Li Jian-Qi

Structural Properties of MgB2 Superconductors with a Critical Current Density Greater than 105A/cm2

The structural features of MgB2 superconductors prepared under different conditions have been analysed by means of transmission electron microscopy. The MgB2 crystal is identified to have a hexagonal structure with the space group of P6/mmm. Superconducting materials synthesized under a high pressure of 6 Cpa are made of regularly stacked grains as large as a few microns in sizes. Stacking faults and microstructure properties of grain boundaries, either with or without impurities, appearing in a sample with the critical current density greater than 10(5)/cm(2), have been systematically investigated.

New Superconductivity Dome in LaFeAsO1−xFx Accompanied by Structural Transition*

High-temperature superconductivity is often found in the vicinity of antiferromagnetism. This is also true in LaFeAsO1−xFx (x ≤ 0.2) and many other iron-based superconductors, which leads to proposals that superconductivity is mediated by fluctuations associated with the nearby magnetism. Here we report the discovery of a new superconductivity dome without low-energy magnetic fluctuations in LaFeAsO1−xFx with 0.25 ≤ x ≤ 0.75, where the maximal critical temperature Tc at xopt = 0.5−0.55 is even higher than that at x ≤ 0.2. By nuclear magnetic resonance and transmission electron microscopy, we show that a C4 rotation symmetry-breaking structural transition takes place for x > 0.5 above Tc. Our results point to a new paradigm of high temperature superconductivity.

Laser Molecular Beam Epitaxy Growth of BaTiO3 in Seven Thousands of Unit-Cell Layers

BaTiO3 thin films in seven thousands of unit-cell layers have been successfully fabricated on SrTiO3 (001) substrates by laser molecular beam epitaxy. The fine streak pattern and the undamping intensity oscillation of reflection high-energy electron diffraction indicate that the BaTiO3 film was layer-by-layer epitaxial growth. The measurements of scanning electron microscopy and atomic force microscopy show that surfaces of the BaTiO3 thin film are atomically smooth. The measurements of x-ray diffraction and transmission electron microscopy, as well as selected-area electron diffraction reveal that the BaTiO3 thin film is a c-oriented epitaxial crystalline structure.

Laser-Driven Silver Nanowire Formation: Effect of Femtosecond Laser Pulse Polarization

We report our laser-driven method used to make large quantities of straight thin silver nanowires, and experimentally demonstrate that femtosecond laser pulse polarization has a prominent effect on formation of non-spherical shapes of nanoscale particles. Further, our experiment directly reveals that the underlying mechanism is plasmon-plasmon interaction, which can be controlled by polarization and plays a decisive role in this non-synthetic method for metal nanowire formation.

TEM study on hollow and porous Cu2O nanoparticles prepared from solution phase

In this paper, hollow and porous Cu2O nanoparticles were prepared by adjusting the cationic surfactant cetyltrimethylammonium (CTAB) concentration in the solution-phase reaction. Structural investigations reveal that Cu2O nanoparticles can be either well-defined hollow nanoboxes or porous nanocubes depending on the synthesis conditions. The transmission electron microscopy (TEM) observations demonstrated that the nanoparticles in general are composed of small grains coherently growing along certain preferred orientations.

Preparation, structure and ferromagnetic properties of the nanocrystalline Ti1-xMnxO2 thin films grown by radio frequency magnetron co-sputtering

This paper reported that the Mn-doped TiO2 films were prepared by radio frequency (RF) magnetron co-sputtering. X-ray diffraction measurements indicate that the samples are easy to form the rutile structure, and the sizes of the crystal grains grow big and big as the Mn concentration increases. X-ray photoemission spectroscopy measurements and high resolution transmission electron microscope photographs confirm that the manganese ions have been effectively doped into the TiO2 crystal when the Mn concentration is lower than 21%. The magnetic property measurements show that the Ti1-xMnxO2 (x = 0.21) films are ferromagnetic at room temperature, and the saturation magnetization, coercivity, and saturation field are 16.0emu/cm3, 167.5 × 80A/m and 3740 × 80A/m at room temperature, respectively. The room-temperature ferromagnetism of the films can be attributed to the new rutile Ti1-xMnxO2 structure formed by the substitution of Mn4+ for Ti4+ into the TiO2 crystal lattice, and could be explained by O vacancy (VO)-enhanced ferromagnetism model.

The influence of interfacial barrier engineering on the resistance switching of In2O3:SnO2/TiO2/In2O3:SnO2 device

The I—V characteristics of In2O3:SnO2/TiO2/In2O3:SnO2 junctions with different interfacial barriers are investigated by comparing experiments. A two-step resistance switching process is found for samples with two interfacial barriers produced by specific thermal treatment on the interfaces. The nonsynchronous occurrence of conducting filament formation through the oxide bulk and the reduction in the interfacial barrier due to the migration of oxygen vacancies under the electric field is supposed to explain the two-step resistive switching process. The unique switching properties of the device, based on interfacial barrier engineering, could be exploited for novel applications in nonvolatile memory devices.

Structural phase transitions, phase separation and physical properties for the 122-system iron-based superconductors

The discovery of iron-based superconductors stimulated the second research upsurge for high-temperature superconductors, which have been considered as one of the significant research field from both academic and technological points of view. It is noted that the studies of iron-based superconductors have brought a great development of a large number of experimental technologies and theoretical researches in superconductivity physics. The remarkable correlations between structural transitions and multiple ordered states have been observed in the typical iron-based superconductors. According to the microstructure analyses and in-situ transmission electron microscopy (TEM) investigations, AFe2As2 (A=Ba, Sr and Ca) iron-based superconducting materials often show up a structural transition from tetragonal phase to orthorhombic phase from room temperature down to 20 K, resulting in visible twinning domains in the orthorhombic phase. The tetragonal SrFe2As2 samples, consistent with X-ray and neutron-diffraction data, undergo the tetragonal- orthorhombic phase transition at about 205 K and show clear twin domains in the orthorhombic phase. On the other hand, TEM observations of CaFe2As2 reveal the presence of a pseudoperiodic structural modulation with a periodicity of around 40 nm at room temperature. This quasi-periodicity structural modulation is likely related to the local structural distortions within the Ca layers. In situ cooling TEM observations of CaFe2As2 reveal the presence of complex domain structures in the low-temperature orthorhombic phase. Phase separation and structural inhomogeneity as critical structural issues have been extensively investigated in a variety of strongly correlated systems. The phase separations associated with structural domains result visibly structural alterations in K y Fe2− x Se2 system. Structural investigations by means of TEM on K0.8Fe x Se2 and KFe x Se2, with 1.5≤ x ≤1.8, have revealed a rich variety of microstructure phenomena. Materials with 1.5≤ x ≤1.6 often show a superstructure modulation along the [310] zone-axis direction, and this modulation can be well interpreted by the Fe-vacancy order, which likely yields a superstructure phase of K2Fe4Se5. The superconducting K0.8Fe x Se2 and KFe x Se2 (1.7≤ x ≤1.8) materials contain clear phase separation, in particular, along the c-axis direction, recognizable as visible parallel lamellae in the crystals; this fact suggests that the superconducting phase could have the Fe-vacancy disordered state. The main changes of physical properties in the AFe2As2 materials have also been discussed. The substitution of A-element in AFe2As2 have a significant effect on the structural properties and spin density wave (SDW), then leads to the appearance and change of superconductivities. Polycrystalline samples of Ba1− x Sr x Fe2As2 (0≤ x ≤1) and Ba1− x Sr x Fe1.8Co0.2As2 (0≤ x ≤1) were synthesized by a solid state reaction method. Structural analysis by means of X-ray diffraction shows that the lattice parameters and unit cell volume decrease monotonically with the increase of x for Ba1– x Sr x Fe2As2. The measurements of transport properties demonstrate that the average size of the Ba(Sr)-site cations could evidently influence the SDW behavior in Ba1− x Sr x Fe2As2 and superconductivity in Ba1− x Sr x Fe1.8Co0.2As2 as well. The critical temperature for SDW ( T SDW) increases with the Sr substitution for Ba in Ba1− x Sr x Fe2As2 and, on the other hand, the superconducting T c decreases with the increase of Sr content in Ba1− x Sr x Fe1.8Co0.2As2. The inhomogeneous distributions of Ba/Sr ions and structural distortions in Ba0.5Sr0.5Fe2As2 have been investigated by TEM observations.

Cs-corrected HAADF-STEM observations on the structural modulations caused by charge density wave and Te-vacancy ordering in LaTe 2 − δ

Microstructural features play a critical role for the understanding of the essential properties of novel functional materials and new devices. Atomic-resolution scanning transmission electron microscopy (STEM) is invaluable for determining the average atomic structure and local structural defects. For strongly correlated electron materials, STEM can be applied to distinguish the structural modulation caused by charge density wave (CDW), chemical ordering (vacancy or impurity atoms). RTe2− δ (R=La, Ce) compounds have attracted recent attention due to their effective low dimensionality. The materials play host to a CDW state above room temperature and can be described in terms of a modulated Cu2Sb-type structure (P4/ nmm ) based on alternating layers of square-planar Te sheets and a corrugated RTe slab. Furthermore, pressure-induced superconductivity in CeTe1.82 with T C of 2.7 K has been reported, suggesting that the nonstoichiometric Te defects are correlated to superconductivity in this material system. Here, we report the study of the structural modulations in LaTe2− δ using STEM. The LaTe2− δ single crystal was grown by self-flux technique. The TEM samples used in the present study were prepared by crushing the well- characterized single crystal, and then the resultant suspensions were dispersed on a holey carbon-covered Cu grid. Electron diffraction experiments were performed in the FEI Tecnai F20 microscope, and HRTEM and high angle annular dark field (HAADF) STEM were performed in the JEOL ARM200F equipped with double aberration correctors and cold field emission gun at room temperature. The experiment result revealing the charge density wave in LaTe2− δ can be tuned by the Te content; the structural modulation correlated with charge density wave can be characterized by a modulation wave vector of q CDW=(1/2− α ) a ∗, where α is the incommensurate parameter determined by the chemical composition. Our experiment data demonstrate that the Cs-corrected HAADF-STEM image can directly reveal the atomic displacements in the Te plane due to electron-phonon coupling. Detailed analysis suggests that the Te atomic displacements adopt an incommensurate wave-pocket structure along each Te-chain with a long periodicity determined by the CDW incommensurability. In addition to the q CDW=(1/2− α ) a ∗ CDW modulation, a superstructure with the vector q 2=1/5(3 a ∗+ b ∗) has been also observed in some regions. In previous study, this modulation was proposed to be correlated with CDW instability, however, recent experimental and theoretical analysis on the electronic structure (FS) shows that there is no such nesting wave vector could be identified. In this paper, our Cs-STEM observations directly demonstrated that the q 2 superstructure actually originates from an imperfect stoichiometry in this layered system. We proposed a 5 × 5 supercell associated with the Te vacancy ordering with the chemical composition of LaTe1.85 based on detailed STEM data analysis. From HRTEM images, it is also noted that the CDW modulation ( q CDW) totally disappears in the region with Te vacancy ordering, suggesting that the CDW can not coexist in the crystals with the Te vacancy ordering. The possible correlation between the ordering of the nonstoichiometric Te defects and the superconductivity in this material system needs to be further investigated.

Splitting Process of Na-Birnessite Nanosheet via Transmission Electron Microscopy

The intermediate phase of hydrothermal synthesis of the Na0.44MnO2 (NMO) nanowires is systematically investigated by means of transmission electron microscopy (TEM). The coexistence of Na-birnessite and NMO is commonly observed in the nanosheets. The NMO nanobelts in general have the width of ~15 nm embedded in the (001) oriented Na-birnessite nanosheet. It is also found that the nanosheets of this intermediate phase often split along the NMO and Na-birnessite interface. Our structural study also shows that the NMO nanobelts prefer to grow along the [001] direction and gives rise to the [001] elongated NMO nanowires. Based on our TEM observations, the visible lattice mismatch and the resultant strain at the NMO/Na-birnessite interface play a critical role for formation of notable splitting structures in this kind of nanomaterial. The mechanism for the formation of the [001] NMO nanobelt is briefly discussed.