246 research outputs found
Comment on "Theory of metal-insulator transitions in gated semiconductors" (B. L. Altshuler and D. L. Maslov, Phys. Rev. Lett. 82, 145 (1999))
In a recent Letter, Altshuler and Maslov propose a model which attributes the
anomalous temperature and field dependence of the resistivity of
two-dimensional electron (or hole) systems to the charging and discharging of
traps in the oxide (spacer), rather than to intrinsic behavior of interacting
particles associated with a conductor-insulator transition in two dimensions.
We argue against this model based on existing experimental evidence.Comment: 1 page; submitted to PR
Cell Adhesion to Crystal Surfaces: A Model for Initial Stages in the Attachment of Cells to Solid Substrates
This study addresses the mechanism of the chirally-restricted, ROD-independent adhesion of A6 epithelial cells to the {011} faces of calcium {R,R)-tartrate tetrahydrate crystals. The extensive and rapid adhesion of the cells to these surfaces, in the presence or absence of serum proteins, is distinctly different from the extracellular matrix-mediated adhesion to conventional tissue culture surfaces or to the {101} faces of the same crystals. The differences are manifested by insensitivity to ATP depletion, to disruption of microfilaments and microtubules and even to formaldehyde fixation of the cells. Furthermore, trypsin pretreatment does not affect cell attachment to the {011} faces, nor does trypsin post-treatment cause cell detachment from the crystals. We also noticed that the rapid adhesion to the crystal surface bears several lines of similarity to the early temporal stages in cell adhesion to regular tissue culture surfaces. Based on these observations and additional theoretical considerations, it is proposed that ·the molecular interactions responsible for the cell adhesion to the {011} surfaces may serve as models for an early engagement stage in cell adhesion which precedes, and may be essential for, the formation of stable and long-term contacts
Deconstruction of the Trap Model for the New Conducting State in 2D
A key prediction of the trap model for the new conducting state in 2D is that
the resistivity turns upwards below some characteristic temperature, . Altshuler, Maslov, and Pudalov have argued that the reason why no upturn
has been observed for the low density conducting samples is that the
temperature was not low enough in the experiments. We show here that within the Altshuler, Maslov, and Pudalov trap model actually increases
with decreasing density, contrary to their claim. Consequently, the trap model
is not consistent with the experimental trends.Comment: Published version of Deconstructio
Novel Properties of The Apparent Metal-Insulator Transition in Two-Dimensional Systems
The low-temperature conductivity of low-density, high-mobility,
two-dimensional hole systems in GaAs was studied. We explicitly show that the
metal-insulator transition, observed in these systems, is characterized by a
well-defined critical density, p_0c. We also observe that the low-temperature
conductivity of these systems depends linearly on the hole density, over a wide
density range. The high-density linear conductivity extrapolates to zero at a
density close to the critical density.Comment: 4 Figure
On the Theory of Metal-Insulator Transitions in Gated Semiconductors
It is shown that recent experiments indicating a metal-insulator transition
in 2D electron systems can be interpreted in terms of a simple model, in which
the resistivity is controlled by scattering at charged hole traps located in
the oxide layer. The gate voltage changes the number of charged traps which
results in a sharp change in the resistivity. The observed exponential
temperature dependence of the resistivity in the metallic phase of the
transition follows from the temperature dependence of the trap occupation
number. The model naturally describes the experimentally observed scaling
properties of the transition and effects of magnetic and electric fields.Comment: 4 two-column pages, 4 figures (included in the text
Observation of the Metal-Insulator Transition in Two-Dimensional n-type GaAs
The observation of a carrier-density driven metal-insulator transition in
n-type GaAs-based heterostructure is reported. Although weaker than in
comparable-quality p-type GaAs samples, the main features of the transition are
rather similar.Comment: 3 pages, 3 figure
Alite calcium sulfoaluminate cement: chemistry and thermodynamics
Calcium sulfoaluminate (CA) cements can combine the favourable characteristics of Portland cement (PC) with those of CA clinkers. The first is a thermodynamic study demonstrating that the production of a-CA clinker can be readily produced in a standard process by controlling the oxygen and sulfur dioxide fugacity in the atmosphere. This allows for the stabilisation of ye’elimite to the higher temperatures required for alite stability. The second result establishes that when using fluorine to mineralise a-C$A clinker production, the iron content in the clinker is also an important variable. Although the exact mechanism of alite stabilisation is not known, it is shown that alite formation increases with the combination of calcium fluoride and iron (III) oxide in the mix
Charged impurity scattering limited low temperature resistivity of low density silicon inversion layers
We calculate within the Boltzmann equation approach the charged impurity
scattering limited low temperature electronic resistivity of low density
-type inversion layers in Si MOSFET structures. We find a rather sharp
quantum to classical crossover in the transport behavior in the K
temperature range, with the low density, low temperature mobility showing a
strikingly strong non-monotonic temperature dependence, which may qualitatively
explain the recently observed anomalously strong temperature dependent
resistivity in low-density, high-mobility MOSFETs.Comment: 5 pages, 2 figures, will appear in PRL (12 July, 1999
Parallel Magnetic Field Induced Transition in Transport in the Dilute Two-Dimensional Hole System in GaAs
A magnetic field applied parallel to the two-dimensional hole system in the
GaAs/AlGaAs heterostructure, which is metallic in the absence of an external
magnetic field, can drive the system into insulating at a finite field through
a well defined transition. The value of resistivity at the transition is found
to depend strongly on density
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