Harvesting of photo-excitations in organic solar-cells is fundamentally governed by the quantum mechanical property of spin. Indeed, spin determines the generation and recombination pathways for a particular species, which ultimately determines device performance. Crucial to solar cell operation are spin-triplet excitons, which have the potential to overcome conventional efficiency limits through the singlet fission mechanism in which two triplet excitons can be generated from a single singlet exciton. The unique spin signatures of triplet excitons makes them ideally suited for investigations using spin-resonance techniques. We propose to study the interaction between spin dynamics and transport properties using a novel technique where triplet excitons are coupled to a microwave superconducting resonator which can simultaneously probe transport and spin-resonance signals. Performing measurements at sub-Kelvin temperatures, we will search for new quartet states created through spin-locking of fission generated excitons by the electromagnetic field in the resonator. This experiment will extend the concept of Majorana-Brossel resonances developed in atomic physics and that we recently demonstrated in an organic context by showing the formation of an overall spin S = 1 from two S = 1/2 polarons due to a strong interaction with the electromagnetic field in a superconducting cavity. Since exction fission strongly depends on the local molecular environment, we also propose a combination of optical and microwave techniques to study exciton fission in the limit of a small number of triplet pairs. This will enable quantitative investigations on the influence of molecular packing and morphology. Triplet excitons are neutral spin carriers, they can thus provide a perfect spin current source without associated charge transport. We describe several device architectures that can be used to harness spin current from triplet-excitons, starting from inverse-spin Hall effect geometries to mesoscopic devices where mesoscopic superconductivity and carbon nanotube quantum dots can be very sensitive probes of their local magnetic environment. We show preliminary results from devices using an inverse-spin Hall effect geometry suggesting spin-transfer between triplet excitons and a platinum thin film. In an attempt to bridge the gap between our fundamental results and the problem of photovoltaic energy generation we will start investigations on dielectric whispering mode gallery resonators that should allow to extend our experimental techniques all the way to room temperature. As an application of his approach we plan to study the problem of energy and charge transfer between triplet excitons and semiconducting nanocrystals which is very important for photovoltaic devices taking advantage of exciton fission. Spin properties are also actively investigated in the context of quantum information processing where long coherence times are essential. The spin of electrons trapped on a liquid helium 4 surface are expected to show record coherence times due to the complete absence of hyperfine interactions and magnetic impurities. We propose to couple electronic spins on liquid helium and photo-excited spins in organic conductors. Electronic spins can then form an ultra-sensitive probe for the spintronics in organic conductors, while the optical manipulation of spin processes in the organic materials will allow to achieve a better control of the spins of electrons on helium.

Contrary to received wisdom, conventional superconductors out-of equilibrium (NECSs) can be good ferromagnets and also excellent spin-dependent thermoelectric materials, and the superconducting wavefunction can be composed of spin-aligned pairs of electrons. My recent experiments demonstrating out-of-equilibrium magnetism, spin-charge separation and quasiparticle spin resonance in superconducting aluminium revealed that the magnetic properties of NECSs depend strongly on voltage- and magnetic-field-induced spin/charge currents and can thus be finely tuned. This work has motivated theoretical studies of the thermoelectric properties of NECSs, which are expected to show similar sensitivity to accessible experimental parameters. Thus, out-of-equilibrium spin currents and distributions in superconductors promise enhanced versatility and novel concepts for spin- and heat-based electronics – respectively spintronics and caloritronics – in particular through spin-sensitive distribution-function engineering. Building on my previous work, I propose to demonstrate, and establish techniques to finely control, the inter-conversion of non-equilibrium spin currents with both heat and charge currents in superconductor and topological-insulator-based devices. In doing so, I shall close the heat-spin-charge current triangle of mutual conversion and control in superconductors. This will in turn bring new techniques to bear on fundamental, long-standing problems in superconductivity, including the mechanisms of spin relaxation and decoherence; and the manipulation of the constituent electron spins in Cooper pairs to generate triplet superconductivity or spin-filtered, spatially-separated entangled electrons.

Achieving light emission from group IV elements has been representing a pot of gold in modern Si technology for decades, involving generations of scientists trying to make Si and SiGe alloys optically active. Over the last few years, the emergence of data-intensive technological areas has made the need of implementing telecommunication functionalities (1.3-1.8 µm range) in group IV materials even more urgent, encouraging researchers to develop novel direct bandgap systems — where recombination of electrons and holes occurs with the same momentum — while keeping their compatibility with Si CMOS electronics. Among the variety of materials investigated, hexagonal-diamond Si1-xGex alloyed nanowires (2H-Si1-xGex NWs) are extremely attractive due to their easiness of fabrication and peculiar optical features. Indeed, although 2H-Si bulk is stable only at pressures larger than 12 GPa, recent studies demonstrated that, at the nanoscale, 2H-SiGe crystals can exist as pure wires. These nanostructures offer the unique possibility to combine an indirect semiconductor (2H-Si) with a direct one (2H-Ge) to obtain a light-emitting direct bandgap material between 1.3-1.8 µm. In fact, photoluminescence proved that increasing the Ge fraction (xGe) in 2H-Si lowers the minimum of the conduction band at Γ until the bandgap becomes direct — with an allowed optical transition — for xGe larger than 0.65. Despite these significant advances, the potentialities of 2H-Si1-xGex NWs remain widely unexplored and a unified description of their optical response is fundamentally lacking. On the one hand, current modelling techniques used, though accurate, cannot take into account the influence of intrinsic (size and composition) and extrinsic parameters (strain, substrates and dopants) on the NW dielectric response because of the high computational demand and convergence problems. On the other hand, most of the optical experimental characterization of such NWs is affected by an intrinsic significant difficulty in separating the role of different physical environmental variables. In this regard, since ultra-high-resolution STEM-EELS allows for the investigation of individual nano-objects it can hence provide unique information that to a considerable degree is obscured in many optical spectroscopic methods. The main breakthrough of the AMPHORE project is to investigate, through the effectual combination of ab initio approaches, semi-empirical methods, and computer simulations, the great potential as light emitters of 2H-Si1-xGex NWs in close integration with targeted advanced nanometer-scale optical measurements. The proposal presents two key challenging objectives: (i) the deep theoretical understanding, via precise quantum-mechanical modelling beyond the state-of-the-art, of the dielectric response of 2H-Si1-xGex NWs in a realistic environment, including morphology, substrates, and dopants and (ii) the design and accurate interpretation of targeted experiments using advanced nanometer-scale optical measurements with STEM-EELS. This funding will place the principal investigator in a unique scientific context to create an independent research line in the field of multiscale modelling of nanostructures in close connection with advanced experiments.

The spin-valley physics in transition metal dichalcogenide two-dimensional monolayers has attracted attention due to its possible application in quantum information processing. Two distinct spin-split valleys occur due the non-centrosymmetric structure and spin-orbit coupling. Excitations in each valley can be created by circularly polarized laser and may be sufficiently long-lived to be manipulated. The role of atomic defects and confinement in excitation decay is under intense debate. Understanding decay is hindered by the spatial resolution limit of optical techniques. SpinE will fill this critical gap by designing new spectroscopy approaches coupling electron and photon techniques in an electron microscope. Laser pulses will be used to selectively create excited states, which will be detected using time-resolved electron spectroscopy. The atomic resolution of the electron microscopes will allow the study of the role of defects, interfaces and confinement in excitation decay.

Using atomic scale current noise measurements, we will unambiguously establish the presence or absence of Majorana bound states (MBS) that have been suggested to exist on individual Fe impurities, vortex cores and 1D crystalline defects in the iron based superconductor Fe(Se,Te). Unlike other non-invasive detection techniques, theoretical calculations show that the current noise of MBS is distinctly different from other in-gap modes. Preliminary measurements on the latter with our MHz-enabled scanning tunnelling microscope show that we can access the relevant tunnelling regime, have sufficient signal-to-noise to reliably carry out the measurement and confirm the theoretical predictions for these modes. If MBS are indeed present in one or more of the proposed scenarios, we will directly proceed to attempt readout and manipulation of the MBS, for which numerous proposals have been put forth that are feasible with the proposed system and our setup.