Negative Differential Conductance Regime Research Articles

Electrical measurement of the linewidth of a quantum well bound state

We investigate electron tunneling spectroscopy in the presence of a bound state within a double quantum barrier, single quantum well structure. We demonstrate a new technique to directly measure the intrinsic linewidth of the bound state within the quantum well from the current-voltage signature of the resonant tunneling phenomena and contrast our results with the standing approach in the literature. We then examine the signal behavior for the influence of device temperature and find support for electron-electron interactions within the well. The measured intrinsic bound-state width, ΓE, in the negative differential conductance regime is 1.11± 0.01 meV.

Negative differential conductance and super-Poissonian shot noise in a single quantum dot coupled to two noncollinear polarized ferromagnetic leads

We have theoretically studied the full counting statistics of electron transport through a single quantum dot (QD) weakly coupled to two noncollinearly polarized ferromagnetic leads. We found a strong robust bias-voltage-tunable negative differential conductance (NDC) region, in which the shot noise is dramatically enhanced and reaches up to a super-Poissonian value. In particular, the formed super-Poissonian shot noise can still occur in a wide bias voltage region where only the singly-occupied electronic states entering the bias voltage window. The underlying mechanisms of the observed NDC and super-Poissonian shot noise originate from the quantum coherence between the two singly-occupied electronic states. In addition, the skewness in the NDC regime can be significantly increased up to a large positive value, which is also attributed to the quantum coherence of the QD system, and the variation of the skewness value is more sensitive to the quantum coherence than the shot noise. Our findings suggest a QD-based tunable NDC device, and the predicted properties of high-order current cumulants can provide a deeper understanding of electron transport through the single QD.

Negative differential conductance in chromium based nano rings

The electric current of $$Cr_8$$Cr8 and $$Cr_7Ni$$Cr7Ni molecular rings with two semi-infinite, $$Au$$Au leads have been investigated. We have performed numerical calculations using the non-equilibrium Green's function method combined with the density functional theory. Then the electric current under a given applied bias voltage has been obtained in a two terminal molecular circuit system. Transport characteristics of these molecular-based quantum rings and coherent transport features of the system have been studied. It was shown that negative differential conductance regime arises, in several ranges of the bias voltage.

Hot Brownian carriers in the Langevin picture: Application to a simple model for the Gunn effect in GaAs

We consider a charged Brownian gas under the influence of external, static and uniform electric and magnetic fields, immersed in a uniform bath temperature. We obtain the solution for the associated Langevin equation, and thereafter the evolution of the nonequilibrium temperature towards a nonequilibrium (hot) steady state. We apply our results to a simple yet relevant Brownian model for carrier transport in GaAs. We obtain a negative differential conductivity regime (Gunn effect) and discuss and compare our results with the experimental results.

Charge and spin transport in strongly correlated one-dimensional quantum systems driven far from equilibrium

We study the charge conductivity in one-dimensional prototype models of interacting particles, such as the Hubbard and the $t\text{\ensuremath{-}}V$ spinless fermion models, when coupled to some external baths injecting and extracting particles at the boundaries. We show that, if these systems are driven far from equilibrium, a negative differential conductivity regime can arise. The above electronic models can be mapped into Heisenberg-like spin ladders coupled to two magnetic baths, so that charge transport mechanisms are explained in terms of quantum spin transport. The negative differential conductivity is due to oppositely polarized ferromagnetic domains that arise at the edges of the chain and therefore inhibit spin transport: we propose a qualitative understanding of the phenomenon by analyzing the localization of one-magnon excitations created at the borders of a ferromagnetic region. We also show that negative differential conductivity is stable against breaking of integrability. Numerical simulations of nonequilibrium time evolution have been performed by employing a Monte Carlo wave function approach and a matrix product operator formalism.

Gas sensing with long, diffusively contacted single-walled carbon nanotubes

A carbon nanotube thermal-conductivity-based pressure or gas sensor is described, which utilizes5–10 µm long, diffusively contacted single-walled nanotubes (SWNTs). Low temperatureelectrical transport measurements for these tubes were suggestive of a thermallyactivated hopping mechanism for electron localization, where a hopping energy of∼39 meV was computed. A negative differential conductance regime was also detected insuspended tubes, released using critical point drying, at high bias voltages. The pressure orgas sensitivity increased more dramatically as the bias power was increased up to14 µW, which was interpreted in the context of the high optical phonon density in the suspendedSWNTs. Such devices are promising for use as pressure sensors, as well as for the chemicalidentification of species having differing gas thermal conductivities.

Electrically driven thermal light emission from individual single-walled carbon nanotubes

Light emission from nanostructures exhibits rich quantum effects and has broad applications. Single-walled carbon nanotubes (SWNTs) are one-dimensional (1D) metals or semiconductors, in which large number of electronic states in a narrow range of energies, known as van Hove singularities, can lead to strong spectral transitions. Photoluminescence and electroluminescence involving interband transitions and excitons have been observed in semiconducting SWNTs, but are not expected in metallic tubes due to non-radiative relaxations. Here, we show that in the negative differential conductance regime, a suspended quasi-metallic SWNT (QM-SWNT) emits light due to joule-heating, displaying strong peaks in the visible and infrared corresponding to interband transitions. This is a result of thermal light emission in 1D, in stark contrast with featureless blackbody-like emission observed in large bundles of SWNTs or multi-walled nanotubes. This allows for probing of the electronic temperature and non-equilibrium hot optical phonons in joule-heated QM-SWNTs.

Theoretical investigation of negative differential conductance regime of silicon nanocrystal single-electron devices

The current-voltage characteristics of metal-insulator-Si quantum dot (QD)-insulator-metal structures are numerically simulated to investigate the design and the possible applications of single-electron devices taking advantage of Coulomb-blockade phenomenon. The simulation technique is based on a physical description of the devices and only requires fundamental quantities of the system but no fitting parameter. One of the originality of this work lies in the accurate calculation of tunneling rates by a perturbation method, which allows us to properly include the effect of a bias voltage on the wave functions in the QD. As a consequence, we show that the bias influence on the wave function may lead to negative-differential-conductance effects depending on the design of the structure.

Theory of terahertz electric oscillations by supercooled superconductors

We predict that below Tc a regime of negative differential conductivity (NDC) can be reached. The superconductor should be supercooledto T<Tc in the normal phase under DC voltage. In such a nonequilibrium situation the NDC of thesuperconductor is created by the excess conductivity of the fluctuation Cooperpairs. We propose NDC of supercooled superconductors to be used as an activemedium for generation of electric oscillations. Such generators can be used in thesuperconducting electronics as a new type THz source of radiation. Oscillations can bemodulated by the change of the bias voltage, electrostatic doping by a gate electrodewhen the superconductor is the channel of a field effect transistor, or by light.When small amplitude oscillations are stabilized near the critical temperatureTc the generator can be used as a bolometer. NDC, which is essential for the applications, ispredicted on the basis of analysis of known results for fluctuation conductivity, obtained inprevious papers by solving the Boltzmann kinetic equation for the Cooper pairs metastablein the normal phase. The Boltzmann equation for fluctuation Cooper pairs is a result ofstate-of-the-art application of the microscopic theory of superconductivity. Our theoreticalconclusions are based on some approximations like time dependent Ginzburg–Landautheory initially derived for gapless superconductors, but nevertheless can reliably predictthe appearance of NDC. NDC is the main ingredient of the proposed technical applications.The maximal frequency at which superconductors can operate as generators is determinedby the critical temperature . For high-Tc superconductors this maximal frequency falls well inside the terahertz range. Technical conditions toavoid nucleation of the superconducting phase are briefly discussed. We suggest that nanostructuredhigh-Tc superconductors patterned in a single chip can give the best technical performance of theproposed oscillator.

Shot noise in metallic double dot structures with a negative differential conductance

The shot noise of current through a metallic double quantum dot structure exhibiting negative differential conductance is studied. We can exactly solve the master equation and derive an analytical expression of the spectral density of current fluctuations as a function of frequency in the first Coulomb staircase region. For a large range of bias voltage the noise is calculated by Monte Carlo simulation. We show that the noise is always sub-Poissonian though it is considerably enhanced in the negative differential conductance regime.

Transport in weakly and strongly modulated two‐dimensional electron systems realized by Cleaved‐Edge‐Overgrowth

Periodic modulation of a free two-dimensional electron system changes the bandstructure in which the electrons move. A weak modulation opens up small energy gaps in the free electron bandstructure while a strong, short-period modulation leads to the formation of well separated cosine-like minibands. We study transport in both cases using structures grown with the Cleaved-Edge-Overgrowth method. It allows to fabricate high quality two-dimensional electron systems with arbitrary modulation periods which are precise on an atomic scale. Perturbing the system only weakly with a long-period superlattice that is located some distance away from the electron layer leads to magneto-transport characteristics which exhibit each a number of different 1/B-periodic oscillations. These can be very well explained by semiclassical theory and allow the extraction of the Fermi surface of the system. The strong modulation with a short-period superlattice leads to the two-dimensional equivalent of a conventional superlattice. One very important difference lies in the stability of the expected negative differential conductance regime with respect to electric field instabilities. Geometrical considerations show that the observation of stable Bloch oscillations is possible in these systems. We will show that density dependent transport studies confirm the formation of a two-dimensional superlattice. Although negative diffential conductance, also observed in those samples, must be attributed to an inhomogeneous density distribution.

Self-stabilization of chaotic domain oscillations in superlattices by time-delayed feedback control

Semiconductor superlattices driven into the regime of negative differential conductivity often exhibit complex or even chaotic self-sustained current oscillations due to travelling field domains. For a reliable operation of a superlattice as an electronic oscillator, such unpredictable and irregular conditions should be avoided. In this work we present a theoretical analysis, proposing a novel scheme to stabilize high-frequency domain oscillations in semiconductor superlattices by a time-delayed feedback loop.

High-frequency impedance of driven superlattices

The complex impedance of a semiconductor superlattice biased into the regime of negative differential conductivity and driven by an additional gigahertz ac voltage is computed. From a simulation of the nonlinear spatiotemporal dynamics of traveling field domains we obtain strong variations of the amplitude and phase of the impedance with increasing driving frequency. These serve as fingerprints of the underlying quasiperiodic or frequency locking behavior. An anomalous phase shift appears as a result of phase synchronization of the traveling domains. If the imaginary part of the impedance is compensated by an external inductor, both the frequency and the intensity of the oscillations strongly increase.

Self-consistent hopping transport in superlattices: non-equilibrium distribution functions and electron heating

The hopping transport model (Phys. Rev. B 12 (1975) 680) for vertical transport in superlattices has been extended towards a self-consistent calculation of the Wannier–Stark state distribution functions for plane–parallel electron motion. This way, the influence of field-induced electron heating could be investigated in the negative differential conductivity regime of nonequilibrium hopping transport. Variations of the mean electron temperature are observed near the optical phonon resonance which occurs if the Wannier–Stark ladder spacing is equal to the LO phonon energy.

Additional Coulomb blockade and negative differential conductance in closed two-dimensional tunnel junction arrays

We have theoretically investigated the transport properties of a two-dimensional array of small tunnel junctions having two different paths between the source and the drain electrodes. We have found that at zero temperature the Coulomb interaction between tunneling electrons in the upper and the lower branches of the array can bring about stationary configurations of electrons which completely block further influx of electrons from the source at voltages above the usual Coulomb blockade. This “additional” Coulomb gap is transformed into a negative differential conductance regime at finite temperature. We also discuss the effect of imperfection of arrays on the additional Coulomb blockade phenomena.

Boundary effect induced nonhysteretic transition caused by saddle-node bifurcation on a limit cycle in n-GaAs

Simulation study of nonlinear behavior of spatio-temporal oscillations of n-type GaAs at room temperatures is studied when the applied dc bias is in the negative differential conductivity regime. Modern MBE technique makes it possible that the electron density in the metal-semiconductor contact at the cathode, n( x = 0), may be controlled experimentally. It is found that when n( x = 0) is varied, the system may have a transition from an oscillating (travelling domains) to a non-oscillating (stationary domains) state. The transition is a global bifurcation known as saddle-node bifurcation on a limit cycle. The bifurcation feature of this transition is studied in detail.

Effect of elastic scattering on miniband transport in semiconductor superlattices.

Within the quasiclassical Boltzmann-Bloch approach to nonlinear miniband transport in superlattices, elastic scattering is included in a relaxation-time approximation. Elastic scattering makes the problem truly three dimensional and leads in the regime of negative differential conductance to results that are qualitatively different from those of the quasi-one-dimensional models considered previously and compare favorably with recent experiments and balance-equation calculations.

Tunneling current through a resonant structure

AbstractTunneling transport through a resonant double‐barrier structure is examined, with attention focused on the suppression of transmission current due to Pauli exclusion arising from population growth of the resonant state. A quasi‐analytic theory is developed to self‐consistently determine the population of the resonant state and its exclusion suppression of tunneling. Furthermore, the role of resonant state population in inducing an electrostatic increase of the resonant level energy, which tends to enhace tunneling, is considered. Taking all these features in joint consideration, a quasi‐analytic description of intrinsic bistability in the regime of negative differential conductance is achieved, exhibiting hysteresis in the I‐U characteristic.

A new method of determination of the I-V characteristics of negative differential conductance devices

A method for mapping the complete I-V characteristic of a negative differential conductance (NDC) device has been investigated. This method employs the measurable positive differential conductance (PDC) portions of the DC I-V curve together with the measured conductances at a fixed DC bias voltage in the PDC region with different RF signal levels using a standard semiconductor analyzer. The NDC regime of the I-V curve is numerically constructed from the measured conductances at a fixed DC bias voltage in the PDC region with different signal levels using a large-signal nonlinear-circuit analysis. >

Hot-electron transient transport in a superlattice miniband

The authors analyse hot-electron transient transport in a Bloch-type superlattice miniband, under conditions leading to saturation in the regime of negative differential conductance (NDC), using a balance equation approach with a dynamical force-balance equation for the centre-of-mass acceleration, and a dynamical energy balance. They solve these balance equations numerically for a GaAs-based superlattice, considering Gamma -valley electrons with a tight-binding miniband structure. Both impurity scattering and phonon scattering are taken into account. For a step electric field in the NDC regime, the drift velocity exhibits very pronounced overshoot. In GaAs systems, the maximum drift velocities occur within 0.1-0.3 ps after turning on an electric field of 10 kV cm-1. About this time, the instantaneous inverse effective CM mass for perpendicular transport falls within the range of negative values. The rise of the electron temperature, however, is much slower, such that the behaviour of the drift velocity response to an impulsive electric field is markedly dependent on the duration of the impulse.