Organic superconductivity studied by positron annihilation spectroscopy

The positron lifetimes of two organic superconductors, {kappa}-(ET){sub 2}Cu(NCS){sub 2} and {kappa}-(ET){sub 2}Cu[N(CN){sub 2}]Br, are measured as a function of temperature across T{sub c}. A drop of positron lifetime below T{sub c} is observed. Positron-electron momentum densities are measured by using 2D-ACAR to search for the Fermi surface in {kappa}-(ET){sub 2}Cu[N(CN){sub 2}]Br. Positron density distributions and positron-electron overlaps are calculated by using the orthogonalized linear combination atomic orbital (OLCAO) method to interpret the temperature dependence due to the local charge transfer, which is inferred to relate to the superconducting transition. 2D-ACAR results in {kappa}-(ET){sub 2}CU[N(CN){sub 2}]Br are compared with the calculated Fermi surface based on a first-principles local density approximation.