Two-dimensional geometry of spin excitations in the high-transition-temperature superconductor YBa2Cu3O6+x

The fundamental building block of the copper oxide superconductors is a Cu4O4 square plaquette. The plaquettes in most of these materials are slightly distorted to form a rectangular lattice, for which an influential theory predicts that high-transition-temperature (high-T(c)) superconductivity is nucleated in 'stripes' aligned along one of the axes. This theory received strong support from experiments that indicated a one-dimensional character for the magnetic excitations in the high-T(c) material YBa2Cu3O6.6 (ref. 4). Here we report neutron scattering data on 'untwinned' YBa2Cu3O6+x crystals, in which the orientation of the rectangular lattice is maintained throughout the entire volume. Contrary to the earlier claim, we demonstrate that the geometry of the magnetic fluctuations is two-dimensional. Rigid stripe arrays therefore appear to be ruled out over a wide range of doping levels in YBa2Cu3O6+x, but the data may be consistent with liquid-crystalline stripe order. The debate about stripes has therefore been reopened.     

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.     

NdB2Cu3O7-x/YBa2Cu3O7-y高温超导薄膜的稳定性研究

采用脉冲激光沉积方法 ,在钇稳定氧化锆 (YSZ)衬底上制备了YBa2 Cu3 O7-y高温超导薄膜和NdB2 Cu3 O7-x/YBa2 Cu3 O7-y双层高温超导薄膜。X射线衍射分析结果表明 ,NdB2 Cu3 O7-x/YBa2 Cu3 O7-y双层膜在结晶度、表面光滑平整度和稳定性方面优于YBa2 Cu3 O7-y薄膜 ;电阻温度曲线测量结果表明 ,NdB2 Cu3 O7-x/YBa2 Cu3 O7-y双层膜的转变温度为 87.6 6K ,转变宽度为 0 .34K ;YBa2 Cu3 O7-y薄膜的则分别为 86 .6 4K和 0 .95K。与YBa2 Cu3 O7-y薄膜相比 ,NdB2 Cu3 O7-x/YBa2 Cu3 O7-y双层膜稳定性更高 ,在超导电子器件领域更具有应用潜力。     

NdB2Cu3O7—x／YBa2Cu3O7—y高温超导薄膜的稳定性研究

采用脉冲激光沉积方法，在钇稳定氧化锆（YSZ）衬底上制备了YBa2Cu3O7-y高温超导薄膜和NdB2Cu3O7-x/YBa2Cu3O7-y双层高温超导薄膜。X射线衍射分析结果表明，NdB2Cu3O7-x/YBa2Cu3O7-y双层膜在结晶度、表面光滑平整度和稳定性方面优于YBa2Cu3O7-x薄膜；电阻－温度曲线测量结果表明，NdB2Cu3O7-x/YBa2Cu3O7-y双层膜的转变温度为87.66K，转变宽度为0.34K;YBa2Cu3O7-y薄膜的则分别为86.64K和0.95K。与YBa2Cu3O7-y薄膜相比，NdB2Cu3O7-x/YBa2Cu3O7-y双层膜稳定性更高，在超导电子器件领域更具有应用潜力。     

An unusual phase transition to a second liquid vortex phase in the superconductor YBa2Cu3O7

A magnetic field penetrates a superconductor through an array of 'vortices', each of which carries one quantum of flux that is surrounded by a circulating supercurrent. In this vortex state, the resistivity is determined by the dynamical properties of the vortex 'matter'. For the high-temperature copper oxide superconductors (see ref.1 for a theoretical review), the vortex phase can be a 'solid', in which the vortices are pinned, but the solid can 'melt' into a 'liquid' phase, in which their mobility gives rise to a finite resistance. (This melting phenomenon is also believed to occur in conventional superconductors, but in an experimentally inaccessible part of the phase diagram.) For the case of YBa2Cu3O7, there are indications of the existence of a critical point, at which the character of the melting changes. But neither the thermodynamic nature of the melting, nor the phase diagram in the vicinity of the critical point, has been well established. Here we report measurements of specific heat and magnetization that determine the phase diagram in this material to 26 T, well above the critical point. Our results reveal the presence of a reversible second-order transition above the critical point. An unusual feature of this transition-namely, that the high-temperature phase is the less symmetric in the sense of the Landau theory-is in accord with theoretical predictions of a transition to a second vortex-liquid phase.     

Addition of nanoparticle dispersions to enhance flux pinning of the YBa2Cu3O7-x superconductor

Following the discovery of type-II high-temperature superconductors in 1986 (refs 1, 2), work has proceeded to develop these materials for power applications. One of the problems, however, has been that magnetic flux is not completely expelled, but rather is contained within magnetic fluxons, whose motion prevents larger supercurrents. It is known that the critical current of these materials can be enhanced by incorporating a high density of extended defects to act as pinning centres for the fluxons. YBa2Cu3O7 (YBCO or 123) is the most promising material for such applications at higher temperatures (liquid nitrogen). Pinning is optimized when the size of the defects approaches the superconducting coherence length ( approximately 2-4 nm for YBCO at temperatures < or =77 K) and when the areal number density of defects is of the order of (H/2) x 10(11) cm(-2), where H is the applied magnetic field in tesla. Such a high density has been difficult to achieve by material-processing methods that maintain a nanosize defect, except through irradiation. Here we report a method for achieving a dispersion of approximately 8-nm-sized nanoparticles in YBCO with a high number density, which increases the critical current (at 77 K) by a factor of two to three for high magnetic fields.     

Antiferromagnetic order induced by an applied magnetic field in a high-temperature superconductor.

One view of the high-transition-temperature (high-Tc) copper oxide superconductors is that they are conventional superconductors where the pairing occurs between weakly interacting quasiparticles (corresponding to the electrons in ordinary metals), although the theory has to be pushed to its limit. An alternative view is that the electrons organize into collective textures (for example, charge and spin stripes) which cannot be 'mapped' onto the electrons in ordinary metals. Understanding the properties of the material would then need quantum field theories of objects such as textures and strings, rather than point-like electrons. In an external magnetic field, magnetic flux penetrates type II superconductors via vortices, each carrying one flux quantum. The vortices form lattices of resistive material embedded in the non-resistive superconductor, and can reveal the nature of the ground state-for example, a conventional metal or an ordered, striped phase-which would have appeared had superconductivity not intervened, and which provides the best starting point for a pairing theory. Here we report that for one high-Tc superconductor, the applied field that imposes the vortex lattice also induces 'striped' antiferromagnetic order. Ordinary quasiparticle models can account for neither the strength of the order nor the nearly field-independent antiferromagnetic transition temperature observed in our measurements.     

单畴YBa2Cu3Oy(123)(YBCO)超导体晶粒取向对磁悬浮力的影响

制备了具有不同晶粒取向的单畴YBa2Cu3Oy(YBCO)大块超导体．在液氮温度下，研究了晶粒取向对YBCO块材磁悬浮力的影响．发现在样品和磁场固定不变的条件下，磁悬浮力的大小与磁场和晶粒方向之间的夹角密切相关．磁场强度H平行于样品c轴时，磁悬浮力的值比H垂直c轴时高出两倍多，因此在实际应用中必须考虑材料的这一各向异性．另外，在分析实验结果的基础上，发现永久磁体与超导体间的磁悬浮力可以很好地用双指数函数描述。     

One-dimensional nature of the magnetic fluctuations in YBa2Cu3O6.6

There is increasing evidence that inhomogeneous distributions of charge and spin--so-called 'striped phases'--play an important role in determining the properties of the high-temperature superconductors. For example, recent neutron-scattering measurements on the YBa2Cu3O(7-x) family of materials show both spin and charge fluctuations that are consistent with the striped-phase picture. But the fluctuations associated with a striped phase are expected to be one-dimensional, whereas the magnetic fluctuations observed to date appear to display two-dimensional symmetry. We show here that this apparent two-dimensionality results from measurements on twinned crystals, and that similar measurements on substantially detwinned crystals of YBa2Cu3O6.6 reveal the one-dimensional character of the magnetic fluctuations, thus greatly strengthening the striped-phase interpretation. Moreover, our results also suggest that superconductivity originates in charge stripes that extend along the b crystal axis, where the superfluid density is found to be substantially larger than for the a direction.     

Phase transitions in the incoherent lattice fluctuations in YBa2Cu3O(7-delta)

The growing body of experimental evidence for the existence of complex textures of charges and spins in the high-temperature superconductors has drawn attention to the so-called 'stripe-phase' models as a possible basis for the mechanism of superconductivity in these materials. Such observations have until now been restricted to systems where the texture dynamics are slow or suppressed altogether, and do not include the important case of YBa2Cu3O(7-delta). It seems likely that the dynamic behaviour of stripes, which has been suggested to undergo several phase transitions as a function of temperature, should also be reflected in the lattice properties of the host materials, and this forms the motivation for our present experiments. Specifically, we use MeV helium ion channelling, an ultrafast real-space probe of atomic displacements (with sub-picometre resolution), to probe incoherent lattice fluctuations in YBa2Cu3O(7-delta) as a function of temperature and oxygen doping. We detect lattice fluctuations that are larger than the expected thermal vibration component, and which show anomalies characteristic of the phase transitions anticipated for a dynamic stripe phase. Comparison of our lattice results with single-particle-tunnelling and photoemission data highlights the importance of spin-charge separation phenomena in the copper oxide superconductors.     

YBa2Cu3O6+x光电导效应的研究进展

本就高温超导体YB2Cu3O6 x的光电导效应的研究现状进行了介绍,并就机理解释及未来的研究工作做了简单的评述。     

Ca、Zn掺杂对高温超导体YBa2Cu3O7-δ晶体结构的影响

采用固相反应法成功的制备出了Y1-yCayBa2Cu3-xZnxO7-δ(y=0,x=0;y=0.1,x=0,0.05,0.1,0.2)的系列样品.利用X射线衍射(XRD)和扫描电镜(SEM)对所制备系列样品结构进行了研究.结果表明,在整个掺杂范围内样品的单相性很好.随着锌离子掺杂量的增加,样品的晶格常数和表面形貌都发生了较大的变化,这是因为掺杂使样品的电子局域化增加,从而使样品的结构发生变化.     

高温超导体中不同临界态模型下的磁弛豫

基于Anderson-Kim临界态模型,考虑涡旋玻璃态模型和集体磁通蠕动理论以及考虑磁通线反跳的热激活模型等目前几种关于高温超导体中磁弛豫现象的理论模型,推导出弛豫率和约化弛豫率的理论计算公式并在此基础上分析各个理论模型下弛豫率和约化弛豫率的理论计算结果和实验数据之间的关系,对各个理论模型的适用范围进行分析和比较.     

No mixing of superconductivity and antiferromagnetism in a high-temperature superconductor

There is still no universally accepted theory of high-temperature superconductivity. Most models assume that doping creates 'holes' in the valence band of an insulating, antiferromagnetic 'parent' compound, and that antiferromagnetism and high-temperature superconductivity are intimately related. If their respective energies are nearly equal, strong antiferromagnetic fluctuations (temporally and spatially restricted antiferromagnetic domains) would be expected in the superconductive phase, and superconducting fluctuations would be expected in the antiferromagnetic phase; the two states should 'mix' over an extended length scale. Here we report that one-unit-cell-thick antiferromagnetic La2CuO4 barrier layers remain highly insulating and completely block a supercurrent; the characteristic decay length is 1 A, indicating that the two phases do not mix. We likewise found that isolated one-unit-cell-thick layers of La1.85Sr0.15CuO4 remain superconducting. The latter further implies that, on doping, new electronic states are created near the middle of the bandgap. These two findings are in conflict with most proposed models, with a few notable exceptions that include postulated spin-charge separation.     

Unusual magnetic order in the pseudogap region of the superconductor HgBa2CuO4+delta

The pseudogap region of the phase diagram is an important unsolved puzzle in the field of high-transition-temperature (high-T(c)) superconductivity, characterized by anomalous physical properties. There are open questions about the number of distinct phases and the possible presence of a quantum-critical point underneath the superconducting dome. The picture has remained unclear because there has not been conclusive evidence for a new type of order. Neutron scattering measurements for YBa(2)Cu(3)O(6+delta) (YBCO) resulted in contradictory claims of no and weak magnetic order, and the interpretation of muon spin relaxation measurements on YBCO and of circularly polarized photoemission experiments on Bi(2)Sr(2)CaCu(2)O(8+delta)(refs 12, 13) has been controversial. Here we use polarized neutron diffraction to demonstrate for the model superconductor HgBa(2)CuO(4+delta) (Hg1201) that the characteristic temperature T* marks the onset of an unusual magnetic order. Together with recent results for YBCO, this observation constitutes a demonstration of the universal existence of such a state. The findings appear to rule out theories that regard T* as a crossover temperature rather than a phase transition temperature. Instead, they are consistent with a variant of previously proposed charge-current-loop order that involves apical oxygen orbitals, and with the notion that many of the unusual properties arise from the presence of a quantum-critical point.     

Signature of optimal doping in Hall-effect measurements on a high-temperature superconductor

High-temperature superconductivity is achieved by doping copper oxide insulators with charge carriers. The density of carriers in conducting materials can be determined from measurements of the Hall voltage--the voltage transverse to the flow of the electrical current that is proportional to an applied magnetic field. In common metals, this proportionality (the Hall coefficient) is robustly temperature independent. This is in marked contrast to the behaviour seen in high-temperature superconductors when in the 'normal' (resistive) state; the departure from expected behaviour is a key signature of the unconventional nature of the normal state, the origin of which remains a central controversy in condensed matter physics. Here we report the evolution of the low-temperature Hall coefficient in the normal state as the carrier density is increased, from the onset of superconductivity and beyond (where superconductivity has been suppressed by a magnetic field). Surprisingly, the Hall coefficient does not vary monotonically with doping but rather exhibits a sharp change at the optimal doping level for superconductivity. This observation supports the idea that two competing ground states underlie the high-temperature superconducting phase.     

Special variation of infiltration-growth processed bulk YBCO fabricated using new liquid source: Ba3Cu5O8 (1:1.3) and YbBa2Cu3Oy

Utilization of novel materials, particularly high-Tc (critical temperature) superconductors, is essential to pursue the United Nations' Sustainable Goals, as well as to meet the increasing worldwide demand for clean and carbon-free electric power technologies. Superconduct-ing magnets are beneficial in several real-life applications including transportation, energy production, magnetic resonance imaging (MRI), and drug delivery systems. To achieve high performance, one must develop uniform, large-grain, infiltration-growth (IG) processed bulk YBa2Cu3Oy (Y-123) super-magnets. In this study, we report the magnetic and microstructural properties of a large-grain, top-seeded, IG-pro-cessed Y-123 pellet, which is 20 mm in diameter and 6 mm in height; the pellet is produced utilizing liquid Yb-123+Ba3Cu5O8 as the liquid source. All the samples cut from the top of the bulk exhibit a sharp superconducting transition (approximately 1 K wide) with the onset Tc of approximately 90 K. However, in the samples cut from the bottom surface, the onset Tc values slightly decreased to between 88 and 90 K, al-though still exhibiting a sharp superconducting transition. The top and bottom samples exhibited the highest remnant value of Jc (critical cur-rent density) at 77 K H//c-axis of 50 and 55 kA/cm2, respectively. The remnant Jc and irreversibility field values significantly fluctuated, being fairly low in some bottom samples. Scanning electron microscopy identified nanometer size Y-211 (Y2BaCuO5) secondary-phase particles dis-persed in the Y-123 matrix. Energy-dispersive spectroscopy clarified that the decreased both Tc and Jc for the bottom samples were attributed to liquid phase dispersion within the Y-123 phase.     

Bi—Sr—Ca—Cu—O系超导电性的探讨

用固态反应法制备不含稀土元类的BiSrCaCu_2Oy陶瓷高温超导体。该系统可能存在多个高温超导相,不同工艺可改变各超导相成份;150K附近可能有一更高温超导相。     

钙钛矿La2/3Ca1/3MnO3/YBa2Cu4O8/La2/3Ca1/3MnO3三层薄膜的磁输运性能

高温超导材料YBa2Cu4O8(YBCO)和铁磁材料La2／3Ca1／3MnO3(LCMO)形成的三层薄膜LCMO／YBCO／LCMO由对靶溅射技术制得．与YBCO单层薄膜相比,由于超导／铁磁系统中的磁性邻近效应,三层薄膜表现出较低的超导转变温度(Tc,ON)．与LCMO单层膜相比,三层薄膜的金属半导体转变温度(TMS)被提高并且强烈依赖于YBCO层的厚度．随中间层厚度的变化,磁电阻显示出非单调行为,长振荡周期被发现．结果表明,当YBCO处于正常态时两层LCMO膜之间存左着磁性自旋相互作用．