Kinetic Inductance and Coulomb Blockade in One Dimensional Josephson Junction Arrays

One dimensional Josephson junction arrays have been fabricated andcurrent-voltage characteristics (IVC) have been measured at cryogenic temperatures. The arrays were fabricated in a SQUID-geometry which allowed an in situ tuning of the Josephson energy by application of a magnetic field. The IVC of the arrays shows a clear Coulomb blockade state. In the Coulomb blockade regime the IVC are hysteretic. The array is modeled using a serial resistive-inductive-junction model which is able to qualitatively explain the IVC. In this model an inductance of the order of 0.1–10 mH per junction is needed to account for the hysteresis. Kinetic inductance, stemming from the inertia of the Cooper pairs, gives the correct order of magnitude. The problem of self-heating is also discussed as an alternative explanation of the hysteresis.     

Single electron charging effects in high-resistance In2O#x2212;x#x2212;x#x2212;x wires

We report on our measurements of the transport properties of 0.75 m long insulating indium oxide wires and rings. These devices have no apparent tunnel barriers, yet they exhibit two properties at low temperatures which are characteristic of series arrays of small capacitance tunnel junctions: highly non-linear IV characteristics and a zero-bias conductance which is periodic in a voltage applied by means of a lateral gate. Two types of samples can be distinguished, based on the behaviour of the conductance oscillations at low temperatures. For the first type, the structure of the oscillations remains periodic down to our lowest temperatures, similar to the data from the tunnel junction arrays. For the second type, lowering the temperature results in a transition from periodic to quasi-periodic conductance peaks. A phenomelogical model based on the orthodox theory of the Coulomb blockade is able to account for most of our observations. The temperature and magnetic field dependence of these effects suggest that they are due to the influence of single electron charging on transport through the localized electron states in the indium oxide.     

Coulomb blockade induced negative differential resistance effect in a self-assembly Si quantum dots array at room temperature

We report a Coulomb blockade induced negative differential resistance (NDR) effect at room temperature in a self-assembly Si quantum dots (Si-QDs) array (Al/SiO₂/Si-QDs/SiO₂/p-Si), which is fabricated in a plasma enhanced chemical vapor deposition system by using layer-by-layer deposition and in-situ plasma oxidation techniques. Obvious NDR effects are directly observed in the current-voltage characteristics, while corresponding capacitance peaks are also identified at the same voltage positions in the capacitance-voltage characteristics. The NDR effect in dot array, arising from the Coulomb blockade effect in the nanometer-sized Si-QDs, exhibits distinctive scan-rate and scan-direction dependences and differs remarkably from that in the quantum well structure in the formation mechanism. Better understanding of the observed NDR effect in Si-QDs array is obtained in a master-equation-based numerical model, where both the scan-rate and scan-direction dependences are well explained.     

Optimal computer simulation of single electron device response

Recently the control of single electrons by Coulomb blockade could be achieved up to high temperatures with small multidot arrays. The size of the dots must lie in the nanometer range; up to now, it is not possible to realize regular lattice of metallic dots on insulators with these dimensions, and the arrays are necessarily highly disordered, with a very large range of tunnel junction resistances. Monte Carlo or rate equation techniques lead to cumbersome calculations for more than two dots. We will show how we are able to reduce drastically the computation time by optimal procedures. This allows us to increase the size of the arrays we can handle. Our target has been to identify the critical parameters which determine the dispersion of the disordered array response, a crucial parameter for device application. First results will be shown.     

Temperature Dependence of Coulomb Oscillations on DNA-Mediated Au Nanoparticle Assembly

We fabricated a single-electron transistor using DNA-assisted assembly of Au nanoparticles. Most devices exhibited clear Coulomb blockade and oscillations. In contrast to conventional single-electron transistors, however, the period of Coulomb oscillations was observed to depend on the temperature. This temperature dependence is probably ascribed to the temperature dependence of gate capacitance.     

Transport model of controlled molecular rectifier showing unusual negative differential resistance effect

We investigate theoretically the charge accumulated Q in a three-terminal molecular device in the presence of an external electric field. Our approach is based on ab initio Hartree-Fock and density functional theory methodology contained in Gaussian package. Our main finding is a negative differential resistance (NDR) in the charge Q as a function of an external electric field. To explain this NDR effect we apply a phenomenological capacitive model based on a quite general system composed of many localized levels (that can be LUMOs of a molecule) coupled to source and drain. The capacitance accounts for charging effects that can result in Coulomb blockade (CB) in the transport. We show that this CB effect gives rise to a NDR for a suitable set of phenomenological parameters, like tunneling rates and charging energies. The NDR profile obtained in both ab initio and phenomenological methodologies are in close agreement.     

Comparison of Coulomb Blockade Thermometers with the International Temperature Scale PLTS-2000

The operation of the primary Coulomb blockade thermometer (CBT) is based on a measurement of the bias-voltage-dependent conductance of arrays of tunnel junctions between normal metal electrodes. Here, a comparison of a CBT with a high-accuracy realization of the PLTS-2000 in the range from 0.008?K to 0.65?K is reported. An overall agreement of about 1?% was found for temperatures above 0.25?K. For lower temperatures, differences increase because the CBT does not reach full thermal equilibrium with its surroundings. This can be accounted for by numerical calculations based on electron?Cphonon decoupling.     

Electron Thermalization in Metallic Islands Probed by Coulomb Blockade Thermometry

We describe a systematic series of experiments on thermalization of electrons in lithographic metallic thin films at millikelvin temperatures using Coulomb blockade thermometry (CBT). Joule dissipation due to biasing of the CBT sensor tends to drive the electron system into non-equilibrium. Under all experimental conditions tested, the electron-electron relaxation is fast enough to ensure thermal electron distribution, which is also in agreement with the theoretical arguments we present. On the other hand, poor electron-phonon relaxation plays a dominant role in lifting the electron temperature above that of the bath. From a comparison of the results with the theoretical current-voltage characteristics of the thermometers we precisely determine the electron-phonon coupling constant for the common metals used. Our experiments show that it is a formidable task to attain thermal equilibrium with the bath using single-electron devices under non-zero bias conditions at 20–50 mK temperatures that are typically encountered in experiments. The conclusion concerning Coulomb blockade thermometry is more optimistic and two-fold: (1) One can now correct the errors due to bias heating in a satisfactory manner based on known material properties and the size of the metal films in the sensor. (2) Reliable thermometry down to 20 mK requires islands whose volumes are >10^?15 m³, which is still acceptable both from the parameter (capacitance) and fabrication points of view.     

Topographie und elektrische Eigenschaften von InAs‐Quantenpunkten

Topography and electrical properties of InAs quantum dots Self assembled InAs‐islands were grown on GaAs with molecular beam epitaxy in the Stranski‐Krastanow growth mode. The topography of surface quantum dots was investigated by atomic force (AFM) and scanning electron microscopy (SEM). While the AFM enables to determine the dot height of ≈ 10 nm the SEM is best suited to study the lateral dimensions of uncapped islands. The latter technique gives a dot diameter of ≈ 30 nm. Although the size distribution of the islands is convoluted in the capacitance measurements on a dot ensemble, it was possible to determine roughly a Coulomb blockade energy of ≈ 20 meV for the ground state and ≈ 10 meV for the first excited dot level. Taking advantage of AFM‐lithography we were able to study electron transport through a single InAs island. Here we got a Coulomb blockade energy of 12 meV when electrons tunnel through the first excited state of the dot.     

Position resolution in silicon sampling calorimeters

Due to the coupling between closely spaced detectors, the position resolution of a silicon sampling calorimeter can be degraded. In a capacitance coupling model, a Green's function is found which quantitatively describes signal spreading over two-dimensional detector arrays. The result provides some theoretical guidance in choosing calorimeter parameters to get the best performance.     

Realization of multiple valued logic and memory by hybrid SETMOS architecture

A novel complimentary metal-oxide-semiconductor (CMOS) single-electron transistor (SET) hybrid architecture, named SETMOS, is proposed, which offers Coulomb blockade oscillations and quasi-periodic negative differential resistance effects at much higher current level than the traditional SETs. The Coulomb blockade oscillation characteristics are exploited to realize the multiple valued (MV) literal gate and the periodic negative differential resistance behavior is utilized to implement capacitor-less multiple valued static random access memory (MV SRAM) cell. The SETMOS literal gate is then used to build up other MV logic building blocks, e.g., transmission gate, binary to MV logic encoder, and MV to binary logic decoder. Analytical SET model simulations are employed to verify the functionalities of the proposed MV logic and memory cells for quaternary logic systems. SETMOS MV architectures are found to be much faster and less temperature-sensitive than previously reported hybrid CMOS-SET based MV circuits.     

Analysis of Energy Quantization Effects on Single-Electron Transistor Circuits

In this paper, the effects of energy quantization on different single-electron transistor (SET) circuits (logic inverter, current-biased circuits, and hybrid MOS-SET circuits) are analyzed through analytical modeling and Monte Carlo simulations. It is shown that energy quantization mainly increases the Coulomb blockade area and Coulomb blockade oscillation periodicity, and thus, affects the SET circuit performance. A new model for the noise margin of the SET inverter is proposed, which includes the energy quantization effects. Using the noise margin as a metric, the robustness of the SET inverter is studied against the effects of energy quantization. An analytical expression is developed, which explicitly defines the maximum energy quantization (termed as “ quantization threshold”) that an SET inverter can withstand before its noise margin falls below a specified tolerance level. The effects of energy quantization are further studied for the current-biased negative differential resistance (NDR) circuit and hybrid SETMOS circuit. A new model for the conductance of NDR characteristics is also formulated that explains the energy quantization effects.      

Primary Thermometry in the Intermediate Coulomb Blockade Regime

We investigate Coulomb blockade thermometers (CBT) in an intermediate temperature regime, where measurements with enhanced accuracy are possible due to the increased magnitude of the differential conductance dip. Previous theoretical results show that corrections to the half width and to the depth of the measured conductance dip of a sensor are needed, when leaving the regime of weak Coulomb blockade towards lower temperatures. In the present work, we demonstrate experimentally that the temperature range of a CBT sensor can be extended by employing these corrections without compromising the primary nature or the accuracy of the thermometer.     

Control of electron localization in a coupled quantum dot structure

Abstract

We systematically investigate the dynamical behaviour of an electron in a double quantum dot system under the influence of an external AC field. It is assumed that the quantum dot confined structure exhibits a non-negligible Coulomb charging energy, inversely proportional to its small capacitance. The dynamic evolution of the system is obtained by numerically solving the coupled, nonlinear, equations derived from the time-dependent Schrödinger equation. We find cases where the electron is localized in the initially placed dot when both effects of the Coulomb charging energy and the external field are present, even though if either effect is absent the electron will tunnel between dots. We also show that we can pre-select the shape and rise time of a semi-infinite, pulsed, AC field in order to transfer an electron from the initially placed dot to the other dot and localize it there.     

Conducting Polymer Nanowire Arrays for High Performance Supercapacitors

This Review provides a brief summary of the most recent research developments in the fabrication and application of one‐dimensional ordered conducting polymers nanostructure (especially nanowire arrays) and their composites as electrodes for supercapacitors. By controlling the nucleation and growth process of polymerization, aligned conducting polymer nanowire arrays and their composites with nano‐carbon materials can be prepared by employing in situ chemical polymerization or electrochemical polymerization without a template. This kind of nanostructure (such as polypyrrole and polyaniline nanowire arrays) possesses high capacitance, superior rate capability ascribed to large electrochemical surface, and an optimal ion diffusion path in the ordered nanowire structure, which is proved to be an ideal electrode material for high performance supercapacitors. Furthermore, flexible, micro‐scale, threadlike, and multifunctional supercapacitors are introduced based on conducting polyaniline nanowire arrays and their composites. These prototypes of supercapacitors utilize the high flexibility, good processability, and large capacitance of conducting polymers, which efficiently extend the usage of supercapacitors in various situations, and even for a complicated integration system of different electronic devices.     

Single-electron transistors based on gate-induced Si island for single-electron logic application

The island size dependence of the capacitance components of single-electron transistors (SETs) based on gate-induced Si islands was extracted from the electrical characteristics. In the fabricated SETs, the sidewall gate tunes the electrically induced tunnel junctions, and controls the phase of the Coulomb oscillation. The capacitance between the sidewall gate and the Si island extracted from the Coulomb oscillation phase shift of the SETs with sidewall depletion gates on a silicon-on-insulator nanowire was independent of the Si island size, which is consistent with the device structure. The Coulomb oscillation phase shift of the fabricated SETs has the potential for a complementary operation. As a possible application to single-electron logic, the complementary single-electron inverter and binary decision diagram operation on the basis of the Coulomb oscillation phase shift and the tunable tunnel junctions were demonstrated.     

Integration of single-electron transistors using field-emission-induced electromigration

A novel technique for the integration of planar-type single-electron transistors (SETs) composed of nanogaps is presented. This technique is based on the electromigration procedure, which is caused by a field emission current. The technique is called "activation." By applying the activation to the nanogaps, SETs can be easily obtained. Furthermore, the charging energy of the SETs can be controlled by adjusting the magnitude of the applied current during the activation process. The integration of two SETs was achieved by passing a field emission current through two series-connected initial nanogaps. The current-voltage (I(D)-V(D)) curves of the simultaneously activated devices exhibited clear electrical-current suppression at a low-bias voltage at 16 K, which is known as the Coulomb blockade. The Coulomb blockade voltage of each device was also obviously modulated by the gate voltage. In addition, the two SETs, which were integrated by the activation procedure, exhibited similar electrical properties, and their charging energy decreased uniformly with increasing the preset current during the activation. These results indicate that the activation procedure allows the simple and easy integration of planar-type SETs.     

Charging and Spin Effects in Triple Dot Artificial Molecules

We have fabricated artificial molecules consisting of three coupled quantum dots defined in the two-dimensional electron gas of a GaAs/AlGaAs heterostructure using lithographically patterned gates and trenches. The three dots are arranged in a ring structure, where each dot is coupled to the other two dots. We find that, when tuned to the Coulomb blockade regime, the triple quantum dot device acts as a charge rectifier: an electron enters the third dot where it is trapped, producing a jamming effect where no other electron may enter the first dot. Triple quantum dots coupled in a ring will allow for the study of new molecular phases using artificial molecules and may also serve as building blocks of two-dimensional arrays for quantum computation.     

Charging and Spin Effects in Triple Dot Artificial Molecules

We have fabricated artificial molecules consisting of three coupled quantum dots defined in the two-dimensional electron gas of a GaAs/AlGaAs heterostructure using lithographically patterned gates and trenches. The three dots are arranged in a ring structure, where each dot is coupled to the other two dots. We find that, when tuned to the Coulomb blockade regime, the triple quantum dot device acts as a charge rectifier: an electron enters the third dot where it is trapped, producing a jamming effect where no other electron may enter the first dot. Triple quantum dots coupled in a ring will allow for the study of new molecular phases using artificial molecules and may also serve as building blocks of two-dimensional arrays for quantum computation.