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

A hydrogenic molecular atmosphere of a neutron star

A model of a hydrogenic content of atmosphere of the isolated neutron star 1E1207.4-5209 is proposed. It is based on the assumption that the main component in the atmosphere is the exotic molecular ion $H_3^{2+}$ and that there exists a magnetic field in the range of $(4 \pm 2) \times 10^{14}$ G. Photoionization H_3^{2+} \rar e + 3p and photodissociation H_3^{2+} \rar H + 2p correspond to two absorption features at 0.7 KeV and 1.4 KeV, respectively, discovered by {\it Chandra} observatory (Sanwal et al, 2002). The model predicts one more absorption feature at 80-150 eV corresponding to photodissociation H_3^{2+} \rar H_2^+ + p.Comment: 8 pages, 1 figur

Universal Behaviour of Metal-Insulator Transitions in the p-SiGe System

Magnetoresistance measurements are presented for a strained p-SiGe quantum well sample where the density is varied through the B=0 metal-insulator transition. The close relationship between this transition, the high field Hall insulator transition and the filling factor $\nu$=3/2 insulating state is demonstrated.Comment: 6 pages, 4 figures. Submitted to EP2DS XIII conference 199

Superconductivity in correlated disordered two-dimensional electron gas

We calculate the dynamic effective electron-electron interaction potential for a low density disordered two-dimensional electron gas. The disordered response function is used to calculate the effective potential where the scattering rate is taken from typical mobilities from recent experiments. We investigate the development of an effective attractive pair potential for both disordered and disorder free systems with correlations determined from existing numerical simulation data. The effect of disorder and correlations on the superconducting critical temperature Tc is discussed.Comment: 4 pages, RevTeX + epsf, 4 figure

Classical versus Quantum Effects in the B=0 Conducting Phase in Two Dimensions

In the dilute two-dimensional electron system in silicon, we show that the temperature below which Shubnikov-de Haas oscillations become apparent is approximately the same as the temperature below which an exponential decrease in resistance is seen in B=0, suggesting that the anomalous behavior in zero field is observed only when the system is in a degenerate (quantum) state. The temperature dependence of the resistance is found to be qualitatively similar in B=0 and at integer Landau level filling factors.Comment: 3 pages, 3 figure

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

Magnetic Field Suppression of the Conducting Phase in Two Dimensions

The anomalous conducting phase that has been shown to exist in zero field in dilute two-dimensional electron systems in silicon MOSFETs is driven into a strongly insulating state by a magnetic field of about 20 kOe applied parallel to the plane. The data suggest that in the limit of T -> 0 the conducting phase is suppressed by an arbitrarily weak magnetic field. We call attention to striking similarities to magnetic field-induced superconductor-insulator transitions

On a complex differential Riccati equation

We consider a nonlinear partial differential equation for complex-valued functions which is related to the two-dimensional stationary Schrodinger equation and enjoys many properties similar to those of the ordinary differential Riccati equation as, e.g., the famous Euler theorems, the Picard theorem and others. Besides these generalizations of the classical "one-dimensional" results we discuss new features of the considered equation like, e.g., an analogue of the Cauchy integral theorem

Magnetic-Field-Driven Superconductor-Insulator-Type Transition in Graphite

A magnetic-field-driven transition from metallic- to semiconducting-type behavior in the basal-plane resistance takes place in highly oriented pyrolytic graphite at a field $H_c \sim 1~$kOe applied along the hexagonal c-axis. The analysis of the data reveals a striking similarity between this transition and that measured in thin-film superconductors and Si MOSFET's. However, in contrast to those materials, the transition in graphite is observable at almost two orders of magnitude higher temperatures.Comment: 4 Figure

Universal scaling, beta function, and metal-insulator transitions

We demonstrate a universal scaling form of longitudinal resistance in the quantum critical region of metal-insulator transitions, based on numerical results of three-dimensional Anderson transitions (with and without magnetic field), two-dimensional quantum Hall plateau to insulator transition, as well as experimental data of the recently discovered two-dimensional metal-insulator transition. The associated reflection symmetry and a peculiar logarithmic form of the beta function exist over a wide range in which the resistance can change by more than one order of magnitude. Interesting implications for the two-dimensional metal-insulator transition are discussed.Comment: 4 pages, REVTEX, 4 embedded figures; minor corrections to figures and tex