  The Nernst effect is the transverse electric field produced by a longitudinal
thermal gradient in presence of magnetic field. In the beginning of this
century, Nernst experiments on cuprates were analyzed assuming that: i) The
contribution of quasi-particles to the Nernst signal is negligible; and ii)
Gaussian superconducting fluctuations cannot produce a Nernst signal well above
the critical temperature. Both these assumptions were contradicted by
subsequent experiments. This paper reviews experiments documenting multiple
sources of a Nernst signal, which, according to the Brigman relation, measures
the flow of transverse entropy caused by a longitudinal particle flow. Along
the lines of Landauer's approach to transport phenomena, the magnitude of the
transverse magneto-thermoelectric response is linked to the quantum of
thermoelectric conductance and a number of material-dependent length scales:
the mean-free-path, the Fermi wavelength, the de Broglie thermal wavelength and
the superconducting coherence length. Extremely mobile quasi-particles in
dilute metals generate a widely-documented Nernst signal. Fluctuating Cooper
pairs in the normal state of superconductors have been found to produce a
detectable Nernst signal with an amplitude conform to the Gaussian theory,
first conceived by Ussishkin, Sondhi and Huse. In addition to these microscopic
sources, mobile Abrikosov vortices, mesoscopic objects carrying simultaneously
entropy and magnetic flux, can produce a sizeable Nernst response. Finally, in
metals subject to a magnetic field strong enough to truncate the Fermi surface
to a few Landau tubes, each exiting tube generates a peak in the Nernst
response. The survey of these well-established sources of the Nernst signal is
a helpful guide to identify the origin of the Nernst signal in other
controversial cases.
