Raman scattering experiments on LaFeAsO with splitted antiferromagnetic (TAF M = 140 K) and tetragonal-orthorhombic (TS = 155 K) transitions show a quasi-elastic peak (QEP) in B2g symmetry (2 Fe tetragonal cell) that fades away below ∼ TAF M and is ascribed to electronic nematic fluctuations. A scaling of the reported shear modulus with the T −dependence of the QEP height rather than the QEP area indicates that magnetic degrees of freedom drive the structural transition. The large separation between TS and TAF M in LaFeAsO compared with their coincidence in BaFe2As2 manifests itself in slower dynamics of nematic fluctuations in the former. The discovery of Fe-based superconductors (FeSCs) with high transition temperatures (above 100 K in FeSe films [1]) triggered much interest on these materials [2][3][4][5]. Nematicity, characterized by large in-plane electronic transport anisotropy [6], is normally observed below a tetragonal-orthorhombic transition temperature T S , and seems to be also present in other high-T c superconductors [7]. Also, divergent nematic susceptibility in the optimal doping regime suggests that nematic fluctuations play an important role in the superconducting pairing mechanism [8]. Thus, investigations of the nematic order and fluctuations in FeSCs and their parent materials are pivotal to unraveling the origin of high-T c superconductivity. Clearly, it is necessary to identify the primary order parameter associated with the nematic phase [4, 5]. A relation between nematicity and magnetism is suggested by the near coincidence between T S and the antiferromagnetic (AFM) ordering temperature T AF M in some materials, most notably BaFe 2 As 2 with T AF M ∼ T S = 138 K [9,10]. In fact, the magnetic ground state is a stripe AFM phase that breaks the 4-fold tetragonal symmetry of the lattice (see Fig. 1(a)), providing a natural mechanism for electronic anisotropy. On the other hand, T S and T AF M are significantly separated for LaFeAsO (LFAO) (T AF M = 140 K and T S = 155 K) [11][12][13], while FeSe does not order magnetically at ambient pressure but still shows a nematic transition at T S = 90 K [14], motivating suggestions that the nematic transition may be driven by charge/orbital degrees of freedom rather than magnetism in the latter [16,17]. However, even for FeSe the magnetic scenario may still apply [18]. In * Present address: Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.Ba(Fe 1−x Co x ) 2 As 2 and other doped systems, the splitting between T AF M and T S increases with doping [9,15]. Overall, the primary order parameter that drives the structural/nematic transition at T S and the dominating mechanism of T AF M /T S separation in parent FeSCs are not fully settled yet.Raman scattering was recently employed as a probe of nematic fluctuations in FeSCs and their parent materials. In A(Fe 1−x Co x ) 2 As 2 (A = Ca, Sr, Ba, Eu) [1, 4,19,21,22,24,25], Ba 1−p K p Fe 2 As 2 [25], FeSe [26,27] and NaFe 1−x Co x As [28], a quasi-elastic ...

Low-dimensional metal halide compounds, usually described as low-dimensional perovskites, present exciting properties as functional materials for a broad range of optoelectronic applications. These compounds are characterized by intense photoluminescence (PL), a narrow emission line width, and a high exciton binding energy. In particular, the mechanism behind the strong green emission of the zero-dimensional compound Cs 4 PbBr 6 has been the subject of intense debate. As a propertytuning tool, hydrostatic pressure was used to investigate the structural and optical properties of bulk Cs 4 PbBr 6 through synchrotron X-ray diffraction combined with Raman and PL spectroscopies. As a result, two structural phase transitions at 3.2 and 4.6 GPa were identified, with the latter not observed in previous investigations performed on nanocrystals. Also, the pressure dependence of the PL emission was recorded and compared with the previous results on Cs 4 PbBr 6 and CsPbBr 3 nanocrystals. Under the ambient conditions, strong green emission exhibits a subtle redshift, followed by a blueshift under pressure, being associated first with an intensity enhancement and subsequent quenching above 3 GPa. These results support the CsPbBr 3 luminescent inclusions as the PL emission mechanism in Cs 4 PbBr 6 .

A remarkable hardening (∼ 30 cm −1 ) of the normal mode of vibration associated with the symmetric stretching of the oxygen octahedra for the Ba 2 FeReO 6 and Sr 2 CrReO 6 double perovskites is observed below the corresponding magnetic ordering temperatures. The very large magnitude of this effect and its absence for the anti-symmetric stretching mode provide evidence against a conventional spin-phonon coupling mechanism. Our observations are consistent with a collective excitation formed by the combination of the vibrational mode with oscillations of local 3d and 5d occupations and spin magnitudes.

We report a structural/magnetic investigation by X-ray absorption spectroscopy (XAS), neutron diffraction, dc-susceptibility (χ dc ) and electron spin resonance (ESR) of the 12R-type perovskite BaTi 1/2 Mn 1/2 O3. Our structural analysis by neutron diffraction supports the existence of structural trimers with chemically disordered occupancy of Mn 4+ and Ti 4+ ions, with the valence of the Mn ions confirmed by the XAS measurements. The magnetic properties are explored by combining dc-susceptibility and X-band (9.4 GHz) electron spin resonance, both in the temperature interval of 2 ≤ T ≤ 1000 K. A scenario is presented under which the magnetism is explained by considering magnetic dimers and trimers, with exchange constants Ja/kB = 200(2) K and J b /kB = 130(10) K, and orphan spins. Thus, BaTi 1/2 Mn 1/2 O3 is proposed as a rare case of an intrinsically disordered S = 3/2 spin gap system with a frustrated ground state.

The crystal lattice of Sr2IrO4 is investigated with synchrotron X-ray powder diffraction under hydrostatic pressures up to P = 43 GPa and temperatures down to 20 K. The tetragonal unit cell is maintained over the whole investigated pressure range, within our resolution and sensitivity. The c-axis compressibility κc(P, T ) ≡ −(1/c)(dc/dP ) presents an anomaly with pressure at P1 = 17 GPa at fixed T = 20 K that is not observed at T = 300 K, whereas κa(P, T ) is nearly temperatureindependent and shows a linear behavior with P . The anomaly in κc(P, T ) is associated with the onset of long-range magnetic order, as evidenced by an analysis of the temperature-dependence of the lattice parameters at fixed P = 13.7 ± 0.5 GPa. At fixed T = 20 K, the tetragonal elongation c/a(P, T ) shows a gradual increment with pressure and a depletion above P2 = 30 GPa that indicates an orbital transition and possibly marks the collapse of the J ef f = 1/2 spin-orbit-entangled state. Our results support pressure-induced phase transitions or crossovers between electronic ground states that are sensed, and therefore can be probed, by the crystal lattice at low temperatures in this prototype spin-orbit Mott insulator. arXiv:1912.07330v1 [cond-mat.str-el]

Raman scattering, synchrotron x-ray diffraction, specific heat, resistivity and magnetic susceptibility measurements were performed in Sr(Fe 1−x Co x ) 2 As 2 [x = 0.20(3)] single crystals with superconducting critical temperature T c = 22 K and two additional transitions at 132 and 152 K observed in both specific heat and resistivity data. A quasielastic Raman signal with B 2g symmetry (tetragonal cell) associated with electronic nematic fluctuations is observed. Crucially, this signal shows maximum intensity at T nem ∼ 132 K, marking the nematic transition temperature. X-ray diffraction shows evidence of coexisting orthorhombic and tetragonal domains between T nem and T o ∼ 152 K, implying that precursor orthorhombic domains emerge over an extended temperature range above T nem . While the height of the quasielastic Raman peak is insensitive to T o , the temperature-dependence of the average nematic fluctuation rate indicates a slowing down of the nematic fluctuations inside the precursor orthorhombic domains. These results are analogous to those previously reported for the LaFeAsO parent oxypnictide (Kaneko et al 2017 Phys. Rev. B 96 014506). We propose a scenario where the precursor orthorhombic phase may be generated within the electronically disordered regime (T > T nem ) as long as the nematic fluctuation rate is sufficiently small in comparison to the optical phonon frequency range. In this regime, the local atomic structure responds adiabatically to the electronic nematic fluctuations, creating a net of orthorhombic clusters that, albeit dynamical for T > T nem , may be sufficiently dense to sustain long-range phase coherence in a diffraction process up to T o .

A Raman spectroscopy study on high quality single crystals of SrCr2As2 (SCA) in the temperature T range 4 K < T < 300 K and high applied magnetic fields up to H = 9 T is presented. The chromium B1g phonon analysis reveals two anomalous shifts in the frequency, the first below T = 250 K at H = 0 T in the saturated AFM G-type order likely due to an enhanced electronphonon coupling by the magnetic order, whereas the second anomaly occurs above H = 4 T at T = 4 K likely as a consequence of a magnetostructural displacive transition. Renormalization of the electronic Raman spectra in both studies reveals a decrease in the electronic density of states with decreasing T and increasing H, respectively, with consequent changes in the Fermi surface, which are intrinsically related to the observed anomalies.

Experimentally achieving extreme thermodynamical conditions of temperature, pressure and magnetic field such as the ones found in the interior of planets and stars has been a dream to many scientists seeking to reproduce those conditions on earth to study and produce unconventional materials. The advent of the 4th generation Brazilian synchrotron source (named after the “Sirius” star) allows us to get closer to this dream by implementing a state-of-the-art beamline facility to study samples under extreme thermodynamical conditions by means of a multitude of synchrotron x-ray techniques. The EMA Beamline (Extreme condition Methods of Analysis) will be able to do this by coupling both microfocus (1x1 µm2) and nanofocus (100x100 nm2) beamsizes to x-ray magnetic spectroscopy, x-ray diffraction and x-ray coherent imaging in multiple experimental instruments, placed along the beam path for optimization. Support laboratories (thermodynamical conditions, nuclear materials, laser and optics) were also planned to fulfil all requirements for the experiments under extreme. The EMA beamline, as overviewed here, should open a plethora of opportunities for diverse studies of materials at extreme conditions with synchrotron x-ray techniques.