High-temperature superconductivity stabilized by electron-hole interband coupling in collapsed tetragonal phase ofKFe2As2under high pressure

Abstract

We report a high-pressure study of simultaneous low-temperature electrical resistivity and Hall effect measurements on high quality single-crystalline KFe2As2 using designer diamond anvil cell techniques with applied pressures up to 33 GPa. In the low pressure regime, we show that the superconducting transition temperature Tc finds a maximum onset value of 7 K near 2 GPa, in contrast to previous reports that find a minimum Tc and reversal of pressure dependence at this pressure. Upon applying higher pressures, this Tc is diminished until a sudden drastic enhancement occurs coincident with a first-order structural phase transition into a collapsed tetragonal phase. The appearance of a distinct superconducting phase above 13 GPa is also accompanied by a sudden reversal of dominant charge carrier sign, from hole-to electron-like, which agrees with our band structure calculations predicting the emergence of an electron pocket and diminishment of hole pockets upon Fermi surface reconstruction. Our results suggest the high-temperature superconducting phase in KFe2As2 is substantially enhanced by the presence of nested electron and hole pockets, providing the key ingredient of high-Tc superconductivity in iron pnictide superconductors.Superconductivity in iron-based compounds has introduced a new paradigm in our understanding of unconventional pairing mechanisms in which the repulsive interaction between different Fermi surfaces play a major role [1]. In the iron superconductors, a wide versatility is found in the symmetry of the superconducting (SC) gap function, including sign-reversed full gap (s ± ) [2-5] and symmetry-imposed (d) [6] or accidental nodal states (nodal s ± ) [7,8]. These paring symmetries have indeed been considered theoretically and experimentally [9,10], and can undergo a transition from one to another by chemical substitution or pressure [8,[11][12][13]. Capturing universal traits in these symmetries is widely thought to give us the key to understanding high-temperature superconductivity in these fascinating materials.Located at the end of phase diagram in hole-doped (Ba,K)Fe 2 As 2 [14], the stoichiometric intermetallic compound KFe 2 As 2 is a promising platform for exploring the evolution of rich pairing symmetries in iron-pnictide superconductors. While in (Ba,K)Fe 2 As 2 , the gap symmetry is believed to be of the fully-gapped s-wave type [5,15], in KFe 2 As 2 both symmetry-imposed [6] and accidental nodal [7,8] gap functions have been reported, which suggests a transition or crossover of SC gap function with chemical doping. In addition to chemical manipulation, recent pressure studies on KFe 2 As 2 have proposed a possible symmetry change from d-to s ± -wave state to explain the sudden reversal of T c pressure dependence at P c ∼ 2 GPa [12, 13].Here we report a high pressure study of transport and structural properties of KFe 2 As 2 up to 33 GPa, far beyond previous work. We find two striking features: first, an initial enhancement of T c with pressure reaches up to 7 K around P c , opposit...

Fermi-Surface Topological Phase Transition and Horizontal Order-Parameter Nodes in CaFe2As2 Under Pressure

“…It is intriguing to notice that the same phase transition with disappearance of the holelike FS at Γ that here produces a dramatic increase of the coupling in both bands seems to be responsible for the high- T c phase at 47 K observed in rare-earth doped CaFe 2 As 2 47, and for the second, higher- T c superconducting dome in LaFeAsO 1− x H x 48. Recently, a study of KFe 2 As 2 in an extended pressure range (up to 30 GPa) has shown two superconducting phases in the phase diagram49. A first superconducting dome (with a maximum around 2.5 GPa) occurs in the T structure, and is extended up to about 7 GPa.…”

Origin of Pressure-induced Superconducting Phase in KxFe2−ySe2 studied by Synchrotron X-ray Diffraction and Spectroscopy

“…The phase diagram of the 122 system is similar to that of the 122* system37. The DFT calculations showed the change in the electronic structure between the T phase and cT phase38.…”

Structural, electronic, and dynamical properties of the tetragonal and collapsed tetragonal phases of KFe2As2

“…External pressure changes the superconducting properties of the K122 tetragonal phase. T c shows a nonmonotonic behavior on pressure [11,12,[22][23][24][25], with a minimum at p c ∼ 1.8 GPa [25]. Around this point, T c shows a universal V-shape dependence in the weak pressure regime [26], what has been reported not only in the case of K122 [25], but also for Rb122 [26] and Cs122 [27].…”