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曹光旱, Guanghan
PHYSICAL REVIEW B, VOLUME 64, 214514,-0001,():
-1年11月30日
The thiospinel system Cu1-xZnxIr2S4 (0≤x≤0.9) has been studied by x-ray diffraction, electrical resistivity, and magnetic susceptibility measurements. The parent compound CuIr2S4, being metallic at room temperature, undergoes a structural phase transition towards lower symmetry around 230 K and becomes an insulator at the low temperature. The Zn substitution for Cu was found to drastically suppress the metal-to-insulator (MI) transition, resulting in the appearance of superconductivity. The MI transition temperature TMI and the extent of the structural distortion both decrease with increasing x until the phase transition is completely suppressed at x~0.4. In the region of x≤0.4, the cubic spinel phase coexists with the low-symmetry phase below TMI. For the metallic phase, the change of the Pauli paramagnetic susceptibility indicates the hole-filling mechanism due to an excess electron from the Zn substitution for Cu. The insulating state of the low-symmetry phase is tentatively explained in terms of charge ordering combined with the Ir4+ dimerization. Bulk type-II superconductivity below 3.4 K is observed for 0.25≤x≤0.8 samples. The superconducting transition temperature decreases with increasing the Zn content. The abnormal behavior of the normal-state resistivity below 200 K for 0.3≤x≤0.5 samples suggests modification of the electronic states, which might be related to the occurrence of superconductivity.
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曹光旱, Guanghan
J. Phys.: Condens. Matter 15 (2003) L519-L525,-0001,():
-1年11月30日
We report the electrical, magnetic and thermal measurements on a layered cobalt oxyhydrate Na0.31CoO2·1.3H2O. Bulk superconductivity at 4.3 K has been confirmed; however, the measured superconducting fraction is relatively low probably due to the sample's intrinsic two-dimensional characteristic. The compound exhibits weak-coupled and extreme type-II superconductivity with an average energy gap △a(0) and a Ginzburg–Landau parameter κof ∼0.50 meV and 140, respectively. The temperature dependence of the normalized electronic specific heat in the superconducting state gives a clue to the superconducting gap structure.
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【期刊论文】Suppression of the metal-insulator transition in the spinel Cu1−xInxIr2S4 system
曹光旱, Guanghan
PHYSICAL REVIEW B 72, 125128 (2005),-0001,():
-1年11月30日
The thiospinel Cu1−xInxIr2S4(0≤x≤0.25) system was studied by the measurements of crystal structure, electrical resistivity, and magnetic susceptibility. The parent compound was known to exhibit an intriguing first-order metal-insulator (MI) transition with a simultaneous spin-dimerization and charge ordering at ~230 K with decreasing temperature. Upon indium doping on the copper site, the conduction holes of the metallic phase are depleted, or the doped electrons occupy the antibonding state of the insulating phase, suppressing the MI transition. Moreover, the first-order transition is changed into a higher-order one for X≥0.2. Our experimental data suggest that the higher-order phase transition is associated with an electronic transformation from small polarons to small bipolarons. Comparing the doping effects of Zn, Cd, and In, we found that the variations of the electrical and magnetic properties depend on the lattice size, i.e., the suppression of the MI transition becomes weaker for an enlarged lattice. This lattice size effect is mainly explained in terms of the electron-phonon interactions, which is enhanced by the band narrowing due to the larger ionic size of the dopants.
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