794 research outputs found
Phonon emission and absorption in the fractional quantum Hall effect
We investigate the time dependent thermal relaxation of a two-dimensional
electron system in the fractional quantum Hall regime where ballistic phonons
are used to heat up the system to a non-equilibrium temperature. The thermal
relaxation of a 2DES at can be described in terms of a broad band
emission of phonons, with a temperature dependence proportional to . In
contrast, the relaxation at fractional filling is characterized by
phonon emission around a single energy, the magneto-roton gap. This leads to a
strongly reduced energy relaxation rate compared to with only a weak
temperature dependence for temperatures 150 mK 400 mK.Comment: 4 pages, 3 figures; 14th International Conference on High Magnetic
Fields in Semiconductor Physics, September 24-29, 2000, Matsue, Japa
Recommended from our members
Terahertz sensor for non-contact thickness and quality measurement of automobile paints of varying complexity
In this paper, we propose to use terahertz pulsed imaging (TPI) as a novel tool to measure the thickness and quality of up to four layers of car paint on both metallic and non-metallic substrates. Using a rigorous one-dimensional electromagnetic model for terahertz propagation in a multi-layered medium combined with a numerical fitting method, the refractive index, extinction coefficient, and thickness of individual paint layers were determined. This proposed method was shown to be able to resolve coating layers down to a thickness of 18 and was validated for both single- and multi-layer automobile paint samples. Results of the terahertz measurements were benchmarked against other techniques that are currently used for non-destructive testing during car manufacture: ultrasound and eddy current measurements, as well as two reference techniques, X-ray microcomputed tomography and surface profilometry. Good consistency was found between the techniques. Compared to conventional techniques, TPI has the advantage that it is a non-contact method and that it is able to spatially resolve the thickness uniformity distribution information by two-dimensional mapping.This work was supported in part by the Technology Strategy Boardunder Award 101262, and by the U.K. Engineering and Physical Sciences Research Council under EP/K503721/1.Full published version, which can also be found at: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=682722
Lifting of the Landau level degeneracy in graphene devices in a tilted magnetic field
We report on transport and capacitance measurements of graphene devices in
magnetic fields up to 30 T. In both techniques, we observe the full splitting
of Landau levels and we employ tilted field experiments to address the origin
of the observed broken symmetry states. In the lowest energy level, the spin
degeneracy is removed at filling factors and we observe an enhanced
energy gap. In the higher levels, the valley degeneracy is removed at odd
filling factors while spin polarized states are formed at even . Although
the observation of odd filling factors in the higher levels points towards the
spontaneous origin of the splitting, we find that the main contribution to the
gap at , and is due to the Zeeman energy.Comment: 5 pages, 4 figure
Uplift of Central Mongolia Recorded in Vesicular Basalts
Epeirogenic histories of highland areas have confounded earth scientists for decades, as there are few sedimentary records of paleoelevation in eroding highlands. For example, mechanisms that have led to the high elevations of the Hangay Mountains in central Mongolia are not clear, nor is it well understood how the epeirogenic history of central Mongolia is connected to that of a broader region of high elevation that extends hundreds of kilometers to the north, east, and west. However, preserved basaltic lava flows record paleoelevation in the size distributions of vesicles at the tops and bottoms of flow units. As an initial step toward better understanding the tectonics of this part of Asia, we collected and analyzed samples from several basaltic lava flows from throughout the Hangay Mountains to use as a paleoaltimeter on the basis of lava flow vesicularity. Samples were dated and scanned with x-ray tomography to provide quantitative information regarding their internal vesicle size distributions. This yielded the pressure difference between the top and bottom of each flow for paleoelevation calculation. Results suggest that the Hangay Mountains experienced uplift of more than 1 km sometime during the past 9 m.yr. The magnitude of uplift of the Hangay, in addition to the composition of its lavas, the geomorphology of the region, its drainage pattern history, and other proxies, bears on possible mechanisms for uplift of this part of central Asia
Tuning the onset voltage of resonant tunneling through InAs quantum dots by growth parameters
We investigated the size dependence of the ground state energy in
self-assembled InAs quantum dots embedded in resonant tunneling diodes.
Individual current steps observed in the current-voltage characteristics are
attributed to resonant single-electron tunneling via the ground state of
individual InAs quantum dots. The onset voltage of the first step observed is
shown to decrease systematically from 200 mV to 0 with increasing InAs
coverage. We relate this to a coverage-dependent size of InAs dots grown on
AlAs. The results are confirmed by atomic force micrographs and
photoluminescence experiments on reference samples.Comment: 3 pages, 3 figure
Long-Term Efficacy and Safety of Chronic Globus Pallidus Internus Stimulation in Different Types of Primary Dystonia
Background: Deep brain stimulation (DBS) of the globus pallidus internus (GPi) offers a very promising therapy for medically intractable dystonia. However, little is known about the long-term benefit and safety of this procedure. We therefore performed a retrospective long-term analysis of 18 patients (age 12-78 years) suffering from primary generalized (9), segmental (6) or focal (3) dystonia (minimum follow-up: 36 months). Methods: Outcome was assessed using the Burke-Fahn-Marsden (BFM) scores (generalized dystonia) and the Tsui score (focal/segmental dystonia). Follow-up ranged between 37 and 90 months (mean 60 months). Results: Patients with generalized dystonia showed a mean improvement in the BFM movement score of 39.4% (range 0 68.8%), 42.5% (range -16.0 to 81.3%) and 46.8% (range-2.7 to 83.1%) at the 3- and 12-month, and long-term follow-up, respectively. In focal/ segmental dystonia, the mean reduction in the Tsui score was 36.8% (range 0-100%), 65.1% (range 16.7-100%) and 59.8% (range 16.7-100%) at the 3- and 12-month, and long-term follow-up, respectively. Local infections were noted in 2 patients and hardware problems (electrode dislocation and breakage of the extension cable) in 1 patient. Conclusion: Our data showed Gpi-DBS to offer a very effective and safe therapy for different kinds of primary dystonia, with a significant long-term benefit in the majority of cases. Copyright (c) 2008 S. Karger AG, Base
High frequency conductivity in the quantum Hall effect
We present high frequency measurements of the diagonal conductivity sigma_xx
of a two dimensional electron system in the integer quantum Hall regime. The
width of the sigma_xx peaks between QHE minima is analyzed within the framework
of scaling theory using both temperature T=100-700 mK and frequency f <= 6 GHz
in a two parameter scaling ansatz. For the plateau transition width we find
scaling behaviour for both its temperature dependence as well as its frequency
dependence. However, the corresponding scaling exponent for temperature
kappa=0.42 significantly differs from the one deduced for frequency scaling
(c=0.6). Additionally we use the high frequency experiments to suppress the
contact resistances that strongly influences DC measurements. We find an
intrinsic critical conductivity sigma_c=0.17e^2/h, virtually independent of
temperature and filling factor, and deviating significantly from the proposed
universal value 0.5e^2/h.Comment: Proceedings of the '14th international conference on high magnetic
fields in semiconductor physics' (Semimag-2000) in Matsue, Japa
Scaling of the quantum-Hall plateau-plateau transition in graphene
The temperature dependence of the magneto-conductivity in graphene shows that
the widths of the longitudinal conductivity peaks, for the N=1 Landau level of
electrons and holes, display a power-law behavior following with a scaling exponent . Similarly the
maximum derivative of the quantum Hall plateau transitions
scales as with a scaling exponent
for both the first and second electron and hole Landau
level. These results confirm the universality of a critical scaling exponent.
In the zeroth Landau level, however, the width and derivative are essentially
temperature independent, which we explain by a temperature independent
intrinsic length that obscures the expected universal scaling behavior of the
zeroth Landau level
- …