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Register a new userThermal origin of light emission in non-resonant and resonant tunnel junctions
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Electron tunneling is associated with light emission. In order to elucidate its generating mechanism, we provide a novel experimental ansatz that employs fixed-distance epitaxial graphene as metallic electrodes. In contrast to previous experiments, this permits an unobscured light spread from the tunnel junction, enabling both a reliable calibration of the visible to infrared emission spectrum and a detailed analysis of the dependence of the parameters involved. In an open, non-resonant geometry, the emitted light is perfectly characterized by a Planck spectrum. In an electromagnetically resonant environment, resonant radiation is added to the thermal spectrum, both being strictly proportional in intensity. In full agreement with a simple heat conduction model, we provide evidence that in both cases the light emission stems from a hot electronic subsystem in interaction with its linear electromagnetic environment. These very clear results should resolve any ambiguity about the mechanism of light emission in nano contacts.
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The resonant tunneling model is the simplest model for describing electronic transport through nanoscale objects like individual molecules. A complete understanding includes not only charge transport but also thermal transport and their intricate interplay. Key linear response observables are the electrical conductance G and the Seebeck coefficient S. Here we present experiments on unspecified resonant tunnel junctions and molecular junctions that uncover correlations between $G$ and $S$, in particular rigid boundaries for $S(G)$. We find that these correlations can be consistently understood by the single-level resonant tunneling model, with excellent match to experiments. In this framework, measuring $I(V)$ and $S$ for a given junction provides access to the full thermoelectric characterization of the electronic system. A remarkable result is that without targeted chemical design, molecular junctions can expose thermoelectric conversion efficiencies which are close to the Carnot limit. This insight allows to provide design rules for optimized thermoelectric efficiency.
We propose energy band engineering to enhance tunneling electroresistance (TER) in ferroelectric tunnel junctions (FTJs). We predict that an ultrathin dielectric layer with a smaller band gap, embedded into a ferroelectric barrier layer, acts as a switch controlling high and low conductance states of an FTJ depending on polarization orientation. Using first-principles modeling based on density functional theory, we investigate this phenomenon for a prototypical SrRuO3/BaTiO3/SrRuO3 FTJ with a BaSnO3 monolayer embedded in the BaTiO3 barrier. We show that in such a composite-barrier FTJ, ferroelectric polarization of BaTiO3 shifts the conduction band minimum of the BaSnO3 monolayer above or below the Fermi energy depending on polarization orientation. The resulting switching between direct and resonant tunneling leads to a TER effect with a giant ON/OFF conductance ratio. The proposed resonant band engineering of FTJs can serve as a viable tool to enhance their performance useful for device application.
The thermal spin-transfer torque (TSTT) is an effect to switch the magnetic free layer in a magnetic tunnel junction by a temperature gradient only. We present ab initio calculations of the TSTT. In particular, we discuss the influence of magnetic layer composition by considering $text{Fe}_text{x}text{Co}_{text{1-x}}$ alloys. Further, we compare the TSTT to the bias voltage driven STT and discuss the requirements for a possible thermal switching. For example, only for very thin barriers of 3 monolayers MgO a thermal switching is imaginable. However, even for such a thin barrier the TSTT is still too small for switching at the moment and further optimization is needed. In particular, the TSTT strongly depends on the composition of the ferromagentic layer. In our current study it turns out that at the chosen thickness of the ferromagnetic layer pure Fe gives the highest thermal spin-transfer torque.
We theoretically investigate heat transport in temperature-biased Josephson tunnel junctions in the presence of an in-plane magnetic field. In full analogy with the Josephson critical current, the phase-dependent component of the heat flux through the junction displays coherent diffraction. Thermal transport is analyzed in three prototypical junction geometries highlighting their main differences. Notably, minimization of the Josephson coupling energy requires the quantum phase difference across the junction to undergo pi-slips in suitable intervals of magnetic flux. An experimental setup suited to detect thermal diffraction is proposed and analyzed.
We study the behavior of shot noise in resonant tunneling junctions far from equilibrium. Quantum-coherent elastic charge transport can be characterized by a transmission function, that is the probability for an incoming electron at a given energy to tunnel through a potential barrier. In systems such as quantum point contacts, electronic shot noise is oftentimes calculated based on a constant (energy independent) transmission probability, a good approximation at low temperatures and under a small bias voltage. Here, we generalize these investigations to far from equilibrium settings by evaluating the contributions of electronic resonances to the electronic current noise. Our study extends canonical expressions for the voltage-activated shot noise and the recently discovered delta-T noise to the far from equilibrium regime, when a high bias voltage or a temperature difference is applied. In particular, when the Fermi energy is located on the shoulder of a broad resonance, we arrive at a formula for the shot noise revealing anomalous-nonlinear behavior at high bias voltage.
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