Resonant tunneling via localized impurity states in metal-insulator-metal junctions

Strongly temperature dependent peaks in the tunnel conductance of Al-Al oxide-Pd diodes appearing at low temperatures are interpreted in terms of resonant tunneling via impurity states. Such structure was not observed when non-transition metals (Ag, Au, Pb, Sn) were used as counter electrodes.     

Resonant tunneling in magnetic semiconductor tunnel junctions with arbitrary magnetic alignments

Combining an extended Julliere model with transfer matrix method, we study the spin-polarized resonant tunneling in GaMnAs/AlAs/GaAs/AlAs/GaMnAs double barrier ferromagnetic semiconductor (FS) tunnel junctions with the arbitrary angle θ between the magnetic directions of two FS's. It is shown that tunneling magnetoresistance (TMR) ratio linearly varies with sin²(θ/2). We also demonstrate that for the heavy and light holes, the properties of the spin-polarized resonant tunneling are obviously different. The present results are expected to be instructive for manufacturing the relevant semiconductor spintronic devices.     

Spin-dependent resonant tunneling through quantum-well states in magnetic metallic thin films

Quantum-well (QW) states in nonmagnetic metal films between magnetic layers are known to be important in spin-dependent transport, but QW states in magnetic films remains elusive. Here we identify the conditions for resonant tunneling through QW states in magnetic films and report first principles calculations of Fe/MgO/FeO/Fe/Cr and Co/MgO/Fe/Cr. We show that, at resonance, the current increases by 1 to 2 orders of magnitude. The tunneling magnetoresistance ratio is much larger than in simple spin tunnel junctions and is positive (negative) for majority- (minority-) spin resonances, with a large asymmetry between positive and negative biases. The results can serve as a basis for novel spintronic devices.     

Resonant Josephson tunneling through S-I-S junctions of arbitrary size

The Josephson tunneling current in S-I-S structures where the main current transport channel is resonant tunneling through an isolated localized state is calculated using the Bogolyubov-de Gennes equations. It is shown that the efficiency of equilibrium Josephson resonant tunneling is determined only by the ratio of the width of the resonance level to the absolute value of the order parameter for the superconducting electrodes with arbitrary relationships among the system parameters. Zh. éksp. Teor. Fiz. 112, 342–352 (July 1997)     

Detecting electronic states at stacking faults in magnetic thin films by tunneling spectroscopy

Co islands grown on Cu(111) with a stacking fault at the interface present a conductance in the empty electronic states larger than the Co islands that follow the stacking sequence of the Cu substrate. Electrons can be more easily injected into these faulted interfaces, providing a way to enhance transmission in future spintronic devices. The electronic states associated with the stacking fault are visualized by tunneling spectroscopy, and its origin is identified by band structure calculations.     

铁磁金属量子阱态的共振隧穿

非铁磁金属层中的量子阱态在磁输运过程中的重要性已被广泛认识.铁磁金属层中自旋极化的量子阱态以前并没有详尽的理论研究;实验上也没有清晰地观测到自旋极化量子阱态的隧穿.文章介绍了最近由卢仲毅、张晓光和Pantelides［1］预言的Fe/MgO/FeO/Fe/Cr和其他铁磁量子阱隧道结中的共振隧穿,并解释铁、钴、铬的Δ1能带的对称性在这种共振隧穿中的作用.     

铁磁金属量子阱态的共振隧穿

非铁磁金属层中的量子阱态在磁输运过程中的重要性已被广泛认识．铁磁金属层中自旋极化的量子阱态以前并没有详尽的理论研究；实验上也没有清晰地观测到自旋极化量子阱态的隧穿．文章介绍了最近由卢仲毅、张晓光和Pantelides预言的Fe／MgO／FeO／Fe／Cr和其他铁磁量子阱隧道结中的共振隧穿，并解释铁、钴、铬的△1能带的对称性在这种共振隧穿中的作用．     

Resonant transport through midgap states in voltage-biased Josephson junctions of d-wave superconductors

We study theoretically the ac Josephson effect in voltage-biased planar junctions of d-wave superconductors. For some orientations of the superconductors a current peak is found at finite voltage in the current–voltage characteristics. We pick out the relevant physical processes and write down an analytical formula for the current which clearly shows how the midgap state acts as a resonance and produces the peak. We present a possible explanation for the zero-bias conductance peak, recently found in experiments on grain boundary junctions of high-temperature superconductors, in terms of resonant transmission through midgap state of quasiparticles undergoing multiple Andreev reflections. We note that within our framework the zero-bias conductance peak appears in rather transparent Josephson junctions of d-wave superconductors.     

Spin-flip assisted tunneling through quantum dot based magnetic tunnel junctions

We theoretically study the spin-polarized transport through double barrier magnetic tunnel junction (DBMTJ) consisting of the quantum dot sandwiched by two ferromagnetic (FM) leads. The tunneling current through the DBMTJ is evaluated based on the Keldysh nonequilibrium Green’s function approach. The self-energy and Green’s function of the dot are analytically obtained via the equation of motion method, by systematically incorporating two spin-flip phenomena, namely, intra-dot spin-flip, and spin-flip coupling between the lead and the central dot region. The effects of both spin-flip processes on the spectral functions, tunneling current and tunnel magnetoresistance (TMR) are analyzed. The spin-flip effects result in spin mixing, thus contributing to the spectral function of the off-diagonal Green’s function components ( G_s[`(s)] ^r )\left( {G_{\sigma \bar \sigma }^r } \right). Interestingly, the spin-flip coupling between the lead and dot enhances both the tunneling current and the TMR for applied bias above the threshold voltage V _th . On the other hand, the intra-dot spin-flip results in an additional step in the I-V characteristics near V _th . Additionally, it suppresses the tunneling current but enhances the TMR. The opposing effects of the two types of spin-flip on the tunneling current means that one spin-flip mechanism can be engineered to counteract the other, so as to maintain the tunneling current without reducing the TMR. Their additive effect on the TMR enables the DBMTJ to attain a large tunneling current and high TMR for above threshold bias values.     

Resonant tunneling through Andreev levels

We use a semiclassical approach for analyzing the tunneling transport through a normal conductor in contact with superconducting mirrors. Our analysis of the electron–hole propagation along semiclassical trajectories shows that resonant transmission through Andreev levels is possible resulting in an excess, low-energy quasiparticle contribution to the conductance. The excess conductance oscillates with the phase difference between the superconductors having maxima at odd multiples of π for temperatures much below the Thouless temperature.     

Resonant tunneling through a double-barrier quantum well in a transverse magnetic field

We theoretically analyze the tunneling of electrons through a heterostructure with two barriers and a quantum well between them in a magnetic field perpendicular to the current. We take into account the contribution from electrons with various positions of the magnetic oscillator center to the current. The region of the Z-shaped current-voltage characteristic for the heterostructure is shown to narrow as the magnetic field strengthens. Our analysis reveals a critical magnetic field strength at which the Z-shaped current-voltage characteristic transforms into an N-shaped one. We compare our results with experimental data.     

Resonant spin tunneling in small magnetic particles

A new macroscopic test of spin tunneling in single-domain magnetic particles is proposed. It is shown that quantization of spin, even in particles of a considerable size, may lead to a detectable zero-field maximum in the dependence of the rate of the magnetic relaxation on the magnetic field. This effect must be especially pronounced in antiferromagnetic particles. It is argued that recent observation by several groups of the non-monotonic field dependence of the blocking temperature in ferritin may, in fact, present the experimental evidence of this effect.     

Spin-polarized electron tunneling in bcc FeCo/MgO/FeCo(001) magnetic tunnel junctions

In combining spin- and symmetry-resolved photoemission, magnetotransport measurements and ab initio calculations we detangled the electronic states involved in the electronic transport in Fe(1-x)Co(x)(001)/MgO/Fe(1-x)Co(x)(001) magnetic tunnel junctions. Contrary to previous theoretical predictions, we observe a large reduction in TMR (from 530 to 200% at 20 K) for Co content above 25 atomic% as well as anomalies in the conductance curves. We demonstrate that these unexpected behaviors originate from a minority spin state with Δ(1) symmetry that exists below the Fermi level for high Co concentration. Using angle-resolved photoemission, this state is shown to be a two-dimensional state that occurs at both Fe(1-x)Co(x)(001) free surface, and more importantly at the interface with MgO. The combination of this interface state with the peculiar density of empty states due to chemical disorder allows us to describe in details the complex conduction behavior in this system.     

Resonant tunneling transport through GaAs/AlGaAs superlattices in strong tilted magnetic field

An analysis is presented of the transverse resonant tunneling transport through GaAs/AlGaAs superlattices due to tunneling between Landau levels in quantum wells in a strong tilted magnetic field. A high tunneling rate is demonstrated between Landau levels with Δn ≠ 0 in a magnetic field with a nonzero in-plane component. This leads to substantial broadening and shift of the tunneling resonance and significant changes in the current-voltage characteristics of superlattices. The predicted behavior of the current-voltage characteristics of superlattices in tilted magnetic fields is demonstrated experimentally.     

Resonant exciton trapping at impurity states in silver bromide

Excitation spectra near the indirect exciton edge of AgBr at 1.8K are reported for several luminescence lines from weakly localized excitons. Excitation below the exciton absorption threshold reveals several excited bound exciton states the energetic positions of which are determined. For excitation above the threshold, strong energy dependent structure is observed. It is interpreted in terms of resonant trapping of free excitons in both ground and excited bound exciton states associated with emission of LO(Γ), long wavelength acoustic and intervalley TA(X) and LA(X) phonons as well as combinations and overtones of these. From measurements in doped crystals two bound exciton systems are found to be correlated with Cd²⁺ and Ca²⁺, respectively.