1,549 research outputs found
Microspectroscopy and Imaging in the THz Range Using Coherent CW Radiation
A novel THz near-field spectrometer is presented which allows to perform
biological and medical studies with high spectral resolution combined with a
spatial resolution down to l/100. In the setup an aperture much smaller than
the used wavelength is placed in the beam very close to the sample. The sample
is probed by the evanescent wave behind the aperture. The distance is measured
extremely accurate by a confocal microscope. We use monochromatic sources which
provide powerful coherent cw radiation tuneable from 50 GHz up to 1.5 THz.
Transmission and reflection experiments can be performed which enable us to
study solids and molecules in aqueous solution. Examples for spectroscopic
investigations on biological tissues are presented.Comment: 4 pages, 5 figures, email: [email protected]
DC and high-frequency conductivity of the organic metals beta"-(BEDT-TTF)2SF5RSO3 (R = CH2CF2 and CHF)
The temperature dependences of the electric-transport properties of the
two-dimensional organic conductors beta"-(BEDT-TTF)2SF5CH2CF2SO3,
beta"-(d8-BEDT-TTF)2SF5CH2CF2SO3, and beta"-(BEDT-TTF)2SF5CHFSO3 are measured
by dc methods in and perpendicular to the highly-conducting plane. Microwave
measurements are performed at 24 and 33.5 GHz to probe the high-frequency
behavior from room temperature down to 2 K. Superconductivity is observed in
beta"-(BEDT-TTF)2SF5CH2CF2SO3 and its deuterated analogue. Although all the
compounds remain metallic down to low-temperatures, they are close to a
charge-order transition. This leads to deviations from a simple Drude behavior
of the optical conductivity which become obvious already in the microwave
range. In beta"-(BEDT-TTF)2SF5CH2CF2SO3, for instance, charge fluctuations
cause an increase in microwave resistivity for T < 20 K which is not detected
in dc measurements. beta"-(BEDT-TTF)2SF5CHFSO3 exhibits a simple metallic
behavior at all frequencies. In the dc transport, however, we observe
indications of localization in the perpendicular direction.Comment: 8 pages, 9 figure
Temperature and frequency dependent optical properties of ultra-thin Au films
While the optical properties of thin metal films are well understood in the
visible and near-infrared range, little has been done in the mid- and
far-infrared region. Here we investigate ultra-thin gold films prepared on
Si(111)(7 x 7) in UHV by measuring in the frequency range between 500 cm-1 and
7000 cm-1 and for temperatures between 300 K and 5 K. The nominal thickness of
the gold layers ranges between one monolayer and 9 nm. The frequency and
temperature dependences of the thicker films can be well described by the Drude
model of a metal, when taking into account classical size effects due to
surface scattering. The films below the percolation threshold exhibit a
non-metallic behavior: the reflection increases with frequency and decreases
with temperature. The frequency dependence can partly be described by a
generalized Drude model. The temperature dependence does not follow a simple
activation process. For monolayers we observe a transition between surface
states around 1100 cm-1.Comment: 7 pages, 10 figure
Deconfinement transition and dimensional crossover in the Bechgaard-Fabre salts: pressure- and temperature-dependent optical investigations
The infrared response of the organic conductor (TMTSF)PF and the Mott
insulator (TMTTF)PF 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)PF, 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)PF exhibits a crossover from a
quasi-one-dimensional to a higher-dimensional metal upon reducing the
temperature. For (TMTSF)PF 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
Signatures of polaronic charge ordering in optical and dc conductivity using dynamical mean field theory
We apply dynamical mean field theory to study a prototypical model that
describes charge ordering in the presence of both electron-lattice interactions
and intersite electrostatic repulsion between electrons. We calculate the
optical and d.c. conductivity, and derive approximate formulas valid in the
limiting electron-lattice coupling regimes. In the weak coupling regime, we
recover the usual behavior of charge density waves, characterized by a transfer
of spectral weight due to the opening of a gap in the excitation spectrum. In
the opposite limit of very strong electron-lattice coupling, instead, the
charge ordering transition is signaled by a global enhancement of the optical
absorption, with no appreciable spectral weight transfer. Such behavior is
related to the progressive suppression of thermally activated charge defects
taking place below the critical temperature. At intermediate values of the
coupling within the polaronic regime, a complex behavior is obtained where both
mechanisms of transfer and enhancement of spectral weight coexist.Comment: 1 figure added, illustrating the optical sum rul
ROCKY BOTTOM ROUGHNESS ALONG THE CALIFORNIA COAST
The roughness of ninety-nine rocky bottoms is measured for nearshore bathymetric surveys (depths >5m to several km offshore) resolved at 1 or 2m extending the length of California. Visual inspection shows a wide variety of rock formations comprising the bottoms. Many flat areas are found in depressions amongst the rocks that presumably are filled with sediments. Roughness is measured as the standard deviation of the residual elevation, σ_z, about a mean planform that is averaged over 750m in the alongshore and cross-shore. The σ_z vary from 0.35m to 8.6m with a mean of 2.1m. The measured rocky bottoms are significantly rougher than previously measured coral reef bottoms. Roughness scale is determined from ensemble-averaged wavenumber (k) spectra of elevation, (G_zz ) ̂(k), and of bottom slopes, ( G_ββ ) ̂(k) in both the cross-shore (x) and alongshore (y). All (G_zz ) ̂(k) are red with maximum variance at low wavenumber with slopes increasing from -1 to -3, have similar shapes, and appear near isotropic in x and y. Assuming a power law relationship in k, the slopes of (G_zz ) ̂(k), denoted as (α_z ) ̂(k), are shown to be related to the slopes of (G_ββ ) ̂(k), denoted as(〖 α〗_β ) ̂(k), by log (α_β ) ̂(k)=log (α_z ) ̂(k) +2 in x and y. Both (α_z ) ̂(k) and (α_β ) ̂(k) are isotropic in x and y evidenced by their 95% confidence intervals overlapping across the intermediate wavenumber scales <1/666m.ONR, Arlington, VA 22203Lieutenant Commander, United States NavyApproved for public release. Distribution is unlimited
Non-Fermi liquid behavior in nearly charge ordered layered metals
Non-Fermi liquid behavior is shown to occur in two-dimensional metals which
are close to a charge ordering transition driven by the Coulomb repulsion. A
linear temperature dependence of the scattering rate together with an increase
of the electron effective mass occur above T*, a temperature scale much smaller
than the Fermi temperature. It is shown that the anomalous temperature
dependence of the optical conductivity of the quasi-two-dimensional organic
metal alpha-(BEDT-TTF)2MHg(SCN)4, with M=NH4 and Rb, above T*=50-100 K, agrees
qualitatively with our predictions for the electronic properties of nearly
charge ordered two-dimensional metals.Comment: accepted in Phys. Rev. Let
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
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