Jin-An Shi

Molecular Beam Epitaxy‐Grown SnSe in the Rock‐Salt Structure: An Artificial Topological Crystalline Insulator Material

Z. Wang, J. Wang, Y. Zang, Dr. T. Jiang, Y. Gong, Prof. C.-L. Song, Prof. S.-H. Ji, Prof. L.-L. Wang, Prof. K. He, Prof. W. Duan, Prof. X. Ma, Prof. X. Chen, Prof. Q.-K. Xue State Key Laboratory of Low-Dimensional Quantum Physics Department of Physics Tsinghua University Beijing 100084 , China E-mail: kehe@tsinghua.edu.cn; qkxue@tsinghua.edu.cn Q. Zhang, J.-A. Shi, Prof. L. Gu Beijing National Laboratory for Condensed Matter Physics Institute of Physics Chinese Academy of Sciences Beijing 100190 , China Dr. T. Jiang College of Opto-Electronic Science and Engineering National University of Defense Technology Changsha , Hunan 410073 , China Prof. S.-H. Ji, Prof. L.-L. Wang, Prof. L. Gu, Prof. K. He, Prof. W. Duan, Prof. X. Ma, Prof. X. Chen, Prof. Q.-K. Xue Collaborative Innovation Center of Quantum Matter Beijing 100084 , P. R. China

In Situ Atomic-Scale Observation of Electrochemical Delithiation Induced Structure Evolution of LiCoO2 Cathode in a Working All-Solid-State Battery

We report a method for in situ atomic-scale observation of electrochemical delithiation in a working all-solid-state battery using a state-of-the-art chip based in situ transmission electron microscopy (TEM) holder and focused ion beam milling to prepare an all-solid-state lithium-ion battery sample. A battery consisting of LiCoO2 cathode, LLZO solid state electrolyte and gold anode was constructed, delithiated and observed in an aberration corrected scanning transmission electron microscope at atomic scale. We found that the pristine single crystal LiCoO2 became nanosized polycrystal connected by coherent twin boundaries and antiphase domain boundaries after high voltage delithiation. This is different from liquid electrolyte batteries, where a series of phase transitions take place at LiCoO2 cathode during delithiation. Both grain boundaries become more energy favorable along with extraction of lithium ions through theoretical calculation. We also proposed a lithium migration pathway before and after polycrystallization. This new methodology could stimulate atomic scale in situ scanning/TEM studies of battery materials and provide important mechanistic insight for designing better all-solid-state battery.

Atomically resolved FeSe/SrTiO3(001) interface structure by scanning transmission electron microscopy

Interface-enhanced high-temperature superconductivity in one unit-cell (UC) FeSe films on SrTiO3(001) (STO) substrate has recently attracted much attention in condensed matter physics and material science. By combined in-situ scanning tunneling microscopy/spectroscopy (STM/STS) and ex-situ scanning transmission electron microscopy (STEM) studies, we report on atomically resolved structure including both lattice constants and actual atomic positions of the FeSe/STO interface under both non-superconducting and superconducting states. We observed TiO2 double layers (DLs) and significant atomic displacements in the top two layers of STO, lattice compression of the Se-Fe-Se triple layer, and relative shift between bottom Se and topmost Ti atoms. By imaging the interface structures under various superconducting states, we unveil a close correlation between interface structure and superconductivity. Our atomic-scale identification of FeSe/STO interface structure provides useful information on investigating the pairing mechanism of this interface-enhanced high-temperature superconducting system.

A new non-destructive readout by using photo-recovered surface potential contrast

Ferroelectric random access memory is still challenging in the feature of combination of room temperature stability, non-destructive readout and high intensity storage. As a non-contact and non-destructive information readout method, surface potential has never been paid enough attention because of the unavoidable decay of the surface potential contrast between oppositely polarized domains. That is mainly due to the recombination of the surface movable charges around the domain walls. Here, by introducing a laser beam into the combination of piezoresponse force microscopy and Kelvin probe force microscopy, we demonstrate that the surface potential contrast of BiFeO3 films can be recovered under light illumination. The recovering mechanism is understood based on the redistribution of the photo-induced charges driven by the internal electric field. Furthermore, we have created a 12-cell memory pattern based on BiFeO3 films to show the feasibility of such photo-assisted non-volatile and non-destructive readout of the ferroelectric memory.

Flexible strain sensors with high performance based on metallic glass thin film

Searching strain sensitive materials for electronic skin is of crucial significance because of the restrictions of current materials such as poor electrical conductivity, large energy consumption, complex manufacturing process, and high cost. Here, we report a flexible strain sensor based on the Zr55Cu30Ni5Al10 metallic glass thin film which we name metallic glass skin. The metallic glass skin, synthesized by ion beam deposition, exhibits piezoresistance effects with a gauge factor of around 2.86, a large detectable strain range (∼1% or 180° bending angle), and good conductivity. Compared to other e-skin materials, the temperature coefficient of resistance of the metallic glass skin is extremely low (9.04 × 10−6 K−1), which is essential for the reduction in thermal drift. In addition, the metallic glass skin exhibits distinct antibacterial behavior desired for medical applications, also excellent reproducibility and repeatability (over 1000 times), nearly perfect linearity, low manufacturing cost, and neg...

One-step synthesis of van der Waals heterostructures of graphene and two-dimensional superconducting α−Mo2C

Assembling different two-dimensional (2D) crystals, covering a very broad range of properties, into van der Waals (vdW) heterostructures enables the unprecedented possibilities for combining the best of different ingredients in one objective material. So far, metallic, semiconducting, and insulating 2D crystals have been used successfully in making functional vdW heterostructures with properties by design. Here, we expand 2D superconducting crystals as a building block of the vdW hererostructures. A one-step growth of large-scale high-quality vdW heterostructures of graphene and 2D superconducting a-Mo2C by using chemical vapor deposition (CVD) method is reported. The superconductivity and its 2D nature of the heterostructures are characterized by our scanning tunneling microscopy (STM) measurements. This adds the 2D superconductivity, the most attractive property of condensed matter physics, to the vdW heterostructures.

Longitudinal wave function control in single quantum dots with an applied magnetic field

Controlling single-particle wave functions in single semiconductor quantum dots is in demand to implement solid-state quantum information processing and spintronics. Normally, particle wave functions can be tuned transversely by an perpendicular magnetic field. We report a longitudinal wave function control in single quantum dots with a magnetic field. For a pure InAs quantum dot with a shape of pyramid or truncated pyramid, the hole wave function always occupies the base because of the less confinement at base, which induces a permanent dipole oriented from base to apex. With applying magnetic field along the base-apex direction, the hole wave function shrinks in the base plane. Because of the linear changing of the confinement for hole wave function from base to apex, the center of effective mass moves up during shrinking process. Due to the uniform confine potential for electrons, the center of effective mass of electrons does not move much, which results in a permanent dipole moment change and an inverted electron-hole alignment along the magnetic field direction. Manipulating the wave function longitudinally not only provides an alternative way to control the charge distribution with magnetic field but also a new method to tune electron-hole interaction in single quantum dots.

Atomic-resolution imaging of electrically induced oxygen vacancy migration and phase transformation in SrCoO 2.5-σ

Oxygen ion transport is the key issue in redox processes. Visualizing the process of oxygen ion migration with atomic resolution is highly desirable for designing novel devices such as oxidation catalysts, oxygen permeation membranes, and solid oxide fuel cells. Here we show the process of electrically induced oxygen migration and subsequent reconstructive structural transformation in a SrCoO2.5−σ film by scanning transmission electron microscopy. We find that the extraction of oxygen from every second SrO layer occurs gradually under an electrical bias; beyond a critical voltage, the brownmillerite units collapse abruptly and evolve into a periodic nano-twined phase with a high c/a ratio and distorted tetrahedra. Our results show that oxygen vacancy rows are not only natural oxygen diffusion channels, but also preferred sites for the induced oxygen vacancies. These direct experimental results of oxygen migration may provide a common mechanism for the electrically induced structural evolution of oxides.Information on how oxygen ions transport is crucial to understanding field-induced phase transformations in materials. Here, Zhang et al. directly image atomic-scale oxygen migration and the subsequent structural reconstruction in a SrCoO2.5-σ film in the presence of an electric field.

C/L-band emission of InAs QDs monolithically grown on Ge substrate

In recent years, the growing demand for silicon based light sources has boosted the research field of III-V/IV hybrid lasers. Here, the C/L-band light emission (1.53 μm-1.63 μm) of InAs/In0.25Ga0.75As quantum dots (QDs) epitaxially grown on Ge substrate by solid-source molecular beam epitaxy (MBE) is reported. By hybrid III-V/IV epitaxial growth, ultra-thin and anti-phase domains (APD) free III-V materials are achieved on Ge substrate. Step-graded InGaAs metamorphic buffer layers are applied to reduce the strain in InAs QDs in order to extend the emission wavelength. At last, a high quality InAs/In0.25Ga0.75As QD structure on Ge(001) substrate is obtained, which has a strong C/L-band emission centered at the wavelength of 1.6 μm with a full-width-half-maximum (FWHM) of 57 meV at room temperature.

The effects of oxygen in spinel oxide Li1+xTi2−xO4−δ thin films

The evolution from superconducting LiTi2O4-δ to insulating Li4Ti5O12 thin films has been studied by precisely tuning the oxygen pressure in the sample fabrication process. In superconducting LiTi2O4-δ films, with the increase of oxygen pressure, the oxygen vacancies are filled gradually and the c-axis lattice constant decreases. When the oxygen pressure increases to a certain critical value, the c-axis lattice constant becomes stable, which implies that the sample has been completely converted to Li4Ti5O12 phase. The two processes can be manifested by the angular bright-field images of the scanning transmission electron microscopy techniques. The transition temperature (Tch) of magnetoresistance from the positive to the negative shows a nonmonotonic behavior, i.e. first decrease and then increase, with the increase of oxygen pressure. We suggest that the decrease Tch can be attributed to the suppressing of orbital-related state, and the inhomogeneous phase separated regions contribute positive MR and thereby lead to the reverse relation between Tch and oxygen pressure.