Distinct pressure evolution of coupled nematic and magnetic orders in FeSe

Abstract

FeSe, despite being the structurally simplest compound in the family of iron-based superconductors, shows an astoundingly rich interplay of physical phenomena including nematicity and pressure-induced magnetism. Here, we present a microscopic study of these two phenomena by high-energy x-ray diffraction and time-domain Mössbauer spectroscopy on FeSe single crystals over a wide temperature and pressure range. The topology of the pressure-temperature phase diagram is a surprisingly close parallel to the well-known doping-temperature phase diagram of BaFe2As2 generated through partial Fe/Co and Ba/Na substitution. In FeSe with pressure p as a control parameter, the magneto-structural ground state can be tuned from "pure" nematic -paramagnetic with an orthorhombic lattice distortion -through a strongly coupled magnetically ordered and orthorhombic state to a magnetically ordered state without an orthorhombic lattice distortion. The magnetic hyperfine field increases monotonically over a wide pressure range. However, the orthorhombic distortion initially decreases under increasing pressure, but is stabilized by cooperative coupling to the pressure-induced magnetic order. Close to the reported maximum of the superconducting critical temperature Tc (occuring at p = 6.8 GPa), the orthorhombic distortion suddenly disappears and FeSe remains tetragonal down to the lowest temperature measured. Analysis of the structural and magnetic order parameters suggests an independent origin of the structural and magnetic ordering phenomena, and their cooperative coupling leads to the similarity with the canonical phase diagram of iron pnictides. arXiv:1803.09449v1 [cond-mat.supr-con]

Intertwined spin-orbital coupled orders in the iron-based superconductors

“…The most promising iron-based compounds where these unusual A 2u and B 2u vestigial phases may be realized are those for which the primary C 4 magnetic phases have been observed. Interestingly, not only doping, but also pressure, have been shown to be capable of tuning the magnetic ground state from stripe to C 4 [34,36]. Recently, indirect evidence for charge-order, presumably due to a non-zero Φ B2u , has been reported in ARPES experiments in hole-doped compounds [101].…”

“…Typically, vestigial order is triggered by the correlation length of the primary order reaching an often large threshold value [7]. Moreover, the fact that upon application of pressure the nematic transition temperature is suppressed whereas the magnetic transition temperature is enhanced [34] suggests a scenario of competing phases rather than intertwined vestigial phases. However, as shown by the firstprinciple calculations of Ref.…”

“…In particular, besides stripe magnetism and nematicity, other types of magnetic order were observed to proliferate as optimal doping is approached [25]. These are magnetic configurations that do not break tetragonal symmetry [26][27][28][29][30][31][32][33][34][35][36], and have thus been dubbed C 4 magnetic phases. This phenomenon is observed quite broadly in hole-doped pnictides, and also in certain materials under pressure [34][35][36][37].…”

Orbital transmutation and the electronic spectrum of FeSe in the nematic phase

“…The intriguing physical properties of FeSe continue to attract the attention of the correlated electron systems community [1][2][3]. This material has the simplest structure among the Fe-based superconductors (FeSCs), yet its phase diagram is rather complex, particularly under pressure [4,5], and is quite different from that of other FeSCs. The most notable distinction is a wide temperature range where the tetragonal symmetry of the lattice is spontaneously broken down to C 2 (the nematic phase).…”

Magnetic phase diagram of the iron pnictides in the presence of spin-orbit coupling: Frustration between C2 and C4 magnetic phases

“…The other has M 1 ⊥ M 2 (also with |M 1 | = |M 2 |) and is dubbed the spin-vortex crystal (SVC) phase due to the vortex-like structures arising in the real-space magnetization profiles [32]. The C 4 magnetic orders have been observed to appear with hole-doping or pressure in a diverse range of materials such as Ba 1−x Na x Fe 2 As 2 [4,6], Ba 1−x K x Fe 2 As 2 [13,15,18,35], Sr 1−x Na x Fe 2 As 2 [14,36], Ca 1−x Na x Fe 2 As 2 [37], FeSe [38], and Ni-and Co-doped CaKFe 4 As 4 [5]. Determining which type of C 4 magnetic order (CSDW or SVC) is present in these systems is experimentally challenging.…”