J. Q. Li

Microstructure and ordering of iron vacancies in the superconductor system KyFexSe2 as seen via transmission electron microscopy

Structural investigations by means of transmission electron microscopy (TEM) on KFexSe2 with 1.5 \leq x \leq 1.8 have revealed a rich variety of microstructure phenomena, the KFe1.5Se2 crystal often shows a superstructure modulation along the [310] zone-axis direction, this superstructure can be well interpreted by the Fe-vacancy order within the a-b plane. Increase of Fe-concentration in the KFexSe2 materials could not only result in the appearance of superconductivity but also yield clear alternations of microstructure. Structural inhomogeneity, the complex superstructures and defect structures in the superconducting KFe1.8Se2 sample have been investigated based on the high-resolution TEM.

Resistive switching effect in SrTiO3−δ∕Nb-doped SrTiO3 heterojunction

The authors report on the fabrication and properties of SrTiO3−δ∕Nb-doped SrTiO3 heterojunctions. The current-voltage curves of these junctions show hysteresis and remarkable resistive switching behavior. Hysteresis was also observed in the capacitance-voltage curves of these junctions. Upon applying voltage pulses, the resistance of the heterojunctions can be switched between different states and the relaxation of the junction current after switching follows the Curie–Von Schweidler law. The results were discussed by considering the role of defects in the interfacial depletion region of the heterojunctions. This work indicates that heterojunctions composed of two oxides can also show the switching effect, which is useful for applications.

Strain induced magnetoelectric coupling between magnetite and BaTiO3

Magnetite (Fe3O4) thin films have been grown on ferroelectric BaTiO3 (BTO) single crystal substrates by pulsed laser deposition. Transmission-electron microscope observations demonstrate the orientation relationship between Fe3O4 and BTO as [400]Fe3O4∥[200]BTO and [004]Fe3O4∥[002]BTO. Experimental measurements of magnetization, coercivity and remanent magnetization of the films show abrupt jumps at around phase transition temperatures of BTO and opposite jump signs are observed for the in-plane and out-of-plane measurements. The magnetization jumps can be suppressed by a strong external magnetic field. These results were discussed in terms of the interface strain induced changes of magnetic domain structure in the Fe3O4 film. This work demonstrates the presence of strong magnetoelectric coupling between Fe3O4 and BTO.

Coexistence of reentrant-spin-glass and ferromagnetic martensitic phases in the Mn2Ni1.6Sn0.4 Heusler alloy

A giant exchange bias field of up to 1170 Oe was observed in the Mn2Ni1.6Sn0.4 Heusler alloy. A reentrant spin glass phase and a ferromagnetic martensitic phase coexist below the blocking temperature as confirmed by dc magnetization and ac susceptibility measurements. Exchange bias in Mn2Ni1.6Sn0.4 is thought to originate from the interface exchange interaction between the reentrant spin glass phase and the ferromagnetic martensitic phase. X-ray diffraction and selected area electron diffraction results demonstrate that excess Mn atoms occupy Ni and Sn sites randomly. In this way, Mn-Mn clusters are formed and constitute the reentrant-spin-glass phase.

Microstructure and ferroelectric properties of sol-gel derived Bi3.15Nd0.85Ti3O12 thin films on Pt∕Ti∕SiO2∕Si(100)

Bi3.15Nd0.85Ti3O12 (BNdT) thin films were deposited on Pt∕Ti∕SiO2∕Si substrates using a sol-gel process. The film annealed at 750°C is composed of grains of 50–100nm in diameter. The fine grains show nearly random orientations. “Micropores” were frequently observed at the junctions of the grains and they are mostly amorphous, while sometimes containing very fine crystalline particles with sizes of only a few nanometers. The BNdT film capacitors with a Pt top electrode showed excellent ferroelectric properties. The remanent polarization and the coercive field were in the range of 41–43μC∕cm2 and 70–84kV∕cm, respectively. The BNdT capacitors did not show any significant fatigue up to 5×109 switching cycles at a frequency of 1MHz.

Synthesis and structure of nanocrystal-assembled bulk GaN

A new condensed form of GaN, nanocrystal-assembled bulk (NAB) GaN, was obtained directly from reactions of metal Ga and NH4Cl in liquid ammonia at 350-500 degrees C. High-resolution transmission electron microscopy observations reveal that the NAB GaN consists of well-crystallized nanocrystals with wurtzite structure. The synchronous densificated NAB GaN is transparent to visible light while the constituted nanocrystals have an average size of about 12 nm. A possible synthesis mechanism is discussed

Phase transition, superstructure and physical properties of K2Fe4Se5

The structural features and physical properties of the antiferromagnetic K0.8Fe1.6Se2 (so-called K2Fe4Se5 phase) have been studied in the temperature range from 300 K up to 600 K. Resistivity measurements on both single crystal and polycrystalline samples reveal a semiconducting behavior. Structural investigations of K2Fe4Se5 by means of transmission electron microscopy (TEM) and powder X-ray diffraction (XRD) demonstrate the presence of a well-defined superstructure within the a-b plane originating from a Fe-vacancy order along the [1−10] direction (indexed based on the supercell with space group of I4/m). Moreover, in situ heating structural analysis shows that K0.8Fe1.6Se2 undergoes a transition of the Fe-vacancy order to disorder at about 600 K. The phase separation and the Fe-vacancy ordering in the superconducting materials of KxFe2-ySe2 (0.2 ≤y ≤ 0.3) has been briefly discussed.

MODULATED STRUCTURE OF THE SUPERCONDUCTING COMPOUNDS BI2SR2CAN-1CUNOY WITH N=1 AND 2

The incommensurate modulated structures of high-Tc superconducting phases Bi2Sr2Can−1CunOy (n=1 and 2) have been studied using symmetry properties of four-dimensional super-spacegroups and electron diffraction as well as high resolution electron microscopy are used to describe the properties of the incommensurate modulated structure in the compounds. The main results are planar monoclinic symmetryPB2/b11 orPBb1 for Bi2Sr2CuOy (n=1) and orthorhombic symmetryN111Bbmb orN111Bb2b for Bi2Sr2CaCu2Oy (n=2). The temperature dependence of the modulated structure of Bi2Sr2CaCu2Oy has been investigated from −190°C to 800°C. Some structure properties of Bi2Sr2Ca2Cu3Oy (n=3) are presented for comparison.

Superconductivity enhancement in the S-doped Weyl semimetal candidate MoTe2

Two-dimensional (2D) transition-metal dichalcogenide (TMDs) MoTe2 has attracted much attention due to its predicted Weyl semimetal (WSM) state and a quantum spin Hall insulator in bulk and monolayer form, respectively. We find that the superconductivity in MoTe2 single crystal can be much enhanced by the partial substitution of the Te ions by the S ones. The maximum of the superconducting temperature TC of MoTe1.8S0.2 single crystal is about 1.3 K. Compared with the parent MoTe2 single crystal (TC=0.1 K), nearly 13-fold in TC is improved in MoTe1.8S0.2 one. The superconductivity has been investigated by the resistivity and magnetization measurements. MoTe2-xSx single crystals belong to weak coupling superconductors and the improvement of the superconductivity may be related to the enhanced electron-phonon coupling induced by the S-ion substitution. A dome-shape superconducting phase diagram is obtained in the S-doped MoTe2 single crystals. MoTe2-xSx materials may provide a new platform for our understanding of superconductivity phenomena and topological physics in TMDs.

Raman spectroscopy study of Na x Co O 2 and superconducting Na x Co O 2 ∙ y H 2 O

The Raman spectra of the parent compound Na{sub x}CoO{sub 2} (x = 0.75) and the superconducting oxyhydrates Na{sub x}CoO{sub 2} {center_dot} yH{sub 2}O with different superconducting temperatures (T{sub c}) have been measured. Five Raman active phonons around 195 cm{sup -1} (E{sub 1g}), 482 cm{sup -1}, 522 cm{sup -1}, 616 cm{sup -1} (3E{sub 2g}), and 663 cm{sup -1} (A{sub 1g}) appear in all spectra. These spectra change systematically along with the intercalation of H{sub 2}O and superconducting properties. In particular, the Raman active phonons (A{sub 1g} and E{sub 1g}) involving the oxygen motions within the Co-O layers show up monotonous decrease in frequency along with superconducting temperature T{sub c}. The fundamental properties and alternations of other active Raman phonons in the superconducting materials have also been discussed.