Inelastic neutron scattering from superconducting k -(BEDT-TTF)2 Cu(NCS)2: possible proton tunneling enhanced in superconducting state?

Summary form only given. Since a hydrogen has quite a large incoherent-scattering-cross-section for neutrons, time-of-flight inelastic neutron scattering from the title superconducting salt with Tc=9.5K has been measured for the first time in order to study local vibrational states of hydrogens at the terminal ethylene. We report measurements on (1) polycrystalline samples of neutral and salt crystals (energy transfer < 300meV) between 90K and 6K, and (2) a piece of salt single crystal (<10meV) between 12K and 2.6K. The data are analyzed to get the excitation spectrum for the generalized density-of-states (GPDOS) as a function of energy transfer. In the high energy region (>10meV), totally symmetric and asymmetric modes of BEDT-TTF molecular vibrations which have been studied by Raman and infrared spectroscopies and calculations are well reproduced. In the low energy region up to about 3-4meV, however, a broad peak appears in the GPDOS below Tc. The single crystal data have revealed that the peak shifts to low energy transfer with decreasing temperature and the integrated intensity takes a maximum around 4K. These phenomena might indicate strong correlation of the proton motion with the superconductivity, associated with the opening of the energy gap 2,&=3.5-4meV, just in the same energy interval where the GPDOS appears. The most likely explanation for these anomalies, which have never been observed so far in any superconductor, is based on a proton tunneling model between almost equivalent hydrogen bonding sites, which assumes that the tunneling might be drastically enhanced due to the ffiction-les protan matin when the surrounding electrons are superfluid.