二维玻色系统超流密度的重整化群研究

利用Holzmann-Baym超流密度公式以及准确到一圈图的重整化群方法,计算了零温下二维弱相互作用玻色体系的超流密度和超流刚度。计算表明,当高能涨落的贡献占主导地位时,超流密度与超流刚度对化学势的依赖关系会呈现标度行为, 与前人的研究结果相一致。     

Coherent zero-state and pi-state in an exciton-polariton condensate array

The effect of quantum statistics in quantum gases and liquids results in observable collective properties among many-particle systems. One prime example is Bose-Einstein condensation, whose onset in a quantum liquid leads to phenomena such as superfluidity and superconductivity. A Bose-Einstein condensate is generally defined as a macroscopic occupation of a single-particle quantum state, a phenomenon technically referred to as off-diagonal long-range order due to non-vanishing off-diagonal components of the single-particle density matrix. The wavefunction of the condensate is an order parameter whose phase is essential in characterizing the coherence and superfluid phenomena. The long-range spatial coherence leads to the existence of phase-locked multiple condensates in an array of superfluid helium, superconducting Josephson junctions or atomic Bose-Einstein condensates. Under certain circumstances, a quantum phase difference of pi is predicted to develop among weakly coupled Josephson junctions. Such a meta-stable pi-state was discovered in a weak link of superfluid 3He, which is characterized by a 'p-wave' order parameter. The possible existence of such a pi-state in weakly coupled atomic Bose-Einstein condensates has also been proposed, but remains undiscovered. Here we report the observation of spontaneous build-up of in-phase ('zero-state') and antiphase ('pi-state') 'superfluid' states in a solid-state system; an array of exciton-polariton condensates connected by weak periodic potential barriers within a semiconductor microcavity. These in-phase and antiphase states reflect the band structure of the one-dimensional polariton array and the dynamic characteristics of metastable exciton-polariton condensates.     

克尔非线性黑体中光超流态的研究

研究了克尔非线性黑体中一种新的光超流态.研究表明:非线性黑体中具有相反波矢和旋量的裸光子结合成对,未成对的裸光子则转换成一种新的准粒子--非极化激元;光子对系统是一个超流态,而非极化激元系统是一个正常流态.正常流态的光谱能量密度和辐射压强都比普通黑体大并且随温度单调增加.在理论上证明了克尔非线性黑体中光超流态的存在并给出了测量这种光超流态的方法.     

微观相互作用玻色子模型中热166,168,170Er核比热容及其相变的数值研究

应用微观相互作用玻色子模型，制备了偶166，168，170Er同位素的零温能谱，其值与最新的实验结果符合得比较好；采用正则系综理论，计算出核的有限温度比热容，基地比热峰是相变信号和对序参量的分析，有可能统一描述核发生的热激发模式相变，核形状相变和从超流相到正常相相变。     

Strong dipolar effects in a quantum ferrofluid

Symmetry-breaking interactions have a crucial role in many areas of physics, ranging from classical ferrofluids to superfluid (3)He and d-wave superconductivity. For superfluid quantum gases, a variety of new physical phenomena arising from the symmetry-breaking interaction between electric or magnetic dipoles are expected. Novel quantum phases in optical lattices, such as chequerboard or supersolid phases, are predicted for dipolar bosons. Dipolar interactions can also enrich considerably the physics of quantum gases with internal degrees of freedom. Arrays of dipolar particles could be used for efficient quantum information processing. Here we report the realization of a chromium Bose-Einstein condensate with strong dipolar interactions. By using a Feshbach resonance, we reduce the usual isotropic contact interaction, such that the anisotropic magnetic dipole-dipole interaction between 52Cr atoms becomes comparable in strength. This induces a change of the aspect ratio of the atom cloud; for strong dipolar interactions, the inversion of ellipticity during expansion (the usual 'smoking gun' evidence for a Bose-Einstein condensate) can be suppressed. These effects are accounted for by taking into account the dipolar interaction in the superfluid hydrodynamic equations governing the dynamics of the gas, in the same way as classical ferrofluids can be described by including dipolar terms in the classical hydrodynamic equations. Our results are a first step in the exploration of the unique properties of quantum ferrofluids.     

Has the Higgs boson been discovered?

The standard model of particle physics describes the strong and electroweak interactions of fermions (spin-1/2), gauge bosons (spin-1) and a final vital ingredient--the spin-0 Higgs boson, which gives masses to the other particles. But the Higgs boson has yet to be discovered, and its own mass is not specified by the theory. There is some evidence (although statistically not very significant) for its detection at a mass of about 115 GeV/c2, from electron-positron interactions at LEP (the Large Electron Positron collider). Indirect methods can also be used to constrain the mass of the Higgs boson, because it affects other observable quantities (for example, the mass of the W boson and some measurable properties of the Z boson). An indirect determination of the Higgs boson mass from the most recent measurements of such quantities yields a value compatible with 115 GeV/c2, but with some important caveats arising from inconsistencies in the present data.     

Determination of the fermion pair size in a resonantly interacting superfluid

Fermionic superfluidity requires the formation of particle pairs, the size of which varies from the femtometre scale in neutron stars and nuclei to the micrometre scale in conventional superconductors. Many properties of the superfluid depend on the pair size relative to the interparticle spacing. This is expressed in 'BCS-BEC crossover' theories, describing the crossover from a Bardeen-Cooper-Schrieffer (BCS)-type superfluid of loosely bound, large Cooper pairs to Bose-Einstein condensates (BECs) of tightly bound molecules. Such a crossover superfluid has been realized in ultracold atomic gases where high-temperature superfluidity has been observed. The microscopic properties of the fermion pairs can be probed using radio-frequency spectroscopy. However, previous work was difficult to interpret owing to strong final-state interactions that were not well understood. Here we realize a superfluid spin mixture in which such interactions have negligible influence and present fermion pair dissociation spectra that reveal the underlying pairing correlations. This allows us to determine that the spectroscopic pair size in the resonantly interacting gas is 20 per cent smaller than the interparticle spacing. These are the smallest pairs so far observed in fermionic superfluids, highlighting the importance of small fermion pairs for superfluidity at high critical temperatures. We have also identified transitions from fermion pairs to bound molecular states and to many-body bound states in the case of strong final-state interactions.     

Quantum phase transition from a superfluid to a Mott insulator in a gas of ultracold atoms.

For a system at a temperature of absolute zero, all thermal fluctuations are frozen out, while quantum fluctuations prevail. These microscopic quantum fluctuations can induce a macroscopic phase transition in the ground state of a many-body system when the relative strength of two competing energy terms is varied across a critical value. Here we observe such a quantum phase transition in a Bose-Einstein condensate with repulsive interactions, held in a three-dimensional optical lattice potential. As the potential depth of the lattice is increased, a transition is observed from a superfluid to a Mott insulator phase. In the superfluid phase, each atom is spread out over the entire lattice, with long-range phase coherence. But in the insulating phase, exact numbers of atoms are localized at individual lattice sites, with no phase coherence across the lattice; this phase is characterized by a gap in the excitation spectrum. We can induce reversible changes between the two ground states of the system.     

Gluon propagator in a new perturbative expansion of Quantum Chromodynamics with a non-perturbative background

Using a new perturbative expansion method in Quantum Chromodynamics with a non-perturbative gluon background, the gluon propagator is calculated up to the one-loop level, and renormalized in the modified minimal subtraction scheme. The resultant renormalization constants of the quantum gluon field and the gauge parameter receive a non-perturbative contribution coming from the gluon condensate <FF> besides the usual perturbative one, respectively. Supported by the National Natural Science Foundation of China Wang Wenyu: born in 1973, Graduate student     

弱CM环的性质

主要研究特殊Noether交换环弱CM环的性质.这类环包括Cohen-Macaulay环、优秀环和广义Cohen-Macaulay环,能够用局部上同调模来刻画.证明了如果R是弱CM环,则R的有限生成代数, 以及R关于理想I的I-adic完备化环均为弱CM环.     

液氦超流转变温度复现性研究

利用液氦超流转变时超流氦（He Ⅱ）与正常氦（He Ⅰ）热导率突变的性质复现液氦超流转变温度T_λ。采用带毛细管结构的小型密封瓶,通过控温将微小热流通过小型密封瓶毛细管,实现毛细管中He Ⅰ/He Ⅱ两相共存,并使He Ⅰ/He Ⅱ界面停留在毛细管中,从而获得稳定、平坦的液氦超流转变温坪。利用毛细管热流对液氦超流转变温度的下压效应,得到不同热流的多个温坪,进一步利用外推法求得零热流下真实的T_λ值。24次液氦超流转变温度复现实验结果表明,标准偏差为0.022mK,证明了液氦超流转变温度具有良好的稳定性和复现性,推荐将液氦超流转变温度作为国际温标的固定点使用。     

Phonon interpretation of the 'boson peak' in supercooled liquids

Glasses are amorphous solids, in the sense that they display elastic behaviour. In crystalline solids, elasticity is associated with phonons, which are quantized vibrational excitations. Phonon-like excitations also exist in glasses at very high (terahertz; 10(12) Hz) frequencies; surprisingly, these persist in the supercooled liquids. A universal feature of such amorphous systems is the boson peak: the vibrational density of states has an excess compared to the Debye squared-frequency law. Here we investigate the origin of this feature by studying the spectra of inherent structures (local minima of the potential energy) in a realistic glass model. We claim that the peak is the signature of a phase transition in the space of the stationary points of the energy, from a minima-dominated phase (with phonons) at low energy to a saddle-point-dominated phase (without phonons). The boson peak moves to lower frequencies on approaching the phonon-saddle transition, and its height diverges at the critical point. Our numerical results agree with the predictions of euclidean random matrix theory on the existence of a sharp phase transition between an amorphous elastic phase and a phonon-free one.     

Observation of the ideal Josephson effect in superfluid 4He

Superfluids and superconductors are the only states of condensed matter that can be described by a single wavefunction, with a coherent quantum phase Phi. The mass flow in a superfluid can be described by classical hydrodynamics for small flow velocity, but above a critical velocity, quantized vortices are created and the classical picture breaks down. This can be observed for a superfluid flowing through a microscopic aperture when the mass flow is measured as a function of the phase difference across the aperture; the curve resembles a hysteretic sawtooth where each jump corresponds to the creation of a vortex. When the aperture is made small enough, the system can enter the so-called 'ideal' Josephson regime, where the superfluid mass flow becomes a continuous function of the phase difference. This regime has been detected in superfluid 3He, but was hitherto believed to be unobservable, owing to fluctuations, in 4He. Here we report the observation of the ideal Josephson effect in 4He. We study the flow of 4He through an array of micro-apertures and observe a transition to the ideal Josephson regime as the temperature is increased towards the superfluid transition temperature, Tlambda.     

On extra neutral and charged Higgs bosons at the LHC

The properties of 125 GeV new particle, which was discovered in 2012 at the Large Hadron Collider (LHC), are found to be consistent with those of the Higgs boson in the standard model (SM). Hereafter the new particle is dubbed as SM-like Higgs boson. However there is still spacious room for physics beyond the SM (BSM) due to the limited energy and luminosity of the LHC. With more data, experiments will scrutinize whether the new particle is indeed the SM one or not. At the same time, one believes that discovery of the SM-like Higgs boson is just the start of the new era of particle physics. The predominant topic is whether there are other new Higgs bosons as speculated in various BSM models. In this short review, we will describe the current status of Higgs physics at the LHC and several BSM models which contain more Higgs sectors. In literatures, there are numerous studies on extended Higgs sector and a comprehensive review is beyond the scope of this review. Instead, we will present two latest studies on Higgs physics: (1) how to detect the charged Higgs boson and measure \(\tan \beta \) after including the top polarization information, and (2) how to discover the extra neutral Higgs boson via the pair production of SM-like Higgs boson.     

Van der Pol振子在随机激励下的Hopf分叉

研究了Van der Pol振子在宽带随机外部激励和宽带随机参数激励联合作用下的Hopf分叉.文中采用随机平均法将Van del Pol方程的幅值响应过程逼近为一维的Markov扩散过程.利用FPK方程的直接积分,得到了幅值响应的稳态概率密度函数.在此基础上,分析了系统在分叉点附近由于随机扰动的影响带来的系统局域行为的变化.从本文的研究中发现,非线性系统在随机外部激励以及两种不同的随机参数激励作用下,其分叉行为与确定性系统相比会有明显的变化。     

Direct observation of Anderson localization of matter waves in a controlled disorder

In 1958, Anderson predicted the localization of electronic wavefunctions in disordered crystals and the resulting absence of diffusion. It is now recognized that Anderson localization is ubiquitous in wave physics because it originates from the interference between multiple scattering paths. Experimentally, localization has been reported for light waves, microwaves, sound waves and electron gases. However, there has been no direct observation of exponential spatial localization of matter waves of any type. Here we observe exponential localization of a Bose-Einstein condensate released into a one-dimensional waveguide in the presence of a controlled disorder created by laser speckle. We operate in a regime of pure Anderson localization, that is, with weak disorder-such that localization results from many quantum reflections of low amplitude-and an atomic density low enough to render interactions negligible. We directly image the atomic density profiles as a function of time, and find that weak disorder can stop the expansion and lead to the formation of a stationary, exponentially localized wavefunction-a direct signature of Anderson localization. We extract the localization length by fitting the exponential wings of the profiles, and compare it to theoretical calculations. The power spectrum of the one-dimensional speckle potentials has a high spatial frequency cutoff, causing exponential localization to occur only when the de Broglie wavelengths of the atoms in the expanding condensate are greater than an effective mobility edge corresponding to that cutoff. In the opposite case, we find that the density profiles decay algebraically, as predicted in ref. 13. The method presented here can be extended to localization of atomic quantum gases in higher dimensions, and with controlled interactions.     

三腿梯子上硬核玻色-哈伯德模型的量子蒙特卡罗研究

运用集团平均场和密度矩阵重整化群方法得到三腿梯子中玻色-哈伯德模型基于z方向密度交错排布的超固体相,比单层三角晶格多一种超固体.然而平均场超流序参量没有考虑超流的方向,这种超固体的本质有待于进一步探索.本文运用可靠的有向圈随机序列展开量子蒙特卡罗方法模拟了三腿梯子上硬核玻色-哈伯德模型,通过测量层内和层间的结构因子和超流刚硬度,发现系统具有密度ρ=1/2的固体,增加量子隧穿后,z方向不存在超固体.计算结果有助于冷原子实验寻找新的量子相.     

Structural diversity and reactivity of d^10 metal- （4-pyridylthio）acetate system

The hydrothermal synthesis and crystal structures of four coordination polymers, namely, 2D [Zn（μ3-ta）（pytaH）]n （1）, 2D [Zn（μ3-pyta）Cl]n （2）, 1D [Cd（μ-pyta）2（H2O）]n （3）, and 3D [Cd（μ3-pyta）（μ-Cl）]n （4） （pyta = （4-pyridylthio）acetate, ta = thioglycolate）, are reported. They are based on （4-pyridylthio）acetate and its derived ligand. The ta^2- ligand present in 1 was generated from an in situ C（sp^2）-S bond cleavage of the pyta ligand. In these compounds, versatile intermolecular interactions, such as close S…S interactions and strong （O-H…O/N/S） or weak （C-H…O/S, C-H…Cl） hydrogen bonding interactions, play an important role in the formation of three-dimensional supramolecular networks in the solid state.