Interplay of magnetic and structural transitions in iron-based pnictide superconductors

The interplay between the structural and magnetic phase transitions occurring in the Fe-based pnictide superconductors is studied within a Ginzburg-Landau approach. We show that the magnetoelastic coupling between the corresponding order parameters is behind the salient features observed in the phase diagram of these systems. This naturally explains the coincidence of transition temperatures observed in some cases as well as the character (first or second order) of the transitions. We also show that magnetoelastic coupling is the key ingredient determining the collinearity of the magnetic ordering and we propose an experimental criterion to distinguish between a pure elastic from a spin-nematic-driven structural transition.

Figure 1

(Color online) Schematic $T$ vs doping diagrams for the structural and magnetic phase transitions (top panels) and general phase diagram $T$ vs $T_S^0-T_N^0$ determined by our Ginzburg-Landau theory Eq. (1) (bottom panel). Empirically $T_S^0-T_N^0$ increases with doping. In the regions I the transitions are simultaneous while in regions II they occur at different temperatures $T_S$ and $T_N$. Continuous (dotted) lines indicate second- (first-) order transitions. The background-color intensity indicates the strength of the discontinuity.

Figure 2

(Color online) (a) Tetragonal Fe lattice (drawn as two interpenetrating sublattices) at temperatures $T>T_S,T_N$. (b) A monoclinic distortion of the tetragonal unit cell and a collinear Néel order in the two Fe sublattices take place at $T<T_S,T_N$.

Figure 3

(Color online) Expected temperature dependence of the squared spontaneous strain $u_{xy}^2$ and the elastic modulus $c_{66}$ in scenarios (a) proper ferroelastic instability and (b) driven by spin-nematic ordering. Large deviations from a linear behavior in $c_{66}$ as compared to $u_{xy}^2$ indicate spin-nematic ordering.