金属玻璃(Cu_(1-x)Ni_x)_(33)Zr_(67)的超导电性

本文研究了金属玻璃(Cu_(1-x)Ni_x)_(33)Zr_(67)的超导电性.测量了超导转变温度T_c、上临界磁场H_c_2和其它重要物理量.结果分析表明:这些金属玻璃属于弱耦合超导体.随着Ni含量增多,T_c提高.由(dH_c_2/dT)_(T_c)的测量得到费米面上电子态密度N(0).Varma-Dynes参数δ粗略地是常数.较高的N(0)相应于较高的T_c.N(0)是支配超导行为的基本参量.     

U(N) SP(N) O(3)的分支律

利用群的直乘分解公式,考虑U(N)群的[2a1b]表示按群链U(N)SP(N)O(3)的约化规则,给出了相应的比较简单的分支律递推公式.该公式在用计算机计算分支律时,不受秩和表示维数的限制.为求解这类问题的分支律提供了一种比较简单的算法.在简化同位旋1/2的单j费米子体系的母分系数计算中具有十分重要的意义.用同样的方法也可以求出群链U(N)O(N)O(3)的分支律.     

Structural relaxation of Zr_(41)Ti_(14)Cu_(12.5)Ni_(10)Be_(22.5) bulk metallic glass under high pressure

Zr41Ti14Cu12.5Ni10Be22.5bulk metallic glass (BMG) is annealed at 573 K under 3 GPaand its structural relaxation is investigated by X-ray diffraction, ultrasonic study, compression as well as sliding wear measurements. It is found that after the ZrTiCuNiBe BMG sample was annealed under high pressure, the mechanical properties were improved. Moreover, the BMG with relaxed structure exhibits markedly different acoustic properties. These results are attributed to the fact that relaxation under high-pressure results in a microstructural transformation in the BMG.     

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编辑同志: 看了贵刊1998年第6期上陈印椿同志的<关于光子角动量问题的来信>,觉得其中的几个基本论点是不妥当的.现提出讨论.     

Negative thermal expansion and photoluminescence in solid solution(HfSc)_(0.83)W_(2.25)P_(0.83)O_(12-δ)

A solid solution of(HfSc)_(0.83)W_(2.25)P_(0.83)O_(12-δ)is synthesized by the high-temperature, solid-state reaction and fastquenching method. It is shown that it possesses an orthorhombic structure with space group Pmmm(47) and exhibits negative thermal expansion(NTE) property with low anisotropy in thermal expansion. The coefficients of thermal expansion(CTEs) for a, b, and c axes are-1.41 × 10~(-6) K~(-1),-2.23 × 10~(-6) K-1, and-1.87 × 10~(-6) K-1, respectively. This gives rise to volume and linear CTEs of-3.10 × 10-6 K-1 and-1.03 × 10-6 K-1, respectively. Besides, it exhibits also intense photoluminescence from 360 nm to about 600 nm. The mechanism of NTE and the correlation of the PL with axial thermal expansion property are discussed.     

~(59)Co(n,p)~(59)Fe,~(59)Co(n,α)~(56)Mn,~(59)Co(n,2n)~(58)Co反应截面的测量

本实验用活化法测量了中子能量在12.8MeV到17.8MeV的~(59)Co(n,p)~(59)Fe,~(59)Co(n,α)~(56)Mn,~(59)Co(n,2n)~(58)Co三个反应道的反应截面值,实验的测量误差在3.3％—6.9％范围内. 本文还计算了反应截面测量误差的协方差矩阵,并将实验测量值与理论计算值进行了比较.另外,还对上述三个反应道的截面进行编评,给出了推荐的激发曲线.     

La_(0.7-x)Gd_xSr_(0.3)MnO_3(x=0.20,0.30,0.40,0.50)的TCR研究(英文)

研究了La位Gd掺杂对La0.7-xGdxSr0.3MnO3(x=0.20,0.30,0.40,0.50)体系的电阻温度系数(TCR)的影响.实验结果表明:Gd掺杂将引起电阻率曲线的急剧变化,导致出现大的TCR;而且随Gd掺杂的增加,TCR在x=0.30出现峰值,然后随掺杂量增加逐步降低.体系出现大的TCR,来源于Gd掺杂引起的额外磁性耦合.     

Pb_(0.50)Sr_(0.75)Y_(0.70)Ca_(0.25)Cu_(1.70)O_y单晶的磁阻研究

对低掺杂区非超导的Pb_(6.56)Sr_(?)Y_(?)Ca_(?)Cu_(?)O_y单晶样品进行了电阻和磁阻的测量,发现在低温下电阻温度关系遵从变程跳跃传导的R=R_(?)exp[(T_(?)/T)~(?)]的变化规律,这说明该样品处于强局域化区域.在低温下的磁阻为负并有明显的各向异性,这表明这类体系的磁阻主要来自于轨道效应的贡献,并且可能受到自旋-轨道散射的影响.     

重离子引起的单质子转移反应~(40)Ar(~(11)B,~(10)Be)~(41)K

测量了50MeV的“B束在~(40)Ar上的弹性散射角分布和单质子转移反应~(40)Ar(~(11)B,~(10)Be)~(41)K的微分截面.用光学模型拟合了弹性散射截面.用包含反冲效应的精确有限程扭曲波玻恩近似(EFR-DWBA)分析了微分截面,提取了谱因子.     

~(15)O-~(15)N,~(17)F-~(17)O库仑移位能

从Paris势和电磁相互作用出发,应用格林函数方法微观地计算了~(15)O-~(15)N,~(17)F-~(17)O库仑移位能.G矩阵的电荷非对称性的贡献较重要,比通常唯象的核力电荷非对称成分的贡献大一倍左右.     

The study of Xe adsorption behavior in meso-size pores of carbon black materials using laser-polarized 129Xe NMR spectroscopy

The meso size pores of carbon black materials with Pt critically affect catalysts which play an important role for fuel cells of electric vehicles. Time-consuming BET methods are usually used to measure the physisorption enthalpy which determines the characteristics of catalysts. The laser polarized method enhances 129Xe polarization by 4 orders of magnitude, overcoming a low sensitivity, making this measurement technique faster than conventional experiments. In this paper, we first demonstrate Laser-Polarized 129Xe NMR Spectroscopy for studying carbon black materials with Pt of fuel cells of electric vehicles in order to determine the physisorption enthalpy. At the same time, T1 experiments using Laser-Polarized 129Xe will be discussed in order to clarify the surface condition and adsorption behavior.     

129Xe and131Xe NMR of xenon on silica surfaces at 77 K

Little is known about¹²⁹Xe NMR spectral features and spin-lattice relaxation behavior, and the dynamics of xenon atoms, for xenon adsorbed on solid surfaces at cryogenic temperatures (≤77 K), where exchange with gas-phase atoms is not a significant complication. We report¹²⁹Xe NMR experiments at 9,4 T that provide such information for xenon adsorbed onto the hydroxylated surface of a number of microporous silica samples at 77 K. A convenient design for these cryogenic experiments is described. Dynamics of surface-adsorbed xenon atoms on the time scale of seconds can be observed by¹²⁹Xe NMR hole-burning experiments; much slower dynamics occurring over hours and days are evidenced from changes with time of the¹²⁹Xe NMR chemical shifts. The peak maxima occur in the region ca. 180–316 ppm, considerably downfield of¹²⁹Xe shifts previously reported on surfaces at higher temperatures, and closer to the shift of xenon bulk solid (316.4±1 ppm). The¹²⁹Xe spin-lattice relaxation timesT ₁ range over five orders of magnitude; possible explanations for both nonexponential relaxation behavior and extremely shortT ₁ values (35 ms) are discussed. Preliminary¹³¹Xe and¹H NMR results are presented, as well as a method for greatly increasing the sensitivity of¹²⁹Xe NMR detection at low temperatures by using closely-spaced trains of rf pulses.     

Single-Shot Diffusion Measurement in Laser-Polarized Gas

A single-shot pulsed gradient stimulated echo sequence is introduced to address the challenges of diffusion measurements of laser polarized 3He and 129Xe gas. Laser polarization enhances the NMR sensitivity of these noble gases by >10(3), but creates an unstable, nonthermal polarization that is not readily renewable. A new method is presented which permits parallel acquisition of the several measurements required to determine a diffusive attenuation curve. The NMR characterization of a sample's diffusion behavior can be accomplished in a single measurement, using only a single polarization step. As a demonstration, the diffusion coefficient of a sample of laser-polarized 129Xe gas is measured via this method.     

Gas flow MRI using circulating laser-polarized 129Xe.

We describe an experimental approach that combines multidimensional NMR experiments with a steadily renewed source of laser-polarized 129Xe. Using a continuous flow system to circulate the gas mixture, gas phase NMR signals of laser-polarized 129Xe can be observed with an enhancement of three to four orders of magnitude compared to the equilibrium 129Xe NMR signal. Due to the fact that the gas flow recovers the nonequilibrium 129Xe nuclear spin polarization in 0.2 to 4 s, signal accumulation on the time scale of seconds is feasible, allowing previously inaccessible phase cycling and signal manipulation. Several possible applications of MRI of laser-polarized 129Xe under continuous flow conditions are presented here. The spin density images of capillary tubes demonstrate the feasibility of imaging under continuous flow. Dynamic displacement profiles, measured by a pulsed gradient spin echo experiment, show entry flow properties of the gas passing through a constriction under laminar flow conditions. Further, dynamic displacement profiles of 129Xe, flowing through polyurethane foams with different densities and pore sizes, are presented.     

129Xe NMR in study of tissues and plants

The high sensitivity of the¹²⁹Xe nuclear magnetic resonance (NMR) chemical shift to the environment was used for characterization of biological tissues and plants. The xenon gas was dissolved under moderate pressure by means of a special device in small pieces of human and animal tissues (heart, muscle, lung, kidney, liver, spleen, brain, sinew, cartilage and hypodermic fat) or plants (leaves, stems, grains, fruits) and the NMR parameters were measured in vitro. The observed line with the chemical shift ～ 180 ppm was attributed to the xenon located in various cellular structures such as lipid shells, intracellular formations. A xenon spectrum in the lungs obtained in vitro coincides with that in the lungs of a mouse measured in vivo by other investigators. The NMR parameters were found to reveal noticeable distinctions between normal and tumour-affected tissues. The analysis of the¹²⁹Xe NMR spectra of the sinew and the cartilage revealed the dependence of the magnetic parameters on the age of the substance. This fact could be accounted for by the changes of the absorption ability of a biological system due to age transformations. The results obtained in comparison with biochemical data reveal the promissory outlook of¹²⁹Xe NMR for the investigation of the state of biological tissues and for medical diagnostics.     

129Xe NMR研究MCM-22分子筛中的Xe-Xe作用

¹²⁹Xe NMR 测量得到Xe-Xe碰撞产生的δ_Xe-Xe，该值要比自由 的Xe和在NaY、CaA分 子筛中的值小很多. 通过一定的理论分析，证明在MCM-22分子筛超笼中的Xe-Xe碰撞作用类似于一维气体碰撞，同时表明δ_Xe-Xe也和δ_S（由Xe与孔壁碰撞产生 的化学位移）一样与分子筛的孔道结构密切相关.      

The role of polarization of Xe by di- and monovalent cations in Xe NMR studies in zeolite A

We consider the role of polarization in the adsorption of Xe in zeolites of type A by direct comparative analysis of the adsorption isotherms, distributions of occupancies, and ¹²⁹Xe NMR chemical shifts of Xe_n in cages containing Ca_xNa_{12 −2x} ions per alpha cage (x = 0, 1, 2, 3, 5). We find that the qualitative trends in the adsorption isotherms, and in the progressions of Xe_n chemical shifts (for n = 0–8 in cages with x = 0, 1 Ca²⁺ ions and for n = 0–5 in cages with x = 2, 3 Ca²⁺ ions) upon increasing the level of Ca²⁺ ion for Na⁺ ion substitution could only be accounted for by including polarization of the Xe atom by the zeolite framework and its ions. This system, which permits observation of individual Xe_n peaks and of directly comparable adsorption isotherms in several cage types, provides a good model system for the interpretation of the more general case in which only the overall average ¹²⁹Xe NMR chemical shift is observed in open network zeolites, arising from free exchange of Xe among cavities of variable occupancy and variable cation distribution.     

An apparatus for NMR of laser polarized 129Xe on single crystal surfaces

Due to a lack of at least 2 orders of magnitude in the amount of sample nuclei, single crystal surfaces are out of reach for conventional NMR measurements. Our aim is to prove that highly polarized ¹²⁹Xe provides a technique to overcome this restriction. Therefore an apparatus for polarizing ¹²⁹Xe up to 0.7 by spin transfer from optically pumped Rb has been designed as well as an NMR spectrometer in combination with a UHV chamber with sample cleaning, cooling and characterization abilities and a special manifold of glass stopcocks with a liquid nitrogen cooled trap for dosing nitrogen free polarized Xe into the chamber onto the surface. This revised version was published online in August 2006 with corrections to the Cover Date.     

激光极化129Xe的生物磁共振成像研究进展

激光光抽运自旋交换方法能够极大地增强＾１２９Ｘｅ核自旋极化,其获得的非平衡极化度远远高于在相同磁场里玻尔兹曼平衡值。     

Enhanced NMR signal from the laser-polarized129Xe produced by spin exchange at a high magnetic field

Nuclear-magnetic-resonance (NMR) measurement of laser-polarized gaseous¹²⁹Xe produced by spin-exchange optical pumping with a narrow-linewidth laser at a high magnetic field of 4.7 T is reported. The samples are contained in the glass tubes. The nuclear spin polarization of the laserpolarized¹²⁹Xe is 3.9%, and this corresponds to an enhancement of 9· 10³ compared to the equilibrium value at 311 K and at the same magnetic field. The laser-enhanced¹²⁹Xe NMR signals can be used in MR imaging.