Nematic quantum critical point without magnetism in FeSe1-xSx superconductors.

In most unconventional superconductors, the importance of antiferromagnetic fluctuations is widely acknowledged. In addition, cuprate and iron-pnictide high-temperature superconductors often exhibit unidirectional (nematic) electronic correlations, including stripe and orbital orders, whose fluctuations may also play a key role for electron pairing. In these materials, however, such nematic correlations are intertwined with antiferromagnetic or charge orders, preventing the identification of the essential role of nematic fluctuations. This calls for new materials having only nematicity without competing or coexisting orders. Here we report systematic elastoresistance measurements in FeSe1-xSx superconductors, which, unlike other iron-based families, exhibit an electronic nematic order without accompanying antiferromagnetic order. We find that the nematic transition temperature decreases with sulfur content x; whereas, the nematic fluctuations are strongly enhanced. Near [Formula: see text], the nematic susceptibility diverges toward absolute zero, revealing a nematic quantum critical point. The obtained phase diagram for the nematic and superconducting states highlights FeSe1-xSx as a unique nonmagnetic system suitable for studying the impact of nematicity on superconductivity.

Field-induced superconducting phase of FeSe in the BCS-BEC cross-over.

Fermi systems in the cross-over regime between weakly coupled Bardeen-Cooper-Schrieffer (BCS) and strongly coupled Bose-Einstein-condensate (BEC) limits are among the most fascinating objects to study the behavior of an assembly of strongly interacting particles. The physics of this cross-over has been of considerable interest both in the fields of condensed matter and ultracold atoms. One of the most challenging issues in this regime is the effect of large spin imbalance on a Fermi system under magnetic fields. Although several exotic physical properties have been predicted theoretically, the experimental realization of such an unusual superconducting state has not been achieved so far. Here we show that pure single crystals of superconducting FeSe offer the possibility to enter the previously unexplored realm where the three energies, Fermi energy εF, superconducting gap Δ, and Zeeman energy, become comparable. Through the superfluid response, transport, thermoelectric response, and spectroscopic-imaging scanning tunneling microscopy, we demonstrate that εF of FeSe is extremely small, with the ratio Δ/εF ~ 1(~0.3) in the electron (hole) band. Moreover, thermal-conductivity measurements give evidence of a distinct phase line below the upper critical field, where the Zeeman energy becomes comparable to εF and Δ. The observation of this field-induced phase provides insights into previously poorly understood aspects of the highly spin-polarized Fermi liquid in the BCS-BEC cross-over regime.

Two distinct superconducting pairing states divided by the nematic end point in FeSe1-x S x.

Unconventional superconductivity often competes or coexists with other electronic orders. In iron-based superconductors, a central issue has been the relationship between superconductivity and electronic nematicity, spontaneous breaking of the lattice rotational symmetry. Using spectroscopic-imaging scanning tunneling microscopy, we simultaneously investigated the electronic structure and the superconducting gap in FeSe1-x S x , where the nematicity diminishes above the nematic end point (NEP) at x = 0.17. The nematic band structure appears as anisotropic quasiparticle-interference patterns that gradually become isotropic with increasing x without anomalies at the NEP. By contrast, the superconducting gap, which is intact in the nematic phase, discontinuously shrinks above the NEP. This implies that the presence or absence of nematicity results in two distinct pairing states, whereas the pairing interaction is insensitive to the strength of nematicity.