82 research outputs found
Field induced quantum-Hall ferromagnetism in suspended bilayer graphene
We have measured the magneto-resistance of freely suspended high-mobility
bilayer graphene. For magnetic fields T we observe the opening of a field
induced gap at the charge neutrality point characterized by a diverging
resistance. For higher fields the eight-fold degenerated lowest Landau level
lifts completely. Both the sequence of this symmetry breaking and the strong
transition of the gap-size point to a ferromagnetic nature of the insulating
phase developing at the charge neutrality point.Comment: 7 pages, 5 figure
Microwave assisted heterogeneous catalysis: effects of varying oxygen concentrations on the oxidative coupling of methane
The oxidative coupling of methane was investigated over alumina supported La2O3/CeO2 catalysts under microwave dielectric heating conditions at different oxygen concentrations. It was observed that, at a given temperature using microwave heating, selectivities for both ethane and ethylene were notably higher when oxygen was absent than that in oxygen/methane mixtures. The differences were attributed to the localised heating of microwave radiation resulting in temperature inhomogeneity in the catalyst bed. A simplified model was used to estimate the temperature inhomogeneity; the temperature at the centre of the catalyst bed was 85 °C greater than that at the periphery when the catalyst was heated by microwaves in a gas mixture with an oxygen concentration of 12.5% (v/v), and the temperature difference was estimated to be 168 °C in the absence of oxygen
A bright impulsive solar burst detected at 30 THz
Ground- and space-based observations of solar flares from radio wavelengths
to gamma-rays have produced considerable insights but raised several unsolved
controversies. The last unexplored wavelength frontier for solar flares is in
the range of submillimeter and infrared wavelengths. Here we report the
detection of an intense impulsive burst at 30 THz using a new imaging system.
The 30 THz emission exhibited remarkable time coincidence with peaks observed
at microwave, mm/submm, visible, EUV and hard X-ray wavelengths. The emission
location coincides with a very weak white-light feature, and is consistent with
heating below the temperature minimum in the atmosphere. However, there are
problems in attributing the heating to accelerated electrons. The peak 30 THz
flux is several times larger than the usual microwave peak near 9 GHz,
attributed to non-thermal electrons in the corona. The 30 THz emission could be
consistent with an optically thick spectrum increasing from low to high
frequencies. It might be part of the same spectral component found at sub-THz
frequencies whose nature remains mysterious. Further observations at these
wavelengths will provide a new window for flare studies.Comment: 9 pages, 11 figures, accepted by Astrophysical Journal, March 23,
201
Spin dependent quantum interference in non-local graphene spin valves
Spin dependent electron transport measurements on graphene are of high
importance to explore possible spintronic applications. Up to date all spin
transport experiments on graphene were done in a semi-classical regime,
disregarding quantum transport properties such as phase coherence and
interference. Here we show that in a quantum coherent graphene nanostructure
the non-local voltage is strongly modulated. Using non-local measurements, we
separate the signal in spin dependent and spin independent contributions. We
show that the spin dependent contribution is about two orders of magnitude
larger than the spin independent one, when corrected for the finite
polarization of the electrodes. The non-local spin signal is not only strongly
modulated but also changes polarity as a function of the applied gate voltage.
By locally tuning the carrier density in the constriction we show that the
constriction plays a major role in this effect and indicates that it can act as
a spin filter device. Our results show the potential of quantum coherent
graphene nanostructures for the use in future spintronic devices
Unusual Emissions at Various Energies Prior to the Impulsive Phase of the Large Solar Flare and Coronal Mass Ejection of 4 November 2003
The GOES X28 flare of 4 November 2003 was the largest ever recorded in its class. It produced the first evidence for two spectrally separated emission components, one at microwaves and the other in the THz range of frequencies.We analyzed the pre-flare phase of this large flare, twenty minutes before the onset of the major impulsive burst. This periodis characterized by unusual activity in X-rays, sub-THz frequencies, H, and microwaves.The CME onset occurred before the onset of the large burst by about 6 min
Bright 30 THz Impulsive Solar Bursts
Impulsive 30 THz continuum bursts have been recently observed in solar
flares, utilizing small telescopes with a unique and relatively simple optical
setup concept. The most intense burst was observed together with a GOES X2
class event on October 27, 2014, also detected at two sub-THz frequencies,
RHESSI X-rays and SDO/HMI and EUV. It exhibits strikingly good correlation in
time and in space with white light flare emission. It is likely that this
association may prove to be very common. All three 30 THz events recently
observed exhibited intense fluxes in the range of 104 solar flux units,
considerably larger than those measured for the same events at microwave and
sub-mm wavelengths. The 30 THz burst emission might be part of the same
spectral burst component found at sub-THz frequencies. The 30 THz solar bursts
open a promising new window for the study of flares at their originComment: 11 pages, 4 Figures J. Geophys. Res - Space Physics, accepted, May
21, 201
Sub-THz and H{\alpha} activity during the preflare and main phases of a GOES class M2 event
Radio and optical observations of the evolution of flare-associated phenomena
have shown an initial and rapid burst at 0.4 THz only followed subsequently by
a localized chromospheric heating producing an H{\alpha} brightening with later
heating of the whole active region. A major instability occurred several
minutes later producing one impulsive burst at microwaves only, associated with
an M2.0 GOES X-ray flare that exhibited the main H{\alpha} brightening at the
same site as the first flash. The possible association between long-enduring
time profiles at soft X-rays, microwaves, H{\alpha} and sub-THz wavelengths is
discussed. In the decay phase the H{\alpha} movie shows a disrupting magnetic
arch structure ejecting dark, presumably chromospheric, material upwards. The
time sequence of events suggests genuine interdependent and possibly
non-thermal instabilities triggering phenomena, with concurrent active region
plasma heating and material ejection.Comment: Accepted by Astrophysical Journal, October 13, 201
Terahertz photometer to observe solar flares in continuum
Solar observations at sub-THz frequencies detected a new flare spectral
component peaking in the THz range, simultaneously with the well known
microwaves component, bringing challenging constraints for interpretation.
Higher THz frequencies observations are needed to understand the nature of the
mechanisms occurring in flares. A THz photometer system was developed to
observe outside the terrestrial atmosphere on stratospheric balloons or
satellites, or at exceptionally transparent ground stations. The telescope was
designed to observe the whole solar disk detecting small relative changes in
input temperature caused by flares at localized positions. A Golay cell
detector is preceded by low-pass filters to suppress visible and near IR
radiation, a band-pass filter, and a chopper. A prototype was assembled to
demonstrate the new concept and the system performance. It can detect
temperature variations smaller than 1 K for data sampled at a rate of
10/second, smoothed for intervals larger than 4 seconds. For a 76 mm aperture,
this corresponds to small solar burst intensities at THz frequencies. A system
with 3 and 7 THz photometers is being built for solar flare observations on
board of stratospheric balloon missions.Comment: 11 pages, 8 figures, accepted by Journal of Infrared, Millimeter, and
Terahertz Waves, 9 December 201
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