Zhaosheng Wang

Surface phase separation in nanosized charge-ordered manganites

Recent experiments showed that the robust charge ordering in manganites can be weakened by reducing the grain size down to nanoscale. Weak ferromagnetism was evidenced in both nanoparticles and nanowires of charge-ordered manganites. To explain these observations, a phenomenological model based on surface phase separation is proposed. The relaxation of superexchange interaction on the surface layer allows formation of a ferromagnetic shell, whose thickness increases with decreasing grain size. Possible exchange bias and softening of the ferromagnetic transition in nanosized charge-ordered manganites are predicted.

Superconductivity at 36 K in gadolinium-arsenide oxides GdO 1− x F x FeAs

In this paper we report the fabrication and superconducting properties of GdO1−xFxFeAs. It was found that when x is equal to 0.17, GdO0.83F0.17FeAs is a superconductor with the onset transition temperature Tcon ≈ 36.6 K. Resistivity anomaly near 130 K was observed for all samples up to x = 0.17, and such a phenomenon is similar to that of LaO1−xFxFeAs. Hall coefficient indicates that GdO1−xFxFeAs is conducted by electron-like charge carriers.

Fishtail effect and the vortex phase diagram of single crystal Ba0.6K0.4Fe2As2

By measuring the magnetization hysteresis loops of superconducting Ba0.6K0.4Fe2As2 single crystals, we obtained the high upper critical field and large current carrying ability, which point to optimistic applications. The fishtail (or second peak) effect is also found in the material, and the position of the vortex pinning force shows a maximum at 1∕3 of the reduced field, being consistent with the picture of vortex pinning by small size normal cores in the sample. Together with the resistive measurements, for the first time the vortex phase diagram is obtained for superconductor Ba0.6K0.4Fe2As2.By measuring the magnetization hysteresis loops of superconducting Ba0.6K0.4Fe2As2 single crystals, we obtained the high upper critical field and large current carrying ability, which point to optimistic applications. The fishtail (or second peak) effect is also found in the material, and the position of the vortex pinning force shows a maximum at 1∕3 of the reduced field, being consistent with the picture of vortex pinning by small size normal cores in the sample. Together with the resistive measurements, for the first time the vortex phase diagram is obtained for superconductor Ba0.6K0.4Fe2As2.

Roles of multiband effects and electron-hole asymmetry in the superconductivity and normal-state properties of Ba(Fe(1-x)Cox)(2)As-2

We report a systematic investigation, together with a theoretical analysis, of the resistivity and Hall effect in single crystals of Ba(Fe1-xCox)(2)As-2 over a wide doping range. We find a surprisingly great disparity between the relaxation rates of the holes and the electrons in excess of one order of magnitude in the low-doping, low-temperature regime. The ratio of the electron to hole mobilities diminishes with temperature and doping (away from the magnetically ordered state) and becomes more conventional. We also find a straightforward explanation of the large asymmetry (compared to cuprates) of the superconducting dome: in the underdoped regime the decisive factor is the competition between antiferromagnetism and superconductivity, while in the overdoped regime the main role is played by degradation of the nesting that weakens the pairing interaction. Our results indicate that spin fluctuations due to interband electron-hole scattering play a crucial role not only in the superconducting pairing but also in the normal transport.

Evidence for two energy gaps in superconducting Ba0.6K0.4Fe2As2 single crystals and the breakdown of the Uemura plot.

We report a detailed investigation on the lower critical field Hc1 of the superconducting Ba0.6K0.4Fe2As2 (FeAs-122) single crystals. A pronounced kink is observed on the Hc1(T ) curve, which is attributed to the existence of two superconducting gaps. By fitting the data Hc1(T ) to the two-gap BCS model in full temperature region, a small gap of ∆a(0) = 2.0 ± 0.3 meV and a large gap of ∆b(0) = 8.9± 0.4 meV are obtained. The in-plane penetration depth λab(0) is estimated to be 105 nm corresponding to a rather large superfluid density, which points to the breakdown of the Uemura plot in FeAs-122 superconductors.

Flux dynamics and vortex phase diagram in Ba(Fe1-xCox)(2)As-2 single crystals revealed by magnetization and its relaxation

Magnetization and its relaxation have been measured in Ba(Fe1-xCox)(2)As-2 single crystals at various doping levels ranging from very underdoped to very overdoped regimes. A sizable magnetization-relaxation rate has been observed in all samples, indicating a moderate vortex motion and relatively small characteristic pinning energy. Detailed analysis leads to the following conclusions. (1) A prominent second-peak (SP) effect was observed in the samples around the optimal doping level (x approximate to 0.08), but it becomes invisible or very weak in the very underdoped and overdoped samples. (2) The magnetization-relaxation rate is inversely related to the transient superconducting current density revealing the nonmonotonic field and temperature dependence through the SP region. (3) A very sharp magnetization peak was observed near zero field which corresponds to a much reduced relaxation rate. (4) A weak temperature dependence or a plateau of relaxation rate was found in the intermediate-temperature region. Together with the treatment of the generalized inversion scheme, we suggest that the vortex dynamics is describable by the collective pinning model. Finally, vortex phase diagrams were drawn for all the samples showing a systematic evolution of vortex dynamics.

Growth and characterization of A1-xKxFe2As2 (A = Ba, Sr) single crystals with x = 0-0.4

Single crystals of A1−xKxFe2As2 (A = Ba, Sr) with high quality have been grown successfully by an FeAs self-flux method. The samples have sizes up to 4 mm with flat and shiny surfaces. The x-ray diffraction patterns suggest that they have high crystalline quality and c-axis orientation. The non-superconducting crystals show a spin-density-wave (SDW) instability at about 173 and 135 K for the Sr-based and Ba-based compound, respectively. After doping K as the hole dopant into the BaFe2As2 system, the SDW transition is smeared, and superconducting samples of the compound Ba1−xKxFe2As2 (0<x≤0.4) are obtained. The superconductors, characterized by AC susceptibility and resistivity measurements, exhibit very sharp superconducting transitions at about 36, 32, 27 and 23 K for x = 0.40,0.28,0.25 and 0.23, respectively.

Upper critical field, anisotropy, and superconducting properties of Ba1-xKxFe2As2 single crystals

The temperature-dependent resistivity of Ba1-xKxFe2As2 (x=0.23, 0.25, 0.28, and 0.4) single crystals and the angle-dependent resistivity of superconducting Ba0.6K0.4Fe2As2 single crystals were measured in magnetic fields up to 9 T. The data measured on samples with different doping levels revealed very high upper critical fields, which increase with the transition temperature, and a very low superconducting anisotropy ratio Gamma=H-c2(ab)/H-c2(c)approximate to 2. By scaling the resistivity within the framework of the anisotropic Ginzburg-Landau theory, the angle-dependent resistivity of the Ba0.6K0.4Fe2As2 single crystal measured with different magnetic fields at a certain temperature collapsed onto one curve. As the only scaling parameter, the anisotropy Gamma was alternatively determined for each temperature and was found to be between two and three.

Low temperature specific heat of the hole-doped Ba0.6K0.4Fe2As2 single crystals

We report the specific heat (SH) measurements on single crystals of hole doped

Transport properties and asymmetric scattering in Ba1-xKxFe2As2 single crystals

FeAs