1,181 research outputs found
Superconductivity up to 29 K in SrFe2As2 and BaFe2As2 at high pressures
We report the discovery of superconductivity at high pressure in SrFe2As2 and
BaFe2As2. The superconducting transition temperatures are up to 27 K in
SrFe2As2 and 29 K in BaFe2As2, making these the highest pressure-induced
superconducting materials discovered thus far.Comment: Accepted in Journal of Physics: Condensed Matte
Superconductivity at 23 K in Pt doped BaFe2As2 single crystals
We report superconductivity in single crystals of the new iron-pnictide
system BaFe1.9Pt0.1As2 grown by a self-flux solution method and characterized
via x-ray, transport, magnetic and thermodynamic measurements. The magnetic
ordering associated with a structural transition at 140 K present in BaFe2As2
is completely suppressed by substitution of 5% Fe with Pt and superconductivity
is induced at a critical temperature Tc=23 K. Full diamagnetic screening in the
magnetic susceptibility and a jump in the specific heat at Tc confirm the bulk
nature of the superconducting phase. All properties of the superconducting
state including transition temperature Tc, the lower critical field Hc1=200 mT,
upper critical field Hc2~65 T, and the slope dHc2/dT are comparable in value to
the those found in other transition-metal-substituted BaFe2As2 series,
indicating the robust nature of superconductivity induced by substitution of
Group VIII elements.Comment: 6 pgs, 4 figs, and 1 tbl, slightly revised, updated reference
Pressure Induced Changes in the Antiferromagnetic Superconductor YbPd2Sn
Low temperature ac magnetic susceptibility measurements of the coexistent
antiferromagnetic superconductor YbPd2Sn have been made in hydrostatic
pressures < 74 kbar in moissanite anvil cells. The superconducting transition
temperature is forced to T(SC) = 0 K at a pressure of 58 kbar. The initial
suppression of the superconducting transition temperature is corroborated by
lower hydrostatic pressure (p < 16 kbar) four point resisitivity measurements,
made in a piston cylinder pressure cell. At ambient pressure, in a modest
magnetic field of ~ 500 G, this compound displays reentrant superconducting
behaviour. This reentrant superconductivity is suppressed to lower temperature
and lower magnetic field as pressure is increased. The antiferromagnetic
ordering temperature, which was measured at T(N) = 0.12 K at ambient pressure
is enhanced, to reach T(N) = 0.58 K at p = 74 kbar. The reasons for the
coexistence of superconductivity and antiferromagnetism is discussed in the
light of these and previous findings. Also considered is why superconductivity
on the border of long range magnetic order is so much rarer in Yb compounds
than in Ce compounds. The presence of a new transition visible by ac magnetic
susceptibility under pressure and in magnetic fields greater than 1.5 kG is
suggested.Comment: 5 pages, 6 figure
Theory of Initialization-Free Decoherence-Free Subspaces and Subsystems
We introduce a generalized theory of decoherence-free subspaces and
subsystems (DFSs), which do not require accurate initialization. We derive a
new set of conditions for the existence of DFSs within this generalized
framework. By relaxing the initialization requirement we show that a DFS can
tolerate arbitrarily large preparation errors. This has potentially significant
implications for experiments involving DFSs, in particular for the experimental
implementation, over DFSs, of the large class of quantum algorithms which can
function with arbitrary input states
High pressure study of BaFe2As2 - role of hydrostaticity and uniaxial stress
We investigate the evolution of the electrical resistivity of BaFe2As2 single
crystals with pressure. The samples used were from the same batch grown from
self flux and showed properties that were highly reproducible. Samples were
pressurised using three different pressure media: pentane-isopentane (in a
piston cylinder cell), Daphne oil (in an alumina anvil cell) and steatite (in a
Bridgman cell). Each pressure medium has its own intrinsic level of
hydrostaticity, which dramatically affects the phase diagram. An increasing
uniaxial pressure component in this system quickly reduces spin density wave
order and favours the appearance of superconductivity, similar to what is seen
in SrFe2As2.Comment: 11 page
A Multi-scale Approach for Simulations of Kelvin Probe Force Microscopy with Atomic Resolution
The distance dependence and atomic-scale contrast observed in nominal contact
potential difference (CPD) signals recorded by KPFM on surfaces of insulating
and semiconducting samples, have stimulated theoretical attempts to explain
such effects. We attack this problem in two steps. First, the electrostatics of
the macroscopic tip-cantilever-sample system is treated by a finite-difference
method on an adjustable nonuniform mesh. Then the resulting electric field
under the tip apex is inserted into a series of atomistic wavelet-based density
functional theory (DFT) calculations. Results are shown for a realistic neutral
but reactive silicon nano-scale tip interacting with a NaCl(001) sample.
Bias-dependent forces and resulting atomic displacements are computed to within
an unprecedented accuracy. Theoretical expressions for amplitude modulation
(AM) and frequency modulation (FM) KPFM signals and for the corresponding local
contact potential differences (LCPD) are obtained by combining the macroscopic
and atomistic contributions to the electrostatic force component generated at
the voltage modulation frequency, and evaluated for several tip oscillation
amplitudes A up to 10 nm. Being essentially constant over a few Volts, the
slope of atomistic force versus bias is the basic quantity which determines
variations of the atomic-scale LCPD contrast. Already above A = 0.1 nm, the
LCPD contrasts in both modes exhibit almost the same spatial dependence as the
slope. In the AM mode, this contrast is approximately proportional to
, but remains much weaker than the contrast in the FM mode, which
drops somewhat faster as A is increased. These trends are a consequence of the
macroscopic contributions to the KPFM signal, which are stronger in the AM-mode
and especially important if the sample is an insulator even at sub-nanometer
separations where atomic-scale contrast appears.Comment: 19 pages, 13 figure
Chemical Pressure and Physical Pressure in BaFe_2(As_{1-x}P_{x})_2
Measurements of the superconducting transition temperature, T_c, under
hydrostatic pressure via bulk AC susceptibility were carried out on several
concentrations of phosphorous substitution in BaFe_2(As_{1-x}P_x)_2. The
pressure dependence of unsubstituted BaFe_2As_2, phosphorous concentration
dependence of BaFe_2(As_{1-x}P_x)_2, as well as the pressure dependence of
BaFe_2(As_{1-x}P_x)_2 all point towards an identical maximum T_c of 31 K. This
demonstrates that phosphorous substitution and physical pressure result in
similar superconducting phase diagrams, and that phosphorous substitution does
not induce substantial impurity scattering.Comment: 5 pages, 4 figures, to be published in Journal of the Physical
Society of Japa
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