Magnetic susceptibility of the frustrated spinels ZnCr2O4, MgCr2O4 and CdCr2O4

We analyzed the magnetic susceptibilities of several Cr spinels using two recent models for the geometrically frustrated pyrochlore lattice, the Quantum Tetrahedral Mean Field model and a Generalized Constant Coupling model. Both models can describe the experimental data for ACr2 O4 (with A = Zn, Mg, and Cd) satisfactorily, with the former yielding a somewhat better agreement with experiment for A = Zn, Mg. The obtained exchange constants for nearest and next-nearest neighbors are discussed.Comment: 4 pages, 1 figure, 1 table, conferenc

Tuning orbital-selective correlation effects in superconducting Rb0.75_{0.75}Fe1.6_{1.6}Se2−z_{2-z}Sz_z

We report on terahertz time-domain spectroscopy on superconducting and metallic iron chalcogenides Rb$_{0.75}$Fe$_{1.6}$Se$_{2-z}$S$_z$. The superconducting transition is reduced from $T_c=$ 32 K ($z=0$) to 22 K ($z=1.0$), and finally suppressed ($z=1.4$) by isoelectronic substitution of Se with S. Dielectric constant and optical conductivity exhibit a metal-to-insulator transition associated with an orbital-selective Mott phase. This orbital-selective Mott transition appears at higher temperature $T_{met}$ with increasing sulfur content, identifying sulfur substitution as an efficient parameter to tune orbital-dependent correlation effects in iron-chalcogenide superconductors. The reduced correlations of the $d_{xy}$ charge carriers can account for the suppression of the superconductivity and the pseudogap-like feature between $T_c$ and $T_{met}$ that was observed for $z=0$.Comment: 6 pages, 4 figure

Large and Small Polaron Excitations in La2/3(Sr/Ca)1/3MnO3 Films

We present detailed optical measurements of the mid-infrared (MIR) excitations in thin films of La2/3Sr1/3MnO3 (LSMO) and La2/3Ca1/3MnO3 (LCMO) across the magnetic transition. The shape of the excitation at about 0.2 eV in both samples is analyzed in terms of polaron models. We propose to identify the MIR resonance in LSMO as the excitation of large polarons and that in LCMO as a small polaron excitation. A scaling behavior for the low-energy side of the polaronic MIR resonance in LSMO is established

Evolution of optical properties of chromium spinels CdCr2_2O4_4, HgCr2_2S4_4, and ZnCr2_2Se4_4 under high pressure

We report pressure-dependent reflection and transmission measurements on ZnCr$_2$Se$_4$, HgCr$_2$S$_4$, and CdCr$_2$O$_4$ single crystals at room temperature over a broad spectral range 200-24000 cm$^{-1}$. The pressure dependence of the phonon modes and the high-frequency electronic excitations indicates that all three compounds undergo a pressure-induced structural phase transition with the critical pressure 15 GPa, 12 GPa, and 10 GPa for CdCr$_2$O$_4$, HgCr$_2$S$_4$, and ZnCr$_2$Se$_4$, respectively. The eigenfrequencies of the electronic transitions are very close to the expected values for chromium crystal-field transitions. In the case of the chalcogenides pressure induces a red shift of the electronic excitation which indicates a strong hybridization of the Cr d-bands with the chalcogenide bands.Comment: Accepted for publication in Phys. Rev.

Correlation of structural and spectroscopic properties of a photosynthetic reaction center

Polarized spectra of absorption and light-induced absorbance changes are presented for the crystallized reaction centers of Rhodopseudomonas viridis. We find that a model based on extended dipole interaction between all six pigments is capable of interpreting detailed features such as the contributions from the individual pigments to the various absorption peaks. Even though the pigments are arranged in approximate C2 symmetry, the optical spectra together with the calculations reflect deviations from this symmetry, which may be important in understanding the electron pathway