A Bragg glass phase in the vortex lattice of a type II superconductor

Although crystals are usually quite stable, they are sensitive to a disordered environment: even an infinitesimal amount of impurities can lead to the destruction of crystalline order. The resulting state of matter has been a long-standing puzzle. Until recently it was believed to be an amorphous state in which the crystal would break into 'crystallites'. But a different theory predicts the existence of a novel phase of matter: the so-called Bragg glass, which is a glass and yet nearly as ordered as a perfect crystal. The 'lattice' of vortices that contain magnetic flux in type II superconductors provide a good system to investigate these ideas. Here we show that neutron-diffraction data of the vortex lattice provides unambiguous evidence for a weak, power-law decay of the crystalline order characteristic of a Bragg glass. The theory also predicts accurately the electrical transport properties of superconductors; it naturally explains the observed phase transitions and the dramatic jumps in the critical current associated with the melting of the Bragg glass. Moreover, the model explains experiments as diverse as X-ray scattering in disordered liquid crystals and the conductivity of electronic crystals.     

Spatially resolved electronic structure inside and outside the vortex cores of a high-temperature superconductor

Puzzling aspects of high-transition-temperature (high-Tc) superconductors include the prevalence of magnetism in the normal state and the persistence of superconductivity in high magnetic fields. Superconductivity and magnetism generally are thought to be incompatible, based on what is known about conventional superconductors. Recent results, however, indicate that antiferromagnetism can appear in the superconducting state of a high-Tc superconductor in the presence of an applied magnetic field. Magnetic fields penetrate a superconductor in the form of quantized flux lines, each of which represents a vortex of supercurrents. Superconductivity is suppressed in the core of the vortex and it has been suggested that antiferromagnetism might develop there. Here we report the results of a high-field nuclear-magnetic-resonance (NMR) imaging experiment in which we spatially resolve the electronic structure of near-optimally doped YBa2Cu3O7-delta inside and outside vortex cores. Outside the cores, we find strong antiferromagnetic fluctuations, whereas inside we detect electronic states that are rather different from those found in conventional superconductors.     

An unusual isotope effect in a high-transition-temperature superconductor

In conventional superconductors, the electron pairing that allows superconductivity is caused by exchange of virtual phonons, which are quanta of lattice vibration. For high-transition-temperature (high-T(c)) superconductors, it is far from clear that phonons are involved in the pairing at all. For example, the negligible change in T(c) of optimally doped Bi2Sr2CaCu2O8+delta (Bi2212; ref. 1) upon oxygen isotope substitution (16O --> 18O leads to T(c) decreasing from 92 to 91 K) has often been taken to mean that phonons play an insignificant role in this material. Here we provide a detailed comparison of the electron dynamics of Bi2212 samples containing different oxygen isotopes, using angle-resolved photoemission spectroscopy. Our data show definite and strong isotope effects. Surprisingly, the effects mainly appear in broad high-energy humps, commonly referred to as 'incoherent peaks'. As a function of temperature and electron momentum, the magnitude of the isotope effect closely correlates with the superconducting gap--that is, the pair binding energy. We suggest that these results can be explained in a dynamic spin-Peierls picture, where the singlet pairing of electrons and the electron-lattice coupling mutually enhance each other.     

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.     

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.     

二维Josephson结阵列超导的涡旋Nernst效应

借助于与时间相关的Gintzburg-Landau动力学,数值计算了二维Josephson结阵列的Nernst信号,获得了Nemst信号随温度及磁场变化的规律．在超导转变温度以上发现了较大的Nernst信号．而对于固定的温度而言,Nernst信号则随外加磁场的升高呈现出非单调变化的行为．本研究获得的数值模拟结果与一些铜氧化物高温超导体的实验结果在定性上吻合．     

驻涡燃烧室内涡系分布研究

为清晰了解驻涡燃烧室内流场的涡系分布,对中心驻体为67%燃烧室当量宽度的驻涡燃烧室流场,在驻涡腔无喷射和有喷射两种状况下分别进行冷态数值模拟,详细研究流场内旋涡分布情况.结果表明:流道前部存在4个影响很小、沿流动方向运动、分布于驻体与端壁角区的螺旋状旋涡;流道后部存在1个由2个旋涡构成、面积较大的回流区;流道中部的驻涡区旋涡情况复杂,在驻涡腔无喷射情况下,主流与驻涡区之间悬挂驻留2个稳定的驻涡,驻涡腔内存在不稳定且数目不确定的旋涡;当驻涡腔前驻体有喷射存在时,驻涡腔内形成4个均匀且稳定的旋涡.     

The structure of the high-energy spin excitations in a high-transition-temperature superconductor

In conventional superconductors, lattice vibrations (phonons) mediate the attraction between electrons that is responsible for superconductivity. The high transition temperatures (high-T(c)) of the copper oxide superconductors has led to collective spin excitations being proposed as the mediating excitations in these materials. The mediating excitations must be strongly coupled to the conduction electrons, have energy greater than the pairing energy, and be present at T(c). The most obvious feature in the magnetic excitations of high-T(c) superconductors such as YBa2Cu3O6+x is the so-called 'resonance'. Although the resonance may be strongly coupled to the superconductivity, it is unlikely to be the main cause, because it has not been found in the La2-x(Ba,Sr)(x)CuO4 family and is not universally present in Bi2Sr2CaCu2O8+delta (ref. 9). Here we use inelastic neutron scattering to characterize possible mediating excitations at higher energies in YBa2Cu3O6.6. We observe a square-shaped continuum of excitations peaked at incommensurate positions. These excitations have energies greater than the superconducting pairing energy, are present at T(c), and have spectral weight far exceeding that of the 'resonance'. The discovery of similar excitations in La2-xBa(x)CuO4 (ref. 10) suggests that they are a general property of the copper oxides, and a candidate for mediating the electron pairing.     

A coherent three-dimensional Fermi surface in a high-transition-temperature superconductor

All conventional metals are known to possess a three-dimensional Fermi surface, which is the locus in reciprocal space of the long-lived electronic excitations that govern their electronic properties at low temperatures. These excitations should have well-defined momenta with components in all three dimensions. The high-transition-temperature (high-T(c)) copper oxide superconductors have unusual, highly two-dimensional properties above the superconducting transition. This, coupled with a lack of unambiguous evidence for a three-dimensional Fermi surface, has led to many new and exotic models for the underlying electronic ground state. Here we report the observation of polar angular magnetoresistance oscillations in the overdoped superconductor Tl2Ba2CuO6+delta in high magnetic fields, which firmly establishes the existence of a coherent three-dimensional Fermi surface. Analysis of the oscillations reveals that at certain symmetry points, however, this surface is strictly two-dimensional. This striking form of the Fermi surface topography, long-predicted by electronic band structure calculations, provides a natural explanation for a wide range of anisotropic properties both in the normal and superconducting states. Our data reveal that, despite their extreme electrical anisotropy, the high-T(c) materials at high doping levels can be understood within a framework of conventional three-dimensional metal physics.     

Breakdown of Fermi-liquid theory in a copper-oxide superconductor.

The behaviour of electrons in solids is well described by Landau's Fermi-liquid theory, which predicts that although electrons in a metal interact, they can still be treated as well defined fermions, which are called 'quasiparticles'. At low temperatures, the ability of quasiparticles to transport heat is given strictly by their ability to transport charge, as described by a universal relation known as the Wiedemann-Franz law, which hitherto no material has been known to violate. High-temperature superconductors have long been thought to fall outside the realm of Fermi-liquid theory, as suggested by several anomalous properties, but this has yet to be shown conclusively. Here we report an experimental test of the Wiedemann-Franz law in the normal state of a copper-oxide superconductor, (Pr,Ce)2CuO4, which reveals that the elementary excitations that carry heat in this material are not fermions. This is compelling evidence for the breakdown of Fermi-liquid theory in high-temperature superconductors.     

激励横向射流对超声速混合中流向涡影响的数值研究

为增强燃料与超声速气流的混合,以达到较好的混合效果,利用FLUENT DES方法对1 k Hz激励横向射流对超声速混合中流向涡的影响进行了数值模拟。通过与无激励射流的工况进行对比发现,因大尺度结构的提早形成,激励射流将射流核心中的流向涡对的能量迅速打散,底部涡对迅速扩大并且向上抬升,并夹挤射流核心涡对,最终将之取代,造成流场剧烈变化。对横向射流进行激励有利于增强混合。     

永磁体与超导体之间水平间距对磁悬浮力的影响

研究了长方体永磁体与超导体之间不同水平间距对磁悬浮力的影响,当长方体永磁体与超导体之间处于5个不同水平间距时,永磁体组合和长方体永磁体在竖直方向往返运动一次测量磁悬浮力.结果表明,长方体永磁体与超导体之间水平间距从10mm减小到2mm时,最大磁悬浮力从92.3N减小到10.5N,衰减为原来的11.4%.这与永磁体磁场分布和超导体捕获磁通紧密相关,对磁悬浮应用系统设计提供一些可参考的实验依据.     

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.     

Quantum critical behaviour in a high-T(c) superconductor

Quantum criticality is associated with a system composed of a nearly infinite number of interacting quantum degrees of freedom at zero temperature, and it implies that the system looks on average the same regardless of the time- and length scale on which it is observed. Electrons on the atomic scale do not exhibit such symmetry, which can only be generated as a collective phenomenon through the interactions between a large number of electrons. In materials with strong electron correlations a quantum phase transition at zero temperature can occur, and a quantum critical state has been predicted, which manifests itself through universal power-law behaviours of the response functions. Candidates have been found both in heavy-fermion systems and in the high-transition temperature (high-T(c)) copper oxide superconductors, but the reality and the physical nature of such a phase transition are still debated. Here we report a universal behaviour that is characteristic of the quantum critical region. We demonstrate that the experimentally measured phase angle agrees precisely with the exponent of the optical conductivity. This points towards a quantum phase transition of an unconventional kind in the high-T(c) superconductors.     

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.     

铪对FGH97合金平衡相影响的评估

利用材料相图计算和材料性能模拟软件JMatPro,计算了粉末高温合金FGH97热力学平衡相及相组成,研究了不同Hf含量对平衡相和相组成的影响,并结合物理化学相分析实验和显微组织观察进行了验证分析,进而揭示FGH97合金在750℃下各相析出和Hf在相间分配的规律,认清Hf在FGH97合金的作用.结果表明:FGH97合金在750℃下的平衡相包括γ、γ’、MC、M23C6、M3B2和μ相,Hf主要存在于γ’和MC相中,并且随着Hf含量的增大,Hf在γ’和MC相中分配不同.     

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.     

共存组分参与的反应的平衡处理

共存组分参与的反应的化学平衡,是把组分与共存组分的生成物作为反应物,再与另一组分反应处理的。其平衡常数等于组分间反应的平衡常数与组分和共存组分的平衡常数之比。     

Hidden magnetic excitation in the pseudogap phase of a high-T(c) superconductor

The elucidation of the pseudogap phenomenon of the high-transition-temperature (high-T(c)) copper oxides-a set of anomalous physical properties below the characteristic temperature T* and above T(c)-has been a major challenge in condensed matter physics for the past two decades. Following initial indications of broken time-reversal symmetry in photoemission experiments, recent polarized neutron diffraction work demonstrated the universal existence of an unusual magnetic order below T* (refs 3, 4). These findings have the profound implication that the pseudogap regime constitutes a genuine new phase of matter rather than a mere crossover phenomenon. They are furthermore consistent with a particular type of order involving circulating orbital currents, and with the notion that the phase diagram is controlled by a quantum critical point. Here we report inelastic neutron scattering results for HgBa(2)CuO(4+δ) that reveal a fundamental collective magnetic mode associated with the unusual order, and which further support this picture. The mode's intensity rises below the same temperature T* and its dispersion is weak, as expected for an Ising-like order parameter. Its energy of 52-56?meV renders it a new candidate for the hitherto unexplained ubiquitous electron-boson coupling features observed in spectroscopic studies.     

Unraveling the mystery of enhanced cell-killing effect around the Bragg peak region of a double irradiation source ^9C-ion beam

Clinical trials of heavy ion cancer therapy, especially carbon ion radiotherapy, have hitherto achieved unprece- dented success against tumors resistant to conventional radiations and hard to cure with other modalities[1―3]. In the final analysis, the remarkable curative efficacy of the treatments with heavy ions is attributed to the inverted depth-dose distribution of the heavy ion beam and the elevated relative biological effectiveness (RBE) across the Bragg peak[4]. Radioactive ion beam p…