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S. V. Kravchenko

Publications and source records attributed to S. V. Kravchenko.

At least 73 records · Page 4Linked to original sources
arXiv1999

Response to Parallel Magnetic Field of a Dilute 2D Electron System across the Metal-Insulator Transition

The response to a parallel magnetic field of the very dilute insulating two-dimensional system of electrons in silicon MOSFET's is dramatic and similar to that found on the conducting side of the metal-insulator transition: there is a large initial increase in resistivity with increasing field, followed by saturation to a value that is approximately constant above a characteristic magnetic field of about one Tesla. This is unexpected behavior in an insulator that exhibits Efros-Shklovskii variable-range hopping in zero field, and appears to be a general feature of very dilute electron systems.

cond-mat.str-el↗
arXiv1999

Novel Phenomena in Dilute Electron Systems in Two Dimensions

We review recent experiments that provide evidence for a transition to a conducting phase in two dimensions at very low electron densities. The nature of this phase is not understood, and is currently the focus of intense theoretical and experimental attention.

cond-mat.str-el↗
arXiv1999

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.

cond-mat.str-el↗
arXiv1998

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.

cond-mat.str-el↗
arXiv1998

Comment on "Charged impurity scattering limited low temperature resistivity of low density silicon inversion layers" (Das Sarma and Hwang, cond-mat/9812216)

In a recent preprint cond-mat/9812216, Das Sarma and Hwang propose an explanation of the sharp decrease in resistivity at low temperatures which has been attributed to a transition to an unexpected conducting phase in dilute high-mobility two-dimensional systems at B=0. In this Comment, we examine whether their model is supported by the available experimental data.

cond-mat.str-el↗
arXiv1998

Hopping Conduction in Uniaxially Stressed Si:B near the Insulator-Metal Transition

Using uniaxial stress to tune the critical density near that of the sample, we have studied in detail the low-temperature conductivity of p-type Si:B in the insulating phase very near the metal-insulator transition. For all values of temperature and stress, the conductivity collapses onto a single universal scaling curve. For large values of the argument, the scaling function is well fit by the exponentially activated form associated with variable range hopping when electron-electron interactions cause a soft Coulomb gap in the density of states at the Fermi energy. The temperature dependence of the prefactor, corresponding to the T-dependence of the critical curve, has been determined reliably for this system, and is proportional to the square-root of T. We show explicitly that nevlecting the prefactor leads to substantial errors in the determination of the scaling parameters and the critical exponents derived from them. The conductivity is not consistent with Mott variable-range hopping in the critical region nor does it obey this form for any range of the parameters. Instead, for smaller argument of the scaling function, the conductivity of Si:B is well fit by an exponential form with exponent 0.31 related to the critical exponents of the system at the metal- insulator transition.

cond-mat.str-el↗
arXiv1998

H/T Scaling of the Magnetoconductance in Two Dimensions near the Conductor-Insulator Transition

For an electron density near the H=0 insulator-to-conductor transition, the magnetoconductivity of the low-temperature conducting phase in high-mobility silicon MOSFETs is consistent with the form $Δσ(H_{||},T)\equivσ(H_{||},T)-σ(0,T) = f(H_{||}/T)$ for magnetic fields $H_{||}$ applied parallel to the plane of the electron system. This sets a valuable constraint on theory and provides further evidence that the electron spin is central to the anomalous H=0 conducting phase in two dimensions.

cond-mat.dis-nn↗
arXiv1997

Effect of Tilted Magnetic Field on the Anomalous H=0 Conducting Phase in High-Mobility Si MOSFETs

The suppression by a magnetic field of the anomalous H=0 conducting phase in high-mobility silicon MOSFETs is independent of the angle between the field and the plane of the 2D electron system. In the presence of a parallel field large enough to fully quench the anomalous conducting phase, the behavior is similar to that of disordered GaAs/AlGaAs heterostructures: the system is insulating in zero (perpendicular) field and exhibits reentrant insulator-quantum Hall effect-insulator transitions as a function of perpendicular field. The results demonstrate that the suppression of the low-T phase is related only to the electrons' spin.

cond-mat.str-el↗
arXiv1997

The metal-insulator transition in Si:X: Anomalous response to a magnetic field

The zero-temperature magnetoconductivity of just-metallic Si:P scales with magnetic field, H, and dopant concentration, n, lying on a single universal curve. We note that Si:P, Si:B, and Si:As all have unusually large magnetic field crossover exponents near 2, and suggest that this anomalously weak response to a magnetic field is a common feature of uncompensated doped semiconductors.

cond-mat.str-el↗
arXiv1997

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.

cond-mat.str-el↗
arXiv1996

Reflection symmetry at a B=0 metal-insulator transition in two dimensions

We report a remarkable symmetry between the resistivity and conductivity on opposite sides of the B=0 metal-insulator transition in a two-dimensional electron gas in high-mobility silicon MOSFET's. This symmetry implies that the transport mechanisms on the two sides are related.

cond-mat.mes-hall↗
arXiv1996

Electric field scaling at a B=0 metal-insulator transition in two dimensions

The non-linear (electric field-dependent) resistivity of the 2D electron system in silicon exhibits scaling as a function of electric field and electron density in both the metallic and insulating phases, providing further evidence for a true metal-insulator transition in this 2D system at B=0. Comparison with the temperature scaling yields separate determinations of the correlation length exponent, ν=1.5, and the dynamical exponent, z=0.8, close to the theoretical value z=1.

cond-mat↗
arXiv1996

Comment on ``Observation of the $ν=1$ quantum Hall effect in a strongly localized two-dimensional system''

In a recent Rapid Communication ``Observation of the $ν=1$ quantum Hall effect in a strongly localized two-dimensional system,'' Shahar, Tsui, and Cunningham reported a disorder --- magnetic field phase diagram for the integer quantum Hall effect that appeared to differ qualitatively from theoretical predictions as well as from experimental results obtained by others. In this Comment, I suggest a possible origin of this difference.

cond-mat↗
arXiv1995

Global phase diagram for the quantum Hall effect: An experimental picture

We study the behavior of the extended states of a two-dimensional electron system in silicon in a magnetic field, B. Our results show that the extended states, corresponding to the centers of different Landau levels, merge with the lowest extended state as B --> 0. Using our data, we construct an experimental-based ``disorder vs filling factor'' phase diagram for the integer quantum Hall effect (QHE). Generalizing this diagram to the case of the fractional QHE, we show that it is consistent with the recently observed direct transitions between insulator and FQHE at 2/5, 2/7, and 2/9.

cond-mat↗
arXiv1995

Experimental Evidence for a Coulomb Gap in Two Dimensions

We have studied the resistivity, $ρ$, of a two-dimensional electron system in silicon in the temperature range 200 mK < T < 7.5 K at zero magnetic field at low electron densities, when the electron system is in the insulating regime. Our results show that at an intermediate temperature range $ρ=ρ_0 exp[(T_0/T)^{1/2}]$ for at least four orders of magnitude up to 3x10^9 Ohms. This behavior is consistent with the existence of a Coulomb gap. Near the metal/insulator transition, the prefactor was found to be close to $h/e^2$, and resistivity scales with temperature. For very low electron densities, $n_s$, the prefactor diminishes with diminishing $n_s$. A comparison with the theory shows that a specific set of conditions are necessary to observe the behavior of resistivity consistent with the existence of the Coulomb gap.

cond-mat↗
arXiv1995

DESTRUCTION OF THE QUANTUM HALL EFFECT WITH INCREASING DISORDER

We report experimental studies of disorder-induced transitions between quantum-Hall, metallic, and insulating states in a very dilute two-dimensional electron system in silicon at a magnetic field corresponding to Landau level filling factor $ν=1$. At low disorder, the lowest extended state at $ν=1$ is below the Fermi energy so that the system is in the quantum Hall state. Our data show that with increasing disorder (but at constant electron density and magnetic field), the extended state DOES NOT DISAPPEAR BUT FLOATS UP IN ENERGY SO THAT THE SYSTEM BECOMES INSULATING. As the extended state crosses the Fermi energy, the conductivity $σ_{xx}\sim e^2/2h$ has temperature dependence characteristic of a metallic system.

cond-mat↗
arXiv1994

"Scaling of an anomalous metal/insulator transition in a 2D system in silicon at zero magnetic field"

We have studied the temperature dependence of resistivity, $ρ$, for a two-dimensional electron system in silicon at low electron densities, $n_s\sim10^{11}$ cm$^{-2}$, near the metal/insulator transition. The resistivity was empirically found to scale with a single parameter, $T_0$, which approaches zero at some critical electron density, $n_c$, and increases as a power $T_0\propto|n_s-n_c|^β$ with $β=1.6\pm0.1$ both in metallic ($n_s>n_c$) and insulating ($n_s<n_c$) regions. This dependence was found to be sample-independent. We have also studied the diagonal resistivity at Landau level filling factor $ν=3/2$ where the system is known to be in a metallic state at high magnetic field and in an insulating state at low magnetic field. The temperature dependencies of resistivity at $B=0$ and at $ν=3/2$ were found to be identical. These behaviors suggest a true metal/insulator transition in the two dimensional electron system in silicon at $B=0$, in contrast with the well-known scaling theory.

cond-mat↗