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1.
Some of the Bi-Ca-Sr-Cu-O compositions show indications of onset of superconductivity in the 200–300 K region, possibly due to the intergrowth of different layered sequences. Contribution No. 529 from the Solid State and Structural Chemistry Unit.  相似文献
2.
    
Members of the PrBa2Cu3O7– system possessing the orthorhombic structure over a wide range of oxygen stoichiometry (=–0.5 to +0.5) have been prepared and characterized. Similar compositions with a tetragonal structure have also been prepared. None of the oxides is superconducting, independent of the structure or stoichiometry. Praseodymium seems to be present to a small extent in the 4+ state in oxygen-excess (negative ) samples. Orthorhombic PrBa2Cu3O7– samples show the presence of twins suggesting that twins arise from orthorhombicity and have no relation to the superconductivity. A neutron diffraction study of a near-stoichiometric sample has shown a disordered orthorhombic structure with 72% occupancy of the 01 (chain) sites and with no interchange between Pr and Ba sites.Contribution No. 503 from the Solid State and Structural Chemistry Unit  相似文献
3.
Analysis of EXAFS data of complex systems containing more than one phase and one type of coordination, has been discussed. It is shown that a modified treatment of EXAFS function as well as the amplitude ratio plots provide useful means of obtaining valuable structural information. The systems investigated are: biphasic Ni+NiO mixture, NiAl2O4 with two coordinations for Ni, NiO+NiAl2O4 mixture, CoS+CoO system and Ni dispersed on Al2O3. The results obtained with these systems have been most satisfactory and serve to illustrate the utility and the applicability of the innovations described in this paper.Contribution No. 522 from Solid State and Structural Chemistry Unit  相似文献
4.
Monte Carlo simulations of a binary alloy with impurity concentrations between 20 and 45 at.% have been carried out. The proportion of large clusters relative to that of small clusters increases with the number of MC diffusion steps as well as impurity concentration. Magnetic susceptibility peaks become more prominent and occur at higher temperatures with increasing impurity concentration. The different peaks in the susceptibility and specific heat curves seem to correspond to different sized clusters. A freezing model would explain the observed behaviour with the large clusters freezing first and the small clusters contributing to susceptibility (specific heat) peaks at lower temperatures.Contribution No. 153 from the Solid State and Structural Chemistry Unit  相似文献
5.
Tunneling studies have been carried out on single crystals of Bi2Ca1−xSrxCu2O8+δ over a wide range of compositions wherein the hole concentration varies by a factor of 2.5. The 2Δ value varies between 25 meV and 75 meV over the composition range studied, but scales with 2Δ/kBTc≈9.5 throughout.  相似文献
6.
FMR measurements have been carried out on several members of the Ln1−xSrxCoO3 (Ln = Rare earth) system. The results show that geff in these systems is around 1.25 independent of x as well as the rare earth ion. It is suggested that this unusual value of geff is due to the localized intermediate-spin Co3+ ions (t52g e1g) located at the top of the π* band.  相似文献
7.
While YSr2Cu3O7 cannot be prepared under ambient conditions, partial substitution of the phosphate group for copper, as in YSr2Cu2.8(PO4)0.2Oy, stabilizes this phase in the orthorhombic structure, but the material is not superconducting. Superconductivity in YSr2Cu2.8(PO4)0.2Oy is obtained by increasing the hole concentration through partial substitution of Y by Ca, as in Y0.7Ca0.3Sr2Cu2.8(PO4)0.2Oy (Tc≈40 K). By incorporating the phosphate group in orthorhombic YBaSrCu3O 7, a stable tetragonal derivative of the formula YBaSrCu2.8(PO4)0.2Oy (Tc≈ 47 K) has been prepared; the Tc increases to 70 K by partial substitution of Y by Ca as in Y0.7Ca0.3BaSrCu2.8(PO4)0.2Oy.  相似文献
8.
Nanoparticles of superconducting (YBCO) () exhibit ferromagnetism at room temperature while the bulk YBCO, obtained by heating the nanoparticles at high temperature (940 °C), shows a linear magnetization curve. Across the superconducting transition temperature, the magnetization curve changes from that of a soft ferromagnet to a superconductor. Furthermore, our experiments reveal that not only nanoparticles of metal oxides but also metal nitrides such as NbN () and δ-MoN () exhibit room-temperature ferromagnetism.  相似文献
9.
Brillouin scattering studies have been carried out on high-quality single crystals of undoped and 0.9% Cr-doped V2O3. The observed modes in both the samples at ∼12 and ∼60 GHz are associated with the surface Rayleigh wave (SRW) and bulk acoustic wave (BAW), respectively. In the undoped sample, the mode frequencies of the SRW and BAW modes decrease as the temperature is lowered from room temperature to the insulator-metal transition temperature (TIM=TN=∼130 K). Below the transition, the modes show hardening. In the doped sample, the SRW mode shows a similar temperature-dependence as the undoped one, but the BAW mode shows hardening from room temperature down to the lowest temperature (50 K). This is the first measurement of the sound velocity below TIM in the V2O3 system. The softening of the SRW frequency from 330 K to TIM can be qualitatively understood on the basis of the temperature-dependence of C44, which, in turn, is related to the orbital fluctuations in the paramagnetic metallic phase. The hardening of the mode frequencies below TIM suggests that C44 must increase in the antiferromagnetic insulating phase, possibly due to the orbital ordering.  相似文献
10.
Magnetic and electrical properties of well-characterized Gd0.5Ba0.5CoO2.9 have been studied carefully in order to compare them with those of other analogous cobaltates of the type Ln0.5A0.5CoO3 (Ln=La, Nd and A=Sr, Ba) which are ferromagnetic. The results show that Gd0.5Ba0.5CoO2.9, which has A-site cation ordering at room temperature, does not become a genuine ferromagnet at low temperatures, but the ferromagnetic interactions observed at 280 K give over to an antiferromagnetic (AFM) state on cooling to 230 K. The AFM state is rendered ferromagnetic on the application of high magnetic fields. The properties can be understood on the basis of phase separation induced by the large A-site cation-disorder, arising from the size mismatch.  相似文献
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