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 D 0 energy of the donor. Such microwave assisted transport at zero bias enhances the resolution in the spectroscopy of the energy levels of the donor. 1 I. INTRODUCTION Single and double atom impurities in semiconductor nanostructures are of major interest for their key role in atomic scale devices for electronics and spintronics [1, 2, 3, 4, 5, 6]. The first observation of a single As atom in a Silicon nanostructure [7, 8] opened the route to the spectroscopy of donor atoms by means of quantum transport measurements. Isolated dopants in the channel of a silicon NanoFET behave as atomic-like systems with one or two electrons in a central 1/r attracting Coulomb potential. Donors can be characterized in terms of energy levels, associated to the D 0 and D − states, known in bulk Silicon [9]. A direct measure of the degree of hybridization of the electron wavefunction between the donor potential and the interfacial well has been performed in a Si nanostructure [8]. The separation of the conduction peaks, observed by applying a gate voltage to a nanostructure, and the threshold voltage, is proportional to the energy level position with respect to the conduction band edge [7, 8]. An isolated, diffused dopant embedded in the Si inversion layer of a nanostructure acts as a quantum dot (Single Donor Quantum Dot, SDQD) at cryogenic temperatures [7, 10]. Above the conduction band edge, further quantized energy levels are due to the quantum dot formed in the Si channel by electrostatic confinement.A microwave field, thanks to the variety of its couplings with the single atom and its environment, provides an unique tool to explore quantum transport and spin properties. In quantum information processing applications, microwave irradiation should drive electron spin resonance of Zeeman spin doublets and single spin manipulation with pulses of appropriate duration as already observed in GaAs systems [11]. In Si the observation and the control of single electron spin resonance has not been demonstrated yet due to the small linewidth (expected to be of the order on one gauss) [12] if compared to the typical resolution of superconducting cryomagnets ordinarily used for quantum transport measurements, contrarily to GaAs where the spin orbit effect broadens the line to tens of gauss [11,13,14], and to the competition of such effect with photon assisted tunneling in terms of current variation [20].When a quantum dot is i...

We present experimental results concerning both the fabrication and characterization of superconducting tunnel junctions containing superconductor/ferromagnet ͑S/F͒ bilayers made by niobium ͑S͒ and a weak ferromagnetic Ni 0.50 Cu 0.50 alloy. Josephson junctions have been characterized down to T = 1.4 K in terms of current-voltage I-V characteristics and Josephson critical current versus magnetic field. By means of a numerical deconvolution of the I-V data the electronic density of states on both sides of the S/F bilayer has been evaluated at low temperatures. Results have been compared with theoretical predictions from a proximity model for S/F bilayers in the dirty limit in the framework of Usadel equations for the S and F layers, respectively. The main physical parameters characterizing the proximity effect in the Nb/ NiCu bilayer, such as the coherence length and the exchange field energy of the F metal, and the S/F interface parameters have been also estimated.

Superconductors are highly suitable materials for radiation detection. Several detector types have been proposed, with properties of fast detection or high wavelength resolution over a wide range of optical frequencies. Their performances depend on the relaxation processes involving phonons, quasiparticles and Cooper pairs occurring during the energy cascade following the absorption of the radiation in the superconductor. The energy down-conversion processes are related to the electron–phonon scattering strength λe−ph which is usually expressed in terms of the electron–phonon coupling time, τ0, which is characteristic for each material. In this paper we estimate the value of τ0 for several classes of superconducting materials not yet investigated. It is calculated in the framework of the McMillan model for the superconducting critical temperature Tc within the Debye approximation. The values obtained for τ0 are discussed as regards new possibilities for unexplored materials in the field of superconducting detectors. In particular, we focus our attention on materials with τ0 values that could play a significant role in both hot electron photodetectors and superconducting tunnel junction devices.

We report the observation of the ratchet effect for a relativistic flux quantum trapped in an annular Josephson junction embedded in an inhomogeneous magnetic field. In such a solid state system mechanical quantities are proportional to electrical quantities, so that the ratchet effect represents the realization of a relativistic-flux-quantum-based diode. Mean static voltage response, equivalent to directed fluxon motion, is experimentally demonstrated in such a diode for deterministic current forcing both in the overdamped and in the underdamped dynamical regime. In the underdamped regime, the recently predicted phenomenon of current reversal is also recovered in our fluxon ratchet.

Decay of a zero-voltage state of a Josephson junction at low temperature occurs via quantum tunneling through an effective potential barrier. An extremely small probability of quantum tunneling may become not very small under the action of an ac component of the bias current. The tunneling rate has a peak as a function of a dc component of the bias current (Euclidean resonance). An analysis of this extremely multiphoton process is done on the basis of classical trajectories in imaginary time. The studied phenomenon does not involve transitions between energy levels and, thus, is distinctly different from the well-known process of photon-assisted tunneling