Evidence for the Presence of the Fulde-Ferrell-Larkin-Ovchinnikov State in CeCu₂Si₂ Revealed Using Cu⁶³ NMR

Abstract

Nuclear magnetic resonance measurements were performed on CeCu₂Si₂ in the presence of a magnetic field close to the upper critical field μ₀Hc₂ in order to investigate its superconducting (SC) properties near pair-breaking fields. In lower fields, the Knight shift and nuclear spin-lattice relaxation rate divided by temperature 1/T₁T abruptly decreased below the SC transition temperature Tc(H), a phenomenon understood within the framework of conventional spin-singlet superconductivity. In contrast, 1/T₁T was enhanced just below Tc(H) and exhibited a broad maximum when magnetic fields close to μ₀Hc₂ (0) were applied parallel or perpendicular to the c axis; although the Knight shift decreased just below Tc(H). This enhancement of 1/T₁T, which was recently observed in the organic superconductor κ−(BEDT−TTF)₂Cu(NCS)₂, suggests the presence of high-density Andreev bound states in the inhomogeneous SC region, a hallmark of the Fulde-Ferrell-Larkin-Ovchinnikov phase.