New progress was achieved in the Fe-based superconductivity by the research group of Professor Qiu Xianggang of the Institute of Physics (Beijing) by using the low-temperature X-ray diffractometer (LT-XRD) of the Steady High Magnetic Field Facilities (SHMFF) in the High Magnetic Field Laboratory of Chinese Academy of Sciences (CHMFL). They performed studies on the Fe-based superconductor Ca_0.86Pr_0.14Fe₂As₂, and clarified the key roles of As-As and Fe-As bonds in the superconductivity of this system.

In the exploration of the high-temperature superconducting mechanism, electron correlation and magnetism are widely believed to be intimately related to the pairing mechanism of unconventional high-temperature superconductors. The iron pnictides are newly discovered superconductors with T_C as high as 55 K. Similar to the cuprates, superconductivity in iron pnictides appears in the vicinity of an antiferromagnetic (AFM) phase by doping or pressure. However, the parent compounds of iron pnictides are bad metals with moderate correlation, and the local magnetic moment in iron pnictides is much larger than that in the cuprates. Therefore, the strong magnetism and moderate correlation provide us an alternative material to investigate the interplay among electron correlation, magnetism, and superconductivity.

By using the low-temperature X-ray diffractometer (LT-XRD) of the SHMFF in CHMFL, Dr. Yang from the research group of Professor Qiu Xianggang of the Institute of Physics (Beijing) performed detailed study on the Fe-based superconductor Ca_0.86Pr_0.14Fe₂As₂. In addition to the superconductivity transition, a structure phase transition is found from tetragonal to collapsed tetragonal cell. The bulk superconductivity does not exist in the collapsed tetragonal phase. Meanwhile, in the collapsed tetragonal phase, the electronic correlation is weakened, and the spin fluctuation is suppressed. The results of the experiments indicate that the As ions in the interlayer plane forms strong As-As bonds in collapsed tetragonal phase. The As-As bond strongly affects the Fe-As hybridizations and, in turn, drastically changes the Ca_0.86Pr_0.14Fe₂As₂ into a nonmagnetic Fermi liquid system without bulk superconductivity in the collapsed tetragonal phase. Since the superconductivity in iron pnictides is widely believed to be mediated by the spin fluctuation, the degree of hybridization between iron and arsenic atoms can provide a possible explanation for the lack of bulk superconductivity in the collapsed tetragonal phase.

The Findings were published in Physical Review B.

Open access on the paper is available via:

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.224507

Figure: The X-ray diffraction patterns at selected temperatures for Fe-based superconductor Ca0.86Pr0.14Fe2As2 【the inset panel (a) shows the crystal lattice for CaFe2As2; the inset panel (b) plots the temperature dependence of the lattice constant c】