2026-09-23T16:09:08Z https://www.ideals.illinois.edu/oai-pmh

oai:www.ideals.illinois.edu:2142/45598 2023-07-11 col_2142_5131 col_2142_8859 com_2142_5130 com_2142_8858 com_2142_234

Item withdrawn by Mark Zulauf (zulauf@illinois.edu) on 2013-05-10T21:50:06Z Item was in collections: University of Illinois Theses & Dissertations (ID: 1) No. of bitstreams: 1 Lui_TakKei.pdf: 11830561 bytes, checksum: 5477b7fed52668dabb3a0d337cc0bf19 (MD5) Made available in DSpace on 2013-08-22T16:49:07Z (GMT). No. of bitstreams: 2 Tak Kei_Lui.pdf: 11832199 bytes, checksum: c2808356c2f2604c451c877d0506286c (MD5) license.txt: 4057 bytes, checksum: 72976bc6bdfe5028205ab30017321e8d (MD5) Restriction data tranferred 2014-07-01T11:34:39-05:00 Original Data Group with Access UIUC Users [automated] Release Date: 2015-08-22 11:49:27 UTC Reason: Author requested U of Illinois access only (OA after 2yrs) in Vireo ETD system Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Seth Robbins (srobbins@illinois.edu) on 2013-08-22T16:49:45Z Item is restricted until 2015-08-22T16:49:27Z U of I Only Restriction Lifted for Item 45580 on 2015-08-22T10:00:52Z. Giannetta, Russell W. Phillips, Philip W. Giannetta, Russell W. Slichter, C.P. Stack, John D. Lui, Tak Kei 2013-08-22T16:49:07Z 2013-08-22T16:49:07Z 2015-08-22T10:00:52Z 2013-08 2013-08-22T16:49:07Z 2013-08 The $^{13}$C NMR absorption lines in the organic superconductors $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]X, X~=~Br ($T_C=11.6$~K), Cl ($T_C=12.8$~K at 0.3~kbar) show an abrupt increase in the linewidth at 170~K at 9.4~T. This line broadening transition is believed to be caused by the abrupt slowing down of some ``motion'' as suggested by a sharp peak in the exponential component of the transverse relaxation rate ($1/T_{2E}$) at a slightly lower temperature at 9.4~T. To investigate the characteristics of this motion, we applied the multi-pulse ``S-shape'' sequence. Two preparation pulses label the spins according to their precession frequencies and modulate the NMR spectrum. We then allow the spins to evolve for a time $T_{ev}$, after which inspection pulses are applied to observe changes in the spectrum. The modulation in the NMR spectrum shows a decay as $T_{ev}$ increases. The decay is described by 2 time constants, $R_\mathrm{fast}(T)$ and $R_\mathrm{slow}(T)$. Each rate shows a sharp peak at some temperature near to, but not at, the linewidth transition. Outside the transition region, $R_\mathrm{fast}$ is independent of $T$ and is close to the Gaussian component of the transverse relaxation rate $1/T_{2G}$. $R_\mathrm{fast}$ represents the relaxation of the spectrum modulations by dipolar coupling. $R_\mathrm{slow}$ is also largely $T$ independent outside the transition region. Unlike $R_\mathrm{fast}$, $R_\mathrm{slow}$ shows a strong $T_p$ dependence, namely, $R_\mathrm{slow}$ varies as $T_p^2$. $T_p$ is the delay between the 2 preparation pulses, and $\pi/T_p$ is equal to the separation between the adjacent peaks and troughs in the spectrum modulations. The $T_p^2$ dependence of $R_\mathrm{slow}$ reflects the diffusive nature of the motion relaxing the modulations. It has been proposed that the motion causing the linewidth transition is the rotation of the ethylene end groups. But a second moment calculation reveals that the ethylene end groups contribute only a small fraction of the total measured $^{13}$C linewidth below the linewidth transition. An anisotropic Knight shift calculation suggests that the (ET)$_2$ dimers only need to deviate from its equilibrium position by $2.5^\circ$ to produce a large enough spread in Knight shifts to account for the total measured linewidth below the linewidth transition. We believe a diffusive rotational motion of the (ET)$_2$ dimers together with a BPP style relaxation model is the cause of the peak of $R_\mathrm{slow}$, the $T_p^2$ dependence of $R_\mathrm{slow}$, the linewidth transition, as well as the peak in $1/T_{2E}$. http://hdl.handle.net/2142/45598 en Copyright 2013 Tak Kei Lui Nuclear magnetic resonance (NMR) Organic superconductor Bis(ethylenedithio)tetrathiafulvalene(BEDT-TTF) Motion NMR studies of slow motion in the organic superconductors k-(BEDT-TTF)2Cu[N(CN)2]X, X= Br, Cl text Physics 1244 Physics 0240 University of Illinois at Urbana-Champaign Dissertation Ph.D. PHD:Physics -UIUC 10KS0240PHD