Activation gaps for the fractional quantum Hall effect: realistic treatment of transverse thickness

The activation gaps for fractional quantum Hall states at filling fractions $\nu=n/(2n+1)$ are computed for heterojunction, square quantum well, as well as parabolic quantum well geometries, using an interaction potential calculated from a self-consistent electronic structure calculation in the local density approximation. The finite thickness is estimated to make $\sim$30% correction to the gap in the heterojunction geometry for typical parameters, which accounts for roughly half of the discrepancy between the experiment and theoretical gaps computed for a pure two dimensional system. Certain model interactions are also considered. It is found that the activation energies behave qualitatively differently depending on whether the interaction is of longer or shorter range than the Coulomb interaction; there are indications that fractional Hall states close to the Fermi sea are destabilized for the latter.Comment: 32 pages, 13 figure

The key role of smooth impurity potential in formation of hole spectrum for p-Ge/Ge_{1-x}Si_x heterostructures in the quantum Hall regime

We have measured the temperature (0.1 <= T <= 15 K) and magnetic field (0 <= B <= 12 T) dependences of longitudinal and Hall resistivities for the p-Ge_0.93Si_0.07/Ge multilayers with different Ge layer widths 10 <= d_w <= 38 nm and hole densities p_s = (1-5)10^11 cm^-2. Two models for the long-range random impurity potential (the model with randomly distributed charged centers located outside the conducting layer and the model of the system with a spacer) are used for evaluation of the impurity potential fluctuation characteristics: the random potential amplitude, nonlinear screening length in vicinity of integer filling factors nu = 1 and nu = 2 and the background density of state (DOS). The described models are suitable for explanation of the unusually high value of DOS at nu = 1 and nu = 2, in contrast to the short-range impurity potential models. For half-integer filling factors the linear temperature dependence of the effective QHE plateau-to-plateau transition width nu_0(T) is observed in contrast to scaling behavior for systems with short-range disorder. The finite T -> 0 width of QHE transitions may be due to an effective low temperature screening of smooth random potential owing to Coulomb repulsion of electrons.Comment: Accepted for publication in Nanotechnolog

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

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

Anomalous behaviour of the in-plane electrical conductivity of the layered superconductor κ\kappa-(BEDT-TTF)2_2Cu(NCS)2_2

The quasiparticle scattering rates in high-quality crystals of the quasi-two-dimensional superconductor $\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$ ~are studied using the Shubnikov-de Haas effect and MHz penetration-depth experiments. There is strong evidence that the broadening of the Landau-levels is primarily caused by spatial inhomogeneities, indicating a quasiparticle lifetime for the Landau states $\gg 3$ ps. In contrast to the predictions of Fermi-liquid theory, the scattering time derived from the intralayer conductivity is found to be much shorter ($0.14-0.56$ ps)

Effective Mass of the Four Flux Composite Fermion at ν=1/4\nu = 1/4

We have measured the effective mass ($m^*$) of the four flux composite fermion at Landau level filling factor $\nu = 1/4$ ($^4$CF), using the activation energy gaps at the fractional quantum Hall effect (FQHE) states $\nu$ = 2/7, 3/11, and 4/15 and the temperature dependence of the Shubnikov-de Haas (SdH) oscillations around $\nu = 1/4$. We find that the energy gaps show a linear dependence on the effective magnetic field $B_{eff}$ ($\equiv B-B_{\nu=1/4}$), and from this linear dependence we obtain $m^* = 1.0 m_e$ and a disorder broadening $\Gamma \sim$ 1 K for a sample of density $n = 0.87 \times 10^{11}$ /cm$^2$. The $m^*$ deduced from the temperature dependence of the SdH effect shows large differences for $\nu > 1/4$ and $\nu < 1/4$. For $\nu > 1/4$, $m^* \sim 1.0 m_e$. It scales as $\sqrt{B_{\nu}}$ with the mass derived from the data around $\nu =1/2$ and shows an increase in $m^*$ as $\nu \to 1/4$, resembling the findings around $\nu =1/2$. For $\nu < 1/4$, $m^*$ increases rapidly with increasing $B_{eff}$ and can be described by $m^*/m_e = -3.3 + 5.7 \times B_{eff}$. This anomalous dependence on $B_{eff}$ is precursory to the formation of the insulating phase at still lower filling.Comment: 5 pages, 3 figure

Vertical Confinement and Evolution of Reentrant Insulating Transition in the Fractional Quantum Hall Regime

We have observed an anomalous shift of the high field reentrant insulating phases in a two-dimensional electron system (2DES) tightly confined within a narrow GaAs/AlGaAs quantum well. Instead of the well-known transitions into the high field insulating states centered around $\nu = 1/5$, the 2DES confined within an 80\AA-wide quantum well exhibits the transition at $\nu = 1/3$. Comparably large quantum lifetime of the 2DES in narrow well discounts the effect of disorder and points to confinement as the primary driving force behind the evolution of the reentrant transition.Comment: 5 pages, 4 figure

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

Two-Dimensional Wigner Crystal in Anisotropic Semiconductor

We investigate the effect of mass anisotropy on the Wigner crystallization transition in a two-dimensional (2D) electron gas. The static and dynamical properties of a 2D Wigner crystal have been calculated for arbitrary 2D Bravais lattices in the presence of anisotropic mass, as may be obtainable in Si MOSFETs with (110) surface. By studying the stability of all possible lattices, we find significant change in the crystal structure and melting density of the electron lattice with the lowest ground state energy.Comment: 4 pages, revtex, 4 figure

Experimental study of weak antilocalization effect in a high mobility InGaAs/InP quantum well

The magnetoresistance associated with quantum interference corrections in a high mobility, gated InGaAs/InP quantum well structure is studied as a function of temperature, gate voltage, and angle of the tilted magnetic field. Particular attention is paid to the experimental extraction of phase-breaking and spin-orbit scattering times when weak anti- localization effects are prominent. Compared with metals and low mobility semiconductors the characteristic magnetic field $B_{tr} = \hbar/4eD \tau$ in high mobility samples is very small and the experimental dependencies of the interference effects extend to fields several hundreds of times larger. Fitting experimental results under these conditions therefore requires theories valid for arbitrary magnetic field. It was found, however, that such a theory was unable to fit the experimental data without introducing an extra, empirical, scale factor of about 2. Measurements in tilted magnetic fields and as a function of temperature established that both the weak localization and the weak anti-localization effects have the same, orbital origin. Fits to the data confirmed that the width of the low field feature, whether a weak localization or a weak anti-localization peak, is determined by the phase-breaking time and also established that the universal (negative) magnetoresistance observed in the high field limit is associated with a temperature independent spin-orbit scattering time.Comment: 13 pages including 10 figure