Penetration Depth in Organic Superconductors

1. Penetration Depth in Organic Superconductors Russell W. Giannetta, University of Illinois, Urbana-Champaign, DMR 0503882 D D Rapid cooling rate ………..Slow cooling rate S H S S S H C C C C C We are using magnetic penetration depth measurements to explore the mechanism for superconductivity in materials made from the organic molecule “ET” , shown in the top right. For ordinary materials such as Al or Pb, the vibrations of the crystalline lattice are responsible for superconductivity, but in high temperature, heavy electron and organic superconductors, it is believed that interactions between electrons themselves are the cause. To distinguish between the two, we substitute deuterium (2D) for hydrogen (1H) in the ET molecule. We observe two dramatic changes. (1) The volume fraction of material that becomes superconducting depends strongly upon how rapidly the material is cooled to its transition temperature. (2) The transition temperature falls in direct proportion to the volume fraction. The cooling rate effect strongly suggests the formation of a two-phase mixture of superconducting (blue) and magnetic regions (green). The rapid cooling produces regions of higher internal pressure which in turn affects the interactions between electrons, thus lowering TC. Penetration depth is also being used to determine the size of the superconducting regions and how their microscopic properties change with cooling rate. We find that the temperature dependence of the penetration depth has the functional form expected for an electronic (as opposed to lattice vibration) mechanism for organic superconductivity.

2. Penetration Depth in Organic Superconductors Russell W. Giannetta, University of Illinois, Urbana-Champaign, DMR 0503882 Education Two PhD students, Tyson Olheiser and Nicholai Salovich, are involved in the experiments. Two undergraduates were involved in the early phase of the project: Andrew McCormick (currently at Harvard), Zane Shi (currently at Princeton). Nathaniel Burdick, currently an undergraduate at UIUC, is now working on improving the electronic instrumentation for our penetration depth measurements. Societal Impact The mechanism for superconductivity in many newly synthesized materials is one of the major unsolved problems in physics. It appears to be intimately related to magnetism. Understanding this problem will lead to new ways of controlling the passage of electrons through matter, devices with increased functionality, and insights into the design and synthesis of sophisticated new materials. The organic conductors are a large class of materials where these problems can be studied and tested against current theories of matter.