144 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
Spin polarization of strongly interacting 2D electrons: the role of disorder
In high-mobility silicon MOSFET's, the inferred indirectly from
magnetoconductance and magnetoresistance measurements with the assumption that
are in surprisingly good agreement with obtained by
direct measurement of Shubnikov-de Haas oscillations. The enhanced
susceptibility exhibits critical behavior of the form
. We examine the significance of the field
scale derived from transport measurements, and show that this field
signals the onset of full spin polarization only in the absence of disorder.
Our results suggest that disorder becomes increasingly important as the
electron density is reduced toward the transition.Comment: 4 pages, 3 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
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 yeāelimite 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
Two-species percolation and Scaling theory of the metal-insulator transition in two dimensions
Recently, a simple non-interacting-electron model, combining local quantum
tunneling via quantum point contacts and global classical percolation, has been
introduced in order to describe the observed ``metal-insulator transition'' in
two dimensions [1]. Here, based upon that model, a two-species-percolation
scaling theory is introduced and compared to the experimental data. The two
species in this model are, on one hand, the ``metallic'' point contacts, whose
critical energy lies below the Fermi energy, and on the other hand, the
insulating quantum point contacts. It is shown that many features of the
experiments, such as the exponential dependence of the resistance on
temperature on the metallic side, the linear dependence of the exponent on
density, the scale of the critical resistance, the quenching of the
metallic phase by a parallel magnetic field and the non-monotonic dependence of
the critical density on a perpendicular magnetic field, can be naturally
explained by the model.
Moreover, details such as the nonmonotonic dependence of the resistance on
temperature or the inflection point of the resistance vs. parallel magnetic are
also a natural consequence of the theory. The calculated parallel field
dependence of the critical density agrees excellently with experiments, and is
used to deduce an experimental value of the confining energy in the vertical
direction. It is also shown that the resistance on the ``metallic'' side can
decrease with decreasing temperature by an arbitrary factor in the degenerate
regime ().Comment: 8 pages, 8 figure
Coexistence of Weak Localization and a Metallic Phase in Si/SiGe Quantum Wells
Magnetoresistivity measurements on p-type Si/SiGe quantum wells reveal the
coexistence of a metallic behavior and weak localization. Deep in the metallic
regime, pronounced weak localization reduces the metallic behavior around zero
magnetic field without destroying it. In the insulating phase, a positive
magnetoresistivity emerges close to B=0, possibly related to spin-orbit
interactions.Comment: 4 pages, 3 figure
Unexpected Behavior of the Local Compressibility Near the B=0 Metal-Insulator Transition
We have measured the local electronic compressibility of a two-dimensional
hole gas as it crosses the B=0 Metal-Insulator Transition. In the metallic
phase, the compressibility follows the mean-field Hartree-Fock (HF) theory and
is found to be spatially homogeneous. In the insulating phase it deviates by
more than an order of magnitude from the HF predictions and is spatially
inhomogeneous. The crossover density between the two types of behavior, agrees
quantitatively with the transport critical density, suggesting that the system
undergoes a thermodynamic change at the transition.Comment: As presented in EP2DS-13, Aug. 1999. (4 pages, 4 figures
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