Abstract
FeSe is a unique high-T_c iron-based superconductor in which nematicity, superconductivity, and magnetism are entangled with each other in the P-T phase diagram. We performed ^{77}Se-nuclear magnetic resonance measurements under pressures of up to 3.9 GPa on 12% S-substituted FeSe, in which the complex overlap between the nematicity and magnetism are resolved. A pressure-induced Lifshitz transition was observed at 1.0 GPa as an anomaly of the density of states and as double superconducting (SC) domes accompanied by different types of antiferromagnetic (AF) fluctuations. The low-T_{mathrm{c}} SC dome below 1 GPa is accompanied by strong AF fluctuations, whereas the high-T_{mathrm{c}} SC dome develops above 1 GPa, where AF fluctuations are fairly weak. These results suggest the importance of the d_{xy} orbital and its intra-orbital coupling for the high-T_{mathrm{c}} superconductivity.
Highlights
FeSe is a unique high-Tc iron-based superconductor in which nematicity, superconductivity, and magnetism are entangled with each other in the pressure versus temperature (P-T) phase diagram
The absence of magnetism originates from characteristic unconnected Fermi surfaces: small hole pockets at point Ŵ, k = (0, 0), and anisotropic electron pockets at point X, k = (π, 0) or (0, π ), which are caused by the splitting of the dxz and dyz orbitals[5,6,7,8,9,10]
nuclear magnetic resonance (NMR) measurements on 12% S-substituted FeSe have revealed that the characteristics of low-energy magnetic fluctuations change at 1 GPa20, which is indicative of the reconstitution of Fermi surfaces as well as the band mass change
Summary
The monotonic decrease in K below 3 GPa with decreasing T suggests that the influence of magnetism is absent at low temperatures In this case, χ(0) can be described using the formula for conventional paramagnetic metals and is related to the DOS of free electrons, D(EF). The assumption of Korb ∼ 0.26% leads to an unrealistic result, namely, Kspin or the DOS at high pressures becomes lower than that at ambient pressure. To overcome this difficulty, we focus on a remarkable drop in Kspin below Tc at 2 and 3 GPa (see Fig. 3a).
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