Study of γ d → π + π − d Reaction in an Energy Region of 0.67 ≤ E γ ≤ 1.08 GeV

Double charged pion photoproductions from the deuteron have been studied at Research Center for Electron Photon Science (ELPH), Tohoku University. Tagged photon beams in an energy range of 0.67 ≤ E _γ ≤ 1.08 GeV were impinged on the liquid deuteron target. Produced charged particles were observed with the Neutral Kaon Spectrometer2 (NKS2). The γ d → π ⁺ π ^? d process was clearly separated among the three charged tracks. We obtained the photon energy dependence of the normalized yields of this process.     

200GeV/u的~(238)U+~(238)U核碰撞产生的双轻子谱(M≤4GeV/c~2)的计算

在夸克味动力学的计算中,除包含ｕ、ｄ、ｓ夸克及其反夸克外,还涉及粲夸克及其反夸克（ｃｃ）,采用包含夸克味动力学效应的相对论性流体力学模型,计算了碰撞能量为２００ＧｅＶ／ｕ的２３８Ｕ＋２３８Ｕ核碰撞时形成的膨胀ＱＧＰ及其在相变过程中发射的双轻子谱,轻子对的质量范围Ｍ≤４ＧｅＶ／ｃ２．计算结果与ＣＥＲＮＳＰＳ的实验测量数据进行了定性的比较和分析,得到初步的结论：由于夸克碎块和味动力学效应,Ｊ／ψ→μ＋＋μ－峰值被降低,导致低质量区（阈至１．３５ＧｅＶ／ｃ２）谱的提高．     

200GeV/u的238U+238U核碰撞产生的双轻子谱(M≤4GeV/c2)的计算

在夸克味动力学的计算中,除包含u、d、s夸克及其反夸克外,还涉及粲夸克及其反夸克(cc),采用包含夸克味动力学效应的相对论性流体力学模型,计算了碰撞能量为200GeV/u的²³⁸U+²³⁸U核碰撞时形成的膨胀QGP及其在相变过程中发射的双轻子谱,轻子对的质量范围M≤4GeV/c².计算结果与CERNSPS的实验测量数据进行了定性的比较和分析,得到初步的结论:由于夸克碎块和味动力学效应,J/ψ→μ⁺+μ^－峰值被降低,导致低质量区(阈至1.35GeV/c²)谱的提高.     

2—5GeV能区的R值测量

作为粒子物理中的一个基本参数,R值在发展粒子物理理论及检验标准模型（SM）方面起着重要的作用。在低能区精确测量R值的实验努力对于弱电精确物理是至关重要的。R值测量不仅对α(M^2Z)的计算及αu的解释是重要的,而且对强子产生机制和经e^ e^-湮没产生的粲偶素本身的理解也是必须的。文章简要报道了在北京正负电子对撞机（BEPC）和升级的北京谱仪（BES－Ⅱ）上完成的R扫描及其物理结果。     

La2—xBaxCuO4（0≤x≤0.22）的低温结构不稳定性研究

测量了Ｌａ２－ｘＢａｘＣｕＯ４单相多晶材料在１０Ｋ－３００Ｋ的纵波超声声速和超 声衰减，在高温四方相向低温正交相结合相变温度附近，发现强烈的晶格软化现象并伴有明显的超声内耗峰，温度低于６０Ｋ在所有样品中均发现明显的超声异常现象，这表明在ｘ＝０到ｘ＝０范围内Ｌａ２－ｘＢａｘＣｕＯ４中都可能存在不稳定的低温结构相变。ｘ＝０，０．１２和０．２２几个样品的超声衰减在低温下明显高于其它超志性能好的样品，这可     

Transport properties of the single-crystal La2−xBaxCuO4 (0≤x≤0.11)

We report on the temperature dependence of the in-plane electrical resistivity ρ_ab and the in-plane Hall coefficient R_H in various magnetic fields of the single-crystal La_2−xBa_xCuO₄ with x=0.083 and 0.11. In x=0.11, which is close to x=1/8, where the superconductivity is strongly suppressed, a clear jump in ρ_ab and a drop in R_H have been observed at T_d2∼53 K, where the structural phase transition between the orthorhombic mid-temperature and tetragonal low-temperature phases occurs. Moreover, a sign reversal of R_H has been observed below ∼25 K and the magnitude of the sign reversal increases with increasing magnetic field. In x=0.083, on the other hand, there is neither jump in ρ_ab nor drop in R_H at T_d2, and also no sign reversal in R_H at low temperatures even in magnetic fields up to 9 T. In conclusion, there is no doubt that a static stripe order of holes and spins, observed in La_1.6−xNd_0.4Sr_xCuO₄ with x∼1/8, is formed below T_d2 also in La_2−xBa_xCuO₄ with x∼1/8. The R_H in the stripe-ordered state has a negative value, which is consistent with the recent theory by Prelovšek et al.     

2.22 GeV ηη′ structure observed in 38 GeV / c and 100 GeV / c π−p collisions

A structure has been observed at 2220 MeV in the mass spectrum of ηη′ systems produced by 38GeV/c and 100 GeV/c′ negative pions on protons. The angular distribution of the decay products shows that this structure is presumably a spin J ⩾ 2 meson.     

Helimagnetism of CuxZn1−xCr2Se4 (for 0.0 ≤ × ≤ 0.1)

Abstract

The magnetization, the susceptibility and the magnetic anisotropy field of Cu _x Zn_1?xCr₂Se₄ compounds have been studied at low temperatures (down to 2.9 K) in: high magnetic stationary fields (up to 14 T), high pulsed magnetic fields (up to 25 T), medium magnetic stationary fields (up to 0.6 T). The magnetic structure of these spinels was studied by neutron powder diffraction.

The magnetic properties of Cu_xZn_1?xCr₂Se₄ are explained in terms of the molecular field approximation assuming the existence of 90° exchange interactions, ferromagnetic for Cr-Se-Cr between the nearest Cr ions and antiferromagnetic for Cr-Se-Se-Cr between the second-nearest Cr ions. The exchange parameters and integrals for the whole series under consideration are calculated. Taking into account the three magnetic phase transitions observed in these spinels (Juszczyk, Krok, Okońska-Koz?owska, Broda, Warczewski, Byszewski, 1981) and the neutron diffraction studies a modification of the simple spin spiral forced by a strong magnetic field is described.     

Mg_n(2≤n≤15)的几何结构和振动频率及光谱研究

采用基于密度泛函理论的B3LYP方法计算了Mgn(2≤n≤15)团簇的键长、能量、二阶能量差分、配位数、能隙、光谱等性质,找到了Mgn(2≤n≤15)团簇的较稳定结构并确定幻数团簇,然后对这些稳定结构进行光谱分析。采用3-21g和lanl2dz一大一小两种基组分别进行计算,结果表明,两种基组计算结果在基态结构上相差不大,只有二阶能量差分有较小的区别,lanl2dz基组计算的键长偏长;Mg4、Mg10、Mg15为较稳定结构,其中Mg4、Mg10为幻数团簇;除Mg2之外,其他团簇都有红外、拉曼活性;对稳定结构的光谱分析发现,Mg10的红外谱峰最多,Mg15的光谱强度最大。     

Tin-mZrmO2n(n=2-7，0≤m≤n)团簇结构和稳定性研究

应用密度泛函理论中的B3LYP方法对Tin-mZrmO2n (n = 2-7,0≤m≤n) 团簇的基态几何结构、相对稳定性和电子结构进行了理论研究.结果表明,与桥氧链接的Ti原子被Zr原子取代后形成的混合团簇较为稳定;在团簇尺寸一定的条件下（即n相同）,随着Zr原子数m的增加,团簇的结合能基本呈线性增大,团簇的稳定性增强;Ti、Zr、O原子之间发生了电荷转移现象,形成了稳定的Ti-O-Zr键.     

高离化Kr~(33+)离子2~2P-n~2S(2≤n≤9)的偶极跃迁行为（英文）

本文构造了高离化Kr~(33+)离子1s~2ns(2≤n≤9)和1s22p态的波函数并利用其计算了体系的非相对论能量.为了得到高精度的理论计算结果,将相对论修正和质量极化效应作为一级微扰计算了体系的总能量和2~2P-n~2S(2≤n≤9)跃迁能,本文的结果与有限的实验数据符合的较好.在此基础上完成了2~2Pn~2S(2≤n≤9)偶极跃迁三种规范下振子强度的理论计算,三者的一致性进一步证明所构造的波函数在整个空间的准确性和可靠性.     

Lithium/vacancy ordering in the monoclinic LixNiO2 (0.50≤x≤0.75) solid solution

Lithium de-intercalation from the nearly stoichiometric LiNiO₂ phase (z=0.02 in Li_1−zNi_1+zO₂) entails a structural transition from the rhombohedral to the monoclinic symmetry. As this macroscopic lattice distortion appears in a wide composition range (0.50<x<0.75, x in `Li_xNiO₂'), a detailed electron diffraction study has been performed on the intermediate Li_0.63Ni_1.02O₂ composition in order to find the driving force of this transition. A superstructure cell, four times bigger than the one deduced from the X-ray Rietveld refinement, has been derived from the electron diffraction data. The existence of such a superstructure is suggestive of a possible ordering of vacancies among the lithium layers. Indeed, the existence of three lithium crystallographic positions in this supercell allows us to understand, on the one hand, why a monoclinic distortion occurs, and, on the other hand, its composition limits (Li_0.50□_0.25□_0.25′NiO₂ and Li_0.50Li_0.25□_0.25′NiO₂). Moreover, the presence of twinned crystals has also been shown in this monoclinic phase and has been related to the structural change which occurs on lithium de-intercalation from the pristine rhombohedral Li_0.98Ni_1.02O₂ phase.     

E_p≤3GeV质子入射铅、铋引起的中子产生双微分截面的模拟(英文)

加速器驱动次临界系统(ADS)液态Pb-Bi散裂靶的设计中,需要可靠的理论计算工具精确地预言几个GeV能量范围的质子引起的散裂反应产生的各种粒子和核素。利用蒙特卡罗模拟软件包Geant4计算研究了800 MeV至3 GeV质子入射铅、铋材料引起的中子产生双微分截面。比较了Geant4不同物理模型得到的模拟结果与现有的实验数据。其中,Geant4的QGSP BERT和QGSP INCL ABLA物理模型模拟结果很好地再现了实验数据。本工作证实了Geant4蒙特卡罗模拟软件包适合用于能量高达3 GeV的质子入射铅、铋引起的中子产生双微分截面的模拟计算。     

K-doping effect of the superconductivity in K2xFeTe1-xSx (0.07≤x≤0.3)

Bulk samples of K doping K_2xFeTe_1-xS_x with x = 0.07, 0.1, 0.2, 0.3 are successfully prepared by using easy-to-use stable compound K₂S as the reactant. The lattice constant calculated from X-ray diffraction patterns indicate that K ions enter the Fe-Te-S layers. K doping is beneficial enhance the superconductivity transition temperature from the R-T curves. The apparent diamagnetic signal is observed in M-T curves when the content of K is smaller than 0.1. However, differential curves (dM/dT) in K-rich samples appear sharp slope mutations, which means that the Meissner effect signal is covered by the increased excess ferromagnetic ions. The number of excess Fe magnetic ions is proportional to K content, which may play an important role in determining the superconductivity.     

Evidence for the production of thermal muon pairs with masses above 1 GeV/c 2 in 158 A GeV indium-indium collisions

The yield of muon pairs in the invariant mass region 1<M<2.5 GeV/c ² produced in heavy-ion collisions significantly exceeds the sum of the two expected contributions, Drell-Yan dimuons and muon pairs from the decays of D meson pairs. These sources properly account for the dimuons produced in proton-nucleus collisions. In this paper, we show that dimuons are also produced in excess in 158 A GeV In-In collisions. We furthermore observe, by tagging the dimuon vertices, that this excess is not due to enhanced D meson production, but made of prompt muon pairs, as expected from a source of thermal dimuons specific to high-energy nucleus-nucleus collisions. The yield of this excess increases significantly from peripheral to central collisions, both with respect to the Drell-Yan yield and to the number of nucleons participating in the collisions. Furthermore, the transverse mass distributions of the excess dimuons are well described by an exponential function, with inverse slope values around 190 MeV. The values are independent of mass and significantly lower than those found at masses below 1 GeV/c ², rising there up to 250 MeV due to radial flow. This suggests the emission source of thermal dimuons above 1 GeV/c ² to be of largely partonic origin, when radial flow has not yet built up.     

高离化Kr33+离子22P -n2S(2≤n≤9)的耦极跃迁行为

本文构造了高离化Kr33+离子 1s2ns (2≤n≤9)和1s22p 态的波函数并利用其计算了体系的非相对论能量。为了得到高精度的理论计算结果,将相对论修正和质量极化效应作为一级微扰计算了体系的总能量和22P -n2S(2≤n≤9)跃迁能,本文的结果与有限的实验数据符合的较好。在此基础上完成了22P -n2S(2≤n≤9)偶极跃迁三种规范下振子强度的理论计算,三者的一致性进一步证明所构造的波函数在整个空间的准确性和可靠性。     

电子型超导体Sm2-xCexCuO4(0.00≤x≤0.21)的异常热电势与电阻率

测量了Sm2-xCexCuO4(0.00≤x≤0.21)多晶样品的热电势S和电阻率ρ的温度依赖关系.在缘绝体-金属转变边界处,观测到热电势从绝缘体区明显的弱温度依赖关系到金属区线性温度依赖关系的转变.当Ce的含量由0.09增加到0.21时,高温下S的斜率发生由负到正的转变,这是能带的填充能级发生改变时电子型和空穴型载流子的贡献发生竞争的表现,由电子型向空穴型的过渡发生在x＝0.17处.S和ρ在200K以下的斜率变化是载流子局域化造成的.x＝0.06-0.21的样品在50K处观察到一个正的曳引峰.室温下的热电势S300K和S0(高温区热电势线性外推到0K的值)与Ce含量在绝缘体、欠掺杂和过掺杂区域有不同的依赖关系.过掺杂区域很小的S300K和S0意味着一个宽带的费米液体的贡献,同时ρ满足T2关系,二者相一致.