Search for a Magnetic-Field Dependence of the Interaction of the Nuclear Quadrupole Moment with the Electric-Field Gradient

It is argued that the nuclear quadrupole–electric field gradient (EFG) interaction is, in principle, dependent on the presence of a magnetic fieldB. A rough estimate of the size of this effect yields 10⁻⁴in fields up to 10 T. However, if the site symmetry of the nucleus in question includes time-reversal symmetry, the linear dependence of the EFG onBvanishes. In diamagnetic compounds, time-reversal symmetry is violated only by the presence of nuclear spins. In such compounds, the dominant dependence of the EFG onBshould be quadratic and should be described by a fourth-rank tensor. In ferro- and antiferromagnetic compounds time-reversal symmetry is strongly violated and a linear dependence of the EFG onB, described by a third-rank tensor, is expected. A search for a magnetic field dependence of the EFG was carried out by measuring the quadrupole coupling constants (QCCs) of the²⁷Al and¹⁴N nuclei in corundum and sodium nitroprusside (SNP) by pure NQR, and by NMR in fields of 6.3 and 11 T. These diamagnetic compounds were selected because previous measurements, done in different fields, yielded differing results for the QCCs. A new technique for measuring QCCs by NMR is introduced that circumvents the necessity of precisely orienting the sample crystals. For the QCCs of both the²⁷Al and¹⁴N nuclei in corundum and SNP, respectively, a precision of distinctly better than 10⁻⁴is reached. The results obtained in 0, 6.3, and 11 T fields fully agree with each other which means that, in fields up to 11 T, any possible field dependence of the QCCs is smaller than 10⁻⁴. These results confirm that in diamagnetic compounds a linear dependence of QCCs onBis largely suppressed.