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Achieving fundamental understanding of insulator-to-metal transitions (IMTs) in strongly correlated systems and their persistent and reversible control via nonequilibrium drive are prime targets of current condensed matter research. Photoinduced switching between competing orders in correlated insulators requires a free-energy landscape with nearly degenerate ground states, which is commonly reached through doping, strain, or static electric field. The associated spatial inhomogeneity leads to a photoinduced phase transition that remains confined near the illuminated region. Here we report optical spectroscopy experiments at the first-order IMT in the 4d-electron compound Ca$_3$(Ru$_{0.99}$Ti$_{0.01}$)$_2$O$_7$ and show that specific Ru d-d interband transitions excited by light with a threshold fluence corresponding to the planar density of Ru atoms can trigger reversible, avalanche-like coherent propagation of phase interfaces across the full extent of a macroscopic sample, in the absence of assisting external stimuli. Based on detailed comparison of spectroscopic data to density functional calculations, we attribute the extraordinary photo-sensitivity of the IMT to an exceptionally shallow free-energy landscape generated by the confluence of electron-electron and electron-lattice interactions. Our findings suggest Ca$_3$(Ru$_{0.99}$Ti$_{0.01}$)$_2$O$_7$ as an ideal model system for building and testing a theory of Mott transition dynamics in the presence of strong electron-lattice coupling and may pave the way towards nanoscale devices with quantum-level photosensitivity., Comment: 24 pages, 8 figures, 3 ancillary video files

Using a combination of infrared ellipsometry, time-domain terahertz spectroscopy, and far-infrared reflectometry we have obtained the $ac$-plane complex dielectric function of monoclinic ($C2/c$) Ta$_2$NiSe$_5$ and orthorhombic ($Cmcm$) Ta$_2$NiS$_5$ single crystals. The identified dipole-active phonon modes polarized along $a$ and $c$ axes are in good agreement with density functional theory calculations. With increasing temperature the $a$-axis phonon modes of Ta$_2$NiSe$_5$ become poorly discernible, as they are superimposed on the electronic background which gradually fills the energy gap near the monoclinic-to-orthorhombic phase transition temperature $T_c$ = 326 K. In Ta$_2$NiS$_5$, which does not exhibit such a structural transition and remains orthorhombic down to low temperatures, the $a$-axis phonon modes are superimposed on a persistent broad electronic mode centered near 16 meV. We attribute this difference to strongly overlapping exciton-phonon complexes in Ta$_2$NiSe$_5$, as opposed to isolated instances of the same in Ta$_2$NiS$_5$, and find this to be in good agreement with an excitonic insulator state below $T_c$ in the former, as compared to the absence of one in the latter., Comment: 10 pages, 7 figures

We report the complex dielectric function of the quasi-one-dimensional chalcogenide Ta$_2$NiSe$_5$, which exhibits a structural phase transition that has been attributed to exciton condensation below $T_c = 326$ K, and of the isostructural Ta$_2$NiS$_5$ which does not exhibit such a transition. Using spectroscopic ellipsometry, we have detected exciton doublets with pronounced Fano lineshapes in both the compounds. The exciton Fano resonances in Ta$_2$NiSe$_5$ display an order of magnitude higher intensity than those in Ta$_2$NiS$_5$. In conjunction with prior theoretical work by E. Rashba, we attribute this observation to the giant oscillator strength of spatially extended exciton-phonon bound states in Ta$_2$NiSe$_5$. The formation of exciton-phonon complexes in Ta$_2$NiS$_5$ and Ta$_2$NiSe$_5$ is confirmed by the pronounced temperature dependence of sharp interband transitions in the optical spectra, whose peak energies and widths scale with the thermal population of optical phonon modes. The description of the optically excited states in terms of strongly overlapping exciton complexes is in good agreement with the hypothesis of an EI ground state., Comment: 10 pages, 7 figures

We report the complex dielectric function along and perpendicular to the IrO2 planes in the layered perovskite Sr2IrO4 determined by spectroscopic ellipsometry in the spectral range from 12 meV to 6 eV. Thin high quality single crystals were stacked to measure the c-axis optical conductivity. In the phonon response we identified 10 infrared-active modes polarized within the basal plane and only four modes polarized along the c-axis, in full agreement with first-principle lattice dynamics calculations. We also observed a strong optical anisotropy in the near-infrared spectra arising from direct transitions between Ir 5d t2g Jeff = 1/2 and Jeff = 3/2 bands, which transition probability is highly suppressed for light polarized along the c-axis. The spectra are analyzed and discussed in terms of relativistic LSDA+U band structure calculations., Comment: 9 pages, 7 figures

The photoresponse of YBa2Cu3O7-d thin film microbridges with thicknesses between 15 and 50 nm was studied in the optical and terahertz frequency range. The voltage transients in response to short radiation pulses were recorded in real time with a resolution of a few tens of picoseconds. The bridges were excited by either femtosecond pulses at a wavelength of 0.8 \mu m or broadband (0.1 - 1.5 THz) picosecond pulses of coherent synchrotron radiation. The transients in response to optical radiation are qualitatively well explained in the framework of the two-temperature model with a fast component in the picosecond range and a bolometric nanosecond component whose decay time depends on the film thickness. The transients in the THz regime showed no bolometric component and had amplitudes up to three orders of magnitude larger than the two-temperature model predicts. Additionally THz-field dependent transients in the absence of DC bias were observed. We attribute the response in the THz regime to a rearrangement of vortices caused by high-frequency currents.

We present measurements of the infrared response of the quasi-one-dimensional organic conductor (TMTSF)2$SO3 along (E||a) and perpendicular (E||b') to the stacking axis as a function of temperature. Above the metal-insulator transition related to the anion ordering the optical conductivity spectra show a Drude-like response. Below the transition an energy gap of about 1500 cm-1 (185 meV) opens, leading to the corresponding charge transfer band in the optical conductivity spectra. The analysis of the infrared-active vibrations gives evidence for the long-range crystal structure modulation below the transition temperature and for the short-range order fluctuations of the lattice modulation above the transition temperature. Also we report about a new infrared mode at around 710 cm-1 with a peculiar temperature behavior, which has so far not been observed in any other (TMTSF)2X salt showing a metal-insulator transition. A qualitative model based on the coupling between the TMTSF molecule vibration and the reorientation of electrical dipole moment of the FSO3 anion is proposed, in order to explain the anomalous behavior of the new mode., Comment: to appear in Eur. Phys. J. B

We investigate the crystal structure of YTiO3 at high pressures up to 30 GPa by synchrotron x-ray powder diffraction (T = 295 K). The variation of the Ti--O bond lengths with pressure evidences a distinct change in the distortion of the TiO_6 octahedra at around 10 GPa, indicating a pressure-driven spatial reorientation of the occupied Ti 3d(t_2g) orbitals. The pressure-induced reduction of the optical band gap of YTiO3 is determined quantitatively by mid-infrared synchrotron micro-spectroscopy and discussed in terms of bond length and orbital orientation changes., Comment: 4 pages, 6 figures

The far-infrared (FIR) c axis conductivity of a Bi2223 crystal has been measured by ellipsometry. Below T$_c$ a strong absorption band develops near 500 cm $^{-1}$, corresponding to a transverse Josephson-plasmon. The related increase in FIR spectral weight leads to a giant violation of the Ferrell-Glover-Tinkham sum-rule. The gain in c axis kinetic energy accounts for a sizeable part of the condensation energy. We also observe phonon anomalies which suggest that the Josephson-currents lead to a drastic variation of the local electric field within the block of closely spaced CuO$_2$ planes., Comment: 8 pages, 3 figures