Effect of magnetic frustration on nematicity and superconductivity in iron chalcogenides

Abstract

Over the past few years Fe chalcogenides (FeSe/Te) have advanced to the forefront of Fe-based superconductors (FeBS) research. The most intriguing results thus far are for intercalated and monolayer FeSe, however experimental studies are still inconclusive. Yet, bulk FeSe itself remains an unusual case when compared with pnictogen-based FeBS, and may hold clues to understanding the more exotic FeSederivatives. The FeSe phase diagram is unlike the pnictides: the orthorhombic distortion, which is likely to be of a "spin-nematic" nature in numerous pnictides, is not accompanied by magnetic order in FeSe, and the superconducting transition temperature Tc rises significantly with pressure before decreasing. In this paper we show that the magnetic interactions in chalcogenides, as opposed to pnictides, demonstrate unusual (and unanticipated) frustration, which suppresses magnetic, but not nematic order, favors ferroorbital order in the nematic phase and can naturally explain the nonmonotonic pressure dependence of the superconducting critical temperature Tc(P ).While full consensus regarding the mechanism of hightemperature superconductivity in Fe-based superconductors (FeBS) remains elusive, nearly all researchers agree that it is unconventional and that it has a magnetic origin 1,2 . However, there is a divergence of opinions on the nature of the electrons responsible for magnetism. There is an itinerant approach based on calculating the spin susceptibility with moderate Coulomb (Hubbard) and Hund's interactions [3][4][5][6][7][8][9][10] as well as a localized approach where itinerant electrons responsible for conduction and the Fermi surface interact with local spins 11,12 . Finally, there is an increasingly popular description where the electrons have a dual character and provide the local moments, the interaction between them, and the electronic conductivity [13][14][15][16] . Within this picture, FeBS can still be reasonably mapped onto a short-range model of pairwise interactions between the local moments.Following the discovery of the FeBS, there were multiple attempts to map the exchange interactions onto the Heisenberg model. The J 1 -J 2 model on the square lattice 17 with nearest-(J 1 ) and next-nearest-neighbor (J 2 ) exchange couplings was a natural starting point 18-21 , but required dramatically different couplings for ferro-and antiferromagnetic neighbors, J 1a J 1b to reproduce the observed spin waves 22,23 and ab-initio calculations 24 ; it also failed to describe the double-stripe configuration in FeTe 25,26 . The model was extended to include third-neighbor exchange J 3 27 to reproduce the FeTe magnetic ground state. However, only the Ising model has this configuration as a solution, and in the Heisenberg model it is not a ground state for any set of parameters 28,29 . Therefore adding J 3 does not solve the problem. Besides, the J 1a J 1b implies an unphysical temperature dependence of the exchange constants (as T approaches T N , by symmetry J 1a → J 1b ).There were attempts to overcome these pr...

Emergent Ising degrees of freedom above a double-stripe magnetic ground state

“…We define the coefficients for NN biquadratic exchange, λ 1 ≡ λ 12,12 = λ 14,14 = λ 23,23 = λ 34,34 ; NNN biquadratic exchange, λ 2 ≡ λ 13,13 = λ 24,24 ; NN ring exchange [38] λ 3 = λ 12,34 = λ 14,32 ; and λ 4 = λ 13,24 involving a "diagonal" ring exchange.…”

“…Next, we turn to the finite g 3 problem, where we similarly find that the Ising bond order transition for ψ is accompanied by a simultaneous magnetic transition at r = −ϕ + ψ, which means that all three transitions are simultaneous. For conciseness, we will directly start with the saddle-point equations including M, (38), and replace r = −ϕ + ψ: We can solve the third equation for ψ(M),…”

“…As this maximum is at a nonzero ϕ, the transition is first order, and the condition for magnetic order is satisfied at the transition, and so the two transitions will be simultaneous. In order to examine the nature of the magnetic transition, we return to the saddle-point equations including M, (38), which simplify for d = 3 and g 3 = 0:…”

Unified spin model for magnetic excitations in iron chalcogenides

“…The conspicuous lack of magnetic ordering under ambient pressure in FeSe has led to several theoretical proposals for the unusual nature of the ground state in this material [22][23][24][25]. For the theory to be consistent, it is important that it should be able to describe not only the lack of magnetic ordering in FeSe, but also the appearance of magnetism under applied pressure and with Te doping.…”

Order on Command

“…DOI: 10.1103/PhysRevLett.119.227001 Iron-based superconductors (FeSCs) have shown intriguing phenomena related to the coexistence of magnetism and superconductivity below the superconducting transition temperature (T c ) [1][2][3]. Although an understanding of their detailed interplay is still under debate, certain magnetic orders seem to be very crucial in realizing coexistent superconductivity [3][4][5][6][7][8][9][10][11][12][13][14][15]. Recent studies have shown new reentrant C 4 symmetric antiferromagnetic phases (C 4 magnetism from now on) coexisting with superconductivity and have reported that the superconducting T c is suppressed by C 4 magnetic order [16][17][18][19].…”