共查询到20条相似文献,搜索用时 15 毫秒
1.
D. Brinkmann 《Applied magnetic resonance》2004,27(1-2):207-213
This presentation gives a personal review of nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) spin-lattice relaxation studies in cuprate superconductors mainly dealing with the YBa2Cu4O8 compound with many examples from the Zürich laboratory. The studies were performed in both the normal and the superconducting state with various NMR isotopes (e.g.,17O,63,65Cu,135,137Ba). The relatively broad signals were mostly obtained by a phase-alternating add-subtract spin-echo technique. We will discuss the general behavior of spin-lattice relaxation in the normal state and the calculation of the dynamic spin including an approach (on the basis of thet-J model) to calculate the relaxation for plane copper, oxygen, and yttrium. An application of the Luttingerliquid model to the relaxation of chain copper in YBa2Cu3O7 and YBa2Cu4O8 is also given. We then will deal with characteristic features of the YBa2Cu4O8 structure: the spin gap, an electronic crossover in the normal state, the single-spin fluid model, and the d-wave pairing. 相似文献
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The Zeeman spin-lattice relaxation is measured in a single crystal and in powdered samples of lithium acetate (CH3COOLi·2D2O). Measurements have also been performed in powdered samples of lithium acetate which contain 50% and 95% of CD3-groups besides the CH3-groups. A non-exponential behaviour, i.e. time dependence, of the Zeeman spin-lattice relaxation is clearly observed at temperatures above the T1-minimum in all samples. The results are explained within the framework of the symmetry-restricted spin-diffusion model of Emid and Wind. 相似文献
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Specially chosen pulse sequences have been used to isolate the contribution to the nuclear magnetic resonance signal from nearest neighbor oH2 molecules. Measurements of the rotational diffusion were shown to be consistent with earlier measurements where only oH2 pairs or only isolated singles could be observed. It was found that the pair magnetization so obtained did not relax exponentially. 相似文献
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We report measurements of nuclear spin-lattice relaxation and dynamic polarization of protons in ethyleneglycol-alcohol mixtures at 9 kG and 4·2 K and 1·3 K. Comparison of experimental results with a proposed model of spin-lattice relaxation indicates that the role of dipolar and exchange reservoirs of paramagnetic ions in nuclear relaxation is not necessarily negligible, particularly in the presence of high concentration of paramagnetic impurities. The influence of methanol is to enhance the dynamic polarization of protons compared to pure ethyleneglycol, in the most favourable cases by almost 50% in our experimental conditions. 相似文献
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Samuel M. Myers Horst Meyer
J. P. Remeika
《Journal of Physics and Chemistry of Solids》1969,30(12):2687-2692Pulse measurements of T1 for 57Fe nuclei in very pure, single crystals of YIG are reported. The temperature was varied from 2° to 292°K, and the externally applied field ranged from 0 to 6000 Oe. The temperature variation of T1 is quite strong, being three orders of magnitude in the range 2°–40°K. At constant temperature, T1 changed approximately one order of magnitude between saturation field and 6000 Oe. The data are compared with the results of a calculation by Beeman and Pincus, in which a second-order Raman process and the three-magnon process are assumed to predominate below 50°K. Agreement is only qualitative, the experimental values of T1 being larger than predicted. At 4.2°K in zero field, it is found that a polycrystalline sample containing particles of ≈ 5 × 10−4 cm dia. has a value of 1/T1 which is some two orders of magnitude larger than for a macroscopic crystal. The presence of a relaxation mechanism associated with surface effects is suggested. 相似文献
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Fluorine-19 spin-lattice relaxation of electron-beam-irradiated poly(tetrafluoroethylene) (PTFE) has been investigated in the temperature range from 250 to 315 K. As shown before, in the initial step, radicals are produced by the electron-beam irradiation and chain scission takes place. The concentrations of radicals and chain end groups after irradiation of PTFE strongly depend on the irradiation conditions. Radicals like other paramagnetic species decrease the spin-lattice relaxation times. In addition, decreased polymer chain lengths shift theT 1 minimum to lower temperatures. Tetrafluorosuccinic acid in solution was used as a model system and paramagnetic copper sulphate CuSO4 added to quantify the effect on the relaxation times. The shift of the minima inT 1 versus temperature in PTFE are compared with the chain length determined from high-resolution solid-state nuclear magnetic resonance spectra and with the concentration of paramagnetic species. 相似文献
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A. F. Klimachev 《Russian Physics Journal》1969,12(8):998-1002
The Van Vleck method is used with the point-charge model to calculate the spin-lattice relaxation times of the Ce3+ ion in binary nitrates. Normal coordinates are found for the icosahedral XY12 complex containing the paramagnetic ion and its nearest neighborhood. The theoretical transition probabilities for the purely ionic model turn out to be slightly above the experimental results, so covalent effects are important.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 8, pp. 31–37, August, 1969.The author thanks L. K. Aminov for supervision of and assistance in this study. 相似文献
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I. S. Solodovnikov 《Journal of Experimental and Theoretical Physics》2002,94(4):770-778
Spin-lattice relaxation times T 1 were measured for solid 3He at temperatures of 0.22 to 0.73 K in a 44-Oe magnetic field. An increase in T 1 at temperatures higher than approximately 0.4 K was related to switching on the vacancy mechanism of atomic mobility in the crystal. At a melting curve minimum, in the region of predominance of exchange motions of atoms in the crystal, measurements of T 1 were performed in magnetic fields of 2 to 71 Oe. The data obtained in fields higher than 5 Oe were in agreement with the Cowan-Fardis theory. 相似文献
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Charnaya EV Loeser T Michel D Tien C Yaskov D Kumzerov YA 《Physical review letters》2002,88(9):097602
Nuclear spin relaxation for liquid gallium embedded into nanoporous matrices was found to accelerate remarkably compared to the bulk melt. NMR measurements on two gallium isotopes showed that the dominant mechanism of relaxation was changed from magnetic to quadrupolar and the relation rate depended on the Larmor frequency. The correlation time of electric field gradient fluctuations was estimated using data for quadrupolar relaxation contribution and was found to increase drastically compared to bulk, which corresponded to slowing down mobility in confined liquid gallium. 相似文献
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《Magnetic resonance imaging》1995,13(3):379-385
Spin-lattice relaxation time T1 and relaxation parameters in magnetization transfer (MT) imaging were measured in 11 intracranial tumors before and after injection of Gd-DTPA at 0.1 T by using the inversion recovery method and the saturation transfer technique, respectively. Preinjection T1 relaxation times of the tumors were longer than those of white matter, but after Gd-enhancement the relaxation times of most tumors were in the same range as those of white matter. Gd-DTPA shortened the apparent relaxation time in the presence of off-resonance saturation pulse (T1α) due to marked shortening of the relaxation time of mobile water (T1w). Gd-DTPA decreased the magnetization transfer contrast (MTC) but did not influence on the magnetization transfer rate (Rwm). The parameters MTC and Rwm differed clearly between Gd-enhanced tumors and normal brain, whereas the relaxation time T1α was in many Gd-enhanced tumors in the same range as in normal brain. 相似文献
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The proton spin-lattice relaxation times, T1, of methyl groups in (CH3CO)2O, CH3COCl, CH3 COBr and (CH3)2S2 have been measured below melting points at 52 MHz. The observed T1 minima display the presence of tunnelling rotation. From the fit of the experimental results the ground, the first excited state tunnelling frequencies and the energy difference between the ground and the first excited states of the compounds have been estimated. 相似文献
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Prof. Dr. T. G. Castner Jr. 《Zeitschrift für Physik B Condensed Matter》1970,12(2):104-130
An experimental and theoretical study of the spin-lattice relaxation (SLR) of the O2 ? molecule-ion in the potassium halides has been made. The SLR rate was measured by several methods between 1.32°K and 36°K with a 3 cm. ESR spectrometer. The results indicate a one phonon process with a large anisotropy dominates forT<2°K.T 1 is much longer for the magnetic field parallel to the molecular axis. Detailed theoretical calculations indicate a new SLR process, namely phonon-induced libration of the molecular axis, can probably explain the field parallel case. The Van Vleck SLR process can account for the field-perpendicular case. 相似文献
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The solid state diffusion of hydrogen, or of its pseudo-isotope muonium, provides an interesting example of spin-lattice relaxation in a 2-spin, 4-level system. The local field experienced by the interstitial atom fluctuates as it moves, inducing transitions between the coupled electron and nuclear spin states. Rate equations governing the populations of these states may be solved numerically to simulate the different relaxation functions which would be displayed by ESR, ENDOR and μSR spectroscopies and to assist in extracting motional correlation times from the experimental data. Spin relaxation in molecular radicals may be treated similarly, with different selection rules for different mechanisms: this paper treats the spin rotation mechanism and perturbation to anisotropic or isotropic components of the hyperfine interaction, caused by inter or intra-molecular motion. Conventional magnetic resonance monitors the population differences appropriate to particular transitions; only in sufficiently high fields do these distinguish the electronic and nuclear response. Muon spin relaxation is remarkable in separating out the nuclear spin projection whatever the degree of mixing of the spin states,via the asymmetry in the muon radioactive decay. Experimentally it has the advantage that measurements can be made over a wide range of field, from null external field up to thelevel crossing where the relaxation rate exhibits a striking peak. 相似文献
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《Journal of Magnetic Resonance (1969)》1978,29(2):191-199
Using the quadrupolar echo Fourier transform technique (Davis et al., Chem. Phys. Lett.42, 390 (1976)), T1 has been measured, in a sample of 70% potassium palmitate (d31)/30% H2O, as a function of chain position and temperature in the Lα liquid-crystalline phase. At temperatures near the transition from the liquid-crystal to the gel phase, T1 is roughly constant for the first eight or nine chain positions but increases rapidly on moving further out along the chain. As the temperature is increased the relaxation times for all positions increase but those nearer the head group increase more rapidly, resulting, of temperatures above ∼100°C, in a minimum in the plot of T1 versus chain position at the center of the chain (the 8 or 9 position). These results are in general accord with the temperature dependence of chain disorder (Davis and Jeffrey, Chem. Phys. Lipids20, 87 (1977) and offer some suggestions on the kind of model required to explain relaxation in these systems. 相似文献
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T1 measurements have been done on pure, NH3 doped and HF doped ice crystals. Three relaxation processes can be distinguished: a low temperature process, an extrinsic and an intrinsic process at high temperatures with activation energies of 0.08 eV, 0.35 eV and 0.62 eV respectively. Vested vacancies are proposed as defects effective in the low temperature process. The extrinsic high temperature process is due to vacancies which are formed by shifting a HF molecule to an interstitial site. Schottky defects are responsible for the intrinsic high temperature relaxation mechanism. Outdiffusion of HF is found to occur in concentration steps and at high concentrations by an interstitial mechanism. 相似文献