Imaging the vortex-lattice melting process in the presence of disorder

General arguments suggest that first-order phase transitions become less sharp in the presence of weak disorder, while extensive disorder can transform them into second-order transitions; but the atomic level details of this process are not clear. The vortex lattice in superconductors provides a unique system in which to study the first-order transition on an inter-particle scale, as well as over a wide range of particle densities. Here we use a differential magneto-optical technique to obtain direct experimental visualization of the melting process in a disordered superconductor. The images reveal complex behaviour in nucleation, pattern formation, and solid-liquid interface coarsening and pinning. Although the local melting is found to be first-order, a global rounding of the transition is observed; this results from a disorder-induced broad distribution of local melting temperatures, at scales down to the mesoscopic level. We also resolve local hysteretic supercooling of microscopic liquid domains, a non-equilibrium process that occurs only at selected sites where the disorder-modified melting temperature has a local maximum. By revealing the nucleation process, we are able to experimentally evaluate the solid-liquid surface tension, which we find to be extremely small.     

层状超导体磁通格子固相熔化的温度效应

发展了层状超导体中3D磁通线模型,用动力学模拟方法数值研究了磁通格子的有序-无序熔化相变.在无序强度温度相图中,发现从有序的布拉格玻璃相到无序的磁通玻璃相之间的固-固相变线在中等温度区域有一个突起,与最近实验上得到的"反向熔化"现象一致.这一反向熔化行为起因于磁通之间相互作用的异常温度效应.     

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.     

The order of the quantum chromodynamics transition predicted by the standard model of particle physics

Quantum chromodynamics (QCD) is the theory of the strong interaction, explaining (for example) the binding of three almost massless quarks into a much heavier proton or neutron--and thus most of the mass of the visible Universe. The standard model of particle physics predicts a QCD-related transition that is relevant for the evolution of the early Universe. At low temperatures, the dominant degrees of freedom are colourless bound states of hadrons (such as protons and pions). However, QCD is asymptotically free, meaning that at high energies or temperatures the interaction gets weaker and weaker, causing hadrons to break up. This behaviour underlies the predicted cosmological transition between the low-temperature hadronic phase and a high-temperature quark-gluon plasma phase (for simplicity, we use the word 'phase' to characterize regions with different dominant degrees of freedom). Despite enormous theoretical effort, the nature of this finite-temperature QCD transition (that is, first-order, second-order or analytic crossover) remains ambiguous. Here we determine the nature of the QCD transition using computationally demanding lattice calculations for physical quark masses. Susceptibilities are extrapolated to vanishing lattice spacing for three physical volumes, the smallest and largest of which differ by a factor of five. This ensures that a true transition should result in a dramatic increase of the susceptibilities. No such behaviour is observed: our finite-size scaling analysis shows that the finite-temperature QCD transition in the hot early Universe was not a real phase transition, but an analytic crossover (involving a rapid change, as opposed to a jump, as the temperature varied). As such, it will be difficult to find experimental evidence of this transition from astronomical observations.     

钛团簇的融化行为

采用经验分子动力学方法研究了几个典型Ti团簇的热力学稳定性和融化行为.Ti团簇的融化行为清楚地依赖于体系的结构和尺寸.对于较小的团簇,融化曲线没有出现清楚的一级相变特征,意味着由于尺寸效应导致的固态液态共存.对于较大尺寸的团簇,观察到其融化过程伴随着表面融化、局部融化、结构共存和相变行为.     

An inverse transition of magnetic domain patterns in ultrathin films

Inverse freezing and inverse melting are processes where a more symmetric phase is found at lower temperatures than at higher temperatures. Such inverse transitions are very rare. Here we report the existence of an inverse transition effect in ultrathin Fe films that are magnetized perpendicular to the film plane. The magnetization of these films is not uniform, but instead manifests itself as stripe domains with opposite perpendicular magnetization. Predictions relating to the disordering of this striped ground state in the limit of monolayer film thicknesses are controversial. Mean-field arguments predict a continuous reduction of the stripe width when the temperature is increased; other studies suggest that topological defects, such as dislocations and disclinations, might penetrate the system and induce geometrical phase transitions. We find, from scanning electron microscopy imaging, that when the temperature is increased, the low-temperature stripe domain structure transforms into a more symmetric, labyrinthine structure. However, at even higher temperatures and before the loss of magnetic order, a re-occurrence of the less symmetric stripe phase is found. Despite the widespread theoretical and experimental work on striped systems, this phase sequence and the microscopic instabilities driving it have not been observed before.     

Berezinskii-Kosterlitz-Thouless crossover in a trapped atomic gas

Any state of matter is classified according to its order, and the type of order that a physical system can possess is profoundly affected by its dimensionality. Conventional long-range order, as in a ferromagnet or a crystal, is common in three-dimensional systems at low temperature. However, in two-dimensional systems with a continuous symmetry, true long-range order is destroyed by thermal fluctuations at any finite temperature. Consequently, for the case of identical bosons, a uniform two-dimensional fluid cannot undergo Bose-Einstein condensation, in contrast to the three-dimensional case. However, the two-dimensional system can form a 'quasi-condensate' and become superfluid below a finite critical temperature. The Berezinskii-Kosterlitz-Thouless (BKT) theory associates this phase transition with the emergence of a topological order, resulting from the pairing of vortices with opposite circulation. Above the critical temperature, proliferation of unbound vortices is expected. Here we report the observation of a BKT-type crossover in a trapped quantum degenerate gas of rubidium atoms. Using a matter wave heterodyning technique, we observe both the long-wavelength fluctuations of the quasi-condensate phase and the free vortices. At low temperatures, the gas is quasi-coherent on the length scale set by the system size. As the temperature is increased, the loss of long-range coherence coincides with the onset of proliferation of free vortices. Our results provide direct experimental evidence for the microscopic mechanism underlying the BKT theory, and raise new questions regarding coherence and superfluidity in mesoscopic systems.     

化学复合镀Ni—P—PTFE共沉积机理研究

通过测定钢试片上化学镀Ｎｉ－Ｐ－ＰＴＦＥ（聚四氟乙烯）复合镀层中ＰＴＦＥ粒子沉积量及分布特征，研究各种因素对ＰＴＦＥ粒子沉积的影响规律．试验表明，表面活性剂在化学镀Ｎｉ－Ｐ－ＰＴＦＥ过程中起着重要作用；同时，镀面状态及施镀工艺也有明显影响．提出了化学镀Ｎｉ－Ｐ－ＰＴＦＥ复合镀层的共沉积机理，即ＰＴＦＥ粒子通过机械碰撞及静电吸附的联合作用被试件表面俘获，实现与Ｎｉ－Ｐ共沉积     

质量比为0．2％ZnO掺杂Ba0．2Sr0．8TiO3与Ba0．2Sr0．8TiO3陶瓷的结构和变温变频复介电常数的对比测量分析

采用固相反应方法合成了（1-x％）Ba0.2Sr0.8TiO3＋x％ZnO（x=0,0．2）陶瓷材料,并分别用x射线衍射和变温变频介电谱方法,对它们的结构和复介电常数进行了对比测量分析．结果表明,质量比为0．2％ZnO的掺杂：（1）对样品的室温晶系类型没有影响,仍然为立方晶系,晶格常数仅仅减小了0．18％;（2）弥散铁电相变的相变温度由未掺杂的136K向高温移动至140K;（3）低温铁电相的复介电常数的实部ε′减小而虚部ε”增加,高温顺电相的ε’和ε″几乎不变;（4）样品中弥散铁电相变过程的ε″与测量频率f几乎无关;（5）样品中出现在高温区的离子扩散过程向低温移动．     

位错引起晶体熔化的统计理论

建立三维晶体中缺陷引起晶体熔化的统计理论，讨论了三维品体中的缺陷—位错对的形成与长大过程，通过对无相互作用位错对“成核－长大”过程的统计计算，得到了理论相变温度Ｔｃ，文中还从理论上证明了此相变为一级相交，并得到了相交潜热的计算公式，最后还对理论预言进行了实验验证。     

A superconductor to superfluid phase transition in liquid metallic hydrogen

Although hydrogen is the simplest of atoms, it does not form the simplest of solids or liquids. Quantum effects in these phases are considerable (a consequence of the light proton mass) and they have a demonstrable and often puzzling influence on many physical properties, including spatial order. To date, the structure of dense hydrogen remains experimentally elusive. Recent studies of the melting curve of hydrogen indicate that at high (but experimentally accessible) pressures, compressed hydrogen will adopt a liquid state, even at low temperatures. In reaching this phase, hydrogen is also projected to pass through an insulator-to-metal transition. This raises the possibility of new state of matter: a near ground-state liquid metal, and its ordered states in the quantum domain. Ordered quantum fluids are traditionally categorized as superconductors or superfluids; these respective systems feature dissipationless electrical currents or mass flow. Here we report a topological analysis of the projected phase of liquid metallic hydrogen, finding that it may represent a new type of ordered quantum fluid. Specifically, we show that liquid metallic hydrogen cannot be categorized exclusively as a superconductor or superfluid. We predict that, in the presence of a magnetic field, liquid metallic hydrogen will exhibit several phase transitions to ordered states, ranging from superconductors to superfluids.     

Size-dependent solid-solid phase transition process of Ag2S nanoparticles

It is well known that for a nanoparticle the solid-solid phase transition begins with the appearance of a high temperature disordered phase at the surface and the phase interface moves inward gradually with the increase of temperature. However, the size-dependent phase transition behavior remains unclear. Here we report an in-situ TEM study of the phase transition process of different-sized Ag₂S nanoparticles at atomic resolution. The onset temperature of disordered phase of the small nanoparticle is found to be lower than that of the big nanoparticle. And, the disordered phase thickness of small nanoparticle is always thicker than that of big nanoparticle. By considering surface and interface free-energy, a phenomenological model based on the minimization of system free-energy is established, which could well explain our experimental results. These discoveries extend our understanding of size dependent phase transition mechanism.     

相变潜热测量的扫描速率依赖性：RbNo3在结构相变中的热耗散

用差分扫描量热仪(DSC)测量了硝酸铷在380K至460K温度范围内升、降温过程中的三角(α)立方(β)结构相变。所用的扫描速率为1.25～40K/min。结果表明,升温相变的潜热(吸热)大于降温相变的潜热(放热),其差值△Q(Cal/g)随扫描速率的增大而增大。用结构相变中的热耗散对实验结果进行了解积。因此,可以用本文的方法来研究一级相变过程的不可逆耗散。     

Phase-slip-induced dissipation in an atomic Bose-Hubbard system

Phase-slips control dissipation in many bosonic systems, determining the critical velocity of superfluid helium and the generation of resistance in thin superconducting wires. Technological interest has been largely motivated by applications involving nanoscale superconducting circuit elements, such as standards based on quantum phase-slip junctions. Although phase slips caused by thermal fluctuations at high temperatures are well understood, controversy remains over the role of phase slips in small-scale superconductors--in solids, problems such as uncontrolled noise sources and disorder complicate their study and application. Here we show that phase slips can lead to dissipation in a clean and well-characterized Bose-Hubbard system, by experimentally studying the transport of ultracold atoms trapped in an optical lattice. In contrast to previous work, we explore a low-velocity regime described by the three-dimensional Bose-Hubbard model that is unaffected by instabilities, and we measure the effect of temperature on the dissipation strength. The damping rate of atomic motion (the analogue of electrical resistance in a solid) in the confining parabolic potential is well fitted by a model that includes finite damping at zero temperature. The low-temperature behaviour is consistent with the theory of quantum tunnelling of phase slips, whereas at higher temperatures a crossover consistent with a transition to thermal activation of phase slips is evident. Motion-induced features reminiscent of vortices and vortex rings associated with phase slips are also observed in time-of-flight imaging. These results clarify the role of phase slips in superfluid systems. They may also be of relevance in understanding the source of metallic phases observed in thin films, or serve as a test bed for theories of bosonic dissipation based upon variants of the Bose-Hubbard model.     

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.     

无序和有序GaInP2的光致发光谱

对无序和有序ＧａＩｎＰ２样品分别作了变温和变激发功率密度光致发光谱的测量．无序样品表现为与激发功率密度无关的单谱峰结构，在不同温度区有不同的激活能．有序样品则表现为双峰结构，其中高能端发光峰强度随温度升高先增强而后热猝灭．根据样品有序度及取向超晶格模型解释了实验现象     

原子无序对half-Heusler合金CoCrAl电子结构及Seebeck系数的影响

采用真空电弧熔炼和放电等离子烧结技术制备了half-Heusler合金CoCrAl,利用XRD对合金的物相结构进行表征,借助相关设备测试了合金的饱和磁矩及Seebeck系数,并将实验结果与分别基于C1b有序、XY及YZ无序结构的CoCrAl合金的模拟计算值进行比较。结果表明,所制CoCrAl合金样品中除了完全有序的C1b结构相外还存在XY及YZ无序相。合金样品表现出典型的磁性金属特性且其Seebeck系数(绝对值)随着温度的升高呈先增加后降低的趋势,这与利用VASP和BoltzTraP软件包结合计算得到的CoCrAl合金的Seebeck系数随温度(300～800K)的变化趋势相符合。     

A possible Zpif's law in the nuclear disassembly

Zpif's law in the field of linguistics is tested in the nuclear disassembly within the framework of isospin dependent lattice gas model. It is found that the average cluster charge (or mass) of rank n in the charge (or mass) list shows exactly an inverse order to its rank, i.e., Zpif's law appears at the phase transition temperature. This novel criterion shall be helpful in searching the nuclear liquid gas phase transition.     

Macroscopically ordered state in an exciton system

There is a rich variety of quantum liquids -- such as superconductors, liquid helium and atom Bose-Einstein condensates -- that exhibit macroscopic coherence in the form of ordered arrays of vortices. Experimental observation of a macroscopically ordered electronic state in semiconductors has, however, remained a challenging and relatively unexplored problem. A promising approach for the realization of such a state is to use excitons, bound pairs of electrons and holes that can form in semiconductor systems. At low densities, excitons are Bose-particles, and at low temperatures, of the order of a few kelvin, excitons can form a quantum liquid -- that is, a statistically degenerate Bose gas or even a Bose-Einstein condensate. Here we report photoluminescence measurements of a quasi-two-dimensional exciton gas in GaAs/AlGaAs coupled quantum wells and the observation of a macroscopically ordered exciton state. Our spatially resolved measurements reveal fragmentation of the ring-shaped emission pattern into circular structures that form periodic arrays over lengths up to 1 mm.     

不同一级相变材料的内耗峰形成机制

试验研究了3种一级相变材料的相变过程,即非晶PdNiCuP的晶化与无序和有序型相变、形状记忆合金Cu 11.9Al 2.5Mn的无扩散位移型相变以及共析合金Zn 22Al的共析相变,设计并测量了3种材料的电阻和内耗,并采用插值法获得不同温度下内耗与频率的关系曲线.结果表明,3种材料的相变内耗峰不是弛豫型内耗峰,而是弛豫参数发生突变的结果,相变内耗峰高度与频率的关系是弛豫的表现.在低温区域,观测到其相变内耗峰的结构弛豫现象,并可用结构失稳和结构弛豫的概念定性解释升温速率对相变内耗峰高度的影响.