The puzzle of high temperature superconductivity in layered iron pnictides and chalcogenides

Abstract

The response of the worldwide scientific community to the discovery in 2008 of superconductivity at Tc = 26 K in the Fe-based compound LaFeAsO1−xFx has been very enthusiastic. In short order, other Fe-based superconductors with the same or related crystal structures were discovered with Tc up to 56 K. Many experiments were carried out and theories formulated to try to understand the basic properties of these new materials and the mechanism for Tc. In this selective critical review of the experimental literature, we distill some of this extensive body of work, and discuss relationships between different types of experiments on these materials with reference to theoretical concepts and models. The experimental normal-state properties are emphasized, and within these the electronic and magnetic properties because of the likelihood of an electronic/magnetic mechanism for superconductivity in these materials. Contents

Phase Diagram of (Li1–xFex)OHFeSe: A Bridge between Iron Selenide and Arsenide Superconductors

“…Like the parent compounds of FeAsbased superconductors, the non-SC (Li 1-x Fe x )OHFeSe samples displays a AFM SDW order below 127 K in the absence of the well-known √5×√5 ordered iron vacancies. Moreover, the unique superstructure, dominant in the SC phase with the modulation wave vector q=1/2 (1,1,0), and the comparable optimal T c of 40 K are very similar to previously reported for K y Fe 2-x Se 2 superconductors. Hence, we conclude that the high-T c superconductivity in (Li 1-x Fe x )OHFeSe emerges from the similarly weak AFM fluctuations, as it does in FeAs-based superconductors.…”

“…We find that the physical properties of the non-SC samples of (Li 1-x Fe x )OHFeSe are very similar to those of FeAsbased superconductors. [1][2][3][4]29 An emergent AFM SDW transition is observed at ~127 K in the absence of the well-known √5×√5 Fe vacancy ordered state associated with the AFM order in A y Fe 2-x Se 2 . In the optimal SC sample of (Li 1-x Fe x )OHFeSe with T c = 40 K, one dominant superstructure with a unique modulation wave vector q=1/2 (1,1,0) is observed by means of selected-area electron diffraction and high-resolution TEM.…”

“…The SDW transition temperatures are much lower than that of the AFM transition of A y Fe 2-x Se 2 (its Neel temperature T N ~ 500 K). 5,13,[19][20][21]24,25 By contrast, such a SDW transition at 100-150 K has been commonly seen in the FeAs-based parent compounds of REFeAsO (RE = rare earth element), [1][2][3][4] whose structural skeleton containing FeAs layers is of the same P4/nmm symmetry as the present (Li 1-x Fe x )OHFeSe samples. The similar SDW transition indicates that its magnetic coupling strength is comparable to the FeAs-based counterparts, in consistent with the fact that their T c 's are similar.…”

“…Second, a superstructure is dominant in the optimal SC sample as illustrated in Figure 3c, in contrast to the non-SC sample (Figure 3b). The satellite spots, which generally are clearly visible in the a*-b* plane of reciprocal space, can be characterized by a unique modulation wave vector q =1/2 (1,1,0 axis direction. The former shows the basic tetragonal structure of the non-superconducting sample (SDW127), and the latter exhibits clearly visible satellite spots in the optimal superconducting sample (SC40-2), characterized by a unique modulation wave vector of q =1/2 (1,1,0).…”

Soft x-ray absorption and high-resolution powder x-ray diffraction study of superconducting Ca La1−Ba1.75−La0.25+Cu3O system

“…Indeed, recently it has been argued that electronic inhomogeneity controlled by organization of defects, lattice distortions, interface states, nanoscale phase separation in the layered oxides has substantial influence in the superconducting properties of cuprates [4][5][6][7][8][9]. This appears to be common to all layered superconductors as diborides [10][11][12][13] and more recently pnictides [14][15][16][17][18][19]. Here it should be specified that large structural differences from one cuprate family to another one hinders the investigation of what drives the T c max in these materials.…”