122 research outputs found
Infrared Spectroscopy of Graphene in Ultrahigh Magnetic Fields
Graphene – a two-dimensional honeycomb lattice of sp2-bonded carbon atoms – possesses unusual zero-gap band structure with linear band dispersions, accommodating photon-like, massless electrons that have exhibited a variety of surprising phenomena, primarily in DC transport, in the last several years. In this thesis dissertation, we investigate graphene’s AC or infrared properties in the presence of an ultrahigh magnetic field, produced by a destructive pulsed method. The linear dispersions of graphene lead to unequally spaced Landau levels in a magnetic field, which we probe through cyclotron resonance (CR) spectroscopy in the magnetic quantum limit. Specifically, using magnetic fields up to 170 T and polarized midinfrared radiation with tunable wavelengths from 9.22 to 10.67 μm, we experimentally investigated CR in large-area graphene grown by chemical vapor deposition. Circular-polarization-dependent studies revealed strong p-type doping for as-grown graphene, and the dependence of the CR fields on the radiation wavelength allowed for an accurate determination of the Fermi energy. Upon annealing the sample to remove physisorbed molecules, which shifts the Fermi energy closer to the Dirac point, we made the unusual observation that hole and electron CR emerges in the magnetic quantum limit, even though the sample is still p-type. We theoretically show that this non-intuitive phenomenon is a direct consequence of the unusual Landau level structure of graphene. Namely, if the Fermi energy lies in the n = 0 Landau level, then CR is present for both electron-active and hole-active circular polarizations. Furthermore, if the Fermi level lies in the n = 0 Landau level, the ratio of CR absorption between the electron-active and hole-active peaks allows one to accurately determine the Fermi level and carrier density. Hence, high-field CR studies allow not only for fundamental studies but also for characterization of large-area, low-mobility graphene samples
Ultrafast and magneto-optical spectroscopy of excitons and phonons in carbon nanotubes
Understanding how electrons and phonons relax in energy and momentum is one of the current goals in carbon nanotube spectroscopy as well as an important step toward developing novel electronic and optoelectronic devices based on carbon nanotubes. Here, we investigate the polarization anisotropy of coherent phonon (CP) dynamics of radial breathing mode (REM) phonons in highly-aligned single-walled carbon nanotubes (SWNTs). Using CP spectroscopy, we measure REM CPs as a function of angle for two different geometries and in both cases, we observe quenching of the RBM when polarization is perpendicular to the nanotubes. We also make progress in understanding the role of dark excitons in SWNTs at ultralow temperatures. Measuring the magnetic field dependence to 5 T, we obtained an unexpected zero-field photoluminescence (PL) and PL brightening at 50 mK. To explain this contradiction with current theory, we introduced a non-thermal distribution of excitons into current theory
Non-Physical Causes of the Loss of Imami Sources of History until the Thirteenth Century
Several major Imami sources of history, dating back to the early Islamic centuries, are not available to us. This article is concerned with the main non-physical causes of the loss of these sources, such as formation of hadith communities, negligence of historical works, and excessive focus on theology and jurisprudence. The research is done with the descriptive-analytical method based on library sources, book catalogues, bibliographies, and the views of scholars. The hypothesis put forward in this article is that change of attitude on the part of Imami scholars led to the enrichment of some fields of study and impoverishment of others. The findings confirm this hypothesis for the main non-physical cause of the loss of historical sources, as opposed to certain alleged physical causes. Since citation is a significant criterion for accreditation of research data, then if these missing sources were available to us, the findings of historians could be more accurate and cogent and there would be no ground for reliance on famous historical accounts
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
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
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
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