17 research outputs found
Tuning Co valence state in cobalt oxyhydrate superconductor by post reduction
We report a successful tuning of Co valence state in cobalt oxyhydrate
superconductor via a facile post reduction using NaOH as reducing agent. The
change in Co valence was precisely determined by measuring the volume of the
released oxygen. The possible hydronium-incorporation was greatly suppressed in
concentrated NaOH solution, making the absolute Co valence determinable. As a
result, an updated superconducting phase diagram was obtained, which shows that
the superconducting transition temperature increases monotonically with
increasing Co valence in a narrow range from +3.58 to +3.65.Comment: 17 pages, 5 figures and 1 table. Chem. Mat. in pres
Spin gap and magnetic resonance in superconducting BaFeNiAs
We use neutron spectroscopy to determine the nature of the magnetic
excitations in superconducting BaFeNiAs ( K).
Above the excitations are gapless and centered at the commensurate
antiferromagnetic wave vector of the parent compound, while the intensity
exhibits a sinusoidal modulation along the c-axis. As the superconducting state
is entered a spin gap gradually opens, whose magnitude tracks the
-dependence of the superconducting gap observed by angle resolved
photoemission. Both the spin gap and magnetic resonance energies are
temperature \textit{and} wave vector dependent, but their ratio is the same
within uncertainties. These results suggest that the spin resonance is a
singlet-triplet excitation related to electron pairing and superconductivity.Comment: 4 pages, 4 figure
Proton incorporations and superconductivity in a cobalt oxyhydrate
We report the evidence of proton incorporations in a newly-discovered cobalt
oxyhydrate superconductor. During the hydration process for
NaCoO by the direct reaction with water liquid, it was shown
that substantial NaOH was gradually liberated, indicating that H is
incorporated into the hydrated compound. Combined with the thermogravimetric
analysis, the chemical composition of the typical sample is
NaHCoOHO, which shows bulk
superconductivity at 4.4 K.Comment: 9 pages, 4 figure
CeNiAsO: an antiferromagnetic dense Kondo lattice
A cerium containing pnictide, CeNiAsO, crystallized in the ZrCuSiAs type
structure, has been investigated by measuring transport and magnetic
properties, as well as specific heat. We found that CeNiAsO is an
antiferromagnetic dense Kondo lattice metallic compound with Kondo scale 15 K and shows an enhanced Sommerfeld coefficient of 203
mJ/molK. While no superconductivity can been observed down to 30
mK, Ce ions exhibit two successive antiferromagnetic (AFM) transitions. We
propose that the magnetic moment of Ce ion could align in the G type AFM order
below the first transition at =9.3 K, and it might be modified into the
C type AFM order below a lower transition at =7.3 K. Our results
indicate that the 3 interlayer Kondo interactions play an important role
in Ni-based Ce-containing pnictide.Comment: 13 pages, 5 figures, to appear in J. Phys.: Condens. Matte
Effect of Zn doping on magnetic order and superconductivity in LaFeAsO
We report Zn-doping effect in the parent and F-doped LaFeAsO oxy-arsenides.
Slight Zn doping in LaFeZnAsO drastically suppresses the
resistivity anomaly around 150 K associated with the antiferromagnetic (AFM)
spin density wave (SDW) in the parent compound. The measurements of magnetic
susceptibility and thermopower confirm further the effect of Zn doping on AFM
order. Meanwhile Zn doping does not affect or even enhances the of
LaFeZnAsOF, in contrast to the effect of Zn
doping in high- cuprates. We found that the solubility of Zn content ()
is limited to less than 0.1 in both systems and further Zn doping (i.e.,
0.1) causes phase separation. Our study clearly indicates that the
non-magnetic impurity of Zn ions doped in the FeAs layers
affects selectively the AFM order, and superconductivity remains robust against
the Zn doping in the F-doped superconductors.Comment: 7 figures, 13 pages; revised version with more dat
Three-dimensional Resonance in superconducting BaFeNiAs
We use inelastic neutron scattering to study magnetic excitations of the
FeAs-based superconductor BaFeNiAs above and below its
superconducting transition temperature K. In addition to gradually
open a spin gap at the in-plane antiferromagnetic ordering wavevector
, the effect of superconductivity is to form a three dimensional
resonance with clear dispersion along the c-axis direction. The intensity of
the resonance develops like a superconducting order parameter, and the mode
occurs at distinctively different energies at and . If the
resonance energy is directly associated with the superconducting gap energy
, then is dependent on the wavevector transfers along the
c-axis. These results suggest that one must be careful in interpreting the
superconducting gap energies obtained by surface sensitive probes such as
scanning tunneling microscopy and angle resolved photoemission.Comment: 5 pages, 4 figure
Heavy Fermion Quantum Criticality and Destruction of the Kondo Effect in a Nickel Oxypnictide
A quantum critical point arises at a continuous transformation between
distinct phases of matter at zero temperature. Studies in antiferromagnetic
heavy fermion materials have revealed that quantum criticality has several
classes, with an unconventional type that involves a critical destruction of
the Kondo entanglement. In order to understand such varieties, it is important
to extend the materials basis beyond the usual setting of intermetallic
compounds. Here we show that a nickel oxypnictide, CeNiAsO, displays a
heavy-fermion antiferromagnetic quantum critical point as a function of either
pressure or P/As substitution. At the quantum critical point, non-Fermi liquid
behavior appears, which is accompanied by a divergent effective carrier mass.
Across the quantum critical point, the low-temperature Hall coefficient
undergoes a rapid sign change, suggesting a sudden jump of the Fermi surface
and a destruction of the Kondo effect. Our results imply that the enormous
materials basis for the oxypnictides, which has been so crucial to the search
for high temperature superconductivity, will also play a vital role in the
effort to establish the universality classes of quantum criticality in strongly
correlated electron systems.Comment: 4 figures, Supplementary Information on NPG websit