4,550 research outputs found
Observation of Quantized Hall Effect and Shubnikov-de Hass Oscillations in Highly Doped Bi2Se3: Evidence for Layered Transport of Bulk Carriers
Bi2Se3 is an important semiconductor thermoelectric material and a prototype
topological insulator. Here we report observation of Shubnikov-de Hass (SdH)
oscillations accompanied by quantized Hall resistances (Rxy) in highly-doped
n-type Bi2Se3 with bulk carrier concentrations of few 10^19 cm^-3. Measurements
under tilted magnetic fields show that the magnetotransport is 2D-like, where
only the c-axis component of the magnetic field controls the Landau level
formation. The quantized step size in 1/Rxy is found to scale with the sample
thickness, and average ~e2/h per quintuple layer (QL). We show that the
observed magnetotransport features do not come from the sample surface, but
arise from the bulk of the sample acting as many parallel 2D electron systems
to give a multilayered quantum Hall effect. Besides revealing a new electronic
property of Bi2Se3, our finding also has important implications for electronic
transport studies of topological insulator materials.Comment: accepted by Physical Review Letters (2012
Spectrum-Efficient Triple-Layer Hybrid Optical OFDM for IM/DD-Based Optical Wireless Communications
In this paper, a triple-layer hybrid optical orthogonal frequency division multiplexing
(THO-OFDM) for intensity modulation with direct detection (IM/DD) systems with a high spectral efficiency is proposed. We combine N-point asymmetrically clipped optical orthogonal frequency division
multiplexing (ACO-OFDM), N/2-point ACO-OFDM, and N/2-point pulse amplitude modulated discrete
multitoned (PAM-DMT) in a single frame for simultaneous transmission. The time- and frequency-domain
demodulation methods are introduced by fully exploiting the special structure of the proposed THO-OFDM.
Theoretical analysis show that, the proposed THO-OFDM can reach the spectral efficiency limit of the
conventional layered ACO-OFDM (LACO-OFDM). Simulation results demonstrate that, the time-domain
receiver offers improved bit error rate (BER) performance compared with the frequency-domain with ∼40%
reduced computation complexity when using 512 subcarriers. Furthermore, we show a 3 dB improvement
in the peak-to-average power ratio (PAPR) compared with LACO-OFDM for the same three layers
Steady State Solution and Stability of an Age-Structured MSIQR Epidemic Model
The importance of epidemiology in our life has stimulated researchers to extend the classic Susceptibles-Infectives-Removed (SIR) model to sophisticated models by including more factors in order to give detailed transmission dynamics of epidemic diseases. However, the integration of the quarantine policy and age-structure is less addressed. In this work we propose an age-structured MSIQR (temporarily immune-susceptibles-infectives-quarantined-removed) model to study the impact of quarantine policies on the spread of epidemic diseases. Specifically, we investigate the existence of steady state solutions and stability property of the proposed model. The derived explicit expression of the basic reproductive number shows that the disease-free equilibrium is globally asymptotically stable if, and that the unique endemic equilibrium exists if. In addition, the stability conditions of the endemic equilibrium are derived
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