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Register a new userIrreversibility on the Level of Single-Electron Tunneling
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We present a low-temperature experimental test of the fluctuation theorem for electron transport through a double quantum dot. The rare entropy-consuming system trajectories are detected in the form of single charges flowing against the source-drain bias by using time-resolved charge detection with a quantum point contact. We find that these trajectories appear with a frequency that agrees with the theoretical predictions even under strong nonequilibrium conditions, when the finite bandwidth of the charge detection is taken into account.
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An important consequence of the discovery of giant magnetoresistance in metallic magnetic multilayers is a broad interest in spin dependent effects in electronic transport through magnetic nanostructures. An example of such systems are tunnel junctions -- single-barrier planar junctions or more complex ones. In this review we present and discuss recent theoretical results on electron and spin transport through ferromagnetic mesoscopic junctions including two or more barriers. Such systems are also called ferromagnetic single-electron transistors. We start from the situation when the central part of a device has the form of a magnetic (or nonmagnetic) metallic nanoparticle. Transport characteristics reveal then single-electron charging effects, including the Coulomb staircase, Coulomb blockade, and Coulomb oscillations. Single-electron ferromagnetic transistors based on semiconductor quantum dots and large molecules (especially carbon nanotubes) are also considered. The main emphasis is placed on the spin effects due to spin-dependent tunnelling through the barriers, which gives rise to spin accumulation and tunnel magnetoresistance. Spin effects also occur in the current-voltage characteristics, (differential) conductance, shot noise, and others. Transport characteristics in the two limiting situations of weak and strong coupling are of particular interest. In the former case we distinguish between the sequential tunnelling and cotunneling regimes. In the strong coupling regime we concentrate on the Kondo phenomenon, which in the case of transport through quantum dots or molecules leads to an enhanced conductance and to a pronounced zero-bias Kondo peak in the differential conductance.
We report on the realization of a single-electron source, where current is transported through a single-level quantum dot (Q), tunnel-coupled to two superconducting leads (S). When driven with an ac gate voltage, the experiment demonstrates electron turnstile operation. Compared to the more conventional superconductor - normal metal - superconductor turnstile, our SQS device presents a number of novel properties, including higher immunity to the unavoidable presence of non-equilibrium quasiparticles in superconducting leads. In addition, we demonstrate its ability to deliver electrons with a very narrow energy distribution.
A recent mean-field approach to the fractional quantum Hall effect (QHE) is reviewed, with a special emphasis on the application to single-electron tunneling through a quantum dot in a high magnetic field. The theory is based on the adiabatic principle of Greiter and Wilczek, which maps an incompressible state in the integer QHE on the fractional QHE. The single-particle contribution to the addition spectrum is analyzed, for a quantum dot with a parabolic confining potential. The spectrum is shown to be related to the Fock-Darwin spectrum in the integer QHE, upon substitution of the electron charge by the fractional quasiparticle charge. Implications for the periodicity of the Aharonov-Bohm oscillations in the conductance are discussed.
Recent scanning tunnelling microscopy (STM) experiments reported single-molecule fluorescence induced by tunneling currents in the nanoplasmonic cavity formed by the STM tip and the substrate.The electric field of the cavity mode couples with the current-induced charge fluctuations of the molecule, allowing the excitation of the mode. We investigate theoretically this system for the experimentally relevant limit of large damping rate $kappa$ for the cavity mode and arbitrary coupling strength to a single-electronic level. We find that for bias voltages close to the first inelastic threshold of photon emission, the emitted light displays anti-bunching behavior with vanishing second-order photon correlation function. At the same time, the current and the intensity of emitted light display Franck--Condon steps at multiples of the cavity frequency $omega_c$ with a width controlled by $kappa$ rather than the temperature $T$. For large bias voltages, we predict strong photon bunching of the order of the $kappa/Gamma$ where $Gamma$ is the electronic tunneling rate. Our theory thus predicts that strong coupling to a single level allows current-driven non-classical light emission.
We devise a scheme to characterize tunneling of an excess electron shared by a pair of tunnel-coupled dangling bonds on a silicon surface -- effectively a two-level system. Theoretical estimates show that the tunneling should be highly coherent but too fast to be measured by any conventional techniques. Our approach is instead to measure the time-averaged charge distribution of our dangling-bond pair by a capacitively coupled atomic-force-microscope tip in the presence of both a surface-parallel electrostatic potential bias between the two dangling bonds and a tunable midinfrared laser capable of inducing Rabi oscillations in the system. With a nonresonant laser, the time-averaged charge distribution in the dangling-bond pair is asymmetric as imposed by the bias. However, as the laser becomes resonant with the coherent electron tunneling in the biased pair the theory predicts that the time-averaged charge distribution becomes symmetric. This resonant symmetry effect should not only reveal the tunneling rate, but also the nature and rate of decoherence of single-electron dynamics in our system.
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