Symmetry-unprotected nodes or gap minima in the s++ state of monocrystalline FeSe

Abstract

We report the study on superconducting pairing mechanism of FeSe via the pair-breaking effects induced by H + -irradiation combined with low-temperature specific heat measurements. A multigap structure with nodes or gap minima is suggested in a clean FeSe by the specific heat results. The suppression of critical temperature Tc with increasing the defect density manifests a two-step behavior. When the increase in the residual resistivity is small, ∆ρ0 < ∼4.5 µΩcm, Tc is gradually suppressed with increasing the density of scattering centers, suggesting the presence of symmetryunprotected nodes or gap minima. However, for ∆ρ0 > ∼4.5 µΩcm, Tc is almost independent of the scattering, which indicates that the nodes or gap minima are lifted and the order parameter becomes almost isotropic without sign change. Thus, the superconductivity in FeSe is found to be realized in symmetry-unprotected nodal or highly anisotropic s++ state.

Structural-transition-induced quasi-two-dimensional Fermi surface in FeSe

“…For comparison, we also show the temperature dependence of resistivity and Hall coefficient in the inset. Details about the transport properties can be found in our previous report [24]. An obvious kink-like behavior related to the structural transition (breaking the tetragonal C 4 lattice symmetry down to orthorhombic C 2 symmetry) is observed at the temperature of ∼86 K similar to the previous report [14], and is also marked by the arrow in the main panel of Fig.…”

“…∼33 as reported in our previous publications [23,24]. Transport measurements were performed by using the six-lead method with a physical property measurement system (PPMS, Quantum Design).…”

Effects of heavy-ion irradiation on FeSe

“…The obtained crystals are of high quality with a sharp superconducting transition width ∆T c < 0.5 K from susceptibility measurements, and large residual resistivity ratio (RRR) ∼33 as reported in our previous publications [6,35].…”

“…The δl-pinning associated with charge-carrier mean free path fluctuations, and the δTc-pinning associated with spatial fluctuations of the transition temperature are found to coexist in the pristine FeSe, while the irradiation increases the contribution from δl-pinning, and makes it dominant over B φ = 4 T. FeSe composed of only stackings Fe-Se layers [1] has the simplest crystal structure in iron-based superconductors (IBSs), and is usually regarded as the parent compound. It also manifests very intriguing properties such as the nematic state without long-range magnetic order [2], crossover from Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein-condensation (BEC) [3], Dirac-cone-like state [4][5][6], and quasi-2D Fermi surface at temperatures below the structure transition [7][8][9]. Recently, an unexpected high T c with a sign of superconductivity over 100 K observed in monolayered FeSe [10] makes this system a promising candidate for achieving high-temperature superconductivity and probing the mechanism of superconductivity.…”

Observation of Ising spin-nematic order and its close relationship to the superconductivity in FeSe single crystals

“…with α ~ 0.052 and T min ~ 4.8 K. Moreover, we also collect other three sets of T c and T sn given by either the onset temperature of MR or the cusp temperature of the in-plane magnetic magnetization on FeSe single crystal samples of the (001) orientation [25,30,31]. All the collected T c and T sn well satisfy this linear relationship as well.…”