  In recent years, the study of heat to work conversion has been re-invigorated
by nanotechnology. Steady-state devices do this conversion without any
macroscopic moving parts, through steady-state flows of microscopic particles
such as electrons, photons, phonons, etc. This review aims to introduce some of
the theories used to describe these steady-state flows in a variety of
mesoscopic or nanoscale systems. These theories are introduced in the context
of idealized machines which convert heat into electrical power (heat-engines)
or convert electrical power into a heat flow (refrigerators). In this sense,
the machines could be categorized as thermoelectrics, although this should be
understood to include photovoltaics when the heat source is the sun. As quantum
mechanics is important for most such machines, they fall into the field of
quantum thermodynamics. In many cases, the machines we consider have few
degrees of freedom, however the reservoirs of heat and work that they interact
with are assumed to be macroscopic. This review discusses different theories
which can take into account different aspects of mesoscopic and nanoscale
physics, such as coherent quantum transport, magnetic-field induced effects
(including topological ones such as the quantum Hall effect), and single
electron charging effects. It discusses the efficiency of thermoelectric
conversion, and the thermoelectric figure of merit. More specifically, the
theories presented are (i) linear response theory with or without magnetic
fields, (ii) Landauer scattering theory in the linear response regime and far
from equilibrium, (iii) Green-Kubo formula for strongly interacting systems
within the linear response regime, (iv) rate equation analysis for small
quantum machines with or without ..... (SEE THE PDF FOR THE REST OF THIS
ABSTRACT)
