铜氧化物高温超导体的比热容

 基于Nozieres Schmitt Rink的强耦合超导理论和推广的Luttinger理论,文中分别计算了低电子浓度和高电子浓度体系的电子比热容及线性比热系数,描绘出线性比热系数随温度的变化曲线。电子线性比热系数的计算结果表明,在Nozieres Schmitt Rink理论和推广的Luttinger理论框架下,铜氧化物高温超导体中产生赝隙。     

铜氧化物高温超导体的晶体结构和物理性能研究

阐述了铜氧化物高温超导体的晶体结构 ,指出导电区、蓄电区的划分和电子相图是铜氧化物高温超导体重要的共性特征 .介绍了分块模型及其在研究晶体结合能、弹性模量中的应用 ,对铜氧化物高温超导体结构和性能之间的关系作了探讨 .最后对当前高温超导体研究的几个热点问题及其应用前景予以简介 .     

高温超导体YBaCuO光掺杂效应的正电子寿命谱研究

报道了YBaCuO高温超导体进行光掺杂的基本情况,提供了正电子寿命与光掺杂量之间的变化关系,讨论了光掺杂机理,并将实验结果与理论模型进行了对比.     

未掺杂SI-GaAs退火行为的正电子湮没研究

用正电子湮没技术,研究了未掺杂的SI-GaAs退火行为。结果表明,长寿命分量τ_2的变化不仅与镓空位而且和多镓空位络合物有关。本文还讨论了电子和中子辐照的影响。     

高温超导体中的一个正电子陷阱湮没模型

从简单的正电子物理图像出发，引入正电子在陷阱中湮没竞争机制，提出了一个新的陷阱湮没模型，并采用这个模型分析了多陷阱的高温超导材料中的正电子湮没特征，此模型可用于探索非平衡缺陷问题的普遍情况．     

用正电子湮没研究CaO掺杂的CeO2氧离子导体

测量了不同ＣａＯ掺杂量的ＣｅＯ２氧离子导体（ＣａＯ２）１－ｘ（ＣｅＯ２）ｘ的正电子寿命谱。发现正电子寿命随ｘ的增加（ｘ从０．０５到０．５）先增大后减小,峰值出现在ｘ＝０．１２附近。从掺杂离子与缺陷的相互作用所造成的缺陷组态变化对该现象进行了解释。     

锰氧化物La0.67Ca0.33Mn0.9A0.1O3的正电子湮没研究

采用固相反应法制备了La0.67Ca0.33Mn0.9A0.1O3(其中A为Cr、Co、Fe、Al、Cu)系列Mn位替代的氧化物样品;利用正电子湮没对该系列样品的结构缺陷进行了分析,结果表明:τ1的变化范围不大,表明样品主要受大尺寸点缺陷的影响,τ2的变化比较明显,说明样品内存在一定数量的空洞及微空洞等缺陷.类比合金中正电子长寿命与缺陷簇之间的关系,估算了元素替代后样品的缺陷半径大小,可能大的空位团簇、位错和晶界在样品中起着重要作用.按Cr、Co、Fe、Al、Cu的掺杂顺序,平均寿命τavg逐渐增加,而电子密度ne则逐渐减小.说明元素掺杂引起锰离子局域环境的改变,样品中的铁磁与反铁磁作用的相互竞争及样品内部电子局域化所形成的极化行为等因素的影响,导致了正电子寿命各参数的变化.     

钆掺杂巨磁阻材料Sr_2FeMoO_6正电子湮没谱

采用固相反应法成功制备了钆掺杂样品Sr2-xGdxFeMoO6(x=0,0.05,0.1,0.15,0.2).X射线衍射对样品的检测结果表明,在整个掺杂范围内样品单相性很好;利用正电子湮没技术对样品缺陷进行了研究,掺杂使微缺陷复合成大的空位团,导致缺陷尺寸变大;采用标准四引线法测量了样品电阻率随温度变化,反位缺陷、缺陷尺寸及GdFeO3和GdMoO3团簇是影响样品电阻率的重要因素.     

高温氧化物超导体氧含量的测定

用碘滴定法测定了高温氧化物超导体LLa2-xBaxCuO4-y在不同掺杂浓度下和YBa2Cu3O7-y在不同淬火温度下的氧含量，得到了氧含量与掺杂浓度x和淬火温度T的关系．     

Quantum oscillations and the Fermi surface in an underdoped high-Tc superconductor

Despite twenty years of research, the phase diagram of high-transition-temperature superconductors remains enigmatic. A central issue is the origin of the differences in the physical properties of these copper oxides doped to opposite sides of the superconducting region. In the overdoped regime, the material behaves as a reasonably conventional metal, with a large Fermi surface. The underdoped regime, however, is highly anomalous and appears to have no coherent Fermi surface, but only disconnected 'Fermi arcs'. The fundamental question, then, is whether underdoped copper oxides have a Fermi surface, and if so, whether it is topologically different from that seen in the overdoped regime. Here we report the observation of quantum oscillations in the electrical resistance of the oxygen-ordered copper oxide YBa2Cu3O6.5, establishing the existence of a well-defined Fermi surface in the ground state of underdoped copper oxides, once superconductivity is suppressed by a magnetic field. The low oscillation frequency reveals a Fermi surface made of small pockets, in contrast to the large cylinder characteristic of the overdoped regime. Two possible interpretations are discussed: either a small pocket is part of the band structure specific to YBa2Cu3O6.5 or small pockets arise from a topological change at a critical point in the phase diagram. Our understanding of high-transition-temperature (high-T(c)) superconductors will depend critically on which of these two interpretations proves to be correct.     

Spontaneous breaking of time-reversal symmetry in the pseudogap state of a high-Tc superconductor

A change in 'symmetry' is often observed when matter undergoes a phase transition-the symmetry is said to be spontaneously broken. The transition made by underdoped high-transition-temperature (high-Tc) superconductors is unusual, in that it is not a mean-field transition as seen in other superconductors. Rather, there is a region in the phase diagram above the superconducting transition temperature Tc (where phase coherence and superconductivity begin) but below a characteristic temperature T* where a 'pseudogap' appears in the spectrum of electronic excitations. It is therefore important to establish if T* is just a cross-over temperature arising from fluctuations in the order parameter that will establish superconductivity at Tc (refs 3, 4), or if it marks a phase transition where symmetry is spontaneously broken. Here we report that, for a material in the pseudogap state, left-circularly polarized photons give a different photocurrent from right-circularly polarized photons. This shows that time-reversal symmetry is spontaneously broken below T*, which therefore corresponds to a phase transition.     

A multi-component Fermi surface in the vortex state of an underdoped high-Tc superconductor

To understand the origin of superconductivity, it is crucial to ascertain the nature and origin of the primary carriers available to participate in pairing. Recent quantum oscillation experiments on high-transition-temperature (high-T(c)) copper oxide superconductors have revealed the existence of a Fermi surface akin to that in normal metals, comprising fermionic carriers that undergo orbital quantization. The unexpectedly small size of the observed carrier pocket, however, leaves open a variety of possibilities for the existence or form of any underlying magnetic order, and its relation to d-wave superconductivity. Here we report experiments on quantum oscillations in the magnetization (the de Haas-van Alphen effect) in superconducting YBa(2)Cu(3)O(6.51) that reveal more than one carrier pocket. In particular, we find evidence for the existence of a much larger pocket of heavier mass carriers playing a thermodynamically dominant role in this hole-doped superconductor. Importantly, characteristics of the multiple pockets within this more complete Fermi surface impose constraints on the wavevector of any underlying order and the location of the carriers in momentum space. These constraints enable us to construct a possible density-wave model with spiral or related modulated magnetic order, consistent with experimental observations.     

Resonance as a measure of pairing correlations in the high-Tc superconductor YBa2Cu3O6.6

One of the most striking properties of the high-transition-temperature (high-Tc) superconductors is that they are all derived from insulating antiferromagnetic parent compounds. The intimate relationship between magnetism and superconductivity in these copper oxide materials has intrigued researchers from the outset, because it does not exist in conventional superconductors. Evidence for this link comes from neutron-scattering experiments that show the unambiguous presence of short-range antiferromagnetic correlations (excitations) in the high-Tc superconductors. Even so, the role of such excitations in the pairing mechanism for superconductivity is still a subject of controversy. For YBa2Cu3O(6+x), where x controls the hole-doping level, the most prominent feature in the magnetic excitation spectrum is a sharp resonance (refs 6-11). Here we show that for underdoped YBa2Cu3O6.6, where x and Tc are below their optimal values, modest magnetic fields suppress the resonance significantly, much more so for fields approximately perpendicular to the CuO2 planes than for parallel fields. Our results indicate that the resonance measures pairing and phase coherence, suggesting that magnetism plays an important role in high-Tc superconductivity. The persistence of a field effect above Tc favours mechanisms in which the superconducting electron pairs are pre-formed in the normal state of underdoped copper oxide superconductors, awaiting transition to the superconducting state.     

Microscopic electronic inhomogeneity in the high-Tc superconductor Bi2Sr2CaCu2O8+x

The parent compounds of the copper oxide high-transition-temperature (high-Tc) superconductors are unusual insulators (so-called Mott insulators). Superconductivity arises when they are 'doped' away from stoichiometry. For the compound Bi2Sr2CaCu2O8+x, doping is achieved by adding extra oxygen atoms, which introduce positive charge carriers ('holes') into the CuO2 planes where the superconductivity is believed to originate. Aside from providing the charge carriers, the role of the oxygen dopants is not well understood, nor is it clear how the charge carriers are distributed on the planes. Many models of high-Tc superconductivity accordingly assume that the introduced carriers are distributed uniformly, leading to an electronically homogeneous system as in ordinary metals. Here we report the presence of an electronic inhomogeneity in Bi2Sr2CaCu2O8+x, on the basis of observations using scanning tunnelling microscopy and spectroscopy. The inhomogeneity is manifested as spatial variations in both the local density of states spectrum and the superconducting energy gap. These variations are correlated spatially and vary on the surprisingly short length scale of approximately 14 A. Our analysis suggests that this inhomogeneity is a consequence of proximity to a Mott insulator resulting in poor screening of the charge potentials associated with the oxygen ions left in the BiO plane after doping, and is indicative of the local nature of the superconducting state.     

铜氧化物的晶体结构参量与超导电性

以ｄＡ（ＣｕＯ２面上Ｃｕ（Ｐ）和项角氧Ｏ（Ａ）键长）对Ｖｄ［沿Ｃｕ（Ｐ）－Ｏ（Ａ）键和沿Ｃｕ（Ｐ）－Ｏ（Ｐ）键方向一个空穴的平均Ｍａｄｅｌｕｎｇ位势差］做图，发现所有铜氧化物超导体的转变温度和结构参量存在一确定的变化规律，不同超导系统被明显地分离开来，Ｔｃ既随ｄＡ也随Ｖｄ的增大而增大．本文从晶体结构的角度分析了产生该现象的原因．     

Dy、Yb掺杂稀土锰氧化物的磁性和电性

在La0.67Ga0.33MnO3中用Dy和Yb对La进行了部分替代，随替代量的增加，材料的居里温度单调下降。掺Dy使峰值电阻迅速增加，而Yb的掺杂对电阻率的影响较小，说明样品的磁性和输运性质受掺杂离子的影响。     

High-temperature interface superconductivity between metallic and insulating copper oxides

The realization of high-transition-temperature (high-T(c)) superconductivity confined to nanometre-sized interfaces has been a long-standing goal because of potential applications and the opportunity to study quantum phenomena in reduced dimensions. This has been, however, a challenging target: in conventional metals, the high electron density restricts interface effects (such as carrier depletion or accumulation) to a region much narrower than the coherence length, which is the scale necessary for superconductivity to occur. By contrast, in copper oxides the carrier density is low whereas T(c) is high and the coherence length very short, which provides an opportunity-but at a price: the interface must be atomically perfect. Here we report superconductivity in bilayers consisting of an insulator (La(2)CuO(4)) and a metal (La(1.55)Sr(0.45)CuO(4)), neither of which is superconducting in isolation. In these bilayers, T(c) is either approximately 15 K or approximately 30 K, depending on the layering sequence. This highly robust phenomenon is confined within 2-3 nm of the interface. If such a bilayer is exposed to ozone, T(c) exceeds 50 K, and this enhanced superconductivity is also shown to originate from an interface layer about 1-2 unit cells thick. Enhancement of T(c) in bilayer systems was observed previously but the essential role of the interface was not recognized at the time.     

不同化学结合剂对铜氧化物的团聚和浸出行为

The chemical binder is one of the critical factors affecting ore agglomeration behavior and leaching efficiency. In this study, we investigated the effect of the type of binder and mass fraction of the H₂SO₄ solution used on the curing, soaking, and leaching behavior of agglomerations. The results revealed that Portland cement (3CaO·SiO₂, 2CaO·SiO₂, and 3CaO·Al₂O₃) was the optimal binder for obtaining a well-shaped, stable agglomeration structure. A higher extraction rate was achieved when using Portland cement than that obtained using sodium silicate, gypsum, or acid-proof cement. An excessive geometric mean size is not conducive to obtaining well-shaped agglomerations and desirable porosity. Using computed tomography (CT) and MATLAB, the porosity of two-dimensional CT images in sample concentrations L1–L3 was observed to increase at least 4.5vol% after acid leaching. Ore agglomerations began to be heavily destroyed and even to disintegrate when the sulfuric acid solution concentration was higher than 30 g/L, which was caused by the excessive accumulation of reaction products and residuals.