金电极下二元化合物氮化镓链结构演化和电导的从头计算

基于从头密度泛函理论结合非平衡格林函数方法,首次研究了二元化合物GaN链介于两个金电极之间的电子输运性质。模拟了Au-(GaN)2-Au节点断裂过程,计算了相应的结合能,获得了平衡态电导以及平衡态下节点的投影态密度。此外,计算了平衡态下节点在微小偏压下的电流和电导。结果发现,随着偏压增强,GaN分子链的电导也随之下降。最后给出的伏安特性曲线呈现一个非线性关系,表明节点具有类似半导体的特征。     

Tunneling Conductance in Normal Metal/Insulator/Triplet Superconductor Junction

Tunneling conductance in normal metal/insulator/triplet superconductor junctions is studied theoretically as a function of the bias voltage at zero temperature and finite temperature. The results show there are zero-bias conductance peak, zero-bias conductance dip and double-minimum structures in the spectra for p-wave superconductor junctions. The existence of such structures in the conductance spectrum can be taken as evidence that the pairing symmetry of Sr2RuO4 is p-wave symmetry.     

Tunneling Conductance in Normal Metal/Insulator/Triplet Superconductor Junction

Tunneling conductance in normal metal/insulator/triplet superconductor junctions is studied theoretically as a function of the bias voltage at zero temperature and finite temperature. The results show there are zero-bias conductance peak, zero-bias conductance dip and double-minimum structures in the spectra for p-wave superconductor junctions. The existence of such structures in the conductance spectrum can be taken as evidence that the pairing symmetry of Sr2RuO4 is p-wave symmetry.     

Multiple Andreev reflections in ballistic Nb-InGaAs/InP-Nb junctions

The current–voltage characteristics of ballistic Nb-InGaAs/InP-Nb Josephson junctions have been investigated. At temperatures below 1 K a negative differential conductance, which usually leads to a hysteresis in the current–voltage characteristics, was resolved by connecting an additional external shunt resistor to the junction. The negative differential conductance is explained by heating and conductance enhancement due to multiple Andreev reflections. The structures observed in the differential resistance measurements as a function of the bias voltage are explained by self-detection of Josephson radiation at low bias voltages and subharmonic gap structures at higher bias voltages.     

Possibility of superconductivity in rapidly quenched silicon

We observed the symmetric negative conductance spike structures at non-zero bias voltages in Si-Si point contact junctions. This observation will be understood in terms of the characteristics of a network of micro-Josephson junctions of ultrarapidly quenched granules of Si.     

正常金属/自旋三重态p波超导体结隧道谱的奇异性

利用Blonder，Tinkham和Klapwijk理论计算了正常金属/绝缘层/正常金属/自旋三重态的p波超导体结的隧道谱和平均电流.计算结果表明：在自旋三重态p波超导结的隧道谱中存在零偏压电导峰、零偏压电导凹陷和双凹陷结构，并有微分电导随偏压震荡的现象出现，在I-V曲线上出现电流台阶.这些结果在理论上支持Sr₂RuO₄的超导态是自旋三重态p波超导态. 关键词： 自旋三重态超导体 p波超导体 隧道谱     

Elastic electron tunneling study of the metal-insulator transition in TTF-TCNQ

Using a single crystal of TTF-TCNQ as a moving electrode and by approaching to to an oxidized Al counter-electrode we realized tunnel junctions in the temperature range of about 30 to 295 K. By selecting junctions with minimum thermionic background current and measuring the tunnel conductance by a standard constant voltage lock-in detection we observed a deepening of the zero bias conductance abruptly reaching zero at about 53 K. We suggest that this reflects a progressive depletion of the electron density of states at the Fermi level and gives a direct image of the opening of a Peierls gap at the transition temperature.     

Spin-polarized quasiparticle transport in ferromagnet/d-wave superconductor junctions

Within a scattering framework, a theoretical study is presented for the spin-polarized quasiparticle transport in ferromagnet/d-wave superconductor junctions. We find that the subgap conductance behavior is qualitatively different from a nonmagnetic junction, and can also be significantly different from those of a ferromagnet/s-wave junction. For a ballistic ferromagnet/d-wave superconductor junction, under appropriate conditions, a zero-bias conductance minimum could be achieved. In addition, a conductance maximum at finite bias could be evolved by interfacial scattering. For a normal-metal/ferromagnet/d-wave superconductor junction, conductance resonances are predicted.     

Tunneling through narrow-gap semiconductor Sb2Te3 barrier

Tunnel experiments have been performed on Au/Sb₂Te₃/Al tunnel junctions to study elastic interelectrode tunneling through the small energy gap of a narrow-gap semiconductor. Tunnel conductance exhibited narrow width conductance peak at zero bias voltage. This behaviour is in accordance with the result of the theoretically calculated tunnel conductance, in which the nonparabolic dispersion relation within the energy gap of the narrow-gap semiconductor used as a tunnel barrier in a metal/narrow-gap semiconductor/metal tunnel structure is included. And some interesting structures are also observed in the conductance curves.     

量子线/绝缘层/p波超导体结的隧道谱

用Bogoliubov-de Gennes方程来研究量子线/绝缘层/p波超导体结(q/I/p)中的准粒子输运过程，利用推广的Blonder，Tinkham和Klapwijk模型计算绝对零度和有限温度下q/I/p的一级谐波隧道谱.和量子线/绝缘层/d波超导体结的一级谐波隧道谱不同的是q/I/p的一级谐波隧道谱中存在零偏压电导峰.随着q/I/p中绝缘层的势垒散射增强，q/I/p的一级谐波隧道谱中零偏压电导峰变高.     

Conductance anomalies of small tunnel junctions inside the Coulomb gap

A small-capacitance normal tunnel deviates significantly from equilibrium because each tunneling event turns the junction voltage almost upside-down. If such a sudden perturbation occurs locally, Fermi liquid theory guarantees that infinitely many electron-hole pairs should be created near the Fermi surface. It is predicted that such an infrared-divergent shake-up combined with the electromagnetic environment leads to subgap conductance anomalies for two categories of junctions. For symmetric junctions whose electrodes have the same electronic properties, a nonvanishing subgap conductance is shown to be inevitable even if the environmental impedance is infinite. This effect smoothes the current-voltage (I–V) characteristic and shifts the Coulomb offset extrapolated back from the high-voltage part of theI–V curve. For asymmetric junctions, whose electrodes have different electronic affinities, tunneling conductance is enhanced in one direction and suppressed in the other; that is, the junctions exhibit a diode effect. In particular, when the tunneling resistance is much smaller than the resistance quantum and the current flows in the favorable direction, a strong tendency towards establishing phase coherence is shown to emerge, as in Josephson junctions, resulting in infinite differential conductance at zero bias voltage.     

The influence of the accumulation layer on the conductance characteristics of MIS tunnel junctions

The inflex structure found by Tsui in conductance characteristics of InAsInAs oxidePb tunnel junctions at the bias equal to the bulk Fermi degeneracy is explained as a consequence of the modification of the semiconductor band wave functions by the accumulation layer.     

Adsorption of 7-ethynyl-2,4,9-trithia-tricyclo[3.3.1.1]decane on ultra-thin CdS films

The adsorption mechanism for the new compound, 7-ethynyl-2,4,9-trithia-tricyclo[3.3.1.1^3,7]decane (7ETTD), on ultra-thin films (∼3 nm) of CdS is investigated. Multiple reflection absorption IR spectroscopy, in conjunction with inelastic electron tunneling spectroscopy, indicates that this compound forms a self-assembled monolayer adsorbed on the CdS surface via each molecule’s trithia-adamantane anchor. Conductance-voltage data are recorded for tunnel junctions of the type Al/CdS/7ETTD/Pb over a temperature range of 4 K to room temperature and they indicate that the presence of the 7ETTD layer on the CdS dramatically modifies the conductance-voltage behavior of the junctions. These measurements show that different conduction mechanisms, including tunneling and possibly hopping, are responsible for charge transfer through the junctions depending on current, temperature, and voltage. WKB fits to the data are used to determine barrier parameters (height and width) for Al/CdS/Pb junctions with and without adsorbed 7ETTD layers on the CdS. Analysis of the fits shows that tunneling occurs at low bias (less than ∼0.2 V) but, at higher bias voltages, modification of the barrier parameters alone is insufficient to account for the observed conductance changes. A frontier orbital model is invoked which does offer a plausible explanation for these conductance changes. The model assumes bias-dependent coupling between HOMO and LUMO states of the adsorbed 7ETTD and the surface states on the CdS. The present work suggests that, because of the marked effect on the conductance of CdS ultra-thin films, 7ETTD and other similar compounds may be candidates for use in molecular electronic device fabrication.     

Tunneling into multiwalled carbon nanotubes: coulomb blockade and the Fano resonance

Tunneling spectroscopy measurements of single tunnel junctions formed between multiwalled carbon nanotubes (MWNTs) and a normal metal are reported. Intrinsic Coulomb interactions in the MWNTs give rise to a strong zero-bias suppression of a tunneling density of states that can be fitted numerically to the environmental quantum-fluctuation theory. An asymmetric conductance anomaly near zero bias is found at low temperatures and interpreted as Fano resonance in the strong tunneling regime.     

Intrinsic tunnelling effects in self-doped La0.89MnO3 single crystals

Transport properties of self-doped La_0.89MnO₃ single crystals with Néel temperature of T_N ≈139 K have been investigated in wide temperature range 10–300 K. Data suggests that current at low temperature is conducted through a strongly temperature-dependent, but almost bias independent channel operating in parallel with a bias controlled but temperature independent channel. The first channel is associated with transport across an insulating antiferromagnetic matrix while the latter one represents tunnel conductivity through intrinsic tunnel junctions appearing due to interruption of conducting percolating paths by phase separated insulating inclusions. Tunnel character of the conductivity manifests itself in nonlinear current-voltage characteristics and appearance of a zero-bias anomaly in the form of a prominent conductance peak in the vicinity of zero bias. Zero bias anomaly and V-shaped characteristics of the differential conductance at high voltages are ascribed to the formation of local magnetic states in the insulating region of the tunneling junction.     

Bias Dependence in Spin-Polarized Tunneling of Ferromagnet/Ferromagnetic Insulator (Semiconductor)/Ferromagnet Junctions

Based on the nearly-free-electron approximation, the bias dependencies of electron transport properties of ferromagnet/ferromagnetic insulator (semiconductor)/ferromagnet junctions have been studied. Resonances appear in electron transmission probability. These resonances cause oscillations in the zero-temperature tunnel current and the resonances occur in tunnel conductance. Tunnel magnetoresistance (TMR) is an oscillatory function of bias. The TMR can reach a value as high as 100%. The bins dependencies of electron transport properties relate to the magnetic configurations of the junctions.     

Nonequilibrium charge susceptibility and dynamical conductance: identification of scattering processes in quantum transport

We calculate the nonequilibrium charge transport properties of nanoscale junctions in the steady state and extend the concept of charge susceptibility to the nonequilibrium conditions. We show that the nonequilibrium charge susceptibility is related to the nonlinear dynamical conductance. In spectroscopic terms, both contain the same features versus applied bias when charge fluctuation occurs in the corresponding electronic resonances. However, we show that, while the conductance exhibits features at biases corresponding to inelastic scattering with no charge fluctuations, the nonequilibrium charge susceptibility does not. We suggest that measuring both the nonequilibrium conductance and charge susceptibility in the same experiment will permit us to differentiate between different scattering processes in quantum transport.     

Evidence of size quantization in Bi films by tunnel spectroscopy

We have studied the tunnelling characteristics of junctions having as one of the electrodes a size-quantized Bi film. We observe a series of peaks in the tunnel conductance as a function of the bias voltage. Their position is explained in terms of a nonellipsoidal nonparabolic (NENP) model.     

正常金属/铁磁绝缘层/p波超导体结的隧道谱

利用Blonder-Tinkham-Klapwijk理论,计算了正常金属/铁磁绝缘层/p波超导体结的隧道谱.结果表明:(1)在正常金属/铁磁绝缘层/p波超导体结的隧道谱中存在零偏压电导峰、零偏压电导凹陷;(2)在Px波结的隧道谱中,磁散射能导致零偏压电导峰的劈裂,而界面的粗糙散射却可以阻止其劈裂;(3)界面的势垒散射,磁散射及其与粗糙散射的共同作用对px、py波结零偏压电导的影响是不同的.     

Tunneling conductance of Al-oxide-Al junctions doped with Ni at the oxide-Al interface

The effect of a Ni impurity layer at the Oxide-Al interface has been studied for thickness in the range between 0.3 nm and 100 nm. The results obtained from the temperature dependence of the zero bias conductance and the voltage dependence of the dynamic resistance exhibit structures being due to the dynamics of spin-flip scattering and impurity interaction. We show that at a layer thickness of 1.5 nm Ni the behaviour of the tunnel junctions changes drastically because of the occurence of the magnetic interaction between the impurity atoms.