磁场对La2－xSrxCuO4单晶热导的影响研究

报道了系列欠掺杂La_2-xSr_xCuO₄（x=0063,0070,0 090,0110,0125）单晶的零场和加磁场情况下ab面和c方向的热导率与温度的关系曲线，测量 温度范围从2到45K. 研究发现ab面和c方向的热导率都受到磁场的压制. 而且在磁场 的作用下，热导压制率随温度变化的关系和场致反铁磁有序的增强与温度的依赖关系有高度 相似性. 认为磁场引起的ab面的热导压制主要是电子热导的变化所致，而c方向的 压制则可能主要来源于声子热导的变化，它们均可能与磁场诱导下欠掺杂La_2-x Sr_xCuO₄的某种电荷有序和磁有序的增强密切相关. 关键词： 热导 电荷有序 磁有序     

LaFe11.5Si1.5中的磁不稳定性

通过对LaFe_11.5Si_1.5化合物进行自旋极化和固定磁矩(FSM)的能带 计算，发现LaFe_11.5Si_1.5化合物具有磁不稳定性的特征，无磁态和铁磁 态的能量 差别很小，FSM计算表明LaFe_11.5Si_1.5化合物具有低自旋态和高 自旋态的双 磁性态特征，在一定条件下能够在两磁性态之间发生变磁转变.通过计算的结果，定性地分 析了实验上所观察的一些现象. 关键词： 磁不稳定性 变磁转变     

La2/3Ca1/3MnO3/Eu2CuO4/La2/3Ca1/3MnO3磁性隧道结的制备与表征

采用磁控溅射，紫外线光刻和离子束刻蚀制备了La_2/3Ca_1/3MnO₃/Eu₂CuO₄/La_2/3Ca_1/3MnO₃磁性隧道结.通过对获得的磁性隧道结的I-V特性测量，发现非线性的I-V特性，显示结样品的隧穿特性.有趣的是发现在电极材料La_2/3Ca_1/3MnO₃的金属-绝缘体转变温度(Tp)以下，I-V曲线出现一个跳变.随着温度降低，开始出现跳变的临界电流增大，但是跳变都发生在同样的电压下～209mV.当电流增大或减小在跳变点附近出现回滞.这一跳变只发生在铁磁金属态，表明这是一个磁性相关联的效应，可能对应一种新的磁性开关过程.虽然，目前对这一现象背后的物理机理还不清楚，但是，这一现象有可能在未来自旋电子学器件方面具有潜在的应用价值. 关键词： 庞磁电阻 磁性隧道结 开关效应     

Zn掺杂La2CuO4热电势和磁化率研究

测量了La2CuO4掺Zn样品在不同降温速率下(330K保温05h,然后分别以6Kh,02Ks的速度降到8K)的直流磁化率和热电势.实验结果表明,反铁磁温度TN不随降温速率变化而变化,其直流磁化率也未受很大影响.高温热电势弱的温度依赖关系表明为极化子气体的贡献.热电势在转折温度Tdrop之下的快速降低是由于二维反铁磁涨落的贡献.热电势在更低温度的拐点TS与载流子的局域化有关.降温速率变化时,Tdrop和TS都有明显的变化.Zn掺杂对Tdrop和TS没有明显影响,但导致了更强的载流子局域化.讨论了上述现象产生的物理图像 关键词： La2CuO4 直流磁化率 热电势     

Zn,Ni,Mg高掺杂La1.85Sr0.15CuO4的晶体结构比较

对Zn,Ni,Mg高掺杂的La_1.85Sr_0.15CuO₄(LSCO)超导体的X射线衍射谱进行了仔细的分析,从而获得了各掺杂样品的晶格常量、原子结构参量及衍射的峰形参量,比较了不同掺杂所引起的晶体结构的差异,计算了由于掺杂所产生的晶格内部的微应变,结果表明非磁性金属Zn,Mg掺杂比磁性金属Ni掺杂更强烈地抑制了LSCO的超导电性可能与材料中晶格的微应变有关. 关键词： 1.85Sr_0.15CuO₄')" href="#">La_1.85Sr_0.15CuO₄ 磁、非磁掺杂 原子结构参量 晶格微应变     

混合场中 (Pr1－yNdy)2/3Sr1/3MnO3体系磁转变行为研究

对(Pr1-yNdy)2/3Sr1/3MnO3(y=2/8,4/8,6/8)体系在42K—300K温度范围内混合场（即交流驱动场Hac与附加直流磁场H dc的叠加）下的磁转变行为进行了系统的实验研究. 结果表明，随着外加直流磁场强 度的增大，样品的直流磁化强度增大，交流磁化率逐渐减小；在混合场作用下，磁转变温度 （居里温度）附近样品交流磁化率的实部出现一个特征的尖峰. 对于(Pr1-yNdy)2/3Sr1/3MnO3体系，随着Nd掺杂量的增加，交流磁化率实部 的相对变化 (Eχ′=Δχ′/χ′dc)的峰值对应的温度降低，这一点与该系 统的磁电阻的变化规律一致. 对锰氧化物体系在混合场中的磁转变现象进行了讨论. 关键词： 锰氧化物 混合场 磁特性     

La2CuO4掺锌样品的低温电阻率与热导率研究

对La2CuO4掺锌样品在不同降温速率下(330K保温30min后,分别以6Kh和02Ks的速率冷却至42K)电阻率(42—330K)和热导率(80—300K)随温度的变化关系进行了研究.实验结果表明,在不同降温速率下,热导率和电阻率都受到很大影响.快速降温过程使得130K以上的热导率减小,而热导率最小值出现在130K,且与降温速率无关.而低温下的热导率不受降温速率变化的影响.样品在高温区(T高于125K)电阻率随降温速率的增大而增大,低温区电阻率的非线性行为可用变程跳跃行为来描述.所有样品的热导率和电阻率在反铁磁相变温度都没有出现反常,这与能带理论框架下预期的结果和Anderson电荷自旋分离理论发生了矛盾,对此进行了讨论,并用极化子理论进行了自洽解释. 关键词： La2CuO4 热导率 电阻率     

La2-xSrxCuO4单晶膜的热电势与电阻率

测量了高质量的单晶膜La2-xSrxCuO4(x=010,020,025)的电阻率和热电势.La19Sr01CuO4电阻率呈现S型行为,表明存在一个赝能隙,在赝能隙态可以用公式ρ=ρ0+βexp(-ΔT)很好地拟合.热电势的测量表明,在超导转变前样品的残余热电势值非常小,这是膜的高质量引起的,三个样品在200K以上都出现一个宽峰,对其进行了一些理论模型分析,并与电子型超导体热电势结果作了比较. 关键词： 薄膜 输运性质 热电势     

YBa2Cu3O7-δ熔融织构样品的磁响应研究

对YBa₂Cu₃O_7-δ熔融织构样品在不同温度、不同磁场不同外场扫描速率下进行了磁滞回线测量,获得了电流密度J(T,B),lnE-lnJ曲线以及热激活能U₀(T,B)随温度、磁场的变化关系,从而分析了该样品磁响应特性,特别是第二峰(简称峰)受外场扫描速率的影响,探讨了峰形成的真正原因,以及影响峰位置、大小的各种因素 关键词： 磁响应 熔融织构 峰效应 临界电流密度     

La2O3对Yb:Y2O3透明陶瓷光谱性能的影响

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     

ENHANCEMENT OF FERROMAGNETIC CLUSTER INDUCED BY MAGNETIC FIELD IN THE PHASE-SEPARATED La0.5Ca0.5MnO3

Magnetic and transport properties of La_0.5Ca_0.5MnO₃ have been investigated by measuring the magnetization and resistance in zero-field-cooled (ZFC) and field-cooled (FC) modes. Conspicuously irreversible behaviors of magnetization/resistance in the two different modes were observed below the charge ordering transition temperature (T_CO). The ZFC and FC magnetizations at 5K, as functions of the magnetic field, coincide for μ₀H≤1T. Afterwards, the ZFC magnetization tends to an approximate constant, but the FC one increases linearly with increasing field. There exists an excellent correspondence between magnetization and resistance below T_CO. All the results suggest that the ferromagnetic clusters embed in the charge-ordered matrix. The phenomenon of ferromagnetic clusters growing up easily in the FC procedure has been interpreted according to the model of thermally activated two-level system.     

TEMPERATURE-DEPENDENT ANISOTROPY IN La1.86Sr0.14CuO4

Based on the scaling rule: R(T, H, θ) = R(T, h), where h = H(sin²θ+cos²θ/γ²)^1/2 and θ is field angle with respect to the ab-plane, the scaling approach is performed to examine the c-axis resistances of single crystalline La_1.86Sr_0.14CuO₄ as functions of field and its orientation at different temperatures. The anisotropy parameter γ of the system is extracted from the scaling approach, but shows an unusual temperature-dependent behavior, which is not expected by conventional theory. A phenomenological explanation of this temperature dependence is presented.     

Anisotropic polarons in lightly doped La2CuO4

The unusual anisotropy of an activation energy for the conductivity of highly pure La₂CuO₄ is explained in terms of disk-like polarons with large in-plane radius and small interplane one. For such a polaron the anisotropy of the mobility mechanism appears in a wide range of T: the in-plane hops are thermoactivated, while the interplane ones are due to the quantum tunneling. The T-dependence of Hall effect and the possibilities for the microscopic structure of a polaron in La₂CuO₄ are briefly discussed.     

ELECTROCHEMICAL OXIDATION OF La2CuO4 SINGLE CRYSTALS

Bulk superconducting La₂CuO_4+δ single crystals are obtained by using electrochemical intercalation technique from the as-grown insulating samples. Oxidation is carried out by constant current I=10μA at temperature T=70℃ and room temperature, respectively. Structure and magnetic properties are studied by low-temperature X-ray diffraction and susceptibility measurements. A superconducting phase with T_c of 19K and δ-0.12 can be attributed to the formation of oxygen clusters. Room temperature oxidation is inhomogeneous: two superconducting phases with T_c1 of 24K and T_c2 of 8K and an antiferromagnetic phase are coexisting in the crystal. It is found that the appearance of T_c in this system has the "step" tendency.     

Disruption of antiferromagnetic order in Zn-doped La2CuO4

The magnetic phase diagram of La₂(Cu_1−xZn_x)O₄ has been determined from zero-field muon-spin-rotation (ZF-μSR) data taken at LAMPF for 0 ≤ x ≤ 0.10. Antiferromagnetic onset temperatures follow T_N(x) from susceptibility measurements on the same samples. However, the order becomes long range, as evidenced by a well-defined internal magnetic field, only at temperatures well below T_N. Extrapolation of our results yields T_N → 0 K at x = 0.11 for a single (Cu_1−xZn_x)O₂ plane, and comparison with YBa₂(Cu_1−xZn_x)₃O₆ implies identical disruption of magnetism by Zn doping in the single- and double-plane systems.     

Dissipative quantum-creep studies in single-crystal La2CuO4+δ

Temperature-dependent magnetic relaxation was studied in a La₂CuO_4+δ crystal in the low-temperature range 2–10 K. The experimental data exhibited a non-vanishing magnetic relaxation in the low-temperature limit. These data could be well interpreted by a quantum-creep theory, suggesting the non-vanishing relaxation at low temperatures due to quantum tunneling of vortices. The effective Euclidean action determined as /|d ln M/d ln t| showed a quadratic temperature dependence. The quantitative analysis of the effective Euclidean action yields a classical activation energy, crossover temperature and zero-temperature quantum-tunneling rate.     

Gd2CuO4的远红外反铁磁共振理论

提出一个适用于奈耳温度6.4K以下,Gd₂CuO₄的远红外反铁磁共振理论.理论预言的结果与Kaplan等的实验符合. 关键词：     

Higher-order elastic constants of La1.84Sr0.16CuO4 high-temperature superconductor

Finite deformation theory is used to obtain the strain energy density of a tetragonal 2–1–4-type single crystal of the high-temperature superconductor La_2−xSr_xCuO₄. The complete set of second and third-order elastic constants of the high-temperature superconductor La_2−xSr_xCuO₄ (x = 0.16) is calculated by taking into account the interactions between nine nearest-neighbour atoms in the lattice and using Mie–Grüneisen interatomic potential. For the sake of comparison we have also computed the values of these constants for x = 0.13–0.20. The values of third-order elastic constants of La_2−xSr_xCuO₄ (x = 0.13–0.20) are negative and their absolute magnitudes are one order higher than those of the second-order elastic constants.     

Defects in Nd2CuO4 and Nd1.815Ce0.185CuO4 single crystals

We describe here four kinds of defects found in Nd₂CuO₄ and Nd_1.815Ce_0.185CuO₄ single crystals grown by the traveling solvent floating zone method. These defects include: (1) cracks, (2) inclusions, (3) gas bubbles and (4) uneven distribution of Ce⁴⁺. The formation mechanism of these defects and the measures of eliminating these defects are discussed. It is thought that the preparation of raw materials and stringent growth conditions are necessary to obtain high quality of Nd₂CuO₄ and Nd_1.815Ce_0.185CuO₄ single crystals.