Articles you may be interested inEffects of bias and temperature on the intersubband absorption in very long wavelength GaAs/AlGaAs quantum well infrared photodetectors High speed photodetectors based on a two-dimensional electron/hole gas heterostructure Growth of p -type Ga As ∕ Al Ga As ( 111 ) quantum well infrared photodetector using solid source molecularbeam epitaxy J. Appl. Phys. 98, 054905 (2005);

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The relaxation dynamics of photoexcited carriers in a chemical vapor deposited graphene transferred on quartz substrate has been investigated by using ultrafast optical-pump terahertz (THz)-probe spectroscopy. Terahertz transmission through graphene sample is reduced by optical pumping. The change of transmission decays exponentially after the optical pulse. We find the relaxation time is insensitive to the substrate temperature from 10 K to 300 K but increases sublinearly with pump fluence. We model the relaxation process involving electron-phonon coupling together with a set of rate equations to describe the transient responses of quasi-particles and optical phonons. The increases of the extracted carrier temperature and the measured relaxation time with pump fluence are associated with the fact that high pump fluence significantly increases the carrier temperature and broadens the carrier distribution. As a result, it leads to the reduction of optical phonon emission efficiency and the decrease of cooling rate as well.

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We investigate the acoustoelectric properties of graphene and extract its acoustoelectric attenuation Γ as a function of the carrier density n, tuned via ionic liquid gating. Acoustoelectric effects in graphene are induced by launching surface acoustic waves (SAWs) on a piezoelectric LiNbO3 substrate. We measure the acoustoelectric current Iae through graphene and extract the SAW attenuation factor Γ as a function of n. The magnitude of Iae increases with decreasing n when the n is far from the charge neutral point (CNP). When n is tuned across the CNP, Iae first exhibits a local maximum, vanishes at the CNP, and then changes sign in accordance with the associated change in the carrier polarity. By contrast, Γ monotonically increases with decreasing n and reaches a maximum at the CNP. The extracted values of Γ, calibrated at the central frequency of 189 MHz, vary from ∼0.4 m−1 to 6.8 m−1, much smaller than the values for known two-dimensional systems. Data analysis suggests that the evolution of Iae and Γ with n manifests the electronic states of graphene. Our experimental findings provide insightful information for developing innovative graphene-based devices.

As urban and suburban areas expand, the problem of sewage disposal spreads as well. Inappropriate planning of a sewage management system could impair water quality, destroy habitat, and threaten public health. Simply building a sewage interceptor system along the urban river corridor to handle the wastewater effluents without regard to the impacts from combined-sewer overflows (CSOs) in the storm events cannot fulfill the ultimate goal of environmental restoration in the receiving water body. This study therefore carries out a system-based assessment to search for the optimal operating strategy of the interceptor facilities with respect to biocomplexity or biodiversity in an urban river system. In particular, it focuses on the richness of the fish community in the biological systems, the effect of stress on the fish community by storm events, and their capacity for adaptive behavior in response to the CSOs' impact in the Love River estuarine system, South Taiwan. By integrating the biological indicators in an environmental context, two simulation models describing the quality and quantity of storm water and their impact on the river water quality are calibrated and verified. The interactions of natural systems and engineered systems covering both spatial and temporal aspects can then be explored in terms of the predicted levels of dissoved oxygen (DO) along the river reaches so as to strengthen an ultimate optimal search for the best operational alternative for the interceptor system. In view of the inherent complexity of integrating simulation outputs at various scales to aid in building the optimization step, three regression submodels were derived beforehand. These submodels present a high potential for exhibiting, eliciting, and summarizing the nonlinear behavior between the CSO impacts and the DO levels in the river reaches. With the aid of such findings, this study finally applies a linear programming model to determine the optimal size of a constructed storage pond (i.e., a detention pond), based on several types of storm events in the study area. This is proved essential for minimizing the ecological risk in such a way so as to indirectly improve the biodiversity in the estuarine river system.

We investigate the effects of defect scatterings on the electric transport properties of chemical vapor deposited (CVD) graphene by measuring the carrier density dependence of the magneto-conductivity. To clarify the dominant scattering mechanism, we perform extensive measurements on large-area samples with different mobility to exclude the edge effect. We analyze our data with the major scattering mechanisms such as short-range static scatters, short-range screened Coulomb disorders, and weak-localization (WL). We establish that the charged impurities are the predominant scatters because there is a strong correlation between the mobility and the charge impurity density. Near the charge neutral point (CNP), the electron-hole puddles that are induced by the charged impurities enhance the inter-valley scattering, which is favorable for WL observations. Away from the CNP, the charged-impurity-induced scattering is weak because of the effective screening by the charge carriers. As a result, the local static structural defects govern the charge transport. Our findings provide compelling evidence for understanding the scattering mechanisms in graphene and pave the way for the improvement of fabrication techniques to achieve high-quality CVD graphene. V C 2013 AIP Publishing LLC. [http://dx.

We have constructed a scanning probe microscope for magnetic imaging, which can function as a scanning Hall probe microscope (SHPM) and as a scanning SQUID microscope (SSM). The scanning scheme, applicable to SHPM and SSM, consists of a mechanical positioning (sub) micron-XY stage and a flexible direct contact to the sample without a feedback control system for the Z-axis. With the interchangeable capability of operating two distinct scanning modes, our microscope can incorporate the advantageous functionalities of the SHPM and SSM with large scan range up to millimeter, high spatial resolution (⩽4 μm), and high field sensitivity in a wide range of temperature (4.2 K-300 K) and magnetic field (10(-7) T-1 T). To demonstrate the capabilities of the system, we present magnetic images scanned with SHPM and SSM, including a RbFeB magnet and a nickel grid pattern at room temperature, surface magnetic domain structures of a La(2/3)Ca(1/3)MnO3 thin film at 77 K, and superconducting vortices in a striped niobium film at 4.2 K.