Nernst effect

In the ordinary Nernst effect, heat flows through an electrically conducting sample because of a temperature gradient. When a magnetic field is applied perpendicular to that gradient, charge carriers experience magnetic deflection as they move. The resulting transverse charge separation produces a measurable voltage across the sample. In a common geometry, the thermal gradient is along one axis, the magnetic field is normal to the sample plane, and the Nernst electric field is along the remaining axis.

The effect is often described using a Nernst coefficient: a transverse electric-field response normalized by both temperature gradient and magnetic field, with sign and numerical conventions varying between fields of study. At steady state, the measured voltage is generated by the balance of carrier transport and the induced electric field; it is not simply a transverse current under all measurement conditions.

The ordinary Nernst effect (ONE) is associated with an externally applied magnetic field acting on charge carriers in a conductor. In magnetic conductors, a transverse thermoelectric response can also be linked to magnetization and the electronic structure. This contribution is called the anomalous Nernst effect (ANE). It can remain when the applied field is zero if the material retains spontaneous magnetization.

The ANE is connected to transverse electrical transport phenomena, including the anomalous Hall effect, but its response is driven by a temperature gradient rather than solely by an applied electrical current. In modern accounts, spin–orbit coupling and Berry curvature in electronic bands can be important to its magnitude. These microscopic explanations are material-dependent; a large observed signal alone does not uniquely identify a particular mechanism.

Experiments impose a known temperature gradient across a sample, apply or vary a magnetic field, and measure the voltage across the perpendicular direction. Since other thermoelectric and magnetotransport signals may appear in the same geometry, researchers commonly compare measurements at opposite field directions to isolate the part of the response that is odd in magnetic field. Accurate work also requires controlling temperature drift, contact geometry, and electrical noise.

The Nernst response can help characterize materials with multiple types of charge carriers, unusual scattering, superconducting fluctuations, or distinctive magnetic band structures. Interpretation generally requires comparison with other measurements, such as the Hall and Seebeck effects, rather than treating a Nernst voltage as a standalone identifier of a material's properties.

Because the electrical response is perpendicular to the imposed temperature gradient, Nernst-based thermoelectric devices use a transverse geometry, unlike conventional Seebeck devices where voltage develops along the gradient. This geometry has drawn interest for harvesting heat distributed over broad areas and for simplifying some device layouts. Reviews identify topological semimetals and magnetic materials among actively studied material families.

The possibility of a useful transverse voltage does not by itself establish high conversion efficiency or commercial readiness. Device performance depends on material transport properties, magnetic-field requirements, contacts, thermal losses, and scalable fabrication. The field remains an active area of materials and device research.