Changqing Jin

Structure and magnetoresistance of the double perovskite Sr2FeMoO6 doped at the Fe site

The structural, magnetic and transport properties of the compounds Sr2(Fe1−xMnx)MoO6 (0 ≤ x ≤ 0.45) have been studied. The saturated magnetization, Curie temperature and low-field magnetoresistance of the compounds decrease with x, while the resistivity increases by several orders of magnitude when x exceeds a critical value. A positive magnetoresistance has been observed for x = 0.45. A possible percolation mechanism is proposed to elucidate the observations; it also suggests a coexistence of (Mn3+, Mo5+) and (Mn2+, Mo6+) valence pairs and a saturated substitution of Mn3+ for Fe3+.

Structural stability and electrical properties of Sr2FeMoO6 under high pressure

The structural stability and electrical properties of Sr2FeMoO6 under high pressure at room temperature have been studied using energy dispersive x-ray diffraction with synchrotron radiation and resistance and capacitance measurements. The x-ray diffraction results show that the structure of Sr2FeMoO6 remains stable up to 40 GPa. The equation of state of Sr2FeMoO6 is obtained from the V/V0–P relationship. The bulk modulus B0 and its first-order derivative B0′ of Sr2FeMoO6 were calculated based on the Birch–Murnaghan equation. The electrical resistance undergoes a metallic transition at about 2.1 GPa. The metallic transition may be caused by a change in the electronic structure induced by high pressure.

Evolution of magnetic behaviour in the graphitization process of glassy carbon

We have carried out DC magnetization measurements on spherical glassy carbon exposed to high temperatures and high pressures. The observed magnetic signals clearly depend on the sintering temperature. On the basis of the graphitization temperature (around 1400oC), the behaviour can be classified into three regions: (1) paramagnetism in the no-graphitization region; (2) ferromagnetism in the near-graphitization region; (3) diamagnetic behaviour after graphitization. The magnetic transitions associated with the process of graphitization are discussed.

Pressure induced structural transitions in nanocrystalline grained selenium

Using diamond anvil cell technique, energy-dispersive X-ray diffraction and Raman spectroscopy were employed to study pressure induced phase transitions in nanocrystalline selenium up to 30 GPa. A complex phase transition behavior was observed, with lower transition pressure and more phase transitions than in conventional coarse-grained Se. This is considered a new kind of pressure induced phase transition behavior for a nanocrystalline material.

High critical current density of a MgB2 bulk superconductor high-pressure synthesized directly from the elements

We report the property studies of a MgB2 superconductor with high critical current density. The MgB2 superconductor was readily fabricated through a direct high-pressure synthesis of the respective elements. The obtained high-density MgB2 undergoes a sharp superconducting transition at 39 K. The bulk critical current density (J(C)) of the sample was calculated on the basis of Beans critical state model, and rather high critical current densities over a wide temperature range were obtained in comparison with the ambient prepared samples. The results highlight that high-pressure sintering would be a promising way to produce and search for this kind of intermetallic boride and the related superconductors.

Electron energy-loss spectroscopy characterization of the boron p-like density of states in MgB2

The microstructure and boron K-edge fine structure in MgB2 produced under high pressure are studied using high-resolution transmission electron microscopy and electron energy-loss spectroscopy in a transmission electron microscope. A pre-peak located at 188 eV in the boron K-edge energy-loss near edge structure (ELNES) provides a direct evidence for the existence of a high and unfilled p-like density of states (DOS) across the Fermi level. The σ(px+py) states dominate in this unfilled p-like DOS. The theoretical simulation results of the ELNES using the linearized augmented plane wave method can explain the experimental spectrum very well.

Transmission electron microscopy studies of C3N4H4 treated at high pressure and high temperature

C3N4H4 was treated at 6.0 GPa and 1500 °C for 2.5 min. Powder x-ray measurement shows that the sample is decomposed and a hexagonal graphite phase forms. Transmission electron microscopy studies show that small amounts of diamond and amorphous carbon phase coexist with the graphite phase. Parallel electron energy-loss spectroscopy analysis was also carried out for these phases.

Studies of the Ca4Mn3O10 structure obtained using high pressure and high temperature

An ideal n = 3 member of the group of Ruddlesden?Popper (RP) phases Ca4Mn3O10 was obtained by solid-state reaction under high pressure. This phase has been studied by transmission electron microscopy, convergent beam electron microscopy, high-resolution transmission electron microscopy, and parallel electron energy-loss spectroscopy. The lattice parameters are derived as a = b = 0.37 nm and c = 2.69 nm with the space group I4/mmm, which is the space group of the ideal RP phase.

Acoustic properties of MgB2 superconductor under pressure

The pressure dependence of sound velocity of longitudinal and shear waves were measured up to 0.5 GPa using a pulse echo overlap method on polycrystalline single-phase MgB2 superconductor which was high pressure synthesized and with a Tc of 39 K. The pressure dependence of Debye temperature and specific heat were calculated based on sound velocity data. The results indicate that Debye temperature increases with the applied pressures while specific heat decreases. The Gruneisen constant was calculated as well. From the sound velocities, the pressure coefficient of the averaged phonon frequency was obtained, which is consistent with the value estimated by Vogt et al. This indicates that the importance of phonon hardening in the variation of Tc under pressure. Upon changing the Debye temperature, unit cell volume and transition temperature with the pressure, volume dependence of electron-phonon coupling constant are also calculated.

Quantum tunneling of vortices in MgB2 superconductor

Magnetic relaxation in a MgB2 superconductor was measured. The temperature dependence of the normalized relaxation rate was determined for three different magnetic fields. By extrapolating these rates to T=0 K, we find that these extrapolations do not approach zero, indicating quantum tunneling of vortices in the superconductor. A quantum correction of the relaxation rate, followed by the correction of the magnetic moment, is proposed. Using the quantum correction, we find that U0 increases with decreasing temperature and approaches a maximum at T=0.