Hole density dependent magnetic structure and anisotropy in Fe-pnictide superconductor

The competition between different magnetic structures in hole-doped Fe-pnicitides is explored based on an extended five-orbital Hubbard model including long-range Coulomb interactions.Our results show that the stabilized magnetic structure evolves with increasing hole doping level.Namely,the stripe antiferromagnetic phase dominates at zero doping,while magnetic structures with more antiferromagnetic linking numbers such as the staggered tetramer,staggered trimer,and staggered dimer phases become energetically favorable as the hole density increases.At a certain doping level,energy degeneracy of different magnetic structures appears,indicating strong magnetic frustration and magnetic fluctuations in the system.We suggest that the magnetic competition induced by the hole doping may explain the fast decrease of the Neel temperature TNand the moderately suppressed magnetic moment in the hole doped Fe-pnicitides.Moreover,our results show a sign reversal of the kinetic energy anisotropy as the magnetic ground state evolves,which may be the mechanism behind the puzzling sign reversal of the in-plane resistivity anisotropy in hole-doped Fe-pnicitides.

Research on dependence of resistivity changing anisotropy on microcracks extending in rock with experiment

Multiple electrodes are arranged on the surfaces of cubic granite samples of different sizes according to pre-designed patterns. Smples are fully saturated with water in vacuum. Waterproof insulation glue is coated on the measuring surfact and asmall parts (including two arrises) of the adjacent surfeces of the saturated sample to ensure that the electric current flows only within the sample through the connection between the electrodes. The multiple electrodes are combined form arrays of different direction and specing with symmetrical four-clectrode method according to need of measuring of resistivity changing anisotropy, electric profiling and electric sounding. The samples are placed into container filled with water. The samples are uniaxially compressed along the direction parallel to the longest dimension of the cubic, and the variation of resistivity during the whole loading process is observed. In the experiments, some samples are loaded to rupture with macro-fractures, some are only loaded to the Stage, which shows obvious Precursors in variation of resistivity associated with the indication of forthcoming rupture. Finally a quantitative comparison batween the dominant orientation of pre-existing cracks in photo-micrography of unruptured Samles and those macro-fractures in ruptured sample is made, together with theirrespective resistivity changing anisotropy behaviors. The experimental results are the following: ① For measuring points in areas that are passed by craks or rupture bands, the directions of principal anisotropy axes dedued from four kinds of combined equation sets are essentially identical with each other, and accord with the orientation of cracks or main rupture bands approximately. For measuring points in areas without crack or rupture band passing through, either the directions of calculated principal anisotropy axes by different combinatorial arrays are inconsistent with each other, or the principal anisotropy axis cannot be determined, especially in the cases where the crack plane is parallel to the measuring surface.② The dominant orientation of microfractures or rupture bands shown from micrographs is close to the direction of principal anisotropy axis along which the variation in resistivity is the greaest.③ The results of electric profiling can be used for detecting the localization of cracks.

The variations of the degree of ground resistivity anisotropy during the Tangshan earthquake

Based on the geoelectric observation data of the 1976 Tangshan earthquake of M=7. 8 compiled by the Research Group on the Geoelectricity of the Tangshan Great Earthquake, a dimensionless factor S has been defined as the degree of ground resistivity anisotropy. The S values during the generation process of that earthquake have been calculated and their variations have been analyzed. The result has showed that the variation of the degree of ground resistivity anisotropy existed throughout the process of generation and occurrence of the Tangshan earthquake and the features of its pattern are representative. The S value can therefore be taken as a new precursory factor of earthquakes which can be applied together with other dimensionless factors in the analysis and prediction of earthquakes. A physical explanation of the variation of the S value has also been given.

Dynamic evolution patterns of the degree of ground resistivity anisotropy and the sets mogenic process

Following a new train of thinking, this paper has explored first the potential information in the ground resistivitydata observed by the existing geoelectric observation system, investigated and proposed a new dimensionlessgeoelectric precursor factor, the degree of ground resistivity anisotropy, S, and studied the characteristics of dynamic evolution pattern of S during the seismogenic process. The results show that, during the seismogenic process, the degree of ground resistivity anisotropy (S) displays h process of 'normal' → 'abnormal strengthening(amplitude, range)' → 'abnormal weakening'→ 'earthquake occurrence'→ 'normal'. The earthquake wouldoccur at the time when the S value has entered the late stage of strengthening and turns to weaken and in the gradient belt on the margin ofS anomaly region. The dynamic evolution pattern ofS reflects the changes of the tectonicstress field during the seismogenic process. Therefore, it would be possible to trace the process of earthquake generation and occurrence from the dynamic evolution pattern ofS so as to service eaJ'thquake prediction.

Experiments on exploring and monitoring landslip-mass using geoelectric resistivity observations

In the experiments, a high-density resistivity method is used to explore the electric structure of landslip mass, and a resistivity-changing anisotropy method is used to monitor the orientation and speed of main fracture extending of landslip mass. The results are as follows. 1 The exploring experiments have verified a part of creep deformation borderline, the depth and thickness of groundwater horizon, and the property of superstrata in the landslip mass investigated formerly, which have proved that the landslip belts contain rich groundwater; 2 The main fracture extending orientation inferred from the resistivity-changing anisotropy accords with the strike of fracture belt of landslip mass deduced from GPS displacement. Moreover, the changing rates of resistivity-changing anisotropy coefficient matches with the changing speeds of deep displacement of landslip mass were measured by suing clinometer in the borehole.