103 research outputs found
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 2 GPa due to a
structural phase transition.Comment: 13 pages, 15 figure
Wide-range optical studies on various single-walled carbon nanotubes: the origin of the low-energy gap
We present wide-range (3 meV - 6 eV) optical studies on freestanding
transparent carbon nanotube films, made from nanotubes with different diameter
distributions. In the far-infrared region, we found a low-energy gap in all
samples investigated. By a detailed analysis we determined the average
diameters of both the semiconducting and metallic species from the near
infrared/visible features of the spectra. Having thus established the
dependence of the gap value on the mean diameter, we find that the frequency of
the low energy gap is increasing with increasing curvature. Our results
strongly support the explanation of the low-frequency feature as arising from a
curvature-induced gap instead of effective medium effects. Comparing our
results with other theoretical and experimental low-energy gap values, we find
that optical measurements yield a systematically lower gap than tunneling
spectroscopy and DFT calculations, the difference increasing with decreasing
diameter. This difference can be assigned to electron-hole interactions.Comment: 9 pages, 8 figures, to be published in Physical Review B,
supplemental material attached v2: Figures 1, 7 and 8 replaced, minor changes
to text; v3: Figures 3, 4 and 5 replaced, minor changes to tex
Pressure-induced transition from the dynamic to static Jahn-Teller effect in (PhP)IC
High-pressure infrared transmission measurements on \PhC60 were performed up
to 9 GPa over a broad frequency range (200 - 20000 cm) to monitor the
vibrational and electronic/vibronic excitations under pressure. The four
fundamental T 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 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
Study of charge dynamics in transparent single-walled carbon nanotube films
We report the transmission over a wide frequency range (far infrared -
visible) of pristine and hole-doped, free-standing carbon nanotube films at
temperatures between 50 K and 300 K. Optical constants are estimated by
Kramers-Kronig analysis of transmittance. We see evidence in the far infrared
for a gap below 10 meV. Hole doping causes a shift of spectral weight from the
first interband transition into the far infrared. Temperature dependence in
both the doped and undoped samples is restricted to the far-infrared region.Comment: 6 pages, 4 figures, submitted to Phys. Rev. B v3: Fig. 2 replaced,
changes in caption of Table II, minor changes in tex
Far-infrared study of the Jahn-Teller distorted C60 monoanion in C60 tetraphenylphosphoniumiodide
We report high-resolution far-infrared transmission measurements on C(60)-tetraphenylphosphoniumiodide as a function of temperature. In the spectral region investigated (20-650 cm(-1)), we assign intramolecular modes of the C(60) monoanion and identify low-frequency combination modes. The well-known F(1u)(1) and F(1u)(2) modes are split into doublers at room temperature, indicating a D(5d) or D(3d) distorted ball. This result is consistent with a dynamic Jahn-Teller effect in the strong-coupling limit or with a static distortion stabilized by low-symmetry perturbations. The appearance of silent odd modes is in keeping with symmetry reduction of the hall, while activation of even modes is attributed to interband electron-phonon coupling and orientational disorder in the fulleride salt. Temperature dependences reveal a weak transition in the region 125-150 K in both C(60)(-) and counterion modes, indicating a bulk, rather than solely molecular, effect. Anomalous softening (with decreasing temperature) in several modes may correlate with the radial character of those vibrations. [S0163-1829(98)03245-7]
Giant microwave absorption in fine powders of superconductors
Enhanced microwave absorption, larger than that in the normal state, is
observed in fine grains of type-II superconductors (MgB and KC)
for magnetic fields as small as a few of the upper critical field. The
effect is predicted by the theory of vortex motion in type-II superconductors,
however its direct observation has been elusive due to skin-depth limitations;
conventional microwave absorption studies employ larger samples where the
microwave magnetic field exclusion significantly lowers the absorption. We show
that the enhancement is observable in grains smaller than the penetration
depth. A quantitative analysis on KC in the framework of the
Coffey--Clem (CC) theory explains well the temperature dependence of the
microwave absorption and also allows to determine the vortex pinning force
constant
Static and dynamic Jahn-Teller effect in the alkali metal fulleride salts A4C60 (A = K, Rb, Cs)
We report the temperature dependent mid- and near-infrared spectra of K4C60,
Rb4C60 and Cs4C60. The splitting of the vibrational and electronic transitions
indicates a molecular symmetry change of C604- which brings the fulleride anion
from D2h to either a D3d or a D5d distortion. In contrast to Cs4C60, low
temperature neutron diffraction measurements did not reveal a structural phase
transition in either K4C60 and Rb4C60. This proves that the molecular
transition is driven by the molecular Jahn-Teller effect, which overrides the
distorting potential field of the surrounding cations at high temperature. In
K4C60 and Rb4C60 we suggest a transition from a static to a dynamic Jahn-Teller
state without changing the average structure. We studied the librations of
these two fullerides by temperature dependent inelastic neutron scattering and
conclude that both pseudorotation and jump reorientation are present in the
dynamic Jahn-Teller state.Comment: 13 pages, 10 figures, to be published in Phys. Rev.
Direction-dependent secondary bonds and their stepwise melting in a uracil-based molecular crystal studied by infrared spectroscopy and theoretical modeling
Three types of supramolecular interactions are identified in the three
crystallographic directions in crystals of
1,4-bis[(1-hexylurac-6-yl)ethynyl]benzene, a uracil-based molecule with a
linear backbone. These three interactions, characterized by their strongest
component, are: intermolecular double H-bonds along the molecular axis, London
dispersion interaction of hexyl chains connecting these linear assemblies, and
-- stacking of the aromatic rings perpendicular to the molecular
planes. On heating, two transitions happen, disordering of hexyl chains at 473
K, followed by H-bond melting at 534 K. The nature of the bonds and transitions
was established by matrix-isolation and temperature-dependent infrared
spectroscopy and supported by theoretical computations
Structure and properties of the stable two-dimensional conducting polymer Mg5C60
We present a study on the structural, spectroscopic, conducting,
and
magnetic properties of Mg5C60, which is a two-dimensional (2D)
fulleride polymer. The polymer phase is stable up to the
exceptionally
high temperature of 823 K. The infrared and Raman studies
suggest the
formation of single bonds between the fulleride ions and
possibly
Mg-C-60 covalent bonds. Mg5C60 is a metal at ambient
temperature, as
shown by electron spin resonance and microwave conductivity
measurements. The smooth transition from a metallic to a
paramagnetic
insulator state below 200 K is attributed to Anderson
localization
driven by structural disorder
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