Impurity scattering effects on the superconducting properties and the tetragonal-to-orthorhombic phase transition in FeSe

Abstract

A comprehensive study of the doping dependence phase diagram of FeSe-based superconductors is still required due to the lack of a clean and systematic means of doping control. Here, we report on the magneto-optical imaging, thermodynamic and transport properties, as well as in-situ angleresolved photoemission spectroscopy (ARPES) studies, on the impurity scattering in stoichiometric FeSe single crystals. Co doping at the Fe site, is found to decrease the superconducting transition temperature (Tc). The upper critical field and specific heat all indicate a possible multiband with strong coupling superconductivity in the Co-doped system. A remarkable feature in FeSe is that its temperature dependent resistivity exhibits a wide hump at high temperatures, signature of a crossover from a semiconducting-like behavior to metallic behavior. ARPES data between 180 K and 282 K indicates the existence of chemical potential shift with increasing thermal excitations, resulting in a change of the Fermi surface topology and exhibiting a semi-metal behavior. We found that the temperature induced-Lifshitz transition is much higher than the temperature for the nematic order. A structural tetragonal-to-orthorhombic phase transition (Ts) (consequence of the electronic nematicity) is suppressed by either physical or chemical pressures. Due to the reconstruction of the Fermi surface at Ts, specific heat anomalies at Ts present ∆Cp/Ts ≈ γn, the Sommerfield coefficient at low temperature. This reflects additional electronic instability in the FeSe1−xSx system.

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

“…Such a result is consistent with the ARPES observation that S doping reduces the Fermi surface anisotropy, and suppresses the orbital ordering [36]. It should be noted that a possible temperature-induced Lifshitz transition is suggested in FeSe 1−x S x (x = 0.055) at a temperature higher than T s by recent ARPES measurements [37]. Another possible mechanism is the Pomeranchuk instability, which will spontaneously deform the FS along a specific direction, is also proposed as the origin of the nematic transition in FeSe based on the recent electronic Raman scattering measurements [38].…”

Electronic structure of FeS

“…Moreover, our samples exhibit quite high residual resistivity ratios (RRR) of about 40 [26], which is better than the typical RRR of FeCh (Ch = Te, Se, S) and close to the one as reported in Refs. [27][28][29]. Therefore, the x and y should be quite small.…”

Upper critical fields in Ba2Ti2Fe2As4O single crystals: Evidence for dominant Pauli paramagnetic effect

“…With T c up to 55 K, high critical fields, and moderate anisotropy of their superconducting properties, these compounds show potential for applications [2][3][4]. As to fundamental research, they offer rich phase diagrams resulting from several competing or cooperative order parameters that have attracted particular attention [5][6][7][8][9]. Furthermore, the multiband nature of iron-based compounds, which are, for the most part, moderately correlated electron systems, adds to their complexity.…”