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

Metal-Insulator Transition and Spin Degree of Freedom in Silicon 2D Electron Systems

Magnetotransport in 2DES's formed in Si-MOSFET's and Si/SiGe quantum wells at low temperatures is reported. Metallic temperature dependence of resistivity is observed for the n-Si/SiGe sample even in a parallel magnetic field of 9T, where the spins of electrons are expected to be polarized completely. Correlation between the spin polarization and minima in the diagonal resistivity observed by rotating the samples for various total strength of the magnetic field is also investigated.Comment: 3 pages, RevTeX, 4 eps-figures, conference paper (EP2DS-13

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

Production of belite calcium sulfoaluminate cement using sulfur as a fuel and as a source of clinker sulfur trioxide : pilot kiln trial

The authors gratefully acknowledge the financial support provided by the Gulf Organization for Research and Development (GORD), Qatar, through research grant number ENG016RGG11757. The authors would also like to acknowledge Thomas Matschei and Guanshu Li for the stimulating and fruitful discussions concerning the development of this work. The continuous support prior to, during and after the pilot kiln trial from Vadym Kuznietsov and the entire team at IBU-tec is also greatly appreciated.Peer reviewedPublisher PD

In-plane Magnetoconductivity of Si-MOSFET's: A Quantitative Comparison between Theory and Experiment

For densities above $n=1.6 \times 10^{11}$ cm$^{-2}$ in the strongly interacting system of electrons in two-dimensional silicon inversion layers, excellent agreement between experiment and the theory of Zala, Narozhny and Aleiner is obtained for the response of the conductivity to a magnetic field applied parallel to the plane of the electrons. However, the Fermi liquid parameter $F_0^\sigma(n)$ and the valley splitting $\Delta_V(n)$ obtained from fits to the magnetoconductivity, although providing qualitatively correct behavior (including sign), do not yield quantitative agreement with the temperature dependence of the conductivity in zero magnetic field. Our results suggest the existence of additional scattering processes not included in the theory in its present form

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 $n$-type inversion layers in Si MOSFET structures. We find a rather sharp quantum to classical crossover in the transport behavior in the $0 - 5$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

Thermodynamic Signature of a Two-Dimensional Metal-Insulator Transition

We present a study of the compressibility, K, of a two-dimensional hole system which exhibits a metal-insulator phase transition at zero magnetic field. It has been observed that dK/dp changes sign at the critical density for the metal-insulator transition. Measurements also indicate that the insulating phase is incompressible for all values of B. Finally, we show how the phase transition evolves as the magnetic field is varied and construct a phase diagram in the density-magnetic field plane for this system.Comment: 4 pages, 4 figures, submitted to Physical Review Letters; version 1 is identical to version 2 but didn't compile properl

Spin polarization of strongly interacting 2D electrons: the role of disorder

In high-mobility silicon MOSFET's, the $g^*m^*$ inferred indirectly from magnetoconductance and magnetoresistance measurements with the assumption that $g^*\mu_BH_s=2E_F$ are in surprisingly good agreement with $g^*m^*$ obtained by direct measurement of Shubnikov-de Haas oscillations. The enhanced susceptibility $\chi^* \propto (g^*m^*)$ exhibits critical behavior of the form $\chi^* \propto (n - n_0)^{-\alpha}$. We examine the significance of the field scale $H_s$ 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

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

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