198 research outputs found
Flow diagram of the metal-insulator transition in two dimensions
The discovery of the metal-insulator transition (MIT) in two-dimensional (2D)
electron systems challenged the veracity of one of the most influential
conjectures in the physics of disordered electrons, which states that `in two
dimensions, there is no true metallic behaviour'; no matter how weak the
disorder, electrons would be trapped and unable to conduct a current. However,
that theory did not account for interactions between the electrons. Here we
investigate the interplay between the electron-electron interactions and
disorder near the MIT using simultaneous measurements of electrical resistivity
and magnetoconductance. We show that both the resistance and interaction
amplitude exhibit a fan-like spread as the MIT is crossed. From these data we
construct a resistance-interaction flow diagram of the MIT that clearly reveals
a quantum critical point, as predicted by the two-parameter scaling theory
(Punnoose and Finkel'stein, Science 310, 289 (2005)). The metallic side of this
diagram is accurately described by the renormalization group theory without any
fitting parameters. In particular, the metallic temperature dependence of the
resistance sets in when the interaction amplitude reaches gamma_2 = 0.45 - a
value in remarkable agreement with the one predicted by the theory.Comment: as publishe
Mobility-Dependence of the Critical Density in Two-Dimensional Systems: An Empirical Relation
For five different electron and hole systems in two dimensions (Si MOSFET's,
p-GaAs, p-SiGe, n-GaAs and n-AlAs), the critical density, that marks the
onset of strong localization is shown to be a single power-law function of the
scattering rate deduced from the maximum mobility. The resulting curve
defines the boundary separating a localized phase from a phase that exhibits
metallic behavior. The critical density in the limit of infinite
mobility.Comment: 2 pages, 1 figur
Large Bychkov-Rashba spin-orbit coupling in high-mobility GaN/AlGaN heterostructures
We present low temperature magnetoconductivity measurements of a
density-tunable and high mobility two-dimensional electron gas confined in the
wide bandgap GaN/AlGaN system. We observed pronounced anti-localization minima
in the low-field conductivity, indicating the presence of strong spin-orbit
coupling. Density dependent measurements of magnetoconductivity indicate that
the coupling is mainly due to the Bychkov-Rashba mechanism. In addition, we
have derived a closed-form expression for the magnetoconductivity, allowing us
to extract reliable transport parameters for our devices. The Rashba spin-orbit
coupling constant is 6 10eVm, while the
conduction band spin-orbit splitting energy amounts to
0.3meV at n=1m.Comment: Accepted for publication in PR
Energy-resolved spatial inhomogeneity of disordered Mott systems
We investigate the effects of weak to moderate disorder on the T=0 Mott
metal-insulator transition in two dimensions. Our model calculations
demonstrate that the electronic states close to the Fermi energy become more
spatially homogeneous in the critical region. Remarkably, the higher energy
states show the opposite behavior: they display enhanced spatial inhomogeneity
precisely in the close vicinity to the Mott transition. We suggest that such
energy-resolved disorder screening is a generic property of disordered Mott
systems.Comment: 6 pages, 6 figures. Submitted to the Proceedings of the SCES 200
Typical-Medium Theory of Mott-Anderson Localization
The Mott and the Anderson routes to localization have long been recognized as
the two basic processes that can drive the metal-insulator transition (MIT).
Theories separately describing each of these mechanisms were discussed long
ago, but an accepted approach that can include both has remained elusive. The
lack of any obvious static symmetry distinguishing the metal from the insulator
poses another fundamental problem, since an appropriate static order parameter
cannot be easily found. More recent work, however, has revisited the original
arguments of Anderson and Mott, which stressed that the key diference between
the metal end the insulator lies in the dynamics of the electron. This physical
picture has suggested that the "typical" (geometrically averaged) escape rate
from a given lattice site should be regarded as the proper dynamical order
parameter for the MIT, one that can naturally describe both the Anderson and
the Mott mechanism for localization. This article provides an overview of the
recent results obtained from the corresponding Typical-Medium Theory, which
provided new insight into the the two-fluid character of the Mott-Anderson
transition.Comment: to be published in "Fifty Years of Anderson localization", edited by
E. Abrahams (World Scientific, Singapore, 2010); 29 pages, 22 figures
Interaction effects on magnetooscillations in a two-dimensional electron gas
Motivated by recent experiments, we study the interaction corrections to the
damping of magnetooscillations in a two-dimensional electron gas (2DEG). We
identify leading contributions to the interaction-induced damping which are
induced by corrections to the effective mass and quantum scattering time. The
damping factor is calculated for Coulomb and short-range interaction in the
whole range of temperatures, from the ballistic to the diffusive regime. It is
shown that the dominant effect is that of the renormalization of the effective
electron mass due to the interplay of the interaction and impurity scattering.
The results are relevant to the analysis of experiments on magnetooscillations
(in particular, for extracting the value of the effective mass) and are
expected to be useful for understanding the physics of a high-mobility 2DEG
near the apparent metal-insulator transition.Comment: 24 pages; subsection adde
Absence of backscattering at integrable impurities in one-dimensional quantum many-body systems
We study interacting one dimensional (1D) quantum lattice gases with
integrable impurities. These model Hamiltonians can be derived using the
quantum inverse scattering method for inhomogeneous models and are by
construction integrable. Absence of backscattering at the impurities is shown
to be the characteristic feature of these disordered systems. The value of the
effective carrier charge and the Sutherland-Shastry relation are derived for
the half-filled XXX model and are shown to be independent of the impurity
concentration and strength. For the half-filled XXZ model we show that there is
no enhancement of the persistent currents for repulsive interactions. For
attractive interactions we identify a crossover regime beyond which enhancement
of the currents is observed.Comment: 14 RevTeX 3.0 pages with 1 PS-figure include
ZnO Nanoparticles Modulate the Ionic Transport and Voltage Regulation of Lysenin Nanochannels
Background: The insufficient understanding of unintended biological impacts from nanomaterials (NMs) represents a serious impediment to their use for scientific, technological, and medical applications. While previous studies have focused on understanding nanotoxicity effects mostly resulting from cellular internalization, recent work indicates that NMs may interfere with transmembrane transport mechanisms, hence enabling contributions to nanotoxicity by affecting key biological activities dependent on transmembrane transport. In this line of inquiry, we investigated the effects of charged nanoparticles (NPs) on the transport properties of lysenin, a pore-forming toxin that shares fundamental features with ion channels such as regulation and high transport rate.
Results: The macroscopic conductance of lysenin channels greatly diminished in the presence of cationic ZnO NPs. The inhibitory effects were asymmetrical relative to the direction of the electric field and addition site, suggesting electrostatic interactions between ZnO NPs and a binding site. Similar changes in the macroscopic conductance were observed when lysenin channels were reconstituted in neutral lipid membranes, implicating protein-NP interactions as the major contributor to the reduced transport capabilities. In contrast, no inhibitory effects were observed in the presence of anionic SnO2 NPs. Additionally, we demonstrate that inhibition of ion transport is not due to the dissolution of ZnO NPs and subsequent interactions of zinc ions with lysenin channels.
Conclusion: We conclude that electrostatic interactions between positively charged ZnO NPs and negative charges within the lysenin channels are responsible for the inhibitory effects on the transport of ions. These interactions point to a potential mechanism of cytotoxicity, which may not require NP internalization
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
- …