Low-temperature anomalies in the thermoelectric power of highly oriented graphite

The thermoelectric power (TEP) of kish graphite and highly oriented pyrolytic graphite exhibits three distinct anomalies. A pronounced negative dip around 30--40 K is observed in most samples and was ascribed to an ordinary phonon-drag effect. A sharp peak around 10--15 K is attributed to the two-stage drag effect in contrast with the normal drag effect at 30--40 K and this peak disappears as the structural perfection is lowered. A negative dip in the TEP observed below 4.2 K is a new finding by Hishiyama and Ono; and the dip strongly depends on the structural perfection. The two anomalies at 30--40 K and at 10--15 K are associated with the phonon-drag effect due to the in-plane vibration, while the newly observed anomaly is due to the interaction with out-of-plane vibrations. The phonon-drag TEP is proportional to C₄₄ which is sensitive to the structural perfection, especially to the degree of stacking faults. This is consistent with the observed results and the calculation qualitatively explains the observed temperature dependence and its order of magnitude.