25 research outputs found
Dephasing of G-Band Phonons in Single-Wall Carbon Nanotubes Probed via Impulsive Stimulated Raman Scattering
We have studied the coherent dynamics of G-band phonons in single-walled
carbon nanotubes through impulsive stimulated Stokes and anti-Stokes Raman
scattering. The probe energy dependence of phonon amplitude as well as
preferential occurrence between Stokes and anti-Stokes components in response
to chirped-pulse excitation are well explained within our model. The
temperature dependence of the observed dephasing rate clearly exhibits a
thermally-activated component, with an activation energy that coincides with
the frequency of the radial breathing mode (RBM). This fact provides a clear
picture for the dephasing of optical phonons by random frequency modulation via
interaction with the RBM through anharmonicity.Comment: 4 pages, 4 figure
Chirality-Selective Excitation of Coherent Phonons in Carbon Nanotubes
Using pre-designed trains of femtosecond optical pulses, we have selectively
excited coherent phonons of the radial breathing mode of specific-chirality
single-walled carbon nanotubes within an ensemble sample. By analyzing the
initial phase of the phonon oscillations, we prove that the tube diameter
initially increases in response to ultrafast photoexcitation. Furthermore, from
excitation profiles, we demonstrate that an excitonic absorption peak of carbon
nanotubes periodically oscillates as a function of time when the tube diameter
undergoes radial breathing mode oscillations.Comment: 4 pages, 4 figure
Polarization dependence of coherent phonon generation and detection in highly-aligned single-walled carbon nanotubes
We have investigated the polarization dependence of the generation and
detection of radial breathing mode (RBM) coherent phonons (CP) in
highly-aligned single-walled carbon nanotubes. Using polarization-dependent
pump-probe differential-transmission spectroscopy, we measured RBM CPs as a
function of angle for two different geometries. In Type I geometry, the pump
and probe polarizations were fixed, and the sample orientation was rotated,
whereas, in Type II geometry, the probe polarization and sample orientation
were fixed, and the pump polarization was rotated. In both geometries, we
observed a very nearly complete quenching of the RBM CPs when the pump
polarization was perpendicular to the nanotubes. For both Type I and II
geometries, we have developed a microscopic theoretical model to simulate CP
generation and detection as a function of polarization angle and found that the
CP signal decreases as the angle goes from 0 degrees (parallel to the tube) to
90 degrees (perpendicular to the tube). We compare theory with experiment in
detail for RBM CPs created by pumping at the E44 optical transition in an
ensemble of single-walled carbon nanotubes with a diameter distribution
centered around 3 nm, taking into account realistic band structure and
imperfect nanotube alignment in the sample
Circular-Polarization Dependent Cyclotron Resonance in Large-Area Graphene in Ultrahigh Magnetic Fields
Using ultrahigh magnetic fields up to 170 T and polarized midinfrared
radiation with tunable wavelengths from 9.22 to 10.67 um, we studied cyclotron
resonance in large-area graphene grown by chemical vapor deposition.
Circular-polarization dependent studies reveal strong p-type doping for
as-grown graphene, and the dependence of the cyclotron resonance on radiation
wavelength allows for a determination of the Fermi energy. Thermal annealing
shifts the Fermi energy to near the Dirac point, resulting in the simultaneous
appearance of hole and electron cyclotron resonance in the magnetic quantum
limit, even though the sample is still p-type, due to graphene's linear
dispersion and unique Landau level structure. These high-field studies
therefore allow for a clear identification of cyclotron resonance features in
large-area, low-mobility graphene samples.Comment: 9 pages, 3 figure
Resonant Coherent Phonon Spectroscopy of Single-Walled Carbon Nanotubes
Using femtosecond pump-probe spectroscopy with pulse shaping techniques, one
can generate and detect coherent phonons in chirality-specific semiconducting
single-walled carbon nanotubes. The signals are resonantly enhanced when the
pump photon energy coincides with an interband exciton resonance, and analysis
of such data provides a wealth of information on the chirality-dependence of
light absorption, phonon generation, and phonon-induced band structure
modulations. To explain our experimental results, we have developed a
microscopic theory for the generation and detection of coherent phonons in
single-walled carbon nanotubes using a tight-binding model for the electronic
states and a valence force field model for the phonons. We find that the
coherent phonon amplitudes satisfy a driven oscillator equation with the
driving term depending on photoexcited carrier density. We compared our
theoretical results with experimental results on mod 2 nanotubes and found that
our model provides satisfactory overall trends in the relative strengths of the
coherent phonon signal both within and between different mod 2 families. We
also find that the coherent phonon intensities are considerably weaker in mod 1
nanotubes in comparison with mod~2 nanotubes, which is also in excellent
agreement with experiment.Comment: 21 pages, 22 figure
Malnutrition Has No Effect on the Timing of Human Tooth Formation
The effect of nutrition on the timing of human tooth formation is poorly understood. Delays and advancements in dental maturation have all been reported as well as no effect. We investigated the effect of severe malnutrition on the timing of human tooth formation in a large representative sample of North Sudanese children. The sample (1102 males, 1013 females) consisted of stratified randomly selected healthy individuals in Khartoum, Sudan, aged 2-22 years using a cross-sectional design following the STROBE statement. Nutritional status was defined using WHO criteria of height and weight. Body mass index Z-scores and height for age Z-scores of ≤-2 (cut-off) were used to identify the malnourished group (N = 474) while the normal was defined by Z-scores of ≥0 (N = 799). Clinical and radiographic examination of individuals, with known ages of birth was performed including height and weight measurements. Mandibular left permanent teeth were assessed using eight crown and seven root established tooth formation stages. Mean age at entry and mean age within tooth stages were calculated for each available tooth stage in each group and compared using a t-test. Results show the mean age at entry and mean age within tooth stages were not significantly different between groups affected by severe malnutrition and normal children (p>0.05). This remarkable finding was evident across the span of dental development. We demonstrate that there is little measurable effect of sustained malnutrition on the average timing of tooth formation. This noteworthy finding supports the notion that teeth have substantial biological stability and are insulated from extreme nutritional conditions compared to other maturing body systems
Nerve Conduction Study in the Extremity Carrying Fistula in Hemodialysis Patients
Introduction: Arteriovenous fistulas (AVF) make chronic hemodialysis easy, but the complications are unpredictable. Ischemic neuropathy due to arterial insufficiency is one of the most important complications of AVF. This study was designed to evaluate ischemic neuropathy using electro diagnostic method in the extremity carrying fistula. Methods: In this cross-sectional study, 37 hemodialysis patients at the hemodialysis center of Shahid-Mohammadi hospital (Bandar-Abbas) were enrolled. All of the patients had only one AVF in one extremity. NCV of median and ulnar nerves of both upper extremities were evaluated and the intact extremity of all patients served as the control group. Results: In patients with distal AVF, ulnar motor conduction velocities were significantly increased (p: 0.034). Distal and proximal amplitudes of ulnar nerve were decreased only in non-diabetic and hypertensive patients (p<0.05). Median sensory nerve action potential was increased, distal amplitude decreased (p: 0.04) and sensory latency was increased (p: 0.02). In patients with proximal AVF, ulnar proximal amplitude and median motor conduction velocities were decreased (p: 0.05). There was a negative correlation between the age of AVF and development of neuropathy. In most patients with neuropathy, AVF age was less than one year. Conclusion: Ulnar nerve is more sensitive to ischemia than median nerve and sensory fibers are more sensitive than motor fibers. The site of fistula had no affect on the rate of development of ischemic neuropathy. We recommend using electrodiagnostic study for screening of ischemic neuropathy every 6 months in hemodialysis patients
Cell-specific Deletion of NLRP3 Inflammasome Identifies Myeloid Cells as Key Drivers of Liver Inflammation and Fibrosis in Murine Steatohepatitis.
Background & aimsNonalcoholic fatty liver disease (NAFLD) is the leading cause of chronic liver disease worldwide. The NLRP3 inflammasome, a platform for caspase-1 activation and release of interleukin 1β, is increasingly recognized in the induction of inflammation and liver fibrosis during NAFLD. However, the cell-specific contribution of NLRP3 inflammasome activation in NAFLD remains unknown.MethodsTo investigate the role of NLRP3 inflammasome activation in hepatocytes, hepatic stellate cells (HSCs) and myeloid cells, a conditional Nlrp3 knock-out mouse was generated and bred to cell-specific Cre mice. Both acute and chronic liver injury models were used: lipopolysaccharide/adenosine-triphosphate to induce in vivo NLRP3 activation, choline-deficient, L-amino acid-defined high-fat diet, and Western-type diet to induce fibrotic nonalcoholic steatohepatitis (NASH). In vitro co-culture studies were performed to dissect the crosstalk between myeloid cells and HSCs.ResultsMyeloid-specific deletion of Nlrp3 blunted the systemic and hepatic increase in interleukin 1β induced by lipopolysaccharide/adenosine-triphosphate injection. In the choline-deficient, L-amino acid-defined high-fat diet model of fibrotic NASH, myeloid-specific Nlrp3 knock-out but not hepatocyte- or HSC-specific knock-out mice showed significant reduction in inflammation independent of steatosis development. Moreover, myeloid-specific Nlrp3 knock-out mice showed ameliorated liver fibrosis and decreased HSC activation. These results were validated in the Western-type diet model. In vitro co-cultured studies with human cell lines demonstrated that HSC can be activated by inflammasome stimulation in monocytes, and this effect was significantly reduced if NLRP3 was downregulated in monocytes.ConclusionsThe study provides new insights in the cell-specific role of NLRP3 in liver inflammation and fibrosis. NLRP3 inflammasome activation in myeloid cells was identified as crucial for the progression of NAFLD to fibrotic NASH. These results may have implications for the development of cell-specific strategies for modulation of NLRP3 activation for treatment of fibrotic NASH