Magnetocrystalline anisotropic effect in GdCo1−x_{1-x}Fex_xAsO (x=0,0.05x = 0, 0.05)

From a systematic study of the electrical resistivity $\rho(T,H)$, magnetic susceptibility $\chi(T,H)$, isothermal magnetization $M(H)$ and the specific heat $C(T,H)$, a temperature-magnetic field ($T$-$H$) phase diagram has been established for GdCo$_{1-x}$Fe$_x$AsO ($x = 0$ and $0.05$) polycrystalline compounds. GdCoAsO undergoes two long-range magnetic transitions: ferromagnetic (FM) transition of Co $3d$ electrons ($T_\textup{C}^\textup{Co}$) and antiferromagnetic (AFM) transition of Gd $4f$ electrons ($T_\textup{N}^\textup{Gd}$). For the Fe-doped sample ($x=0.05$), an extra magnetic reorientation transition takes place below $T_\textup{N}^\textup{Gd}$, which is likely associated with Co moments. The two magnetic species of Gd and Co are coupled antiferromagnetically to give rise to ferrimagnetic (FIM) behavior in the magnetic susceptibility. Upon decreasing the temperature ($T < T_\textup{C}^\textup{Co}$), the magnetocrystalline anisotropy breaks up the FM order of Co by aligning the moments with the local easy axes of the various grains, leading to a spin reorientation transition at $T_\textup{R}^\textup{Co}$. By applying a magnetic field, $T_\textup{R}^\textup{Co}$ monotonically decreases to lower temperatures, while the $T_\textup{N}^\textup{Gd}$ is relatively robust against the external field. On the other hand, the applied magnetic field pulls the magnetization of grains from the local easy direction to the field direction via a first-order reorientation transition, with the transition field ($H_\textup{M}$) increasing upon cooling the temperature.Comment: accepted by physical Review B 6 figures and 7 page

Theoretical Analysis of Pressure-Dependent K₀ for Normally Consolidated Clays Using Critical State Soil Models

The coefficient of earth pressure at rest (K0) for normally consolidated clays increases nonlinearly with increasing consolidation pressure toward a steady value under high pressure rather than remaining constant. Analytical expressions for evaluating pressure-dependent K0 were derived from three representative critical state soil models: modified Cam-clay model (MCC), original Cam-clay model (OCC), and clay and sand model (CASM). In formulations, the authors relaxed a well-adopted assumption that stress ratio is kept constant during one-dimensional (1D) compression. It is found that the constant stress ratio, corresponding to the well-adopted assumption, is essentially a limit value of the stress ratio as predicted by MCC and CASM under high pressure during 1D compression. The predicted relationship between K0 and consolidation pressure is significantly affected by the critical state stress ratio. Without considering the effect of high pressure, the value of K0 may be considerably underestimated. The results predicted by the proposed formula based on CASM agree well with experimental data, showing the capability of this formula for predicting pressure-dependent K0

Upper critical field and thermally activated flux flow in single crystalline Tl0.58_{0.58}Rb0.42_{0.42}Fe1.72_{1.72}Se2_2

The upper critical field $\mu_0H_{c2}(T_c)$ of Tl$_{0.58}$Rb$_{0.42}$Fe$_{1.72}$Se$_2$ single crystals has been determined by means of measuring the electrical resistivity in both a pulsed magnetic field ($\sim$60T) and a DC magnetic field ($\sim$14T). It is found that $H_{c2}$ linearly increases with decreasing temperature for $\textbf{H}$$\parallel$$c$, reaching $\mu_0H_{c2}^{\textbf{H}\parallel c}(0\textrm{K})\simeq60$ T. On the other hand, a larger $\mu_0H_{c2}(0\textrm{K})$ with a strong convex curvature is observed for $\textbf{H}$$\perp$$c$ ($\mu_0H_{c2}^{\textbf{H}\perp c}$(18K)$\simeq$60T). This compound shows a moderate anisotropy of the upper critical field around $T_c$, but decreases with decreasing temperature. Analysis of the upper critical field based on the Werthamer-Helfand-Hohenberg (WHH) method indicates that $\mu_0H_{c2}(0\textrm{K})$ is orbitally limited for $\textbf{H}$$\parallel$$c$, but the effect of spin paramagnetism may play an important role on the pair breaking for $\textbf{H}$$\perp$$c$. All these experimental observations remarkably resemble those of the iron pnictide superconductors, suggesting a unified scenario for the iron-based superconductors. Moreover, the superconducting transition is significantly broadened upon applying a magnetic field, indicating strong thermal fluctuation effects in the superconducting state of Tl$_{0.58}$Rb$_{0.42}$Fe$_{1.72}$Se$_2$. The derived thermal activation energy for vortex motion is compatible with those of the 1111-type iron pnictides.Comment: 7 pages, 6 figure

Optimization of Tertiary Alkaloids Separation from Corydalis yanhusuo by Macroporous Resins

Corydalis yanhusuo is used widely for the treatment of gastralgia, costalgia and dysmenorrhea in Chinese medicine. The alkaloid is the main active ingredient of C. yanhusuo. Response surface methodology was applied to optimize the separation and purification process for alkaloids by AB-8 resin-packed chromatogram column. The optimal conditions were found to be as follows: height-diameter ratio of AB-8 resin-packed chromatogram column, 10.50; concentration and pH of feed sample solution, 1.12 mg mL–1 and 7.16, respectively. The gradient elution program was 30 % ethanol for 2 BV (bed volume) followed by 80 % of ethanol for 5 BV at flow rate of 3 mL min–1. After the AB-8 resin treatment, the contents of alkaloids and tetrahydropalmatine were increased respectively from 25.20 % and 2.12 % to 58.25 % and 6.58 %, the recovery of alkaloids and tetrahydropalmatine were 85.40 % and 65.21 %, respectively. The results indicated that the optimization of alkaloid separation from C. yanhusuo by macroporous resins is feasible and efficient