表面和界面处的强Rashba效应在基础科学探索和实际应用中引起了极大关注。本文的第一性原理研究表明，通过对KTaO$_3$（KTO）超薄膜施加双轴应力，可以实现巨大且可调的Rashba效应。当面内压缩应变接近-5%时，Rashba自旋劈裂能达到$E_{R}=140$ meV，约对应Rashba耦合常数$α_{R}=1.3$ eV Å。我们研究了其随应变变化的晶体结构、能带及相关性质，从而阐明了巨Rashba效应的机理。此外，我们展示了在SrTiO$_3$覆盖层和/或Si衬底存在的情况下，巨Rashba自旋劈裂可以保持，并且可以实现强环光电流效应以在KTO薄膜或相关异质结构中产生自旋极化电流，这对于未来的自旋电子学和光电子学应用具有前景。
英文摘要：
Strong Rashba effects at surfaces and interfaces have attracted great attention for basic scientific exploration and practical applications. Here, the first-principles investigation shows that giant and tunable Rashba effects can be achieved in KTaO$_3$ (KTO) ultrathin films by applying biaxial stress. When increasing the in-plane compressive strain nearly to -5\%, the Rashba spin splitting energy reaches $E_{R}=140$ meV, approximately corresponding to the Rashba coupling constant $α_{R}=1.3$ eV Å. We investigate its strain-dependent crystal structures, energy bands, and related properties, and thereby elucidate the mechanism for the giant Rashba effects. Furthermore, we show that giant Rashba spin splitting can be kept in the presence of SrTiO$_3$ capping layer and/or Si substrate, and strong circular photogalvanic effect can be achieved to generate spin-polarized currents in the KTO thin films or related heterostructures, which are promising for future spintronic and optoelectronic applications.
In this work we are investigating the effect of strain in ultrathin Mn-Ga thin films on the magnetic properties at room temperature. Two different Mn-Ga compositions of which one is furthermore doped with Co were grown on Cr buffered MgO (001) substrates. Films with a thickness below 12nm are highly strained and it was observed that the ratio c/a vs. thickness is depending on composition. Using c/a as an order parameter the PMA is shown to be drastically reduced with increasing strain. These findings should be considered when generalizing and downscaling results obtained from films >20nm. Furt…
We report results of large-scale molecular-dynamics (MD) simulations of dynamic deformation under biaxial tensile strain of pre-strained single-crystalline nanometer-scale-thick face-centered cubic (fcc) copper films. Our results show that stacking faults, which are abundantly present in fcc metals, may play a significant role in the dissociation, cross-slip, and eventual annihilation of dislocations in small-volume structures of fcc metals. The underlying mechanisms are mediated by interactions within and between extended dislocations that lead to annihilation of Shockley partial dislocations…
This work shows that the combination of ultrathin highly strained GaN quantum wells embedded in an AlN matrix, with controlled isotopic concentrations of Nitrogen enables a dual marker method for Raman spectroscopy. By combining these techniques, we demonstrate the effectiveness in studying strain in the vertical direction. This technique will enable the precise probing of properties of buried active layers in heterostructures, and can be extended in the future to vertical devices such as those used for optical emitters, and for power electronics.
Fundamental understanding of interfacial magnetic properties in ferromagnetic heterostructures is essential to utilize ferromagnetic materials for spintronic device applications. In this paper, we investigate the interfacial magnetic and electronic structures of epitaxial single-crystalline LaAlO$_3$ (LAO)/La$_{0.6}$Sr$_{0.4}$MnO$_3$ (LSMO)/Nb:SrTiO$_3$ (Nb:STO) heterostructures with varying LSMO-layer thickness, in which the magnetic anisotropy strongly changes depending on the LSMO thickness due to the delicate balance between the strains originating from both the Nb:STO and LAO layers, usin…
Observation of mirror-odd and mirror-even spin texture in ultrathin epitaxially strained RuO2 films
University of Minnesota-Twin Cities（明尼苏达大学双城分校）；Kyung Hee University（庆熙大学）；Rice University（莱斯大学）；Paul Scherrer Institute（保罗谢勒研究所）；Gwangju Institute of Science and Technology (GIST)（光州科学技术院）
Time-resolved two color pump-probe polarization spectroscopy was performed at room temperature on SrTiO3 films grown directly on Si with film thickness varying from 2 nm to 7.8 nm. The E soft mode with a characteristic frequency of 0.2 THz is impulsively generated and measured in these coherently strained tetragonal phase SrTiO3 thin films. Another over-damped signal observed indicates the possible relaxational hopping of Ti ion between double potential wells. The dependence of the coherent phonon signal on pump and probe laser polarization helps to identify the phonon modes.
We present a synchrotron grazing incidence x-ray diffraction analysis of the domain structure and polar symmetry of highly strained BiFeO3 thin films grown on LaAlO3 substrate. We revealed the existence of periodic elastic nanodomains in the pure tetragonal-like BFO ultrathin films down to a thickness of 6 nm. A unique shear strain accommodation mechanism is disclosed. We further demonstrated that the periodicity of the nanodomains increases with film thickness but deviates from the classical Kittel's square root law in ultrathin thickness regime (6 - 30 nm). Temperature-dependent experiments …
Epitaxial strain plays an important role in determining physical properties of perovskite ferroelectric oxide thin films. However, it is very challenging to directly measure properties such as polarization in ultrathin strained films using traditional sandwich capacitor devices, because of high leakage current. We employed a planar electrode device with different crystallographical orientations between electrodes along different electric field orientation to directly measure the in-plane polarization-electric field (P-E) hysteresis loops in fully strained thin films. At high misfit strains suc…
We report on the exchange coupling and magnetic properties of a strained ultrathin CoO/PtFe double-layer with perpendicular magnetic anisotropy. The cobalt oxide growth by reactive molecular beam epitaxy on a Pt-terminated PtFe/Pt(001) surface gives rise to an hexagonal surface and a monoclinic distorted CoO 3nm film at room temperature. This distorted ultrathin CoO layer couples with the PtFe(001) layer establishing a robust perpendicular exchange bias shift. Soft x-ray absorption spectroscopy provides a full description of the spin orientations in the CoO/PtFe doublelayer. The exchange bias …
We have investigated the electrical transport properties of 8 nm thick La0.67Ca0.33MnO3 films, sputter-deposited on SrTiO3 (STO), and etched into 5 micrometer-wide bridges by Ar-ion etching. We find that even slight overetching of the film leads to conductance of the STO substrate, and asymmetric and non-linear current-voltage (I-V) characteristics. However, a brief oxygen plasma etch allows full recovery of the insulating character of the substrate. The I-V characteristics of the bridges are then fully linear over a large range of current densities. We find colossal magnetoresistance properti…
Thermodynamic theory is developed for the ferroelectric phase transition of an ultrathin film in equilibrium with a chemical environment that supplies ionic species to compensate its surface. Equations of state and free energy expressions are developed based on Landau-Ginzburg-Devonshire theory, using electrochemical equilibria to provide ionic compensation boundary conditions. Calculations are presented for a monodomain PbTiO$_3$ (001) film coherently strained to SrTiO$_3$ with its exposed surface and its electronically conducting bottom electrode in equilibrium with a controlled oxygen parti…
NiFe$_2$O$_4$ thin films with varying thickness were grown on SrTiO$_3$(001) by reactive molecular beam epitaxy. Soft and hard x-ray photoelectron spectroscopy measurements reveal a homogeneous cation distribution throughout the whole film with stoichiometric Ni:Fe ratios of 1:2 independent of the film thickness. Low energy electron diffraction and high resolution (grazing incidence) x-ray diffraction in addition to x-ray reflectivity experiments were conducted to obtain information of the film surface and bulk structure, respectively. For ultrathin films up to 7.3 nm, lateral tensile and vert…
Ultrathin bismuth-yttrium iron garnet films with tunable magnetic anisotropy
ETH Zurich（苏黎世联邦理工学院）；Université Grenoble Alpes, CNRS, Institut Néel（格勒诺布尔阿尔卑斯大学，法国国家科学研究中心，内耳研究所）；Kyoto University（京都大学）；University of Konstanz（康斯坦茨大学）；Instituto de Nanociencia y Materiales de Aragón, CSIC（阿拉贡纳米科学与材料研究所，西班牙国家研究委员会）；Universidad de Zaragoza（萨拉戈萨大学）
Lattice structure can dictate electronic and magnetic properties of a material. Especially, reconstruction at a surface or heterointerface can create properties that are fundamentally different from those of the corresponding bulk material. We have investigated the lattice structure on the surface and in the thin films of epitaxial SrRuO3 with the film thickness up to 22 pseudo-cubic unit cells (u.c.), using the combination of surface sensitive low energy electron diffraction and bulk sensitive scanning transmission electron microscopy. Our analysis indicates that, in contrast to many perovski…
Ag(111) films were deposited at room temperature onto H-passivated Si(111)-(1x1) substrates, and subsequently annealed at 300 C. An abrupt non-reactive Ag/Si interface is formed, and very uniform non-strained Ag(111) films of 6-12 monolayers have been grown. Angle resolved photoemission spectroscopy has been used to study the valence band electronic properties of these films. Well-defined Ag sp quantum-well states (QWS) have been observed at discrete energies between 0.5-2eV below the Fermi level, and their dispersions have been measured along the GammaK, GammaMM'and GammaL symmetry directions…
We have grown untwinned YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) films on (110) MgO substrates that were pre-annealed at high temperature in oxygen atmosphere. The annealing results in surface reconstruction with shallow facets, which induce the suppression of the YBCO twinning domains, and the preferential alignment of the CuO chains along one of the in-plane directions of the substrate. Because of the large mismatch between the in-plane lattice parameters of film and substrate, the strain induced by the MgO into the YBCO layer is strong and very peculiar. The YBCO film is compressed, with respect to th…
