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Mesoscopic heattronics: thermal and nonequilibrium effects and fluctuations in nanoelectronics

Final Report Summary - HEATTRONICS (Mesoscopic heattronics: thermal and nonequilibrium effects and fluctuations in nanoelectronics)

HEATTRONICS project studied how heat is distributed in small electronic conductors. The emphasis was on fundamental processes and at low temperatures where different heat conduction channels can be separately characterized, but the outcomes are also relevant for applications. The major outcomes of this project are (i) prediction of a novel mechanism for highly efficient thermoelectric conversion between heat and electricity in systems composed of superconductors and ferromagnets, and simultaneously providing an explanation of the measured long spin-flip lengths in these systems (ii) understanding fluctuations of the temperature in non-linear electronic components, (iii) providing insight into high-frequency phenomena in superconducting devices, and their use for extremely accurate sensing, (iv) finding out how electrons lose heat to phonons in graphene, (v) showing how the energy transfer between a microwave cavity and a nanomechanical resonator can be used for quantum-limited microwave amplification and long-time storage of optical information, (vi) providing the first realistic prediction of reaching the single-photon strong coupling limit of optomechanics, and contributing to the first realisation of this scheme, and (vii) prediction and description of mechanisms for high-temperature surface superconductivity in certain types of layered systems. Besides helping to understand fundamental heat transfer processes and fluctuations, these findings help in improving nanoelectronic devices used for amplification, detection of charge and heat, and quantum computing.
Electronics is based on the flow of charge in response to varying fields. Along with the charge, its carriers, electrons, also carry heat. Operation of electronics thus results into heat being dissipated and carried through the devices. One of the major limitations to the miniaturization of electronics is the problem of overheating: the unavoidable heating related to the driving of the electronic components eventually heats them up too much, and either the process which was utilized in the device ceases to work or the heated device becomes unpractical to use. Along with heating comes another problem: thermal fluctuations of observables, resulting into errors in the operations. Moreover, these fluctuations are especially problematic for finding of quantum mechanical effects in large systems, as the quantum features tend to be washed out by fluctuations.
Nanoelectronic devices are proposed and already used in various types of applications, ranging from amplifiers and detectors to quantum computing. Reducing their size typically allows for improving their sensitivity or coherence. The small size means a small heat capacity and therefore strong sensitivity to outside influence. This property employed in detection applications also means that fluctuations couple strongly to the device. One type of fluctuations often overlooked is related to the time-dependent fluctuations of the total (thermal) energy stored in the device. Often these fluctuations can be described by temperature fluctuations. In this project we have devised new theoretical schemes to study the fluctuations of temperature in out-of-equilibrium nanoscale electronic systems and shown how they may become strongly non-Gaussian and furthermore, typically at the parameter values of interest for detection or amplification purposes, induce strong fluctuations of the charge current.
One way of tackling the problem with excessive heating is to use the waste heat and convert it back to electrical energy. This is what is done with thermoelectric devices. The problem typically is that such thermoelectric conversion is quite inefficient. Within the project, we have shown how combinations of superconductors and ferromagnets can be used to cure this problem, as a proper combination of them can be used to build an almost ideally efficient thermoelectric device. Our prediction of the huge thermoelectric effect was also recently confirmed experimentally.
Besides making the nanoelectronics accurate, an overall wish is to make them also fast by operating them with high-frequency fields. Therefore we have studied superconducting heterostructures, relevant especially for ultrasensitive detection and for quantum computing, in the presence of microwave driving. We have uncovered novel phenomena related to this driving, and characterized the response to such fields by utilizing and extending modern quantum-mechanical theories of out-of-equilibrium effects in inhomogeneous systems.
Together with another ERC project, NEMSQED, we have studied macroscopic quantum effects in nanomechanical resonators and for example shown how these systems can be used as quantum-limited microwave amplifiers.
We have also studied heat transfer processes in novel nanomaterials, such as graphene. Our results there are relevant especially in the use of graphene as a detector of high-frequency radiation.
As a side step from heattronics, we have studied topological media and for example predicted the occurrence of high-temperature surface superconductivity in certain layered systems. Our predictions may explain the recent findings of superconductivity in graphite and provide routes for achieving higher operating temperatures for superconductors.