55 research outputs found
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 10 GPa, demonstrating the
enhanced mechanical stability due to the inner tube already at low pressures.
Anomalies at the critical pressure P12 GPa signal the onset of the
pressure-induced deformation of the tubular cross-sections. The value of P
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.
A Raman study of the Charge-Density-Wave State in AMoO (A = K,Rb)
We report a comparative Raman spectroscopic study of the
quasi-one-dimensional charge-density-wave systems \ab (A = K, Rb). The
temperature and polarization dependent experiments reveal charge-coupled
vibrational Raman features. The strongly temperature-dependent collective
amplitudon mode in both materials differ by about 3 cm, thus revealing the role
of alkali atom. We discus the observed vibrational features in terms of
charge-density-wave ground state accompanied by change in the crystal symmetry.
A frequency-kink in some modes seen in \bb between T = 80 K and 100 K supports
the first-order lock-in transition, unlike \rb. The unusually sharp Raman
lines(limited by the instrumental response) at very low temperatures and their
temperature evolution suggests that the decay of the low energy phonons is
strongly influenced by the presence of the temperature dependent charge density
wave gap.Comment: 13 pages, 7 figure
Crystal structure of LaTiO_3.41 under pressure
The crystal structure of the layered, perovskite-related LaTiO_3.41
(La_5Ti_5O_{17+\delta}) has been studied by synchrotron powder x-ray
diffraction under hydrostatic pressure up to 27 GPa (T = 295 K). The
ambient-pressure phase was found to remain stable up to 18 GPa. A sluggish, but
reversible phase transition occurs in the range 18--24 GPa. The structural
changes of the low-pressure phase are characterized by a pronounced anisotropy
in the axis compressibilities, which are at a ratio of approximately 1:2:3 for
the a, b, and c axes. Possible effects of pressure on the electronic properties
of LaTiO_3.41 are discussed.Comment: 5 pages, 6 figure
Pressure-induced metallization and structural phase transition of the Mott-Hubbard insulator TiOBr
We investigated the pressure-dependent optical response of the
low-dimensional Mott-Hubbard insulator TiOBr by transmittance and reflectance
measurements in the infrared and visible frequency range. A suppression of the
transmittance above a critical pressure and a concomitant increase of the
reflectance are observed, suggesting a pressure-induced metallization of TiOBr.
The metallic phase of TiOBr at high pressure is confirmed by the presence of
additional excitations extending down to the far-infrared range. The
pressure-induced metallization coincides with a structural phase transition,
according to the results of x-ray powder diffraction experiments under
pressure.Comment: 4 pages, 3 figure
Extremely Small Energy Gap in the Quasi-One-Dimensional Conducting Chain Compound SrNbO
Resistivity, optical, and angle-resolved photoemission experiments reveal
unusual one-dimensional electronic properties of highly anisotropic
SrNbO. Along the conducting chain direction we find an extremely small
energy gap of only a few meV at the Fermi level. A discussion in terms of
typical 1D instabilities (Peierls, Mott-Hubbard) shows that neither seems to
provide a satisfactory explanation for the unique properties of SrNbO.Comment: 4 pages, 3 figure
Signatures of polaronic excitations in quasi-one-dimensional LaTiO
The optical properties of quasi-one-dimensional metallic LaTiO are
studied for the polarization along the and axes. With decreasing
temperature modes appear along both directions suggestive for a phase
transition. The broadness of these modes along the conducting axis might be due
to the coupling of the phonons to low-energy electronic excitations across an
energy gap. We observe a pronounced midinfrared band with a temperature
dependence consistent with (interacting) polaron models. The polaronic picture
is corroborated by the presence of strong electron-phonon coupling and the
temperature dependence of the dc conductivity.Comment: 5 pages, 5 figure
Comparative magnetotransport and Tc measurements on kappa-(BEDT-TTF)2Cu(SCN)2 under pressure
We compare magnetotransport measurements under pressure on the organic
superconductor -(BEDT-TTF)Cu(SCN) with different
pressure-media and discover that the results are pressure media dependent. This
pressure-medium dependence is thought to originate from the difference in
thermal contraction between the very soft and highly anisotropic sample and the
isotropically contracting, but solid pressure medium, thus resulting in
non-hydrostatic pressure on the sample. However, comparison of pressure
measurements with different media reveals a pressure-medium independent
correlation between the superconducting transition temperature, T,
and the size of the quasi 2-dimensional Fermi surface pocket and thus the quasi
2-dimensional carrier density in -(BEDT-TTF)Cu(SCN). The
observed pressure-induced increase in the quasi 2-dimensional carrier density
can be interpreted as a transfer of carriers from quasi 1-dimensional Fermi
surface sections, reminiscent of a mechanism in cuprate superconductors, where
pressure is known to transfer carriers from the insulating charge reservoir
layers into the conducting cuprate sheets.Comment: 11 pages, 6 figure
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