13,411 research outputs found
Thermodynamic evidence for pressure-induced bulk superconductivity in the Fe-As pnictide superconductor CaFe2As2
We report specific-heat and resistivity experiments performed in parallel in
a Bridgman-type of pressure cell in order to investigate the nature of
pressure-induced superconductivity in the iron pnictide compound CaFe2As2. The
presence of a pronounced specific-heat anomaly at Tc reveals a bulk nature of
the superconducting state. The thermodynamic transition temperature differs
dramatically from the onset of the resistive transition. Our data indicates
that superconductivity occurs in the vicinity of a crystallographic phase
transition. We discuss the discrepancy between the two methods as caused by
strain-induced superconducting precursors formed above the bulk thermodynamic
transition due to the vicinity of the structural instability
Design and implementation of automatic control system for steel wire stretching stress relaxation testing machine
Author name used in this publication: K. W. E. ChengVersion of RecordPublishe
The Unusual Superconducting State at 49 K in Electron-Doped CaFe2As2 at Ambient
We report the detection of unusual superconductivity up to 49 K in single
crystalline CaFe2As2 via electron-doping by partial replacement of Ca by
rare-earth. The superconducting transition observed suggests the possible
existence of two phases: one starting at ~ 49 K, which has a low critical field
~ 4 Oe, and the other at ~ 21 K, with a much higher critical field > 5 T. Our
observations are in strong contrast to previous reports of doping or
pressurizing layered compounds AeFe2As2 (or Ae122), where Ae = Ca, Sr or Ba. In
Ae122, hole-doping has been previously observed to generate superconductivity
with a transition temperature (Tc) only up to 38 K and pressurization has been
reported to produce superconductivity with a Tc up to 30 K. The unusual 49 K
phase detected will be discussed.Comment: 11 pages, 8 figure
Boron-mediated directed aromatic C–H hydroxylation
Transition metal-catalysed C–H hydroxylation is one of the most notable advances in synthetic chemistry during the past few decades and it has been widely employed in the preparation of alcohols and phenols. The site-selective hydroxylation of aromatic C–H bonds under mild conditions, especially in the context of substituted (hetero)arenes with diverse functional groups, remains a challenge. Here, we report a general and mild chelation-assisted C–H hydroxylation of (hetero)arenes mediated by boron species without the use of any transition metals. Diverse (hetero)arenes bearing amide directing groups can be utilized for ortho C–H hydroxylation under mild reaction conditions and with broad functional group compatibility. Additionally, this transition metal-free strategy can be extended to synthesize C7 and C4-hydroxylated indoles. By utilizing the present method, the formal synthesis of several phenol intermediates to bioactive molecules is demonstrated
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