1,636 research outputs found
Evidence for Unconventional Superconductivity in Arsenic-Free Iron-Based Superconductor FeSe : A ^77Se-NMR Study
We report the results of Se--nuclear magnetic resonance (NMR) in
-FeSe, which exhibits a similar crystal structure to the
LaFeAsOF superconductor and shows superconductivity at 8 K. The
nuclear-spin lattice relaxation rate shows behavior below the
superconducting transition temperature without a coherence peak. The
const. behavior, indicative of the Fermi liquid state, can be seen in a
wide temperature range above . The superconductivity in -FeSe is
also an unconventional one as well as LaFeAsOF and related
materials. The FeAs layer is not essential for the occurrence of the
unconventional superconductivity.Comment: 4pages, 4figures, to be published in J. Phys. Soc. Jpn. 77 No.11
(2008
Scattering states of a particle, with position-dependent mass, in a symmetric heterojunction
The study of a particle with position-dependent effective mass (pdem), within
a double heterojunction is extended into the complex domain --- when the region
within the heterojunctions is described by a non Hermitian
symmetric potential. After obtaining the exact analytical solutions, the
reflection and transmission coefficients are calculated, and plotted as a
function of the energy. It is observed that at least two of the characteristic
features of non Hermitian symmetric systems --- viz., left / right
asymmetry and anomalous behaviour at spectral singularity, are preserved even
in the presence of pdem. The possibility of charge conservation is also
discussed.Comment: 12 pages, including 6 figures; Journal of Physics A : Math. Theor.
(2012
Intrinsic Properties of AFe2As2 (A = Ba, Sr) Single Crystal under Highly Hydrostatic Pressure Conditions
We measured the electrical resistivity and ac magnetic susceptibility of
BaFe2As2 and SrFe2As2 single crystals under pressure using a cubic anvil
apparatus. For BaFe2As2, the antiferromagnetic (AF) and structural transitions
are suppressed with increasing pressure. Unexpectedly, these transitions
persist up to 8 GPa, and no signature of a superconducting transition was
observed in the pressure range investigated here. On the other hand, the AF and
structural transitions of SrFe2As2 collapse at around the critical pressure Pc
~ 5 GPa, resulting in the appearance of bulk superconductivity. The
superconducting volume fraction abruptly increases above Pc, and shows a dome
centered at approximately 6 GPa. Our results suggest that the bulk
superconducting phase competes with the AF/orthorhombic phase and only appears
in the narrow pressure region of the tetragonal phase.Comment: 4 pages, 4 figures; accepted for publication in J. Phys. Soc. Jp
Mechanism of spin crossover in LaCoO3 resolved by shape magnetostriction in pulsed magnetic fields
In the scientific description of unconventional transport properties of
oxides (spin-dependent transport, superconductivity etc.), the spin-state
degree of freedom plays a fundamental role. Because of this, temperature- or
magnetic field-induced spin-state transitions are in the focus of solid-state
physics. Cobaltites, e.g. LaCoO3, are prominent examples showing these spin
transitions. However, the microscopic nature of the spontaneous spin crossover
in LaCoO3 is still controversial. Here we report magnetostriction measurements
on LaCoO3 in magnetic fields up to 70 T to study the sharp, field-induced
transition at Hc ~ 60 T. Measurements of both longitudinal and transversal
magnetostriction allow us to separate magnetovolume and magnetodistortive
changes. We find a large increase in volume, but only a very small increase in
tetragonal distortion at Hc. The results, supported by electronic energy
calculations by the configuration interaction cluster method, provide
compelling evidence that above Hc LaCoO3 adopts a correlated low spin/high spin
state.Comment: 14 pages, 4 figure
Possible superconductivity above 25 K in single crystalline Co-doped BaFeAs
We present superconducting properties of single crystalline
Ba(FeCo)As by measuring magnetization, resistivity,
upper critical field, Hall coefficient, and magneto-optical images. The
magnetization measurements reveal fish-tail hysteresis loop at high
temperatures and relatively high critical current density above
A/cm at low temperatures. Upper critical field determined by resistive
transition is anisotropic with anisotropic parameter 3.5. Hall effect
measurements indicate that Ba(FeCo)As is a
multiband system and the mobility of electron is dominant. The magneto-optical
imaging reveals prominent Bean-like penetration of vortices although there is a
slight inhomogeneity in a sample. Moreover, we find a distinct
superconductivity above 25 K, which leads us to speculate that higher
transition temperature can be realized by fine tuning Co-doping level.Comment: 4 pages, 5 figure
Effect of K Doping on Phonons in Ba1-xKxFe2As2
The lattice dynamics of Ba1-xKxFe2As2 (x = 0.00, 0.27) have been studied by
inelastic X-ray scattering measurement at room temperature. K doping induces
the softening and broadening of phonon modes in the energy range E = 10-15 meV.
Analysis with a Born-von Karman force-constant model indicates that the
softening results from reduced interatomic force constants around (Ba,K) sites
following the displacement of divalent Ba by monovalent K. The phonon
broadening may be explained by the local distortions induced by the K
substitution. Extra phonon modes are observed around the wave vector q =
(0.5,0,0) at E = 16.5 meV for the x = 0.27 sample. These modes may arise either
from the local disorder induced by K doping or from electron-phonon coupling.Comment: J. Phys. Soc. Jpn. (in press
Pressure-induced superconductivity in Iron pnictide compound SrFe2As2
Electrical resistivity under high pressure have been measured on nominally
pure SrFe2As2 up to 14 GPa. The resistivity drop appeared with increasing
pressure, and we clearly observed zero resistivity. The maximum of
superconducting transition temperature (Tc) is 38 K. The value is corresponding
to the one of optimally doping AFe2As2 (A=Sr, Ba) system with K+ ions at the
A2+ site.Comment: 10 pages, 2 figure
Pressure-induced superconductivity in EuCaFeAs : FeAs-based superconductivity hidden by antiferromagnetism of Eu sublattice
To clarify superconductivity in EuFe2As2 hidden by antiferromagnetism of
Eu2+, we investigated a Ca-substituted sample, Eu0.5Ca0.5Fe2As2, under high
pressure. For ambient pressure, the sample exhibits a spin-density-wave (SDW)
transition at TSDW = 191 K and antiferromagnetic order at TN = 4 K, but no
evidence of superconductivity down to 2 K. The Ca-substitution certainly
weakens the antiferromagnetism. With increasing pressure, TSDW shifts to lower
temperature and becomes more unclear. Above 1.27 GPa, pressure-induced
superconductivity with zero resistivity is observed at around Tc = 20 K. At
2.14 GPa, Tc reaches a maximum value of 24 K and the superconducting transition
becomes the sharpest. These features of emergence of the superconductivity are
qualitatively similar to those observed in AFe2As2 (A = Ba, Ca).Comment: 4 pages, 4 figure
Iron pnictides: Single crystal growth and effect of doping on structural, transport and magnetic properties
We demonstrate the preparation of large, free standing iron pnictide single
crystals with a size up to 20 x 10 x 1 mm3 using solvents in zirconia crucibles
under argon atmosphere. Transport and magnetic properties are investigated to
study the effect of potassium doping on the structural and superconducting
property of the compounds. The spin density wave (SDW) anomaly at Ts ~138 K in
BaFe2As2 single crystals from self-flux shifts to Ts ~85 K due to Sn solvent
growth. We show direct evidence for an incorporation of Sn on the Fe site. The
electrical resistivity data show a sharp superconducting transition temperature
Tc~38.5 K for the single crystal of Ba0.68K0.32Fe2As2. A nearly 100% shielding
fraction and bulk nature of the superconductivity for the single crystal were
confirmed by magnetic susceptibility data. A sharp transition Tc~25 K occurred
for the single crystal of Sr0.85K0.15Fe2As2. There is direct evidence for a
coexistence of the SDW and superconductivity in the low doping regime of
Sr1-xKxFe2As2 single crystals. Structural implications of the doping effects as
well as the coexistence of the two order parameters are discussed.Comment: 22 pages, 9 figure
Superconductivity at 38 K in the iron arsenide (Ba1-xKx)Fe2As2
The ternary iron arsenide BaFe2As2 becomes superconducting by hole doping,
which was achieved by partial substitution of the barium site with potassium.
We have discovered bulk superconductivity up to Tc = 38 K in (Ba1-xKx)Fe2As2
with x = 0.4. The parent compound BaFe2As2 as well as KFe2As2 both crystallize
in the tetragonal ThCr2Si2-type structure, which consists of (FeAs)- iron
arsenide layers separated by barium or potassium ions. BaFe2As2 is a poor metal
and exhibits a SDW anomaly at 140 K. By substituting Ba2+ for K+ ions we have
introduced holes in the (FeAs)- layers, which suppress the SDW anomaly and
induce superconductivity. This scenario is very similar to the recently
discovered arsenide-oxide superconductors. The Tc of 38 K in (Ba1-xKx)Fe2As2 is
the highest observed critical temperature in hole doped iron arsenide
superconductors so far. Therefore, we were able to expand this class of
superconductors by oxygen-free compounds with the ThCr2Si2-type structure. Our
results suggest, that superconductivity in these systems essentially evolves
from the (FeAs)- layers and may occur in other related compounds.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let
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