Coulomb Blockade Of Tunneling Research Articles

Coulomb-promoted spintromechanics in magnetic shuttle devices

Exchange forces on the movable dot ("shuttle") in a magnetic shuttle device depend on the parity of the number of shuttling electrons. The performance of such a device can therefore be tuned by changing the strength $U$ of Coulomb correlations to block or unblock parity fluctuations. We show that by increasing $U$ the spintro-mechanics of the device crosses over, at $U=U_c(T)$, from a mechanically stable regime to a regime of spin-induced shuttle instabilities. This is due to enhanced spin-dependent mechanical forces as parity fluctuations are reduced by a Coulomb blockade of tunneling and demonstrates that single-electron manipulation of single-spin controlled nano-mechanics is possible.

Nanomechanics of a magnetic shuttle device

We show that self sustained mechanical vibrations in a model magnetic shuttle device can be driven by both the charge and the spin accumulated on the movable central island of the device. Different scenarios for how spin- and charge-induced shuttle instabilities may develop are discussed and shown to depend on whether there is a Coulomb blockade of tunneling or not. The crucial role of electronic spin flips in a magnetically driven shuttle is established and shown to cause giant magnetoresistance and dynamic magnetostriction effects.

Coulomb blockade of spin-dependent shuttling

We show that nanomechanical shuttling of single electrons may enable qualitatively new functionality if spin-polarized electrons are injected into a nanoelectromechanical single-electron tunneling (NEM-SET) device. This is due to the combined effects of spin-dependent electron tunneling and Coulomb blockade of tunneling, which are phenomena that occur in certain magnetic NEM-SET devices. Two effects are predicted to occur in such structures. The first is a reentrant shuttle instability, by which we mean the sequential appearance, disappearance and again the appearance of a shuttle instability as the driving voltage is increased (or the mechanical dissipation is diminished). The second effect is an enhanced spin polarization of the nanomechanically assisted current flow.

Self-excited oscillations of charge-spin accumulation due to single-electron tunneling

We theoretically study electronic transport through a layer of quantum dots connecting two metallic leads. By the inclusion of an inductor in series with the junction, we show that steady electronic transport in such a system may be unstable with respect to temporal oscillations caused by an interplay between the Coulomb blockade of tunneling and spin accumulation in the dots. When this instability occurs, a new stable regime is reached, where the average spin and charge in the dots oscillate periodically in time. The frequency of these oscillations is typically on the order of 1GHz for realistic values of the junction parameters.

Nanothermometer Using Single Crystal Silver Nanospheres

Conventional thermometers have been widely employed in scientific researches and industrial applications. Thermometers with nanometer scaled spatial resolution attract more and more attentions recently with the rapid development of nanotechnology and nanoengineering. Many kinds of nanothermometers have been designed by duplicating the conventional thermometers at nanoscale through decreasing the geometrical size of the conventional thermometers. For example, nanoscale thermocouples are fabricated from nano-junctions based on Seebeck effect, liquid-in-tube nanothermometers from nanotubes based on temperature-dependent thermal expansion of liquid, nanosized fluorescence thermometers from nanoparticles based on temperature-dependent photoluminescence, nanoscale infrared thermometers from metal nanoparticles based on blackbody radiation, Coulomb Blockade nanothermometers from nanosized superconductor-insulator-metal tunnel junctions based on the Coulomb blockade of tunneling, and complex structured nanothermometers from MicroElectro-Mechanical-Systems based on temperature-dependent resonator quality factor or Fermi-level shift, etc. In all of these nanothermometers, the physical properties such as the voltage in nanoscale thermocouples, volume of liquid in liquid-in-tube nanothermometers, or photoluminescence spectrum in nanosized fluorescence thermometers are restored to their original state at room temperature after the temperature drops from high temperature. These nanothermometers are usually employed in real-time and in situ temperature detection, but not for recording and readout later, which is not useful for the situation where real-time readout is not possible like in the case of explosion, but the open-ended gallium-filled carbon nanotube thermometers can in principle also be a readout device after the event. Here another kind of nanothermometers, ex situ nanothermometers, which can record the temperature they were exposed to and be read later when the event is over, are demonstrated to measure temperature based on temperature-dependent size distribution and areal density of metal nanospheres. Compared with the reported nanothermometers, the nanothermometers made of nanospheres with a nanometer scaled spatial resolution, described in this paper, can record the highest temperature in the event and be read at a later time after the event is over. Figure 1a shows the deposited silver nanoparticles before being heated. Silver nanoparticles aggregate on the carbon supporting film coated on TEM grid. The shape of the nanoparticles is irregular and the size varies from 10nm to 100 nm. After heating to a certain temperature, smaller silver nanoparticles nucleate and grow on the whole carbon film (Fig. 1b). It is speculated that the surface diffusion causes the nucleation and growth of the new nanoparticles. Statistical analysis shows that all the nucleated nanoparticles are spherical with an average circularity M1⁄4 0.82–0.85. So these nucleated nanoparticles are named as nanospheres. The spherical morphology of the nanospheres should come from the surface melting because of melting point depression. The inset in Figure 1c shows a high-resolution TEM (HRTEM) image of a nanosphere with diameter of 16 nm observed at 500 8C. The surface of the silver nanosphere melts at 500 8Cwhile the melting point of bulk silver is 961 8C, a direct observation of the significant melting point reduction of nanosized silver particles. Themelting liquid layer covers the nanospheres, conceals the lowest-energy growth facets, and forms perfect spheres because of surface tension. When the surface-melted nanospheres cool down quickly from heating temperature, the main spherical shape is kept. High magnification TEM images (Fig. 1c) show that the nucleated nanospheres distribute uniformly on the carbon film and the size distribution of the nanospheres is narrow. HRTEM image of the nanospheres (Fig. 1d) indicates that the nanospheres are single crystalline at room temperature. EDS spectrum (Fig. 1e) and selected area electron diffraction of the nanospheres (Fig. 1f) confirm that the nucleated nanospheres are silver with face-centered cubic structure. Figure 2 shows the room temperature TEM images, size distribution, and the average diameter of the nanospheres after heating at different annealing temperatures and cooling down. Each sample was heated at a certain heating temperature for 5min and then cooled down to room temperature for TEM examination. TEM images (Fig. 2a) show that the nucleated nanospheres are smaller with heating at 300 8C than those at 500 8C. Histogram of the nanospheres (Fig. 2b) indicates that the diameter of most nanospheres is 4 nm after heating at 300 8C and 14 nm at 700 8C. The average diameter is systematically larger with higher heating temperature (Fig. 2c) because of coalescence.

Discrete tunneling in electronic transport properties of nanograined porous silicon and similar heterophase systems

Experimental data obtained in the study of transverse current transport in a number of nanosized grained or similar media, such as porous silicon layers, anodic silicon oxide layers, and silicon nitride layers prepared by ion implantation of nitrogen into silicon, have been analyzed within the theory of discrete tunneling. It has been demonstrated that the measurements of current-voltage characteristics of diode structures with dielectric interlayers and embedded grains make it possible to obtain useful information on the character and sizes of grains or quantum dots in the nanosized grained medium. Amorphous dielectrics can be considered a grained medium with nanosized composition fluctuations. The current-voltage characteristics of real structures are determined by both the current nonlinearity associated with the charge carrier injection and the field nonlinearity caused by the Coulomb blockade of tunneling.

Electron-electron interactions in antidot-based Aharonov-Bohm interferometers

We present a microscopic picture of quantum transport in quantum antidots in the quantum Hall regime taking electron interactions into account. We discuss the edge state structure, energy-level evolution, charge quantization and linear-response conductance as the magnetic field or gate voltage is varied. Particular attention is given to the conductance oscillations due to Aharonov-Bohm interference and their unexpected periodicity. To explain the latter, we propose the mechanisms of scattering by point defects and Coulomb blockade tunneling. They are supported by self-consistent calculations in the Hartree approximation, which indicate pinning and correlation of the single-particle states at the Fermi energy as well as charge oscillation when antidot-bound states depopulate. We have also found interesting phenomena of antiresonance reflection of the Fano type.

Single electron pumping at higher frequency in a novel semiconductor-based quantum dot device

The π-shift sinusoidal signals with different amplitudes applied directly to the two metallic finger gates on a shallow etched GaAs/AlGaAs quantum wire，tune unequally the two barriers of a static quantum dot induced by applying negative dc voltages on these two finger gates. Single electrons are transported through a periodically formed quantum dot without source-drain bias. Since single electron pumping in the novel semiconductor-based quantum dot device does not rely on Coulomb blockade of tunnelling，the device can not be limited to much lower frequency by the fixed tunneling time constant. Current plateaus due to single electron transport can be observed at 1.7K with frequencies up to 3GHz and a current level of 0.5nA. This novel device represents another possible path in the realization of a high current high accuracy quantum standard for electrical current，and also provides another means for high specd high accuracy transport of single electrons.

Coulomb-blockade transport in quasi-one-dimensional polymer nanofibers

We report the low temperature current-voltage (I-V) characteristics studies in quasi-one-dimensional conducting polymer nanofibers. We find a threshold voltage ${V}_{t}$ below which little current flows at temperatures below $30--40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. For $V>{V}_{t}$ current scales as ${(V∕{V}_{t}\ensuremath{-}1)}^{\ensuremath{\zeta}}$ where $\ensuremath{\zeta}\ensuremath{\sim}1.8--2.1$ at high biases. Differential conductance oscillations are observed whose magnitude increases as temperature decreases below $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We attributed the observed low temperature I-V behavior to Coulomb-blockade effects with a crossover to Luttinger liquidlike behavior at high temperature. We demonstrate that at low temperatures such a doped conjugated polymer fiber can be considered as an array of small conducting regions separated by nanoscale barriers, where the Coulomb-blockade tunneling is the dominant transport mechanism.

Optimization of electron cooling by SIN tunnel junctions

We report on the optimization of electron cooling by SIN tunnel junctions due tothe advanced geometry of superconducting electrodes and very effective normalmetal traps for more efficient removal of quasiparticles at temperatures from 25 to500 mK. The maximum decrease in electron temperature of about 200 mK has beenobserved at bath temperatures 300–350 mK. We used four-junction geometry withAl–AlOx–Cr/Cu tunnel junctions and Au traps. Efficient electron cooling was realized due to the improvedgeometry of the cooling tunnel junctions (quadrant shape of the superconducting electrode)and optimized Au traps just near the junctions () to reduce reabsorption of quasiparticles after removing them from normalmetal. The maximum cooling effect was increased from a temperature drop ofd T = −56 mK (ordinarycross geometry) to −130 mK (improved geometry of superconducting electrodes) and tod T = −200 mK (improved geometry of superconducting electrodes and effective Au traps).The heating peak (instead of cooling) near the zero voltage across cooling junctions hasbeen observed in practice for all samples at temperatures below 150 mK. For higher coolingvoltages close to the superconducting gap, the heating was converted to coolingwith decreased amplitude. The leakage resistance of the tunnel junctions gives areasonable explanation of the heating peak. The phonon reabsorption due to therecombination of quasiparticles in superconducting electrodes gives an additionalimprovement in the theoretical fitting but could not explain the heating peak.An anomalous zero-bias resistance peak has been observed for all tested structures. Thepeak is explained by Coulomb blockade of tunnelling in transistor-type structures withrelatively small tunnel junctions.The work on electron cooling is devoted to the development of a cold-electron bolometer(CEB) with capacitive coupling by SIN tunnel junctions to the antenna for sensitivedetection in the terahertz region. Direct electron cooling of an absorber plays a crucial rolein supersensitive detection in the presence of a realistic background power load.

Structural and electrical manifestation of ageing in thin-film Fe-Ta-O nanocomposite prepared by plasma jet technique

Electrical transport in the Fe–Ta–O nanocomposite prepared by a plasma jet technique was studied in a 4–300 K temperature range. Since the mean size of the Fe nanoparticles embedded in tantalum oxide is about 4 nm, the Coulomb blockade of tunneling is revealed in differentiated I–V curves below about 180 K, where the charging energy prevails the thermal energy. Below about 13 K the I–V curves are hysteretic, which implies the presence of charging traps in the insulating matrix. Electron diffraction reveals the presence of an additional oxide in the matrix – Fe2O3 regardless of much higher oxygen affinity to Ta than Fe. The occurrence of Fe2O3 is unavoidable due to the technology used. The presence of the traps in the tunneling barrier, which could behave as spin-flip centres, is considered the main reason for the low tunneling magnetoresistivity (TMR = –1.5% at 77 K in the field of 0.6 T). The chemical instability of the nanocomposite film is inferred from significant changes found in tunneling characteristics that were measured several times in the period of four months. This ageing effect is clearly manifested also in an altered nanoparticle size distribution. The measures for the quality improvement of the insulating matrix are proposed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Coulomb blockade of tunneling between disordered conductors.

We determine the zero-bias anomaly of the conductance of tunnel junctions by an approach unifying the conventional Coulomb blockade theory for ultrasmall junctions with the diffusive anomalies in disordered conductors. Both electron-electron interactions within the electrodes and electron-hole interactions between the electrodes are taken into account nonperturbatively. Explicit results are given for one- and two-dimensional junctions, and the crossover to ultrasmall junctions is discussed.

Quantum Description of a Degenerate P-N Junction Coupled to an Electrical Circuit

A theory of Coulomb blockade of tunneling in a degenerate p-n junction is presented. Perturbation theory and temperature Green's functions are used to obtain the current–voltage characteristics. The formulation extends the theory of Coulomb blockade in MIM junctions, to junctions in which the characteristics of the device, is partly determined by the many particle interaction in the electrodes. Exact analytical expressions for the I-V at zero temperature and approximate expressions at nonzero temperatures are obtained. Among the novel features observed are: an asymmetry in the current–voltage characteristics even for voltages on the scale of the Coulomb blockade threshold and threshold like behavior below (e/2C).

Horizon picture for the electrodynamic environment of coulomb blockade devices

The influence of a low impedance electromagnetic environment on Coulomb blockade of tunneling can be explained and qualitatively estimated through a simplified horizon picture, where the environment is reduced to a voltage dependent capacitance. We have studied the offset voltage,Voff, for three different circuit layouts. The measurements are well explained by the horizon picture.

Observation of the resonant tunneling of Cooper pairs.

We demonstrate how the Coulomb blockade of tunneling and the coherent Josephson tunneling of electron pairs can coexist in the same experiment. Measurements were made on a circuit consisting of two dc SQUIDs connected in series, with a gate capacitively coupled to the center electrode. Peaks in the current as a function of bias voltage shifted position with gate voltage in agreement with the theory of resonant Cooper pair tunneling.

Tunneling studies of mesoscopic all-NbN junctions

A scanning tunneling microscope was used to position a NbN tip near a NbN thin film sputtered on a Si substrate. Measurements at 4.2 K clearly show an energy gap of 5.0 mV. The Coulomb blockade of tunneling and the Coulomb staircase, formed by single-electron charging of the central electrode of a double capacitor system consisting of a substrate, a particle lodged in the oxide, and a tip, were observed at a number of points on the film. Experiments were repeated at room temperature. Fitting I-V and dI/dV-V curves to theory yields capacitances on the order of 5*10/sup 19/ to 2*10/sup -18/ F. The granular nature of the sputtered NbN greatly facilitates formation of the requisite double junction structure.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Coulomb blockade of tunneling in a 2D electron gas

The 2D electron gas of a GaAs/GaAlAs heterojunction is confined to a small island by means of Schottky gates. Two gates provide adjustable tunnel barriers and a central gate controls the electron number of the island. Conductance oscillations, periodic with the electron number, are observed in the Coulomb blockade regime with negligible energy level spacing. At low temperature, the blockade of the conductance by the island is shown to be limited by the quantum process of co-tunneling.

Effect of the electromagnetic environment on the Coulomb blockade in ultrasmall tunnel junctions.

The current-voltage characteristic of an ultrasmall tunnel junction is calculated for arbitrary frequency dependence of the impedance presented to the junction by its electromagnetic environment. It is shown that the Coulomb blockade of tunneling is washed out by quantum fluctuations of the charge on the junction capacitor except for ultrahigh impedance environments. Two simple cases where the environment can be treated as an inductor or resistor are examined in detail. Effects of finite temperatures are discussed.