Nuclear Spin Relaxation in Molybdenum Metal

The ⁹⁵Mo and ⁹⁷Mo nuclear spin-spin and spin-lattice relaxation rates in molybdenum metal have been studied in the temperature range 1≤T≤4^∘{}K. The transverse relaxation process is found to have an exponential time dependence (corresponding to a Lorentzian line shape) with characteristic times T₂(⁹⁵Mo)=10±1 msec and T₂(⁹⁷Mo)=14±1 msec which are independent of temperature. At 4.0^∘{}K the longitudinal relaxation times are T₁(⁹⁵Mo)=8.9±0.2 sec and T₁(⁹⁷Mo)=7.6±0.2 sec. The ratio T₁(⁹⁵Mo)T₁(⁹⁷Mo)=1.17±0.03 is larger than the square of the nuclear moment ratio, (μ⁹⁷μ⁹⁵)²=1.0424. This anomaly is attributed to an electric quadrupole process due to d-band conduction electrons. This process contributes significantly to the relaxation rate of ⁹⁷Mo but not to that of ⁹⁵Mo because of the large difference in nuclear quadrupole moments (Q⁹⁷Q⁹⁵=9.2). The known moment ratios are used to partition the observed rates into nuclear magnetic dipole (R_μ) and nuclear electric quadrupole (RQ) contributions. The resulting values of RQ yield quadrupole moment estimates Q⁹⁵=(0.12±0.03)×10^-24 cm² and Q⁹⁷=(1.1±0.2)×10^-24 cm². An approximate separation of R_μ into contact, core-polarization, and orbital rates has been achieved. The principal contribution to the Knight shift and to the conduction-electron susceptibility is shown to arise from the orbital magnetization of the d band. The results of this study provide an upper-limit estimate of about 3 for the electron-phonon enhancement of the s-electron specific heat.