Infrared Investigation of the Charge Ordering Pattern in the Organic Spin Ladder Candidate (DTTTF)2Cu(mnt)2

We measured the variable temperature infrared response of the spin ladder candidate (DTTTF)2Cu(mnt)2 in order to distinguish between two competing ladder models, rectangular versus zigzag, proposed for this family of materials. The distortion along the stack direction below 235 K is consistent with a doubling along b through the metal-insulator transition. While this would agree with either of the ladder models, the concomitant transverse distortion rules out the rectangular ladder model and supports the zigzag scenario. Intramolecular distortions within the DTTTF building block molecule also give rise to on-site charge asymmetry.Comment: 4 pages, 4 figures, submitted to Solid State Science

Spin-phonon coupled modes in the incommensurate phases of doped CuGeO3_{3}

The doping effect of the folded phonon mode at 98 cm$^{-1}$ was investigated on the Si-doped CuGeO$_3$ by magneto-optical measurements in far-infrared (FIR) region under high magnetic field. The folded phonon mode at 98 cm$^{-1}$ appears not only in the dimerized (D) phase but also in the dimerized-anitiferromagnetic (DAF) phase on the doped CuGeO$_3$. The splitting was observed in the incommensurate (IC) phase and the antiferromagnetically ordered incommensurate (IAF) phase above $H_C$. The split-off branches exhibit different field dependence from that of the pure CuGeO$_3$ in the vicinity of $H_C$, and the discrepancy in the IAF phase is larger than that in the IC phase. It is caused by the interaction between the solitons and the impurities.Comment: 7 pages, 4 figures, resubmitted to Phys. Rev.

Layered tungsten bronzes: Tuning the optical properties by changing the layer thickness

We investigated the optical properties of a series of monophosphate tungsten bronzes (P02)4(W03)2m (m=2,4, 6,7) as a function of layer thickness, with special attention on the m=7 density wave superconductor. These materials have several layers of corner-sharing WO6 octahedra separated by one PO4 layer, leading to a tunable octahedral layer thickness with m. In the optical regime, the spectra of the m=2, 4, 6, and 7 materials display an anisotropic electronic excitation, originating from the W intra t2g d to d transition. The intensities and frequencies of these excitations vary with the octahedral layer thickness, consistent with a softer lattice with increasing m. The low-frequency electrodynamics of the monophosphate tungsten bonzes show a gap or pseudogap feature in the infrared, demonstrating a ubiquitous bound camer response. The m=7 density wave superconductor is especially interesting. The variable temperature ab-plane spectra display a suppression of the optical conductivity along the b-axis below 140 K, giving rise to charge localization and anisotropic charge density wave gap formation near 1400 cm-1. This middle infrared charge localization is directly related to the appearance of both flat and dispersive bands along b. Although oscillator strength is redistributed among the free carrier response, charge density wave gap absorption, and d to d transition in the density wave states, the spectral weight is largely conserved below the plasma frequency. Based upon these observations, P4W 14050 is another example of a superconductor with an unusual normal state

Correlation effects in the electronic structure of the Mn4 molecular magnet

Contains fulltext : 72097.pdf (publisher's version ) (Open Access)4 p

Optical properties of complex oxides: Vanadate ladders and related inorganic nanotubes

Optical spectroscopy is a sensitive probe of charge, spin, and lattice degrees of freedom. In this talk, I will detail our magneto-optical work on alpha'-NaV2O5, a prototypical ladder material, as well as the new Na2V3O7 nanotubes. Using changes in electronic structure with temperature and applied magnetic field, we have mapped out the H-T phase diagram of alpha'-NaV2O5. We find a temperature independent phase boundary at 27 T which persists above Tc (34 K), a kink in Tc(H) near 27 T, and H$\wedge$2 behavior of Tc(H) in the high field phase with an unusually small coupling constant. The energy scale of Tc and the newly discovered phase boundary at Hc (27 T) are identical. Confinement effects are investigated in the related nanotubes. Results include a larger gap than in analogous bulk materials as well as a low-energy rattling mode, which corresponds to the motion of Na+ ions inside the nanotube

Magnetic field dependent vibrational modes in κ\kappa-(ET)2_{2}Cu(SCN)2_{2} organic superconductor

The infrared reflectance of the 10.4 K organic superconductor $\kappa$(ET)$_{2}$Cu(SCN)$_{2}$ has been measured as a function of applied magnetic field at 4.2 K. We investigate changes in intramolecular vibrational modes between the superconducting (low field) and normal (high field) states, of interest for mechanistic reasons. It is shown that the $\nu$$_{3}$ (A$_{g})$, $\nu$$_{60 ~}$(B$_{3g})$, and $\nu$$_{21 }$(B$_{1g})$ modes display field dependence. These results suggest that intramolecular vibrational modes are involved in the superconducting to normal state transition in $\kappa$(ET)$_{2}$Cu(SCN)$_{2 }$ below $T_{c}$. Key words. Organic superconductors – infrared spectra – magnetooptical effects

Understanding the gap in polyoxovanadate molecule-based magnets

Contains fulltext : 34814.pdf (publisher's version ) (Open Access)We report a joint experimental and theoretical investigation of the transport gap, optical properties, and electronic structure of two chemically similar, inhomogeneously mixed-valent polyoxovanadate molecule-based magnets. We attribute the substantial gap in [NHEt3](4)[(V8V4As8O40)-V-IV-As-V(H2O)]center dot H2O to weak p-d hybridization and a large on-site Coulomb repulsion (U=6 eV). The reduced gap in [NHEt3](3)[(V6V6As8O40)-V-IV-As-V(HCO2)]center dot 2H(2)O is associated with a smaller value of U (4 eV), at least from a molecular point of view, although the transport properties also reflect subtle organization of the molecular structure and differences between direct and indirect intermolecular charge transfer. A detailed analysis of the vibrational response supports the important role of local molecular distortions and hydrogen bonding in the intramolecular and intermolecular charge transport of [NHEt3](4)[(V8V4As8O40)-V-IV-As-V(H2O)]center dot H2O

High field optical response of

We report the polarized optical reflectance of quasi-two-dimensional η-Mo4O11 in a perpendicular magnetic field up to 30 T at 4.2 K. The in-field measurements are done on the conducting bc plane covering the 6000-26000 cm-1 energy range. The absolute reflectance shows a free carrier response in the near infrared and two electronic transitions in the optical regime : an intraband transition (d → d) near 12000 cm-1 and a charge transfer (oxygen p → molybdenum d) near 30000 cm-1. In the magnetic field, no change in the normalized reflectance is observed in the intraband transition range through the 20 T Landau transition, whereas a notable deviation from unity is observed on the low-energy side of the charge transfer transition along both b and c polarizations. We discuss the results in terms of the effect of the magnetic field on the electronic structure and quantum limit response of η-Mo4O11

Adsorption-controlled growth of BiMnO3 thin films by molecular-beam epitaxy

We have developed the means to grow BiMnO3 thin films with unparalleled structural perfection by reactive molecular-beam epitaxy and determined its band gap. Film growth occurs in an adsorption-controlled growth regime. Within this growth window bounded by oxygen pressure and substrate temperature at a fixed bismuth overpressure, single-phase films of the metastable perovskite BiMnO3 may be grown by epitaxial stabilization. X-ray diffraction reveals phase-pure and epitaxial films with omega rocking curve full width at half maximum values as narrow as 11 arc sec (0.003 degrees). Optical absorption measurements reveal that BiMnO3 has a direct band gap of 1.1 +/- 0.1 eV. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3457786