Thermal evolution of antiferromagnetic correlations and tetrahedral bond angles in superconductingFeTe1−xSex

Abstract

It has recently been demonstrated that dynamical magnetic correlations measured by neutron scattering in iron chalcogenides can be described with models of short-range correlations characterized by particular choices of four-spin plaquettes, where the appropriate choice changes as the parent material is doped towards superconductivity. Here we apply such models to describe measured maps of magnetic scattering as a function of two-dimensional wave vectors obtained for optimally superconducting crystals of FeTe1−xSex. We show that the characteristic antiferromagnetic wave vector evolves from that of the bicollinear structure found in underdoped chalcogenides (at high temperature) to that associated with the stripe structure of antiferromagnetic iron arsenides (at low temperature); these can both be described with the same local plaquette, but with different inter-plaquette correlations. While the magnitude of the low-energy magnetic spectral weight is substantial at all temperatures, it actually weakens somewhat at low temperature, where the charge carriers become more itinerant. The observed change in spin correlations is correlated with the dramatic drop in the electronic scattering rate and the growth of the bulk nematic response on cooling. Finally, we also present powder neutron diffraction results for lattice parameters in FeTe1−xSex indicating that the tetrahedral bond angle tends to increase towards the ideal value on cooling, in agreement with the increased screening of the crystal field by more itinerant electrons and the correspondingly smaller splitting of the Fe 3d orbitals.

Unified spin model for magnetic excitations in iron chalcogenides

“…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. In this paper, we show that the recently proposed theory of the spin ferroquadrupolar (FQ) ground state for FeSe [25] indeed satisfies these requirements and successfully describes the evolution of the INS data as a function of Te doping in Fe(Te 1−x Se x ), in qualitative accord with the recent INS experiments [11].…”

“…Remarkably, it was found that in nonsuperconducting Fe(Te 1−x Se x ) samples (due to excess of Fe), the magnetic correlations remain pinned at (π/2,π/2) [11]. The authors of Ref.…”

“…Intriguingly, the neutron spin structure in superconducting Fe(Te 1−x Se x ) samples undergoes a complicated transformation as a function of temperature, with the hightemperature data (T 100 K) characterized by the DS wave vector (π/2,π/2) and evolving to Q 1,2 upon cooling [11]. Remarkably, it was found that in nonsuperconducting Fe(Te 1−x Se x ) samples (due to excess of Fe), the magnetic correlations remain pinned at (π/2,π/2) [11].…”

“…This state is in stark contrast to the parent compounds of iron pnictides that have a columnar antiferromagnetic (CAFM) ground state [4][5][6]. Upon doping with selenium, the DS magnetism disappears, resulting in a nonmagnetic ground state in Fe(Te 1−x Se x ) (for sufficiently large x) [7][8][9][10][11]. The nature of this state, extending all the way to the stoichiometric FeSe, has been the subject of intense debate recently, with elastic neutron scattering showing no sign of magnetic Bragg peaks in FeSe [12,13].…”

“…In this paper, we show that including the third-neighbor Heisenberg interaction J 3 is necessary to describe the DS magnetic state of Fe 1+y Te and that the increasing J 3 strength describes qualitatively the effect of Te doping in Fe(Te 1−x Se x ). Using the variational mean-field, flavor-wave expansion and the DMRG calculations, we compute the phase diagram and establish that the evolution of the calculated dynamic spin-structure factor S(q,ω) with increasing J 3 mimics that observed in INS data in Fe(Te 1−x Se x ) [11]. Crucially, the obtained phase diagram naturally describes both this evolution and the tendency towards the CAFM ordering under the applied pressure in FeSe [18][19][20][21] within a single unified theory.…”