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

We describe an alternating current method for the measurement of the longitudinal (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 increase 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. The phase difference between two heater currents gives a temperature gradient at 2f(0). The corresponding thermopower and Nernst effect signals are extracted by using a digital signal processing method due to the low frequency of the measurement. An important component of the method involves a superconducting link, YBa2Cu3O7+delta, which is mounted in parallel with sample to remove the background magnetothermopower of the lead wires. The method is demonstrated for the quasi-two-dimensional organic conductor alpha-(BEDT-TTF)(2)KHg(SCN)(4), which exhibits a complex, magnetic field dependent ground state above 22.5 T at low temperatures. (C) 2001 American Institute of Physics.