Separation of quadrupolar and magnetic contributions to spin-lattice relaxation in the case of a single isotope

Finding the true spin-lattice relaxation time for half-integral nuclei with non-zero quadrupole couplings

Measuring true spin-lattice relaxation times T(1) of half-integral quadrupolar nuclei having non-zero nuclear quadrupole coupling constants (NQCCs) presents challenges due to the presence of satellite-transitions (STs) that may lie outside the excitation bandwidth of the central transition (CT). This leads to complications in establishing well-defined initial conditions for the population differences in these multi-level systems. In addition, experiments involving magic-angle spinning (MAS) can introduce spin exchange due to zero-crossings of the ST and CT (or possibly rotational resonance recoupling in the case of multiple sites) and greatly altered initial conditions as well. An extensive comparison of pulse sequences that have been previously used to measure T(1) in such systems is reported, using the (71)Ga (I=3/2) NMR of a Ge-doped h-GaN n-type semiconductor sample as the test case. The T(1) values were measured at the peak maximum of the Knight shift distribution. Analytical expressions for magnetization-recovery of the CT appropriate to the pulse sequences tested were used, involving contributions from both a magnetic relaxation mechanism (rate constant W) and a quadrupolar one (rate constants W(1) and W(2), approximately equal in this case). An asynchronous train of high-power saturating pulses under MAS that is able to completely saturate both CT and STs is found to be the most reliable and accurate method for obtaining the "true T(1)", defined here as (2W+2W1,2)(-)(1). All other methods studied yielded poor agreement with this "true T(1)" value or even resulted in gross errors, for reasons that are analyzed in detail. These methods involved a synchronous train of saturating pulses under MAS, an inversion-recovery sequence under MAS or static conditions, and a saturating comb of pulses on a static sample. Although the present results were obtained on a sample where the magnetic relaxation mechanism dominated the quadrupolar one, the asynchronous saturating pulse train approach is not limited to this situation. The extent to which W(1) and W(2) are unequal does affect the interpretability of the experiment however, particularly when the quadrupolar mechanism dominates. A numerically approximate solution for the I=3/2 recovery case reveals the quantitative effects of any such inequality.

Magnetization-recovery experiments for static and MAS-NMR of I=3/2 nuclei

Multifrequency pulsed NMR experiments on quadrupole-perturbed I=3/2 spins in single crystals are shown to be useful for measuring spin-lattice relaxation parameters even for a mixture of quadrupolar plus magnetic relaxation mechanisms. Such measurements can then be related to other MAS-NMR experiments on powders. This strategy is demonstrated by studies of (71)Ga and (69)Ga (both I=3/2) spin-lattice relaxation behavior in a single-crystal (film) sample of gallium nitride, GaN, at various orientations of the axially symmetric nuclear quadrupole coupling tensor. Observation of apparent single-exponential relaxation behavior in I=3/2 saturation-recovery experiments can be misleading when individual contributing rate processes are neglected in the interpretation. The quadrupolar mechanism (dominant in this study) has both a single-quantum process (T(1Q1)) and a double-quantum process (T(1Q2)), whose time constants are not necessarily equal. Magnetic relaxation (in this study most likely arising from hyperfine couplings to unpaired delocalized electron spins in the conduction band) also contributes to a single-quantum process (T(1M)). A strategy of multifrequency irradiation with observation of satellite and/or central transitions, incorporating different initial conditions for the level populations, provides a means of obtaining these three relaxation time constants from single-crystal (71)Ga data alone. The (69)Ga results provide a further check of internal consistency, since magnetic and quadrupolar contributions to its relaxation scale in opposite directions compared to (71)Ga. For both perpendicular and parallel quadrupole coupling tensor symmetry axis orientations small but significant differences between T(1Q1) and T(1Q2) were measured, whereas for a tensor symmetry axis oriented at the magic-angle (54.74 degrees ) the values were essentially equal. Magic-angle spinning introduces a number of complications into the measurement and interpretation of the spin-lattice relaxation. Comparison of (71)Ga and (69)Ga MAS-NMR saturation-recovery curves with both central and satellite transitions completely saturated by a train of 90 degrees pulses incommensurate with the rotor period provides the simplest means of assessing the contribution from magnetic relaxation, and yields results for the quadrupolar mechanism contribution that are consistent with those obtained from the film sample.

Charge degree of freedom and the single-spin fluid model in YBa2Cu4O8

We present a 17O nuclear magnetic resonance study in the stoichiometric superconductor YBa2Cu4O8. A double irradiation method enables us to show that, below around 200 K, the spin-lattice relaxation rate of plane oxygen is not only driven by magnetic fluctuations, but also significantly by quadrupolar fluctuations, i. e., low-frequency charge fluctuations. In the superconducting state, on lowering the temperature, the quadrupolar relaxation diminishes faster than the magnetic one. These findings show that, with the opening of the spin pseudogap, a charge degree of freedom of mainly oxygen character is present in the electronic low-energy excitation spectrum.