Electrical-resistivity, magnetic-susceptibility, and specific-heat data reveal that UBe13 is superconducting below 0.85 K. Highly anomalous low-temperature electronic properties in both the normal and superconducting states result in an enormous electronic specific-heat coefficient γ=1.1J/mole K2 and a corresponding magnetic susceptibility X =1.5×10-2 emu/mole. The superconducting state appears to be extremely stable with an initial slope of the temperature derivative of the critical field(∂H c2/∂T) Tc =-257kOe/K.

/pdf/ube13-an-unconventional-actinide-superconductor-54sombtjip.pdf

Ube13 - an unconventional actinide superconductor

The upper temperature for superconductivity in organic conductors has increased from 1 kelvin in 1980, when the phenomenon was discovered in the quasi-one-dimensional cation radical salt tetramethyltetraselenafulvalene phosphorus heptafluoride to 12 kelvin in a new series of organic salts that show nearly two-dimensional electronic properties. These superconductors are attracting interest because of the wide range of new phenomena that they exhibit, including the competition between various ground states, the influence of a magnetic field on a quasi-one-dimensional conductor, the quantization of the Hall effect in a three-dimensional material, the giant magnetoresistance effects related to the two-dimensional nature of the Fermi surface of some materials, and the coherent voltage oscillation of a spin-modulated ground state. Furthermore, there is reason to believe that organic conductors with high superconducting transition temperatures could be produced in the near future. The recent finding of superconductivity in "fullerene" doped with alkali metals supports this optimism.

https://science.sciencemag.org/content/sci/252/5012/1509.full.pdf

The physics of organic superconductors.

We introduce the development of conductive organic molecular assemblies including organic metals, superconductors, single component conductors, conductive films, and conductive liquids, particularl...

Development of Conductive Organic Molecular Assemblies: Organic Metals, Superconductors, and Exotic Functional Materials

The authors have evaluated the temperature dependence of the tight-binding transfer integral between neighbouring cation molecules for some representative compounds of the (TMTSF)2X and (TMTTF)2X families using their recently determined low-temperature and/or high-pressure crystallographic structures. The changes in the transverse integrals in going from the new structures to the ambient temperature and pressure structures are largest for those integrals which show an antibonding character compared to the intrastack interaction. These changes lead to a low-temperature electronic picture which differs markedly from the well known high-temperature description. The transverse bonding and antibonding interactions tend to equalise for the atmospheric pressure supraconductor (TMTSF)2ClO4. The results suggest that the structure of the most metallic TMTTF salt, (TMTTF)2Br, is equivalent at a temperature T1 to the structure of (TMTSF)2X at T=T1 or (TMTTF)2PF6 at T=T1+300K with regard to the values of the transverse transfer integrals.

http://iopscience.iop.org/article/10.1088/0022-3719/19/20/016/pdf

Temperature dependence of the transfer integrals in the (TMTSF)2X and (TMTTF)2X families

The static properties of spin density waves (SDW) in (TMTSF) 2 PF 6 were determined by a precise analysis of 1 H-NMR lineshape in the low temperature SDW state. The observed angular dependence of lineshape was well reproduced by a picture of incommensurate sinusoidal SDW's. The wave number was found to be (0.5, 0.24±0.03, -0.06±0.20) in the units of a * , b * and c * . This value is very near the optimal nesting vector recently calculated. The SDW amplitude at low temperature was estimated as 8% µ B /molecule.

Determination of SDW Characteristics in (TMTSF)2PF6 by 1H–NMR Analysis

Anomalous microwave absorption has been observed in the organic superconductor ${\mathrm{TMTSF}}_{2}$${\mathrm{AsF}}_{6}$ (TMTSF: tetramethyltetraselenafulvalene) below its metal-nonmetal transition near 12 K. This absorption is unambiguously identified as antiferromagnetic resonance by the excellent agreement between a spin-wave calculation and the pronounced dependence on temperature and magnetic field orientation. This result dramatically confirms earlier indications of magnetic ordering or spin-density waves and, furthermore, indicates that the spin-density-wave amplitude is considerably larger than previous estimates of 1%.

/pdf/observation-of-antiferromagnetic-resonance-in-an-organic-1vrwz1601o.pdf

H.J. Pedersen

Papers

Observation of antiferromagnetic resonance in an organic superconductor