Rossella Latempa

Microwave-assisted transport in a single-donor silicon quantum dot

Single donors in semiconductor nanostructures represent a key element to develop spin-related quantum functionalities in atomic-scale devices. Quantum transport through a single arsenic donor in the channel of a silicon nano field-effect transistor under microwave irradiation is investigated. The device is characterized at mK temperatures in the regime of Coulomb blockade. Photon-assisted tunneling and microwave-induced electron pumping regimes are revealed, respectively, at low and high microwave power. At sufficiently high power, the microwave irradiation induces tunneling through the first excited energy level of the

Photon-Assisted Tunneling in Quantum Dots

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SWITCHING PHENOMENA AND PULSED OPERATION IN LONG JOSEPHSON DEVICES

energy of the donor. Such microwave-assisted transport at zero bias enhances the resolution of the spectroscopy of the energy levels of the donor.

PHASE LOCKING AND AC AMPLIFICATION OF SMALL JOSEPHSON JUNCTIONS

The manipulation and quantification of the effects produced by an rf field in a mesoscopic structure are fundamental issues in view of developing single-spin-based qubits. Here, we review the experiments on electron transport in quantum dots under microwave irradiation. The electromagnetic vector potential provides excitation of electrons in the leads and in the quantum dot, and an electromotive potential at the leads. The combinations of the two effects go under the name of photon-assisted tunneling. In the present review, the theory of photon-assisted tunneling, based on the Tien–Gordon model applied to the Coulomb-blockade regime of a quantum dot is outlined. An expression for the dc current flowing through the dot in response to a microwave signal is calculated. Then, a classification of different experiments, organized following the different processes adopted to create the dot is presented. Measurements of GaAs split-gate-defined single and double quantum dots as well as lithographically defined SET based on Si/SiGe technology are considered. Finally, recent experiments on a Si/SiO2 commercial flash memory microwave irradiated up to 40 GHz are illustrated, without and with a static magnetic field up to 12 T.

FLUXON DYNAMICS AND RESONANCES IN STACKED ARRAYS OF JOSEPHSON JUNCTIONS

Recent findings by G.Pepe et al. (Appl.Phys.Lett.89, 2770, 2001) have given new insights over the dynamics of Josephson junctions subject to electronic pulse injection. These experiments can be considered important steps toward the study of non-equilibrium effects involved in junctions under very fast (e.g. laser induced) pumping signals. In the governing equations (sine-Gordon or coupled sine-Gordon depending on the system) pulse operation enters via Josephson current distribution or via bias current distribution. By means of numerical simulations here we study the dependence of the switching to resistive state on fast local depression of Josephson current. Next, using simulated electronic pulse train we show how the flip-flop operating mode found in G.Pepe et al. is a common feature of single or stacked Josephson systems without regard for the pulse waveform or the presence of the gap structure in the I-V characteristics. The effect of the length and of the magnetic field is also discussed.

Extreme multiphoton phenomena in josephson junctions : Euclidean resonance

We have addressed the problem of the phase-locking mechanism of a single Josephson junction irradiated with microwaves, and have numerically and theoretically explored the possibility to achieve ac gain in this system. By investigating the capability of Josephson junctions to enhance the signal received form an external rf source we obtain useful information of the possibility of mutual phase-locking of large arrays of junctions.

Microwave effects in silicon low dimensional nanostructures.

We consider a structure consisting of two parallel arrays of long Josephson junctions sharing a common electrode that allows inductive coupling between the arrays. Cavity modes excitations exhibiting 2D synchronization as well as solitonic excitations are inferred from the mathematical model of the system. The stable coherent solitonic solutions of the coupled sine-Gordon equations describing the system is found to consist of a fluxon-antifluxon string travelling in the structure.