140 research outputs found
New type of antiferromagnetic polaron and bipolaron in HTc - superconductors
The possibility of formation of a new type of polaron based on the quantum
aniferromagnet (AF) model is reported. We take into account exchange
interactions between localized d-d spins of the AF, as well as the p-d
interaction of the AF with p-carriers.
The energy minimum is found when maximum charge density occurs on every
second spin. The formation of such ``comb''-like polarons results from the
damping of quantum fluctuations and the appearance of Van Vleck-like staggered
magnetization. Such polarons tend to form pairs coupled by an AF ``glue''.Comment: 3 pages 2 figure
Spin-Valley Kondo Effect in Multi-electron Silicon Quantum Dots
We study the spin-valley Kondo effect of a silicon quantum dot occupied by electrons, with up to four. We show that the Kondo
resonance appears in the Coulomb blockade regimes, but not
in the one, in contrast to the spin-1/2 Kondo effect, which
only occurs at odd. Assuming large orbital level spacings, the
energy states of the dot can be simply characterized by fourfold spin-valley
degrees of freedom. The density of states (DOS) is obtained as a function of
temperature and applied magnetic field using a finite-U equation-of-motion
approach. The structure in the DOS can be detected in transport experiments.
The Kondo resonance is split by the Zeeman splitting and valley splitting for
double- and triple-electron Si dots, in a similar fashion to single-electron
ones. The peak structure and splitting patterns are much richer for the
spin-valley Kondo effect than for the pure spin Kondo effect.Comment: 8 pages, 4 figures, in PRB format. This paper is a sequel to the
paper published in Phys. Rev. B 75, 195345 (2007
Electron mobility in Si δ-doped GaAs with spatial correlationsin the distribution of charged impurities
We present a theoretical study of electron mobility in heavily Si d-doped GaAs in the presence of applied hydrostatic pressure. At low temperature the electron-ionized impurity scattering is the most important scattering mechanism. The presence of DX centers in Si-doped GaAs results in spatial correlations of the charged impurities, which increase the electron mobility through the structure factor of the charged-impurity distribution and/or a decrease in the density of the charged dopants. A Monte Carlo approach has been developed to simulate this distribution in two dimensions for the d+/DX0 and d+/DX- models. In the mobility calculation, both intrasubband and intersubband scatterings are considered with the electron-electron screening within the random-phase approximation. A detailed comparison between experiment and theory shows that theory excluding the correlation effects underestimates the electron mobility systematically. In cooperation with other mechanisms, e.g., self-compensation of Si dopants, in the d layer, both DX-center models can explain the experimental results well. This indicates that in order to effectively study the electronic properties of DX centers via the electron mobility in d-doped structures, the samples must have a relatively low doping concentration in order to prevent self-compensation
Collective character of spin excitations in a system of Mn spins coupled to a two-dimensional electron gas
We have studied the low energy spin excitations in n-type CdMnTe based dilute
magnetic semiconductor quantum wells. For magnetic fields for which the
energies for the excitation of free carriers and Mn spins are almost identical
an anomalously large Knight shift is observed. Our findings suggests the
existence of a magnetic field induced ferromagnetic order in these structures,
which is in agreement with recent theoretical predictions [J. K{\"o}nig and A.
H. MacDonald, submitted Phys. Rev. Lett. (2002)]Comment: 4 figure
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