Pseudogap Behavior of the Nuclear Spin–Lattice Relaxation Rate in FeSe Probed by ⁷⁷Se-NMR

Abstract

We conducted77 Se-nuclear magnetic resonance studies of the iron-based superconductor FeSe in magnetic fields of 0.6 to 19 T to investigate the superconducting and normal-state properties. The nuclear spin-lattice relaxation rate divided by the temperature (T1T ) −1 increases below the structural transition temperature Ts but starts to be suppressed below T * , well above the superconducting transition temperature Tc(H), resulting in a broad maximum of (T1T ) −1 at Tp(H). This is similar to the pseudogap behavior in optimally doped cuprate superconductors. Because T * and Tp(H) decrease in the same manner as Tc(H) with increasing H, the pseudogap behavior in FeSe is ascribed to superconducting fluctuations, which presumably originate from the theoretically predicted preformed pair above Tc(H).The discovery of superconductivity in iron pnictide 1)provided new research systems in which the unconventional superconductivity realized in strongly correlated electron compounds can be studied. Among the Fe-based superconductors (FeSCs), FeSe, which exhibits superconductivity at T c ∼ 9 K, has the simplest crystalline structure, which is called the 11 structure,2) but it exhibits several properties unlike those of other FeSCs. A tetragonal-to-orthorhombic structural transition occurs at T s ∼ 90 K without long-range antiferromagnetic (AFM) ordering down to T c .3) This is in stark contrast with the other FeSCs, such as the 122 and 1111 systems, where static AFM ordering was observed at or slightly below T s . 4,5) We reported that the 77 Se nuclear magnetic resonance (NMR) spectrum splits below T s and that the nuclear spin-lattice relaxation rate divided by T , (T 1 T ) −1 , of 77 Se is enhanced below T s . 6) These results suggest that orbital ordering induces the electronic nematic state below T s and also triggers the development of AFM fluctuations with stripe correlations breaking the C 4 symmetry in FeSe.6, 7) This scenario is consistent with the results of angle-resolved photoemission spectroscopy, 8,9) but it seems to be inconsistent with the AFM-fluctuation-driven nematic scenario.10) In any case, FeSe is an ideal system for studying the origin of the electronic nematic state in FeSCs.It was recently suggested that the superconductivity in FeSe may be in the Bardeen-Cooper-Schrieffer (BCS)-Bose-Einstein condensate (BEC) crossover regime, as an extremely small Fermi energy comparable to the superconducting (SC) condensation energy was revealed by several measurements.11, 12) In a schematic phase diagram of the attractive Hubbard model, the interaction between two fermions (e.g., electrons in solids) is in the weak-coupling regime in the BCS state and in the strongcoupling regime in the BEC state. The characteristic pa-

Anomalous high-magnetic field electronic state of the nematic superconductors FeSe1−xSx

“…Recent NMR data found that anti-ferromagnetic spin fluctuations are present inside the nematic phase of FeSe 1−x S x , being strongest around x ∼ 0.1 [37]. In FeSe, spin fluctuations are rather anisotropic [37,38] and strongly field-dependent below 15 K [11]. Interestingly, the spin-fluctuations relaxation rate is enhanced below T * (Fig.…”

“…The low-temperature regime below T * displays linear resistivity, which is a potential manifestation of scattering induced by critical spin-fluctuations in clean systems [36]. µSR studies place FeSe near an itinerant antiferromagnetic quantum critical point at very low temperatures [12] and spinfluctuations are only found inside the nematic state [11,37]. On the other hand, close to the nematic end point in FeSe 1−x S x we find that resistivity is not linear in temperature but is dominated by a T 1.5 dependence.…”

“…FeSe shows no long-range magnetic order at ambient pressure, but complex magnetic fluctuations are present at high energies over a large temperature range [9]. Below T s , the spin-lattice relaxation rate from NMR experiments is enhanced as it captures the low-energy tail of the stripe spin-fluctuations [10,11]. Furthermore, recent µSR studies invoke the close proximity of FeSe to a magnetic quantum critical point as the muon relaxation rate shows unusual temperature dependence inside the nematic state [12].The changes in the electronic structure and magnetic fluctuations of FeSe can have profound implication on its transport and superconducting properties.…”

Quantum Vortex Core and Missing Pseudogap in the Multiband BCS-BEC Crossover Superconductor FeSe

“…The signatures of the preformed pairs with unusually strong pairing fluctuations above T c have been suggested by torque magnetometry [10] and nuclear magnetic resonance [11]. However, mean-field-like BCS behaviors are reported in other experiments [12,13].…”

Stabilization of the vortex-liquid state by strong pairing interaction

“…In fact, superconductingfluctuation effects on diamagnetic response observed in FeSe are unusually enhanced compared with those in conventional superconductors [4], which may be understood as caused by the strong attractive interaction [5]. In addition, recent NMR measurements have proposed that a pseudogap caused by the preformed-pair formation can exist, and that the onset temperature of the pseudogap depends on the magnetic-field strength [6]. The pair-formation field H * (T ) in FeSe has been estimated based on this NMR result [6] and also on thermodynamic and transport properties [4].…”