Low-frequency method for magnetothermopower and Nernst effect measurementson single crystal samples at low temperatures and high magnetic fields

We describe an alternating current method for the measurement of the longit udinal (S-xx) and transverse (S-xy, i.e., Nernst) thermopower of millimeter -size crystal samples at low temperatures (T <1 K) and high magnetic fields (B similar to 30 T). A low-frequency (33 mHz) heating method is used to in crease the resolution and to determine the temperature gradient reliably in high magnetic fields. Samples are mounted between two thermal blocks which are heated by a sinusoidal frequency f(0) with a pi /2 phase difference. T he phase difference between two heater currents gives a temperature gradien t at 2f(0). The corresponding thermopower and Nernst effect signals are ext racted by using a digital signal processing method due to the low frequency of the measurement. An important component of the method involves a superc onducting link, YBa2Cu3O7+delta, which is mounted in parallel with sample t o remove the background magnetothermopower of the lead wires. The method is demonstrated for the quasi-two-dimensional organic conductor alpha-(BEDT-T TF)(2)KHg(SCN)(4), which exhibits a complex, magnetic field dependent groun d state above 22.5 T at low temperatures. (C) 2001 American Institute of Ph ysics.