103 research outputs found
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
Exact analytical expression for magnetoresistance using quantum groups
We obtain an exact analytical expression for magnetoresistance using
noncommutative geometry and quantum groups.Then we will show that there is a
deep relationship between magnetoresistance and the quantum group ,
from which we understand the quantum interpretation of the quantum corrections
to the conductivity.Comment: 8 pages, 3 figures, replaced with the version published in Physics
Letters
Sharp increase of the effective mass near the critical density in a metallic 2D electron system
We find that at intermediate temperatures, the metallic temperature
dependence of the conductivity \sigma(T) of 2D electrons in silicon is
described well by a recent interaction-based theory of Zala et al. (Phys. Rev.
B 64, 214204 (2001)). The tendency of the slope d\sigma/dT to diverge near the
critical electron density is in agreement with the previously suggested
ferromagnetic instability in this electron system. Unexpectedly, it is found to
originate from the sharp enhancement of the effective mass, while the effective
Lande g factor remains nearly constant and close to its value in bulk silicon
Interaction Corrections to Two-Dimensional Hole Transport in Large Limit
The metallic conductivity of dilute two-dimensional holes in a GaAs HIGFET
(Heterojunction Insulated-Gate Field-Effect Transistor) with extremely high
mobility and large is found to have a linear dependence on temperature,
consistent with the theory of interaction corrections in the ballistic regime.
Phonon scattering contributions are negligible in the temperature range of our
interest, allowing comparison between our measured data and theory without any
phonon subtraction. The magnitude of the Fermi liquid interaction parameter
determined from the experiment, however, decreases with
increasing for r_{s}\agt22, a behavior unexpected from existing
theoretical calculations valid for small .Comment: 6 pages, 4 figure
Negatively Charged Excitons and Photoluminescence in Asymmetric Quantum Well
We study photoluminescence (PL) of charged excitons () in narrow
asymmetric quantum wells in high magnetic fields B. The binding of all
states strongly depends on the separation of electron and hole layers.
The most sensitive is the ``bright'' singlet, whose binding energy decreases
quickly with increasing even at relatively small B. As a result, the
value of B at which the singlet--triplet crossing occurs in the spectrum
also depends on and decreases from 35 T in a symmetric 10 nm GaAs well
to 16 T for nm. Since the critical values of at which
different states unbind are surprisingly small compared to the well
width, the observation of strongly bound states in an experimental PL
spectrum implies virtually no layer displacement in the sample. This casts
doubt on the interpretation of PL spectra of heterojunctions in terms of
recombination
Coulomb effects in granular materials at not very low temperatures
We consider effects of Coulomb interaction in a granular normal metal at not
very low temperatures suppressing weak localization effects. In this limit
calculations with the initial electron Hamiltonian are reduced to integrations
over a phase variable with an effective action, which can be considered as a
bosonization for the granular metal. Conditions of the applicability of the
effective action are considered in detail and importance of winding numbers for
the phase variables is emphasized. Explicit calculations are carried out for
the conductivity and the tunneling density of states in the limits of large
and small tunnelling conductances. It is demonstrated for any
dimension of the array of the grains that at small the conductivity and the
tunnelling density of states decay with temperature exponentially. At large
the conductivity also decays with decreasing the temparature and its
temperature dependence is logarithmic independent of dimensionality and
presence of a magnetic field. The tunnelling density of states for is
anomalous in any dimension but the anomaly is stronger than logarithmic in low
dimensions and is similar to that for disordered systems. The formulae derived
are compared with existing experiments. The logarithmic behavior of the
conductivity at large obtained in our model can explain numerous
experiments on systems with a granular structure including some high
materials.Comment: 30 page
Emergence of quasi-metallic state in disordered 2D electron gas due to strong interactions
The interrelation between disorder and interactions in two dimensional
electron liquid is studied beyond weak coupling perturbation theory. Strong
repulsion significantly reduces the electronic density of states on the Fermi
level. This makes the electron liquid more rigid and strongly suppresses
elastic scattering off impurities. As a result the weak localization, although
ultimately present at zero temperature and infinite sample size, is
unobservable at experimentally accessible temperature at high enough densities.
Therefore practically there exists a well defined metallic state. We study
diffusion of electrons in this state and find that the diffusion pole is
significantly modified due to "mixture" with static photons similar to the
Anderson - Higgs mechanism in superconductivity. As a result several effects
stemming from the long range nature of diffusion like the Aronov - Altshuler
logarithmic corrections to conductivity are less pronounced.Comment: to appear in Phys. Rev.
Binding Energy of Charged Excitons in ZnSe-based Quantum Wells
Excitons and charged excitons (trions) are investigated in ZnSe-based quantum
well structures with (Zn,Be,Mg)Se and (Zn,Mg)(S,Se) barriers by means of
magneto-optical spectroscopy. Binding energies of negatively () and positively
(X+) charged excitons are measured as functions of quantum well width, free
carrier density and in external magnetic fields up to 47 T. The binding energy
of shows a strong increase from 1.4 to 8.9 meV with decreasing quantum well
width from 190 to 29 A. The binding energies of X+ are about 25% smaller than
the binding energy in the same structures. The magnetic field behavior of and
X+ binding energies differ qualitatively. With growing magnetic field strength,
increases its binding energy by 35-150%, while for X+ it decreases by 25%.
Zeeman spin splittings and oscillator strengths of excitons and trions are
measured and discussed
The Parallel Magnetoconductance of Interacting Electrons in a Two Dimensional Disordered System
The transport properties of interacting electrons for which the spin degree
of freedom is taken into account are numerically studied for small two
dimensional diffusive clusters. On-site electron-electron interactions tend to
delocalize the electrons, while long-range interactions enhance localization.
On careful examination of the transport properties, we reach the conclusion
that it does not show a two dimensional metal insulator transition driven by
interactions. A parallel magnetic field leads to enhanced resistivity, which
saturates once the electrons become fully spin polarized. The strength of the
magnetic field for which the resistivity saturates decreases as electron
density goes down. Thus, the numerical calculations capture some of the
features seen in recent experimental measurements of parallel
magnetoconductance.Comment: 10 pages, 6 figure
The Superspace of Geometrodynamics
Wheeler's Superspace is the arena in which Geometrodynamics takes place. I
review some aspects of its geometrical and topological structure that Wheeler
urged us to take seriously in the context of canonical quantum gravity.Comment: 29 pages, 8 figures. To appear in the Wheeler memorial volume of
General Relativity and Gravitatio
- …