20,680 research outputs found
Anisotropic thermal expansion of AEFe2As2 (AE = Ba, Sr, Ca) single crystals
We report anisotropic thermal expansion of the parent, AEFe2As2 (AE = Ba, Sr,
and Ca), compounds. Above the structural/antiferromagnetic phase transition
anisotropy of the thermal expansion coefficients is observed, with the
coefficient along the a-axis being significantly smaller than the coefficient
for the c-axis. The high temperature (200 K < T < 300 K) coefficients
themselves have similar values for the compounds studied. The sharp anomalies
associated with the structural/antiferromagnetic phase transitions are clearly
seen in the thermal expansion measurements. For all three pure compounds the
"average" a-value increases and the c-lattice parameter decreases on warming
through the transition with the smallest change in the lattice parameters
observed for SrFe2As2. The data are in general agreement with the literature
data from X-ray and neutron diffraction experiments
Specific heat jump at the superconducting transition temperature in Ba(Fe(1-x)Cox)2As2 and Ba(Fe(1-x)Nix)2As2 single crystals
We present detailed heat capacity measurements for Ba(Fe(1-x)Cox)2As2 and
Ba(Fe(1-x)Nix)2As2 single crystals in the vicinity of the superconducting
transitions. The specific heat jump at the superconducting transition
temperature (Tc), Delta Cp/Tc, changes by a factor ~ 10 across these series.
The Delta Cp/T$ vs. Tc data of this work (together with the literature data for
Ba(Fe0.939Co0.061)2As2, (Ba0.55K0.45)Fe2As2, and (Ba0.6K0.4)Fe2As2) scale well
to a single log-log plot over two orders of magnitude in Delta Cp/Tc and over
about an order of magnitude in Tc, giving Delta Cp/Tc ~ Tc^2
Pressure induced superconductivity in CaFeAs
CaFeAs has been found to be exceptionally sensitive to the
application of hydrostatic pressure and superconductivity has been found to
exist in a narrow pressure region that appears to be at the interface between
two different phase transitions. The pressure - temperature () phase
diagram of CaFeAs reveals that this stoichiometric, highly ordered,
compound can be easily tuned to reveal all the salient features associated with
FeAs-based superconductivity without introducing any disorder. Whereas at
ambient pressure CaFeAs does not superconduct for K and
manifests a first order structural phase transition near K, the
application of kbar hydrostatic pressure fully suppresses the
resistive signature of the structural phase transition and instead
superconductivity is detected for K. For kbar a different
transition is detected, one associated with a clear reduction in resistivity
and for kbar superconductivity is no longer detected. This higher
pressure transition temperature increases rapidly with increasing pressure,
exceeding 300 K by kbar. The low temperature, superconducting dome
is centered around 5 kbar, extending down to 2.3 kbar and up to 8.6 kbar. This
superconducting phase appears to exist when the low pressure transition is
suppressed sufficiently, but before the high pressure transition has reduced
the resistivity, and possibly the associated fluctuations, too dramatically
An optimal gap theorem
By solving the Cauchy problem for the Hodge-Laplace heat equation for
-closed, positive -forms, we prove an optimal gap theorem for
K\"ahler manifolds with nonnegative bisectional curvature which asserts that
the manifold is flat if the average of the scalar curvature over balls of
radius centered at any fixed point is a function of .
Furthermore via a relative monotonicity estimate we obtain a stronger
statement, namely a `positive mass' type result, asserting that if is
not flat, then for any
Super-ASTROD: Probing primordial gravitational waves and mapping the outer solar system
Super-ASTROD (Super Astrodynamical Space Test of Relativity using Optical
Devices or ASTROD III) is a mission concept with 3-5 spacecraft in 5 AU orbits
together with an Earth-Sun L1/L2 spacecraft ranging optically with one another
to probe primordial gravitational-waves with frequencies 0.1 microHz - 1 mHz,
to test fundamental laws of spacetime and to map the outer solar system. In
this paper we address to its scientific goals, orbit and payload selection, and
sensitivity to gravitational waves.Comment: 7 pages, 1 figure, presented to 7th International LISA Symposium,
16-20 June 2008, Barcelona; submitted to Classical and Quantum Gravity;
presentation improve
Decoupling of the superconducting and magnetic (structural) phase transitions in electron-doped BaFe2As2
Study and comparison of over 30 examples of electron doped BaFe2As2 for
transition metal (TM) = Co, Ni, Cu, and (Co/Cu mixtures) have lead to an
understanding that the suppression of the structural/antiferromagnetic phase
transition to low enough temperature in these compounds is a necessary
condition for superconductivity, but not a sufficient one. Whereas the
structural/antiferromagnetic transitions are suppressed by the number of TM
dopant ions (or changes in the c-axis) the superconducting dome exists over a
limited range of values of the number of electrons added by doping (or values
of the {a/c} ratio). By choosing which combination of dopants are used we can
change the relative positions of the upper phase lines and the superconducting
dome, even to the extreme limit of suppressing the upper structural and
magnetic phase transitions without the stabilization of low temperature
superconducting dome
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