  Thermoelectric effects have been applied to power generators and temperature
sensors that convert waste heat into electricity. The effects, however, have
been limited to electrons to occur, and inevitably disappear at low
temperatures due to electronic entropy quenching. Here, we report
thermoelectric generation caused by nuclear spins in a solid: nuclear-spin
Seebeck effect. The sample is a magnetically ordered material MnCO$_{3}$ having
a large nuclear spin ($I = 5/2$) of $^{55}$Mn nuclei and strong hyperfine
coupling, with a Pt contact. In the system, we observe low-temperature
thermoelectric signals down to 100 mK due to nuclear-spin excitation. Our
theoretical calculation in which interfacial Korringa process is taken into
consideration quantitatively reproduces the results. The nuclear thermoelectric
effect demonstrated here offers a way for exploring thermoelectric science and
technologies at ultralow temperatures.
