Enhanced shot noise in resonant tunneling: theory and experiment

We show that shot noise in a resonant tunneling diode biased in the negative differential resistance regions of the I-V characteristic is enhanced with respect to ``full'' shot noise. We provide experimental results showing a Fano factor up to 6.6, and show that it is a dramatic effect caused by electron-electron interaction through Coulomb force, enhanced by the particular shape of the density of states in the well. We also present numerical results from the proposed theory, which are in agreement with the experiment, demonstrating that the model accounts for the relevant physics involved in the phenomenon.Comment: 4 pages, 4 figure

Mesoscopic Full Counting Statistics and Exclusion models

We calculate the distribution of current fluctuations in two simple exclusion models. Although these models are classical, we recover even for small systems such as a simple or a double barrier, the same distibution of current as given by traditionnal formalisms for quantum mesoscopic conductors. Due to their simplicity, the full counting statistics in exclusion models can be reduced to the calculation of the largest eigenvalue of a matrix, the size of which is the number of internal configurations of the system. As examples, we derive the shot noise power and higher order statistics of current fluctuations (skewness, full counting statistics, ....) of various conductors, including multiple barriers, diffusive islands between tunnel barriers and diffusive media. A special attention is dedicated to the third cumulant, which experimental measurability has been demonstrated lately.Comment: Submitted to Eur. Phys. J.

Theory of charge fluctuations and domain relocation times in semiconductor superlattices

Shot noise affects differently the nonlinear electron transport in semiconductor superlattices depending on the strength of the coupling among the superlattice quantum wells. Strongly coupled superlattices can be described by a miniband Boltzmann-Langevin equation from which a stochastic drift-diffusion equation is derived by means of a consistent Chapman-Enskog method. Similarly, shot noise in weakly coupled, highly doped semiconductor superlattices is described by a stochastic discrete drift-diffusion model. The current-voltage characteristics of the corresponding deterministic model consist of a number of stable branches corresponding to electric field profiles displaying two domains separated by a domain wall. If the initial state corresponds to a voltage on the middle of a stable branch and is suddenly switched to a final voltage corresponding to the next branch, the domains relocate after a certain delay time, called relocation time. The possible scalings of this mean relocation time are discussed using bifurcation theory and the classical results for escape of a Brownian particle from a potential well.Comment: 14 pages, 2 figure

Shot noise suppression in quasi one-dimensional Field Effect Transistors

We present a novel method for the evaluation of shot noise in quasi one-dimensional field-effect transistors, such as those based on carbon nanotubes and silicon nanowires. The method is derived by using a statistical approach within the second quantization formalism and allows to include both the effects of Pauli exclusion and Coulomb repulsion among charge carriers. In this way it extends Landauer-Buttiker approach by explicitly including the effect of Coulomb repulsion on noise. We implement the method through the self-consistent solution of the 3D Poisson and transport equations within the NEGF framework and a Monte Carlo procedure for populating injected electron states. We show that the combined effect of Pauli and Coulomb interactions reduces shot noise in strong inversion down to 23 % of the full shot noise for a gate overdrive of 0.4 V, and that neglecting the effect of Coulomb repulsion would lead to an overestimation of noise up to 180 %.Comment: Changed content, 7 pages,5 figure

Quasiclassical description of transport through superconducting contacts

We present a theoretical study of transport properties through superconducting contacts based on a new formulation of boundary conditions that mimics interfaces for the quasiclassical theory of superconductivity. These boundary conditions are based on a description of an interface in terms of a simple Hamiltonian. We show how this Hamiltonian description is incorporated into quasiclassical theory via a T-matrix equation by integrating out irrelevant energy scales right at the onset. The resulting boundary conditions reproduce results obtained by conventional quasiclassical boundary conditions, or by boundary conditions based on the scattering approach. This formalism is well suited for the analysis of magnetically active interfaces as well as for calculating time-dependent properties such as the current-voltage characteristics or as current fluctuations in junctions with arbitrary transmission and bias voltage. This approach is illustrated with the calculation of Josephson currents through a variety of superconducting junctions ranging from conventional to d-wave superconductors, and to the analysis of supercurrent through a ferromagnetic nanoparticle. The calculation of the current-voltage characteristics and of noise is applied to the case of a contact between two d-wave superconductors. In particular, we discuss the use of shot noise for the measurement of charge transferred in a multiple Andreev reflection in d-wave superconductors

Minimization of phonon-tunneling dissipation in mechanical resonators

Micro- and nanoscale mechanical resonators have recently emerged as ubiquitous devices for use in advanced technological applications, for example in mobile communications and inertial sensors, and as novel tools for fundamental scientific endeavors. Their performance is in many cases limited by the deleterious effects of mechanical damping. Here, we report a significant advancement towards understanding and controlling support-induced losses in generic mechanical resonators. We begin by introducing an efficient numerical solver, based on the "phonon-tunneling" approach, capable of predicting the design-limited damping of high-quality mechanical resonators. Further, through careful device engineering, we isolate support-induced losses and perform the first rigorous experimental test of the strong geometric dependence of this loss mechanism. Our results are in excellent agreement with theory, demonstrating the predictive power of our approach. In combination with recent progress on complementary dissipation mechanisms, our phonon-tunneling solver represents a major step towards accurate prediction of the mechanical quality factor.Comment: 12 pages, 4 figure