光电协同增强的场效应对LaAlO_3/SrTiO_3界面中持续光电导的调控

LaAlO_3/SrTiO_3异质结界面体系具有新奇的二维自由电子气现象、暂态光电导效应、持续光电导效应等丰富的光电性质,是近年来科学界研究的热点之一.本文研究了场效应对LaAlO_3/SrTiO_3界面光电导效应的调控,发现光电协同增强的场效应可以使得LaAlO_3/SrTiO_3界面产生显著的持续光电导效应,进一步研究发现:在光电协同效应的影响下,随着负的背栅门电压的增加,持续光电导的数值增大,在-70 V附近达到极值;随着负的背栅门电压处理时间的增加,持续光电导的数值单调增加.LaAlO_3/SrTiO_3异质结中这种场增强的持续光电导效应可为多参数可调的光电子记忆器件的研发提供参考依据.     

Stability and electronic structure studies of LaAlO_3/SrTiO_3 (110) heterostructures

The first-principles calculations are employed to investigate the stability, magnetic, and electrical properties of the oxide heterostructure of LaAlO3/SrTiO3(110). By comparing their interface energies, it is obtained that the buckled interface is more stable than the abrupt interface. This result is consistent with experimental observation. At the interface of LaAlO3/SrTiO3(110) heterostructure, the Ti–O octahedron distortions cause the Ti t2 gorbitals to split into the twofold degenerate dxz/dyz and nondegenerate dxy orbitals. The former has higher energy than the latter. The partly filled two-fold degenerate t2 gorbitals are the origin of two-dimensional electron gas, which is confined at the interface. Lattice mismatch between LaAlO3 and SrTiO3leads to ferroelectric-like lattice distortions at the interface, and this is the origin of spin-splitting of Ti 3d electrons. Hence the magnetism appears at the interface of LaAlO3/SrTiO3(110).     

Sr掺杂对La_(1-x)Sr_xMnO_3/LaAlO_3/SrTiO_3界面电子结构的影响

采用基于密度泛函理论的第一性原理计算方法,系统地研究了La_(1-x)Sr_xMnO_3层中Sr的掺杂方式和掺杂量对4La_(1-x)Sr_xMnO_3/3LaAlO_3/4SrTiO_3(LSMO/LAO/STO)异质结构原子和电子结构的影响.结果表明:对于相同的Sr掺杂量,掺杂方式的差异对体系电子结构的影响微弱,不会导致体系发生金属-绝缘体转变;掺杂量的不同对体系电子结构有着显著的影响,当Sr的掺杂量较少时,LAO/STO界面处存在着准二维电子气,当Sr的掺杂量高于1/3时,LAO/STO界面处准二维电子气消失.我们相信,Sr的引入以及通过Sr掺杂量的改变可以对LSMO覆盖层极化进行调控,这也是导致体系LAO/STO界面处金属-绝缘体转变的可能原因,进一步为极化灾变机制导致的界面处电子重构提供了证据.     

应力下SmNiO_3钙钛矿氧化物薄膜材料的电导与红外光电导

稀土镍基钙钛矿氧化物RNiO_3(R为稀土元素)可以在温度触发下发生从电子游离态到局域态的金属绝缘体转变,这一特性在传感器,数据存储,调制开关等方面具有可观的应用价值.本文通过脉冲激光沉积法,在钛酸锶(SrTiO_3)、铝酸镧(LaAlO_3)单晶衬底上准外延生长热力学亚稳态镍酸钐(SmNiO_3)薄膜材料,利用薄膜与衬底间晶格失配引入界面应力,实现对SmNiO_3电子轨道结构与金属绝缘体相变温度的调节.结合电输运性质与红外透射实验的综合表征研究,论证了双向拉伸应变引起的晶格双向拉伸畸变,可以引起SmNiO_3的禁带宽度的展宽,从而稳定绝缘体相并提高金属-绝缘相转变温度.进一步结合近边吸收同步辐射实验表征,揭示了拉伸应变稳定SmNiO_3绝缘体相的本质在于Ni—O成键轨道在双向拉伸形变作用下的弱化,使得镍氧八面体中的价电子偏离镍原子从而稳定SmNiO_3的低镍价态绝缘体相.     

First-principles study of electronic structure and magnetic properties of SrTi1−xMxO3 (M= Cr,Mn, Fe,Co, or Ni)

We used first-principles calculations to conduct a comparative study of the structure and the electronic and magnetic properties of SrTiO₃ doped with a transition metal (TM), namely, Cr, Mn, Fe, Co, or Ni. The calculated formation energies indicate that compared with Sr, Ti can be substituted more easily by the TM ions. The band structures show that SrTi_0.875Cr_0.125O₃ and SrTi_0.875Co_0.125O₃ are half metals, SrTi_0.875Fe_0.125O₃ is a metal, and SrTi_0.875Mn_0.125O₃ is a semiconductor. The 3d TM-doped SrTiO₃ exhibits various magnetic properties, ranging from ferromagnetism (Cr-, Fe-, and Co-doped SrTiO₃) to antiferromagnetism (Mn-doped SrTiO₃) and nonmagnetism (Ni-doped SrTiO₃). The total magnetic moments are 4.0_μB, 6.23_μB, and 2.0_μB for SrTi_0.75Cr_0.25O₃, SrTi_0.75Fe_0.25O₃, and SrTi_0.75Co_0.25O₃, respectively. Room-temperature ferromagnetism can be expected in Cr-, Fe-, and Co-doped SrTiO₃, which agrees with the experimental observations. The electronic structure calculations show that the spin polarizations of the 3d states of the TM atoms are responsible for the ferromagnetism in these compounds. The magnetism of TM-doped SrTiO₃ is explained by the hybridization between the TM-3d states and the O-2p states.     

Anisotropic behaviour in the magnetic field dependence of the low temperature electrical resistance of calcium-doped lanthanum manganate thin films grown by RF magnetron sputtering

We report about the magnetoresistive properties of calcium-doped lanthanum manganate thin films grown by RF magnetron sputtering on single crystalline LaAlO₃ and MgO substrates. Two orientations of the magnetic field with respect to the electrical current have been studied: (i) magnetic field in the plane of the film and parallel to the electrical current, and (ii) magnetic field perpendicular to the plane of the film. The film grown on LaAlO₃ is characterised by an unusual magnetoresistive behaviour when the magnetic field is applied perpendicular to the film plane: the appearance of two bumps in the field dependence of the resistance is shown to be related to the occurrence of anisotropic magnetoresistive effects in manganate films.     

Effect of substrate’s nature on the growth of HgBCCO thin films

The synthesis of HgBCCO films by spray pyrolysis on single crystals is investigated. It is shown that in conditions where the precursor is expected to give Hg-1223, Hg-1212 grows on LaAlO₃ (0 0 1) while Hg-1223 grows on MgO (0 0 1). The samples are analysed by X-ray pole figures, SEM and critical current measurements. Both samples were superconducting but the sample on MgO had a negligible I_c while 17 A was measured on LAO at 77 K (sf). These differences are analysed in terms of texture and microstructure and some conclusions concerning the growth are drawn. On LAO, the nucleation is probably epitaxial and the anisotropy of the growth inhibits the transformation into Hg-1223 while some interfacial contribution expected to occur on MgO makes this transformation easier.     

单层FeSe薄膜/氧化物界面高温超导

单层FeSe/SrTiO₃界面增强超导的发现为理解高温超导机理提供了一个新的途径,也为实现新的高温超导体开拓了新思路.本文通过在SrTiO₃（001）表面高温沉积Mg进而沉积单层FeSe薄膜,制备出了FeSe/MgO双层/SrTiO₃异质结.利用扫描隧道显微镜研究了异质结的电学及超导特性,观测到约14–15 meV的超导能隙,比体相FeSe超导能隙值增大了5–6倍,与K掺杂双层FeSe/SrTiO₃的超导能隙值相当.这一结果可理解为能带弯曲造成的界面电荷转移和界面处电声耦合共同作用导致的超导增强.FeSe/MgO界面是继FeSe/TiO₂之后的一个新界面超导体系,为研究界面高温超导机理提供了新载体.     

X-ray diffraction studies of epitaxy in orthorhombic- DyMnO3/Er1Ba2Cu3O7-δ thin film heterostructures on LaAlO3 (100) substrates

The heteroepitaxy in DyMnO₃/Er₁Ba₂Cu₃O_7-δ bilayer thin films on LaAlO₃ (100) substates was characterized by four-circle X-ray diffractometry. The Er₁Ba₂Cu₃O_7-δ thin films on LaAlO₃ (100) substrates were prepared by molecular-beam deposition (MBD) and post-growth annealing in wet and dry O₂ at 880°C, whereas the DyMnO₃ thin films on the Er₁Ba₂Cu₃O_7-δ/LaAlO₃ (100) heterostructure were deposited by MBD and post-growth annealing in dry O₂ at 750°C. The conventional X-ray diffraction (XRD) patterns as well as pole figures (φ-scans) for specific (hkl) reflections were acquired. The Er₁Ba₂Cu₃O_7-δ thin film in the DyMnO₃/Er₁Ba₂Cu₃O_7-δ/LaAlO₃ (100) heterostructure showed [001] oriented epitaxial growth, as expected. The DyMnO₃ thin film on the Er₁Ba₂Cu₃O_7-δ epilayer in the heterostructure grew with (110) epitaxy in its metastable orthorhombic phase (lattice constants: a_o=5.272 Å, b_o=5.795 Å and c_o=7.38 Å). The heteroepitaxial relationships at the orthorhombic-DyMnO O₃ (110) /Er₁Ba₂Cu₃O_7-δ (001) interface was determined as the following: DyMnO₃ (110) Er₁Ba₂Cu₃O_7-δ (001), DyMnO₃ [1 0] ¶r; Er₁Ba₂Cu₃O_7-δ[100] or Er₁Ba₂Cu₃O_7-δ[010], and DyMnO₃ [001] ¶r; Er₁Ba₂Cu₃O _7-δ[010] or Er₁Ba₂Cu₃O_7-δ [100].     

Thermoelectric enhancement in triple-doped strontium titanate with multi-scale microstructure

Strontium titanate(SrTiO_3) is a thermoelectric material with large Seebeck coefficient that has potential applications in high-temperature power generators.To simultaneously achieve a low thermal conductivity and high electrical conductivity,polycrystalline SrTiO_3 with a multi-scale architecture was designed by the co-doping with lanthanum,cerium,and niobium.High-quality nano-powders were synthesized via a hydrothermal method.Nano-inclusions and a nano/microsized second phase precipitated during sintering to form mosaic crystal-like and epitaxial-like structures,which decreased the thermal conductivity.Substituting trivalent Ce and/or La with divalent Sr and substituting pentavalent Nb with tetravalent Ti enhanced the electrical conductivity without decreasing the Seebeck coefficient.By optimizing the dopant type and ratio,a low thermal conductivity of 2.77 W·m~(-1)·K~(-1) and high PF of 1.1 mW·m~(-1)·K~(-2) at 1000 K were obtained in the sample co-doped with 5-mol% La,5-mol% Ce,and 5-mol% Nb,which induced a large ZT of 0.38 at 1000 K.     

Orientation-dependent ferroelectricity of strained PbTiO3 films

PbTiO₃ is a simple but very important ferroelectric oxide that has been extensively studied and widely used in various technological applications. However, most previous studies and applications were based on the bulk material or the conventional [001]-orientated films. There are few studies on PbTiO₃ films grown along other crystalline axes. In this study, a first-principles calculation was performed to compute the polarization of PbTiO₃ films strained by SrTiO₃ and LaAlO₃ substrates. Our results show that the polarization of PbTiO₃ films strongly depends on the growth orientation as well as the monoclinic angles. Further, it is suggested that the ferroelectricity of PbTiO₃ mainly depends on the tetragonality of the lattice, instead of the simple strain.     

过量B的Ta/CoFeB/MgO薄膜垂直各向异性和温度稳定性的增强

以CoFeB/MgO为核心单元的垂直各向异性薄膜体系和相关的垂直磁隧道结已获得广泛研究,其中CoFeB的B含量基本都保持为原子比20%.本文采用磁控溅射制备了Ta/(Co0.5Fe0.5)1-xBx/MgO三明治结构及生长顺序相反的系列薄膜,并在573—623K进行真空退火,研究了样品垂直各向异性随B成分的变化.结果显示,当B含量减小到10%时,Ta/CoFeB/MgO体系的垂直各向异性明显降低;相反,当B含量增加至30%时,该体系的垂直各向异性明显增强;发现在高B含量的情形下,样品的垂直各向异性大小与温度稳定性均与三明治结构的生长顺序密切相关;获得了具有优异温度稳定性的垂直磁化MgO/CoFeB/Ta样品.结果表明适当增加B含量是增强CoFeB/MgO体系垂直各向异性和温度稳定性的有效途径之一.     

Energy Spectra,g Factors and Their Pressure—Induced and ／or Thermal Shifts of SrTiO3：Cr^3＋ and SrTiO3：Mn^4＋ I：energy Spectra and g Factors at Normal Pressure

With the strong-field scheme and cubic bases,the complete d^3 energy matrix in a tetragonally distorted cubic-field has been constructed.By diagonalizing this matrix,the energy spectra of SrTiO3:Cr^3 and SrTiO3:Mn^4 at normal pressure and various temperatures have been calculated.Correspondingly,the FORTRAN program calculating the g factor of the ground state has been worked out.By using the program and the wavefunction obtained from diagonalizing the complete energy matrix,the g factors of the ground state of SrTiO3:Cr^3 and SrTiO3:Mn^4 at normal pressure and room temperature have been evaluated.The calculated results are in good agreement with the optical-spectral and EPR experimental data.The comparison and analysis of the results of two crystals have been made.It is demonstrated that the covalency of the bonding between Mn^4 and ligands(O^2-) in SrTiO3:Mn^4 is stronger than the one of the bonding between Cr^3 and ligands(O^2-)in SrTiO3:Cr^3 .It is shown that the obtained wavefunctions and values of parameters are reasonable.     

SUPERCONDUCTING PROPERTIES AND STRUCTURAL PERFECTION OF EPITAXIAL YBa2Cu3O7-x THIN FILMS

High quality epitaxial YBa₂Cu₃O_7-x thin films have been succcessfully prepared by dc magnetron sputtering deposition, on (100) and (110) aligned SrTiO₃, LaAlO₃ and yttria-stabilized zirconia (YSZ) substrates. The films showed zero resistance around 90 K and had a J_c (at 77 K, H=0) over 10⁶A/cm². It was found that superconducting properties and structures of the films were strongly dependent on oxygen pressure and substrate temperature. The epitaxial structure of the films have been studied by X-ray diffraction. Rutherford backscattering and channeling spectroscopy, X-ray double-crystal diffraction and transmission election microscopy. The experimental results demonstrated that the epitaxial YBa₂Cu₃O_7-x films had excellent superconducting properties and quite perfect structure.     

Metal/insulator manganite multilayers

We have studied the transport and magnetic properties of strongly textured metal/insulator La_1−xSr_xMnO₃ (x=0.4,0.1) bilayers and trilayers, grown by DC magnetron sputtering over MgO and SrTiO₃ substrates. The multilayers present transport properties similar to those of the La_0.6Sr_0.4MnO₃ films, being very sensitive to deposition conditions. Magnetic multilayers show a metal–insulator transition around T_c (250 K) and colossal magnetoresistance which is maximum around T_c. No extrinsic magnetoresistance associated with the multilayered structure was observed, probably due to the presence of ferromagnetic coupling between the metallic layers, as suggested by magnetization measurements.     

Band-gap engineering of La_(1-x)Nd_xAlO_3(x = 0,0.25,0.50,0.75,1)perovskite using density functional theory:A modified Becke Johnson potential study

The structural,electronic,and magnetic properties of the Nd-doped Rare earth aluminate,La_(1-x)Nd_xAlO_3(x = 0%to 100%) alloys are studied using the full potential linearized augmented plane wave(FP-LAPW) method within the density functional theory.The effects of the Nd substitution in La AlO_3 are studied using the supercell calculations.The computed electronic structure with the modified Becke–Johnson(m BJ) potential based approximation indicates that the La_(1-x)Nd_xAlO_3 alloys may possess half-metallic(HM) behaviors when doped with Nd of a finite density of states at the Fermi level(E_F).The direct and indirect band gaps are studied each as a function of x which is the concentration of Nddoped La AlO_3.The calculated magnetic moments in the La_(1-x)Nd_xAlO_3 alloys are found to arise mainly from the Nd-4f state.A probable half-metallic nature is suggested for each of these systems with supportive integral magnetic moments and highly spin-polarized electronic structures in these doped systems at E_F.The observed decrease of the band gap with the increase in the concentration of Nd doping in La AlO_3 is a suitable technique for harnessing useful spintronic and magnetic devices.     

Strain-dependent magnetic configurations in manganite-titanate heterostructures probed with soft X-ray techniques

We present a detailed study on the strain-induced magnetic domain structure of a (La,Sr)MnO₃ thin film epitaxially grown on a BaTiO₃ substrate through the use of polarization-dependent X-ray photoemission electron microscopy and X-ray absorption spectroscopy. Angular-dependent measurements allow us to detect vector magnetization on a single-domain scale, and we relate the strain-induced changes in magnetic anisotropy of the ferromagnetic film to the ferroelectric domain structure of the underlying substrate using X-ray magnetic circular and linear dichroism spectro-microscopy. Comparisons to measurements on a nearly strain free film of (La,Sr)MnO₃ grown on a (La,Sr)(Al,Ta)O₃ substrate illustrate that the BaTiO₃ ferroelectric domain structure imprints specific domain sizes and wall orientations in the (La,Sr)MnO₃/BaTiO₃ artificial multiferroic heterostructure. Furthermore, a change of the BaTiO₃ ferroelectric domain structure either with temperature or with applied electric field results in a corresponding change in the (La,Sr)MnO₃ ferromagnetic domain structure, thus showing a possible route to obtain room-temperature electric field control of magnetic anisotropy at the nanoscale.     

无铅四方相钙钛矿短周期超晶格压电效应机理研究

超晶格压电行为与内部正离子之间的内在联系尚缺乏相关的研究.本文基于密度泛函理论的第一性原理方法,研究了三种无铅四方相钙钛矿铁电超晶格(BaTiO_3/SrTiO_3,KNbO_3/KTaO_3和BaTiO_3/KNbO_3)中A,B位正离子对整体的极化和压电贡献.通过计算超晶格不同轴向应变条件下原子结构和Born有效电荷,获得了超晶格和各个正离子的极化值和压电系数.结果表明,在轴向压缩应变条件下(-0.15—0 A),无铅超晶格中的正离子位移D(A)和D(B)受到抑制,在拉应变时位移才显著增大,因此极化和压电行为不明显.在轴向拉伸应变作用下(0—0.15 A),无铅超晶格中各原子的极化贡献显著增大,特别是B位原子Ti,Nb和Ta的极化贡献使得总的极化强度也显著提高,并当拉应变达到一定值,超晶格才会出现明显的压电行为.无铅超晶格的极化和压电行为主要由B位正离子贡献.     

Luminescence of rare-earth ions in perovskite-type oxides: from basic research to applications

Luminescence of a rare-earth ion in SrTiO₃ or other perovskite-type oxides was used as a probe of physical properties of a host crystal, but not as a phosphor because observed luminescence efficiency was low. Recent investigations have shown that the luminescence efficiency of SrTiO₃:Pr³⁺ is intensified up to 200 times by addition of Al ions. This paper reviews experimental results on this phenomenon and discusses mechanism of the Al-addition effect. A survey was made to find other materials, the efficiency of which can be improved by the impurity addition. Host-to-activator energy transfer by a coupling of the band-edge recombination energy and an allowed transition of a rare-earth ion is essential to provide a high efficiency. A red phosphor, Al-added SrTiO₃:Pr³⁺, is promising for display applications.     

掺铌SrTiO_3中的逆自旋霍尔效应

采用磁控溅射法在未掺杂和掺杂的SrTiO_3基片上沉积了NiFe薄膜,通过翻转测试法分离出掺杂样品中的自旋整流电压和逆自旋霍尔电压.研究结果表明:在未掺杂的SrTiO_3基片中,翻转前后测试的电压曲线基本一致,为NiFe薄膜自旋整流效应产生的电压.对于掺Nb浓度x为0.028, 0.05, 0.1, 0.15, 0.2的SrTiO_3基片,分离出的逆自旋霍尔电压随掺杂浓度增加而减小,在掺杂浓度为0.15和0.2的样品中没有探测到明显的逆自旋霍尔电压.本文的结果表明,在SrTiO_3中掺入强自旋轨道耦合的杂质,通过掺杂浓度可以实现对SrTiO_3中逆自旋霍尔效应的调控,这类可调控的自旋相关研究为自旋电子器件的研究和开发提供了更多的可能性,具有很大的潜在应用价值.