889 research outputs found
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
Statistical properties of the low-temperature conductance peak-heights for Corbino discs in the quantum Hall regime
A recent theory has provided a possible explanation for the ``non-universal
scaling'' of the low-temperature conductance (and conductivity) peak-heights of
two-dimensional electron systems in the integer and fractional quantum Hall
regimes. This explanation is based on the hypothesis that samples which show
this behavior contain density inhomogeneities. Theory then relates the
non-universal conductance peak-heights to the ``number of alternating
percolation clusters'' of a continuum percolation model defined on the
spatially-varying local carrier density. We discuss the statistical properties
of the number of alternating percolation clusters for Corbino disc samples
characterized by random density fluctuations which have a correlation length
small compared to the sample size. This allows a determination of the
statistical properties of the low-temperature conductance peak-heights of such
samples. We focus on a range of filling fraction at the center of the plateau
transition for which the percolation model may be considered to be critical. We
appeal to conformal invariance of critical percolation and argue that the
properties of interest are directly related to the corresponding quantities
calculated numerically for bond-percolation on a cylinder. Our results allow a
lower bound to be placed on the non-universal conductance peak-heights, and we
compare these results with recent experimental measurements.Comment: 7 pages, 4 postscript figures included. Revtex with epsf.tex and
multicol.sty. The revised version contains some additional discussion of the
theory and slightly improved numerical result
Universal relation between longitudinal and transverse conductivities in quantum Hall effect
We show that any critical transition region between two adjacent Hall
plateaus in either integer or fractional quantum Hall effect is characterized
by a universal semi-circle relationship between the longitudinal and transverse
conductivities, provided the sample is homogeneous and isotropic on a large
scale. This conclusion is demonstrated both for the phase-coherent quantum
transport as well as for the incoherent transport.Comment: REVTEX 3.0, 1 figure, 4 pages. SISSA-08179
Synthese, Struktur und Eigenschaften von multinären Chalkogenidometallaten
Die Arbeit gliedert sich in vier Abschnitte: Der erste befasst sich mit der
Synthese und strukturellen Charakterisierung neuer ortho-Chalkogenidostannatsalze. Im zweiten Teil wird das Reaktionsverhalten dieser Salze gegenüber Hauptgruppenelementkationen, sterisch anspruchsvollen Kationen, verschiedenen Lösungsmitteln untersucht. Der dritte Abschnitt befasst sich mit der Synthese und strukturellen Charakterisierung multinärer Chalkogenidometallate. Im vierten Teil wird die Charakterisierung der neu entstandenen Verbindungen mittels NMR-Spektroskopie und UV-Vis-Spektroskopie beschrieben
Studies of the radiation hardness of oxygen-enriched silicon detectors
Detectors of high-energy particles sustain substantial structural defects induced by the particles during the operation period. Some of the defects have been found to be electrically active, degrading the detector's performance. Understanding the mechanisms of the electrical activities and learning to suppress their influence are essential if long 'lifetime' detectors are required. This work report s about radiation hardness of silicon P-I-N devices fabricated from oxygen-enriched, high-resistivity material. The high and nearly uniform concentration of oxygen in float-zone silicon has been achie ved by diffusion of oxygen from SiO2 layers
Non-Universal Behavior of Finite Quantum Hall Systems as a Result of Weak Macroscopic Inhomogeneities
We show that, at low temperatures, macroscopic inhomogeneities of the
electron density in the interior of a finite sample cause a reduction in the
measured conductivity peak heights compared to the
universal values previously predicted for infinite homogeneous samples. This
effect is expected to occur for the conductivity peaks measured in standard
experimental geometries such as the Hall bar and the Corbino disc. At the
lowest temperatures, the decrease in is found to
saturate at values proportional to the difference between the adjacent plateaus
in , with a prefactor which depends on the particular realization
of disorder in the sample. We argue that this provides a possible explanation
of the ``non-universal scaling'' of observed in a
number of experiments. We also predict an enhancement of the ``non-local''
resistance due to the macroscopic inhomogeneities. We argue that, in the Hall
bar with a sharp edge, the enhanced ``non-local'' resistance and the size
corrections to the ``local'' resistance are directly related. Using
this relation, we suggest a method by which the finite-size corrections may be
eliminated from and in this case.Comment: REVTEX 3.0 file (38 pages) + 5 postscript figures in uuencoded
format. Revised version includes an additional figure showing unpublished
experimental dat
Resistivity peak values at transition between fractional quantum Hall states
Experimental data available in the literature for peak values of the diagonal
resistivity in the transitions between fractional quantum Hall states are
compared with the theoretical predictions. It is found that the majority of the
peak values are close to the theoretical values for two-dimensional systems
with moderate mobilities.Comment: 3 pages, 1 figur
Multidisciplinary Treatment of a Patient with Bilateral Cleft Lip-Palate with Severe Functional Disability
In this case report, multidisciplinary treatment of a 12 years old female patient with bilateral complete cleft lip-palate having disability in mastication and speaking functions is presented. In extraoral examination of the patient, it was observed that she had a convex profile, scar tissue causing difficulty in smiling, extremely strained upper lip, a flat nose tip and did not have columella. In intraoral examination, it was observed that the patient was in mixed dentition stage, showing extreme mobility in the premaxilla, had Angle class II molar relationship, congenital absence of upper lateral teeth and increased overjet-overbite. In functional examination, it was observed that the patient had severe velopharyngeal deficiency originating speaking disorder. Treatment of the patient started by using quad-helix appliance for treatment of the maxillary constriction and continued with fixed appliance treatment after eruption of permanent teeth. In this phase, the patient was directed to a speech therapist. At the end of the orthodontic treatment of 32 months in total, the maxilla was expanded, maxillary and mandibular arches were leveled, mastication function was gained by providing a proper occlusion and an aesthetic smile was achieved. With the help of the speech therapist, the speech of the patient improved and at the end of the rhinoplasty operation applied 1 year after the end of the treatment, a substantial contribution was provided in the facial view of the patient
The effect of carrier density gradients on magnetotransport data measured in Hall bar geometry
We have measured magnetotransport of the two-dimensional electron gas in a
Hall bar geometry in the presence of small carrier density gradients. We find
that the longitudinal resistances measured at both sides of the Hall bar
interchange by reversing the polarity of the magnetic field. We offer a simple
explanation for this effect and discuss implications for extracting
conductivity flow diagrams of the integer quantum Hall effect.Comment: 7 pages, 8 figure
Comparison of radiation damage in silicon induced by proton and neutron irradiation
The subject of radiation damage to Si detectors induced by 24-GeV/c protons and nuclear reactor neutrons has been studied. Detectors fabricated on single-crystal silicon enriched with various impurities have been tested. Significant differences in electrically active defects have been found between the various types of material. The results of the study suggest for the first time that the widely used nonionizing energy loss (NIEL) factors are insufficient for normalization of the electrically active damage in case of oxygen- and carbon-enriched silicon detectors. It has been found that a deliberate introduction of impurities into the semiconductor can affect the radiation hardness of silicon detectors. (16 refs)
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