Electric power may, in principle, be generated in a highly efficient manner from heat created by focused solar irradiation, chemical combustion, or nuclear decay by means of thermionic energy conversion. As the conversion efficiency of the thermionic process tends to be degraded by electron space charges, the efficiencies of thermionic generators have amounted to only a fraction of those fundamentally possible. We show that this space-charge problem can be resolved by shaping the electric potential distribution of the converter such that the static electron space-charge clouds are transformed into an output current. Although the technical development of practical generators will require further substantial efforts, we conclude that a highly efficient transformation of heat to electric power may well be achieved.

Graphene, being an atomically thin conducting sheet, is a candidate material for gate electrodes in vacuum electronic devices, as it may be traversed by low-energy electrons. The transparency of graphene to electrons with energies between 2 and 40 eV has been measured by using an optimized vacuum-triode setup. The measured graphene transparency equals ∼60% in most of this energy range. Based on these results, nano-patterned sheets of graphene or of related two-dimensional materials are proposed as gate electrodes for ambipolar vacuum devices.

Transparency of graphene for low-energy electrons measured in a vacuum-triode setup APL Materials 3, 076106 (2015) Mobile energy converters require, in addition to high conversion efficiency and low cost, a low mass. We propose to utilize thermoelectronic converters that use 2D-materials such as graphene for their gate electrodes. Deriving the ultimate limit for their specific energy output, we show that the positive energy output is likely close to the fundamental limit for any conversion of heat into electric power. These converters may be valuable as electric power sources of spacecraft, and with the addition of vacuum enclosures, for power generation in electric planes and cars.

We discuss a scenario for interface-induced superconductivity involving pairing by dipolar excitations proximate to a two-dimensional electron system controlled by a transverse electric field. If the interface consists of transition metal oxide materials, the repulsive on-site Coulomb interaction is typically strong and a superconducting state is formed via exchange of non-local dipolar excitations in the d-wave channel. Perspectives to enhance the superconducting transition temperature are discussed.Introduction. Enormous progress has been made in recent years in the fabrication and control of interfaces of strongly correlated oxide systems. As is well known, these materials exhibit unusual electronic properties already in their bulk form, and the interface of two unlike oxides appears to allow for the formation of additional novel states. The sensitivity of such states to external parameters raises the intriguing possibility of precise quantum control of devices based on interfaces of this type. In particular, field effect devices have been used in LaAlO 3 /SrTiO 3 -heterostructures to switch on and off a two-dimensional (2D) electron liquid localized at the interface 1 . Recently, it was discovered that a superconducting state can be created and electrostatically modulated in such systems 2 , albeit at quite low temperatures. This discovery has once again raised the question of whether or not superconductivity can be influenced by interface phenomena, and indeed whether interfaces with semiconducting or insulating materials might constitute an entirely new mechanism for realizing a hightemperature superconductor. Recent discoveries of superconductivity with transition temperatures near 60 K in Fe-based materials 3 have reinvigorated discussions of novel ways to create higher temperature superconductivity, including many intriguing ideas which were effectively abandoned after they went unrealized in the early years after the advent of BCS theory. In this paper, we discuss a mechanism for interface-mediated superconductivity specific to oxide interfaces and investigate whether high critical temperatures might be possible.Our discussion is very much in the spirit of an early model due to Little, who proposed that high temperature superconductivity with a temperature scale determined by electronic energy scales of the order of the Fermi energy might be realized with metalorganic chain compounds with quasi-1D metallic spines coupled to polarizable (organic) side chains 4 . The electronic excitations in the side chains were assumed to induce Cooper pairing in the metallic spine. A crucial element of Little's scheme was the spatial separation of the metallic electrons from the excitations mediating pairing. This separation prevented the screening of the excitations which otherwise would have lowered the pairing scale. In fact, Little