Emergence of Novel Antiferromagnetic Order Intervening between Two Superconducting Phases in LaFe(As_1−xP_x)O: ³¹P-NMR Studies

Abstract

Systematic31 P-NMR studies of LaFe(As1−xPx)O compounds have revealed the emergence of a novel antiferromagnetic ordered phase (AFM-2) at 0.4≤ x ≤0.7 that intervenes between two superconductivity (SC) phases. This AFM-2 phase with Néel temperature TN= 35 K for x=0.6 is in strong contrast to the AFM order (AFM-1) at x=0 exhibiting TN of 140 K. Previous 31 P-NMR studies of LaFe(As1−xPx)(O1−yFy) have revealed that Tc reaches a maximum of 24 K for x=0.6 as a result of the marked enhancement of AFM spin fluctuations at low energies due to electron doping by the flourine substitution of y=0.05 for oxygen. The reason for this unexpected result has been found in the present work, that is, the emergence of AFM-2 at 0.4 ≤ x ≤0.7 without electron doping. We note that AFM spin fluctuations arising from interband nesting on the dXZ/dY Z orbits must be a key factor for the occurrence of SC around AFM-2.Iron (Fe) oxypnictide LaFeAsO with an orthorhombic structure exhibits antiferromagnetic (AFM) order, and the substitution of F − for O 2− induces superconductivity (SC) with a maximum transition temperature of T c =26 K in LaFeAsO 1−y F y .1 The isostructural compound LaFeP(O 1−y F y ) with P substituted for As also reveals the SC transition at T c =4 -7 K, which is lower than that in the case of LaFeAs(O,F).2 In Fepnictide superconductors, T c reaches a maximum of 55 K 3, 4 when a FeAs 4 block forms a nearly regular tetrahedral structure:5 The optimal values of the As-Fe-As bonding angle (α), the height of pnictogen (h Pn ) from the Fe plane, and the a-axis length (a) are 109.5• , 5∼1.38Å, 6 and ∼3.9Å, 4 respectively. In this context, since the substitution of P for As makes the a-axis length smaller, α wider, and h Pn smaller than the optimal values for high-T c Fe pnictides, it is anticipated that T c might decrease monotonically as x increases in solid solution compounds LaFe(As 1−x P x )(O 1−y F y ). Unexpectedly, T c exhibits a nonmonotonic variation with x in LaFe(As 1−x P x )(O 1−y F y ) compounds.7-9 Previous 31 P-NMR studies of these compounds have revealed that T c reaches its respective maxima of 27 and 24 K for x=0.4 with y=0.1 and for x=0.6 with y=0.05, as a result of the marked enhancement of AFM spin fluctuations(AFMSFs) at low energies.10 The result provides clear evidence that T c is enhanced by AFMSFs at low energies even though the lattice parameters deviate from their optimum values. However, another question should be addressed: Why are AFMSFs enhanced despite the fact that the lattice parameters of the compounds are far from those of the AFM mother compound LaFeAsO.In this Letter, we report the results of our 31 P-NMR studies that a novel AFM ordered phase (AFM-2) emerges at 0.4 ≤ x ≤0.7, intervening between two SC phases (SC-1 and SC-2) in LaFe(As 1−x P x )O. The 31 P- * E-mail: mukuda@mp.es.osaka-u.ac.jp † NMR Knight shift indicates the appearance of a sharp density of states (DOS) at the Fermi level derived from a d 3Z 2 −r 2 orbit, which is less relevant with the onset of SC-2. On the other hand, AFMSF...

Observation of two superconducting domes under pressure in tetragonal FeS

“…3 In the four above-mentioned IBSs (LaFeAsO 1−x F x , LaFeAs(O 1−x H x ), LaFe(As 1−x P x )O, (Ca 4 Al 2 O 6 )Fe 2 (As 1−x P x ) 2 ), the first dome is associated with an antiferromagnetic order. 54,[56][57][58] In this work, however, FeS at ambient pressure is non-magnetic above T c and we note that the normal-state resistance curve of FeS with P around 4 GPa is quite flat at low temperature, which does not show a non-Fermi liquid behavior. Therefore, it is not clear whether the first dome in FeS comes from a magnetic QCP.…”

“…[56][57][58] In LaFe(As 1−x P x )O, the first superconducting dome appear at 0.2 ≤ x ≤ 0.4 while the second dome emerges at x > 0.7. 56,57 As for (Ca 4 Al 2 O 6 )Fe 2 (As 1−x P x ) 2 , a nodeless superconducting phase lies between 0 ≤ x ≤ 0.4 while another nodal one is around x = 1. 58 Actually, a superconducting dome in T-P or T-x phase diagram has been commonly observed in unconventional superconductors, including heavy-fermion/cuprate/organic superconductors, and IBSs.…”

“…For the second dome in heavily doped IBSs, for instance, LaFeAs (O 1−x H x ), LaFe(As 1−x P x )O and (Ca 4 Al 2 O 6 )Fe 2 (As 1−x P x ) 2 , it is associated with an antiferromagnetic phase, 54,[56][57][58] while in LaFeAsO 1−x F x , the second superconducting dome is accompanied by a structural transition. 53 Very recently, the structural and electronic properties of FeS both at ambient condition and high pressure were theoretically studied by Tresca et al 61 They claim a topological change of the Fermi surface as a function of the pressure in FeS, 61 which may be the origin of the observed second superconducting dome in our work.…”

Theoretical study of correlation between spin fluctuations andTcin isovalent-doped 1111 iron-based superconductors

“…However, it has recently been revealed that another local maximum of T c (we will call this the second T c dome) exists in the high phosphorous content regime 7,8,10 . Moreover, for the non-fluorine-doped system, the antiferromagnetic phase sandwiched by the two superconducting phases were found in the finite P doping region [9][10][11] . In order to investigate this problem theoretically, some of the authors of the present paper have recently studied the correlation between the Fe-Pn-Fe bond angle and the spin-fluctuation-mediated superconductivity 12 .…”

Antiferromagnetic Spin Fluctuations; What Else?

“…As seen in Fig. 1, the phase diagram of LaFe(As 1−x P x )-(O 1−y F y ) is very rich; in the x variation at y = 0, the superconductivity is divided into two phases by the antiferromagnetic AFM2 phase [10,11], and at x = 0, the highest T c phase (SC3) in the LaFeAsO system was reported in the H doping [12] and AFM3 with the different magnetic structure than that of AFM1 was found with further H doping [13].…”