Impurities and the anomalous spin-lattice relaxation in copper at submillkelvin temperatures

The spin-lattice relaxation time tau //1 of copper has been studied as a function of the external magnetic field at constant electronic temperatures between 50 and 300 mu K. The ratio r equals tau //1(15 mT)/ tau //1(0) for a high purity copper sample was found to be as high as 100. By selective oxidation r could be reduced to 2. 6 in the same specimen. Present theories seem unable to explain the anomalously rapid relaxation in zero field.