铁基超导体系中的竞争序

文章简要介绍铁基超导体系中存在的一种竞争序,即超导与自旋密度波有序之间的竞争,它是理解铁基超导体物理性质与机理的基础.     

电荷密度波材料K0.3MoO3及W掺杂样品的红外光学响应的研究

研究了准一维材料K03MoO3以及掺杂了3%和15%的W的样品在TCDW=180K以上和以下的温度时的光反射谱和电导率谱.在此基础上讨论了伴随着Peierls相变产生的相声子和单粒子能隙,以及W掺杂对CDW凝聚的影响 关键词： 电荷密度波 Peierls相变     

Low-energy Ce spin excitations in CeFeAsO and CeFeAsO<Subscript>0.84</Subscript>F<Subscript>0.16</Subscript>

We use inelastic neutron scattering to study the low-energy spin excitations of polycrystalline samples of nonsuperconducting CeFeAsO and superconducting CeFeAsO_0.84F_0.16. Two sharp dispersionless modes are found at 0.85 and 1.16 meV in CeFeAsO below the Ce antiferromagnetic (AF) ordering temperature of T _N ^Ce ? 4 K. On warming to above T _N ^Ce ? 4 K, these two modes become one broad dispersionless mode that disappears just above the Fe ordering temperature T _N ^Fe ? 140 K. For superconducting CeFeAsO_0.84F_0.16, where Fe static AF order is suppressed, we find a weakly dispersive mode center at 0.4 meV that may arise from short-range Ce-Ce exchange interactions. Using a Heisenberg model, we simulate powder-averaged Ce spin wave excitations. Our results show that we need both Ce spin wave and crystal electric field excitations to account for the whole spectra of low-energy spin excitations.     

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.     

Low-temperature specific heat and resistance for the heavy-electron metals CeCu6-xMx （M=Ni,Zn）

The specific heat and resistance of CeCu_{6-x}M_x (M=Ni,Zn) have been studied in the concentration range where they still retain the orthorhombic structure. The coefficient c of -ln T term in resistance R=R_0+bT-cln T, as well as the γ, have been found to vary in opposite directions with increasing x for Ni and Zn substitutions. The physical mechanism is discussed.     

Bi2Sr2Ca1-xYxCu2O8+y的晶体结构、电子结构和超导电性

本文研究了Y替代Ca对Bi₂Sr₂Ca_1-xY_xCu₂O_8+y,超导体晶体结构和电子状态的影响,发现:Y替代Ca后至少使得Bi-O双层中所夹的一层额外的氧原子发生了变化,沿b轴方向的无公度超格子的调制周期从4.7b连续减小到大约为4.0b;并发现当Y完全替代Ca时,则同时存在大约为4b和8b的调制周期.用XPS进一步研究其电子状态变化的结果表明,超导电性的变化不是由于晶体结构的微小变化引起的,而主要是由CuO₂层中O2p轨道空穴的逐步填充所致.结果明确表明,对于该替代体系依然是当Cu离子3d轨道存在一个空穴时(3d⁹态)对应于强关联的反铁磁绝缘体,而当CuO₂层中氧离子2p轨道存在额外的空穴时(即3d⁹L态)对应于超导体.     

Bi2Sr2CaCu2O8超导单晶中的电阻反常行为

测量了Bi2 Sr2 CaCu2 O8单晶的ab面和c轴方向电阻 ,在其超导转变温度附近发现了反常的电阻峰出现 .其随外磁场 (>10 0Gs)和电流的增加而逐渐消失 .文章认为这个反常的电阻峰是由于单晶中超导相的不均匀分布而导致的准再进入行为 .     

Optical study on topological superconductor candidate Sr-doped Bi2Se3

Utilizing infrared spectroscopy, we study the charge dynamics of the topological superconductor candidate Sr$_x$Bi$_2$Se$_3$. The frequency-dependent reflectivity $R(\omega$) demonstrates metallic feature and the scattering rate of the free carriers decreases with temperature decreasing. The plasma edge shows a slight blue shift upon cooling, similar to the behavior of Cu$_x$Bi$_2$Se$_3$. As the carrier concentration $n$ obtained by Hall resistivity increases slightly with the decreasing temperature, the effective mass is proved to increase as well, which is in contrast with that of Cu$_x$Bi$_2$Se$_3$.We also perform the ultrafast pump-probe study on the Sr$_{0.2}$Bi$_2$Se$_3$ compounds. Resembling its parent compound Bi$_2$Se$_3$, three distinct relaxation processes are found to contribute to the transient reflectivity. However, the deduced relaxation times are quite different. In addition, the electron-optical-phonon coupling constant is identified to be $\lambda = 0.88$.