Spectral Density Of Shot Noise Research Articles

Shot noise in a quantum dot system coupled with Majorana bound states

We investigate the spectral density of shot noise and current for the system of a quantum dot coupled to Majorana bound states (MBS) employing the nonequilibrium Green’s function. The Majorana bound states at the end of the wire strongly affect the shot noise. There are two types of coupling in the system: dot–MBS and MBS–MBS coupling. The curves of shot noise and current versus coupling strength have novel steps owing to the energy-level splitting caused by dot–MBS coupling. The magnitude of these steps increases with the strength of dot–MBS coupling λ but decreases with the strength of MBS–MBS coupling. The steps shift toward the large ∣eV∣ region as λ or ϵM increases. In addition, dot–MBS coupling enhances the shot noise while MBS–MBS coupling suppresses the shot noise. In the absence of MBS–MBS coupling, a sharp jump emerges in the curve of the Fano factor at zero bias owing to the differential conductance being reduced by a factor of 1/2. This provides a novel technique for the detection of Majorana fermions.

Josephson effect in cuprate superconducting structures

Electron transport and microwave properties of cuprate superconducting structures (bicrystal junctions and hybrid mesa heterostructures) are discussed here. Superconducting current in junctions from cuprate superconductors with the dominant dx2−y2-wave symmetry is determined by the barrier properties, characterized by the mid-gap bound states due to the multiple Andreev reflection. In bicrystal junctions it is revealed via linear dependence of critical current density on square root of the transparency, and an increase of spectral density of shot noise at low voltages are observed. The experiments demonstrate that the superconducting hybrid mesa-heterostructures have the critical current density jс = 1–700 A/cm2 for an antiferromagnetic interlayer with thickness dM = 10–50 nm and the characteristic decay length of superconducting correlations on the order of 7 nm, due to the anomalous long range proximity effect, analyzed in the model of coupled superconductors via multilayer magnetic layer with antiferromagnetic ordering of magnetization in the layers. It is found that the hybrid mesa–heterostructures have much greater sensitivity to external magnetic field than conventional Josephson junctions because of the strong dependence of superconducting current on interlayer spin state.

AC shot noise through a vibrating quantum dot

In this paper we investigate the joint effects of the electron-phonon interaction and an external alternating (ac) gate voltage on the spectral density of shot noise through a vibrating quantum dot system, by applying the Lang-Firsov canonical transformation and the Keldysh nonequilibrium Green's function (NGF) technique. We find that the effects of the electron-phonon and electron-photon interaction on the differential shot noise are different. The main resonant peak of the differential shot noise is decreased only when a time-dependent gate voltage is imposed on quantum dot. With the ac field amplitude increasing, the main resonant peak of the differential shot noise decreases. The Fano factor of the system, which exhibits the deviation of shot noise from the Schottky formula, is also studied. Super-Poissonian shot noise appears due to the photon absorption (emission) processes in the low bias voltage region.

Photon-assisted shot noise due to the charging effect in a quantum dot device

We have investigated the spectral density of shot noise for an ultra-small quantum dot(QD) system in the Coulomb blockade regime when irradiated with microwave fields (MWFs) by employing a nonequilibrium Green’s function technique. The shot noise is sensitive to Coulomb interaction, and the photon-assisted Coulomb blockade behaviour strongly modifies the mesoscopic transport. We have calculated the first and second derivatives of shot noise in the strong and weak coupling regimes to compare the theoretical results with existing experimental results. In the strong coupling regime, the first and second derivatives of shot noise display Fano type peak-valley structures around the charging channel 2E c due to Coulomb interaction. When the magnitudes of the MWFs are sufficiently large, the system displays channel blockade due to photon irradiation. The photon-assisted and Coulomb blockade steps in the noise — as well as the resonant behaviour in the differential noise — are smeared by increasing temperature. The Coulomb interaction suppresses the shot noise, but the ac fields can either suppress the shot noise(balanced case) or enhance the shot noise(unbalanced case). The suppression of shot noise caused by ac fields in the balanced case is greater than that caused by Coulomb interaction in our system. Super-Poissonian shot noise may be induced due to the compound effects of strong Coulomb interaction and photon absorption-emission processes.

A method for measuring the shot noise in Schottky diodes with reduced barrier height

A method for measuring the spectral density of shot noise of one-port networks, in particular, semiconductor diodes, is described. The method is applicable at frequencies of up to 200 MHz. Expressions are obtained that relate the spectral density of the diode’s current noise to the measured noise factor and power gain of the diode fixture with the diode mounted in it. With this method, the dependence of the Fano factor of a GaAs Schottky diode with reduced barrier height on the diode current is determined.

Spin-flip shot noise in a quantum dot coupled to carbon nanotube terminals responded by a rotating magnetic field

We have investigated the spectral density of shot noise of the system with a quantum dot (QD) coupled to two single-wall carbon nanotube terminals, where a rotating magnetic field is applied to the QD. The carbon nanotube (CN) terminals act as quantum wires which open quantum channels for electrons to transport through. The shot noise and differential shot noise exhibit novel behaviors originated from the quantum nature of CNs. The shot noise is sensitively dependent on the rotating magnetic field, and the differential shot noise exhibits asymmetric behavior versus source-drain bias and gate voltage. The Fano factor of the system exhibits the deviation of shot noise from the Schottky formula. The super-Poissonian and sub-Poissonian shot noise can be achieved in different regime of source-drain bias.

Shot noise in a quantum dot coupled to carbon nanotube terminals applied with a microwave field

We have investigated the spectral density of shot noise for the system of a quantum dot (QD) coupled to two single-wall carbon nanotube terminals irradiated with a microwave field on the QD. The terminal features are involved in the shot noise through modifying the self-energy of QD. The contributions of carbon nanotube terminals to the shot noise exhibit obvious behaviors. The novel side peaks are associated with the photon absorption and emission procedure accompanying the suppression of shot noise. The shot noise in balanced absorption belongs to sub-Poissonian, and it is symmetric with respect to the gate voltage. The differential shot noise displays intimate relation with the nature of carbon nanotubes and the applied microwave field. It exhibits asymmetric behavior for the unbalanced absorption case versus gate voltage. The Fano factor of the system exhibits the deviation of shot noise from the Schottky formula, and the structures of terminals obviously contribute to it. The super-Poissonian and sub-Poissonian shot noise can be achieved in the unbalanced absorption in different regime of source-drain bias.

Photon-assisted shot noise in the mesoscopic system with a toroidal carbon nanotube coupled to normal-metal leads perturbed by ac fields

We have investigated the shot noise of a system with a toroidal carbon nanotube (TCN) coupled to two metal leads with applied microwave fields. The tunneling current operator is derived by determining the electron operators in different parts of the system. The Landauer-B\"uttiker-like current operator is also obtained, from which we obtain the time-dependent current fluctuation correlations, and consequently the spectral density of shot noise. The photon-assisted shot noise exhibits interesting behaviors due to the special properties of the TCN and the transport behaviors of electrons in multichannel mesoscopic systems. The quantum steps of the shot noise with respect to the source-drain bias reflect the quantum nature of the TCN, as well as the applied microwave field. The saturated value of the shot noise is suppressed by the applied microwave fields, and the suppression is strongly associated with the structure of the TCN. The obtained shot noises are sub-Poissonian when the source-drain bias increases to a definite value, and super-Poissonian shot noise appears below this value.

Andreev states and shot noise in bicrystal junctions of cuprate superconductors

Experimentally observed features of the electrical and noise characteristics of bicrystal junctions of cuprate superconductors, such as linearity of the critical current density versus square root of the junction transparency and increase in the spectral density of shot noise for small bias voltages (below the superconducting gap), indicate that the superconducting current in cuprate bicrystal junctions is determined by the passage of quasi-particles through a potential barrier at the superconductor boundaries. This process involves bound states appearing as a result of multiple Andreev reflections in superconductors with dominant wavefunction components of the dx2 − y2 symmetry type. At the same time, interpretation of the experimental current-phase and current-magnetic field curves requires that the character of faceting at the bicrystal junctions would be also taken into account.

Analytical and Numerical Investigation of Noise in Nanoscale Ballistic Field Effect Transistors

The current of ballistic nanoscale MOSFETs is expected to exhibit shot noise, essentially because the electron distribution is very far from equilibrium. Here, we derive an analytical expression of shot noise in fully ballistic MOSFETs and show how it can be computed on the basis of numerical simulations of the DC electrical properties. We show that the power spectral density of shot noise of the drain current is strongly suppressed as an effect of both Pauli exclusion and electrostatic interaction. The amount of such suppression depends on the device structure, and in particular on the gate capacitance. Results on shot noise of the gate current are also shown, since such the leakage current might be significant in nanoscale MOSFETs, for small equivalent oxide thickness.

Photon-assisted Fano resonance and corresponding shot noise in a quantum dot

We have studied the Fano resonance in photon-assisted transport in a quantum dot and calculated both the coherent current and spectral density of shot noise. It is predicted, for the first time, that the shape of Fano profile will also appear in satellite peaks. It is found that the variations of Fano profiles with the strengths of nonresonant transmissions are not synchronous in absorption and emission sidebands. The effect of interference on photon-assisted pumped current has been also investigated. We further predict the current and spectral density of shot noise as a function of the phase, which exhibits an intrinsic property of resonant and nonresonant channels in the structures.

Shot noise in the hybrid systems with a quantum dot coupled to normal and superconducting leads

We have investigated the spectral density of shot noise in the tunneling systems composed of a quantum dot coupled with normal and superconducting leads. The spectral density of shot noise is obtained by introducing the Green's functions of quantum dot. The zero-frequency shot noise in the same normal lead is given simply as the form of transmission coefficient associated with two leads. We have performed the numerical calculation on a two-terminal system at zero-temperature, and novel resonant structure due to Andreev reflection is observed. The sub-Poissonian shot noise is obtained by varying the parameters, which denotes that the shot noise can be suppressed by the external parameters.