Structure of BEDT-TTF molecule.

1. S H S S S H C C C C C Superconductivity and magnetism in organic materialsRussell W. Giannetta, University of Illinois at Urbana-Champaign, DMR 0503882 Our goal is to uncover the connections between magnetism and superconductivity. We study organic superconductors, based on the BEDT-TTF molecule as shown. These materials separate into magnetic and superconducting phases as we vary the cooing rate. The thermal properties of the superconducting phase are determined by its excited states, known as quasiparticles. By carefully measuring how far a magnetic field penetrates into the crystal we can study how the quasiparticle population grows with temperature. In the plot, the time to cool the crystal varies from 20 sec to 3 weeks. This changes the relative amount of magnetic and superconducting material by a factor of 30. Nonetheless, each of the curves shown obeys a T3/2 power law. This implies that the quasiparticle energy spectrum is “robust”. This same robustness is observed in the high temperature copper oxides, despite an entirely different chemical composition and a factor of 10 higher transition temperature. The T3/2 power law that we observe in all experiments suggests that the quasiparticles in organic superconductors behave unlike anything predicted by the BCS theory of superconductivity. Structure of BEDT-TTF molecule. Change of London penetration depth of BEDT-TTF superconductor versus T3/2. Curves shown vary in cooling time from 20 (bottom curve ) sec to 3 weeks (top curve). All obey the same power law.

2. Superconductivity and magnetism in organic materialsRussell W. Giannetta (University of Illinois at Urbana-Champaign) DMR 0503882 Broader Impact Understanding the connection between magnetism and superconductivity is a central goal of condensed matter physics. Organic materials hold the promise of controlling these two phenomena by chemical means. Education, outreach, collaborations Graduate students T.Olheiser and N. Salovich are involved in penetration depth experiments, along with undergrads Will Maulbetsch and Matthew Feickert. We are collaborating with C.P. Slichter (UIUC) to do NMR measurements on the organics. Slichter’s group produced the earliest evidence for unconventional superconductivity in these materials. Graduate students Tak-Kei Lui, Mike Murray and Joe Gezo, as well as undergrads Alex Ford and Nate Burdick are continuing this work. Organic superconductor crystals were obtained in a collaboration with P. Tea (UIUC-chemistry dept.) and J. Schleuter (Argonne Labs). Giannetta is also collaborating with former postdocs R. Prozorov (Ames Lab, ISU) and A. Carrington (U. Bristol) to write a book on penetration depth measurements in superconductors. NMR spin-lattice relaxation times of organic and copper oxide superconductors, showing close similarity. Neither material obeys the BCS prediction. [S.M.deSoto, C.P. Slichter, Phys. Rev. B 52, 10364 (1995)] 13C NMR spectrum of organic superconductor recently obtained by Tak-Kei Lui.