New Superconductivity Dome in LaFeAsO _{1− x} F _x Accompanied by Structural Transition

Abstract

High temperature superconductivity is often found in the vicinity of antiferromagnetism. This is also true in LaFeAsO1−xFx (x ≤ 0.2) and many other iron-based superconductors, which leads to proposals that superconductivity is mediated by fluctuations associated with the nearby magnetism. Here we report the discovery of a new superconductivity dome without low-energy magnetic fluctuations in LaFeAsO1−xFx with 0.25≤ x ≤0.75, where the maximal critical temperature Tc at xopt = 0.5∼0.55 is even higher than that at x ≤ 0.2. By nuclear magnetic resonance and Transmission Electron Microscopy, we show that a C4 rotation symmetry-breaking structural transition takes place for x > 0.5 above Tc. Our results point to a new paradigm of high temperature superconductivity.

Two types of superconducting domes in unconventional superconductors

“…The resistivity exponent is shown in color plot for La-1111 at P=0 [37]. F-doped La-1111 is also shown recently to inherit two SC domes (green symbols) [88]. (b) Pressure tuned two SC phases in a 122 family which has only hole-pocket FS [38,89,90].…”

“…In Sm-1111 however, SDW and superconductivity coexist [96]. A two-SC-domes phase diagram is reported in LaFeAsO 1−x F x with a QCP at the first dome and a NFL at the second dome in the absence of a QCP (green symbols in figure 4(a)) [88]. Hosono et al have recently doped 1111 oxypnictides with hydrogen, introducing relatively large disorder to the sample [31,37,87].…”

“…CeFeAsO 1−x F x : a SC state arises only after SDW is suppressed [94]. LnFeAs 1−y P y O 1−x H x : Two SC domes emerge with a SDW QCP in the first dome (n2) and a NFL state (n1 in the second dome, without a QCP [88]. '122' family: Ba 1−x K x Fe 2 As 2 , MFe 2 As 2 (M=K, Cs, Rb), KFe 2 Se 2 , RNi 2 Se 2 (R=Ti, Ba, Sr) Ba 1−x K x Fe 2 As 2 : the two connected SC phases are nodeless and nodal in character, and present at the two extreme doping limits, respectively.…”

Reemergence of superconductivity by 4d transition-metal Pd doping in over-doped 112-type iron pnictide superconductors Ca0.755La0.245FeAs2

“…Accordingly, instead of spin fluctuations, an alternative scenario where fluctuations stemming from orbital degree of freedom play a primary role has proposed and aroused great interest [8,9,11,12]. In 1111-type system, LaFeAsO 1−x F x , a second SC dome at high doping levels centered at x∼0.5 is reemerged, whereas no magnetism or even magnetic fluctuations is found in this regime [13,14]. Furthermore, in H-doped LaFeAsO 1-x H x , another SC dome is also detected, but the AFM order emerges after SC disappears for x beyond 0.5 [3,15,16].…”

Neutron Powder Diffraction Study on the Non-Superconducting Phases of ThFeAsN1−xOx (x = 0.15, 0.6) Iron Pnictide*

“…Surprisingly, a second superconducting dome emerges from x = 0.25 to x = 0.55 with maximum T c about 15 K ( Fig. 1) 41 , which closely resembles the two superconducting domes in the phase diagram of LaFeAsO 1−x H x , LaFeAsO 1−x F x and LaFeAs 1−x P x O systems 19,[22][23][24][25][26][27][28][29]31 . It should be noticed that the hydrogen doped 1111 systems always have double parent compounds with different magnetic structure and ordered moment adjacent to each superconducting dome 24,25,33 , and the intermediate non-superconducting phase of LaFeAs 1−x P x O is magnetically ordered [30][31][32] .…”