Some Important Features of Manganites

Manganites are very complex systems because of interplay among charge, spin, orbital and lattice degrees of freedom. To come closer to the understanding of its nature, we discuss its three important features: 1) correlation between magnetization and electrical resistivity in the same temperature range;2) detection of chemical constitution and the arrangement of Mn3+ and Mn4+ ions at different hole concentrations;and 3) how electrical current flows through double exchange in manganites. The first feature will be discussed for three-dimensional manganies. The features 2 and 3 are inscrutable in three-dimensional manganites. So they will be discussed for one-dimensional manganites and then generalized. One-dimensional solid has been discussed because it may give a see-through picture of various aspects of manganites. All the discussions will be done through a representative example of La1-xCaxMnO3, because it is the intermediate bandwidth manganite;has been most widely investigated and has the highest magnetoresistance. If two things: 1) magnetization and resistivity as a function of temperature at various magnetic fields;and 2) phase diagrams of other manganites are known, their properties can be understood by the discussion of the three features mentioned above.

Model of Preformed Hole-Pairs in Cuprate Superconductors

A model of preformed hole-pairs in cuprate superconductors has been proposed based on some experimental results i.e., 1) electron paramagnetic resonance spectra of quenched superconductors which show very frequently the fragment (CuO)4 broken off from the CuO2 layer in the structure, 2) 41 meV peak observed in neutron diffraction and nuclear magnetic resonance spectra of superconductors, 3) Heisenberg exchange interaction leading to ferromagnetism observed in CuO which is an essential ingredient of all superconductors and some generally accepted conclusions i.e., a) that the order parameter in superconductors has dx2–y2 symmetr and b) coherence length is of the order of 15 - 20 Angstrom. Heisenberg exchange interaction between two (CuO4) plaquettes each containing a lattice hole binds the two holes which are the charge carriers in the cuprate superconductors. It is not very clear whether the hole-pair is in the triplet or singlet state, but the triplet state is supported by the experimental observation of ferromagnetism in the parent material CuO. The proposed hole-pair singlet is different from Zhang-Rice singlet.

在印度门诊病人中比较咳嗽2周与3周为痰检筛选标准对提高涂阳结核病例发现的效果

背景﹕印度六个区的政府卫生机构。目的﹕估算咳嗽的发生率及比较咳嗽≥2周或≥3周的门诊病人涂阳结核病的发现情况。设计﹕由经过培训的卫生人员询问每一病人的咳嗽情况,咳嗽≥2周者进行痰显微镜检查。结果﹕在调查的55561例门诊病人中,咳嗽≥2周者2210例(4%),其中267例为涂阳结核病,而咳嗽≥3周者则为182/1370。31%病人无咳嗽主诉,涂阳可能性显著低于有咳嗽主诉者(各为45/680[7%]、222/1530[15%],P<0.001)。但在267例涂阳病人中占45例。如以咳嗽≥2周为痰显微镜检查筛选标准,估计在初级、二级卫生机构每天进行涂片检查各为8次与19次,如以咳嗽≥3周为标准则各为5次及12次。结论﹕认真询问门诊病人的咳嗽情况以及将咳嗽≥3周改为≥2周作为痰检的筛选标准可以显著提高涂阳结核病人的发现。但在全国实施此改变前需对其进行规划可行性评估。