Pressure-induced high-Tcsuperconducting phase in FeSe: Correlation between anion height andTc

Abstract

In this study, we performed high-pressure electrical resistivity measurements of polycrystalline FeSe in the pressure range of 1-16.0 GPa at temperatures of 4-300 K. A precise evaluation of Tc from zero-resistivity temperatures revealed that Tc shows a slightly distorted dome-shaped curve, with maximum Tc (30 K) at 6 GPa, which is lower than a previously reported Tc value (∼37 K). With the application of pressure, the temperature dependence of resistivity above Tc changes dramatically to a linear dependence; a non-Fermi-liquid-like "high-Tc" phase appears above 3 GPa. We found a striking correlation between Tc and the Se height: the lower the Se height, the more enhanced is Tc. Moreover, this relation is broadly applicable to other iron pnictides, strongly indicating that hightemperature superconductivity can appear only around the optimum anion height (∼1.38Å). On the basis of these results, we suggest that the anion height should be considered as a key determining factor of Tc of iron-based superconductors containing various anions.

Iron-based superconductors: Magnetism, superconductivity, and electronic structure (Review Article)

“…In 111 systems, superconductivity emerges already at zero doping instead of magnetic order (in LiFeAs) or together with it (in NaFeAs). There are several important experimentally established tendencies, which are followed by many representatives of iron-based family with highest c T [146]: large difference in superconducting gap magnitude on different FS pockets [39,43,150,151], / c T Δ value, that is similar to cuprates and much higher than expected from BCS [43,59,150], correlation of c T with anion height [152]. The complexity of the electronic structure of FeSC was originally an obstacle on the way to its understanding [39,40], but at closer look, such a variety of electronic states turned out to be extremely useful for uncovering the correlation between orbital character and pairing strength [146] and, more general, between electronic structure and superconductivity [17].…”

Strain effects on electronic structure and superconductivity in the iron telluride

“…In FeSe tensile strain strongly decreases T c [4] but all kinds of compressive strain lead to an increase of its T c [4,5,6,7,8,9,10,11,12,13]. The influence of strain on SC of iron chalcogenides is somewhat related to the changes of tetrahedral coordination of Fe atoms [10,5,12]. Furthermore, a magnetic order of particular compounds is also connected with chalcogen atom position in the unit cell (u.c.)…”

Strong pressure-dependent electron-phonon coupling in FeSe

“…Electronic and magnetic properties of FeSe are very sensitive to the position (z Se ) of the selenium layers with respect to the iron layers [15,44,[52][53][54][55][56]. The magnetic ordering is found to be strongly affected by the chalcogen height [52].…”

“…The observation of strong electron-phonon and spin-phonon coupling, both in cuprates [32][33][34][35] and iron superconductors [36][37][38][39][40][41] indicates that the electron-phonon coupling (EPC) may play an important role in the unconventional superconductors, at least to explain the observed Fe-isotope effect [42], the anomalous temperature dependence of the local Fe-As displacement [43], gap anisotropy, and the correlation of T c with the Fe-anion height [44]. These observations also suggest polaron and/or bipolaron driven superconductivity in this material [45][46][47][48][49][50].Here we examine the effects of pressure on the electronic structure and electron-phonon interaction (EPI) in FeSe using DMFT in combination with the DFT as implemented in [51].Electronic and magnetic properties of FeSe are very sensitive to the position (z Se ) of the selenium layers with respect to the iron layers [15,44,[52][53][54][55][56]. The magnetic ordering is found to be strongly affected by the chalcogen height [52].…”

“…The role of lattice vibrations in the mechanism of SC in unconventional superconductors is still controversial. The observation of strong electron-phonon and spin-phonon coupling, both in cuprates [32][33][34][35] and iron superconductors [36][37][38][39][40][41] indicates that the electron-phonon coupling (EPC) may play an important role in the unconventional superconductors, at least to explain the observed Fe-isotope effect [42], the anomalous temperature dependence of the local Fe-As displacement [43], gap anisotropy, and the correlation of T c with the Fe-anion height [44]. These observations also suggest polaron and/or bipolaron driven superconductivity in this material [45][46][47][48][49][50].…”