Applied Crystallography

Biography

Biography: Kensuke Kobayashi

Abstract

Iron-based superconductor (iron pnictides) and cuprates are most well-known types of superconductor with critical temperature (T_c) higher than 50 K. In iron-based superconductors, the relation between the maximum T_c and structural parameters of FePn₄ (Pn = pnictide) has been proposed as follows: the highest T_c is achieved when the Pn-Fe-Pn bond angle (α_Pn-Fe-Pn) approaches 109.5° as in a regular tetrahedron of FePn₄ or when the Pn height from Fe plane (h_Pn) ~ 1.38 Å. The application of pressure is a direct and clean way to modify the local geometry of FePn₄ without the degradation of the crystal in comparison to the chemical substitution; hence, the detailed crystal structure under pressure warrants further investigation. A systematic study of the crystal structure of a layered iron oxypnictide LaFeAsO_1-xH_x, with a unique phase diagram of two superconducting phases and two parent phases, as a function of pressure was performed using synchrotron X-ray diffraction. We established that the α_As-Fe-As widens on application of pressure due to the interspace between the layers being nearly infilled by the large La and As atoms. This behavior implies that the FeAs₄ coordination deviates from the regular tetrahedron in our systems, which breaks a widely accepted structural guide albeit the increase of T_c from 18 K at ambient pressure to 52 K at 6 GPa for x = 0.2. In the phase diagram, the second parent phase at x ~ 0.5 is suppressed by low-pressure at ~1.5 GPa in contrast to the first parent phase at x ~ 0, which remains robust to pressure. We suggest that the spin/orbital fluctuation from the second parent phase gives rise to the high-T_c under pressure. The pressure responses of the FeAs₄ modification, the parent phases, and their correlation are previously unexplained peculiarities in 1111-type iron-based superconductors.