Competing magnetic double-Qphases and superconductivity-induced reentrance ofC2magnetic stripe order in iron pnictides

Abstract

We perform a microscopic theoretical study of the generic properties of competing magnetic phases in iron pnictides. As a function of electron filling and temperature, the magnetic stripe (single-Q) order forms a dome, but competing non-collinear and non-uniform double-Q phases exist at the foot of the dome in agreement with recent experiments. We compute and compare the electronic properties of the different magnetic phases, investigate the role of competing superconductivity, and show how disorder may stabilize double-Q order. Superconductivity is shown to compete more strongly with double-Q magnetic phases, which can lead to re-entrance of the C2 (single-Q) order in agreement with recent thermal expansion measurements on K-doped Ba-122 crystals.PACS numbers: 74.70.Xa, 74.62.En, In correlated materials in general, and unconventional superconductors in particular, a microscopic understanding of the magnetism is of paramount importance. Generally, this is because a proper description of the relevant exchange mechanism in these materials is intimately tied to their basic electronic properties. More specifically, it is additionally shown within a wide class of models that the nature of the magnetic fluctuations may be closely linked to the emergence of the superconducting condensate. 1-3Focussing on the iron-based superconductors, the prevalent magnetic structure consists of collinear magnetic stripe (MS) order with in-plane moments oriented antiferromagnetic (ferromagnetic) along the a (b) axis of the orthorhombic Fe lattice as shown in Fig. 1(a). Thus, this configuration of moments singles out the Q 1 = (π, 0) ordering vector (1Q), i.e. M(r) = M 1 exp(iQ 1 · r). An obvious question, however, is why the system does not take advantage of the enhanced susceptibility at both Q 1 and Q 2 = (0, π) to form other magnetic phases, e.g. double-Q (2Q) phases consisting of superpositions of ordering at Q 1 and Q 2 with M(r) = l=1,2 M l exp(iQ l ·r). This question has been studied theoretically mainly using various effective field theories restricted to the vicinity of the magnetic transition temperature T N .4-7 These works have identified two competing magnetic structures of the 2Q type: 1) an orthomagnetic (OM) non-collinear phase with nearest neighbor moments at right angles as shown in Fig. 1(b), and 2) a collinear non-uniform spin and charge ordered (SCO) phase as shown in Fig. 1(c). The favorable magnetic order depends delicately on the band structure, doping level, and interactions. 4-7Experimentally, the dominating magnetic order in the iron pnictides is the 1Q MS state. This phase lowers the C 4 symmetry of the high-T tetragonal phase to orthorhombic C 2 , and causes an associated splitting of the crystal Bragg peaks due to magneto-elastic coupling. Recently, several experiments have, however, reported the discovery of magnetic order without an associated structural splitting, i.e. in the tetragonal phase, 8,9 which has been taken as indirect evidence for a magnetic driven structural transition in the case of 1Q M...

Observation of the magnetic C4 phase in Ca1−xNaxFe2As2

“…Furthermore, we find that T r2 ≤ T c -consistent with recent reports which have suggested the C 4 phase's doubly gapped Fermi surface competes more strongly with superconductivity than the singly gapped C 2 phase. 15,16 D. Comparison of Hole-doped 122's 1. Phase Diagrams and Extent of the C4 phase Fig.…”

“…Consequently, it has become the focus of many recent first principle reports to reveal what parameters are causing the sudden reordering of the single-Q C 2 structure into this novel double-Q structure and how this new phase hosts superconductivity. 12,[15][16][17][18][19] Currently, several different proposed models predict the observed C 4 but they do so through different mechanisms which range from expanded itinerant mean field models to impurity scattering stabilization to spin-orbit coupling driven spin anisotropies. 15,17,18 The different models invoke varying assumptions about the underlying physics in these materials and necessarily predict different, sometimes subtlety so, manifestations of the C 4 phase.…”

Stabilizing the spin vortex crystal phase in two-dimensional iron-based superconductors

“…While most of the iron-based superconductors exhibit an antiferromagnetic stripe order 2 , bulk FeSe appears to be paramagnetic 10 . This has led others to investigate various magnetic configurations to compete with the C 2 stripe order (alternatively, collinear or single-q), particularly C 4 (alternatively, tetragonal or double-q) orders, which have been given various labels; orthomagnetic (OM) and spin charge order (SCO) [11][12][13][14] , C 4 spin density wave (SDW) 15 , spin vortex crystal (SVC) and charge-spin density wave (CSDW) [16][17][18] . The orthomagnetic order is equivalent to the spin vortex crystal, while the spin charge order is equivalent to the charge-spin density wave.…”

Emergent Magnetic Degeneracy in Iron Pnictides due to the Interplay between Spin-Orbit Coupling and Quantum Fluctuations

“…Several microscopic mechanisms have been proposed to explain their origin [4,5,[25][26][27][28][29][30][31][32][33][34][35]. However, they generally suffer from two drawbacks.…”