Collapse of Critical Nematic Fluctuations in FeSe under Pressure.

We report the evolution of the electronic nematic susceptibility in FeSe via Raman scattering as a function of hydrostatic pressure up to 5.8 GPa where the superconducting transition temperature T_{c} reaches its maximum. The critical nematic fluctuations observed at low pressure vanish above 1.6 GPa, indicating they play a marginal role in the fourfold enhancement of T_{c} at higher pressures. The collapse of nematic fluctuations appears to be linked to a suppression of low energy electronic excitations which manifests itself by optical phonon anomalies at around 2 GPa, in agreement with lattice dynamical and electronic structure calculations using local density approximation combined with dynamical mean field theory. Our results reveal two different regimes of nematicity in the phase diagram of FeSe under pressure: a d-wave Pomeranchuk instability of the Fermi surface at low pressure and a magnetic driven orthorhombic distortion at higher pressure.

Singlet Orbital Ordering in Bilayer Sr_{3}Cr_{2}O_{7}.

We perform an extensive study of Sr_{3}Cr_{2}O_{7}, the n=2 member of the Ruddlesden-Popper Sr_{n+1}Cr_{n}O_{3n+1} system. An antiferromagnetic ordering is clearly visible in the magnetization and the specific heat, which yields a huge transition entropy, Rln(6). By neutron diffraction as a function of temperature we have determined the antiferromagnetic structure that coincides with the one obtained from density functional theory calculations. It is accompanied by anomalous asymmetric distortions of the CrO_{6} octahedra. Strong coupling and Lanczos calculations on a derived Kugel-Khomskii Hamiltonian yield a simultaneous orbital and moment ordering. Our results favor an exotic ordered phase of orbital singlets not originated by frustration.

Ba19Cr12O45: A High Pressure Chromate with an Original Structure Solved by Electron Diffraction Tomography and Powder X-ray Diffraction.

We report on the discovery of a Ba-based chromate obtained by high pressure-high temperature treatment of the low pressure orthorhombic Ba2CrO4 phase. By combining transmission electron microscopy and powder X-ray diffraction measurements, we have determined its crystallographic structure. This new Cr-oxide has a cubic lattice with a = 13.3106(6) Å built from a three-dimensional network of two Cr sites, Cr1 and Cr2, both in octahedral environments, with face sharing between Cr1 and Cr2 octahedra and corner-sharing between two Cr1 octahedra. The resulting chemical composition Ba19Cr12O45 and bond valence sum analysis suggest a possible charge disproportion between Cr4+ in the Cr1 site and Cr5+ in the Cr2 site. Finally analysis of magnetization measurements indicates antiferromagnetic correlations between Cr cations and also points toward a probable charge disproportion between Cr sites.

Phosphate tablets or polyethylene glycol for preparation to colonoscopy? A multicentre non-inferiority randomized controlled trial.

BACKGROUND: Adequate bowel preparation is a crucial step in colonoscopy procedure and has been identified as the cornerstone of a quality colonoscopy. Polyethylene glycol (PEG) for bowel cleansing still had up to 10 % unprepared colon. AIM: We herein compare efficacy, acceptability, tolerance and safety of sodium phosphate (NaP) tablets and split-dose PEG for bowel cleansing. PATIENTS AND METHODS: A prospective non-inferiority randomized trial was performed and registered on www.clinicaltrials.gov (NCT01840553). Patients were randomized to either 32 NaP tablets or 4 L of PEG. Blind readers assessed the efficacy of colon cleansing using the Boston Bowel Preparation Scale (BBPS). RESULTS: A total of 461 patients were randomized in groups (NaP group: n = 231; PEG group: n = 230). Median age was 54 and 52 in NaP group and PEG group, respectively (p < 0.01). Patients experienced an overall compliance to the treatment in 99.6 and 94.1 % in the NaP group and in the PEG group, respectively (p < 0.001). The mean time of withdrawal was 15.1 ± 8.9 and 15.4 ± 9.5 min in the NaP group and in the PEG group, respectively (p = 0.95). The good quality of bowel preparation, defined as BBPS score ≥7, was obtained in 86.4 and 89.0 % of cases in the NaP group and in the PEG group, respectively (p = 0.42). In all segment (right colon, transverse colon and left colon and rectum), the NaP group was non-inferior to the PEG group. Bowel prep regimen was more frequently considered as "easy" by patients from the NaP group (54.8 % of patients) than patients from the PEG group (29.0 % of patients; p < 0.001). No serious adverse events were reported. No statistical differences were found between the NaP group and the PEG group concerning the incidence of an adverse event (338 vs. 322, respectively). CONCLUSION: While NaP tablets appeared as efficient as PEG in terms of colon cleansing prior to a colonoscopy, they significantly improved the overall compliance and eased product administration. At an era where bowel cleansing appears to be the cornerstone of a quality colonoscopy, NaP tablets in patients without contraindication might be considered as an option.

Charge-induced nematicity in FeSe.

The spontaneous appearance of nematicity, a state of matter that breaks rotation but not translation symmetry, is one of the most intriguing properties of the iron-based superconductors (Fe SC), and has relevance for the cuprates as well. Establishing the critical electronic modes behind nematicity remains a challenge, however, because their associated susceptibilities are not easily accessible by conventional probes. Here, using FeSe as a model system, and symmetry-resolved electronic Raman scattering as a probe, we unravel the presence of critical charge nematic fluctuations near the structural/nematic transition temperature, [Formula: see text] 90 K. The diverging behavior of the associated nematic susceptibility foretells the presence of a Pomeranchuk instability of the Fermi surface with d-wave symmetry. The excellent scaling between the observed nematic susceptibility and elastic modulus data demonstrates that the structural distortion is driven by this d-wave Pomeranchuk transition. Our results make a strong case for charge-induced nematicity in FeSe.

Jahn-Teller, polarity, and insulator-to-metal transition in BiMnO3 at high pressure.

The interaction of coexisting structural instabilities in multiferroic materials gives rise to intriguing coupling phenomena and extraordinarily rich phase diagrams, both in bulk materials and strained thin films. Here we investigate the multiferroic BiMnO3 with its peculiar 6s2 electrons and four interacting mechanisms: electric polarity, octahedra tilts, magnetism, and cooperative Jahn-Teller distortion. We have probed structural transitions under high pressure by synchrotron x-ray diffraction and Raman spectroscopy up to 60 GPa. We show that BiMnO3 displays under pressure a rich sequence of five phases with a great variety of structures and properties, including a metallic phase above 53 GPa and, between 37 and 53 GPa, a strongly elongated monoclinic phase that allows ferroelectricity, which contradicts the traditional expectation that ferroelectricity vanishes under pressure. Between 7 and 37 GPa, the Pnma structure remains remarkably stable but shows a reduction of the Jahn-Teller distortion in a way that differs from the behavior observed in the archetypal orthorhombic Jahn-Teller distorted perovskite LaMnO3.

Structural trends and chemical bonding in Te-doped silicon clathrates.

The recently discovered tellurium-doped silicon clathrates, Te7+xSi20-x and Te16Si38, both low- and high-temperature forms (cubic and rhombohedral, respectively), exhibit original structures that are all derived from the parent type I clathrate G8Si46 (G = guest atom). The similarities and differences between the structures of these compounds and that of the parent one are analyzed and discussed on the basis of charge distribution and chemical bonding considerations. Because of the particular character of the Te atom, these compounds appear to be at the border between the clathrate and polytelluride families.