Control of relative tunneling rates in single molecule bipolar electron transport

The influence of relative electron tunneling rates on electron transport in a double-barrier single-molecule junction is studied. The junction is defined by positioning a scanning tunneling microscope tip above a copper phthalocyanine molecule adsorbed on a thin oxide film grown on the NiAl(110) surface. By tuning the tip-molecule separation, the ratio of tunneling rates through the two barriers, vacuum and oxide, is controlled. This results in dramatic changes in the relative intensities of individual conduction channels, associated with different vibronic states of the molecule.     

Single electron tunneling at large conductance: The semiclassical approach

We study the linear conductance of single electron devices showing Coulomb blockade phenomena. Our approach is based on a formally exact path integral representation describing electron tunneling nonperturbatively. The electromagnetic environment of the device is treated in terms of the Caldeira-Leggett model. We obtain the linear conductance from the Kubo formula leading to a formally exact expression which is evaluated in the semiclassical limit. Specifically we consider three models. First, the influence of an electromagnetic environment of arbitrary impedance on a single tunnel junction is studied focusing on the limits of large tunneling conductance and high to moderately low temperatures. The predictions are compared with recent experimental data. Second, the conductance of an array of N tunnel junctions is determined in dependence on the length N of the array and the environmental impedance. Finally, we consider a single electron transistor and compare our results for large tunneling conductance with experimental findings. Received 2 February 2000     

Inelastic electron tunneling

Inelastic Electron Tunneling Spectroscopy (IETS) is a new technique for measuring the vibrational spectrum of minute quantities of organic compounds. Sensitivity is its key advantage over the conventional techniques of infrared and Raman spectroscopy. This article will first discuss the technique itself: its theoretical basis, selection rules, sensitivity, vibrational mode shifts due to surface interactions and superconductivity, and sample preparation. Then it will discuss applications of the technique to problems in biology, radiation physics, surface physics, and catalysis.     

Photon-assisted tunneling in electron pumps

We measure photon-assisted tunneling in 4- and 6-junction electron pumps at photon frequencies up to 60 GHz. We determine the microwave voltage at the pumps using noise thermometry. The standard theory of leakage in the electron pump, modified to include photon-assisted tunneling, describes our experiments well. From this test of theory we argue that, in the absence of external microwaves, photon-assisted tunneling driven by 1/f noise is an important error mechanism in electron pumps.     

Single electron on a nanodot in a double-barrier tunneling structure observed by noncontact atomic-force spectroscopy

A single electron has been observed on a nanodot in a double-barrier tunneling structure by noncontact atomic-force microscopy at fixed separation. Frequency shift-voltage dependence of an Au-coated cantilever/vacuum/1-decanethiol protected Au nanodot/1-octanethiol self-assembled monolayer/Au substrate structure deviates from the theoretical parabolic curve, which is attributed to the change in the number of quantized electrons on the Au nanodot caused by the Coulomb blockade phenomena.     

Spin-polarized electron tunneling in lanthanum manganite

Tunneling experiments were performed on ceramic samples with the composition La_0.6Sr_0.4MnO₃, manifesting negative magnetoresistance. Two types of contacts were studied: symmetric (break junction type) and asymmetric ceramic-insulator-metal contact. A high magnetic-field sensitivity of the conductivity σ(H) of the contacts was observed even when only one of the electrodes was magnetic. The effect was explained by the existence of spin-polarized localized states in the tunneling barrier. Their appearance was attributed to the formation of an oxygen-depleted, magnetically two-phase state of localized ferromagnetic nanoregions in an anti-ferromagnetic dielectric matrix in the near-contact region.     

Resonant tunneling single electron transistors

We use a modulation-doped double barrier heterostructure to fabricate a resonant tunneling single electron transistor. Irregular Coulomb blockade oscillations are observed when the gate voltage is swept to vary one-by-one the number of electrons in the dot close to 'pinch-off'. The oscillation period is not regular, and generally becomes longer as the electron number is decreased down to zero, reflecting the growing importance of electron-electron interactions and size quantization. Negative differential resistance associated with resonant tunneling through zero-dimensional states is pronounced for a dot holding just a few electrons. The temperature dependence of the Coulomb blockade oscillations and that for the negative differential resistance are not the same. This highlights the different effects of charging and resonant tunneling on the transport characteristics.     

Asymmetry in inelastic electron tunneling peak intensities

Experiments show that for an organic material placed on an oxide one sees an asymmetry of inelastic electron tunneling peak intensity for bias voltages of opposite sign. When the barrier is composed only of the organic material with no deliberate oxidation such asymmetry disappears.     

Valence band electron tunneling in metal-oxide-silicon structures

Experimental results are presented for the substrate current appearing in thin oxide metal-oxide-silicon capacitors with a shallow n/p junction beneath the gate when a positive gate voltage in the tunneling regime is applied. The analysis of the current-voltage characteristics shows that for an oxide voltage drop lower than about 5 V the substrate current is due to electron tunneling from the silicon valence band. The dispersion relation in the energy range extending 3 eV below the oxide conduction band is determined from the voltage dependence of the current in the direct tunneling regime. An effective mass of about 0.8m_e is found near the edge of the oxide conduction band, while for lower energies a strong decrease of the effective mass is observed.     

Low-temperature phonoemissive tunneling rates in single molecule magnets

Tunneling between the two lowest energy levels of single molecule magnets with Ising type anisotropy, accompanied by the emission or absorption of phonons, is considered. Quantitatively accurate calculations of the rates for such tunneling are performed for a model Hamiltonian especially relevant to the best studied example, Fe₈. Two different methods are used: high-order perturbation theory in the spin–phonon interaction and the non-Ising-symmetric parts of the spin Hamiltonian, and a novel semiclassical approach based on spin-coherent-state-path-integral instantons. The methods are found to be in good quantitative agreement with other, and consistent with previous approaches to the problem. The implications of these results for magnetization of molecular solids of these molecules are discussed briefly.     

自旋相关的电子隧穿和自旋极化

研究了电子的自旋相关的隧穿和极化。在外加磁场的作用下，自旋向上的电子与自旋向下的电子具有不同的隧穿系数。当电子的自旋方向与磁场方向相反时，其隧穿概率受到磁场的抑制而变小；反之，当两平行时，电子的了隧穿系数增大。这种差异可以用本中定义的自旋极化率来表示。本对不同磁场下的自旋极化率进行了计算，结果也表明当电子的动能较小，这种自旋极化的效应越显。     

Size-dependent single electron tunneling effect in Au nanoparticles

We investigated single electron tunneling (SET) behavior of dodecanethiol-coated Au nanoparticles of two different sizes (average sizes are 5 nm and 2 nm) using nanogap electrodes, which have a well-defined gap size, at various temperatures. The Coulomb staircases and the Coulomb gap near-zero bias voltage caused by the suppression of the tunneling electrons due to the Coulomb blockade effect were observed in the current-voltage (I-V) curves of both sizes of nanoparticles at a low temperature (10 K). At room temperature, the Coulomb gap was observed only in the I-V curve of the smaller nanoparticles. This result indicates that the charging energy of the smaller nanoparticles is enough to overcome the thermal energy at room temperature. This suggests that it is possible to operate the SET devices at room temperature using the smaller nanoparticles as a Coulomb island.     

Coherent versus sequential electron tunneling in quantum dots

Manifestations of quantum coherence in the electronic conductance through nearly closed quantum dots in the Coulomb-blockade regime are addressed. We show that quantum coherent tunneling processes explain some puzzling statistical features of the conductance peak heights observed in recent experiments at low temperatures. We employ the constant interaction model and the random matrix theory to model the quantum dot electronic interactions and its single-particle statistical fluctuations, taking full account of the finite decay width of the quantum dot levels.