, some of
the co-doped samples showed large superconducting volume fraction corresponding
to the perfect diamagnetism below Tc2 and high Jc values of 104~105 Acm-2 at 2
K in low field, indicating the bulk superconductivity of (Ca,RE)Fe2As2 phase
occurred below Tc2. On the contrary, the superconducting volume fraction above
Tc2 was always very small, suggesting that 40 K-class superconductivity
observed in this system is originating in the local superconductivity in the
crystal
Second Homologous Series of Iron Pnictide Oxide Superconductors (Fe2As2)(Can+2(Al,Ti)nOy)[n = 2,3,4]
We have discovered a new homologous series of iron pnictide oxides
(Fe2As2)(Can+2(Al,Ti)nOy)[n = 2,3,4]. These compounds have perovskite-like
blocking layers between Fe2As2 layers. The structure of new compounds are
tetragonal with space groups of P4/nmm for n = 2 and 4 and P4mm for n = 3,
which are similar to those of (Fe2As2)(Can+1(Sc,Ti)nOy)[n = 3,4,5] found in our
previous study. Compounds with n = 3 and 4 have new crystal structures with 3
and 4 sheets of perovskite layers, respectively, including a rock salt layer in
each blocking layer. The a-axis lengths of the three compounds are
approximately 3.8 A, which are close to those of FeSe and LiFeAs.
(Fe2As2)(Ca6(Al,Ti)4Oy) exhibited bulk superconductivity in magnetization
measurement with Tc(onset)~36 K and resistivity drop was observed at ~39 K.
(Fe2As2)(Ca5(Al,Ti)3Oy) also showed large diamagnetism at low temperatures.
These new compounds indicate considerable rooms are still remaining for new
superconductors in layered iron pnictides.Comment: 10 pages, 5 figure
Pressure Effects on Superconducting Properties of the BiS2-Based Superconductor Bi2(O,F)S2
Pressure effects on a recently discovered BiS2-based superconductor
Bi2(O,F)S2 (Tc = 5.1 K) were examined via two different methods; high pressure
resistivity measurement and high pressure annealing. The effects of these two
methods on the superconducting properties of Bi2(O,F)S2 were significantly
different although in both methods hydrostatic pressure is applied to the
sample by the cubic-anvil-type apparatus. In high pressure resistivity
measurement, Tc linearly decreased at the rate of -1.2 K GPa-1. In contrast,
the Tc of 5.1 K is maintained after high pressure annealing under 2 GPa and
470{\deg}C of optimally doped sample despite significant change of lattice
parameters. In addition, superconductivity was observed in fluorine-free Bi2OS2
after high pressure annealing. These results suggest that high pressure
annealing would cause a unique effect on physical properties of layered
compounds.Comment: 8 pages, 6 figure