Strong cooperative coupling of pressure-induced magnetic order and nematicity in FeSe

Abstract

A hallmark of the iron-based superconductors is the strong coupling between magnetic, structural and electronic degrees of freedom. However, a universal picture of the normal state properties of these compounds has been confounded by recent investigations of FeSe where the nematic (structural) and magnetic transitions appear to be decoupled. Here, using synchrotron-based high-energy x-ray diffraction and time-domain Mössbauer spectroscopy, we show that nematicity and magnetism in FeSe under applied pressure are indeed strongly coupled. Distinct structural and magnetic transitions are observed for pressures between 1.0 and 1.7 GPa and merge into a single first-order transition for pressures ≳1.7 GPa, reminiscent of what has been found for the evolution of these transitions in the prototypical system Ba(Fe1−xCox)2As2. Our results are consistent with a spin-driven mechanism for nematic order in FeSe and provide an important step towards a universal description of the normal state properties of the iron-based superconductors.

Possible nematic spin liquid in spin-1 antiferromagnetic system on the square lattice: Implications for the nematic paramagnetic state of FeSe

“…Besides, FeSe possesses an electronic nematic order after a tetragonal-to-orthorhombic structural transition at T s 90K [19][20][21][22] . Although the primary origin of this nematic order is still unclear [23][24][25][26][27][28][29][30][31][32][33][34] , neutron scattering measurements indicate the important role of spin degree of freedom 24,25 . These novel properties have triggered wide interests in the magnetic ground state of FeSe [35][36][37][38][39][40][41][42][43][44][45] .…”

Unified spin model for magnetic excitations in iron chalcogenides

“…It has been reported that applying pressure to FeSe leads to the onset of magnetism [18][19][20], reportedly the CAFM phase [21]. Comparing with Fig.…”

“…Since the Néel phase has not been observed in either iron pnictides or chalcogenides, our calculations support the conclusion that K 2 must be present and negative. Above a certain critical value of K 1 , the FQ order is stabilized and a direct transition between the FQ and CAFM phases is achieved [25], mimicking the experimentally observed transition from the nonmagnetic to the antiferromagnetic state in FeSe under applied pressure [18][19][20][21]. For sufficiently large J 3 , a DS magnetic order is obtained in Fig.…”

“…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. This J 1 -J 2 -J 3 -K 1 -K 2 theory, shown earlier to describe semiquantitatively the spin dynamics of BaFe 2 As 2 iron pnictides with very few fitting parameters [27,28], can thus be considered an effective spin model of both iron pnictides and chalcogenides, and is therefore of fundamental importance to the field of iron-based superconductors.…”

“…This suggests that FeSe is close to magnetic ordering, presumably to the CAFM phase. Indeed, it was shown that magnetism can be reached by applying hydrostatic pressure to FeSe, as indicated by the recent transport, ac susceptibility, x-ray scattering, and NMR measurements [18][19][20][21].…”