Double-barrier Tunneling Research Articles

Observation of Displacement Current Staircase and Negative Differential Resistance in Nanomechanical Double-Barrier Tunneling Structures with Scanning Vibrating Probe

Simultaneous measurements of tunneling current and displacement current spectroscopy are demonstrated in nanomechanical double-barrier tunneling structures which consist of scanning vibrating probe/vacuum/colloidal Au dots/self-assembled monolayer (SAM)/Au substrate. The displacement current staircase due to Coulomb blockade is observed together with the tunneling current staircase in the nanomechanical double-barrier tunneling structures. In the case of tunneling current spectroscopy, negative differential resistances (NDRs) are also observed. NDRs are concerned with the change in the number of electrons on the Au dot and the tunneling resistance.

Characterization of InAs/AlSb tunneling double barrier heterostructure by ballistic electron emission microscope with InAs as base electrode

We characterize our InAs/AlSb double tunnel barrier heterostructure as the base electrode with the well thickness of 12 nm and symmetric barrier thickness of 2 nm by ballistic electron emission microscopy/spectroscopy using the top InAs layer of heterostructures. We present the influence of the InAs base layer thickness (3 nm, 10 nm) and the influence of the surface passivation on results of spectroscopic measurement. The mean measured barrier height was 1.11 (±0.03) eV independent of the thickness of the InAs base electrode. The mean measured height of barriers in the case of the passivated InAs base layer was found to be lower and equal to 1.059 (±0.03) eV. We also detect thresholds in spectroscopic characteristics which are in accordance with the energy of the second resonant level in the well, and with energies that are combinations of thresholds for electron–hole pair creation processes in InAs and the energy of the second resonant level in the well. A ballistic electron emission microscopy picture at a constant voltage of 1.25 V is presented.

Displacement Current Staircase in Mechanical Single-Electron Turnstiles

Coulomb-blockade-based displacement current staircases are observed in the displacement current–voltage characteristics of double barrier tunneling junctions with a vibrating probe. Displacement current flows periodically in accordance with the mechanical oscillation of the reservoir and is separated from the tunneling current using a two-phase lock-in amplifier. The theoretical displacement current staircase is successfully fitted to the measurement. This displacement current staircase demonstrates the mechanical single-electron turnstiles in which the quantized number of electrons on the gold dots alternates periodically with the mechanical oscillation.

Thermal equilibrium governing the formation of negatively charged excitons in resonant tunneling diodes

The thermal equilibrium of the formation of negatively charged excitons is studied in this work, by measuring the current and quantum-well photoluminescence in biased resonant tunneling double barrier diodes. We observe that the intensity ratio of negatively charged and neutral excitons depends linearly on the current for fixed temperature and illumination conditions. We propose that the results can be interpreted in terms of a mass-action law governing the concentrations of neutral and charged excitons and free electrons. Measurements at different temperatures and bias yield an electron concentration and a dwell time in the well that are in good agreement with the values previously reported in the literature. We also analyze the dependence of the luminescence on excitation intensity.

Observation of Coulomb staircases of both tunneling current and displacement current in nanomechanical double barrier tunneling structures

Displacement current staircase due to Coulomb blockade is observed together with a tunneling current staircase in nanomechanical double barrier tunneling structures that consist of scanning vibrating probe/colloidal Au particles/vacuum/PtPd substrate. We discuss the motion of single electrons on and through the Au particles from both the tunneling and displacement current staircases, and demonstrate the electron shuttle phenomenon due to nanomechanical probe vibration.

Temperature dependence of tunneling magnetoresistance: Double-barrier versus single-barrier junctions

The temperature dependence of tunneling magnetoresistance (TMR) is studied for spin valve type double-barrier tunnel junctions. Normalized TMR values for double-barrier tunnel junctions (DBTJs) and single-barrier junctions (SBTJs) are plotted as functions of temperature and it is found that the DBTJ shows stronger temperature dependence of TMR than the SBTJ. The strong temperature dependence of TMR for the DBTJ is explained in terms of temperature dependence of the spin polarization of the middle magnetic layer and decrease of the spin coherence length with increasing temperature.

Conductivity oscillations in current-induced metastable states in low-doped manganite single crystals

Deterministic oscillations of current-induced metastable resistivity in changing voltage have been detected in La$_{0.82}$Ca$_{0.18}$MnO$_3$ single crystals. At low temperatures, below the Curie point, application of specific bias procedures switches the crystal into metastable resistivity state characterized by appearance of pronounced reproducible and random structures in the voltage dependence of the differential conductivity. In certain bias range equally spaced broad conductivity peaks have been observed. The oscillating conductivity has been tentatively ascribed to resonances in a quantum well within the double tunnel barrier of intrinsic weak-links associated with twin-like defect boundaries.

Electron transport within resonant tunneling diodes with staggered-bandgap heterostructures

The electron transport physics within the conduction-band of resonant tunneling diodes with staggered-bandgap structure is analyzed. Here, the current–voltage characteristic for AlGaSb/InAs/AlGaSb double-barrier tunneling diodes is calculated in the framework of the six-band Kane model. This work demonstrates that the conduction-band electron transport is extremely dependent on the coupling between the conduction and valence bands and that an accurate estimate of current density requires the application of a multi-band model. In addition, the application of a multi-band model yields results that are in excellent agreement with existing experimental measurements on staggered-bandgap structures when well known material parameters are utilized.

Current and spin torque in double tunnel barrier ferromagnet-superconductor- ferromagnet systems

We calculate the current and the spin-torque in small symmetric double tunnel barrier ferromagnet - superconductor - ferromagnet (F-S-F) systems. Spin-accumulation on the superconductor governs the transport properties when the spin-flip relaxation time is longer than the transport dwell time. In the elastic transport regime, it is demonstrated that the relative change in the current (spin-torque) for F-S-F systems equals the relative change in the current (spin-torque) for F-N-F systems upon changing the relative magnetization direction of the two ferromagnets. This differs from the results in the inelastic transport regime where spin-accumulation suppresses the superconducting gap and dramatically changes the magnetoresistance [S. Takahashi, H. Imamura, and S. Maekawa, Phys. Rev. Lett. 82, 3911 (1999)]. The experimental relevance of the elastic and inelastic transport regimes, respectively, as well as the reasons for the change in the transport properties are discussed.

Spin accumulation in ferromagnetic–superconductor–ferromagnetic double-barrier junctions

We have investigated experimentally the spin transport properties in ferromagnetic–superconductor–ferromagnetic double-barrier tunnel junctions. The transport measurements were conducted at mK temperatures with magnetic fields applied along the leads. When the magnetic moments of the two ferromagnetic leads were in opposite orientations (i.e. antiferromagnetic alignment), majority of the spins accumulated in the superconducting island. This was evidenced by a suppression of the superconducting gap.

Quantum coherent transport versus diode-like effect in semiconductor-free metal/insulator structure

Quantum coherent transport in double barrier tunnel junctions has been exploited for building micrometric-size semiconductor-free diodes. At room temperature, we observe strongly asymmetric current–voltage characteristics with an asymmetry ratio increasing with the bias voltage, reaching a maximum of 20 at 1 V. Our experimental data can be perfectly explained using a theoretical model involving resonance-assisted tunneling. The coherent/resonant tunneling regime is achieved using metallic 3 nm diameter monodisperse Cu clusters, sandwiched between two Al2O3 barriers.

STM study of morphology and electron transport features in cytochrome c and nanocluster molecule monolayers

The morphology and electron tunneling through single cytochrome c and nanocluster Pt 5(CO) 7[P(C 6H 5)] 4 molecules organized as monolayer Langmuir–Blodgett (LB) films on graphite substrate have been studied experimentally using scanning tunneling microscopy (STM) and spectroscopy techniques with sub-nanometer spatial resolution in a double barrier tunnel junction configuration STM tip–monomolecular film–conducting substrate at ambient conditions. STM images of the films revealed globular structures with characteristic diameters (∼3.5 nm for the protein molecule and ∼1.2 nm for the nanocluster). The spectroscopic study by recording the tunneling current–bias voltage ( I– V) curves revealed tunneling I– V characteristics with features as steps of different width and heights that are dependent on the STM tip position over the molecule in the monolayer, giving evidence for sequential discrete electron-tunneling effects with the combination of the single electron Coulomb-charging energy and the electronic energy level separation (molecular spectrum) in such immobilized metalloprotein and nanocluster structures that can be of interest for the development of bioelectronic and hybrid functional nanosystems.

TMR of double spin-valve type AF/FM/I/FM/I/FM/AF magnetic tunneling junctions

An unusually large enhancement of TMR at 77 K was observed in double barrier tunnel junctions (DBTJ). This is explained with extended Julliere's model which yields a twice larger TMR value. When the spin coherence length is much smaller at higher temperature, DBTJ is shown to work as a series of two single barrier tunnel junctions.

Single-electron tunneling in a single PbS nanocrystal nucleated on 11-mercaptoundecanoic acid self-assembled monolayer at room temperature

Nanometer-sized PbS particles in the size of about 3 nm produced by exposing a self-assembled monolayer of 11-mercaptoundecanoic acid salts on gold (111) substrate in a H2S atmosphere were studied by scanning tunneling microscopy (STM) and high-resolution transmission electron microscopy at room temperature in air. The stability of thus-obtained PbS nanoparticles makes STM imaging possible and repeatable scans of the semiconductor nanoparticles were performed. The current-voltage (I–V) characteristics of a single PbS nanoparticle exhibited clearly Coulomb blockade and Coulomb staircase. Furthermore, by varying the gap between a STM tip and the PbS nanoparticle, we also verified the dependence of staircase width on the change of the gap in the local I–V characteristics on the same PbS nanoparticle. The phenomena can be well described by a semi-classical double-barrier tunneling model.

Electron transport on a single CdS nanocrystal formed on self-assembled monolayer at room temperature

Electron transport through a single CdS nanocrystal with the size of 2 nm was investigated using scanning tunneling microscope (STM) at room temperature in air. The nanocrystal was prepared by the exposure of 11-mercaptoundecanoic acid self-assembled monolayer adsorbed bivalent metal ions on Au(1 1 1) substrate to an H 2S atmosphere. STM was employed to image, to position an individual CdS nanoparticle, and finally to detect its current–voltage ( I–V) characteristics. The results demonstrated clear Coulomb blockade and Coulomb staircase effects in the I–V curves. Furthermore, by varying the distance between STM tip and CdS nanoparticle, we also found the dependence of staircase width on the air gap in the local I–V characteristics, which can be well explained in terms of semi-classical double-barrier tunneling model.

Nonclassical behavior in the capacitance of a nanojunction.

The capacitance of a nanojunction formed by a scanning tunneling microscope (STM) tip and a two-dimensional gold cluster was measured through the single electron tunneling spectroscopy of a double-barrier tunnel junction. By decreasing the STM tip-cluster separation, it was observed that the capacitance first increases and then decreases at short separation. This characteristic clearly deviates from the classical behavior and provides evidence for potential quantum effects on the capacitance.

Shot noise in spin-polarized currents

We investigate spin-dependent fluctuations in spin-polarized electronic currents in a semimagnetic double-barrier tunneling structure. We use both quantum-coherent and semi-classical approaches to study the effects of spin-flip scattering on shot noise. In a limited parameter range, we find that for a fully spin-polarized incoming beam, both descriptions yield shot-noise suppression.

中間層にＡｌを用いた二重トンネル接合の磁気抵抗効果

Tunnel magnetoresistance for double barrier tunnel junctions with an intermediate layer of metallic Al was investigated. The junction with a 40-A intermediate layer of Al showed a about 8.5%. TMR ratio. The TMR ratio decreased with increasing Al thickness and became zero at 100 A. At a low temperature, Al thin film was expected to be in transition to superconductor. A superconducting gap was observed in the two junctions with 40-A and 100-A Al at about 0.4 K.

Spin injection in ferromagnet/superconductor/ferromagnet tunnel junctions

We theoretically study the spin-dependent transport in a ferromagnet/superconductor/ferromagnet double barrier tunnel junction. The spin-polarized tunneling currents create spin and charge imbalance in a superconductor. Nonequilibrium effects are dominated by spin imbalance, causing an unusual spin transport and suppression of superconductivity depending on whether the magnetizations of the electrodes are parallel or antiparallel. Recent experimental results on the critical current suppression by spin injection are discussed.

Tunnel current, conductance and magnetoresistance in double-barrier magnetic tunnel junctions

In this paper, we design a new type of magnetic tunnel junction (MTJ), which consists of two ferromagnetic layers separated by two adjacent insulating barriers which have different barrier heights and different dielectric constants. Based on the nearly-free-electron approximation, the tunnel current, conductance and magnetoresistance in the double-barrier MTJ are discussed, respectively, by the treatment of the transfer matrix method. Our numerical results show that there exists an asymmetrical tunneling phenomenon in the double-barrier MTJ under the forward and reverse biases, which is modulated by the configuration of magnetizations of two ferromagnetic layers. We predict that our results will stimulate experimental efforts on fabricating the double-barrier MTJ.