Ultrafast band-gap shift induced by a strain pulse in semiconductor heterostructures

The conventional piezospectroscopic effect is extended to picosecond time scales by using ultrashort strain pulses injected into semiconductor heterostructures. The strain pulses with durations of approximately 10 ps are generated in a metal transducer film by intense femtosecond laser pulses. They propagate coherently in the GaAs/(Al,Ga)As heterostructure over a distance of 100 microm and shift the band gaps by several meV as detected optically for quantum well exciton resonances by pump-probe techniques and time-resolved photoluminescence.     

钙钛矿氧化物异质结的光电特性研究进展

光与物质相互作用可以产生各种光学现象,其中光电效应是非常重要的现象之一.文中集中回顾了文章作者在钙钛矿氧化物异质结的光电效应研究中的进展.在钙钛矿氧化物异质结中,分别观测到了传统的纵向光电效应和反常的横向光电效应,并通过对含时的漂移-扩散方程的自洽求解,从理论上分别揭示了钙钛矿氧化物异质结纵向和横向光电效应的动态过程.文章首先介绍了钙钛矿氧化物异质结纵向光电效应的研究进展,接着概述了钙钛矿氧化物异质结横向光电效应研究的进展.最后对氧化物异质结的纵向和横向光电效应的潜在应用前景进行展望.     

钙钛矿氧化物异质结的光电特性研究进展

光与物质相互作用可以产生各种光学现象,其中光电效应是非常重要的现象之一.文中集中回顾了文章作者在钙钛矿氧化物异质结的光电效应研究中的进展.在钙钛矿氧化物异质结中,分别观测到了传统的纵向光电效应和反常的横向光电效应,并通过对含时的漂移-扩散方程的自洽求解,从理论上分别揭示了钙钛矿氧化物异质结纵向和横向光电效应的动态过程.文章首先介绍了钙钛矿氧化物异质结纵向光电效应的研究进展,接着概述了钙钛矿氧化物异质结横向光电效应研究的进展.最后对氧化物异质结的纵向和横向光电效应的潜在应用前景进行展望.     

Ultrafast charge transfer in dual graphene-WS_2 van der Waals quadrilayer heterostructures

Using dual graphene–WS_2 quadrilayer heterostructures as an example, we find that the ultrafast transfer of electrons from WS_2 to graphene takes place within 114 fs, and the Coulomb field of the charge can effectively affect the interlayer electron transfer. This effect illustrates that the charge transfer in such van der Waals heterostructures may be controlled by an externally applied electric field for promising applications in photoelectric devices.     

Ultrafast carrier dynamics in strained In1−xGaxAs/InP heterostructures

The transfer of electrons and holes from barriers to wells is investigated in strained In_1-xGa_xAs/InP multiple quantum wells by time-resolved luminescence upconversion with 300 fs time resolution. The transfer times are in the range of a few ps and independent of the Ga content. The investigation of Ga-rich structures allows to observe directly the hole transfer.     

Magnetic properties of epitaxial oxide heterostructures

Diluted Magnetic Semiconductors (DMS) are of great interest as injection sources for spin-polarized currents into semiconductors. Epitaxial devices have been synthesized with an intermediate spacer layer of the same semiconductor (zinc oxide, ZnO) used to produce the DMS material (ZnCoO) ensuring a homoepitaxial junction to help reduce the interface states and conduction mismatch. We observe a large magnetoresistance of about 32% in the devices at low temperatures. The present work suggests that spin polarized transport could be achieved with DMS materials acting as the source of injected spins into a non-magnetic host.     

In-situ spectroscopic studies and interfacial engineering on FeSe/oxide heterostructures: Insights on the interfacial superconductivity

The discovery of high temperature superconductivity in single-layer FeSe/SrTiO3 provides a new platform for exploring superconductivity and pursuing higher Tc(superconducting transition temperature) through fabricating artificial heterostructures. In this paper, we review the recent progress in studying and tuning the interfacial superconductivity in single-layer FeSe, through the combined in-situ spectroscopic studies and atomic-scale engineering. By fabricating artificial heterostructures, various interfacial factors were tuned, and the corresponding evolutions of electronic structure and superconducting gap behavior were investigated. These studies enrich the current understanding on the interfacial superconductivity, and provide clues for further enhancing Tc through interface engineering.     

Ultrafast carrier dynamics in In1−x Ga x As/InP heterostructures

We investigate carrier capture and carrier transport in the InGaAs/InP material system by luminescence spectroscopy with femtosecond time resolution. Comparative studies are performed on samples of different well width, barrier width and gallium mole fraction of the InGaAs layers. The investigations focus on excitation conditions that are comparable to those for semiconductor laser operation. Firm data on carrier dynamics are presented for these conditions. We find that the overall transfer rates of electrons and holes are similar and independent of well width. Furthermore, the transfer times show a linear dependence on barrier width. From experimental and model calculation results we derive some guidelines for the design of high-frequency laser devices.     

Simulating resistive switching in heterostructures based on oxide compounds

Simulations are used to study the influence of nonuniform electric field distribution at a heterojunction interface on the effect of bipolar resistive switching in oxide compounds. Computer models show that a conducting channel forms at the edge of a heterojunction area characterized by strong local increase in the electric field intensity. The computations are confirmed by the low temperature properties of metastable phases in heterojunctions based on high temperature superconductors.     

Analysis of colossal magnetoresistance effect in perovskite oxide heterostructures

A systemic study of magnetoresistance (MR) in manganite perovskite oxide p-n junction is performed with experiment and theoretical calculation. The spin-dependent tunneling current is calculated with a model of double-band barrier and MR with reverse bias is explained as a result of competition between tunneling currents with different spins. The reduction of recombination rate at the interface of heterojunction with magnetic field is proposed to explain positive MR at forward bias. Furthermore, negative MR is predicted to be observed in oxide heterostructure without electron filling in t2g↓ band of manganite at the interface region with both forward and reverse bias.     

异质结构的应变和应力分布模型研究

基于组合杆的平衡条件，分别建立了晶格失配、热失配以及由两者共同导致的异质结构应变 和应力分布模型，并获得了异质结构的晶格失配应变、热失配应变、弯曲应变以及曲率半径 的分析解. 同时，运用所建的模型，计算了HgCdTe/CdZnTe异质结构的应变和应力分布.结果 表明：应力最大值均在界面处，而中性面仅是材料厚度和弹性参数的函数，与晶格失配、晶 格弛豫、热失配等参数无关，且该异质结构的曲率半径是衬底厚度的函数，随衬底厚度的减 小而减小，而要保证HgCdTe/CdZnTe器件在液氮温度下不发生断裂，衬底的厚度必须大于临界值. 关键词： 异质结构 应变分布模型 应力分布模型 晶格失配     

Josephson effect in hybrid oxide heterostructures with an antiferromagnetic layer

Josephson coupling between an s- and d-wave superconductor through a 50 nm thick Ca1-xSrxCuO2 antiferromagnetic layer was observed for the hybrid Nb/Au/Ca1-xSrxCuO2/YBa2Cu3O7-delta heterostructures and investigated as a function of temperature, magnetic field, and applied millimeter-wave electromagnetic radiation. The magnetic field dependence of the supercurrent I(c)(H) exhibits anomalously rapid oscillations, which is the first experimental evidence of the theoretically predicted giant magneto-oscillations in Josephson junctions with antiferromagnetic interlayers.     

Tunable magnetic interaction at the atomic scale in oxide heterostructures

We report on a systematic study of a number of structurally identical but chemically distinct transition metal oxides in order to determine how the material-specific properties such as the composition and the strain affect the properties at the interface of heterostructures. Our study considers a series of structures containing two layers of ferromagnetic SrRuO?, with antiferromagnetic insulating manganites sandwiched in between. The results demonstrate how to control the strength and relative orientation of interfacial ferromagnetism in correlated electron materials by means of valence state variation and substrate-induced strain, respectively.     

Oscillatory exchange coupling and positive magnetoresistance in epitaxial oxide heterostructures

Oscillation in the exchange coupling between ferromagnetic La(2/3)Ba(1/3)MnO3 layers with paramagnetic LaNiO3 spacer layer thickness has been observed in epitaxial heterostructures of the two oxides. This behavior is explained within the RKKY model employing an ab initio calculated band structure of LaNiO3, taking into account strong electron scattering in the spacer. Antiferromagnetically coupled superlattices exhibit a positive current-in-plane magnetoresistance.     

Effect of strain on hole tunneling dynamics in double barrier heterostructures

A study of the effects of strain due to external stress and lattice mismatch on hole tunneling times in double barrier heterostructures is presented. The band structure of holes is calculated based on a k·p method within the envelope function approximation, including band mixing effects. The phase delay time is obtained from the energy derivative of the total phase shift of the wave function upon tunneling and is used to estimate the hole tunneling time. The results demonstrate that strain can be utilized to tailor hole tunneling times by changing the energy separation and, consequently, the mixing between heavy hole and light hole states in the quantum well.     

Triplet superconducting correlations in oxide heterostructures with a composite ferromagnetic interlayer

The superconducting current induced by the penetration of the long-range triplet component of superconducting correlations into a composite ferromagnetic interlayer has been detected in mesa-heterostructures based on oxide cuprate superconductors YBa₂Cu₃O_{7 ? δ} and Au/Nb bilayer films with the composite oxide interlayer that is made of ferromagnetic films of manganite La_0.7Sr_0.3MnO₃ and ruthenate SrRuO₃ and has a thickness much larger than the length of correlations determined by the exchange field. The deviation of the superconducting current in the mesa-heterostructure with the fraction of the second harmonic of 13% from a sinusoidal current-phase relation has been detected; this deviation can also be due to the generation of the triplet component of superconducting correlations in the ferromagnet.