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1.
La(2)CuO4-y ceramics containing a few percent of Ca(2+), Sr(2+), and Ba(2+) ions have been prepared. Resistivity and susceptibility measurements exhibit superconductive onsets (as in earlier Ba(2+)-containing samples). The onset temperature La(2)CuO4-y with Sr(2+) is higher and its superconductivity-induced diamagnetism larger than that found with Ba(2+) and Ca(2+). This is proof that the electronic change resulting from alkaline earthdoping, rather than the size effect, is responsible for superconductivity. The ionic radius of Sr(2+) is close to that of La(3+) for which it presumably substitutes. 相似文献
2.
Direct observations of CuO(2) sheet defect structures in superconducting La(2-x)Sr(x)CuO(4), with x in the range 0.05 = x = 0.15, are reported. Electron microscopy shows that extended line defects are generated in {010} planes, along <206> crystallographic directions, by a pure shear mechanism along the edge of the octahedral copper-oxygen units. The line defects are partial screw dislocations, with characteristic displacement vectors of the type <(a/2), 0, (c/6)>, bounding the stacking faults. The existence of this type of defect demonstrates that there is an oxygen deficiency within the CuO(2) layers. However, unlike the open ReO(3) type-related structures, the packing density of the K(2)NiF(4) structure necessarily requires that anion defects be accompanied by the loss of cations of the A type (lanthanum, strontium). Under identical synthesis conditions, no defects are observed in the parent compound La(2)CuO(4). 相似文献
3.
Tarascon JM Greene LH McKinnon WR Hull GW Geballe TH 《Science (New York, N.Y.)》1987,235(4794):1373-1376
Structural, magnetic, and electronic properties of compounds in the series La2-xSrx CuO4-y for 0.05 = x = 1.1, with deltax = 0.025, were studied. Resistance, susceptibility, Meissner, and shielding measurements have revealed superconductivity among several members of the series. For x = 0.15, the transition temperature T(c), measured at the midpoint of the resistive transition, is a maximum at 39.3 K with a width of 2 K. At other compositions the transition is broader and occurs at a lower temperature. Annealing the x = 0.15 sample in oxygen at 500 degrees C increases T(c) to 40.3 K, while annealing at the same temperature under vacuum suppresses the superconductivity. These changes in oxygen content and T(c) are reversible. 相似文献
4.
Crawford MK Farneth WE McCarronn EM Harlow RL Moudden AH 《Science (New York, N.Y.)》1990,250(4986):1390-1394
The oxygen isotope effect on the superconducting transition temperature (alpha(o)) varies as a function of x in La2-xSrxCuO(4) and La2-xBaxCuO(4), with the maximum alpha(o) values (alpha(o) >/= 0.5) found for x near 0.12. This unusual x dependence implies that the isotope effect is influenced by proximity to the Abma --> P4(2)/ncm structural phase transition in these systems. Synchrotron x-ray difaction measurements reveal little change in lattice parameters or orthorhombicity due to isotope exchange in strontium-doped materials where alpha(o) > 0.5, eliminating static structural distortion as a cause of the large isotope effects. The anomalous behavior of alpha(o) in both strontium- and barium-doped materials, in combination with the previously discovered Abma --> P4(2)/ncm structural phase-transition in La(1.88)B(0.12)CuO(4), suggests that an electronic contribution to the lattice instability is present and maximizes at approximately 1/8 hole per copper atom. These observations indicate a dose connection between hole doping of the Cu-O sheets, tilting instabilities of the CuO(6) octahedra, and superconductivity in La(2)CuO(4)-based superconductors. 相似文献
5.
以熟红黄泥为试验材料,研究了在淹水培养过程中不同氧化还原条件下外加Se(Ⅵ)在土壤溶液中的价态转化。结果表明,在两种土壤Eh培养条件下,外加的Se(Ⅵ)约有20%进入土壤固化,80%进入土壤溶液。在低Eh值(175mV )条件下,进入土壤溶液的硒约有90%呈有机态,且较稳定;10%左右的Se(Ⅵ)还原为Se(Ⅳ),此还原以应符合动力学一级方程,半衰期为3.7d。 在高Eh值(450mV)条件下,进入土壤溶液的硒在第一天70.9%呈Se(Ⅵ)态,且稳定;29.0呈有机态,并在培养过程中先氧化成Se(Ⅵ),然后Se(Ⅳ)再氧化成Se(Ⅵ)。 相似文献
6.
Subramanian MA Gopalakrishnan J Torardi CC Askew TR Flippen RB Sleight AW Lin JJ Poon SJ 《Science (New York, N.Y.)》1988,240(4851):495-497
New phases of the type La2-xAx(l+)CUO4-y have been prepared where A(l+) is sodium or potassium. The sodium phases are superconducting for x values from 0.2 to 0.5 at temperatures up to about 40 K. In addition, there are unusual magnetic properties below about 10 K that may be indicative of spin glass behavior. Phases of the type La2-xKxCuO4-y could only be prepared with x values up to about 0.1, and these phases are not superconducting above 4.2 K. 相似文献
7.
对BevalacEOS时间投影室1.0AGeVLa+La碰撞实验,根据以流方向为z轴的质心坐标系中末态粒子方位角分布的非对称性为极小的条件,确定了事件中4种质量范围(A=1、2、3、4)末态粒子的集合侧向流流角。在以流方向为z轴的质心坐标系中研究了不同质量末态粒子极角分布,结果表明质量越大的粒子在流方向分布得越集中,侧向流的“流”值越大。 相似文献
8.
水稻土溶液中Se(Ⅵ)的价态转化条件研究 总被引:3,自引:1,他引:3
为了有效利用土壤硒,以紫潮泥和红黄泥为试验材料,在pH分别为7.5和6.5条件下研究了两种土壤在淹水原原培养和氧化培养过程中Se(Ⅵ)价态转化的条件,结果表明:随着土壤溶液Eh值的变化。Se(Ⅵ)逐渐向有机硒,Se(Ⅵ)转化;Se(Ⅵ)与Se(Ⅵ)相互转化体系的临界电位范围为216.6-74.1mV,同时土壤溶液pH值从6.5上升到7.5,临界电位下降50.1mV;Se(Ⅵ)与Se(Ⅵ)相互转化的氧化还原体系在土壤重要的氧化还原体系序位中处于NO3^--N体系,锰体系之后,大致与铁体系平行。 相似文献
9.
Khalifah P Osborn R Huang Q Zandbergen HW Jin R Liu Y Mandrus D Cava RJ 《Science (New York, N.Y.)》2002,297(5590):2237-2240
We report experimental evidence for a full orbital ordering transition in the two-dimensional lanthanum ruthenate La4Ru2O10. The observable consequences of this orbital ordering include the loss of the Ru local moment, a structural distortion which partitions Ru-O bonds into axially oriented short and long sets, a sharp jump in electrical resistivity, and the opening of a spin gap that is visible in neutron scattering experiments. This is a rare example of a discrete orbital ordering transition in a 4d transition metal oxide and demonstrates that orbital effects can have an influence on the properties of layered ruthenates, a family of compounds that notably includes the p-wave superconductor Sr2RuO4 and the field-tuned quantum critical metamagnet Sr3Ru2O7. 相似文献
10.
Changes in the copper K-edge x-ray absorption spectrum of YBa(2)Cu(3)O(7) across the critical temperature indicate that, accompanying the superconducting transition, the mean square relative displacement of some fraction of the Cu2-O4 bonds becomes smaller or more harmonic (or both), that there may be a slight increase in the associated Cul-O4 distance, and that electronic states involving these atom pairs become more atomic-like. If there is an association between the superconductivity and this lattice instability, then the bridging axial oxygen is of central importnce in determining the high tranition temperature of YBa(2)Cu(3)O(7). Because this structural perturbation will affect the dynamic polrizability of the copper oxygen sublattice, it is consistent with an excitonic pairing mhanism in these materials. 相似文献
11.
12.
Carlin RL 《Science (New York, N.Y.)》1985,227(4692):1291-1295
Each of the following ions with spin S = 3/2, octahedral chromium(III), five-coordinate iron(III), and tetrahedral cobalt(II), possesses a (4)A(2) ground state. The theory describing their paramagnetic properties is therefore the same, though some of the parameters change appreciably from ion to ion. This theory is described along with examples of experimental data. The implications of the paramagnetic properties for the magnetic ordering phenomena exhibited by complexes of these ions are also described. 相似文献
13.
在高定向热解石墨(HOPG)表面上生长少量乙醇分子,在超低温(78K)高真空条件下通过STM针尖进行扫描,在扫描过程中施加4~5V的脉冲电压,乙醇分子或乙醇分子团簇就会带上负电,且能吸引周围的乙醇分子团簇或者乙醇分子链,如果受到吸引的团簇或分子链正好受到一定的扰动,那么该团簇或分子链就会围绕带电团簇转动或摆动,从而构成了靠静电力连接而成的超分子转子和马达.与以往液体中的超分子转子和马达,或者固体表面的单分子转子和马达不一样,这是一种置于固体表面的超分子转子和马达,且在隧穿电子的驱动下维持转动或摆动. 相似文献
14.
在硝酸介质中,Cr(Ⅵ)能显著阻抑甲基红的褪色,据此采用消化法处理土壤样品,甲基红阻抑褪色光度法来测定城固许家庙橘园土壤中的痕量铬.结果显示,橘园土壤中总铬、Cr(Ⅲ)和Cr(Ⅵ)平均含量分别为52.2、8.4、43.8 mg·kg-1.该方法用于土壤中铬的测定,灵敏度高,结果令人满意. 相似文献
15.
以蚕豆为研究对象,探讨了发芽过程中植酸和铁在蚕豆不同部位含量分布及变化情况。植酸随发芽时间延长含量在不断减少,发芽至第8天,子叶中植酸含量减少50%。在发芽过程中,铁含量在子叶中最高,其次是胚轴,而胚根和胚芽中含量最低。蚕豆发芽过程中铁的化学价态发生改变,其中有利于人体吸收的二价铁的比例显著增加。因此,通过发芽处理可提高蚕豆中铁的生物有效性。 相似文献
16.
Matthias BT Geballe TH Andres K Corenzwit E Hull GW Maita JP 《Science (New York, N.Y.)》1968,159(3814):530
Ferromagnetism, antiferromagnetism, or superconductivity has been discovered in most hexa- and dodecaborides. 相似文献
17.
Norton DP Chakoumakos BC Budai JD Lowndes DH Sales BC Thompson JR Christen DK 《Science (New York, N.Y.)》1994,265(5181):2074-2077
Pulsed-laser deposition was used to synthesize artificially layered high-temperature superconductors. Thin-film compounds were formed when the constraint of epitaxy was used to stabilize SrCuO(2)-BaCuO(2) superlattices in the infinite layer structure. Using this approach, two new structural families, Ba(2)Srn-1,Cun+1 O2n+2+delta and Ba(4)Srn-1 Cun+3O2n+6+delta have been synthesized; these families superconduct at temperatures as high as 70 kelvin. 相似文献
18.
F Ronning C Kim DL Feng DS Marshall AG Loeser LL Miller JN Eckstein I Bozovic Z Shen 《Science (New York, N.Y.)》1998,282(5396):2067-2072
An angle-resolved photoemission study is reported on Ca2CuO2Cl2, a parent compound of high-Tc superconductors. Analysis of the electron occupation probability, n(k), from the spectra shows a steep drop in spectral intensity across a contour that is close to the Fermi surface predicted by the band calculation. This analysis reveals a Fermi surface remnant, even though Ca2CuO2Cl2 is a Mott insulator. The lowest energy peak exhibits a dispersion with approximately the &cjs3539;coskxa - coskya&cjs3539; form along this remnant Fermi surface. Together with the data from Dy-doped Bi2Sr2CaCu2O8+delta, these results suggest that this d-wave-like dispersion of the insulator is the underlying reason for the pseudo gap in the underdoped regime. 相似文献
19.
Samples of olivine (Fo(0)Fa(100), Fo(60)Fa(40), Fo(80)Fa(20), and Fo(100)Fa(0)) and of spinel (Fo(50)Fa(50), Fo(2)Fa(100), where Fo is forsterite and Fa is fayalite) were subjected to pressures up to 250 kilobars in a diamond anvil press and were heated in situ up to ~ 1700 degrees C by an infrared beam from a continuous-wave YAG (yttrium-aluminum-garnet) laser. The brightness temperature was determined from the intensity of incandescence of the sample by means of an optical pyrometer. X-ray diffraction patterns of the samples, obtained after quenching and unloading, show conclusively that these compositions disproportionate to (Mg, Fe)O and SiO(s) (stishovite) under these conditions. 相似文献