High-field irreversible moment reorientation in the antiferromagnet Fe1.1_{1.1}Te

Magnetization measurements have been performed on single-crystalline Fe$_{1.1}$Te in pulsed magnetic fields $\mathbf{H}\perp\mathbf{c}$ up to 53 T and temperatures from 4.2 to 65 K. At $T=4.2$ K, a non-reversible reorientation of the antiferromagnetic moments is observed at $\mu_0H_R=48$ T as the pulsed field is on the rise. No anomaly is observed at $H_R$ during the fall of the field and, as long as the temperature is unchanged, during both rises and falls of additional field pulses. The transition at $H_R$ is reactivated if the sample is warmed up above the N\'{e}el temperature $T_N\simeq60$ K and cooled down again. The magnetic field-temperature phase diagram of Fe$_{1.1}$Te in $\mathbf{H}\perp\mathbf{c}$ is also investigated. We present the temperature dependence of $H_R$, as well as that of the antiferromagnetic-to-paramagnetic borderline $H_c$ in temperatures above 40 K.Comment: 5 pages, 4 figure

Magnetic glassy phase in FeSeTe single crystals

The evolution of the magnetic order in FeSeTe crystals as a function of Se content was investigated by means of ac/dc magnetometry and muon-spin spectroscopy. Experimental results and self-consistent DFT calculations both indicate that muons are implanted in vacant iron-excess sites, where they probe a local field mainly of dipolar origin, resulting from an antiferromagnetic (AFM) bicollinear arrangement of iron spins. This long-range AFM phase disorders progressively with increasing Se content. At the same time all the tested samples manifest a marked glassy character that vanishes for high Se contents. The presence of local electronic/compositional inhomogeneities most likely favours the growth of clusters whose magnetic moment "freezes" at low temperature. This glassy magnetic phase justifies both the coherent muon precession seen at short times in the asymmetry data, as well as the glassy behaviour evidenced by both dc and ac magnetometry.Comment: Approved for publication in J. Phys.: Condens. Matte

Electron spin resonance and exchange paths in the orthorhombic dimer system Sr2VO4

We report on magnetization and electron spin resonance (ESR) measurements of Sr$_{2}$VO$_4$ with orthorhombic symmetry. In this dimer system the $V^{4+}$ ions are in tetrahedral environment and are coupled by an antiferromagnetic intra-dimer exchange constant $J/k_B \approx$ 100 K to form a singlet ground state without any phase transitions between room temperature and 2 K. Based on an extended-H\"{u}ckel-Tight-Binding analysis we identify the strongest exchange interaction to occur between two inequivalent vanadium sites via two intermediate oxygen ions. The ESR absorption spectra can be well described by a single Lorentzian line with an effective g-factor $g$ = 1.89. The temperature dependence of the ESR intensity is well described by a dimer model in agreement with the magnetization data. The temperature dependence of the ESR linewidth can be modeled by a superposition of a linear increase with temperature with a slope $\alpha$ = 1.35 Oe/K and a thermally activated behavior with an activation energy $\Delta/k_B$ = 1418 K, both of which point to spin-phonon coupling as the dominant relaxation mechanism in this compound.Comment: 5 pages, 4 figure

Two-dimensional orbital ordering in d1 Mott insulator Sr2VO4

The Mott insulator Sr2VO4 is a unique d1 two-dimensional compound exhibiting an orbital ordering transition. In addition to the orbital ordering transition at about 100 K, we discovered a ferromagnetic transition below 10 K, thus confirming the predictions of recent band structure calculations. The magnetic properties proved to be strongly sensitive to the material purity, the actual oxygen stoichiometry and the crystallographic parameters. An additional transition is observed at 125 K, which is believed to be due to structural modifications

Evidence for core-hole-mediated inelastic x-ray scattering from metallic Fe1.087_{1.087}Te

We present a detailed analysis of resonant inelastic scattering (RIXS) from Fe$_{1.087}$Te with unprecedented energy resolution. In contrast to the sharp peaks typically seen in insulating systems at the transition metal $L_3$ edge, we observe spectra which show different characteristic features. For low energy transfer, we experimentally observe theoretically predicted many-body effects of resonant Raman scattering from a non-interacting gas of fermions. Furthermore, we find that limitations to this many-body electron-only theory are realized at high Raman shift, where an exponential lineshape reveals an energy scale not present in these considerations. This regime, identified as emission, requires considerations of lattice degrees of freedom to understand the lineshape. We argue that both observations are intrinsic general features of many-body physics of metals.Comment: 4 pages, 4 figure

Optical study of phonons and electronic excitations in tetragonal Sr2_2VO4_4

We report on the optical excitation spectra in Sr$_2$VO$_4$. The phonon modes are assigned and their evolution with temperature is discussed in the frame of the different phase transitions crossed upon cooling. Besides the expected infrared-active phonons we observe two additional excitations at about 290 cm$^{-1}$ and 840 cm$^{-1}$ which could correspond to electronic transitions of the V$^{4+}$ ions. Our experimental results are discussed in the context of recent experimental and theoretical studies of this material with a unique spin-orbital ground state.Comment: 7 pages, 6 figure

Pressure induced evolution of superconductivity and magnetic hourglass dispersion in Fe1.02Te0.7Se0.3

Iron based high temperature superconductors have several common features with superconducting cuprates, including the square lattice and the proximity to an antiferromagnetic phase. The magnetic excitation spectrumbelowTc of Fe1.02Te0.7Se0.3 shows an hourglass-shaped dispersion with a resonance around the commensurate point . In a previous inelastic neutron scattering study, we showed that the hourglass-shaped dispersion is most likely a prerequisite for superconductivity, while the consequences are the opening of a gap and a shift of spectral weight. In this paper we follow the evolution of the hourglass shaped dispersion under applied pressure up to 12 kbar. Our results show that that the pressure-induced 37% increase of Tc is concomitant with a change in the magnetic excitation spectrum, with an increase of the hourglass energy by 38%

Liver X receptors, lipids and their reproductive secrets in the male

International audienceLiver X receptor (LXR) a and LXRb belong to the nuclear receptor superfamily. For many years they have been called orphan receptors, as no natural ligand was identified. In the last decade the LXR natural ligands have been shown to be oxysterols, molecules derived from cholesterol. While these nuclear receptors have been abundantly studied for their roles in the regulation of lipid metabolism, it appears that they also present crucial activities in reproductive organs such as testis and epididymis, as well as prostate. Phenotypic analyses of mice lacking LXRs (−/−) pointed out their physiological activies in the various cells and organs regulating reproductive functions. This review summarizes the impact of LXR-deficiency in male reproduction, highlighting the novel information coming from the phenotypic analyses of −/−, −/− and −/− mice

Optical study of phonons and electronic excitations in tetragonal Sr 2VO4

We report on the optical excitation spectra in Sr2VO 4. The phonon modes are assigned and their evolution with temperature is discussed in the frame of the different phase transitions crossed upon cooling. Besides the expected infrared-active phonons, we observe two additional excitations at about 290 and 840 cm-1, which could correspond to electronic transitions of the V4+ ions. Our experimental results are discussed in the context of recent experimental and theoretical studies of this material with a unique spin-orbital ground state. © 2011 American Physical Society