308 research outputs found
Quasiclassical negative magnetoresistance of a 2D electron gas: interplay of strong scatterers and smooth disorder
We study the quasiclassical magnetotransport of non-interacting fermions in
two dimensions moving in a random array of strong scatterers (antidots,
impurities or defects) on the background of a smooth random potential. We
demonstrate that the combination of the two types of disorder induces a novel
mechanism leading to a strong negative magnetoresistance, followed by the
saturation of the magnetoresistivity at a value determined
solely by the smooth disorder. Experimental relevance to the transport in
semiconductor heterostructures is discussed.Comment: 4 pages, 2 figure
Two-Component Scaling near the Metal-Insulator Bifurcation in Two-Dimensions
We consider a two-component scaling picture for the resistivity of
two-dimensional (2D) weakly disordered interacting electron systems at low
temperature with the aim of describing both the vicinity of the bifurcation and
the low resistance metallic regime in the same framework. We contrast the
essential features of one-component and two-component scaling theories. We
discuss why the conventional lowest order renormalization group equations do
not show a bifurcation in 2D, and a semi-empirical extension is proposed which
does lead to bifurcation. Parameters, including the product , are
determined by least squares fitting to experimental data. An excellent
description is obtained for the temperature and density dependence of the
resistance of silicon close to the separatrix. Implications of this
two-component scaling picture for a quantum critical point are discussed.Comment: 7 pages, 1 figur
Dephasing time of composite fermions
We study the dephasing of fermions interacting with a fluctuating transverse
gauge field. The divergence of the imaginary part of the fermion self energy at
finite temperatures is shown to result from a breakdown of Fermi's golden rule
due to a faster than exponential decay in time. The strong dephasing affects
experiments where phase coherence is probed. This result is used to describe
the suppression of Shubnikov-de Haas (SdH) oscillations of composite fermions
(oscillations in the conductivity near the half-filled Landau level). We find
that it is important to take into account both the effect of dephasing and the
mass renormalization. We conclude that while it is possible to use the
conventional theory to extract an effective mass from the temperature
dependence of the SdH oscillations, the resulting effective mass differs from
the of the quasiparticle in Fermi liquid theory.Comment: 14 pages, RevTeX 3.0, epsf, 1 EPS figur
Effective Mass of the Four Flux Composite Fermion at
We have measured the effective mass () of the four flux composite
fermion at Landau level filling factor (CF), using the
activation energy gaps at the fractional quantum Hall effect (FQHE) states
= 2/7, 3/11, and 4/15 and the temperature dependence of the Shubnikov-de
Haas (SdH) oscillations around . We find that the energy gaps show a
linear dependence on the effective magnetic field (), and from this linear dependence we obtain and
a disorder broadening 1 K for a sample of density /cm. The deduced from the temperature dependence of
the SdH effect shows large differences for and . For
, . It scales as with the mass
derived from the data around and shows an increase in as , resembling the findings around . For ,
increases rapidly with increasing and can be described by . This anomalous dependence on is
precursory to the formation of the insulating phase at still lower filling.Comment: 5 pages, 3 figure
Tunneling Between Parallel Two-Dimensional Electron Gases
The tunneling between two parallel two-dimensional electron gases has been
investigated as a function of temperature , carrier density , and the
applied perpendicular magnetic field . In zero magnetic field the
equilibrium resonant lineshape is Lorentzian, reflecting the Lorentzian form of
the spectral functions within each layer. From the width of the tunneling
resonance the lifetime of the electrons within a 2DEG has been measured as a
function of and , giving information about the density dependence of the
electron-impurity scattering and the temperature dependence of the
electron-electron scattering. In a magnetic field there is a general
suppression of equilibrium tunneling for fields above T. A gap in the
tunneling density of states has been measured over a wide range of magnetic
fields and filling factors, and various theoretical predictions have been
examined. In a strong magnetic field, when there is only one partially filled
Landau level in each layer, the temperature dependence of the conductance
characteristics has been modeled with a double-Gaussian spectral density.Comment: LaTeX requires REVTeX macros. Eighteen pages. Fourteen postscript
figures are included. (All figures have been bitmapped to save space. The
original can be requested by email from [email protected]). Accepted for
publication in Phys. Rev.
Analysis of the Metallic Phase of Two-Dimensional Holes in SiGe in Terms of Temperature Dependent Screening
We find that temperature dependent screening can quantitatively explain the
metallic behaviour of the resistivity on the metallic side of the so-called
metal-insulator transition in p-SiGe. Interference and interaction effects
exhibit the usual insulating behaviour which is expected to overpower the
metallic background at sufficiently low temperatures. We find empirically that
the concept of a Fermi-liquid describes our data in spite of the large r_s = 8.Comment: 4 pages, 3 figure
Spin instabilities and quantum phase transitions in integral and fractional quantum Hall states
The inter-Landau-level spin excitations of quantum Hall states at filling
factors nu=2 and 4/3 are investigated by exact numerical diagonalization for
the situation in which the cyclotron (hbar*omega_c) and Zeeman (E_Z) splittings
are comparable. The relevant quasiparticles and their interactions are studied,
including stable spin wave and skyrmion bound states. For nu=2, a spin
instability at a finite value of epsilon=hbar*omega_c-E_Z leads to an abrupt
paramagnetic to ferromagnetic transition, in agreement with the mean-field
approximation. However, for nu=4/3 a new and unexpected quantum phase
transition is found which involves a gradual change from paramagnetic to
ferromagnetic occupancy of the partially filled Landau level as epsilon is
decreased.Comment: 4 pages, 5 figures, submitted to Phys.Rev.Let
Interaction-Induced Enhancement of Spin-Orbit Coupling in Two-Dimensional Electronic System
We study theoretically the renormalization of the spin-orbit coupling
constant of two-dimensional electrons by electron-electron interactions. We
demonstrate that, similarly to the factor, the renormalization corresponds
to the enhancement, although the magnitude of the enhancement is weaker than
that for the factor. For high electron concentrations (small interaction
parameter ) the enhancement factor is evaluated analytically within the
static random phase approximation. For large we use an approximate
expression for effective electron-electron interaction, which takes into
account the local field factor, and calculate the enhancement numerically. We
also study the interplay between the interaction-enhanced Zeeman splitting and
interaction-enhanced spin-orbit coupling.Comment: 18 pages, 2 figures, REVTe
The relative importance of electron-electron interactions compared to disorder in the two-dimensional "metallic" state
The effect of substrate bias and surface gate voltage on the low temperature
resistivity of a Si-MOSFET is studied for electron concentrations where the
resistivity increases with increasing temperature. This technique offers two
degrees of freedom for controlling the electron concentration and the device
mobility, thereby providing a means to evaluate the relative importance of
electron-electron interactions and disorder in this so-called ``metallic''
regime. For temperatures well below the Fermi temperature, the data obey a
scaling law where the disorder parameter (), and not the
concentration, appears explicitly. This suggests that interactions, although
present, do not alter the Fermi-liquid properties of the system fundamentally.
Furthermore, this experimental observation is reproduced in results of
calculations based on temperature-dependent screening, in the context of
Drude-Boltzmann theory.Comment: 5 pages, 6 figure
- …