The phase diagrams of iron-based superconductors: Theory and experiments

Superconductivity in the iron pnictides and chalcogenides arises at the border of antiferromagnetism, which raises the question of the role of quantum criticality. In this topical review, we describe the theoretical work that led to the prediction for a magnetic quantum critical point arising out of a competition between electronic localization and itinerancy, and the proposal for accessing it by using isoelectronic P substitution for As in the undoped iron pnictides. We go on to compile the emerging experimental evidence in support of the existence of such a quantum critical point in isoelectronically-tuned iron pnictides. We close by discussing the implications of these results for the physics of the iron pnictides and chalcogenides.

Quantum criticality in the iron pnictides and chalcogenides

A quantum critical point arises at a continuous transformation between distinct phases of matter at zero temperature. Studies in antiferromagnetic heavy fermion materials have revealed that quantum criticality has several classes, with an unconventional type that involves a critical destruction of the Kondo entanglement. In order to understand such varieties, it is important to extend the materials basis beyond the usual setting of intermetallic compounds. Here we show that a nickel oxypnictide, CeNiAsO, displays a heavy-fermion antiferromagnetic quantum critical point as a function of either pressure or P/As substitution. At the quantum critical point, non-Fermi liquid behavior appears, which is accompanied by a divergent effective carrier mass. Across the quantum critical point, the low-temperature Hall coefficient undergoes a rapid sign change, suggesting a sudden jump of the Fermi surface and a destruction of the Kondo effect. Our results imply that the enormous materials basis for the oxypnictides, which has been so crucial to the search for high temperature superconductivity, will also play a vital role in the effort to establish the universality classes of quantum criticality in strongly correlated electron systems.

Heavy Fermion Quantum Criticality and Destruction of the Kondo Effect in a Nickel Oxypnictide

We revealed novel phase deagram of Fe-pnictide high-Tc superconductor LaFe(As_{1-x}P_{x})O in wide doping level (0.3<x<1) by P-NMR. Systematic 31P-NMR studies revealed the emergence of the antiferromagnetic ordered phase (AFM-2) in 0.4 < x < 0.7 that intervenes between two superconductivity (SC-1/SC-2) phases. The 31P-NMR Knight shift points to the appearance of the sharp density of states at the Fermi level that is derived from d_{3Z^2?r^2} orbit, which is less relevant with the onset of the SC-2. On the other hand, we remark that the AFM spin fluctuations arising from the interband nesting on the d_{XZ}/d_{YZ} orbits must be a key ingredient for the occurrence of SC around AFM-2.

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

We performed $^{31}$P-NMR measurements on LaFe(As$_{1-x}$P$_{x}$)O to investigate the relationship between antiferromagnetism and superconductivity. The antiferromagnetic (AFM) ordering temperature $T_{\rm N}$ and the moment $\mu_{\rm ord}$ are continuously suppressed with increasing P content $x$ and disappear at $x = 0.3$ where bulk superconductivity appears. At this superconducting $x = 0.3$, quantum critical AFM fluctuations are observed, indicative of the intimate relationship between superconductivity and low-energy AFM fluctuations associated with the quantum-critical point in LaFe(As$_{1-x}$P$_{x}$)O. The relationship is similar to those observed in other isovalent-substitution systems, e.g., BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ and SrFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$, with the "122" structure. Moreover, the AFM order reappears with further P substitution ($x > 0.4$). The variation of the ground state with respect to the P substitution is considered to be linked to the change in the band character of Fe-3$d$ orbitals around the Fermi level.

Relationship between Superconductivity and Antiferromagnetism in LaFe(As$_{1-x}$P$_{x}$)O Revealed by $^{31}$P-NMR

Superconductivity in LaFeAs$_{1-x}$P$_{x}$O: effect of chemical pressure

The invention relates to a high temperature superconducting material of gadolinium, thorium, oxygen, iron and arsenic, pertains to a ZrCuSiAs-type structure and has the same crystal structure as LaO1-yFyFeAs. The general formula of the high temperature superconducting material of gadolinium, thorium, oxygen, iron and arsenic of the invention is Gd1-xThx OFeAs, wherein, x is more than or equal to 0.05 and less than or equal to 0.30; the mole ratio of all elements content in the high temperature superconducting material gadolinium, thorium, oxygen, iron and arsenic is that Gd:Th: O: Fe: As is equal to 0.70 to 0.95: 0.3 to 0.05:1: 1: 1. According to the content of every element, the raw materials are mixed, ground, preformed, calcinated and quickly cooled, etc. so as to obtain a superconducting polycrystalline block. The high temperature superconducting material of gadolinium, thorium, oxygen, iron and arsenic of the invention has max superconducting critical temperature Tc which reaches56.5 K, estimated upper critical field Bc2 exceeding 100 Tesla, is superior to the LaO1-yFyFeAs superconductor with similar structure and has obvious advantages in practical use.

Gadolinium thorium oxygen iron arsenic high-temperature superconducting material and preparation thereof

Following the discovery of superconductivity in an iron-based arsenide LaO1-xFxFeAs with a superconducting transition temperature (Tc) of 26 K[1], Tc was pushed up surprisingly to above 40 K by either applying pressure[2] or replacing La with Sm[3], Ce[4], Nd[5] and Pr[6]. The maximum Tc has climbed to 55 K, observed in SmO1-xFxFeAs[7, 8] and SmFeAsO1-x[9]. The value of Tc was found to increase with decreasing lattice parameters in LnFeAsO1-xFx (Ln stands for the lanthanide elements) at an apparently optimal doping level. However, the F- doping in GdFeAsO is particularly difficult[10,11] due to the lattice mismatch between the Gd2O2 layers and Fe2As2 layers. Here we report observation of superconductivity with Tc as high as 56 K by the Th4+ substitution for Gd3+ in GdFeAsO. The incorporation of relatively large Th4+ ions relaxes the lattice mismatch, hence induces the high temperature superconductivity.

Thorium-doping induced superconductivity in Gd1-xThxOFeAs

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Superconductivity in LaFeAs$_{1-x}$P$_{x}$O: effect of chemical pressure

Gadolinium thorium oxygen iron arsenic high-temperature superconducting material and preparation thereof

Thorium-doping induced superconductivity in Gd1-xThxOFeAs