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.     

La_(0.5)Ca_(0.5)MnO_3纳米颗粒的铁磁相变临界行为研究

La0.5Ca0.5MnO3块体陶瓷表现出电荷有序态特性,但是当La0.5Ca0.5MnO3颗粒减小后电荷有序态将被破坏.本文采用磁测量法研究了La0.5Ca0.5MnO3纳米颗粒的铁磁相变临界行为.结果表明La0.5Ca0.5MnO3纳米颗粒的铁磁相变临界行为可用平均场模型很好地拟合,意味着La0.5Ca0.5MnO3纳米颗粒的铁磁相变为长程有序作用特征.输运行为表明,外加磁场可诱导La0.5Ca0.5MnO3纳米颗粒产生绝缘体-金属转变,可归因于外加磁场使得晶界势垒降低所致.     

Hubbard模型中格林函数的数值计算

从Hubbard模型出发,由动态平均场论方法结合量子蒙特卡洛模拟数值求解系统的格林函数,重点讨论在顺磁序和反磁序情形下格林函数的数值结果,给出在不同温度和不同Hubbard能下格林函数的变化情形,从格林函数的变化可以分析得到在温度降低或Hubbard能升高时,系统会发生从金属到绝缘体的Mott转变.     

Cu掺杂AlN的铁磁稳定性的第一性原理计算

文章采用基于密度泛函理论的平面波超软赝势法,结合广义梯度近似计算了Cu掺杂AlN的晶格常数、能带结构、电子态密度、铁磁态和反铁磁态的总能量,并通过平均场近似的海森堡模型估算了居里温度Tc。结果表明,Cu掺杂体系的能带结构呈现半金属性,半金属能隙为0.442eV。铁磁性是Cu原子的3d态与其最近邻的N原子的2p态通过p-d杂化作用而稳定的。当两个Cu原子相距最远且自旋平行排列时,体系具有最低的能量,估算出此时的居里温度高于室温,因此Cu掺杂AlN有望作为稀磁半导体材料。     

高自旋BC模型基态相变的研究

采用相关有效场理论,对具有正方点阵结构的S=2横向BC模型,推导出系统的磁矩公式,并利用数值计算方法得到了基态相图。研究发现三种相变线将相图分成三个区域:有序铁磁相O1、有序铁磁相O2及无序顺磁相P,一级有序-有序相变线将铁磁相分成了O1及O2,并使铁磁相O2的区域非常狭窄。     

掺锰钙钛矿系统中外磁场与转换温度的关系

用渗流模型研究了(La1-xBx)2/3Ca1/3MnO3系统由顺磁性向铁磁性转换过程中外磁场与转换温度的关系.在平均场理论中,电阻是外磁场和温度的函数.研究发现:当外加磁场不太强时,转化温度随着外磁场的增加而线性地增加,理论上的计算与最近的实验观察相当符合.     

含铁磁耦合玻色气体磁化率的研究

采用平均场理论研究三维含铁磁耦合相互作用玻色气体的磁化率, 讨论了磁化率χ 与约化温度t、约化铁磁 耦合常数?I、朗德因子g 以及约化磁场强度h 的关系.结果表明,顺磁性和弱铁磁耦合强度下铁磁性的增加均有利于磁 化率的增大.      

掺杂二维Ising系统的相图研究

用Monte-Carlo模拟方法研究了非磁性掺杂下二维Ising系统的相图.在平面四角点阵上,以晶格Ising模型为框架,在周期性边界条件下详细观测了不同非磁性物质掺杂浓度下系统磁化强度和磁畴生长随温度变化的关系,并根据有限尺寸标度理论得到其相变温度.结果发现:当掺杂浓度较低时,存在铁磁与顺磁间的相变;而掺杂浓度较高时,在仅考虑近邻作用下系统无铁磁相.当考虑次近邻相互作用时,发现相变温度将升高.另外考察了不同掺杂浓度下的自旋密度关联.     

二维含铁磁耦合费米气体磁学性质的研究?

在平均场理论的框架下,推导出外磁场中具有铁磁耦和作用的二维费米气体的自洽方程,在此基础上,运用量子统计力学研究系统的磁学性质.结果表明总磁化强度密度随磁场强度的增大会出现极大值点,且随铁磁耦合强度增大,极大值点向磁场减弱的方向移动.总磁化强度密度也随Lande因子的增大,发生由抗磁性向顺磁性的转变.     

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.     

La0.7Ca0.3Mn1-xCuxO3的电输运特性

通过固相反应法制备不同掺杂浓度的La_0.7Ca_0.3Mn_1-xCu_xO₃（x=0~0.15）样品, 在77~300 K温度范围内测量了铜掺杂后样品电阻随温度的变化关系. 结果表明, 在未引入铜杂质时, 低温铁磁相的电阻率满足T^2.5关系, 顺磁相符合小极化子近邻跳跃模型; 在低掺杂下, 铁磁区可用T^4.5关系解释; 随着掺杂浓度的增加, 在铁磁区和顺磁区, 任何单一模型均与实验不相符, 表明高掺杂样品的电输运性质存在未知模型.      

一维Sznajd舆论模型相变的研究

略去一维标准Sznajd舆论模型的第二个规则,修改后的模型演化的决定时间分布没有发生变化,但弛豫时间延长了;反铁磁性吸引子消失了,铁磁性终态吸引子出现的概率对初始态度人员密度有着重要的依赖关系.进一步在平均场的近似下,修改后的一维Sznajd舆论模型存在相变.     

Observation of scale invariance and universality in two-dimensional Bose gases

The collective behaviour of a many-body system near a continuous phase transition is insensitive to the details of its microscopic physics; for example, thermodynamic observables follow generalized scaling laws near the phase transition. The Berezinskii-Kosterlitz-Thouless (BKT) phase transition in two-dimensional Bose gases presents a particularly interesting case because the marginal dimensionality and intrinsic scaling symmetry result in a broad fluctuation regime and an extended range of universal scaling behaviour. Studies of the BKT transition in cold atoms have stimulated great interest in recent years, but a clear demonstration of critical behaviour near the phase transition has remained elusive. Here we report in situ density and density-fluctuation measurements of two-dimensional Bose gases of caesium at different temperatures and interaction strengths, observing scale-invariant, universal behaviours. The extracted thermodynamic functions confirm the existence of a wide universal region near the BKT phase transition, and provide a sensitive test of the universality predicted by classical-field theory and quantum Monte Carlo calculations. Our experimental results provide evidence for growing density-density correlations in the fluctuation region, and call for further explorations of universal phenomena in classical and quantum critical physics.     

钙钛矿La0.7Pb0.3MnO3的磁电阻效应及输运特性

采用固相反应烧结法成功制备了钙钛矿La0.7Pb0.3MnO3单相多晶样品,利用超导量子干涉仪和直流四探针法主要研究了La0.7Pb0.3MnO3块体的磁电阻效应及输运特性.结果表明,此样品在居里点附近由铁磁向顺磁过渡,且表现出典型的团簇玻璃特征.磁电阻的极大值出现在居里温度附近.当H=0.5T时,磁电阻的极大值为6.8%,当H=1T时,磁电阻的极大值为12.4%.输运结果表明,T1区金属导电是由于电子散射造成的,T2区半导体特性是由于小极化子跃迁引起的.     

磁电耦合对于铁电磁系统的磁性关联的影响

铁电磁系统具有在低温下铁电有序和(反)铁磁有序自发共存的特征,存在内禀的磁电耦合效应.利用了铁电软模理论和基于海森堡模型的平均场近似理论,分别研究了磁电耦合对铁电、铁磁有序共存系统和铁电、反铁磁有序共存系统的磁性质的影响.结果发现在一定的外磁场条件下,无论是铁电、铁磁有序共存的系统还是铁电、反铁磁有序共存的系统,磁电耦合都相当于一个虚拟的外磁场,且系统的铁磁性关联会随着耦合常数的增加而增加.     

La_(0.85)Na_(0.15)MnO_3的结构、磁性质和输运性质

实验研究了由 Pechini方法制备的 L a0 .85Na0 .1 5Mn O3多晶陶瓷样品的结构、磁性质和输运性质 .发现 L a0 .85Na0 .1 5Mn O3的居里温度正好接近室温 ,在该温度附近 ,伴随铁磁相到顺磁相的转变 ,同时发生金属 -半导体转变 ,且在居里温度附近 ,0 .5 T外磁场下的最大磁电阻可达 14 % .理论与实验对比可知 ,在居里温度以上小极化子非对角跳跃为主要导电机制 .     

La2/3Pb1/3MnO3外延膜的结构及磁电特性研究

通过射频磁控溅射法在单晶LaAlO3(100)衬底上成功的沉积了膜厚为300 nm的La2/3Pb1/3MnO3外延膜。利用X射线衍射仪、原子力和超导量子干涉仪、直流四探针法对其结构、磁电特性进行了系统的研究。结果表明,薄膜为赝立方钙钛矿结构,沿(100)方向择优生长,具有良好的单晶外延结构。居里温度TC=345 K,在居里温度附近,发生铁磁-顺磁转变。此材料呈现出一种典型的自旋玻璃特性,是由于应力造成的。对于顺磁态、自旋玻璃态及铁磁态时其磁矩分布给予了合理的解释。在1 T磁场下,其磁电阻极大值为23.4%。     

Mn基钙钛矿样品相图修正及庞磁电阻起因

对具有不同A位平均离子尺寸的AMnO3型Mn基钙钛矿样品的阻温关系和电子顺磁共振谱等进行了实验研究．结果表明,在温度与A位平均离子尺寸的相图上,除了普遍公认的高温顺磁绝缘体相和低温长程铁磁有序的金属相外,在两相之间还存在一过渡相,其起因源于顺磁绝缘体背景上出现短程铁磁有序的金属集团所致．研究结果还表明,外加磁场加速铁磁金属集团的生成和长大是导致庞磁电阻效应的主要原因．     

亚硝酸钠多晶与粉末中铁电相变的研究

对NaNO 2粉末与多晶材料的复介电常数随温度的变化情况进行了讨论,并研究了其铁电相变过程.结果表明：1）高温（420K以上）低频（100Hz以下）条件下,NaNO2粉末及多晶材料复介电常数的变化主要是由离子扩散引起的.而高频（105Hz,106Hz）复介电常数实部的值在相变温度附近基本与频率无关,能够反映材料的极化和结构信息.2）在443K～450K范围内, NaNO 2粉末样品的激活能U为0.70eV,NaNO 2多晶的激活能U为1.58eV,NaNO 2多晶的激活能要大于NaNO2粉末的激活能,这说明NaNO2粉末与多晶中离子扩散可能起源于材料的本征缺陷.3） NaNO2粉末中铁电相变在复介电常数曲线上的表现比NaNO2多晶明显;临界温度Tc以上437K至440K范围内,NaNO 2多晶的复介电常数实部随温度的变化曲线偏离居里-外斯定律,说明该铁电相变过程中存在反常现象,这一现象与NaNO 2单晶的结果相吻合.     

Scaling of entanglement close to a quantum phase transition

Classical phase transitions occur when a physical system reaches a state below a critical temperature characterized by macroscopic order. Quantum phase transitions occur at absolute zero; they are induced by the change of an external parameter or coupling constant, and are driven by quantum fluctuations. Examples include transitions in quantum Hall systems, localization in Si-MOSFETs (metal oxide silicon field-effect transistors; ref. 4) and the superconductor-insulator transition in two-dimensional systems. Both classical and quantum critical points are governed by a diverging correlation length, although quantum systems possess additional correlations that do not have a classical counterpart. This phenomenon, known as entanglement, is the resource that enables quantum computation and communication. The role of entanglement at a phase transition is not captured by statistical mechanics-a complete classification of the critical many-body state requires the introduction of concepts from quantum information theory. Here we connect the theory of critical phenomena with quantum information by exploring the entangling resources of a system close to its quantum critical point. We demonstrate, for a class of one-dimensional magnetic systems, that entanglement shows scaling behaviour in the vicinity of the transition point.