The spontaneous appearance of nematicity, a state of matter that breaks rotation but not translation symmetry, is one of the most intriguing properties of the iron-based superconductors (Fe SC), and has relevance for the cuprates as well. Establishing the critical electronic modes behind nematicity remains a challenge, however, because their associated susceptibilities are not easily accessible by conventional probes. Here, using FeSe as a model system, and symmetry-resolved electronic Raman scattering as a probe, we unravel the presence of critical charge nematic fluctuations near the structural/nematic transition temperature, T S ∼ 90 K. The diverging behavior of the associated nematic susceptibility foretells the presence of a Pomeranchuk instability of the Fermi surface with d-wave symmetry. The excellent scaling between the observed nematic susceptibility and elastic modulus data demonstrates that the structural distortion is driven by this d-wave Pomeranchuk transition. Our results make a strong case for chargeinduced nematicity in FeSe.nematicity | superconductivity | Raman scattering

Shubnikov-de Haas (SdH) oscillations and upper critical magnetic field (Hc2) of the iron-based superconductor FeSe (Tc = 8.6 K) have been studied by tunnel diode oscillatorbased measurements in magnetic fields of up to 55 T and temperatures down to 1.6 K. Several Fourier components enter the SdH oscillations spectrum with frequencies definitely smaller than predicted by band structure calculations indicating band renormalization and reconstruction of the Fermi surface at low temperature, in line with previous ARPES data. The Werthamer-Helfand-Hohenberg model accounts for the temperature dependence of Hc2 for magnetic field applied both parallel (H ab) and perpendicular (H c) to the iron conducting plane, suggesting that one band mainly controls the superconducting properties in magnetic fields despite the multiband nature of the Fermi surface. Whereas Pauli pair breaking is negligible for H c, a Pauli paramagnetic contribution is evidenced for H ab with Maki parameter α = 2.1, corresponding to Pauli field HP = 36.5 T.

Abstract. Based on fine-resolution Geotail magnetometer data, a set of 5-min magnetic field averages was compiled for the period 1993-1997 and merged with 5-min average solar wind parameters, measured by IMP 8 and Wind spacecraft. Using this data set, the shape of the tail current sheet was studied in the interval -100 < XGSM < --10 RE as