Nematicity is ubiquitous in electronic phases of high-T_{c} superconductors, particularly in the Fe-based systems. We used inelastic x-ray scattering to extract the temperature-dependent nematic correlation length ξ from the anomalous softening of acoustic phonon modes in FeSe, underdoped Ba(Fe_{0.97}Co_{0.03})_{2}As_{2}, and optimally doped Ba(Fe_{0.94}Co_{0.06})_{2}As_{2}. In all cases, we find that ξ is well described by a power law (T-T_{0})^{-1/2} extending over a wide temperature range. Combined with the previously reported Curie-Weiss behavior of the nematic susceptibility, these results point to the mean-field character of the nematic transition, which we attribute to a sizable nematoelastic coupling that is likely detrimental to superconductivity.

The magnetic and superconducting properties of a series of underdoped Ba1-xNaxFe2As2 (BNFA) single crystals with 0.19 <= x <= 0.34 have been investigated with the complementary muon-spin-rotation (mu SR) and infrared spectroscopy techniques. The focus has been on the different antiferromagnetic states in the underdoped regime and their competition with superconductivity, especially for the ones with a tetragonal crystal structure and a so-called double-Q magnetic order. Besides the collinear state with a spatially inhomogeneous spincharge-density wave (i-SCDW) order at x = 0.24 and 0.26, that was previously identified in BNFA, we obtained evidence for an orthomagnetic state with a "hedgehog"-type spin vortex crystal (SVC) structure at x = 0.32 and 0.34. Whereas in the former i-SCDW state the infrared spectra show no sign of a superconducting response down to the lowest measured temperature of about 10 K, in the SVC state there is a strong superconducting response similar to the one at optimum doping. The magnetic order is strongly suppressed here in the superconducting state and at x = 0.34 there is even a partial reentrance into a paramagnetic state at T << T-c.

We report inelastic x-ray scattering measurements of the in-plane polarized transverse acoustic phonon mode propagating along $q\ensuremath{\parallel}[100]$ in various hole-doped compounds belonging to the 122 family of iron-based superconductors. The slope of the dispersion of this phonon mode is proportional to the square root of the shear modulus ${C}_{66}$ in the $q\ensuremath{\rightarrow}0$ limit and, hence, sensitive to the tetragonal to orthorhombic structural phase transition occurring in these compounds. In contrast to a recent report for $\mathrm{Ba}{({\mathrm{Fe}}_{0.94}{\mathrm{Co}}_{0.06})}_{2}{\mathrm{As}}_{2}$ [Weber et al., Phys. Rev. B 98, 014516 (2018)], we find qualitative agreement between values of ${C}_{66}$ deduced from our experiments and those derived from measurements of the Young modulus in ${\mathrm{Ba}}_{1\ensuremath{-}x}{(\mathrm{K},\mathrm{Na})}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ at optimal doping. These results provide an upper limit of about 50 \AA{} for the nematic correlation length for the investigated optimally hole-doped compounds. Furthermore, we also studied compounds at lower doping levels exhibiting the orthorhombic magnetic phase, where ${C}_{66}$ is not accessible by volume probes, as well as the ${C}_{4}$ tetragonal magnetic phase.

Compounds BaCe0.7Er0.2In0.1O2.85 and BaCe0.7Er0.2Y0.1O2.85 have been synthesized by solid state reactions. The characterization of compounds has been carried out by X-ray power diffraction. The space group and lattice parameters have been determined. Standard formation enthalpies have been measured by solution calorimetry, stabilization energies and lattice enthalpies have been calculated. It has been shown that lattice enthalpies are increased when yttrium is replaced by indium, which correlates with decreasing ionic radius from yttrium to indium. (C) 2019 Elsevier Ltd.

By means of infrared spectroscopy, we determine the temperature-doping phase diagram of the Fano effect for the in-plane Fe-As stretching mode in Ba1-xKxFe2As2. The Fano parameter 1/q(2), which is a measure of the phonon coupling to the electronic particle-hole continuum, shows a remarkable sensitivity to the magnetic and structural orderings at low temperatures. Most strikingly, at elevated temperatures in the paramagnetic tetragonal state we observe a linear correlation between 1/q(2) and the superconducting critical temperature T-c. Based on theoretical calculations and symmetry considerations, we identify the relevant interband transitions that are coupled to the Fe-As mode. In particular, we show that a sizable xy orbital component at the Fermi level is fundamental for the Fano effect and, thus, possibly also for the superconducting pairing.

Nematicity is ubiquitous in electronic phases of high-T_c superconductors, particularly in the Fe-based systems. While several experiments have probed nematic fluctuations, they have been restricted to uniform or momentum averaged fluctuations. Here, we investigate the behavior of finite-momentum nematic fluctuations by utilizing the anomalous softening of acoustic phonon modes in optimally doped Ba(Fe_0.94Co_0.06)_2As_2. We determine the nematic correlation length and find that it sharply changes its T-dependence at T_c, revealing a strong connection between nematicity and superconductivity.

For the first time solution enthalpies of BaCe0.8Y0.1Gd0.1O2.9, BaCe0.8Y0.1Lu0.1O2.9 and mixtures of BaCl2 + 0.8CeCl(3) + 0.1YCl(3) + 0.1GdCl(3), BaCl2 + 0.8CeCl(3) + 0.1YCl(3) + 0.1LuCl(3) in 1 mol dm(-3) HCl with 0.1 mol dm(-3) KI have been measured. On the basis of experimental data obtained and literature data the standard molar enthalpies of formation and stabilization energies were calculated. Using Born-Haber cycle the lattice energies were calculated. It was established that lattice energy was increased from BaCe0.8Y0.1Gd0.1O2.9 to BaCe0.8Y0.1Lu0.1O2.9. It correlates with decreasing of ion radius of rare earth element from gadolinium to lutetium. Increasing of lattice energy with decreasing of ion radius of rare earth element was explained with modified formula of Kapustinskii. (C) 2017 Elsevier Ltd.

We present a systematic angle-resolved photoemission spectroscopy study of the superconducting gap in FeSe. The gap function is determined in a full Brillouin zone including all Fermi surfaces and kz-dependence. We find significant anisotropy of the superconducting gap in all momentum directions. While the in-plane anisotropy can be explained by both, nematicity-induced pairing anisotropy and orbital-selective pairing, the kz-anisotropy requires additional refinement of theoretical approaches.

Solution calorimetry, using 1moldm−3 HCl as a solvent, has been used to study for the first time the thermochemistry of Bi2.67Er0.33CoO5.83. For the first time, the standard formation enthalpy of the phase has been determined as following: ΔfHo(Bi2.67Er0.33CoO5.83, s, 298.15K)=−1406.9±8.6kJmol−1. The thermodynamic stability at room temperature has been assessed. The results show that investigated phase is thermodynamically stable with respect to decomposition to the constituent binary oxides. It is important to establish thermodynamic stability for solving problem of Bi2O3 stabilization. It is shown that the lattice energies for Bi2.67Er0.33CoO5.83 calculated on the basis of Born-Haber cycle and Kapustinskii rule are in a good agreement. For the first time, we measured the magnetic characteristics of Bi2.67Er0.33CoO5.83 and established that the Bi2.67Er0.33CoO5.83 phase has paramagnetic properties.

We study experimentally the Raman response of the undoped high-${T}_{c}$ parent compound ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6}$, and give a unified theory of the two-magnon Raman peak and optical conductivity based on the Hubbard-Holstein model with electron-phonon coupling (EPC). The Hubbard model without EPC can qualitatively account for the experimentally observed resonance of the Raman response, but only the Hubbard-Holstein model (i) reproduces the asymmetry of the Raman spectrum, (ii) validates the experimental visibility of the two-magnon peak, and (iii) predicts the correct shape and energy of the lower edge of the charge transfer gap in optical conductivity. A comparison of experiments with the theory gives the EPC strength $\ensuremath{\lambda}=0.6$. This result convincingly indicates the vital role of EPC in high-${T}_{c}$ cuprates, providing a clue to the mechanism of high ${T}_{c}$.

B. Xu, ∗ E. Cappelluti, L. Benfatto, B. P. P. Mallett, 4 P. Marsik, E. Sheveleva, F. Lyzwa, Th. Wolf, R. Yang, X. G. Qiu, Y. M. Dai, H. H. Wen, R. P. S. M. Lobo, 9 and C. Bernhard † University of Fribourg, Department of Physics and Fribourg Center for Nanomaterials, Chemin du Musée 3, CH-1700 Fribourg, Switzerland Istituto di Struttura della Materia, CNR, 34149 Trieste, Italy ISC-CNR and Department of Physics, Sapienza University of Rome, P.le A. Moro 5, 00185 Rome, Italy The Photon Factory, Department of Physics, University of Auckland, 38 Princes St, Auckland, New Zealand Institute of Solid State Physics, Karlsruhe Institute of Technology, Postfach 3640, Karlsruhe 76021, Germany Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China LPEM, ESPCI Paris, PSL University, CNRS, F-75005 Paris, France Sorbonne Université, CNRS, LPEM, F-75005 Paris, France (Dated: October 25, 2018)

In the present study, the heat capacities of barium cerate doped by gadolinium and yttrium oxides were measured for the first time in the temperature range of 166–790 K. The differential scanning calorimeter was used for investigation. There was reproducible anomaly with maximum at 601 K and minimum at 679 K. The experimental results were used to calculate the thermodynamic functions: smoothed heat capacities, enthalpy increment (H m o (T) − H m o (298.15)), and entropy (S m o (T)). The heat capacity in the temperature range of 166–602 K was described by a polynomial of the form: C p,m o (T) = 83.140 + 0.14943 T − 1.1537 × 10−4 T2 − 3.6358 × 105/T2 (J mol−1 K−1). Heat capacity of BaCe0.8Y0.1Gd0.1O2.9 in the temperature range of 602–680 K was described by a polynomial: C p,m o (T) = − 118.18 + 0.71961 T − 5.5387 × 10−4 T2 + 5.6948 × 10+6/T2 (J mol−1 K−1). The heat capacity in the temperature range of 680–790 K was well described by the equation: C p,m o (T) = 1987.1 − 6.9263 T + 8.3407 × 10−3 T2 − 3.1992 × 10−6 T3 (J mol−1 K−1).

For the first time the heat capacities of compound Bi12.5Er1.5CoO22.3 have been measured in the temperature range of 193-547 K. Differential scanning calorimetry has been used for measurements. The temperature dependence of heat capacity has been well described by a polynomial C-p,m(o) (T) = 665.46 + 0.82227 T - 3.9692.10(-4) T-2 - 5.3798.10(6)/T-2. On the basis of smoothed heat capacities the enthalpy and entropy increments have been calculated (T = 193-547 K). (C) 2018 Elsevier Ltd.

We present angle-resolved photoemission spectroscopy data taken from the structurally simplest representative of iron-based superconductors, FeSe, in a wide temperature range. Apart from the variations related to the nematic transition, we detect very pronounced shifts of the dispersions on the scale of hundreds of degrees Kelvin. Remarkably, upon warming up the sample, the band structure has a tendency to relax to the one predicted by conventional band structure calculations, directly opposite to what is intuitively expected. Our findings shed light on the origin of the dominant interaction shaping the electronic states responsible for high-temperature superconductivity in iron-based materials.

Using muon spin rotation and infrared spectroscopy, we study the relation between magnetism and superconductivity in Ba1-x KxFe2As2 single crystals from the underdoped to the slightly overdoped regime. We find that the Fe magnetic moment is only moderately suppressed in most of the underdoped region where it decreases more slowly than the Neel temperature T-N. This applies for both the total Fe moment obtained from muon spin rotation and for the itinerant component that is deduced from the spectral weight of the spin-density-wave pair-breaking peak in the infrared response. In the moderately underdoped region, superconducting and static magnetic orders coexist on the nanoscale and compete for the same electronic states. The static magnetic moment disappears rather sharply near optimal doping, however, in the slightly overdoped region there is still an enhancement or slowing down of spin fluctuations in the superconducting state. Similar to the gap magnitude reported from specific-heat measurements, the superconducting condensate density is nearly constant in the optimally and slightly overdoped region, but exhibits a rather pronounced decrease on the underdoped side. Several of these observations are similar to the phenomenology in the electron-doped counterpart Ba(Fe1-yCoy)(2)As-2.

For the first time solution enthalpies of Al, H3PO4, and AlH2P3O10 x 2H(2)O in 2 mol dm (3) NaOH have been measured as following: Delta H-sol(o) (1) = -404.75 +/- 4.36 kJ mol (1); Delta H-sol(o) (2) = -189.48 +/- 0.54 kJ mol (1); Delta H-sol(o) (3) = -238.95 +/- 3.32 kJ mol (1). On the basis of experimental data the standard molar enthalpy of formation and enthalpies of some reactions with participation of AlH2P3O10 x H2O were calculated. The enthalpy of interaction of Al with H3PO4 is Delta H-r(o) = -734.24 +/- 5.56 kJ mol (1). It was established that according to thermodynamic data Al2O3 can react with H3PO4 forming investigated compound at 513 K. Employed compound (AlH2P3O10 x H2O) can react with H2O with formation of AlPO4 and phosphorous acid. All the data were obtained for the first time. (C) 2017 Elsevier Ltd.

Angle-resolved photoemission spectroscopy is used to study the scattering rates of charge carriers from the hole pockets near Gamma in the iron-based high-T-c hole-doped superconductors K-x Ba1-x Fe2As2, x = 0.4, and K-x Eu1-x Fe2As2, x = 0.55, and the electron-doped compound Ba(Fe1-x Co-x)(2)As-2, x = 0.075. The scattering rate for any given band is found to depend linearly on the energy, indicating a non-Fermi-liquid regime. The scattering rates in the hole-doped compound are considerably higher than those in the electron-doped compounds. In the hole-doped systems the scattering rate of the charge carriers of the inner hole pocket is about three times higher than the binding energy, indicating that the spectral weight is heavily incoherent. The strength of the scattering rates and the difference between electron-and hole-doped compounds signals the importance of Hund's exchange coupling for correlation effects in these iron-based high-T-c superconductors. The experimental results are in qualitative agreement with theoretical calculations in the framework of combined density functional dynamical mean-field theory.