2,909 research outputs found
Electrically controllable surface magnetism on the surface of topological insulator
We study theoretically the RKKY interaction between magnetic impurities on
the surface of three-dimensional topological insulators, mediated by the
helical Dirac electrons. Exact analytical expression shows that the RKKY
interaction consists of the Heisenberg-like, Ising-like and DM-like terms. It
provides us a new way to control surface magnetism electrically. The gap opened
by doped magnetic ions can lead to a short-range Bloembergen-Rowland
interaction. The competition among the Heisenberg, Ising and DM terms leads to
rich spin configurations and anomalous Hall effect on different lattices.Comment: 5 pages, 3 figures, 1 tabl
Spin-polarized transport in a lateral two-dimensional diluted magnetic semiconductor electron gas
The transport property of a lateral two-dimensional diluted magnetic
semiconductor electron gas under a spatially periodic magnetic field is
investigated theoretically. We find that the electron Fermi velocity along the
modulation direction is highly spin-dependent even if the spin polarization of
the carrier population is negligibly small. It turns out that this
spin-polarized Fermi velocity alone can lead to a strong spin polarization of
the current, which is still robust against the energy broadening effect induced
by the impurity scattering.Comment: 3 pages, 3 figures, submitted to Appl. Phys. Let
Electric field driven quantum phase transition between band insulator and topological insulator
We demonstrate theoretically that electric field can drive a quantum phase
transition between band insulator to topological insulator in CdTe/HgCdTe/CdTe
quantum wells. The numerical results suggest that the electric field could be
used as a switch to turn on or off the topological insulator phase, and
temperature can affect significantly the phase diagram for different gate
voltage and compositions. Our theoretical results provide us an efficient way
to manipulate the quantum phase of HgTe quantum wells.Comment: 4 pages, 4 figure
Magnetic coupling properties of rare-earth metals (Gd, Nd) doped ZnO: first-principles calculations
The electronic structure and magnetic coupling properties of rare-earth
metals (Gd, Nd) doped ZnO have been investigated using first-principles
methods. We show that the magnetic coupling between Gd or Nd ions in the
nearest neighbor sites is ferromagnetic. The stability of the ferromagnetic
coupling between Gd ions can be enhanced by appropriate electron doping into
ZnO:Gd system and the room-temperature ferromagnetism can be achieved. However,
for ZnO:Nd system, the ferromagnetism between Nd ions can be enhanced by
appropriate holes doping into the sample. The room-temperature ferromagnetism
can also be achieved in the \emph{n}-conducting ZnO:Nd sample. Our calculated
results are in good agreement with the conclusions of the recent experiments.
The effect of native defects (V, V) on the
ferromagnetism is also discussed.Comment: 5 pages, 5 figure
D'yakonov-Perel' spin relaxation in InSb/AlInSb quantum wells
We investigate theoretically the D'yakonov-Perel' spin relaxation time by
solving the eight-band Kane model and Poisson equation self-consistently. Our
results show distinct behavior with the single-band model due to the anomalous
spin-orbit interactions in narrow band-gap semiconductors, and agree well with
the experiment values reported in recent experiment (K. L. Litvinenko, et al.,
New J. Phys. \textbf{8}, 49 (2006)). We find a strong resonant enhancement of
the spin relaxation time appears for spin align along [] at a
certain electron density at 4 K. This resonant peak is smeared out with
increasing the temperature.Comment: 4 pages, 4 figure
Electrically-controllable RKKY interaction in semiconductor quantum wires
We demonstrate in theory that it is possible to all-electrically manipulate
the RKKY interaction in a quasi-one-dimensional electron gas embedded in a
semiconductor heterostructure, in the presence of Rashba and Dresselhaus
spin-orbit interaction. In an undoped semiconductor quantum wire where
intermediate excitations are gapped, the interaction becomes the short-ranged
Bloembergen-Rowland super-exchange interaction. Owing to the interplay of
different types of spin-orbit interaction, the interaction can be controlled to
realize various spin models, e.g., isotropic and anisotropic Heisenberg-like
models, Ising-like models with additional Dzyaloshinsky-Moriya terms, by tuning
the external electric field and designing the crystallographic directions. Such
controllable interaction forms a basis for quantum computing with localized
spins and quantum matters in spin lattices.Comment: 5 pages, 1 figur
Interplay between s-d exchange interaction and Rashba effect: spin-polarized transport
We investigate the spin-polarized transport properties of a two-dimensional
electron gas in a n-type diluted magnetic narrow gap semiconductor quantum well
subjected to a perpendicular magnetic and electric field. Interesting beating
patterns in the magneto resistance are found which can be tuned significantly
by varying the electric field. A resonant enhancement of spin-polarized current
is found which is induced by the competition between the s-d exchange
interaction and the Rashba effect [Y. A. Bychkov and E. I. Rashba, J. Phys. C
17, 6039 (1984)].Comment: 4 pages, 3 figures, Appl. Phys. Lett. (in press
Quantum measurement characteristics of double-dot single electron transistor
Owing to a few unique advantages, double-dot single electron transistor has
been proposed as an alternative detector for charge states. In this work, we
present a further study for its signal-to-noise property, based on a full
analysis of the setup configuration symmetry. It is found that the
effectiveness of the double-dot detector can approach that of an ideal
detector, if the symmetric capacitive coupling is taken into account. The
quantum measurement efficiency is also analyzed, by comparing the measurement
time with the measurement-induced dephasing time.Comment: 7 pages, 5 figure
AN EFFICIENT ITERATIVE DFT-BASED CHANNEL ESTIMATION FOR MIMO-OFDM SYSTEMS ON MULTIPATH CHANNELS
In this paper, an efficient iterative discrete Fourier transform (DFT) -based channel estimator with good performance for multiple-input and multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems such as IEEE 802.11n which retain some sub-carriers as null sub-carriers (or virtual carriers) is proposed. In order to eliminate the mean-square error (MSE) floor effect existed in conventional DFT-based channel estimators, we proposed a low-complexity method to detect the significant channel impulse response (CIR) taps, which neither need any statistical channel information nor a predetermined threshold value. Analysis and simulation results show that the proposed method has much better performance than conventional DFT-based channel estimators and without MSE floor effect
Influence of lateral propagating modes on laser output characteristics in selectively oxidized vertical cavity surface-emitting lasers with double oxide layers
The influence of lateral propagating modes on the threshold current and the spontaneous emission factor in selectively oxidized vertical cavity surface-emitting lasers (VCSELs) is investigated based on the mode behaviors of lateral propagating modes and the rate equation model. The numerical results show that the lateral propagating modes may be trapped in the aperture region for the selectively oxidized VCSEL with two oxide layers, one above and one below the active region. The output characteristics of VCSELs can be affected due to the reabsorption of the quasitrapped lateral propagating modes. A lower threshold current can be expected for a VCSEL with double oxide layers than that with a single oxide layer. The numerical results of rate equations also show that a larger spontaneous emission factor can be obtained by fitting the output-input curves for the VCSEL with double oxide layers. (C) 1999 American Institute of Physics. [S0021-8979(99)07919-0]
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