Shot noise of the spin inelastic tunneling through a quantum dot with single molecule-magnet

We have studied the quantum fluctuations of inelastic spin-electron scattering in quantum dot with an embedded biaxial single molecule-magnet and particularly investigated the zero-frequency shot noise and Fano factor in different magnetic fields. It is found that the shot noise and Fano factor exhibit a stepwise behaviour as bias increases in the presence of interaction between the electron and molecule-magnet for a weak magnetic field. As magnetic field becomes strong, a dip is displayed in the shot-noise-bias curve due to the suppression of inelastic shot noise caused by the quantum tunneling of magnetisation. Because of the spontaneous inelastic tunneling at zero bias, a small shot noise occurs, which results in the case of Fano factor F >> 1. Moreover, our results show that the sweeping speed can also influence the shot noise and Fano factor obviously.     

电-声子耦合强度对量子点系统噪声的影响

利用Lang-Firsov正则变换和Keldysh非平衡格林函数方法研究了低温下具有电子-声子相互作用的量子点系统的噪声。我们特别注意了电-声子耦合强度的变化对量子点系统噪声的影响。数值结果表明：随着电-声子耦合强度的增大,系统的噪声增大,同时微分噪声谱中会出现一系列的声子伴带峰,峰的高度和数目对电-声子耦合强度的变化非常敏感。我们也研究了系统的Fano因子,它显示系统噪声对肖特基(Schottky)公式的偏离。在高偏压区,Fano因子随着电-声子耦合强度的增大而增大。     

电-声子耦合强度对量子点系统噪声的影响

利用Lang-Firsov正则变换和Keldysh非平衡格林函数方法研究了低温下具有电子-声子相互作用的量子点系统的噪声.我们特别注意了电-声子耦合强度的变化对量子点系统噪声的影响.数值结果表明:随着电-声子耦合强度的增大,系统的噪声增大,同时微分噪声谱中会出现一系列的声子伴带峰,峰的高度和数目对电-声子耦合强度的变化非常敏感.我们也研究了系统的Fano因子,它显示系统噪声对肖特基(Schottky)公式的偏离.在高偏压区,Fano因子随着电-声子耦合强度的增大而增大.     

The shot noise of the quantum dot weakly coupled to Luttinger liquid

We study the nonequilibrium transport through a single-level quantum dot weakly coupled to Luttinger liquid leads. A general shot noise expression is derived by using nonequilibrium Green function technique. We find that the differential shot noise and differential conductance demonstrate resonant-like behavior as a function of the bias voltage and the quantum dot's energy level for a weak or moderately strong interaction. In the limit of strong electron-electron interaction, the resonant behavior disappears and shows bias-voltage-dependent power law scalings. And the Fano factor also scales as a power law in high bias voltage region. In addition, the Fano factor is enhanced with the electron-electron interaction increased. It implies that the Fano factor can be controlled by tuning the electron-electron interaction in the leads.     

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 magnetic field modulated graphene superlattice

We investigate the shot noise properties in a monolayer graphene superlattice modulated by N parallel ferromagnets deposited on a dielectric layer. It is found that for the antiparallel magnetization configuration or when magnetic field is zero the new Dirac-like point appears in graphene superlattice. The transport is almost forbidden at this new Dirac-like point, and the Fano factor reaches its maximum value 1/3. In the parallel magnetization configuration as the number of magnetic barriers increases, the shot noise increases. In this case, the transmission can be blocked by the magnetic–electric barrier and the Fano factor approaches 1, which is dramatically distinguishable from that in antiparallel alignment. The results may be helpful to control the electron transport in graphene-based electronic devices.     

Quantum partition noise of photon-created electron-hole pairs

We show experimentally that even when no bias voltage is applied to a quantum conductor, the electronic quantum partition noise can be investigated with GHz radio frequency excitation. Using a quantum point contact configuration as the ballistic conductor we are able to make an accurate determination of the partition noise Fano factor resulting from the photon-assisted shot noise. Applying both voltage bias and rf irradiation we are able to make a definitive quantitative test of the scattering theory of photon-assisted shot noise.     

Mesoscopic stoner instability in metallic nanoparticles revealed by shot noise

We study sequential tunneling through a metallic nanoparticle close to the Stoner instability coupled to parallel magnetized electrodes. Increasing the bias voltage successively opens transport channels associated with excitations of the nanoparticle's total spin. For the current this leads just to a steplike increase. The Fano factor, in contrast, shows oscillations between large super-Poissonian and sub-Poissonian values as a function of bias voltage. We explain the enhanced Fano factor in terms of generalized random-telegraph noise and propose the shot noise as a convenient tool to probe the mesoscopic Stoner instability.     

Observation of photon-assisted noise in a diffusive normal metal-superconductor junction

We report measurements of nonequilibrium noise in a diffusive normal metal-superconductor (N-S) junction in the presence of both dc bias and high-frequency ac excitation. We find that the shot noise of a diffusive N-S junction is doubled compared to a normal diffusive conductor. Under ac excitation of frequency nu the shot noise develops features at bias voltages |V| = hnu/(2e), which bear all the hallmarks of a photon-assisted process. Observation of these features provides clear evidence that the effective charge of the current carriers is 2e, due to Andreev reflection.     

Enhancement of the shot noise of a quantum dot–Luttinger lead system

We investigate the joint effects of the intralead electron interaction and Coulombic dot–lead interaction on the shot noise of a quantum dot coupled to Luttinger liquid leads. A formula of the shot noise is derived by applying the nonequilibrium Green function technique. The shot noise is enhanced by the dot–lead interaction. For a weak or moderately strong interaction the differential shot noise demonstrates resonant-like behavior as a function of bias and gate voltages. In the limit of strong interaction resonant behavior disappears and the differential shot noise and Fano factor scale as a power law in bias voltage. Under some parameters, the differential shot noise may become negative around resonant peaks, and the physical reason is analyzed.     

Shot noise in semiclassical chaotic cavities

We construct a trajectory-based semiclassical theory of shot noise in clean chaotic cavities. In the universal regime of vanishing Ehrenfest time tau(E), we reproduce the random matrix theory result and show that the Fano factor is exponentially suppressed as tau(E) increases. We demonstrate how our theory preserves the unitarity of the scattering matrix even in the regime of finite tau(E). We discuss the range of validity of our semiclassical approach and point out subtleties relevant to the recent semiclassical treatment of shot noise in the universal regime by Braun et al. (cond-mat/0511292).     

Optimal control of shot noise and Fano factor by external fields

A method is devised to control the current, shot noise and Fano factor in a molecular junction using external fields. The tunneling of electrons through a molecular junction weakly coupled to two leads in the presence of a time-dependent external field is studied using a quantum master equation approach. By combining optimal control theory and assuming a predefined time-dependent current pattern, an external field can be determined which does generate a current pattern close to the requested one. With this approach the current flow pattern in time can be chosen in an almost arbitrary fashion. The same technique can be applied to control the shot noise. For minimizing the current, the corresponding shot noise decreases but does not vanish. By minimizing the shot noise, the corresponding current also approaches zero for the present model of spinless electrons. Within certain limits the proposed strategy even works well for the control of the Fano factor.     

Disorder-Induced Rectification in a Molecular System

In this work, We study on the electron transport of an ensemble of coupled sites that simulates an array of quantum dots or a molecular system. By using the Green’s function technique, we calculate current and shot noise for linear and disordered site arrays. While in the linear case the characteristic I–V curve reveals no current rectification, in the disordered configurations a robust rectification is found, thus indicating an operational regime typical of molecular diodes. Additionally, a negative differential resistance is observed due to the drop of the bias voltage along the structure, which yields to an energy mismatch of neighboring sites. Finally, the Fano factor reveals a stronger transport correlation for positive than for negative bias voltages in the disordered site configuration.     

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.     

Shot noise in graphene

We report measurements of current noise in single-layer and multilayer graphene devices. In four single-layer devices, including a p-n junction, the Fano factor remains constant to within +/-10% upon varying carrier type and density, and averages between 0.35 and 0.38. The Fano factor in a multilayer device is found to decrease from a maximal value of 0.33 at the charge-neutrality point to 0.25 at high carrier density. These results are compared to theories for shot noise in ballistic and disordered graphene.     

Coupling strength effect on shot noise in boron devices

The shot noise properties in boron devices are investigated with a tight-binding model and the non-equilibrium Green’s function.It is found that the shot noise and Fano factors can be tuned by changing the structures,the size,and the coupling strength.The shot noise is suppressed momentarily as we switch on the bias voltage,and the electron correlation is significant.The Fano factors are more sensitive to the ribbon width than to the ribbon length in the full coupling context.In the weak-coupling context,the Fano factors are almost invariant with the increase of length and width over a wide bias range.     

Fano Resonances for Tilted Linear and Quadratic Band Touching Dispersions in a Harmonically Driven Potential Well

Considering models with tilted linear and quadratic band touching dispersions, the effect of the transverse linear tilt on the transmission spectra is analyzed through a harmonically driven potential well oriented longitudinally. Employing the Floquet scattering matrix formalism, Fano resonances are found as an outcome of matching between the Floquet sidebands and quasi-bound states, where the tilt renormalizes their energies and wave vectors. It is found that the Fano resonance energy decreases (increases) for linear (quadratic) band touchings as the magnitude of the transverse momentum increases, indicating a distinct signature of the underlying band dispersion in the transmission profile. The sign of the product of the transverse momentum and the tilt also determines the relative shift in the Fano resonance energy with respect to the untilted case for both band dispersions, suggesting a possible tunability of the Fano resonance for tilted systems. Importantly, the tilt strength can also be directly determined by measuring the Fano resonance energy as function of the transverse momenta direction. The shot noise spectra and their differential property are further studied where an inflection region and undulation, respectively, is found around the Fano resonance energy. Interestingly, differential shot noise and transmission spectra both qualitatively behave in a similar fashion and might thus serve as important observables for future experiments on driven solid-state systems.     

AC shot noise through a quantum dot in the Kondo regime

The photon-assisted shot noise through a quantum dot in the Kondo regime is investigated by applying time-dependent canonical transformation and non-crossing approximation technique. A basic formula for the photon-assisted shot noise is obtained. The rich dependence of the shot noise on the external ac field and temperature is displayed. At low temperature and low frequencies, the differential shot noise exhibits staircase behavior. When the temperature increases, the steps are rounded. At elevated frequencies, the photon-assisted tunneling becomes more obvious. We have also found that the Fano factor is enhanced as the ac frequency is enhanced.     

Charge Transport Properties of the Majorana Zero Mode Induced Noncollinear Spin Selective Andreev Reflection

We study the charge transport properties of the spin-selective Andreev reflection(SSAR)effect between a spin polarized scanning tunneling microscope(STM)tip and a Majorana zero mode(MZM).Considering both the MZM and the excited states,we calculate the conductance and the shot noise power of the noncollinear SSAR using scattering theory.We find that the excited states give rise to inside peaks.Moreover,we numerically calculate the shot noise power and the Fano factor of the SSAR effect.Our calculation shows that the shot noise power and the Fano factor are related to the angle between the spin polarization direction of the STM tip and that of the MZM,which provide additional characteristics to detect the MZM via SSAR.     

Spin-dependent shot noise of inelastic transport through molecular quantum dots

Here we present a theoretical analysis of the effect of inelastic electron scattering on spin-dependent transport characteristics (conductance, current–voltage dependence, magnetoresistance, shot noise spectrum, Fano factor) for magnetic nanojunction. Such device is composed of molecular quantum dot (with discrete energy levels) connected to ferromagnetic electrodes (treated within the wide-band approximation), where molecular vibrations are modeled as dispersionless phonons. Non-perturbative computational scheme, used in this work, is based on the Green's function theory within the framework of mapping technique (GFT–MT), which transforms the many-body electron–phonon interaction problem into a single-electron multi-channel scattering problem. The consequence of the localized electron–phonon coupling is polaron formation. It is shown that polaron shift and additional peaks in the transmission function completely change the shape of considered transport characteristics.