Huaixin Yang

Nonlinear current-voltage behavior and electrically driven phase transition in charge-frustrated LuFe2O4

Electric transport measurements of the charge-frustrated LuFe2O4, in which charge ordering (CO) and electronic ferroelectricity are found, reveal a strong nonlinear electric conduction upon application of electric fields in both single-crystalline and polycrystalline samples. The threshold electric fields (Et) in LuFe2O4 are estimated to be about 60u2009V/cm and 10u2009V/cm with E parallel and perpendicular to the c-axis direction, respectively. Experimental measurements also demonstrate that the I-V nonlinearity increases quickly with decreasing temperature. Furthermore, our in situ TEM investigations clearly reveal that the nonlinear I-V behavior is intrinsically in correlation with a current-driven charge ordering insulator-metal transition, and the applied electrical field triggers a visible CO collapse recognizable as the fading of the satellite spots of the CO modulations.

Synthesis and structural characterization of LaOFeP superconductors

High quality LaOFeP superconducting materials?have been successfully synthesized by a two-step solid reaction method. We first prepared a La?Fe?P ternary alloy following a pre-alloying treatment; the resultant La?Fe?P ternary alloy was found to be the key precursor for producing the final LaOFeP superconductors in our experiments. The LaOFeP samples in general have a superconducting transition temperature of about Tc?4.1?K as demonstrated by the measurements of resistivity and magnetic susceptibility. The volume ratio of the superconducting phase in the high quality sample is estimated to be as high as 95% at the temperature of 2?K. Structural analysis by means of x-ray diffraction, electron diffraction and high resolution transmission electron microscopy shows that the LaOFeP crystal has a layered tetragonal structure with a P4/nmm space group. Structural defects such as stacking faults in the LaOFeP sample have also been briefly discussed.

Structural properties and cation ordering in layered hexagonal CaxCoO2

A series of CaxCoO2 (0.15 <= x <= 0.40) materials have been prepared by means of an ion exchange reaction from NaxCoO2. Transmission electron microscopy (TEM) measurements revealed a rich variety of structural phenomena resulting from cation ordering, structural distortion, and twinning. Systematic structural analysis, in combination with the experimental data of NaxCoO2 (0.15 <= x <= 0.8) and SrxCoO2 (1.5 <= x <= 0.4) systems, suggests that there are two common well-defined cation ordered states corresponding, respectively, to the orthorhombic superstructure at around x=1/2 and the 3(1/2)ax3(1/2)a superstructure at around x=1/3 in this kind of system. Multiple ordered states, phase separation, and incommensurate structural modulations commonly appear in the materials with 0.33 < x < 0.5. The TEM observations also reveal an additional periodic structural distortion with q(2)=a(*)/2 in materials for x <= 0.35. This structural modulation also appears in the remarkable superconducting phase Na(0.33)CoO(2)center dot 1.3H(2)O.

The effect of Mg doping on the structural and physical properties of LuFe2O4 and Lu2Fe3O7

The structural and physical properties of the recently discovered electronic ferroelectric materials LuFe(2)O(4) and Lu(2)Fe(3)O(7) have been investigated for Mg substitution of Fe. X-ray diffraction data demonstrate that the lattice parameters in both systems change progressively with increasing Mg content, with a smaller unit cell volume on replacing Fe(2+) by Mg(2+). X-ray absorption near-edge spectroscopy experiments at the Fe K-edge show that the average Fe oxidation state is slightly increased along with Mg doping in Lu(2)Fe(3)O(7) materials, consistent with isomorphous replacement of Fe(2+) by Mg(2+). Measurements of dielectric properties demonstrate that Mg doping could have an effect on the electron hopping energy between Fe(2+) and Fe(3+) ions. Transmission electron microscopy and magnetization analysis reveal that Mg doping in LuFe(2)O(4) has a much greater influence than in Lu(2)Fe(3)O(7) on both the charge ordering and the low-temperature magnetic properties.

Electron energy-loss spectroscopy and ab initio electronic structure of the LaOFeP superconductor

The electronic band structures of the LaOFeP superconductor have been calculated theoretically by the first-principles method and measured experimentally by electron energy loss spectroscopy (EELS). The calculations indicate that the Fe atom in a LaOFeP crystal shows a weak magnetic moment (0.14 mu(B)/atom) and does not form a long-range magnetic ordering. Band structure, Fermi surfaces, and fluorine-doping effects are also analyzed based on the data of the density functional theory. The fine structures of the EELS data have been carefully examined in both the low loss energy region (<60 eV) and the core losses region (O K, Fe L(2,3), and La M(4,5)). A slight bump edge at similar to 44 eV shows notable orientation dependence: it can be observed in the low loss EELS spectra with q parallel to c but becomes almost invisible in the q perpendicular to c spectra. Annealing experiments indicate that low oxygen pressure favors the appearance of superconductivity in LaOFeP: this fact is also confirmed by the changes of Fe L(2,3) and O K excitation edges in the experimental EELS data.

Effects of layered structural features on charge/orbital ordering in (La, Sr)n+1MnnO3n+1 (n = 1 and 2)

The charge/orbital ordering (COO) of the layered mixed-valence manganites (La,Sr)(n+1)Mn(n)O(3n+1) (n = 1 and 2) is examined by first-principles calculations and discussed in comparison with the La(0.5)Ca(0.5)MnO(3) perovskite phase ([Formula: see text]). The results demonstrated that the layered structural features could yield not only visibly weak coupling between Mn-O layers but also various features in the orbital ordering associated with different types of local structural distortions. In both La(0.5)Sr(1.5)MnO(4) (n = 1) and LaSr(2)Mn(2)O(7) (n = 2) phases, the orbital ordering can be chiefly assigned to the d(x(2)-y(2)) orbital, in contrast with the zigzag-type d(z(2)) orbital ordering in the [Formula: see text] perovskite phase. Our theoretical analysis shows that a variety of essential factors, including the local structural distortions of the MnO(6) octahedra, the on-site Coulomb interaction, and magnetic interaction, have to be properly considered in order to achieve acceptable COO ground states for the layered variants in (La,Sr)(n+1)Mn(n)O(3n+1).

Structural modulation in the orbitally induced Peierls state of MgTi2O4

The structural properties associated with the orbitally induced Peierls transition in MgTi(2)O(4) are characterized by in situ cooling TEM observations. A distinctive structural modulation with the wavevector of q(1) = 1/4 (0, 0, 4) has been well demonstrated below a critical temperature of about 260xa0K for MgTi(2)O(4). Systematic analysis demonstrates that this structural modulation can be well explained by an orbital order existing in the low-temperature insulating phase. It is also noted that the nonstoichiometric feature commonly appearing in the present system could yield visible changes in both physical and structural properties. A low-temperature study of the microstructure of Mg[Ti(1.9)Mg(0.1)]O(4) reveals that a little substitution of Mg(2+) for Ti(3+) ions on the octahedral sites can disrupt the long-range order of the t(2g) orbitals, resulting in complex tweed structures in the superstructure phase.

Superconducting dome and microstructure properties of Rb0.8Fe1.6+xSe2 superconductors

High quality Rb0.8Fe1.6+xSe2 single crystals are grown by a one step melting growth method. Superconductivity has been observed in all samples with x more than 0.05 and the maximum critical temperature Tc~32 K has been obtained in samples with x from 0.1 to 0.2 from electronic resistivity measurement. A superconducting dome in the electronic phase diagram of Tc as a function of Fe content has been observed for the first time in this AxFeySe2 (A=K,Rb) superconducting system. Two coexisting phases with structural modulation vectors of q1=1/5 [3a*+b*] and q2=1/2[a*+b*] have been observed in all superconducting samples by TEM observations. Moreover, phase separation in the superconducting crystals, which can be recognized as micro-stripes in the ab plane, are found to be highly dependent on the Fe content.