Abstract
LaFeAsO1-xHx is a prototypical iron-based superconductor. However, its phase diagram presents unique properties compared to conventional iron-based superconductors: (1) the superconducting phase is sandwiched between two antiferromagnetic (AF) phases appearing in lightly (x ≤ 0.05) and heavily (0.49 ≤ x) H-doped regimes. (2) The AF phase (0.49 ≤ x) is strongly suppressed when applying pressure. We measured the relaxation time T1 at 75As sites for x=0.6 under pressures up to 3.7 GPa to investigate the behavior near the quantum critical point (QCP). The relaxation rate divided by temperature (T) 1/T1T exhibited a peak at the AF transition temperature (TN) followed by Curie-Weiss behavior at low temperatures below TN.. The result demonstrates that the gapped excitation originating from the spin-density-wave state and the gapless excitation characteristic to the QCP coexist near the pressure-induced QCP.
Highlights
The H-doped La1111 series LaFeAsO1−xHx (0 ≤ x ≤ 0.6) has offered an intriguing stage in investing the superconducting (SC) pairing mechanism
The second AF phase is significantly suppressed under a pressure of 3.7 GPa, it can still be observed
1/T 1T measured at high pressure exhibits an upturn below T N, implying that the coexistence of gapless excitation with gapped excitation
Summary
The H-doped La1111 series LaFeAsO1−xHx (0 ≤ x ≤ 0.6) has offered an intriguing stage in investing the superconducting (SC) pairing mechanism. It has a distinct phase diagram compared to conventional iron-based pnictide superconductors. As observed in the x-T phase diagram, each SC dome is adjacent to an AF phase appearing in lightly and heavily H-doped regimes. This may imply that AF fluctuations are important for the paring mechanism. Neglecting the wave-vector (q) dependence of the hyperfine interaction, 1/T 1T is proportional to the imaginary part of χ(q,ω)/ω where ω is NMR frequency and χ(q,ω) is q-dependent spin susceptibility
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