利用比热手段证明氧化物超导体正常态有电子库珀对存在

文章简单介绍了氧化物超导体机理问题研究中最核心的问题之一,即超导与赝能隙关系的最新进展状态.介绍了利用高精度比热测量所发现的在欠掺杂样品的正常态有电子配对存在的证据,指出超导凝聚过程是非BCS型.比热数据显示,在重过掺区域,超导转变温度附近的比热跳变非常之陡,超导凝聚所造成的熵变满足守恒规律.然后到欠掺杂区,这个比热跳变变得很矮,而且在正常态仍然测量到一个与磁场相关的比热部分,表现在比热系数有一个很长的尾巴拖到高温区.计算到Tc 附近的超导熵不满足守恒律,但是积分到高温后,即考虑到尾巴部分后,熵就接近守恒.这说明,在欠掺杂超导体的正常态已经有电子配对,但是没有建立起宏观超导相干态.氧化物超导体中的超导凝聚过程不是BCS型的．     

铜氧化物高温超导体正常态反常光电导性质的研究

在全电荷自旋重组的模型下,计算了铜氧化物高温超导体正常态的光电导性质,发现在欠掺杂和最佳掺杂区,铜氧化物高温超导体正常态的光电导谱由低能端的non-Drude峰和较高能量处的中红外带两部分组成,低能峰和中红外带之间存在电荷转移能隙.随着掺杂量的增加,中红外带不断移向低能峰,最后在极过掺杂时完全融入低能峰,光电导谱恢复为Drude形式.全电荷自旋重组模型下的自能体现了部分关联效应,包含了赝能隙的信息,被认为造成了光电导的反常特性.     

关于高T_c氧化物超导体的能隙及其超导态下残余电子比热问题

本文讨论了自由载流子负 U 中心相互作用机制下高 T_c 氧化物超导体的能隙和超导态下极低温时残余线性电子比热项的问题。对 La-Sr-Cu-O 和 Y-Ba-Cu-O 两个系统所得的能隙的数值计算结果分别为2△(0)/k_BT_c=4.00和4.22;两系统的超导态下极低温时残余线性电子比热项的电子比热系数分别为 C_1=2.25和3.04 mJ/mol·K^2.这些数值计算结果与实验数据比较大体一致.     

高临界温度氧化物超导体

 1986年以来,瑞士、美国、日本及中国的科学家相继发现了高温氧化物超导体.这些发现在国际科学界激起巨大反响,给寻找可在液氮温区（即77K以上）工作的超导体的人们带来了极大的“热冲击.从1911年到1986年的75年间,人们发现了数以千计的超导元素、合金和化合物.但它们的超导临界温度都较低,最高的只有23.2K(Nb₃Ge,1973年发现.必须使用技术复杂、代价昂贵的液态氮作冷却剂.超导体的电磁特性所带来极其诱人的优越性被低温技术带来的麻烦大大抵消.     

Experimental evidence for the existence of 7H and for a specific structure of 8He

Experimental search for the superheavy 7H isotope was performed in the reaction p(8He,pp)7H with the 8He beam at 61.3A MeV. The evidence for existence of the 7H state near the t+4n threshold was obtained. In the same experiment, the p(8He,t) reaction populating the ground and excited 2(+) state of 6He was investigated. The obtained results argue on a specific structure of the 8He ground state containing the 6He subsystem in the excited 2(+) state with a large weight.     

The low temperature specific heat of Lu-Cu-Y metallic glasses

The specific heat of a series of amorphous metallic alloys of the form Lu_xCu_0.37Y_0.63-x (x = 0, 0.1, 0.3 and 0.4) has been measured between 2 and 50 K, primarily in order to be able to determine the non-magnetic contributions of the specific heat in magnetic RE-Cu-Y amorphous alloys. The data at low temperature fit the simple form C_p = γT + βT³ from which values of γ and θ_D(0) have been determined. Consideration is given to the error that arises if Y is used rather than Lu or La in forming non-magnetic rare earth intermetallics for purposes of determining the non-magnetic contributions to the specific heat of magnetic samples. A simple procedure is described that allows a useful improvement in accuracy in estimating non-magnetic contributions below 20 K if Y is used. The method may also be useful if only a restricted range of compositions using Lu is possible.     

The low temperature specific heat of Gd-Cu-Y metallic glasses

The specific heat of a series of amorphous aloys of composition Gd_xCu_0.37Y_0.63−x with 0 x 0.63 has been measured in the temperature range 2 to 50 K. A magnetic contribution to the specific heat is apparent in all the samples and has been determined with fair accuracy by subtracting the specific heat of corresponding nonmagnetic amorphous samples prepared with Lu rather than Gd. A qualitative study of the low field ac susceptibility of the magnetic samples has also been made. Samples with less than 35 at% Gd appear to be spin glasses, the results being qualitatively in accord with the predictions of the cluster mean field theory of concentrated spin glasses. As the Gd concentration is increased above 35 at% the samples behave increasingly like ferromagnets. However additional evidence from magnetisation measurements shows that even the sample with the highest concentration of Gd is not a simple ferromagnet.     

A low temperature enhancement in the specific heat of germanium telluride

The specific heat of single crystal germanium telluride has been measured in the interval 1.2 - 22 K. A specific heat enhancement has been found between 1 and 5 K, with a broad maximum at 1.83 K.     

The magnetocaloric effect and low temperature specific heat of SmNi

The magnetocaloric effect (MCE) in a magnetic SmNi sample was evaluated from magnetization and heat capacity measurements. The MCE phenomena in the vicinity of magnetic phase transitions in terms of magnetic entropy change, , and adiabatic temperature change, , are reported. Isothermal magnetization measurements at several temperatures around the transition were carried out and used for versusT calculations. A similar dependence of the magnetic entropy change was evaluated from heat capacity C_p(T) measurements under zero field and 5 T. The SmNi system provides magnetic refrigerants that induce an adiabatic cooling of about during the magnetization process with a field of 5 T in the temperature range of 35-45 K. The temperature dependence of C_p(T) is analyzed in terms of the magnetic and the lattice contributions.     

Experimental evidence for the existence of biexcitons in ZnO

For the first time resonant two-photon Raman scattering via biexcitons in highly excited ZnO is reported. The existence of biexcitons is thus shown. Their binding energy is determined to be (20 ± 1) meV.     

A simple model for the enhancement of the low temperature heat capacity of vitreous silica above 1 K

It is suggested that a simple modification to the Debye model can be made by using a “suitably averaged” sound velocity for vitreous systems, which results in an enhancement of the Debye heat capacity, as seen in experiments.     

Superconductivity under pressure and low temperature specific heat of ThPr alloys

Measurements of superconducting transition temperature under pressure and low temperature specific heat are presented for ThPr alloys. At 18.2 kbar the normal- to superconducting-state phase boundary extends beyond 50 at.% Pr. This system is discussed in terms of a non-magnetic singlet ground state for the Pr³⁺ ions due to crystal field splitting. A splitting scheme which varies with Pr concentration is proposed.     

Lattice specific heat of copper based dilute alloys at low temperature

The low-temperature lattice specific heat of copper-based dilute alloys $\text{Cu}$ Sn, $\text{Cu}$ Ga, $\text{Cu}$ Ge, $\text{Cu}$ Zn and $\text{Cu}$ Ni has been studied theoretically on the basis of Green's function theory. A nearest-neighbour impurity model, with certral and non-central force constant changes has been employed. The effect of volume change is taken into account, and is seen to be important. The calculated results are in excellent agreement with experimental measurements, and the derived force constants are seen to be reasonable.