Pressure-Induced Antiferromagnetic Transition and Phase Diagram in FeSe

Abstract

We report measurements of resistance and ac magnetic susceptibility on FeSe single crystals under high pressure up to 27.2 kbar. The structural phase transition is quickly suppressed with pressure, and the associated anomaly is not seen above ∼18 kbar. The superconducting transition temperature evolves nonmonotonically with pressure, showing a minimum at ∼ 12 kbar. We find another anomaly at 21.2 K at 11.6 kbar. This anomaly most likely corresponds to the antiferromagnetic phase transition found in µSR measurements [M. Bendele et al., Phys. Rev. Lett. 104, 087003 (2010)]. The antiferromagnetic and superconducting transition temperatures both increase with pressure up to ∼ 25 kbar and then level off. The width of the superconducting transition anomalously broadens in the pressure range where the antiferromagnetism coexists.

Unified spin model for magnetic excitations in iron chalcogenides

“…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.…”

“…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.…”

“…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].…”

Spin Ferroquadrupolar Order in the Nematic Phase of FeSe

“…Usually, nematicity appears in close proximity to magnetism above the Néel temperature; however, in FeSe, the nematic phase appears without any accompanying magnetism and coexists with superconductivity [12][13][14][15]. It is thus important to understand the origin of this nonmagnetic nematic phase, in particular, to gain insight into its effect on superconductivity.It turns out that magnetic order can be induced by applying hydrostatic pressure to FeSe [12][13][14]. It has also been suggested based on ab initio calculations that the nonmagnetic phase in FeSe lies in close proximity to the CAFM phase [21][22][23].…”