Electrical measurement of the direct spin hall effect in Fe/InxGa(1-x)As heterostructures

We report on an all-electrical measurement of the spin Hall effect in epitaxial Fe/InxGa(1-x)As heterostructures with n-type (Si) channel doping and highly doped Schottky tunnel barriers. A transverse spin current generated by an ordinary charge current flowing in the InxGa(1-x)As is detected by measuring the spin accumulation at the edges of the channel. The spin accumulation is identified through the observation of a Hanle effect in the voltage measured by pairs of ferromagnetic Hall contacts. We investigate the bias and temperature dependence of the resulting Hanle signal and determine the skew and side-jump contributions to the total spin Hall conductivity.     

Optical and magnetic characterisation of Co3+ and Ni3+ in LaAlO3: interplay between the spin state and Jahn-Teller effect

The coordination, the electronic structures and the spin of the ground state of Ni(3+) (3d(7)) and Co(3+) (3d(6)) introduced as impurities in LaAlO(3) are investigated through optical spectroscopy and magnetic measurements. The unusual trivalent valence state in both transition-metal ions was stabilised via a sol-gel process followed by high oxygen pressure treatments. We show that the crystal-field strength at the nearly O(h) transition-metal site in LaAlO(3) locates Ni(3+) and Co(3+) near the spin state crossover, yielding a low-spin ground state in both cases. We analyse how the interplay between the Jahn-Teller (JT) effect and the spin state affects the magnetic moment of the ion and its temperature dependence. The optical spectra reveal a JT effect associated with a low-spin ground state in Ni(3+) and with a thermally populated high-spin low-lying first excited state in Co(3+). The corresponding JT distortions are derived from structural correlations. We conclude that the JT effect is unable to stabilise the intermediate spin state in Co(3+). A low-spin ground state in thermal equilibrium with a high-spin low-lying first excited state is detected in diluted Co(3+)-doped LaAlO(3). These results are compared with those obtained in the parent pure compounds LaNiO(3) and LaCoO(3).     

Origin of metallic states at the heterointerface between the band insulators LaAlO3 and SrTiO3

We have studied the electronic structure at the heterointerface between the band insulators LaAlO3 and SrTiO3 using in situ photoemission spectroscopy. Our experimental results clearly reveal the formation of a notched structure on the SrTiO3 side due to band bending at the metallic LaAlO3/TiO2-SrTiO3 interface. The structure, however, is absent at the insulating LaAlO3/SrO-SrTiO3 interface. The present results indicate that the metallic states originate not from the charge transfer through the interface on a short-range scale but from the accumulation of carriers on a long-range scale.     

Room Temperature Spin Accumulation Effect in Boron Doped Si Created by Epitaxial Fe<Subscript>3</Subscript>Si/<Emphasis Type="Italic">p</Emphasis>-Si Schottky Contact

To study spin-dependent transport phenomena in Fe₃Si/p-Si structures we fabricated 3-terminal planar microdevices and metal/semiconductor diode using conventional photolithography and wet chemical etching. I?V curve of prepared diode demonstrates rectifying behavior, which indicates the presence of Schottky barrier in Fe₃Si/p-Si interface. Calculated Schottky barrier height is 0.57 eV, which can provide necessary conditions for spin accumulation in p-Si. Indeed, in 3-terminal planar device with Fe₃Si/p-Si Schottky contact Hanle effect was observed. By the analysis of Hanle curves spin lifetime spin diffusion length in p-Si were calculated, which are 145 ps and 405 nm, respectively (at T = 300 K). Spin lifetime strongly depends on temperature which can be related to the fact that spin-dependent transport in our device is realized via the surface states. This gives a perspective of creation of spintronic devices based on metal/semiconductor structure without need for forming tunnel or Schottky tunnel contact.     

LaAlO3/SrTiO3界面的电子结构及光学性质的第一性原理研究

采用基于密度泛函理论框架下的第一性原理平面波超软赝势方法,结合局域密度近似（LDA）研究了钙钛矿结构氧化物LaAlO3 /SrTiO3界面的电子结构及光学性质。能带结构分析表明当形成(AlO2)-/(TiO2)0界面时其禁带宽度为1.888 eV,呈现绝缘体的性质,当形成(LaO)+/(SrO)0界面时其禁带宽度为0.021 eV,呈现半导体或半金属性质。同时,对不同界面的光学性质也进行了研究,结果表明纯相的LaAlO3和SrTiO3的吸收系数、反射系数及能量损失谱强度明显高于由这两种单质形成不同界面的强度。     

Upper limit to magnetism in LaAlO3/SrTiO3 heterostructures

Using polarized neutron reflectometry we measured the neutron spin-dependent reflectivity from four LaAlO(3)/SrTiO(3) superlattices. Our results imply that the upper limit for the magnetization averaged over the lateral dimensions of the sample induced by an 11 T magnetic field at 1.7 K is less than 2 G. SQUID magnetometry of the neutron superlattice samples sporadically finds an enhanced moment, possibly due to experimental artifacts. These observations set important restrictions on theories which imply a strongly enhanced magnetism at the interface between LaAlO(3) and SrTiO(3).     

High mobility in LaAlO3/SrTiO3 heterostructures: origin, dimensionality, and perspectives

We have investigated the dimensionality and origin of the magnetotransport properties of LaAlO3 films epitaxially grown on TiO2-terminated SrTiO3(001) substrates. High-mobility conduction is observed at low deposition oxygen pressures (P(O2)<10(-5) mbar) and has a three-dimensional character. However, at higher P(O2) the conduction is dramatically suppressed and nonmetallic behavior appears. Experimental data strongly support an interpretation of these properties based on the creation of oxygen vacancies in the SrTiO3 substrates during the growth of the LaAlO3 layer. When grown on SrTiO3 substrates at low P(O2), other oxides generate the same high mobility as LaAlO3 films. This opens interesting prospects for all-oxide electronics.     

铝酸镧单晶体中Ce3+的能级结构和荧光特性

采用提拉法成功生长了纯LaAlO₃和掺铈的LaAlO₃单晶体.测试了它们的远红外吸收谱，紫外吸收谱，荧光谱.根据吸收光谱确定了晶体中Ce³⁺的能级结构，利用这一能级结构模型较好地解释了Ce：LaAlO₃晶体的荧光光谱. 关键词： Ce：LaAlO3 吸收光谱 能级 荧光光谱     

Shubnikov-de Haas oscillations in SrTiO3/LaAlO3 interface

Quantum magnetic oscillations in SrTiO3/LaAlO3 interface are observed in the magnetoresistance. We study their frequency as a function of gate voltage and the evolution of their amplitude with temperature. The data are consistent with the Shubnikov-de Haas theory. The Hall resistivity ρ(xy) is nonlinear at low magnetic fields. ρ(xy) is fitted assuming multiple carrier contributions. We infer the density of the mobile charge carriers from the oscillations frequency and from Hall measurements. The comparison between these densities suggests multiple valley and spin degeneracy. The small amplitude of the oscillation is discussed in the framework of the multiple band scenario.     

Electron–Nuclear Spin Dynamics in Semiconductor QDs

This work presents an overview of investigations of the nuclear spin dynamics in nanostructures with negatively charged InGaAs/GaAs quantum dots characterized by strong quadrupole splitting of nuclear spin sublevels. The main method of the investigations is the experimental measurements and the theoretical analysis of the photoluminescence polarization as a function of the transverse magnetic field (effect Hanle). The dependence of the Hanle curve profile on the temporal protocol of optical excitation is examined. Experimental data are analyzed using an original approach based on separate consideration of behavior of the longitudinal and transverse components of the nuclear polarization. The rise and decay times of each component of the nuclear polarization and their dependence on transverse magnetic field strength are determined. To study the role of the Knight field in the dynamic of nuclear polarization, a weak additional magnetic field parallel to the optical axis is used. We have found that, only taking into account the nuclear spin fluctuations, we can accurately describe the measured Hanle curves and evaluate the parameters of the electron–nuclear spin system in the studied quantum dots. A new effect of the resonant optical pumping of nuclear spin polarization in an ensemble of the singly charged (In,Ga)As/GaAs quantum dots subjected to a transverse magnetic field is discussed. Nuclear spin resonances for all isotopes in the quantum dots are detected in that way. In particular, transitions between the states split off from the ±1/2 doublets by the nuclear quadrupole interaction are identified.     

Tuning the electrons at the LaAlO_3/SrTiO_3 interface: From growth to beyond growth

Recently, the quasi-two-dimensional electron gas （q2DEG） confined at the interface between LaAlO3 and SrTiO3 has attracted significant attention. In this paper, we briefly review experimental methods that have been used to tune the carrier density and mobility of this q2DEG. These methods can be classified into two categories： growth-related tuning （i.e. substrate, growth temperature, oxygen pressure, post-annealing, LaAlO3 thickness, stoichiometry, and capping layers） and post-growth tuning （i.e. electrostatic field gating, conductive atomic force microscopy and surface adsorbates）. Taken together, these methods enable the broad tuning of the electronic properties of this interface.     

Proposal of a one-dimensional electron gas in the steps at the LaAlO3-SrTiO3 interface

The two-dimensional electron gas at the interface between LaAlO(3) and SrTiO(3) has become one of the most fascinating and highly debated oxide systems of recent times. Here we propose that a one-dimensional electron gas can be engineered at the step edges of the LaAlO(3)/SrTiO(3) interface. These predictions are supported by first-principles calculations and electrostatic modeling which elucidate the origin of the one-dimensional electron gas as an electronic reconstruction to compensate a net surface charge in the step edge. The results suggest a novel route to increasing the functional density in these electronic interfaces.     

Highly spin-polarized conducting state at the interface between nonmagnetic band insulators: LaAlO3/FeS2 (001)

First-principles density functional calculations demonstrate that a spin-polarized two-dimensional conducting state can be realized at the interface between two nonmagnetic band insulators. The (001) surface of the diamagnetic insulator FeS(2) (pyrite) supports a localized surface state deriving from Fe d orbitals near the conduction band minimum. The deposition of a few unit cells of the polar perovskite oxide LaAlO(3) leads to electron transfer into these surface bands, thereby creating a conducting interface. The occupation of these narrow bands leads to an exchange splitting between the spin subbands, yielding a highly spin-polarized conducting state distinct from the rest of the nonmagnetic, insulating bulk. Such an interface presents intriguing possibilities for spintronics applications.     

Electrostriction at the LaAlO3/SrTiO3 interface

We present a direct comparison between experimental data and ab initio calculations for the electrostrictive effect in the polar LaAlO(3) layer grown on SrTiO(3) substrates. From the structural data, a complete screening of the LaAlO(3) dipole field is observed for film thicknesses between 6 and 20 uc. For thinner films, an expansion of the c axis of 2% matching the theoretical predictions for an electrostrictive effect is observed experimentally.     

Formation of Interfacial Layers in LaAlO3/Silicon during Film Deposition

We have studied the interracial reactions between amorphous LaAlO3 thin films and Si substrates, using high- resolution transmission electron microscopy and x-ray photoelectron spectroscopy. It has been shown that the interracial layer between LaAlO3 film and Si substrate chemical states show that the ratio of La 4d3/2 to Al 2p is SiLaxAlyOz. The depth distributions of La, Si and Al of the interfacial layer remains unchanged with the depth compared to that of the LaAlO3 film. Moreover, the Si content, in the interracial layer gradually decreases with increasing thickness of the interracial layer. These results strongly suggest that the Al element is not deficient in the interracial layer, as previously believed, and the formation of a SiLaxAlyOz interracial layer is mainly due to the diffusion of Si from the substrate during the LaAlO3 film deposition. With the understanding of the interracial layer formation, ones can control the interface characteristics to ensure the desired performances of devices using high-k oxides as gate dielectrics.     

Manifestation of the Hanle effect in submillimeter EPR spectroscopy of thulium impurity ions in synthetic forsterite

A signal related to the spin level crossing in a zero magnetic field—the Hanle effect—has been registered for the first time in the EPR spectrum. It has been shown that, in the general case, the shape of the signal is determined by two qualitatively different mechanisms: (i) the interference of unsteady-state contributions to the dynamics of atomic coherences (electric or magnetic quantum transition moments with certain phases) with close frequencies (“beats at a zero frequency”) and (ii) the summation of resonant signals determined by the steady-state dynamics of the same atomic coherences. The relaxation time of spin coherences has been determined for the EPR transition of Tm³⁺ ions in synthetic forsterite.     

Determination of the sign of the conduction-electron g factor in semiconductor quantum wells by means of the Hanle effect and spin-quantum-beat techniques

A method of measuring the sign of the conduction-electron g factor in semiconductor quantum-wells is proposed, based on determination of the sense of electron-spin Larmor precession by the Hanle effect or spin-quantum-beat techniques under oblique incidence of pump light on the sample, with the luminescence detected at an angle to the pump beam. This method has been used to measure the sign of the transverse electronic g-factor component in GaAs/Al_0.3Ga_0.7As quantum-wells of various widths. It has been shown experimentally that the average spin of electrons photocreated in quantum-wells may not coincide with the pump light direction. Expressions for the oscillations of the luminescence circular polarization in the spin-quantum-beat regime and in the Hanle effect have been obtained, taking into account the electron spin relaxation anisotropy. Fiz. Tverd. Tela (St. Petersburg) 39, 768–773 (1997)     

The Hanle effect for the 3 S1/2 ↔ 3 P1/2 transition of sodium

Experimental results are reported for the Hanle effect on the 3 ²S_1/2 ↔ 3 ²P_1/2 transition of sodium at vapour densities between 10⁹ and 10¹² at/cm³.     

Hanle effect driven by weak localization

The influence of weak localization on the Hanle effect in a two-dimensional system with a spin-split spectrum is considered. We show that weak localization drastically changes the dependence of a stationary spin polarization S on an external magnetic field B. In particular, the nonanalytic dependence of S on B is predicted for III-V-based quantum wells grown in the [110] direction and for the [100]-grown quantum wells having equal strengths of Dresselhaus and Bychkov-Rashba spin-orbit coupling. It is shown that in a weakly localized regime the components of S are discontinuous at B = 0. At low B, the magnetic field-induced rotation of the stationary polarization is determined by quantum interference effects. This implies that the Hanle effect in such systems is totally driven by weak localization.     

Solar blind photoconductivity in three-terminal devices of LaAlO3/SrTiO3 heterostructures

We report observations of photoconductivity in ultrathin films of LaAlO(3) grown on (001) SrTiO(3) at several deposition temperatures. The films show pronounced metallic behavior in the dark. The conductance of this metallic state can be enhanced by ultraviolet light (350-400 nm) of a few μW/cm(2) intensity. The decay of the photoconducting state follows stretched exponential dynamics, which can be accelerated or slowed down on the application of gate voltage, thus imparting a novel functionality to the system.