Fang Minghu

Dielectric and magnetic properties of (Zn, Co) co-doped SnO2 nanoparticles*

Polycrystalline samples of (Zn, Co) co-doped SnO2 nanoparticles were prepared using a co-precipitation method. The influence of (Zn, Co) co-doping on electrical, dielectric, and magnetic properties was studied. All of the (Zn, Co) co-doped SnO2 powder samples have the same tetragonal structure of SnO2. A decrease in the dielectric constant was observed with the increase of Co doping concentration. It was found that the dielectric constant and dielectric loss values decrease, while AC electrical conductivity increases with doping concentration and frequency. Magnetization measurements revealed that the Co doping SnO2 samples exhibits room temperature ferromagnetism. Our results illustrate that (Zn, Co) co-doped SnO2 nanoparticles have an excellent dielectric, magnetic properties, and high electrical conductivity than those reported previously, indicating that these (Zn, Co) co-doped SnO2 materials can be used in the field of the ultrahigh dielectric material, high frequency device, and spintronics.

High-Pressure Single-Crystal Neutron Scattering Study of Magnetic and Fe Vacancy Orders in (Tl,Rb)2Fe4Se5 Superconductor

We investigate the magnetic and iron vacancy orders in superconducting (Tl,Rb)2Fe4Se5 single-crystals by using a high-pressure neutron diffraction technique. Similar to the temperature effect, the block antiferromagnetic order gradually decreases upon increasing pressure while the Fe vacancy superstructural order remains intact before its precipitous disappearance at the critical pressure Pc = 8.3 GPa. Combined with previously determined Pc for superconductivity, our phase diagram under pressure reveals the concurrence of the block AFM order, the √5 × √5 iron vacancy order and superconductivity for the 245 superconductor. Lastly, a synthesis of current experimental data in a coherent physical picture is attempted.

Exploration of iron-chalcogenide superconductors

Iron-chalcogenide compounds with FeSe(Te, S) layers did not attract much attention until the discovery of high-T c superconductivity (SC) in the iron-pnictide compounds at the begining of 2008. Compared with FeAs-based superconductors, iron-chalcogenide superconductors have aroused enormous enthusiasm to study the relationship between SC and magnetisms with several distinct features, such as different antiferromagnetic ground states with relatively large moments in the parents, indicating possibly different superconducting mechanisms, the existence of the excess Fe atoms or Fe vacancies in the crystal lattice. Another reason is that the large single crystals are easily grown for the iron-chalcogenide compounds. This review will focus on our exploration for the iron-chalcogenide superconductors and discussion on several issues, including the crystal structure, magnetic properties, superconductivity, and phase separation. Some of them reach a consensus but some important questions still remain to be answered.Iron-chalcogenide compounds with FeSe(Te, S) layers did not attract much attention until the discovery of high-Tc superconductivity (SC) in the iron-pnictide compounds at the begining of 2008. Compared with FeAs-based superconductors, iron-chalcogenide superconductors have aroused enormous enthusiasm to study the relationship between SC and magnetisms with several distinct features, such as different antiferromagnetic ground states with relatively large moments in the parents, indicating possibly different superconducting mechanisms, the existence of the excess Fe atoms or Fe vacancies in the crystal lattice. Another reason is that the large single crystals are easily grown for the iron-chalcogenide compounds. This review will focus on our exploration for the iron-chalcogenide superconductors and discussion on several issues, including the crystal structure, magnetic properties, superconductivity, and phase separation. Some of them reach a consensus but some important questions still remain to be answered.Iron-chalcogenide compounds with FeSe(Te, S) layers did not attract much attention until the discovery of high-Tc superconductivity (SC) in the iron-pnictide compounds at the begining of 2008. Compared with FeAs-based superconductors, iron-chalcogenide superconductors have aroused enormous enthusiasm to study the relationship between SC and magnetisms with several distinct features, such as different antiferromagnetic ground states with relatively large moments in the parents, indicating possibly different superconducting mechanisms, the existence of the excess Fe atoms or Fe vacancies in the crystal lattice. Another reason is that the large single crystals are easily grown for the iron-chalcogenide compounds. This review will focus on our exploration for the iron-chalcogenide superconductors and discussion on several issues, including the crystal structure, magnetic properties, superconductivity, and phase separation. Some of them reach a consensus but some important questions still remain to be answered.

A new variational calculation for N-dimensional polarons in the strong-coupling limit.

A novel variational approach is presented for the calculation of the ground-state energy of the polaron in arbitrary N dimensions in the strong-coupling limit. By using the phonon coherent state to represent the wavefunction of phonons, a self-consistent integro-differential equation for the electron wavefunction is derived. The calculated results of the ground-state energy for N = 1, 2 and 3 agree well with the best results in the literature. It is also found that, for arbitrary N, the present results are less than the Feynman path integral ones by small percentages. It is proposed that this approach should be universal for systems involving polarons in the strong-coupling regime.

Flux Pinning in Y1-xMxBa2Cu3O7-δ (M=Gd, Eu and Sm) System

In this paper, the flux pinning effects in Y1-xMxBa2Cu3O7-δ (M=Gd, Eu and Sm) samples are reported. It is found that there is another pinning mechanism under higher magnetic fields in addition to that under lower fields. This pinning effect may stem from the lattice stress field resulting from the partial substitution of the rare earth elements with different atomic radii for Y.

Coexistence of Ferromagnetism and Superconductivity in EuSr2 Ru1-xNbxCu2O8

The structural, magnetic and transport properties of EuSr2Ru1-xNbxCu2O8 (x = 0, 0.1, 0.2 and 0.5) samples have been investigated by means of x-ray diffraction, magnetization and resistance measurements. It is found that the EuSr2RuCu2O8 compound exhibits a ferromagnetic order of the Ru moments below the Curie temperature of 130.19 K, and becomes superconducting at a much lower critical temperature of 10 K. The experimental observation that partial substitution of Nb for Ru in this system results in the decrease of the Curie temperature further supports the idea that the ferromagnetic order involves only Ru moments in this compound. We suggest that the decrease of the superconducting critical temperature Tc due to the partial substitution of Nb for Ru originates from the decrease of the carrier concentration in the CuO2 plane because of Nb existing always in the Nb5+ valence state while Ru is in a mixed valence state between Ru4+ and Ru5+. The coexistence of ferromagnetism and superconductivity is also discussed.

Flux pinning effect of small size defects in YBa2Cu3O7-δ

Abstract In this paper, it is reported that the small size defects, which result from the introductions of Y, Ba, Cu and O ionic vacancies and the partial substitutions of Ln (Gd, Sm and Eu) for Y in YBa 2 Cu 3 O 7−δ system, may pin the flux lines. From the experimental results, authors suggest tha the flux pinning effect at lower field relates to the magnetic interaction between the vortex and the surfaces of grains in the samples; and that the flux pinning at higher field belongs to the core interaction mechanism in the samples with ionic vacancies and stems from the elastic interaction between the vortex and the lattice stress field in the partially substituted samples.

Hole Concentration Dependence of Tc in Bi2Sr2CaCu2Oy System

The resistance, ac susceptibility and Hall coefficient measurements were carried out for the high quality single phase Bi2Sr2CaCu2Oy samples with different oxygen contents. It is found that the number of extra oxygen atoms in the samples can be easily modulated when they are annealed in low vacuum, and that the relationship between Tc and P+ is unmonotonic with an optimum P+=0.27 holes/CuO2 plane corresponding to the highest Tc (=95 K). This unmonotonic relationship is discussed briefly.

Polaron effective masses with extended coherent states

The effective mass of a large polaron is calculated using the extended phonon coherent state proposed in our previous paper. In the weak-coupling limit, the mass expansion obtained can reproduce the fourth-order perturbative result. Moreover, in the intermediate-coupling range, the calculated results are in good agreement with those of other approaches.

EFFECT OF THE Ba VACANCY ON THE FLUX PINNING IN YBa x Cu 3 O 7-δ SYSTEM

A series of Y123 single phase samples with various Ba vacancy concentration was prepared by making their Ba contents deviate from the stoichiometric composition. The measurements of their structure, superconductivity and flux pinning behaviour were systematically carried out. It is found that, compared with YBa2Cu3O6.96 sample, the strength of the flux pinning in YBaxCu3O7-δ (1.8 ≤ x<2.0) samples is increased, and that there is an optimum value of Ba vacancy concentration for the maximum flux pinning force density. The possible origin of the flux pinning centers to determine the flux pinning behavior at higher field is discussed in detail. We suggest that the flux pinning effect at lower field may stem from the interaction between the vortex and the surfaces of grains, and that the flux pinning mechanism at higher field belongs to the core interaction.