Nuclear Quadrupole Resonance Spectroscopy

BY K.B. DILLON

1 Introduction

This chapter reports on the pure nuclear quadrupole resonance (NQR) results for quadrupolar (I > 1/2) nuclei in inorganic or organometallic solids. There has been a small decrease in the number of articles published compared with the previous year, with no major conference in this area taking place. One notable feature has been an increase in publications for some nuclei, such as 115In, and a decrease for the halogens Cl, Br and I. The structure and strength of hydrogen bonds in inorganic solids have been reviewed, including results obtained by NQR spectroscopy. Nuclear magnetic resonance techniques, including NQR, as a means for the non-destructive characterisation of materials, have been surveyed. More specialised reviews have appeared on NQR (and NMR) studies of Cu nuclei in YbInCu4 under high pressure, and on recent NQR and high-field NMR results for 25As nuclei in crystalline and glassy samples of arsenic chalcogenides.

Patent applications have been filed for a transmit-receive coil system for NQR signal detection in substances and components thereof, for a receiving system for high q antennas in NQR and a method of detecting substances, for a scanner for NQR measurements and a method of detection for substances containing quadrupolar nuclei, for improvements to apparatus for NQR measurements, and for polarisation-amplified 14N NQR detection of TNT and other explosives in mines by using the quadrupole-quadrupole solid effect.

The normal format is followed in the more detailed sections, with results for main group elements followed by those for transition metals and lanthanides.

2 Main Group Elements

2.1 Group 13 (Gallium-69 and Indium-115). – NQR spectra of 69Ga and 115In nuclei have been recorded for some GaSe and InSe layer compounds. The spectra were analysed, starting from polytypes of these compounds and the probability of formation of ordered structural fragments in the basic crystal modifications. As part of a study of Ni-Y intermetallics (Y = Al, Ga, In, Ti), 69Ga and 115In NQR frequencies (as appropriate) have been measured for Ni2Ga3, ε-Niln and Ni2ln3. The electric field gradient (efg) parameters were computed in advance, enabling rapid location of the experimental signals. The Ni2Y3 structure requires two distinct Y sites, and these were observed for Y = Ga and In. In the Ga compound, the linewidth for the Ga(2) site was more than three times that for the Ga(1) site. The lineshapes appeared distinctly Lorentzian for both compounds, possibly indicating the onset of intersite hopping at room temperature (RT). Line assignments were confirmed from the computed values. Two distinguishable In sites were also found for ε-NiIn, and the results were in excellent agreement with computations using WIEN 97.

Unconventional superconductivity has been deduced in CeIn3 from 115In NQR, including spin-lattice relaxation rates (SLR), and AC-susceptibility measurements, as a function of temperature (T) between 0.05 and 100 K, and a pressure P of 2.65 GPa. A superconducting transition was found at Tc = 95 mK, very much lower than the onset temperature Tonset = 0.15 K at zero resistance. No coherence peak was observed just below Tc in the T-dependence plot, consistent with unconventional superconductivity. 115In NQR and AC-susceptibility data have been similarly reported for CeRhIn5, and 63Cu NQR results for YbInCu4, under pressure. CeRhIn5 showed a P-induced phase transition from antiferromagnetic (AF) ordering to a superconducting state. The studies revealed a homogeneous coexistence of these two types of orderings near the phase boundaries. The results for YbInCu4, which shows a first order valence transition at 42 K and ambient P, indicated a ferromagnetically ordered ground state after the valence transition was suppressed by P. The P-dependence of the 115In NQR parameters has been monitored at various T for CeRhIn5. Some AC-susceptibility measurements were also described. The compound underwent a superconducting transition at Tc ~ 2.1 K for P = 1.6 GPa. The Néel temperature TN was reduced above P = 1.23 GPa, accompanied by emergent pseudogap behaviour. The results at 1.6 GPa revealed that antiferromagnetism and superconductivity coexist microscopically. This coexistent state was suggested to persist down to P ~ 1.5 GPa. The superconductivity did not show any trace of gap opening in the low-lying excitations below the onset temperature 2 K, and the results indicated its unconventional characteristics.

T-dependence of the 115In NQR parameters, including the SLR, from 0-40 K has been monitored for the heavy-fermion superconductor CeCoIn5 (Tc 2.3 K). Some 59Co NMR data were also obtained. SLR measurements revealed that the magnetic nature was characterised by strong AF spin fluctuations in the vicinity of the quantum critical point (QCP). The anomalous T-dependence of the SLR for 115In nuclei could be well explained by the anisotropic spin-fluctuation model. AF spin fluctuations were found to be suppressed by a small magnetic field, reinforcing the view that CeCoIn5 is located close to the QCP. The effect on the Néel temperature TN caused by substitution of Ir for Rh in CeRh1-xIrxIn5 (x = 0, 0.25 or 0.45) has been studied by 115In NQR, including the T-dependence of the SLR. TN was found to increase slightly on substitution of Ir for Rh up to x = 0.45. This resembled the behaviour in hydrostatically pressurised CeRhIn5 below 1.0 GPa. These measurements were extended to CeRh0.5Ir0.5In5 and CeRhIn5 under pressures up to 1.6 GPa; LaRhIn5 and LaIrIn5 were also used as reference compounds. TN decreased with application of P = 1.6 GPa or replacing half of Rh with Ir, probably due to an increase in bandwidth. In CeRh0.5Ir0.5In5, unconventional superconductivity set in well below TN, with Tc ~ 1 K. The 115In SLR as a function of T has been followed for Ce (Ir, Rh) In5, and for analogous La (Ir, Rh) In5. The SLR was higher by an order of magnitude for the Ce compounds, indicating strong magnetic fluctuations. CeIrIn5 was located near the QCP, with quasi-2D spin fluctuations. For this compound, the SLR varied with T3 below Tc = 0.40 K, indicating unconventional superconductivity with a line-node gap. CeIrIn5 was much more itinerant than CeRhIn5.

2.2 Group 14 (Germanium-73). – 73Ge NQR (and NMR) data have been obtained for a 73Ge-enriched sample of UGe2 at ambient P and 1.3 GPa. At the latter P, the T-dependence of the SLR from 0.01-100 K demonstrated the onset of a superconducting transition at Tc = 0.55 K. The lack of a coherence peak just below Tc, followed by T3-like behaviour in the T-dependence plot, provided evidence for an unconventional superconducting state that coexists with the ferromagnetic state...