295 research outputs found
On Steering Swarms
The main contribution of this paper is a novel method allowing an external
observer/controller to steer and guide swarms of identical and
indistinguishable agents, in spite of the agents' lack of information on
absolute location and orientation. Importantly, this is done via simple global
broadcast signals, based on the observed average swarm location, with no need
to send control signals to any specific agent in the swarm
A few electrons per ion scenario for the B=0 metal-insulator transition in two dimensions
We argue on the basis of experimental numbers that the B=0 metal-insulator
transition in two dimensions, observed in Si-MOSFETs and in other
two-dimensional systems, is likely to be due to a few strongly interacting
electrons, which also interact strongly with the random positively ionized
impurities. At the insulating side the electrons are all bound in pairs to the
ions. On the metallic side free electrons exist which are scattered by ions
dressed with electron-pairs and therefore alter the bare scattering potential
of the ions. The physics at the metallic side of the transition is argued to be
controlled by the classical to quantum transport cross-over leading to the
observed non-monotonous dependence of the resistivity on temperature. This few
electrons per ion scenario appears to be an experimentally realistic and
testable scenario, which can also serve as a starting point for further
theoretical analysis of the two-dimensional metal-insulator transition.Comment: 8 pages, revised version, minor change
Interface charged impurity scattering in semiconductor MOSFETs and MODFETs: temperature dependent resistivity and 2D "metallic" behavior
We present the results on the anomalous 2D transport behavior by employing
Drude-Boltzmann transport theory and taking into account the realistic charge
impurity scattering effects. Our results show quantitative agreement with the
existing experimental data in several different systems and address the origin
of the strong and non-monotonic temperature dependent resistivity.Comment: Presented at SIMD, Dec. 1999 in Hawaii. To be published in
Superlattices and Microstructures, May 2000 issu
Charged impurity scattering limited low temperature resistivity of low density silicon inversion layers
We calculate within the Boltzmann equation approach the charged impurity
scattering limited low temperature electronic resistivity of low density
-type inversion layers in Si MOSFET structures. We find a rather sharp
quantum to classical crossover in the transport behavior in the K
temperature range, with the low density, low temperature mobility showing a
strikingly strong non-monotonic temperature dependence, which may qualitatively
explain the recently observed anomalously strong temperature dependent
resistivity in low-density, high-mobility MOSFETs.Comment: 5 pages, 2 figures, will appear in PRL (12 July, 1999
Screening Breakdown on the Route toward the Metal-Insulator Transition in Modulation Doped Si/SiGe Quantum Wells
Exploiting the spin resonance of two-dimensional (2D) electrons in SiGe/Si
quantum wells we determine the carrier-density-dependence of the magnetic
susceptibility. Assuming weak interaction we evaluate the density of states at
the Fermi level D(E_F), and the screening wave vector, q_TF. Both are constant
at higher carrier densities n, as for an ideal 2D carrier gas. For n < 3e11
cm-2, they decrease and extrapolate to zero at n = 7e10 cm-2. Calculating the
mobility from q_TF yields good agreement with experimental values justifying
the approach. The decrease in D(E_F) is explained by potential fluctuations
which lead to tail states that make screening less efficient and - in a
positive feedback - cause an increase of the potential fluctuations. Even in
our high mobility samples the fluctuations exceed the electron-electron
interaction leading to the formation of puddles of mobile carriers with at
least 1 micrometer diameter.Comment: 4 pages, 3 figure
Metal-Insulator Transition of Disordered Interacting Electrons
We calculate the corrections to the conductivity and compressibility of a
disordered metal when the mean free path is smaller than the screening length.
Such a condition is shown to be realized for low densities and large disorder.
Analysis of the stability of the metallic state reveals a transition to the
insulating state in two-dimensions.Comment: 11 pages, REVTEX, 1 figure included; Final versio
Singular Fermi Liquids
An introductory survey of the theoretical ideas and calculations and the
experimental results which depart from Landau Fermi-liquids is presented.
Common themes and possible routes to the singularities leading to the breakdown
of Landau Fermi liquids are categorized following an elementary discussion of
the theory. Soluble examples of Singular Fermi liquids (often called Non-Fermi
liquids) include models of impurities in metals with special symmetries and
one-dimensional interacting fermions. A review of these is followed by a
discussion of Singular Fermi liquids in a wide variety of experimental
situations and theoretical models. These include the effects of low-energy
collective fluctuations, gauge fields due either to symmetries in the
hamiltonian or possible dynamically generated symmetries, fluctuations around
quantum critical points, the normal state of high temperature superconductors
and the two-dimensional metallic state. For the last three systems, the
principal experimental results are summarized and the outstanding theoretical
issues highlighted.Comment: 170 pages; submitted to Physics Reports; a single pdf file with high
quality figures is available from http://www.lorentz.leidenuniv.nl/~saarloo
Stripes and electronic quasiparticles in the pseudogap state of cuprate superconductors
This article is devoted to a discussion of stripe and electron-nematic order
and their connection to electronic properties in the pseudogap regime of
copper-oxide superconductors. We review basic properties of these
symmetry-breaking ordering phenomena as well as proposals which connect them to
quantum-oscillation measurements. Experimental data indicate that these orders
are unlikely to be the cause of the pseudogap phenomenon, implying that they
occur on top of the pseudogap state which itself is of different origin.
Specifically, we discuss the idea that the non-superconducting pseudogap ground
state hosts electron-like quasiparticles which coexist with a spin liquid,
realizing a variant of a fractionalized Fermi liquid. We speculate on how
stripe order in such a pseudogap state might offer a consistent description of
ARPES, NMR, quantum-oscillation, and transport data.Comment: 15 pages, 6 figs. Article prepared for a Physica C special issue on
"Stripes and Electronic Liquid Crystals
How to detect fluctuating order in the high-temperature superconductors
We discuss fluctuating order in a quantum disordered phase proximate to a
quantum critical point, with particular emphasis on fluctuating stripe order.
Optimal strategies for extracting information concerning such local order from
experiments are derived with emphasis on neutron scattering and scanning
tunneling microscopy. These ideas are tested by application to two model
systems - the exactly solvable one dimensional electron gas with an impurity,
and a weakly-interacting 2D electron gas. We extensively review experiments on
the cuprate high-temperature superconductors which can be analyzed using these
strategies. We adduce evidence that stripe correlations are widespread in the
cuprates. Finally, we compare and contrast the advantages of two limiting
perspectives on the high-temperature superconductor: weak coupling, in which
correlation effects are treated as a perturbation on an underlying metallic
(although renormalized) Fermi liquid state, and strong coupling, in which the
magnetism is associated with well defined localized spins, and stripes are
viewed as a form of micro-phase separation. We present quantitative indicators
that the latter view better accounts for the observed stripe phenomena in the
cuprates.Comment: 43 pages, 11 figures, submitted to RMP; extensively revised and
greatly improved text; one new figure, one new section, two new appendices
and more reference
Flux Phase as a Dynamic Jahn-Teller Phase: Berryonic Matter in the Cuprates?
There is considerable evidence for some form of charge ordering on the
hole-doped stripes in the cuprates, mainly associated with the low-temperature
tetragonal phase, but with some evidence for either charge density waves or a
flux phase, which is a form of dynamic charge-density wave. These three states
form a pseudospin triplet, demonstrating a close connection with the E X e
dynamic Jahn-Teller effect, suggesting that the cuprates constitute a form of
Berryonic matter. This in turn suggests a new model for the dynamic Jahn-Teller
effect as a form of flux phase. A simple model of the Cu-O bond stretching
phonons allows an estimate of electron-phonon coupling for these modes,
explaining why the half breathing mode softens so much more than the full
oxygen breathing mode. The anomalous properties of provide a coupling
(correlated hopping) which acts to stabilize density wave phases.Comment: Major Revisions: includes comparisons with specific cuprate phonon
modes, 16 eps figures, revte
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