Heat control in mesoscopic conductors - exploiting quantum effects and size confinement

Abstract: This thesis deals with a theoretical analysis of heat currents, their exploitation and their control in nanoscale devices. The motivation for this study is twofold. (i) The development of nanoscale devices sets up the basis of many applications ranging from nanoelectronics to quantum technology. Such nanodevices, typically operated at low temperatures, are highly sensitive to heating effects. Hence the successful performance of these devices relies on controlling and managing this heat. (ii) Nanostructures provide appealing systems to study quantum and nonequilibrium thermodynamics because, at such small scales, the behavior of systems is highly affected by size confinement and quantum effects. The central purpose of this thesis is to investigate the impact of specific characteristics of quantum systems, in particular of quantum size confinement, nonequilibrium effects and phase coherence, on heat transport quantities. A better understanding of this impact can lead to an improved control and exploitation of heat. This can be used for the evacuation of heat from the system, cooling, or producing power using waste heat. We propose different experimentally accessible setups. In these setups, we theoretically study transport quantities using a scattering formalism. We pursue three main study lines in different setups: (i) We investigate phase-dependent heat transport in normal- and superconducting hybrid junctions. We show how disorder influences this, both in simple junctions as well as in a heat circulator. (ii) We analyze thermodynamical machines, which use nonequilibrium states as their resource instead of heat. Such devices show a "demonic behavior" since they seemingly challenge the second law of thermodynamics. (iii) We analyze how to exploit energy filtering of quantum conductors to perform thermoelectric cooling at the example of a quantum spin Hall device in the whole range from linear to nonlinear response.