Pressure-induced changes in the optical properties of quasi-one-dimensional β\beta-Na0.33_{0.33}V2_2O5_5

The pressure-induced changes in the optical properties of $\beta$-Na$_{0.33}$V$_2$O$_5$ single crystals at room temperature were studied by polarization-dependent Raman and far-infrared reflectivity measurements under high pressure. From the changes in the Raman- and infrared-active phonon modes in the pressure range 9 - 12 GPa a transfer of charge between the different V sites can be inferred. The importance of electron-phonon coupling in the low-pressure regime is discussed.Comment: 7 pages, 5 figure

Suppression of the Charge-Density-Wave State in Sr10_{10}Ca4_{4}Cu24_{24}O41_{41} by External Pressure

The influence of external pressure on the charge-density-wave (CDW) ground state of the quasi-one-dimensional two-leg ladder compound Sr$_{10}$Ca$_{4}$Cu$_{24}$O$_{41}$ has been studied by optical reflectivity measurements as a function of temperature (10 - 300~K) and pressure $P$ (0.3 - 4.3~GPa) over the spectral range 580 - 6000 cm$^1$. With increasing pressure the CDW transition temperature $T_{CDW}$ decreases with the linear pressure coefficient $\approx$-70~K/GPa, and above $\approx$3~GPa the CDW phase is suppressed at all temperatures. This behavior is similar to that in compounds Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ with increasing Ca content $x$ at ambient pressure, with the simple scaling $x \approx 3\cdot P(GPa)$. The size of the CDW gap decreases with increasing pressure, whereas the dimensionality of the high-temperature insulating phase in Sr$_{10}$Ca$_{4}$Cu$_{24}$O$_{41}$ within the ladder plane is hardly affected by external pressure.Comment: accepted for publication in Phys. Rev.

Deconfinement transition and dimensional crossover in the Bechgaard-Fabre salts: pressure- and temperature-dependent optical investigations

The infrared response of the organic conductor (TMTSF)$_2$PF$_6$ and the Mott insulator (TMTTF)$_2$PF$_6$ are investigated as a function of temperature and pressure and for the polarization parallel and perpendicular to the molecular stacks. By applying external pressure on (TMTTF)$_2$PF$_6$, the Mott gap rapidly diminishes until the deconfinement transition occurs when the gap energy is approximately twice the interchain transfer integral. In its deconfined state (TMTTF)$_2$PF$_6$ exhibits a crossover from a quasi-one-dimensional to a higher-dimensional metal upon reducing the temperature. For (TMTSF)$_2$PF$_6$ this dimensional crossover is observed either with increase in external pressure or with decrease in temperature. We quantitatively determine the dimensional crossover line in the pressure-temperature diagram based on the degree of coherence in the optical response perpendicular to the molecular stacks.Comment: 12 pages, 15 figure

Infrared spectroscopic studies on unoriented single-walled carbon nanotube films under hydrostatic pressure

The electronic properties of as-prepared and purified unoriented single-walled carbon nanotube films were studied by transmission measurements over a broad frequency range (far-infrared up to visible) as a function of temperature (15 K - 295 K) and external pressure (up to 8 GPa). Both the as-prepared and the purified SWCNT films exhibit nearly temperature-independent properties. With increasing pressure the low-energy absorbance decreases suggesting an increasing carrier localization due to pressure-induced deformations. The energy of the optical transitions in the SWCNTs decreases with increasing pressure, which can be attributed to pressure-induced hybridization and symmetry-breaking effects. We find an anomaly in the pressure-induced shift of the optical transitions at $\sim$2 GPa due to a structural phase transition.Comment: 13 pages, 15 figure

Optical spectroscopy study on pressure-induced phase transitions in the three-dimensional Dirac semimetal Cd3_3As2_2

We report a room-temperature optical reflectivity study performed on [112]-oriented Cd$_3$As$_2$ single crystals over a broad energy range under external pressure up to 10 GPa. The abrupt drop of the band dispersion parameter ($z$-parameter) and the interruption of the gradual redshift of the bandgap at $\sim$4~GPa confirms the structural phase transition from a tetragonal to a monoclinic phase in this material. The pressure-induced increase of the overall optical conductivity at low energies and the continuous redshift of the high-energy bands indicate that the system evolves towards a topologically trivial metallic state, although a complete closing of the band gap could not be observed in the studied pressure range. Furthermore, a detailed investigation of the low-pressure regime suggests the possible existence of an intermediate state between 2 and 4~GPa , that might be a precursor of the structural phase transition or due to the lifted degeneracy of the Dirac nodes. Several optical parameters show yet another anomaly at 8~GPa, where low-temperature superconductivity was found in an earlier study.Comment: submitted to PR

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.

The (wrong) lenguaje of the caricature: the truth in time of crisis

Ortego publica en 1869 una serie de álbumes, Menestra, en los que hace alarde de una multiplicidad de recursos gráficos e iconográficos, jugando con el idioma, las referencias a la pintura o a la obra grabada y dibujada de Goya, para difundir su convicción democrática y republicana. Partiendo del motivo de la “sombra”, analizamos los procedimientos utilizados en las litografías relacionándolos con los contextos político, estético y cultural.In 1869, Ortego published a series of Menestra albums in which he made prominent use of multiple graphic and iconographic processes, playing with the language, with references to paintings and to Goya’s engravings and drawings, with the aim of disseminating his democratic and republican beliefs. Starting from the “shadow” motif, we shall analyse the processes used in the lithographies and relate them to the political, aesthetic and cultural contexts

Pressure-induced transition from the dynamic to static Jahn-Teller effect in (Ph4_{4}P)2_{2}IC60_{60}

High-pressure infrared transmission measurements on \PhC60 were performed up to 9 GPa over a broad frequency range (200 - 20000 cm$^{-1}$) to monitor the vibrational and electronic/vibronic excitations under pressure. The four fundamental T$_{1u}$ modes of \C60a\ are split into doublets already at the lowest applied pressure and harden with increasing pressure. Several cation modes and fullerene-related modes split into doublets at around 2 GPa, the most prominent one being the G$_{1u}$ mode. The splitting of the vibrational modes can be attributed to the transition from the dynamic to static Jahn-Teller effect, caused by steric crowding at high pressure. Four absorption bands are observed in the NIR-VIS frequency range. They are discussed in terms of transitions between LUMO electronic states in \C60a, which are split because of the Jahn-Teller distortion and can be coupled with vibrational modes. Various distortions and the corresponding symmetry lowering are discussed. The observed redshift of the absorption bands indicates that the splitting of the LUMO electronic states is reduced upon pressure application.Comment: 10 pages, 17 figure

Stabilization of carbon nanotubes by filling with inner tubes: An optical spectroscopy study on double-walled carbon nanotubes under hydrostatic pressure

The stabilization of carbon nanotubes via the filling with inner tubes is demonstrated by probing the optical transitions in double-walled carbon nanotube bundles under hydrostatic pressure with optical spectroscopy. Double-walled carbon nanotube films were prepared from fullerene peapods and characterized by HRTEM and optical spectroscopy. In comparison to single-walled carbon nanotubes, the pressure-induced redshifts of the optical transitions in the outer tubes are significantly smaller below $\sim$10 GPa, demonstrating the enhanced mechanical stability due to the inner tube already at low pressures. Anomalies at the critical pressure P$_d$$\approx$12 GPa signal the onset of the pressure-induced deformation of the tubular cross-sections. The value of P$_d$ is in very good agreement with theoretical predictions of the pressure-induced structural transitions in double-walled carbon nanotube bundles with similar average diameters.Comment: 6 pages, 4 figures; to appear in Phys. Rev.