Experimental observation of nuclear-spin Seebeck effect

Abstract

Seebeck effects, the generation of voltages from temperature gradients via thermal electron motion, have been applied to temperature sensors and power generators that convert thermal energy into electricity. Recently, the electron-spin counterpart of the effect — the spin Seebeck effect — was discovered in spintronics, which generates a thermoelectric voltage from electron-spin fluctuation through a spin current. However, these effects have been limited to electrons, and they inevitably disappear at low temperatures due to electronic entropy quenching. In this article, we report thermoelectric generation driven by nuclear spins in a solid, that is, the nuclear-spin Seebeck effect. The sample is a magnetically ordered material, MnCO₃, having a large nuclear spin (I = 5/2) of ⁵⁵Mn nuclei with a Pt contact. In the system, we observed low-temperature thermoelectric signals reduced to 100 mK owing to nuclear-spin excitation. Theoretical calculation show that the interfacial Korringa process plays an important role. The nuclear thermoelectric effect described here provides an approach for exploring thermoelectric science and technologies at ultralow temperatures.