High-temperature superconductivity appears as a consequence of doping charge carriers into an undoped parent compound exhibiting antiferromagnetic order; therefore, ground-state properties of the parent compound are highly relevant to the superconducting state1,2. On the basis of this logic, spin fluctuations have been considered as the origin of pairing of the superconducting electrons in the cuprates1. As possible pairing mechanisms, there is growing interest in unconventional spin fluctuations or advanced orbital fluctuations owing to the characteristic multi-orbital states in iron pnictides3,4,5,6. Here, we report the discovery of an antiferromagnetic phase as well as a unique structural transition in electron-overdoped LaFeAsO1−xHx (x ∼ 0.5), whereby a second parent phase is uncovered, albeit heavily doped. The unprecedented two-dome superconducting phases observed in this material can be interpreted as a consequence of the carrier doping starting from the original x ∼ 0 and additional x ∼ 0.5 parent phases towards the intermediate region7. The bipartite parent phases with distinct physical properties in the second magnetic phase provide us with an interesting example to illustrate the intimate interplay between the magnetic interaction, structural change and orbital degree of freedom in iron pnictide superconductors.
Iron pnictides comprise a new class of high-temperature (high-Tc) superconductors, which has been extensively studied since the discovery of the iron oxypnictide LaFeAsO (La1111; refs 8, 9, 10, 11, 12, 13, 14). The superconductivity appears as a result of carrier doping to the parent compound in place of the magnetic- and structural-ordered state2,9,15. A recent study reported an advanced doping method using a hydrogen anion instead of fluorine in La1111 that has surpassed the doping limit of fluorine, and uncovered the concealed second superconducting phase (SC2) with a higher Tc of 36 K at x ∼ 0.35, in addition to the first dome (SC1) with the maximum Tc of 26 K at x ∼ 0.1 (refs 7, 16). To investigate the origin of the two SC domes as well as the origin of the higher Tc in SC2, and to determine whether a certain hidden phase exists beyond the SC2 region, we have performed a multi-probe study in the range 0.40 ≤ x ≤ 0.51 using neutron, muon and X-ray beams.
Fig. 1a shows the neutron powder-diffraction pattern in the non-superconducting specimen with x= 0.51. Extra peaks were observed with indices (1/2, 1/2, n)T,M (n= 0, 1, 2) in the low-temperature region, where the subscripts T and M refer to the tetragonal cell and magnetic peak, respectively; these were determined to be unambiguously magnetic in origin because they were unobservable in the X-ray measurement. The (1/2, 1/2, 0)T,M reflection gradually gains intensity below the magnetic transition temperature TN= 89(1) K, estimated by the power law in the x = 0.51 sample, as shown in Fig. 1b, which indicates a continuous second-order transition. Similar magnetic peaks were also observed below TN ∼ 76 K for x= 0.45.
The in-plane magnetic structure is illustrated in the inset of Fig. 1b17. The ordering wavevector q= (1/2,1/2,0) was determined to correspond to the magnetic unit cell of × 2bT× cT, describing a tetragonal axis, which is consistent with the orthorhombic nuclear unit cell in the low-temperature case. The Fe spins are directed to a diagonal axis of the tetragonal cell, and form an antiferromagnetic collinear structure; this result reveals an exceptional stripe-type arrangement among the iron pnictides18,19,20,21. The spins couple ferromagnetically along the c axis. The magnetic moment in x= 0.51 at 10 K is estimated to be 1.21(1) μB per iron atom, which is significantly larger than the value of 0.63 μB for x= 0 (ref. 19). This observation that the long-range magnetic order emerges beyond SC2 is a remarkable finding because the d-electrons on the iron atoms should be delocalized by the large amount of carrier doping and, accordingly, the magnetic interaction is usually expected to be weak.
Fig. 2a shows the muon spin relaxation (μSR) time spectra for x= 0.45. The slow relaxation above 80 K is attributed to a paramagnetic state, whereas the line shapes below 80 K evolve into strongly damped oscillations with increasing amplitude on cooling, reflecting the development of the magnetically ordered volume fraction (MVF). The significant damping of the muon spin precession in contrast to a clear precession in x= 0 (ref. 22) indicates an inhomogeneity in the local magnetic fields, which is presumably due to the heavy doping effect (Supplementary Information).
Fig. 2b is a diagram of the μSR process. The amplitude and frequency of the μSR signal reflect the MVF and the ordered moment of the magnetic region, respectively. Fig. 2c shows the temperature dependence of the resultant MVF. For x= 0.51, the magnetic ordering appears at TN= 92(7) K and the MVF becomes entire towards zero temperature. As the hydrogen content is reduced, the value of TN and the MVF decrease in unison. Fig. 2d shows the MVF and the superconductivity volume fraction (SVF) estimated from susceptibility measurements7, clearly indicating an inverse correlation between the MVF and the SVF. The approximate summation to unity suggests that antiferromagnetic static order and superconductivity coexist in the range 0.40 ≤ x ≤ 0.45 despite the absence of the coexistence state in the underdoped region15. The coexistence state is not attributed to the microscopic scale but to the mesoscopic scale in the spatial inhomogeneity, because if the coexistence state were due to the microscopic scale the MVF obtained by the μSR experiment would become entire even in the superconducting state (Supplementary Information). Further experimental work will clarify any intimate connection between superconductivity and magnetism in the heavier doping region23,24.
Fig. 3a shows the X-ray profiles of the (2, 2, 0)T reflections in the x= 0.45 and 0.51 samples. On cooling, the peak in the x= 0.51 sample was broadened, whereas the peak in x= 0.45 exhibited only slight broadening. No broadening of the (0, 0, l) reflections was observed in the x= 0.51 sample; therefore, the experimental findings imply that the tetragonal to orthorhombic (T–O) structural transition emerges clearly for x= 0.51.
Fig. 3b shows the temperature dependence of the lattice constants in samples with x= 0.45 and 0.51. In x= 0.51, the aT axis length splits in two below the T–O transition of Ts∼ 95 K, and the cT axis length shows an upturn at Ts. In x= 0.45, although an apparent split between aT and bT does not exist, the cT axis anomaly appears to be similar to that in x = 0.51, which may be the result of insufficient coherence of the splitting of aT and bT in the low-temperature phase.
The resultant temperature dependence of the structural and magnetic order parameters is shown in Fig. 3c for x= 0.51. The small gap between the transition temperatures Ts and TN implies a strong correlation between structure and magnetism in comparison with the large gap of ∼20 K for x= 0. On the basis of a structural analysis25, the compound with x= 0.51 crystallizes below Ts in an orthorhombic Aem2 structure without inversion symmetry, in contrast to the universal Cmme structure with inversion symmetry observed in the 1111 materials (Supplementary Information). The Fe atom exhibits an off-centre deformation in the FeAs4 tetrahedron, as shown in the inset of Fig. 4. The present result provides us with an intriguing deduction that the unusual structural distortion, along with the loss of the inversion symmetry, which has not been found in large ensemble of the iron pnictides18,19,20,21, may profoundly affect the d-orbital levels of the iron atom26,27.
Fig. 4 shows the phase diagram of LaFeAsO1−xHx, where the introduction of charge carriers via doping with hydrogen indeed controls the system (Supplementary Information). The features of the physical properties in x= 0.51 (x= 0) can be characterized as follows: the Fe-spin arrangement is a peculiar stripe-type (universal stripe-type)18,19; the magnetic moment is large, 1.21(1) μB (small, 0.63 μB); the gap between Ts and TN is ∼5 K (∼20 K); the structural symmetry is non-centrosymmetric (centrosymmetric); the magnetic and superconducting states coexist (the magnetic and superconducting states are exclusive) ; the behaviour of resistivity is as for a non-Fermi liquid (Fermi liquid)7. These findings strongly suggest the magnetic and electronic correlations are considerably developed in the right-hand region of the phase diagram.
We have thus far regarded the ‘undoped’ antiferromagnet with the structural transition adjacent to the superconducting state as the parent compound in the high-Tc superconductors. Hence, we now reveal that a newly discovered antiferromagnetic phase with a structural transition adhering to the superconducting phase is considered as the ‘doped’ parent phase. This behaviour is unexpected, because magnetic and electronic correlations are widely perceived as being weak in the region with heavy carrier doping. However, the magnetic and electronic correlations in the advanced parent phase at x ∼ 0.5 are indeed rather strong in comparison to the original parent phase at x ∼ 0. We think that a multi-band feature such as an orbitally selective Hund’s coupling or a Mott transition plays a crucial role so as to drive the emergence of the advanced parent phase, which is presumably stimulated by the half-integer number of the d6.5 state in x= 0.5 (refs 28, 29). In the iron pnictides, the ‘parent compound’ not only refers to the undoped material, but also more generally indicates a fingerprint at a certain critical point of the magnetic and electronic correlations. Yamakawa et al. proposed spin and orbital ordered states with the commensurate Fermi surface nesting at x= 0 and the incommensurate Fermi surface nesting at x ∼ 0.4 in LaFeAsO1−xHx from the theoretical calculation30; the latter inconsistency with the present result is presumably due to the sizeable electron correlation with the large magnetic ordered moment. A more sophisticated theory concerning the electron correlation, the multi-band picture, and the heavy doping effect in the crystal is required to specify the origin of the advanced parent phase.
We now discuss the origin of the two-dome SC phases. The electron doping to the d6 state of the x= 0 parent—that is, the chemical substitution of H− to the O2− site—leads to the SC1 phase. From the viewpoint of the x ∼ 0.5 parent, the SC2 phase emerges through hole doping to the d6.5 state, by O2− substitution to the H− site. Consequently, we can definitely state that the two SC domes are generated by carrier doping, starting from the left- and right-hand parent compounds towards the intermediate region of the phase diagram. Moreover, the Tc valley at x ∼ 0.2 is interpreted as a unique crossover region in this phase diagram.
The combination of the distinct feature of the structural instability with the loss of inversion symmetry and the localized nature of the d-electrons relative to the original parent compound plausibly result in a new mechanism of superconductivity as the origin of the significant enhancement of Tc in SC2 relative to SC1, because the ground-state properties in the parent compounds generally persist unabated, even in the superconducting state. Future band structure calculations taking into account the orthorhombic crystal and magnetic structures would confirm and clarify this new ordered state.
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We thank K. Yamada for helpful discussions. The neutron, muon and synchrotron radiation experiments were performed at J-PARC (BL08-SuperHRPD, BL21-NOVA, Muon D1), PSI, KEK-PF (BL-8A/8B), and SPring-8 with the approval of JASRI (BL11XU) (Proposal Nos. 2013S2-002, 2009S05, 2013A3502). This work was supported by MEXT Elements Strategy Initiative to Form Core Research Center.
The authors declare no competing financial interests.
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Hiraishi, M., Iimura, S., Kojima, K. et al. Bipartite magnetic parent phases in the iron oxypnictide superconductor. Nature Phys 10, 300–303 (2014). https://doi.org/10.1038/nphys2906
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