Nuclear magnetic resonance study of organic metals Russell W. Giannetta, University of Illinois at Urbana-Champaign, DMR Our lab uses nuclear magnetic resonance (NMR) to study superconductivity and magnetism in the organic metal κ-(ET) 2 Cu[N(CN) 2 ]Br whose crystal structure (left) consists of layers of ET molecules. Each ET molecule (below) contains H and 13 C nuclei, both of which yield NMR signals. When performing NMR, a large static magnetic field (1 – 9 Tesla) is applied along the a-axis and a smaller radio frequency field is applied along the c-axis to drive the nuclear spins out of their equilibrium state. The spins relax back to equilibrium by exchanging energy with the mobile electrons in the metal. By measuring this relaxation process we can can determine whether the electrons have transformed into a magnetic, superconducting or some other new state. The work here was carried out by graduate students Joseph Gezo and Tak-Kei Lui with the collaboration of Charles P. Slichter. Stretched Exponential Relaxation We have discovered that the relaxation of H spins back to equilibrium takes a new form as the temperature falls below 25 Kelvin. The left figure shows the NMR signal versus time fit to both a sum of two exponential functions and to a “stretched exponential” form: 1 – exp(-(t/T 1 ) b ) as shown by the blue curve. The blue curve is clearly a better fit. The stretched exponential is widely observed in disordered systems. Its observation here suggests that that the electrons have “phase separated” into metallic and magnetic regions, in somewhat the same way that water may separate into liquid and solid phases. H S C S C S C S H C C

Modulation period detectionEvolution period 90 X t 90 Y 90 TpTp T ev FID Sinusoidal modulation of the NMR spectrum Measurements of 13 C NMR also reveal something new. Using the NMR pulse sequence shown at the top, the 13 C spectrum is sinusoidally modulated. This modulated spectrum is then allowed to evolve for a time T ev that can be varied over 7 decades. After the evolution period the signal is detected by a third 90 degree pulse and free induction decay (FID). The middle figure shows the modulated spectrum for evolution times ranging from 0.2 μsec to 900 msec. The bottom figure shows how the amplitude of modulation (A) relaxes toward equilibrium. If each part of the modulated spectrum were to relax independently then A would fit an exponential with a single time constant. If there exists some way for different spectral regions to communicate with each other, then the relaxation will have more than one time constant. The fit (dotted line) clearly shows two “humps” indicative of two distinct time scales. This suggests that magnetic information is being passed from one group of 13 C spins to another. The mechanism may be through a collective motion of the electrons such as a spin density wave. Broader Impacts Organic metals are important since their properties may be engineered through chemical synthesis. To assess the results of this synthesis we require sophisticated tools such as NMR. Both the stretched exponential relaxation and the sinusoidal modulation technique have demonstrated new kinds of behavior that may be generic to these materials. It appears that we must take account of phenomena (1) occurring over many decades in time (2) in which different electronic and magnetic phases may coexist and (3) in which collective motions caused by the interactions between electrons play a central role. Russell W. Giannetta, University of Illinois at Urbana-Champaign, DMR