Elastic properties of iron-based superconductor Ba ( Fe 1-x Co x)2 As 2 with various Co concentrations x were reviewed. Among all elastic constants, C66 shows remarkable softening associated with the structural transition from tetragonal to orthorhombic. The amount of anomaly in C66 is 90% for the underdoped samples of x < 0.07 For the overdoped samples, the anomalies in C66 gradually disappear with the increasing of Co concentration. The elastic compliance S66 (= 1/C66) shows a quantum critical behavior, which behaves just like the magnetic susceptibility of unconventional superconductors. There exists a clear correlation between the superconducting transition temperature and the amount of anomaly in S66. It was suggested that the structural fluctuation, which is measured by S66, plays an important role in the emergence of superconductivity. The elastic anomaly of Ba ( Fe 1-x Co x)2 As 2 is characterized by a strong electron–lattice coupling, which would be originated from the 3d orbitals of iron. This might be a universal phenomenon not only in iron-based superconductors but also d-electron based superconductors. The results on Ba ( Fe 1-x Co x)2 As 2 would reveal relevant roles of the structural fluctuations due to the orbitals, which should be taken into account for the understanding of a whole picture of the superconductivity in iron-based superconductors and related materials.
Electron Transport in Carbon Nanotubes with Adsorbed Chromium Impurities
