1,315 research outputs found
High Frequency Conductivity in the Quantum Hall Regime
We have measured the complex conductivity of a two-dimensional
electron system in the quantum Hall regime up to frequencies of 6 GHz at
electron temperatures below 100 mK. Using both its imaginary and real part we
show that can be scaled to a single function for different
frequencies and for all investigated transitions between plateaus in the
quantum Hall effect. Additionally, the conductivity in the variable-range
hopping regime is used for a direct evaluation of the localization length
. Even for large filing factor distances from the critical
point we find with a scaling exponent
Hopping conductivity in the quantum Hall effect -- revival of universal scaling
We have measured the temperature dependence of the conductivity
of a two-dimensional electron system deep into the localized regime of the
quantum Hall plateau transition. Using variable-range hopping theory we are
able to extract directly the localization length from this experiment. We
use our results to study the scaling behavior of as a function of the
filling factor distance to the critical point of the transition.
We find for all samples a power-law behavior
with a universal scaling exponent as proposed theoretically
Conductance fluctuations at the quantum Hall plateau transition
We analyze the conductance fluctuations observed in the quantum Hall regime
for a bulk two-dimensional electron system in a Corbino geometry. We find that
characteristics like the power spectral density and the temperature dependence
agree well with simple expectations for universal conductance fluctuations in
metals, while the observed amplitude is reduced. In addition, the dephasing
length , which governs the temperature dependence of
the fluctuations, is surprisingly different from the scaling length
governing the width of the quantum Hall plateau
transition
Geographies of Indigenous-based Team Name and Mascot Use in American Secondary Schools
This dissertation identifies the geographic locations and characteristics of high schools using Native American nicknames and, through the examination of Indigenous-based iconography, analyzes the portrayal of Indigenous peoples in these learning environments. Primary and secondary data collected from archives, newspapers, yearbooks, directories, school websites, and fieldwork are utilized in various ways, including cartographic and quantitative analyses of 1,368 school locations and their attributes, numerous case studies highlighting concocted histories and stereotypical depictions of Native Americans, and a content analysis of Indigenous-based imagery photographed in 125 schools with predominately white student bodies. The first chapter defines the dissertation topic and provides a literature review of relevant geographic and anti-mascot literature, and Chapter Two stresses the relevance of studying team names. Chapter Three examines the Indianās role in the development of American identity and argues that the tradition of portraying of Native Americans as āOthersā in literature, art, Wild West shows, film, and television continues to this day through the use of team names and mascots. Chapter Four applies Renato Rosaldoās concept of imperialist nostalgia, Mary Louise Prattās idea of anti-conquest, and other theories to explain the selection and continued use of Native American mascots. National, regional, and local geographic patterns of mascot use are mapped and analyzed in Chapter Five, and Chapter Six critiques the portrayal of diverse Native American groups as stereotypical Plains Indians, noble and ignoble savages, masculine warriors, Indian princesses, and other depictions in secondary schools. The study complements existing geographic and anti-mascot literature in three ways: by approaching the Native American mascot issue from a spatial perspective and mapping the locations and characteristics of these schools for analysis, by strengthening the anti-mascot argument by examining the practice in secondary schools, the next logical step in a movement that has focused largely on professional and collegiate sports, and by advancing knowledge in geography and in the interdisciplinary realm of antimascot literature by using textual evidence from numerous historical periods, and at several geographic scales, to emphasize how race-based ideologies become manifest on the landscape and in life through the use of iconography and ritual
Specific Heat of a Fractional Quantum Hall System
Using a time-resolved phonon absorption technique, we have measured the
specific heat of a two-dimensional electron system in the fractional quantum
Hall effect regime. For filling factors
and 1/3 the specific heat displays a strong exponential temperature dependence
in agreement with excitations across a quasi-particle gap. At filling factor
we were able to measure the specific heat of a composite fermion
system for the first time. The observed linear temperature dependence on
temperature down to K agrees well with early predictions for a Fermi
liquid of composite fermions.Comment: 4 pages, 4 figures (version is 1. resubmission: Added a paragraph to
include the problems which arise by the weak temperature dependence at \nu =
1/2, updated affiliation
Direct measurement of molecular mobility and crystallisation of amorphous pharmaceuticals using terahertz spectroscopy.
Despite much effort in the area, no comprehensive understanding of the formation and behaviour of amorphous solids has yet been achieved. This severely limits the industrial application of such materials, including drug delivery where, in principle, amorphous solids have demonstrated their great usefulness in increasing the bioavailability of poorly aqueous soluble active pharmaceutical ingredients. Terahertz time-domain spectroscopy is a relatively novel analytical technique that can be used to measure the fast molecular dynamics of molecules with high accuracy in a non-contact and non-destructive fashion. Over the past decade a number of applications for the characterisation of amorphous drug molecules and formulations have been developed and it has been demonstrated how this technique can be used to determine the onset and strength in molecular mobility that underpins the crystallisation of amorphous drugs. In this review we provide an overview of the history, fundamentals and future perspective of pharmaceutical applications related to the terahertz dynamics of amorphous systems.We would like to acknowledge the U.K. Engineering and Physical Sciences Research Council (EP/J007803/1) as well as MedImmune for funding.This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.addr.2015.12.02
Transport Gap in Suspended Bilayer Graphene at Zero Magnetic Field
We report a change of three orders of magnitudes in the resistance of a
suspended bilayer graphene flake which varies from a few ks in the high
carrier density regime to several Ms around the charge neutrality point
(CNP). The corresponding transport gap is 8 meV at 0.3 K. The sequence of
appearing quantum Hall plateaus at filling factor followed by
suggests that the observed gap is caused by the symmetry breaking of the lowest
Landau level. Investigation of the gap in a tilted magnetic field indicates
that the resistance at the CNP shows a weak linear decrease for increasing
total magnetic field. Those observations are in agreement with a spontaneous
valley splitting at zero magnetic field followed by splitting of the spins
originating from different valleys with increasing magnetic field. Both, the
transport gap and field response point toward spin polarized layer
antiferromagnetic state as a ground state in the bilayer graphene sample. The
observed non-trivial dependence of the gap value on the normal component of
suggests possible exchange mechanisms in the system.Comment: 8 pages, 5 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
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