Electrostatic carrier doping using a field-effect-transistor structure is an intriguing approach to explore electronic phases by critical control of carrier concentration. We demonstrate the reversible control of the insulator-metal transition (IMT) in a two dimensional (2D) electron gas at the interface of insulating SrTiO$_3$ single crystals. Superconductivity was observed in a limited number of devices doped far beyond the IMT, which may imply the presence of 2D metal-superconductor transition. This realization of a two-dimensional metallic state on the most widely-used perovskite oxide is the best manifestation of the potential of oxide electronics.
The two-dimensional electron system found between LaAlO3 and SrTiO3 hosts a variety of physical phenomena that can be tuned through external stimuli. This allows for electronic devices controlling magnetism, spin-orbit coupling, and superconductivity. Controlling the electron density by varying donor concentrations and using electrostatic gating are convenient handles to modify the electronic properties, but the impact on the microscopic scale, particularly of the former, remains underexplored. Here, we image the current distribution at 4.2 K in amorphous-LaAlO3/SrTiO3 using scanning supercond…
The interplay of electronic reconstructions, lattice distortions, and surface oxygen vacancies in insulator-metal transition of LaAlO$_{3}$/SrTiO$_{3}$
We start by analyzing experimental data of Spinelli [A. Spinelli, M. A. Torija, C. Liu, C. Jan, and C. Leighton, Phys. Rev. B 81, 155110 (2010)] for conductivity of $n$-type bulk crystals of SrTiO$_3$ (STO) with broad electron concentration $n$ range of $4\times 10^{15}$ - $4 \times10^{20} $ cm$^{-3}$, at low temperatures. We obtain good fit of the conductivity data, $σ(n)$, by the Drude formula for $n \geq n_c \simeq 3 \times 10^{16} $ cm$^{-3}$ assuming that used for doping insulating STO bulk crystals are strongly compensated and the total concentration of background charged impurities is $…
The highly mobile electrons at the interface of SrTiO3 with other oxide insulators, such as LaAlO3 or AlOx, are of great current interest. A vertical gate voltage allows controlling a metal/superconductor-to-insulator transition, as well as electrical modulation of the spin-orbit Rashba coupling for spin-charge conversion. These findings raise important questions about the origin of the confined electrons as well as the mechanisms that govern the interfacial electric field. Here we use infrared ellipsometry and confocal Raman spectroscopy to show that an anomalous polar moment is induced at th…
The heterostructure consisting of the Mott insulator LaVO$_3$ and the band insulator SrTiO$_3$ is considered a promising candidate for future photovoltaic applications. Not only does the (direct) excitation gap of LaVO$_3$ match well the solar spectrum, but its correlated nature and predicted built-in potential, owing to the non-polar/polar interface when integrated with SrTiO$_3$, also offer remarkable advantages over conventional solar cells. However, experimental data beyond the observation of a thickness-dependent metal-insulator transition is scarce and a profound, microscopic understandi…
The two-dimensional electron gas found at the surface of SrTiO$_3$ and related interfaces has attracted significant attention as a promising basis for oxide electronics. In order to utilize its full potential, the response of this 2DEG to structural changes and surface modification must be understood in detail. Here, we present a study of the detailed electronic structure evolution of the 2DEG as a function of sample temperature and surface step density. By comparing our experimental results with \textit{ab initio} calculations, we found that a SrO-rich surface layer is a prerequisite for elec…
Using spin polarized neutron reflectivity experiments, we demonstrate an unusual proximity behaviour when the superconductor (SC) and the ferromagnet (FM) are coupled through an insulator (I) in YBa2Cu3O7-δ (SC)/SrTiO3 (I)/La0.67Sr0.33MnO3 (FM) heterostructures. We have observed an unexpected magnetic modulation at the interface region of the FM below the superconducting transition temperature. The magnetization of the FM layer at the I/FM interface was drastically reduced as compared to the magnetization in the rest of the FM layer. This result indicates that the Cooper pairs tunnel across th…
Growth mode control of the free carrier density in SrTiO3-d films
研究了脉冲激光沉积SrTiO_3-d同质外延薄膜的生长动力学和电子性质。我们发现决定生长模式的两个主要因素是表面结晶和氧化的动力学。当匹配时，可以获得数百个单位单元的持续二维逐层生长。通过调整这些动力学因素，氧空位可以被冻结在薄膜中，从而允许在金属-绝缘体过渡期间进行受控的系统掺杂。可以生长金属膜，表现出高达25，000 cm 2/Vs的霍尔迁移率。
Rearrangement of atoms due to broken translational symmetries at the surface of SrTiO$_3$ is scarcely debatable in the present day scenario. Actual concern demands to unveil the true structure and precise mechanism responsible for atomic reconstructions at a unit cell level. Here, we reveal the atomic structure of SrTiO$_3$ surface by using annular bright field imaging in spherical aberration corrected scanning transmission electron microscope. We measure structural parameters such as lattice constant, bond length, atomic displacement and subsequently we find a polarization for the top five un…
Origin of interfacial conductivity at complex oxide heterointerfaces: possibility of electron transfer from water chemistry at surface oxygen vacancies
在SrTiO 3衬底和明显不同的氧化物材料的薄覆盖层之间已经形成了各种导电异质界面，这些氧化物材料可以分为极性带绝缘体（例如LaAl O 3）、极性莫特绝缘体（例如LaTiO 3）、明显非极性带绝缘体（例如γ-Al 2 O 3）和非晶氧化物（例如非晶SrTiO 3）。要问的一个基本问题是，是否有一个共同的机制，管理在所有这些异质界面的界面导电性。在这里，我们研究了不同种类的异质界面的导电性，通过在氧气中退火和表面处理与水。结果发现，所有的异质界面的导电性表现出强烈的依赖于退火，并可以普遍调谐的表面处理，其效果是由退火条件。这些观察，连同环境压力的X射线光电子能谱学测量，表明在表面氧空位的水化学是一种常见的机制，提供电子的接口。
The origin of the 2-dimensional electron system (2DES) appearing at the (001) interface of band insulators $\rm SrTiO_3$ and $\rm LaAlO_3$ has been rationalized in the framework of a polar catastrophe scenario. This implies the existence of a critical thickness of polar $\rm LaAlO_3$ overlayer ($4~\rm u.c.$) for the appearance of the 2DES: polar catastrophe for thick $\rm LaAlO_3$ overlayer is avoided either through a Zener breakdown or a stabilization of donor defects at the $\rm LaAlO_3$ surface, both providing electrons to dope the substrate. The observation of a critical thickness is obser…
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…
In the perovskite oxide SrCrO$_{3}$ the interplay between crystal structure, strain and orbital ordering enables a transition from a metallic to an insulating electronic structure under certain conditions. We identified a narrow window of oxygen partial pressure in which highly strained SrCrO$_{3}$ thin films can be grown using radio-frequency (RF) off-axis magnetron sputtering on three different substrates, (LaAlO$_{3}$)$_{0.3}$-(Sr$_{2}$TaAlO$_{6}$)$_{0.7}$ (LSAT), SrTiO$_{3}$ (STO) and DyScO$_{3}$ (DSO). X-ray diffraction and atomic force microscopy confirmed the quality of the films and a …
