厚度与应变效应对La0.67Ca0.33MnO3薄膜电输运与居里温度的影响

在厚度为25—400nm范围内，系统地研究了 (001)SrTiO_3(STO), (001)LaAlO_3(LAO)衬底上La_0.67Ca_0.33Mn_O.3 (LCMO)薄膜的电输运与居里温度T_C随薄膜厚度及衬底的变化. 结果表明，随薄膜变薄，电阻率ρ增加，T_C降低. 对于同一薄膜厚度，LCMO/STO薄膜的ρ大于LCMO/LAO基上的薄膜的ρ. T_C衬底的依赖关系则与ρ相反. 分析表明，LCMO薄膜的低温区电阻温度(ρ-T) 符合关系式ρ=ρ_0+Bω_s/sin h^2(ω_s/2/k_BT)+CT^n， 其中ρ_0为剩余电阻；等号右端第二项反映软光学模声子对电子散射的贡献；第三项包括其余可能散射机 理在电输运过程中所起的作用；B，ωs（软光学模声子的平均频率）与C都为拟合系数. 高温区的电输运则由小极化子跃迁模型ρ=DT×exp（E_a/k_BT）描述（E_a为极化子激 发能）. 根据ρ_0，ωs，E_a以及T_Ｃ变化，初步讨论了薄膜中的厚度与应变效应. 进一步 研究发现ωs，E_a的变化与T_C相关，从而说明极化子效应为影响T_C变化的主要因素. 关键词： 锰氧化物薄膜 电输运 居里温度 极化子     

Conductivity and band alignment of LaCrO_3/SrTiO_3(111) heterostructure

In this work,we investigate the electrical transport property and electronic structure of oxide heterostructure LaCrO_3/SrTiO_3(111).The interface grown under relatively low oxygen partial pressure is found to be metallic with a conducting critical thickness of 11 unit cells of LaCrO_3.This criticality is also observed by x-ray photoelectron spectroscopy,in which the Ti~(3+) signal intensity at the spectrum edge of the Ti-2p_(3/2) core level increases rapidly when the critical thickness is reached.The variations of the valence band offset and full width at half maximum of the core-level spectrum with LaCrO_3 thickness suggest that the built-in fields exist both in LaCrO_3 and in SrTiO_3.Two possible origins are proposed:the charge transfer from LaCrO_3 and the formation of a quantum well in SrTiO_3.Our results shed light on the understanding of the doping mechanism at the polar/non-polar oxide interface.Moreover,due to the interesting lattice and spin structure of LCO in the(111) direction,our work provides a basis for further exploring the novel topological quantum phenomena in this system.     

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.     

Abundant photoelectronic behaviors of La_(0.67)Sr_(0.33)MnO_3/Nb:SrTiO_3 junctions

Temperature dependence on rectifying and photoelectronic properties of La_(0.67)Sr_(0.33)MnO_3/Nb:SrTiO_3(LSMO/STON) junctions with the thickness values of LSMO film varying from 1 nm to 54 nm are systematically studied. As shown experimentally, the junctions exhibit good rectifying properties. The significant differences in photoemission property among the LSMO/STON junctions are observed. For the junction in a thicker LSMO film, the photocurrent shows a monotonic growth when temperature decreases from 300 K to 13 K. While for the junction in an ultrathin LSMO film, the behaviors of photocurrent are more complicated. The photocurrent increases rapidly to a maximum and then smoothly decreases with the decrease of temperature. The unusual phenomenon can be elucidated by the diffusion and recombination model of the photocarrier.     

基于LabVIEW实时研究辐照效应对锰氧化物薄膜电阻的影响

基于LabVIEW编程开发出了一套自动电输运测量系统,并利用该自动测量系统实时测量了γ射线辐照对钙钛矿锰氧化物薄膜La0.67Ca0.33MnO3(LCMO)的电阻率的影响,结果发现LCMO薄膜电阻率在短时间辐照时降低,随着辐射时间增加,电阻率转而单调增加.在同等条件下还测量了1 wt%Nb掺杂SrTiO3(NSTO)的电阻的射线辐射效应,对比发现NSTO的电阻未出现明显变化.通过辐射损伤机理及锰氧化物的强关联电子特征对结果做出了解释.     

Stabilization variation of organic conductor surfaces induced by π-π stacking interactions

The structures and stabilization of three crystal surfaces of TCNQ-based charge transfer complexes(CTCs) including PrQ(TCNQ) 2,MPM(TCNQ) 2,and MEM(TCNQ) 2,have been investigated by scanning tunneling microscopy(STM).The three bulk-truncated surfaces are all ac-surface,which are terminated with TCNQ molecular arrays.On the ac-surface of PrQ(TCNQ) 2,the TCNQ molecules form a tetramer structure with a wavelike row behavior and a γ angle of about 18° between adjacent molecules.Moreover,the dimer structures are resolved on both ac-surfaces of MPM(TCNQ) 2 and MEM(TCNQ) 2.In addition,the tetramer structure is the most stable structure,while the dimer structures are unstable and easily subject to the STM tip disturbance,which results in changeable unit cells.The main reasons for the surface stabilization variation among the three ac-surfaces are provided by using the ’π-atom model’.     

La2/3Ca1/3MnO3超薄膜应力弛豫过程及性质研究

锰氧化物庞磁电阻材料超薄膜的应力弛豫特性的研究对理解这一强关联体系的特性及应用都具有重要的意义.为此,我们研究了不同温度退火的La2/3Ca1/3MnO3/LaAlO3超薄膜的形貌、结构、电输运特性等.发现退火导致的应力弛豫对薄膜的性质有很强的调制作用,其电输运性质及结构呈现明显的阶段性变化过程,我们提出了一个高温应力弛豫模型对所观察到的实验现象进行了解释.实验中观察到的规则的自相似网状纳米结构,显示了薄膜应力弛豫过程中自组织生长的分形特征.     

厚度与应变效应对La0.67 Ca0.33 MnO3薄膜电输运与居里温度的影响

在厚度为25-400nm范围内,系统地研究了(001)SrTiO3(STO),(001)LaAlO3(LAO)衬底上La0.67Ca0.33MnO3(LCMO)薄膜的电输运与居里温度TC随薄膜厚度及衬底的变化.结果表明,随薄膜变薄,电阻率ρ增加,TC降低.对于同一薄膜厚度,LCMO/STO薄膜的ρ大于LCMO/LAO基上的薄膜的ρ.TC与衬底的依赖关系则与ρ相反.分析表明,LCMO薄膜的低温区电阻温度(ρ-T)符合关系式ρ=ρ0+Bωs/sinh2(hωs/2/kBT)+CTn,其中ρ0为剩余电阻;等号右端第二项反映软光学模声子对电子散射的贡献;第三项包括其余可能散射机理在电输运过程中所起的作用;B,ωs(软光学模声子的平均频率)与C都为拟合系数.高温区的电输运则由小极化子跃迁模型ρ=DT×exp(Ea/kBT)描述(Ea为极化子激发能).根据ρ0,ωs,Ea以及TC变化,初步讨论了薄膜中的厚度与应变效应.进一步研究发现ωs,Ea的变化与TC相关,从而说明极化子效应为影响TC变化的主要因素.     

薄膜厚度对La0.67Ca0.33MnO3/Nb-SrTiO3异质结整流特性及磁阻的影响

利用脉冲激光沉积的方法,在Nb-1wt%:SrTiO3基片上分别生长了5 nm和50 nm的La0.67Ca0.33MnO3薄膜得到异质结.对该异质结电流-电压特性和磁阻的研究表明,50 nm厚的样品表现出良好的整流特性并且在外加1 T磁场的情况下无明显变化,而5nm厚的样品在低温下,整流特性发生变化,并且外加1 T磁场对其整流特性有明显影响,显示负的磁阻.通过考虑p～n结耗尽层的厚度,对观察到的实验现象进行了解释.     

Gate tunable Rashba spin-orbit coupling at CaZrO3/SrTiO3 heterointerface

High mobility quasi two-dimensional electron gas (2DEG) found at the CaZrO₃/SrTiO₃ nonpolar heterointerface is attractive and provides a platform for the development of functional devices and nanoelectronics. Here we report that the carrier density and mobility at low temperature can be tuned by gate voltage at the CaZrO₃/SrTiO₃ interface. Furthermore, the magnitude of Rashba spin-orbit interaction can be modulated and increases with the gate voltage. Remarkably, the diffusion constant and the spin-orbit relaxation time can be strongly tuned by gate voltage. The diffusion constant increases by a factor of ～ 19.98 and the relaxation time is reduced by a factor of over three orders of magnitude while the gate voltage is swept from -50 V to 100 V. These findings not only lay a foundation for further understanding the underlying mechanism of Rashba spin-orbit coupling, but also have great significance in developing various oxide functional devices.