Nematic Crossover inBaFe2As2under Uniaxial Stress

Abstract

Raman scattering can detect spontaneous point-group symmetry breaking without resorting to singledomain samples. Here, we use this technique to study BaFe 2 As 2 , the parent compound of the "122" Febased superconductors. We show that an applied compression along the Fe-Fe direction, which is commonly used to produce untwinned orthorhombic samples, changes the structural phase transition at temperature T s into a crossover that spans a considerable temperature range above T s . Even in crystals that are not subject to any applied force, a distribution of substantial residual stress remains, which may explain phenomena that are seemingly indicative of symmetry breaking above T s . Our results are consistent with an onset of spontaneous nematicity only below T s . DOI: 10.1103/PhysRevLett.115.197002 PACS numbers: 74.25.nd, 74.25.Kc, 74.62.Fj, 74.70.Xa In the phase diagram of iron-based superconductors, superconductivity is commonly situated near the vanishing point of the phase boundary between tetragonal and orthorhombic crystal structures [1][2][3][4]. This commonality has invoked major research efforts to elucidate the nature of the so-called nematic phase, in which the discrete C 4 rotational symmetry is lowered into C 2 but the lattice translational symmetry is intact. While the nematic phase is widely believed to be electronically driven, it is currently under debate whether the electrons' spin or the orbital or charge degree of freedom is in the "driver's seat" [5]. The fact that the tetragonal-to-orthorhombic structural phase transition is closely accompanied by a stripe antiferromagnetic order in the pnictides supports the spin-driven picture, whereas a pronounced uneven occupation of the Fe d xz and d yz orbitals in the orthorhombic phase [6][7][8][9][10] supports the orbital-driven picture. To accommodate the fact that FeSe possesses a nematic phase but no stripe antiferromagnetic order at ambient pressure, alternative scenarios for spindriven nematicity which do not require long-range magnetic order [11][12][13][14] have also been proposed.In order to address the driver's seat question, it is important to study various susceptibilities in the tetragonal phase [5]. A prerequisite for such studies is precise knowledge about the onset temperature of spontaneous nematicity, T nem and, in particular, whether T nem is simply the orthorhombic structural transition temperature T s -or even higher. The latter can be the case if spontaneous nematicity first develops electronically at T nem with minor (although nonzero) influence on the crystal-lattice ground state, and at a lower temperature T s , the lattice reaches a tipping point and responses strongly to the nematic electronic structure, bringing a pronounced C 2 signature to the entire system [15]. In such a scenario, experimental techniques that directly probe the electrons are expected to reveal the nematicity in the temperature range T s < T < T nem .Indeed, resistivity measurements have revealed a pronounced anisotropy in single crystals that are me...

Stress-induced nematicity inEuFe2As2studied by Raman spectroscopy

“…The lowest line width from the cut surface in the high-temperature phase is at 235 K, 60 K above T S . Interestingly, a similar line width broadening is also reported in the E g mode and the (0,2,0) or (2,0,0) Bragg peaks in BaFe 2 As 2 under uniaxial strain [22,31]. Here we stress that the absence of extra broadening for the E g mode in the polished sample indicates that the nematic fluctuations are frozen or negligible above T S , in contrast to previous reports [2,[6][7][8][9].…”

Nematic magnetoelastic effect contrasted betweenBa(Fe1−xCox)2As2and FeSe

“…This shows that the two systems possess similar lattice dynamics in the tetragonal phase, which is not an unexpected result given the similar atomic masses of As and Se and the structural similarity between the FeAs and FeSe layers. As we have recently reported [56], at T = 110 K < T s in BaFe 2 As 2 , the E g peak splits into B 2g and B 3g peaks that differ in energy by 9.4 cm −1 , consistent with a previous report [55]. In contrast, although a splitting of the E g peak is also observed at T = 20 K T s in FeSe, the B 2g and B 3g peaks only differ in energy by 2.6 cm −1 .…”

Combined resistivity and Hall effect study on NaFe1−xRhxAs single crystals

“…[8][9][10][11][12][13][14][15] It turned out, that already far above the structural transition temperature T S such an anisotropy is measurable if uniaxial strain is applied. However, the strain field smears the transition 16 and enhances the fluctuation regime. Thus, in order to study the zero-strain fluctuations other methods were applied.…”