363 research outputs found
Effective viscosity of non-gravitactic Chlamydomonas Reinhardtii microswimmer suspensions
Active microswimmers are known to affect the macroscopic viscosity of
suspensions in a more complex manner than passive particles. For puller-like
microswimmers an increase in the viscosity has been observed. It has been
suggested that the persistence of the orientation of the microswimmers hinders
the rotation that is normally caused by the vorticity. It was previously shown
that some sorts of algaes are bottom-heavy swimmers, i.e. their centre of mass
is not located in the centre of the body. In this way, the algae affects the
vorticity of the flow when it is perpendicular oriented to the axis of gravity.
This orientation of gravity to vorticity is given in a rheometer that is
equipped with a cone-plate geometry. Here we present measurements of the
viscosity both in a cone-plate and a Taylor-Couette cell. The two set-ups
yielded the same increase in viscosity although the axis of gravitation in the
Taylor-Couette cell is parallel to the direction of vorticity. In a
complementary experiment we tested the orientation of the direction of swimming
through microscopic observation of single \textit{Chlamydomonas reinhardtii}
and could not identify a preferred orientation, i. e. our specific strain of
\textit{Chlamydomonas reinhardtii} are not bottom-heavy swimmers. We thus
conclude that bottom heaviness is not a prerequisite for the increase of
viscosity and that the effect of gravity on the rheology of our strain of
\textit{Chlamydomonas reinhardtii} is negligible. This finding reopens the
question of whether origin of persistence in the orientation of cells is
actually responsible for the increased viscosity of the suspension
Upper Pennsylvanian Missourian Corals of Iowa
As in Kansas where distinct zonation of corals has been recognized, the dissepimental Rugosa Dibunophyllum, Neokoninckophyllum and Geyerophyllum are invariably associated with limestones and thin calcareous shale interbeds within limestone units. In both states, the nondissepimental rugose genera Lophamplexus and Stereostyllls have been collected from both limestones and thick intervening shale units. The tabulate genera Syringopora and Cladochonus are restricted to limestones in both areas. Although the vertical distribution of Iowa and Kansas dissepimental corals is similar, Iowa rocks contain fewer and generally smaller corals than correlative Kansas units. Elements of the informal dissepimental coral zones 1, 3 and 4 established by Cocke (1970, 1972) in Kansas are presently known in Iowa
Robust surface electronic properties of topological insulators: Bi2Te3 films grown by molecular beam epitaxy
The surface electronic properties of the important topological insulator
Bi2Te3 are shown to be robust under an extended surface preparation procedure
which includes exposure to atmosphere and subsequent cleaning and
recrystallization by an optimized in-situ sputter-anneal procedure under ultra
high vacuum conditions. Clear Dirac-cone features are displayed in
high-resolution angle-resolved photoemission spectra from the resulting
samples, indicating remarkable insensitivity of the topological surface state
to cleaning-induced surface roughness.Comment: 3 pages, 3 figure
Evaluation of Pure-Tone Thresholds and Distortion Product Otoacoustic Emissions Measured in the Extended High Frequency Region
When the cochlea is stimulated with two primary tones (f1 and f2) some of the energy is reflected back and propagates via the middle ear into the outer ear. Due to cochlear nonlinearities, distortion product otoacoustic emissions (DPOAEs), may be detected by a probe microphone sealed in the ear canal. Reduced DPOAEs may indicate subclinical cochlear lesions. The relationship between hearing sensitivity and the strength of DPOAEs is debatable, especially in the extended high frequency (EHF) region (≥8 kHz). Monitoring cochlea function corresponding to the EHF range is important for detecting early stages of hearing loss, which typically begins above 8 kHz. Complex interactions of high-frequency pure-tones in the ear canal result in standing waves that increases test-retest variability of DPOAEs measured for f2≥6 kHz. The aim of the project was to evaluate reliability of DPOAEs measured up to 12 kHz with a system used routinely in audiology clinics. Thirty-one adults (age, 18-30 yrs) with normal middle-ear function and normal hearing thresholds in the conventional region (≤8 kHz) participated. The EHF audiometry was performed for frequencies up to 16 kHz. The DPOAE data were collected for the f2 frequency varied from 1.5 to 12 kHz, twice for each ear with the probe removed and then repositioned after the first test. The EHF audiometric data of four participants showed elevated thresholds. Their DPOAEs were reduced or absent for f2≥9 kHz, i.e., supporting the sensitivity of DPOAEs for cochlear hearing loss above the conventional audiometry frequency range. Mean and standard deviations of DPOAE levels were calculated separately for the left and the right ears of subjects with normal EHF thresholds. There were no differences between mean DPOAE values in the left and the right ears. The intersubject variability of the DPOAE levels was moderate (SD≈6 dB or lower) but it increased significantly in the 12-kHz region, per the F-test for variances, possibly due to 1. effects of standing waves on the high-frequency DPOAE reliability and/or 2. subclinical pathology in the most basal portion (i.e., high-frequency region) of the cochlea. For each ear, absolute values of differences between test/retest levels of detectable DPOAEs were calculated. ANOVA showed the main effect of frequency for the data collected in the left and the right ears. Post-hoc analyses indicated that test/retest variability of DPOAEs was rather constant for f2 frequencies up to 10 kHz, but a statistically significant increase of test/retest variability for f2 of 11 and 12 kHz was found. This aspect needs to be considered when using DPOAE tests for longitudinal monitoring of cochlear function in the basal portion. Nevertheless, combining behavioral thresholds with DPOAEs collected for the EHF range is vital for detecting the initial stage of the cochlear pathology corresponding to the high-frequency region, e.g., due to ototoxicity or aging of the cochlea
Ultrafast photocurrents at the surface of the three-dimensional topological insulator
Topological insulators constitute a new and fascinating class of matter with
insulating bulk yet metallic surfaces that host highly mobile charge carriers
with spin-momentum locking. Remarkably, the direction and magnitude of surface
currents can be controlled with tailored light beams, but the underlying
mechanisms are not yet well understood. To directly resolve the "birth" of such
photocurrents we need to boost the time resolution to the scale of elementary
scattering events ( 10 fs). Here, we excite and measure photocurrents in
the three-dimensional model topological insulator
with a time resolution as short as 20 fs by sampling the concomitantly emitted
broadband THz electromagnetic field from 1 to 40 THz. Remarkably, the ultrafast
surface current response is dominated by a charge transfer along the Se-Bi
bonds. In contrast, photon-helicity-dependent photocurrents are found to have
orders of magnitude smaller magnitude than expected from generation scenarios
based on asymmetric depopulation of the Dirac cone. Our findings are also of
direct relevance for optoelectronic devices based on topological-insulator
surface currents
Thickness dependence of electron-electron interactions in topological p-n junctions
Electron-electron interactions in topological p-n junctions consisting of
vertically stacked topological insulators are investigated. n-type Bi2Te3 and
p-type Sb2Te3 of varying relative thicknesses are deposited using molecular
beam epitaxy and their electronic properties measured using low-temperature
transport. The screening factor is observed to decrease with increasing sample
thickness, a finding which is corroborated by semi-classical Boltzmann theory.
The number of two-dimensional states determined from electron-electron
interactions is larger compared to the number obtained from
weak-antilocalization, in line with earlier experiments using single layers.Comment: 38 pages, 5 figures, 1 tabl
Disentangling surface and bulk transport in topological-insulator - junctions
By combining -type and -type
topological insulators, vertically stacked - junctions can be formed,
allowing to position the Fermi level into the bulk band gap and also tune
between - and -type surface carriers. Here we use low-temperature
magnetotransport measurements to probe the surface and bulk transport modes in
a range of vertical heterostructures with varying
relative thicknesses of the top and bottom layers. With increasing thickness of
the layer we observe a change from - to -type
behavior via a specific thickness where the Hall signal is immeasurable.
Assuming that the the bulk and surface states contribute in parallel, we can
calculate and reproduce the dependence of the Hall and longitudinal components
of resistivity on the film thickness. This highlights the role played by the
bulk conduction channels which, importantly, cannot be probed using surface
sensitive spectroscopic techniques. Our calculations are then buttressed by a
semi-classical Boltzmann transport theory which rigorously shows the vanishing
of the Hall signal. Our results provide crucial experimental and theoretical
insights into the relative roles of the surface and bulk in the vertical
topological - junctions.Comment: 11 pages, 5 figure
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