Nuclear magnetic resonance studies of the organic superconductor kappa-(bisethylenedithiotetrathiafulvalene)(2) copper(nitrogen(cyanide)(2))bromine
DeSoto, Stewart Martin
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https://hdl.handle.net/2142/19936
Description
Title
Nuclear magnetic resonance studies of the organic superconductor kappa-(bisethylenedithiotetrathiafulvalene)(2) copper(nitrogen(cyanide)(2))bromine
Author(s)
DeSoto, Stewart Martin
Issue Date
1995
Doctoral Committee Chair(s)
Slichter, C.P.
Department of Study
Physics, Condensed Matter
Discipline
Physics, Condensed Matter
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Condensed Matter
Language
eng
Abstract
In this thesis we report $\sp1$H and $\sp{13}$C NMR measurements in the normal and superconducting states of the organic superconductor $\kappa$-(ET)$\sb2$Cu (N(CN)$\sb2$) Br (T$\sb{\rm c}$ = 11.6 K). These measurements include the temperature and magnetic field dependent lineshapes, Knight shifts, spin-spin relaxation rates and spin-lattice relaxation rates for the $\sp{-1}$H and $\sp{13}$C sites located in the organic ET molecules of $\kappa$-(ET)$\sb2$CU (N(CN)$\sb2$) Br for temperatures 2 K $$ 4.3 T). We studied the orientation dependence of this rapid relaxation and found that the extra relaxation disappeared when the magnetic field was oriented very close ($\le3\sp\circ$) to the superconducting layers. This was taken to be strong evidence that fluxoid motion caused the rapid relaxation and that intrinsic pinning of the fluxoids occurred for parallel magnetic fields.
We have also studied samples in which the central two carbon sites of the ET molecule were enriched with $\sp{13}$C (I = 1/2). Due to the pairing of the ET molecules into dimers, there is a breakdown of the molecular inversion symmetry, and we observe two $\sp{13}$C resonance lines, labeled Inner and Outer, with different Knight shifts tensors but the same chemical shift tensor.
There are several key, transition temperatures in which the $\sp{13}$C NMR properties show abrupt changes as the sample is cooled. The first occurs at T = 150 K, where the linewidths suddenly broaden, and the relaxation rates $(\rm 1/T\sb1T, 1/T\sb2)$ increase. There is another transition at T = 50 K, evident in the relaxation rates and Knight shifts. There is another transition near T = 25K before the superconducting transition at T = 11.6K.
In the superconducting state, there is a strong field dependence for the $\sp{13}$C 1/T$\sb1$, with the relaxation rate increasing as the field is increased (opposite to the $\sp1$H field dependence). The relaxation rate below T$\sb{\rm c}$ does not follow the BCS model; there is no increase in the relaxation rate (i.e. coherence peak) just below T$\sb{\rm c}$; nor is there an exponential T dependence at low temperature. Finally, the Knight shift below T$\sb{\rm c}$ does not follow the BCS weak coupling form, but shows an indication of strong coupling in $\kappa$-(ET)$\sb2$Cu (N(CN)$\sb2$) Br.
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