La2CuO4型超导体中电荷有序行为的超声衰减特性研究

本文系统研究了La2CuO4型超导体中电荷有序行为相关的超声衰减特性。在La1．88-y，NdySr0．12CuO4系列样品中观察到由于动态及静态的电荷条纹与晶格之间耦合而导致的超声声速和衰减异常。不同磁场下La1.88Sr0.12-xBaxCuO4系列样品的超声衰减特性表明x≤0．04的样品中存在局域的电荷条纹有序，外加磁场压制超导电性后能够导致局域电荷条纹序的增强。实验结果表明电声子相互作用在铜氧化物超导体的电荷有序现象中扮演着重要的角色。     

La2CuO4型超导体中电荷有序行为的超声衰减特性研究

本文系统研究了La2CuO4型超导体中电荷有序行为相关的超声衰减特性.在La1.88-yNdySr0.12CuO4系列样品中观察到由于动态及静态的电荷条纹与晶格之间耦合而导致的超声声速和衰减异常.不同磁场下La1.88Sr0.12-xBaxCuO4系列样品的超声衰减特性表明x≤0.04的样品中存在局域的电荷条纹有序,外加磁场压制超导电性后能够导致局域电荷条纹序的增强.实验结果表明电声子相互作用在铜氧化物超导体的电荷有序现象中扮演着重要的角色.     

在La_2CuO_(4 y)中超导与长程非公度磁有序的共存

铜氧化物超导体的相图具有如下共同特征 :未掺杂或很低掺杂浓度对应反铁磁区 ;最佳掺杂及欠掺杂的低温相是超导区 ;当掺杂浓度介于上述二者之间时 ,材料既不超导也不具有反铁磁长程序 .一般认为 ,超导与长程磁有序不能共存 .然而 ,在过去的 10年中 ,核磁共振、μ子自旋共振 (μ -ＳＲ)和中子散射实验已经证明 :动态反铁磁关联是高温超导体的固有属性 .最近 ,研究兴趣转移到了观察高温超导体中的静态非公度磁有序 .对于Ｌａ1 6 -ｘＮｄ0 4 ＳｒｘＣｕＯ4 ,Ｔｒａｎｑｕａｄａ等的研究表明 ,存在电荷以及自旋的静态有序 ,并且调制相对于晶格…     

氧掺杂La2CuO4的热导

实验研究了氧掺杂的La2CuO4样品从330K经过慢速冷却和快速冷却后其热导率随温度的变化关系,测量温区为77－300K,在120K以下,两种情况下测得的热导率都随温度的升高而降低。热导率值相同,在120K附近达到极小,120K以上热导率随温度的升高而升高。但经过慢速冷却过程测得的热导值明显低于经过快速冷却过程测得的热导值,分析认为这与两种情况下样品中不同的无序磁散射有关。     

钒掺杂La1.85Sr0.15Cu1-xVxO4＋δ超导体系的结构研究

本文讨论了钒掺杂La1.85Sr0.15Cu1-xVxO4 δ体系的结构.与以往二三价元素掺杂不同,V掺杂的最高掺杂浓度较大,为0.15.精修结果表明,样品中V为 4价,并且全部替代了Cu.晶胞参数a,b随掺杂量x的增加而增加,c随x的增加减小.x=0.10时发生四方-正交相变.随掺杂含量的增加样品结构的变化在红外吸收谱中也有对应.随V含量的增加,500cm-1处的吸收峰的强度和位置都没有变化,这表明V替代在了Cu位.对于350cm-1处的吸收峰,x=0.5时在其附近左侧327cm-1处出现一个吸收峰,当x=0.10时在其右侧400cm-1处又出现一个吸收峰.400cm-1处吸收峰的出现是和四方-正交相变(x=0.10时发生四方-正交相变)相对应的.     

正离子尺寸离散度对掺杂La2CuO4超导电性的影响

固态化学与物理学的结合推动了高温超导体的发现与发展．美国普林斯顿材料研究所的Ｒ．Ｊ．Ｃａｖａ，由于在上述领域创造性的工作，获得了’９７国际超导大会（北京）奖．最近，Ｃａｖａ在一篇为《Ｎａｔｕｒｅ》撰写的短评中讲述了发生于１９８６年１２月的一段亲身经历...     

氧掺杂形成的T‘—R2CuO4（R=Nd,Sm,Eu）化合物中的铁磁性团簇

本文报道了一个由氧掺杂产生的Ｔ’相Ｒ２ＣｕＯ４系列化合物中位于２８Ｋ附近的公有的磁反常规象。     

反尖晶石LiNiVO4中几何失措的抑制与磁有序

对反尖晶石结构的LiNiVO4的磁学性质的研究表明:高温顺磁区研究表明材料存在自旋.轨道耦合作用;在低温区,由于其Li+/Ni2+无序占据八面体间隙位置,从而导致该材料几何失措效应的抑制;相应地体系呈现短程反铁磁有序态,Neel温度为35 K.     

反尖晶石LiNiVO4中几何失措的抑制与磁有序

对反尖晶石结构的LiNiVO₄的磁学性质的研究表明：高温顺磁区研究表明材料存在自旋-轨道耦合作用;在低温区,由于其Li⁺/Ni²⁺无序占据八面体间隙位置,从而导致该材料几何失措效应的抑制;相应地体系呈现短程反铁磁有序态, Neel温度为35K. 关键词： 4')" href="#">LiNiVO₄ 自旋-轨道耦合 短程反铁磁有序 几何失措     

新几何阻挫物质M2(OH)3X的新颖量子磁性——磁有序和涨落在均匀自旋系的共存

几何阻挫引起诸多的未知新颖量子状态,这些新颖量子相的理解预计将带来物理学的突破。笔者通过材料科学研究,偶然发现了新型几何阻挫系列M2（OH）3X[M==Cu,Co,Ni,Mn,Feetc．;X=C1,Br,I]。它们初步展示了新颖的磁性,虽然这些物质是由单一磁性离子组成的均匀晶体,在这些化学均匀系中自旋的有序[如铁磁或反铁磁秩序]和自旋涨落同时共存。因为d电子磁性离子的量子性,本物质系列提供了研究几何阻挫引发的新颖量子特性的绝好舞台。本文综合介绍我们在这一方面最近取得的主要成果。他山之石可以攻玉,新材料的发现往往会带来物理学的新进展,本文同时也例证了材料科学对凝聚态物理的重要性。     

Magnetic correlations and 139La NQR relaxation in La2−x Sr x CuO4−y

Summary ¹³⁹La NQR spin-spin and spin-lattice relaxation measurements in La_2−x Sr _x CuO_4−y are presented, forx ranging from zero to 0.3 and for variable oxygen content, in the temperature range (1.6÷450) K. The data are analysed in terms of the different possible relaxation mechanisms. The dominant relaxation mechanism over most of thex andT range is associated with the Cu⁺⁺−Cu⁺⁺ magnetic correlations and spin dynamics. To explain the data we introduce a heuristic model whereby the degree of magnetic correlations, that would result from the strength of the Cu−Cu exchange coupling, is reduced by mobile charge defects (solitons or holons) causing a liquidlike thermal bath of magnetic excitations described by a concentration-dependent and thermally activated correlation time τ_d. The activation energy for τ_d is inversely proportional to the concentration of mobile defects. It is shown that theT ₁ data yield thex andT dependence of the correlation length, with quantitative estimate in agreement with neutron scattering results. Some implications of the model on other quantities, like magnetic susceptibility,x dependence of the Neel temperature and electric conductivity, are briefly discussed.

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Riassunto Sono presentate misure di rilassamento NQR spin-spin e spin-reticolo per¹³⁹La in La_2−x Sr _x CuO_4−y , perx compreso tra 0 e 0.3 e a contenuto variabile di ossigeno, nell'intervallo di temperaturaT tra 1.6 e 450 K. I risultati delle misure vengono dapprima analizzati alla luce dei possibili meccanismi di rilassamento e si mostra come in un ampio intervallo di valori dix e diT il meccanismo dominante è costituito dalla dinamica e dalle correlazioni degli spin di Cu⁺⁺. Allo scopo di spiegare i risultati ottenuti, viene introdotto un modello euristico in cui il grado di correlazione magnetica che conseguirebbe dall'interazione di superscambio Cu−Cu viene ridotto da difetti mobili (solitoni o holoni) che insediano un bagno termico di eccitazioni magnetiche simile a un liquido, descritto da un tempo di correlazione τ_d che dipende dalla concentrazione di tali difetti ed è termicamente attivato. L'energia di attivazione per τ_d risulta inversamente proporzionale alla concentrazione di difetti mobili. Oltre a giustificare compiutamente le risultanze sperimentali, tale modello consente di ricavare quantitativamente la dipendenza dax e daT della lunghezza di correlazione, che risulta in accordo con quella nota aT=77K dallo scattering di neutroni. Sono anche brevemente discusse le implicazioni del modello su altre grandezze come la suscettività magnetica, la dipendenza della temperatura di Neel dax e la conducibilità elettrica.

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Резюме Приводятся результаты измерений¹³⁹La NQR спин-спиновой и спинрешеточной релаксации в La_2−x Sr _x CuO_4−y дляx в области от 0 доx=0.3 и для различного содержания кислорода, в области температур (1.6÷450)K. Полученные данные анализируются в терминах различных механизмов релаксации. Доминиркющий механизм релаксации для большенства значенийx и в рассматриваемой области температур связан с магнитными корреляциями Cu⁺⁺−Cu⁺⁺ и спиновой динамикой. Для объяснения полученных данных мы вводим эвристическую модель, с помощью которой степень магнитных корреляций, которые зависят от силы Cu−Cu обменной связи, сводится к подвижным заряженным дефектам (солитовы или голоны), обусловленным термостатом магнитных возбужений, описываемых зависящим от концентрации и термически активированным временем корреляции τ_d. Энергия активации для τ_d обратно пропорциональна концентрации подвижных дефектов. Показывается, что данные дляT ₁ определяют зависимость корреляции отx и отT, причем количественная оценка согласуется с данными по рассеянию нейтронов. Вкратце обсуждаются возможные применения предложенной модели к другим величинам: магнитной восприимчивости, зависимости температуры Нееля (точки Кюри) отx и электропроводности.

     

Structural phase transition in substituted La2CuO4 and Pr2CuO4−y

The structural phase transition from orthorhombic (T) phase to tetragonal (T′) phase in substituted La_2−x R _x CuO₄ (R = Pr, Nd, Sm, Eu and Gd) and T′ to T-phase in Pr_2−x M _x CuO_4−y (M = Sr, Ca) has been studied by X-ray diffraction technique. The T-phase of La₂CuO₄ is transferred to T′ phase abruptly atx=0.8, 0.4, 0.4, 0.3 and 0.4 respectively for substitution of Pr, Nd, Sm, Eu and Gd for La in La₂CuO₄ without evidence of the T* phase. The T′ structure of Pr₂CuO₄ (x = 0.0) gets transformed to the T* structure at 30% Ca doping (x=0.6) and then to the T structure at 50% Ca doping (x=1.0), while for Sr-contentx=0.0, 0.4 and 1.0 it shows T′, T* and T structure respectively.     

Superconductivity at 45 K in La2CuO4+δ oxidized by NaClO

An efficient and convenient oxidizing agent, sodium hypochlorite solution, has been used to chemically prepare the superconductor La₂CuO_4+δ at room temperature. The crystallographic data show an enhanced orthorhombic distortion and an increase of the unit-cell volume. The magnetic susceptibility properties reveal a sharp transition beginning at 45 K and the oxidized sample is a homogeneous and single-phase bulk superconductor. The iodometric titration measurements indicate that the oxidized sample has a higher excess-oxygen content of 0.11. Scanning electron microscope observations showed that no grain size difference was observed for the sample before and after oxidation. Received: 15 September 2000 / Accepted: 19 January 2001 / Published online: 3 April 2001     

Neutron diffraction investigation of phase separation in La2CuO4+y single crystals

La₂CuO_4+y single crystals with y=0.03 and 0.04 are investigated with a high-resolution neutron diffractometer in the temperature range 10<T<293 K. Althouogh the excess oxygen concentration y falls in the range of the dissolution gap, the crystal with y=0.03 does not undergo phase separation (PS)—the oxygen distribution in this crystal remains uniform over the entire experimental temperature interval. Macroscopic PS is observed in the crystal with y=0.04. An analysis of the dependence of the widths of the reflection orders on the interplanar separation gives the sizes of the coherent regions of the two coexisting phases: (970±30) Å in the direction of the tetragonal axis c and (1460 645) Å in the basal plane.     

NMR Study on Coexistence of Magnetic Order and Superconductivity in RuSr2GdCu2O8

Electronic and magnetic properties of RuSr₂GdCu₂O₈ have been investigated by resistivity, magnetization and NMR measurements. Magnetic order (T _C=133 K) and superconducting transition (T _S(onset) 52 K) have been confirmed in RuSr₂GdCu₂O₈. We observed two kinds of Ru-NMR signals (the hyperfine fields of ¹⁰¹Ru are 590 kOe and 290 kOe), suggesting a possible charge segregation of Ru⁴⁺(S=1) and Ru⁵⁺(S=3/2) in the RuO₂ layers. Holes can be inherently doped in the CuO₂ layers from the (Ru⁴⁺, Ru⁵⁺)O₂ layers, and the superconductivity can occur under weak magnetic interactions between Ru and Cu spins in RuSr₂GdCu₂O₈.     

Percolative phase separation in La2CuO4+δ and La2−x Sr x CuO4

We report on the formation of conducting phases in slightly doped La₂CuO₄ samples by the existence of a percolative phase separation. Phase separation can be quenched by rapid cooling and can be restored by the application of a 3 T magnetic field. Magnetically polarizable quasiparticles are shown to be formed by hole doping which fuse to form percolative conducting and below 37 K superconducting phases.     

Magnetic properties of La2CuO4 single crystals

Magnetic susceptibility x of La₂CuO₄ crystals have been measured with external magnetic field perpendicular to and within the c plane. A kind of ordering or condensation of the spin system with the spin direction parallel to the c axis has been found although it is difficult to consistently explain all of the observed behaviors. Even above the ordering temperature (⋍ 250K), uniaxial anisotropy of x has also been observed.