Universität Stuttgart
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Item Open Access Nuclear magnetic shielding tensors for 1H, 13C, and 15N in organic solids(1975) Spiess, Hans Wolfgang; Haeberlen, Ulrich; Kempf, Jürgen; Schweitzer, DieterA survey will be given of nuclear magnetic shielding tensors obtained by multiple pulse techniques for 1H in carboxylic acids and by high field NMR for 13C in carbonyl- and carboxyl groups of aromatic compounds and for 15N in pyridine and nitrobenzene.Item Open Access 13C nuclear magnetic relaxation studies at 62 MHz(1971) Jaeckle, H.; Haeberlen, Ulrich; Schweitzer, DieterAn apparatus suitable to measure 13C relaxation times T1 and T2 in liquids at 62 MHz is described. The required field of 58 kG is generated by a superconducting magnet. Results of T1 measurements on a variety of liquids, including benzene, derivatives of benzene, saturated rings, CS2, and others are reported. They are discussed in terms of dipole-dipole interactions, spin-rotation interactions and anisotropic chemical shifts, the latter of which turn out to play only a minor role. Intramolecular dipole-dipole interactions are found to provide by far the most important spin-lattice relaxation mechanism, whenever protons are bound directly to the carbons under investigation, even when the samples contain dissolved oxygen.Item Open Access Proton relaxation in the superconducting organic solid (BEDT-TTF)2Cu(NCS)2: evidence for relaxation by localized paramagnetic centers(1990) Klutz, Thomas; Haeberlen, Ulrich; Schweitzer, DieterThe nonexponential character of the build-up of the nuclear magnetization usually increases on lowering the temperature. Superconducting fluctuations have been offered in a speculative way as an explanation for the nonexponential spin relaxation. We have observed the same relaxation behaviour in our investigation of the proton spin relaxation in (BEDT-TTF)2Cu(NCS)2 which is an organic conductor at room temperature and becomes a superconductor at Tc=10.4 K at ambient pressure. As we noticed during preliminary measurements that the degree of deviation from exponential relaxation depends on the sample under study we decided to do experiments under controlled conditions of sample preparation.Item Open Access Proton spin-lattice relaxation in the organic superconductor (BEDT-TTF)2Cu(NCS)2 : evidence for relaxation by localized paramagnetic centres(1990) Klutz, Thomas; Haeberlen, Ulrich; Schweitzer, DieterThe spin-lattice relaxation of the protons in the compound was investigated at nu L=13.5 MHz and 270 MHz for 4.2 K10 K and becomes non-exponential for T<10 K. At nu L=13.5 MHz the transition from exponential to noticeably non-exponential relaxation occurs already at T approximately=25 K. The cause for the non-exponential proton relaxation in the coarse grains is finite penetration of the RF-field (skin-effect) into the electrically conducting crystals. Powdering the crystals suppresses the skin-effect; this procedure, however, generates relaxation sinks in the form of localized paramagnetic centres. There is reason to believe that the skin-effect in coarse grains and generation of relaxation sinks by a powdering procedure complicate proton relaxation studies as well in other organic superconductors.Item Open Access NMR investigations of the organic metals and superconductors α-(BEDT-TTF)2I3, β-(BEDT-TTF)2I3 and α t-(BEDT-TTF)2I3(1988) Hennig, Ingolf; Haeberlen, Ulrich; Heinen, Ilsabe; Schweitzer, Dieter; Keller, Heimo J.1H and 13C spin lattice relaxation rates as well as 13C Knight shifts of the quasi two dimensional organic metals α-, β- and αt-(BEDT-TTF)2I3 are reported. In β- and αt-(BEDT-TTF)2I3 at temperatures below 100 K the Korringa relation is fulfilled. Near the critical temperature of 8K of the stable superconducting state of αt-(BEDT-TTF)2I3 an enhancement of the proton spin lattice relaxation rate due to superconducting fluctuations is observed. The observed individual 13C Knight shifts, as measured by magic angle sample spinning and cross polarisation methods range between -6 and +244 ppm for α- phase and -10 and +125 ppm for β- and αt- phase crystals.Item Open Access NMR study of the quasi one dimensional organic metal (perylene)2(AsF6)0,75(PF6)0,35x0,85CH2Cl2(1987) Wieland, Jürgen; Haeberlen, Ulrich; Schweitzer, Dieter; Keller, Heimo J.Measurements of the temperature and frequency dependences of the proton spin lattice relaxation rate and of the proton and carbon Knight shifts in the title compound are presented. The results are interpreted in terms of a quasi-one dimensional microscopic conduction electron mobility and of the local unpaired spin densities at the carbon sites of the perylene donor molecules.Item Open Access A stable superconducting state at 8K and ambient pressure in αt-(BEDT-TTF)2I3(1987) Schweitzer, Dieter; Bele, Petra; Brunner, Hermann; Gogu, Emil; Haeberlen, Ulrich; Hennig, Ingolf; Klutz, Thomas; Swietlik, Roman; Keller, Heimo J.We report bulk superconductivity at 8 K and ambient pressure in crystals of α t (BEDT-TTF)2I3. In contrast to the earlier observed metastable superconducting state at 8 K in crystals of β-(BEDT-TTF)2I3 here the superconducting state is stable and the crystals can be prepared by tempering α-(BEDT-TTF)2I3 above 70 °C for several days. ac-susceptibility measurements show that the observed superconducting state at 8 K is a bulk property of the crystals. Resistivity measurements indicate a sharp superconducting transition at 8 K with an onset temperature of about 9 K. The upper critical fields Hc2 at 1.3 K lie between 3 and 11 T depending on the direction of the magnetic field with respect to the crystal axes. ESR- as well as NMR-measurements indicate a total transformation of the α-phase crystals into the new superconducting α t -crystals after tempering.Item Open Access Spin rotation interaction and anisotropic chemical shift in 13CS2(1971) Spiess, Hans Wolfgang; Schweitzer, Dieter; Haeberlen, Ulrich; Hausser, Karl H.The 13C nuclear spin-lattice relaxation time T1 was studied in liquid CS2 from -106°C to +35°C at resonance frequencies of 14, 30, and 62 MHz. The relaxation is caused by anisotropic chemical shift and spin-rotation interaction. It is shown that for linear molecules the spin-rotation constant C and the anisotropy of the chemical shift Δσ can be obtained from the relaxation rates without use of adjustable parameters. The analysis yields: C = -13.8 ± 1.4 kHz and Δσ = 438 ± 44 ppm.Item Open Access Superconductivity at ambient pressure in BEDT-TTF radical salts(1988) Schweitzer, Dieter; Polychroniadis, K.; Klutz, Thomas; Keller, Heimo J.; Hennig, Ingolf; Heinen, Ilsabe; Haeberlen, Ulrich; Gogu, Emil; Gärtner, StephanCrystals of (BEDT-TTF)2Cu(NCS)2 were prepared by several methods and the superconducting transitions investigated by resistivity and ac susceptibility measurements. Depending on the preparation of the crystals a variation of the temperature of the superconducting transition is observed. This variation is manifested in the upper critical fields Hc2 and proton NMR relaxation measurements at temperatures below Tc show it as well. The upper critical fields Hc2 of crystals of αt(BEDT-TTF)2I3 were determined in dependence of the temperature and of the direction of the magnetic field with respect to the various crystal axes by measuring the mid transition of the resistivity and of the rf penetration depth. The data are analyzed with the anisotropic effective mass model in the picture of the Ginsburg Landau (Gl) theory as well as in the picture of a layered superconductor. 13C Knight shifts measured by magic angle sample spinning and NMR cross polarisation methods support the picture of the layered superconductor.Item Open Access Molecular motion in liquid toluene from a study of 13C and 2D relaxation times(1973) Spiess, Hans Wolfgang; Schweitzer, Dieter; Haeberlen, UlrichThe 13C nuclear spin-lattice relaxation times for ring and methyl carbons in liquid toluene were studied from −95°C to +60°C at frequencies of 14 and 61 MHz. Data were taken for protonated as well as deuterated toluene. The results were analyzed in terms of three relaxation mechanisms: intramolecular dipole-dipole coupling, spin-rotation interaction, and anisotropic chemical shift. The last mechanism gives a significant contribution only to the relaxation rate of the ring carbons of the deuterated species at 61 MHz and low temperatures. A tentative value of Δσ = 295 ppm is obtained in this case. In order to separate the contributions of the dipole-dipole and spin-rotation interactions the 13C data are compared with deuteron relaxation times. Comparison of the 13C data in the protonated and deuterated form of toluene shows that the correlation times for the ring differ by 20% and an even larger effect of isotopic substitution is found for the methyl group. It is demonstrated that the fast internal motion of the methyl group cannot be studied quantitatively using deuteron or 13C intramolecular dipole-dipole relaxation rates alone because of the sensitivity of the results to the angle, varpi, the Z-axis of the electric field gradient, or the internuclear vector, respectively, forms with the C3 axis. Analysis of the relaxation rates due to spin-rotation interaction yields τj (internal), the correlation time of angular momentum of the internal motion directly. The correlation time of reorientation τc (internal) is calculated from τj (internal) using Gordon's extended diffusion model which is applied to a symmetric rotor with a fixed axis. It is found that both τj (internal) and τc (internal) are of the same magnitude as the correlation time of the free rotor. The ratio of correlation times of the overall and internal reorientation ranges from approximately 200 at the melting point to approximately 13 at +60°.