Negative excess noise in quantum conductors

We predict that in quantum conductors the excess noise can be absent or even negative provided the energy dependence of the electron transmission probability at the Fermi energy is sufficiently sharp. In other words the current (or voltage) fluctuations under transport conditions can be less than in equilibrium. As examples for this surprising behavior we consider resonant tunneling, ballistic point contacts and the integer quantum Hall effect.Work performed within the research program of the Sonderforschungsbereich 341, Köln-Aachen-Jülich     

Low-frequency excess noise characterizations of laser-assisted de-bonded GaN films

Systematic characterization of flicker noise was conducted on GaN-based metal-semiconductor-metal (MSM) interdigitated devices. The devices were fabricated on both the regular GaN-on-sapphire (type A) and laser de-bonded films followed by layer transfer of hydride vapor phase epitaxy-grown GaN films to Si substrates (type B). Experimental results indicated no significant degradation in the I-V characteristics for Schottky MSM devices fabricated on type B films compared to those fabricated on type A films. However, substantial increase in the flicker noise level, particularly in the low-temperature regime, is observed among the ohmic MSM devices fabricated on type B films. The experimental data suggest that material degradation occurs at the vicinity of the GaN-sapphire interface, while in regions close to the GaN film surface there is practically no change in the film quality. This is supported by finite element simulation of the temperature of the film during laser irradiation. The results indicate that the temperature dropped from 1400 K at the GaN-sapphire interface to about 1000 K within 0.5 μm away from the interface stipulating that material degradation occurs only within 0.5 μm from the GaN-sapphire interface.     

A comparison of confining potentials in the quantum wire problem

A simple numerical method for solving a two-terminal quantum electronic waveguide problem is presented. The method can be adapted to a quantum wire cavity of irregular geometry and/or non-constant potential field. We compare the circular bend wire with parabolic confining potential profile to the commonly used hard wall confinement. We find an energy scaling which makes the results correspond closely.     

On the additivity of Flicker contributions to the excess noise in quantum conductors

We investigate the quantum interference induced non-additive contribution to the excess noise due to several mobile scatters in the diffusion/cooperon approximation. For weak impurity coupling and standard assumption on impurity hopping the relative correction to the noise spectrum is proportional to logf. Although the constant of proportionality is small the logarithmic deviation from the main additive part showing 1/f behavior seems rather remarkable.Work performed within the research program of the Sonderforschungsbereich 341, Köln-Aachen-Jülich     

Hot electron relaxation in one-dimensional models: exact polaron dynamics versus relaxation in the presence of a Fermi sea

We present the exact solution for the time evolution of the electron and phonon momentum distribution for a one-dimensional polaron model with alinear electronic energy dispersion. The electron momentum distribution is shown to obey aMarkovian quantum kinetic equation. Numerical results for the polaron model are compared to the corresponding exact results, when the negative momentum states are filled in the initial state. The presence of this Fermi sea modifies the dynamics except in the short time regime. The different, long time dynamics might show up in comparison of hot electron relaxation of undoped and doped semiconductors.     

On the gate- and drain-voltage dependence of the RTS amplitude in submicron MOSTs

The experimental gate- (V _GS) and drain-voltage (V _DS) dependence of the fractional Random Telegraph Signal (RTS) amplitude I _D/I _D, obtained on a large series of submicron Metal-Oxide Semiconductor Transistors (MOSTs), is reported. The observed variation of the RTS amplitude in linear operation is discussed in view of recently published models. As will be shown, the large spread in weak-inversion amplitudes can only be explained by taking into account the microscopic nature of the oxide trap and its environment. The position of a trap along the channel can in principle be retrieved from studying the so-called RTS amplitude asymmetry, defined as the V _DS dependence of the amplitude in both normal and reverse operation of the transistor. Widely different asymmetry behaviour is observed in this work. Here, a qualitative model will be derived which gives a more refined analysis and offers some deeper insight than existing theories. However, to fully understand the RTS amplitude in weak inversion, more microscopic detail is needed.     

Current and Shot Noise in a Quantum Dot Coupled to Ferromagnetic Leads in the Kondo Regime

Using the Keldysh nonequilibrium Green function technique, we study the current and shot noise spectroscopy of an interacting quantum dot coupled to two ferromagnetic leads with different polarizations in the Kondo regime. General formulas of current and shot noise are obtained, which can be applied in both the parallel （P） and antiparallel （AP） alignment cases. For large polarization values, it is revealed that the behaviour of differential conductance and shot noise are completely different for spin up and spin down configurations in the P alignment case. However, the differential conductance and shot noise have similar properties for different spin configurations in the P alignment case with the small polarization value and in the AP alignment case with any polarization value.     

Experimental progress in the nonlinear behavior of semiconductors

During the past five years spontaneous oscillations and chaotic behavior have been observed in the electronic transport of many semiconductors. Whereas the temperature range of some experiments extends up to room temperature, the majority of the measurements have been performed at liquid helium temperatures. We summarize these experimental developments and discuss their impact on the field of nonlinear dynamics which is rapidly progressing at present.     

Classification of current instabilities during low-temperature breakdown in germanium

We present experimental investigations on the spatio-temporal nonlinear current flow in the post-breakdown regime of p-germanium at liquid-helium temperatures. The basic nonlinear effects are characterized in terms of the underlying semiconductor physics, taking into account the influence of different experimental parameters.     

Metastable state of sliding CDW in K0.3MoO3

The narrow band noise and the transient voltage oscillation were investigated in a k_0.3MoO₃ sample, which showed different I–V characteristics in the non-linear conductivity region for dc and pulse methods. For repeated current pulses, after a sufficiently long duration of dc current, the voltage response showed relaxation behavior with a relaxation time of about 30 min at 77 K. Similar relaxation was also observed for the opposite case of a dc current applied after repeated pulses. The slope of frequency of voltage oscillation against CDW current was constant through these relaxation process in both dc and pulse cases. But the peak of narrow band noise is larger and sharper after repeated pulses than in the dc stationary state. This result was understood as an enhancement of the coherent-phase region for voltage oscillation in the case of repeated pulses.     

Conductance and shot noise of inelastic tunneling through a quantum dot in the Kondo regime

We investigate the inelastic transport properties of a quantum dot connected to two leads, based on the combination of a recently developed nonperturbative technique and slave-boson methods involving the approximate mapping of the many-body electron–phonon coupling problem onto a multichannel scattering problem in the Kondo regime. The nonequilibrium Green's function method is adopted in calculations for the inelastic transport processes of electrons in the limit of large Coulomb interaction U→∞$U\to \infty$ under nonequilibrium conditions. The electron–phonon interactions, which are the main source of the inelasticity, are taken into account. For a single quantum dot, we find that the differential conductance and the shot noise exhibit new structures of peaks and dips which are absent in the case without electron–phonon interactions.     

Low-Temperature Thermal Conductance in Superlattice Nanowire with Structural Defect

Using the scattering-matrix cascading method, we investigate the effect of structural defect on the acoustic phonon transmission and thermal conductance in the superlattice nanowire at low temperatures. In the present system, the phonon transmissions exhibit quite complex oscillatory behaviour. It is found that a lateral defect in an otherwise periodic structure significantly decrease the thermal conductance and completely washes away the transmission quantization. However, the appreciable transmission quantization survives in the presence of a longitudinal defect whereas a good quantization plateau of thermal conductance emerges below the universal level in a wide temperature range with the lateral defect.     

Conductance in an Aharonov-Bohm Interferometer with Parallel-Coupled Double Dots

We present a theoretical study of the conductance in an Aharonov-Bohm interferometer containing two coupled quantum dots. The interdot tunneling divides the interferometer into two coupled subrings, where opposite magnetic fluxes are threaded separately while the net flux is kept zero. Using the Green function technique we derive the expression of the linear conductance. It is found that the Aharonov-Bohm effect still exists, and when the level of each dot is aligned, the exchange of the Fano and Breit-Wigner resonances in the conductance can be achieved by tuning the magnetic flux. When the two levels are mismatched the exchange may not happen. Further, for some specific asymmetric systems where the coupling strengths between the two dots and the leads are not equal, the flux can change the Fano resonance into an antiresonance, which is absent in symmetric systems.     

Entanglement degree of electron pairs out of Andreev reflection

We study the entanglement degree of electron pairs emitted from an s-wave superconductor, which couples to two normal leads via a single-level quantum dot. Within the framework of scattering matrix theory, the concurrence is used to quantify the entanglement. And the result shows that the entanglement degree is generally influenced by the initial separation of the two electrons in a Cooper pair and the normal transmission eigenvalues T₁, T₂. But it is only determined by the eigenvalues in the tunnelling limit, T₁,T₂?1, what is more, it is measurable.     

Spreading resistance and 1/f noise of embedded ellipsoidal electrodes in a conductor

Results of calculations for the spreading resistance and the resistance fluctuations due to 1/f conductivity fluctuations are presented. The resistor consists of a conducting medium between a small embedded ellipsoidal electrode and a large electrode at a large distance from the small one. The resistance and resistance fluctuations are compared with the simple results for an embedded hemispherical electrode. The results are presented graphically and can be used for ohmic contacts on semiconductors or for embedded ellipsoidal electrodes in an ionic solution.     

Photocurrent spin diode: Electrically controlled sign reversal of current polarization

We report a study of spin-dependent transport through a quantum dot irradiated by continuous circularly polarized light resonant to the electron-heavy hole transition. We use the nonequilibrium Green's function to calculate the spin accumulation, spin-resolved currents, and current polarization in the presence of an external bias and intradot Coulomb interaction. It is found that for a range of external biases sign reversal of the current polarization can be modulated. The system thus operates as a rectifier for spin current polarization. This effect follows from the interplay between the external irradiation and the Coulomb repulsion. The spin-polarized transport through a three-terminal device is also discussed. Spin current with high polarization could be obtained due to spin filter effect.     

Double-resonant structure in the high-frequency transresistance of coupled electron-hole systems

The linear dc and high-frequency transresistivity of coupled electron-hole systems are investigated using the Lei-Ting balance equations approach extended to include many-body corrections. A possible indirect method of experimentally measuring the dynamical transresistivity in the high frequency (terahertz) regime is designed basing on the detailed analysis on the relationship between the directly measurable resistivities in the electron- and hole-layer and the dynamical transresistance. The theoretically predicted dc transresistance is in good agreement with the experimental data for the given electron-hole system experimentally investigated. The calculated dynamical transresistance exhibits pronounced double-resonant structure, which can be attributed to the cooperation and competition between the two plasmon modes. It is pointed out that the behavior of the frequency-dependent transresistance is temperature-sensitive and the dynamical transport properties are essentially influenced by the short range correlations. Received: 1st April 1998 / Revised: 22 June 1998 / Accepted: 6 August 1998     

Shot Noise in a Mesoscopic Interferometer

The charge conductance and the shot noise in an Aharonov--Bohm interferometer with double quantum dots embedded and coupled to each other by a capacity are studied in the framework of the equation of motion of Green’s function. From the impurity Anderson model Hamiltonian, the equations of motion of nonequilibrium Green functions are derived and solved including the effects of two body correlations under Lacroix’s approximation. Our results show that the conductance, the shot noise, and the Fano factor (the ratio of the shot noise to the Poisson noise) as functions of the magnetic flux oscillate with the period of h/e, and their oscillation behaviour is similar to the results of the experiment replacing the capacitive coupling by tunnelling between the two dots. The experiment is suggested to test the results.     

Temperature dependence of the noise amplitude in graphene and graphene oxide

Graphene and related materials such as carbon nanotubes and graphene oxide are promising materials for future applications in chemical sensing and electronics. Electronic noise in these materials is typically very high due to the low number of carriers and the inverse dependence of 1/f noise on the number of carriers. We have investigated the changes in 1/f noise amplitude with temperature in exfoliated graphene and reduced graphene oxide devices. We show that using reduced graphene oxide results in an intriguing environmental coupling to noise amplitude. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Noise and correlation functions of hot carriers in semiconductors

We present a unifying theory of electronic noise appropriate to semiconductor materials in the presence of electric fields of arbitrary strength. In addition to thermal noise, a classification scheme for excess noise indicating different microscopic sources of fluctuations responsible for number and mobility fluctuations is provided. On the basis of simple two-level models, numerical calculations using a Monte Carlo technique are performed for the case of p-type Si at 77 K. The primary quantity which is evaluated by the theory is the auto-correlation function of current fluctuations which, subsequently, is analyzed in terms of correlation functions of the relevant physical variables. Accordingly, the corresponding current spectral-densities are determined and then compared with direct experimental results and/or analytical expressions. Important subjects which have been investigated are: (i) the effect of field assisted ionization on generation-recombination noise from shallow impurity levels; (ii) the contribution to the total noise spectrum of cross-correlation terms coupling fluctuations in velocity with those in energy and number; (iii) the current random telegraph signal and the corresponding spectral density associated with a mobility fluctuator. In all cases the numerical calculations are found to be in satisfactory agreement with experiments and/or analytical expressions thus fully supporting the physical reliability of the theoretical approach here proposed.List of the Symbols Used e Absolute value of the electron charge - f Frequency - f Distribution function - g ₁ Scattering strength with the scatter in state 1 - g ₂ Scattering strength with the scatter in state 2 - Reduced Planck constant - j Total current density - j _c Conduction current density - j _d Displacement current density - j _x Component along the x direction of the total current density - k Carrier wavevector - m Carrier effective mass - m ₀ Free electron mass - r Position vector - s Average sound velocity - t Time - u Fraction of ionized carriers - u _i Random telegraph signal related to carrier state - u _m Random telegraph signal related to scatterer state - v _d Ensemble average of the free carrier drift-velocity - v _i Carrier group velocity - v _t Ensemble average of the carrier velocity in the direction transverse to the applied field - v _ix Component along the x direction of the carrier group velocity - v _d ^r Ensemble average of the reduced drift-velocity - v ^r _i Reduced velocity component in the field direction of the i-th particle - v _ix ^j Reduced velocity component along the x axis of the i-th particle in band j - v ^r _ix Reduced velocity component along the x axis of the i-th particle - x _d Ensemble average of the carrier displacement along the x direction from the initial position - x _i Displacement along the x direction of the i-th carrier from the initial position - y _i i-th stochastic parameter - A Cross-sectional area of a homogeneous sample - C _I Auto-correlation function of the total current fluctuations - Auto-correlation function of the total current fluctuations due to mobility fluctuations - D Diffusion coefficient - D _t K Optical deformation potential - E Electrical field strength - E Electric field - E _x Component of the electric field along the x direction - E ₁ ⁰ Acoustic deformation potential - G Conductance - I Total current - I ₀ Total current in the voltage noise operation - I _m Total current associated with mobility fluctuations - I ^V Total current in the current noise operation - K _B Boltzmann constant - L Length of a homogeneous sample - N Number of free carriers which are instantaneously present in the device - N _A Acceptor concentration - N _I Total number of carriers inside the device participating in the transport (here assumed to be constant in time) - N _T Total number of carriers which are instantaneously present in the device - S _I Spectral density of current fluctuations - S _V Spectral density of voltage fluctuations - Spectral density of current fluctuations associated with the mobility fluctuations - Spectral density of current fluctuations due to correlations between fluctuations in number and velocity - Spectral density of current fluctuations due to generation-recombination processes - Spectral density of current fluctuations due to free carrier drift-velocity fluctuations - S _I ^l Longitudinal component with respect to the applied field of the current spectral-density - S _I ^t Transverse component with respect to the applied field of the current spectral-density - T Absolute temperature - T _e Electron temperature - V Electrical potential - V ^I Electrical potential in the voltage noise operation - W Collision rate - Z Small signal impedance - Poole-Frenkel factor - Equilibrium generation rate - _E Field dependent generation rate - Typical energy for thermally escaping from the impurity level - v _d ⁽⁰⁾ Fluctuation of the ensemble average of the driftvelocity associated with Brownian-like motion - v _d ^r(0) Fluctuation of the ensemble average of the reduced drift-velocity associated with Brownian-like motion - Carrier energy - ₀ Vacuum permittivity - _a Energy of the acceptor level - _r Relative static dielectric constant - Angle between initial and final k states - _op Optical phonon equivalent temperature - Mobility - ₀ Chemical potential - ₁ Mobility with the fluctuating scatterer in state 1 - ₂ Mobility with the fluctuating scatterer in state 2 - ₀ Crystal density - _E Field dependent volume recombination rate - _eq Equilibrium volume recombination rate - Conductivity - _g Cross-section for impact ionization - _c Average scattering time - _g Generation time - _l Carrier lifetime - _m Scatterer lifetime - _m1 Mean value of the time spent by the fluctuating scatterer in state 1 - _m2 Mean value of the time spent by the fluctuating scatterer in state 2 - _r Average recombination time - _T Transit time - Scattering rate - _AB Correlation function of the two variables A and B