Low-frequency current fluctuations in individual semiconducting single-wall carbon nanotubes

We present a systematic study on low-frequency current fluctuations of nanodevices consisting of one single semiconducting nanotube, which exhibit significant 1/f-type noise. By examining devices with different switching mechanisms, carrier types (electrons vs holes), and channel lengths, we show that the 1/f fluctuation level in semiconducting nanotubes is correlated to the total number of transport carriers present in the system. However, the 1/f noise level per carrier is not larger than that of most bulk conventional semiconductors, e.g., Si. The pronounced noise level observed in nanotube devices simply reflects on the small number of carriers involved in transport. These results not only provide the basis to quantify the noise behavior in a one-dimensional transport system but also suggest a valuable way to characterize low-dimensional nanostructures based on the 1/f fluctuation phenomenon.     

Ballistic transport and electrostatics in metallic carbon nanotubes

We calculate the current and electrostatic potential drop in metallic carbon nanotube wires self-consistently by solving the Green's function and electrostatics equations in the ballistic case. About one-tenth of the applied voltage drops across the bulk of a nanowire, independent of the lengths considered here. The remaining nine-tenths of the bias drops near the contacts, thereby creating a nonlinear potential drop. The scaling of the electric field at the center of the nanotube with length (L) is faster than 1/L (roughly 1/L/sup 1.25-1.75/). At room temperature, the low bias conductance of larger-diameter nanotubes is larger than 4e/sup 2//h due to occupation of noncrossing subbands. The physics of conductance evolution with bias due to Zener tunneling in noncrossing subbands is discussed.     

Switching of current channels and new mechanism of magnetoresistance in a tunneling structure

We have experimentally studied the transport properties of a planar La_0.7Sr_0.3MnO₃ (LSMO)/Mn-depleted LSMO/MnSi tunneling structure, in which the Mn-depleted LSMO layer plays the role of a potential barrier between the conducting layers of LSMO and MnSi. The measurements were performed in geometry with the current direction parallel to the planes of interfaces in the tunneling structure. It is established that the structure exhibits a nonlinear current-voltage characteristic and possesses a positive magnetoresistance, the value of which depends on the tunneling current. It is suggested that specific features of the transport properties of this structure are related to the phenomenon of current channel switching between the conducting layers. The switching mechanism is based on the dependence of the resistance of the tunneling junction between the conducting layers on the bias voltage and the applied magnetic field.     

ZnO Nanorods on a LaAlO3–SrTiO3 Interface: Hybrid 1D–2D Diodes with Engineered Electronic Properties

Integrating nanomaterials with different dimensionalities and properties is a versatile approach toward realizing new functionalities in advanced devices. Here, a novel diode‐type heterostructure is reported consisting of 1D semiconducting ZnO nanorods and 2D metallic LaAlO₃–SrTiO₃ interface. Tunable insulator‐to‐metal transitions, absent in the individual components, are observed as a result of the competing temperature‐dependent conduction mechanisms. Detailed transport analysis reveals direct tunneling at low bias, Fowler–Nordheim tunneling at high forward bias, and Zener breakdown at high reverse bias. Our results highlight the rich electronic properties of such artificial diodes with hybrid dimensionalities, and the design principle may be generalized to other nanomaterials.     

Ultra low-noise current sources

The solid state DC current sources available on the market are not suitable for applications in low noise measurement systems because of the high level of low-frequency noise introduced in the measurement chain. The most important cause of low-frequency noise in such instruments is the solid state device used as a voltage reference (usually a Zener diode). This problem has been solved, in the instrument described in this paper, by using a new circuit topology in which the solid-state voltage reference has been substituted by a low-noise battery. The instrument, capable of supplying a current as high as 100 mA, is characterized by a low-frequency noise level some orders of magnitude lower than that of similar commercial instrumentation     

Topological Electronic Transitions in Vortex Cores in Type-II Superconductors

On the basis of microscopic theory, we study topological transitions in quasiparticle spectra of vortex systems, governed by an external magnetic field and transport current. We analyze two generic examples of such transitions: (i) opening of Fermi surface segments corresponding to the creation of a vortex-antivortex pairs; (ii) merging of different Fermi surface segments via the Landau–Zener tunneling. The basic properties of vortex matter such as pinning and transport characteristics, heat transport in the vortex state and peculiarities of the local density of states are strongly affected by these changes in the Fermi surface topology.     

In0.53Ga0.47As PIN光电探测器的温度特性分析

从理论和实验上分析了双异质结In0.53Ga0.47As PIN光电探测器在不同的反向偏置电压下暗电流在甚宽温度范围内的温度特性。结果表明：在反向偏置低压与高压段,产生一复合电流与隧道电流(缺陷隧道电流与带带间隧道电流)分别占主导地位,并呈现出相应的温度特性。还从理论与实验两方面探讨了噪声对探测器R0A的影响,结果表明：在低温段,产生一复合噪声起主要作用,在高温段,俄歇复合噪声起主要作用。     

Fast Transient Current Response to Switching Events in Short Chains of Molecular Islands

We consider current flow between two metallic leads joined by tunneling junctions to a molecular island. The junctions are assumed to be “wiggly,” that is, switching on and off intermittently. The resulting sequence of current transients overlaps in dependence on the interplay between the switching times and the decay time of “initial” correlations induced by each switching event. The process is described in terms of nonequilibrium Green’s functions.     

Highly sensitive PtSi/porous Si Schottky detectors

Presents the first experimental results on PtSi/porous Si Schottky detectors. Si pores have been filled by Pt through electrodeposition. Under proper temperature treatment, Pt reacts with Si creating a PtSi layer that uniformly covers the walls of the pores. The excess unreacted Pt inside the pores is etched away leaving empty spaces behind. The spectral response of such a detector is very wide, covering from 0.9 up to at least 7 /spl mu/m of IR radiation in back illumination mode. Excellent responsivities, such as 60 A/W at 1 /spl mu/m and 0.96 A/W at 4 /spl mu/m of IR radiation is exhibited. Reverse bias current-voltage characteristics exhibit a breakdown type behavior with a breakdown voltage at about 10 V. The general shape of the reverse bias I-V curve, the wide spectral range, and high responsivity are explained through tunneling and avalanche multiplication. It is proposed that large fringing fields developed at sharp edges of the porous surface cause tunneling and avalanche multiplication.     

Low-noise automated measurement system for low-frequency currentfluctuations in thin-oxide silicon structures

The design of an automated system for low-noise measurement of low-frequency current fluctuations in thin-oxide silicon devices is presented. The aim of these measurements is to study the current tunneling through the oxide, and to investigate its correlations with the oxide breakdown. The dedicated system is realized by integrating a personal computer commercial acquisition board with custom designed low-noise preamplifiers     

Reducing of the time drift of Zener stabilization voltages

A technology is described that decreases the time drift of the Zener stabilization voltage, shortens the run-in time, and reduces the low-frequency noise amplitude. Translated from Izmeritel'naya Tekhnika, No. 2, pp. 34–36, February, 1997.     

单壁碳纳米管量子点的传导电流与电流噪声谱

近来，碳纳米管尤其是单壁碳纳米管越来越引起人们的注意，并已经广泛应用于实践中。本文采用开放量子系统的研究方法，利用量子主方程给出了与扫描隧道显微镜耦合的单壁碳纳米管量子点的输运电流与电流噪声谱的一般性计算方法，并研究了强弛豫条件下输运电流的性质。     

Low-frequency intensity noise in semiconductor lasers

The low-frequency intensity noise at 25 MHz of a Fabry-Perot semiconductor laser is measured as a function of injection current. All the measurements are taken at room temperature and the laser is operated with a commercial current source (the conditions under which laser diodes are often used). At the highest injection current of twice threshold, the intensity noise is 5.5 dB above the shot-noise limit. When the longitudinal side mode suppression of the laser is 20 dB or larger, the intensity noise is modeled adequately by an expression derived from the single-mode, small-signal, linearized, semiclassical rate equations. All the parameters used in the theory are derived or referenced.     

Reversible bistable switching in nanoscale thiol-substituted oligoaniline molecular junctions

Single molecular monolayers of oligoaniline dimers were integrated into sub-40-nm-diameter metal nanowires to form in-wire molecular junctions. These junctions exhibited reproducible room temperature bistable switching with zero-bias high- to low-current state conductance ratios of up to 50, switching threshold voltages of approximately +/-1.5 V, and no measurable decay in the high-state current over 22 h. Such switching was not observed in similarly fabricated saturated dodecane (C12) or conjugated oligo(phenylene ethynylene) (OPE) molecular junctions. The low- and high-state current versus voltage was independent of temperature (10-300 K), suggesting that the dominant transport mechanism in these junctions is coherent tunneling. Inelastic electron tunneling spectra collected at 10 K show a change in the vibrational modes of the oligoaniline dimers when the junctions are switched from the low- to the high-current state. The results of these measurements suggest that the switching behavior is an inherent molecular feature that can be attributed to the oligoaniline dimer molecules that form the junction.     

Non-Gaussian fluctuation in the charge transport of Si nanochains

The stability of the tunneling charge transport of a tangle of Si nanochains is investigated at high bias voltages using a micromanipulator in a scanning electron microscope. We confirm that the influence of electron injection due to the electron beam of a scanning electron microscope on the charge transport properties of nanochains is negligible when the electrode gap is small and the bias voltage is large. Under such conditions, current-time curves show large fluctuations. We find that the fluctuation is not a simple Brownian motion, but its distribution function can be fitted well by a Lévy distribution. Its origin is discussed in terms of percolation theory.     

Crossover from Kondo-assisted suppression to co-tunneling enhancement of tunneling magnetoresistance via ferromagnetic nanodots in MgO tunnel barriers

The dependence of the tunneling magnetoresistance (TMR) of planar magnetic tunnel junctions on the size of magnetic nanodots incorporated within MgO tunnel barriers is explored. At low temperatures, in the Coulomb blockade (CB) regime, for smaller nanodots the conductance of the junction is increased at low bias consistent with Kondo-assisted tunneling and the TMR is suppressed. For slightly larger nanodots but within the CB regime, the TMR is enhanced at low bias, consistent with co-tunneling. Magnetic tunnel junctions (MTJ) exhibit giant magnetoresistance in small magnetic fields that arises from the flow of spin-polarized current through an ultrathin tunnel barrier separating two magnetic electrodes. The current through an MTJ device depends on the magnetic orientation of the electrodes and is typically higher when the electrode moments are parallel than when they are antiparallel. It has recently been demonstrated that the spin polarization of the tunneling current can be greatly enhanced by using crystalline tunnel barriers formed from MgO as compared with conventional amorphous barriers formed from alumina, due to spin filtering across the MgO layer. The magneto-transport properties of magnetic granular alloys and magnetic tunnel junction devices with magnetic nanodots embedded in amorphous dielectric matrices, and tunnel barriers, respectively, have been studied by several groups, but no systematic studies of the dependence on these properties on the nanodot size have been made.     

Possible Spin Pumping Effects on Spin Torque Induced Magnetization Switching in Magnetic Tunneling Junctions

Dependence of spin torque induced magnetization switching upon interfacial insulating layers properties of magnetic tunneling junctions (MTJ) are studied. For the same magnetic properties and patterning geometric dimensions, changes in MTJ interfacial insulating layers properties reveal interesting magnetization switching behaviors. These behaviors cannot be explained by conventional Landau-Lifshitz-Gilbert equation with a spin torque term and an intrinsic ferromagnetic relaxation damping. However the magnetization switching dynamics can be understood through assumption of spin pumping effects in magnetic tunneling junctions. This is not only important for fundamental understanding of spin and electronic transport in MTJ but also important for practical trade-offs between critical switching current and MTJ resistance for spin torque random access memory.     

Precise in situ tuning of the critical current of a superconducting nanowire using high bias voltage pulses

We present a method for in situ tuning of the critical current (or switching current) and critical temperature of a superconducting MoGe nanowire using high bias voltage pulses. Our main finding is that as the pulse voltage is increased, the nanowire demonstrates a reduction, a minimum and then an enhancement of the switching current and critical temperature. Using controlled pulsing, the switching current of a superconducting nanowire can be set exactly to a desired value. These results correlate with in situ transmission electron microscope imaging where an initially amorphous nanowire transforms into a single crystal nanowire by high bias voltage pulses. We compare our transport measurements to a thermally activated model of Little's phase slips in nanowires.     

Detecting Current Noise with a Josephson Junction in the Macroscopic Quantum Tunneling Regime

We discuss the use of a hysteretic Josephson junction to detect current fluctuations with frequencies below the plasma frequency of the junction. These adiabatic fluctuations are probed by switching measurements observing the noise-affected average rate of macroscopic quantum tunneling of the detector junction out of its zero-voltage state. In a proposed experimental scheme, frequencies of the noise are limited by an on-chip filtering circuit. The third cumulant of current fluctuations at the detector is related to an asymmetry of the switching rates.