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本文报道,通过对Y1-xCaxBa2Cu3-xMxO7-δ(M=Fe,Ni)体系样品的晶体结构、氧含量、正常态电阻率与温度的关系,以及超导转变温度等测量,并与YBa2Cu3-xMxO7-δ(M=Fe,Ni)体系进行比较,发现Y1-xCaxBa2Cu3-xFexO7-δ体系的Tc显著地高于相应x值的YBa2Cu3-xFexO7-δ体系,而Y1-xCaxBa2Cu3-xNixO7-δ体系则相反,Tc低于仅Ni替代的体系,表明Ca和Fe同时替代时两者引起的载流子浓度(nH)变化相互补偿,抑制了仅Fe替代时引起的nH和Tc急剧下降;而作Ca和Ni同时替代时主要的不是两者引起载流子浓度变化的相互补偿,Ca和Ni替代效应之间的关联较弱。作者认为,对Y1-xCaxBa2Cu3-xFexO7-δ体系属于CuO2平面外的元素替代,这时载流子浓度是决定Tc的主要因素;而对Y1-xCaxBa2Cu3-xNixO7-δ体系,由于Ni2+离子主要占据Cu(Ⅱ)位,它导致磁拆对效应,Ni2+离子的拆对效应是引起Tc下降的直接原因。
关键词: 相似文献
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本文报道了TixY1-xBa2Cu3O7-δ(x=0.2和0.4)在4.2—300K的红外、远红外光谱。在50—360cm-1波段内发现七个反射峰。它们分别与Ba,Cu,O离子团,Y,O离子团。Ti,O离子团,以及Cu—O键的振动有关,对于x=0.2的样品,存在两个反转结构。对于x=0.4的样品,则仅发现一个反转结构。在红外光谱中观察到六个反对峰和三个吸收峰,它们的强度大多随Ti含量的增加而增强。与Y1Ba2Cu3O7-δ的光谱结果比校,讨论了声子峰及反转结构的物理起因。
关键词: 相似文献
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本文报道了对PrxY1-xBa2Cu3O7-δ体系超导体红外光谱研究的结果。发现在整个光谱范围内(中红外和远红外),Pr含量增加的直接结果是大部分反射峰的强度增强。在远红外区,Y原子的“指纹”性声子190cm-1峰的频率位置与Pr含量无关,证明了Pr在该体系中价态要比Y的+3价高,介于+3价与+4价之间。通过对各个声子结构的指认,我们发现:随Pr含量增高,声子活性增强的振动模必定是属于CuO2面内的或与CuO2平面平行的振动模,与YBa2Cu3O7-δ体系中减少氧的情况类似。这意味着CuO2面内的振动对于高温超导性的重要作用。
关键词: 相似文献
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Using 1+1 soliton-like polaron model of a deformable continuum, the critical temperature and the energy gap in high-temperature superconductors Y1-xCaxBa2Cu3-xMxO7-δ(M=Fe,Ni) are studied in the framework of finite temperature Green's function theory. The ratio 2Δ/kBTc=3.9 is obtained. Theoretical results are found in good agreement with the experimental data. 相似文献
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SUPERCONDUCTING TRANSITION TEMPERATURES AND NORMAL-STATE ELECTRICAL RESISTIVITIES OF Tm1-xPrxBa2Cu3O7-δ 下载免费PDF全文
曹效文 《物理学报(海外版)》1994,3(8):583-588
Superconducting transition temperature Tc and normal-state electrical resistivities ρ of the Tm1-xPrxBa2Cu3O7-δ system have been measured. The results indicate that Tc remains constant for x = 0 to 0.08 before it drops steadily with higher x. Following the Abrikosov-Gor'kov model, a total suppression of superconductivity occurs at a critical Pr concentration xcr≈0.56. It is found for the first time, as far as we know, that the logarithmic resistivity at 270 K increases linearly with increasing x, yielding ρ= ρ0eax. Judging from the relation between dρ/dT and x, a metal to insulator transition occurs just beyond xcr. 相似文献
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In this paper it is reported that the measurement of the bulk positron lifetime as a function of substitution content x in the temperature range from 70 to 220K was performed in two high-Tc superconducting systems, Y1-xCaxBa2Cu3-xFexO7-δ and Y1-xCaxBa2Cu3O7-δ. It was found that τB of both systems decreases significantly with x and the temperature dependence of τB is very weak in normal state. In lower temperature region (Tc), a dramatic x-dependent temperature variation of τB was observed in the Ca-substituted system: from a decrease of τB with decreasing T to an increase of τB. With increasing x, the temperature dependence of τB remains weak in the Ca- and Fe- substituted systems. Compared with the experimental data of positron lifetime in other substituted systems and the calculation of the positron density distribution, the authors suggest that positron bulk lifetime spectra behaviour can be interpreted by the physical model based on the transfer of electron density between the CuO2 planes and Cu-O chains. Therefore, the study of positron lifetime spectra provides a useful means to detect the local charge density and to study the correlation between the electronic structure and the high-Tc superconductivity. 相似文献
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利用正电子湮没实验,结合x射线衍射(XRD)结构分析,研究了具有混合稀土特征的(Y1 -xGdx)Ba2Cu3O7-δ系列样品. XRD 实验结果表明,半径较大的Gd离子Y位替代使得样品晶胞参数和晶胞体积增大,但所有样品 仍保持与YBa2Cu3O7-δ(YBCO)样品相同的单相正交结 构. 正电子湮没实验表明,正电子各寿命参数表现出很强的Gd替代依赖关系. 从正电子实验 结果出发,计算了Cu-O链区局域电子密度ne的变化. 结果表明,局域电子密度n e随Gd含量x的增加而减小,而超导转变温度Tc随局域电子密度ne的减小而增加,这种局域电子密度ne与超导电性的关联是与铜位替代 完全不同的,且可能是近年来人们关于混合稀土铜氧化物体系具有较高临界电流密度的原因 之一. 该研究结果为铜氧化物超导体的应用和机理研究提供了相应的正电子实验资料.
关键词:
超导电性
正电子湮没
相结构
局域电子密度 相似文献
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曹效文 《物理学报(海外版)》1994,3(9):697-701
The temperature dependence of resistivity in Dy1-xPrxBa2Cu3O7-δ system with x>0.6 was measured. The experimental results show that Pr substitution leads to the localization of mobile holes and such a localization is enhanced with increasing Pr concentration. The gradually enhancing of localization induces Anderson transitions one by one in this system, including the transition from the conduction by excitation of holes to the one by thermal activation hopping between localized states, the so called Anderson transition type-I, and the transition from nearest neighbor hopping (NNH) to variable range hopping (VRH), the Anderson transition type-II, and the Anderson transition type-lI from 3D to 2D. 相似文献
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采用高温固相法制备出Ca1.97-xSrxNb2O7:3%Eu3+(x=0.10,0.50,1.0,1.5,1.97)红色荧光粉,研究了Ca1.97-xSrxNb2O7:3%Eu3+的发光特性及Sr2+的浓度对该荧光粉发光性质的影响。随着Sr2+浓度的改变,Ca2-xSrxNb2O7:Eu3+的XRD呈现不同的相。Ca1.97Nb2O7:3%Eu3+(x=0)的激发光谱中,302 nm附近的强宽带来自于O2-→Eu3+电荷转移跃迁,272 nm附近的肩峰来自于NbO7-6基团的电荷转移跃迁,350~600 nm范围内的锐锋属于Eu3+的特征4f-4f组内跃迁。在398 nm激发下,发射光谱的最强峰位于616 nm,属于Eu3+的5D0→7F2电偶极跃迁,发射出强烈的红光。当Ca2+逐渐被Sr2+取代时,Ca2-xSrxNb2O7:Eu3+的各激发峰的强度先提高后降低,且O2-→Eu3+电荷转移跃迁发生明显红移。少量Sr2+的掺杂可以有效提高Ca2-xSrxNb2O7:Eu3+ 的红光发射强度,当x=0.01时该荧光粉的红光发射达到最强,可以被紫外LED芯片激发。 相似文献
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研究了La2O3对Yb:Y2O3透明陶瓷光谱性能的影响,添加适量La2O3以后,Yb:Y2O3透明陶瓷的吸收峰和发射峰的位置不变,但由于La3+的离子半径大于Y3+的离子半径,在Y2O3中引入La3+离子后,导致Y2O3晶格常数变大,晶场强度变弱,同时降低了Y2O3晶体的有序度,致使发射峰强度有所下降,发射截面变小.过量的La2O3(x=0.16)造成Yb3+激活离子发射强度明显下降;其荧光寿命在添加La2O3后总体增大45%—60%.
关键词:
氧化镧
氧化钇
透明陶瓷
光谱性能 相似文献
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磁通钉扎性能对GdBa2Cu3O7-δ超导块材的实际应用具有重要的影响, 而引入合适的第二相粒子可以改善GdBa2Cu3O7-δ 超导块材的磁通钉扎性能.本文采用顶部籽晶熔融织构法成功地制备出纳米微粒BaFe12O19(<100 nm)掺杂的超导块材, 样品的最终组分为Gd123+ 0.4 Gd211+ x BaFe12O19 (x=0, 0.2 mol%, 0.4 mol%, 0.8 mol%)+ 10 wt%Ag2O+ 0.5 wt%Pt. 通过研究不同掺杂量的BaFe12O19微粒对GdBa2Cu3O7-δ 超导块材微观结构和超导性能的影响, 结果表明当掺杂量为0.2 mol%时, 样品的临界电流密度几乎在整个外加磁场下都有明显的提高.在零场下, 临界电流密度达到5.5× 104 A/cm2. 纳米微粒BaFe12O19不仅可以保持掺杂前的化学组成, 作为有效的钉扎中心存在于超导块材中, 并且能够改善Gd2BaCuO5粒子的分布和细化Gd2BaCuO5粒子, 使Gd2BaCuO5粒子的平均粒径由未掺杂时的1.4 μ m减小到掺杂后的0.79 μ m, 进而提高了超导块材的临界电流密度和俘获磁场, 明显提高了GdBa2Cu3O7-δ 超导块材的超导性能.临界温度TC也有所提升, 并能够维持在92.5 K左右. 该结果为进一步研究纳米磁通钉扎中心的引入并改善GdBa2Cu3O7-δ 超导块材的性能有着重要的意义. 相似文献