Asymmetry and separation of spin tunneling time in ZnSe/Zn1-xMnxSe multilayers

We investigate characteristics of spin tunneling time in ZnSe/Ze_1-xMn_xSe multilayers under the influence of both an electric field and a magnetic field. The results indicate that the tunneling time shows complicated oscillations and significant spin separation for electrons with different spin orientations traversing semimagnetic semiconductor heterostructures. It is also shown that the tunneling time exhibits obvious asymmetry in opposite tunneling directions for electrons tunneling through asymmetric heterostructures, which mainly occurs in resonant regions. The degree of the asymmetry of the tunneling time is not only spin-polarization dependent but also external-field induced. Received 10 July 2001     

Magneto-tunneling spectroscopy of GaAs---AlGaAs double barrier tunneling devices in pulsed high magnetic fields up to 40T

The magneto-tunneling effect was investigated in GaAs---AlGaAs double barrier resonant tunneling devices in pulsed high magnetic fiels up to 40T applied parallel(B) and perpendicular (B) to the barrier layers. In a sample with , oscillatory structures due to the 2D electrons in the emitter and the LO phonon assisted resonant tunneling were observed when the magnetic field (B) was swept at constant bias voltages. A large drop of the current was found in the quantum limit at applied voltages below the negative differential conductivity region. A striking hysteresis was observed in the voltage-current (V - I) curves. In a wide well sample with , rich structures were observed in the V - I curve for B, corresponding to the tunneling to different cyclotron orbits from the emitter.     

Wave function mapping of self-assembled quantum dots by capacitance spectroscopy

We use frequency-dependent capacitance–voltage spectroscopy to study the dynamic charging of self-assembled InAs quantum dots. With increasing frequency, the AC charging becomes suppressed, beginning with the low-energy states. By applying an in-plane magnetic field, we generate an additional magnetic confinement that alters the tunneling barrier and hence the charging dynamics. In traveling through the potential barrier, the electrons acquire an additional momentum k₀, proportional to the magnetic field B. As the tunneling is enhanced, when k₀ matches the maximum of the electronic wave function Ψ (in momentum representation), we are able to map out the shape of Ψ by varying B.     

Sr2FeMoO6 nanosized compound with dielectric sheaths for magnetically sensitive spintronic devices

Single-phase agglomerated Sr₂FeMoO₆-_δ powders with the iron and molybdenum cations superstructural ordering of 88% were synthesized by sol-gel technique from the Sr(NO₃)₂ and Fe(NO₃)₃·9H₂O solutions with pH = 4. The ultrasound dispersion enabled us to obtain 75 nm grains. Powders were pressed with 4 GPa to receive the ceramics. The additional annealing at 700 K promoted the appearance of 7.5% SrMoO₄ phase. The nanocomposite with dielectric sheaths around the grains was obtained. Magnetization temperature dependences in zero-field cooled mode revealed inhomogeneous magnetic states. At temperature below 19 K, the superparamagnetic state is observed. Temperature increase leads to a realization of the stable superparamagnetic and metastable ferrimagnetic states, blocked by magnetic anisotropy energy. The resistivity temperature dependences have the semiconducting conductivity type. The charge transfer due to the hopping conductivity on the localized states in the energy band near the Fermi level dominates at 260–300 K. At 130–200 K the charge transfer is realized by electrons tunneling through the energy barrier. The electrons inelastic tunneling on conducting channels between grains, through the localized states in the dielectic interlayer dominates at low temperatures. The resistivity decreases in magnetic fields and the negative tunneling magnetoresistive effect reaching 41% occurs.     

Interlayer tunneling spectroscopy of graphite at high magnetic field oriented parallel to the layers

Interlayer tunneling in graphite mesa-type structures is studied at a strong in-plane magnetic field H up to 55 T and low temperature T = 1.4 K. The tunneling spectrum dI/dV vs. V has a pronounced peak at a finite voltage V ₀. The peak position V ₀ increases linearly with H. To explain the experiment, we develop a theoretical model of graphite in the crossed electric E and magnetic H fields. When the fields satisfy the resonant condition E = vH, where V is the velocity of the two-dimensional Dirac electrons in graphene, the wave functions delocalize and give rise to the peak in the tunneling spectrum observed in the experiment.     

Magnetotunneling relaxation of electrons in double quantum wells

Electron tunneling relaxation in double quantum wells subject to a transverse magnetic field is studied. The resonant peaks in the tunneling relaxation rate appear when the energy splitting Δ of the tunnel-coupled pair of the left- and right- well electron states is a multiple of the cyclotron energy ℏω_c and two series of the Landau levels coincide. The shape of such resonant oscillations of the relaxation rate is determined by the Landau levels' broadening (which is associated with the intrawell scattering in the case of small tunnel coupling), but it is not expressed through the electron density of states directly. The dependence of the tunneling relaxation rate on ℏω_c and Δ is calculated taking into account elastic scattering of the electrons by the inhomogeneities of the structure in the limit when the scattering potential is slowly changing on the magnetic length scale.     

Theory of the upper critical field in layered superconductors with magnetic intercalates

We propose a model for the layered superconductors with electronically isolated magnetic intercalates in which the electrons propagate freely in the dirty superconducting layers with arbitrarily strong spin-orbit scattering, and spin-flip scatter off the magnetic ions during interlayer tunneling. We calculate the upper critical field H_c2 including demagnetization effects. In a parallel field below the dimensional-crossover temperature T¹, a new type of spin ordering is predicted.     

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.     

Spin-polarized transport and thermopower of organometallic nanocontacts

We present a theoretical study of the length dependence of both conductance and thermopower of organometallic vanadium-benzene molecules (V _nBz_n+1) sandwiched between magnetic Co(100) electrodes. We show that the molecules with n≥3 are efficient spin filters. Namely, we find that the zero bias conductance of the majority electrons is small and decays exponentially with increasing length of the molecule and is in the tunneling regime while the minority electrons show metallic conductance. We show furthermore that the thermopower strongly depends on the length of the molecules and can even change sign as a function of length and temperature.     

铁磁/半导体/铁磁结构的双量子环自旋输运的特性

研究了与铁磁/半导体/铁磁结构相关的双量子环自旋输运的规律,研究结果表明：总磁通为零条件下,铁磁电极磁化方向反平行时,双量子环与单量子环相比提高了自旋电子透射概率的平均值.铁磁电极磁化方向平行时,双量子环对提高自旋向下电子平均透射概率的效果更明显;双量子环受到Rashba自旋轨道耦合作用影响时,自旋电子的平均透射概率明显高于单量子环,即使再加上外加磁场的影响,透射概率较高这一特征依然存在;双量子环所含的δ势垒具有阻碍自旋电子输运的作用,随δ势垒强度Z的增大透射概率 关键词： 双量子环 Rashba自旋轨道耦合 透射概率 δ势垒')" href="#">δ势垒     

Hysteresis of magnetoresistance in granular La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at low temperatures

The magnetoresistance of granular La_0.7Ca_0.3MnO₃ is studied experimentally over wide ranges of temperatures and magnetic fields. The emphasis is on anomalously large hysteresis of magnetoresistance at low temperatures (T = 4.2 K). The observed ρ(H) dependence can be qualitatively explained by spin-dependent tunneling of electrons through the dielectric boundaries of conducting granules characterized by a wide spread in the magnetic-moment magnitudes.     

Features of electronic transport in relaxed Si/Si1−XGeX heterostructures with high doping level

The low-temperature electrical and magnetotransport characteristics of partially relaxed Si/Si_1−xGe_x heterostructures with two-dimensional electron channel (n_e≥10¹² cm⁻²) in an elastically strained silicon layer of nanometer thickness have been studied. The detailed calculation of the potential and of the electrons distribution in layers of the structure was carried out to understand the observed phenomena. The dependence of the tunneling transparency of the barrier separating the 2D and 3D transport channels in the structure, was studied as a function of the doping level, the degree of blurring boundaries, layer thickness, degree of relaxation of elastic stresses in the layers of the structure. Tunnel characteristics of the barrier between the layers were manifested by the appearance of a tunneling component in the current–voltage characteristics of real structures. Instabilities, manifested during the magnetotransport measurements using both weak and strong magnetic fields are explained by the transitions of charge carriers from the two-dimensional into three-dimensional state, due to interlayer tunneling transitions of electrons.     

Twin-microstructure enhanced magnetoresistance in La0.67Ba0.33MnO3 oxides

The grain growth dependence of microstructure and its effects on magnetic and transport properties are studied in the polycrystalline La_0.67Ba_0.33MnO₃ oxides. It is found that a lateral growth manner along a certain direction and a concentric terrace pattern along three orthogonal axes occur in the samples sintered at 1573 and 1673 K, respectively. Lamella-like twin microstructure forms in the concentric terrace growth pattern and the magnetoresistance properties can be enhanced by the twin microstructure. It suggests that the twin-boundaries in twin-grains may possibly induce spin-dependent scattering of electrons that is field reduced, or spin-polarized tunneling of electrons that is field enhanced, thus strengthening the effect of grain boundaries.     

Intercluster conduction in lightly doped La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> manganites in the paramagnetic temperature range

The magnetotransport and magnetic properties of La _{1 ? x} Ca _x MnO₃ polycrystalline samples (x = 0–0.3) annealed under vacuum and in the oxygen environment are investigated in the temperature range from 77 to 400 K. The magnetic studies of lightly doped manganites reveal persistence of short-range magnetic order up to a temperature T* ≈ 300 K, which is about 2–3 times higher than their Curie temperature T _C. The temperature dependence of the electrical resistivity measured from T* down to nearly T ≈ T _C is fitted by the relation logρ ～ T ^?1/2, which is characteristic of granular metals with electrons tunneling among nanoclusters of magnetic metals embedded in a dielectric host. The magnetoresistance of polycrystalline samples annealed in the oxygen environment has been observed to increase. The electrical, magnetic, and magnetotransport properties of the manganites can be accounted for by the formation of magnetic nanoclusters below T*, tunneling (or hopping) of carriers among the nanoclusters, variation in the magnetic cluster size, and tunneling barrier thickness with variations in temperature and magnetic field strength, as well as by the effect of annealing in different media on the cluster properties.     

Wigner crystallization of electrons and holes in amorphous silicon nitride. Antiferromagnetic ordering of localized electrons and holes as a result of a resonance exchange interaction

The predicted quantitative relation between the density and trapping cross section of traps in Si₃N₄ and the Coulomb repulsion radius in the Wigner crystallization of carriers in localized states is observed experimentally. The absence of ESR for localized electrons and holes in Si₃N₄ is interpreted on the basis of a model of a resonance exchange interaction of electrons on account of tunneling via localized states. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 7, 489–494 (10 October 1996)     

Spin tunneling properties in mesoscopic magnets: effects of a magnetic field

The tunneling of a giant spin at excited levels is studied theoretically in mesoscopic magnets with a magnetic field at an arbitrary angle in the easy plane. Different structures of the tunneling barriers can be generated by the magnetocrystalline anisotropy, the magnitude and the orientation of the field. By calculating the nonvacuum instanton solution explicitly, we obtain the tunnel splittings and the tunneling rates for different angle ranges of the external magnetic field ( θ _H = π/2 and π/2 < θ _H < π). The temperature dependences of the decay rates are clearly shown for each case. It is found that the tunneling rate and the crossover temperature depend on the orientation of the external magnetic field. This feature can be tested with the use of existing experimental techniques. Received 12 March 2001 and Received in final form 18 October 2001     

超薄栅氧化层n-MOSFET软击穿后的导电机制

研究了恒压应力下超薄栅氧化层n型金属-氧化物-半导体场效应晶体管（n-MOSFET）软击穿 后的导电机制.发现在一定的栅电压V_g范围内，软击穿后的栅电流I_g符合Fowl er-Nordheim隧穿公式，但室温下隧穿势垒_b的平均值仅为0936eV，远小于S i/Si O₂界面的势垒高度315eV.研究表明，软击穿后，处于Si/SiO₂界 面量子化能级上的 电子不隧穿到氧化层的导带，而是隧穿到氧化层内的缺陷带上._b与缺陷带能 级和电 子所处的量子能级相关；高温下，激发态电子对隧穿电流贡献的增大导致_b逐 渐降低. 关键词： 软击穿 栅电流 类Fowler-Nordheim隧穿 超薄栅氧化层     

Magneto-transport and magnetic properties of (1-x)La0.7Ca0.3MnO3+xAl2O3 composites

We report magneto-transport and magnetic properties of (1-x)La_0.7Ca_0.3MnO₃+xAl₂O₃ composites synthesized through a solid-state reaction method combined with a high energy milling method. Most interestingly, the effective magnetic anisotropy is found to decrease with increase in the non-magnetic insulating Al₂O₃ phase fraction in the composites. In addition, we observed that the magnitude of low-field magnetoresistance arising from spin-polarized tunneling of conduction electrons, as well as that of high-field magnetoresistance, displays a Curie-Weiss law-like behavior. Finally, we found that the temperature dependence of low and high-field magnetoresistance is controlled predominantly by the nature of temperature response of surface magnetization of the particles.     

Phase transitions in a ferromagnetic semiconductor. III

We investigate the ferromagnetic phase transition in a heavily doped semiconductor or semimetal containing magnetic ions. The coupling between conduction electrons and magnetic ions is treated in the molecular field approximation. We show that the system undergoes a transition to an ordered state for arbitrarily small concentrations of the magnetic ions and the conduction electrons. In the limitJ/? _F?1 we obtain the Ruderman-Kittel result for the Curie temperature, and forJ/? _F?1 a generalization of Thompson’s strong coupling result. (J is the exchange coupling constant between conduction electrons and magnetic ions and? _F is the kinetic Fermi energy at absolute zero.) The intermediate range is evaluated numerically.     

Suppression of the equilibrium tunneling current between slightly disordered two-dimensional electron systems with different electron concentrations in a high magnetic field

Tunneling between parallel two-dimensional electron gases (2DEG) in accumulation layers formed on both sides of the single doped AlGaAs barrier are examined in both zero and high magnetic field. Accumulation layers are separated from highly n-doped contact regions which freely supply electrons to the 2DEGs via 80 nm thick lightly n-doped spacer layers. Strongly oscillating current with magnetic field along the 2DEGs is absent in this arrangement. Without magnetic field resonant tunneling between 2DEGs with different as grown electron concentration could be settle by application of external voltage bias. High magnetic fields (ν<1) shift resonant tunneling to zero external bias and suppresses tunneling current, creating wide gap in the tunneling density of states at the Fermi level arisen from the in-plane Coulomb interaction in the 2DEGs. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 3, 236–241 (10 February 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.