LOW-TEMPERATURE RADIO-FREQUENCY SHIELDING OF EXFOLIATED GRAPHITE

The results of experiments using exfoliated graphite as a low temperature substrate for NMR studies of physisorbed molecules at ultra-high frequencies (100-300 MHz) are reported. The radio frequency shielding was observed to be anomalously high at very high frequencies for the graphite samples studied and precluded the use of this material for NMR studies requiring high sensitivity in this frequency range at liquid helium temperatures. It is shown that this behaviour can be understood in terms of the in- and out-of-plane conductivities and the capacitive coupling of the microcrystalline structure of the graphite.