457 research outputs found
Electric Field Effect Tuning of Electron-Phonon Coupling in Graphene
Gate-modulated low-temperature Raman spectra reveal that the electric field
effect (EFE), pervasive in contemporary electronics, has marked impacts on long
wavelength optical phonons of graphene. The EFE in this two dimensional
honeycomb lattice of carbon atoms creates large density modulations of carriers
with linear dispersion (known as Dirac fermions). Our EFE Raman spectra display
the interactions of lattice vibrations with these unusual carriers. The changes
of phonon frequency and line-width demonstrate optically the particle-hole
symmetry about the charge-neutral Dirac-point. The linear dependence of the
phonon frequency on the EFE-modulated Fermi energy is explained as the
electron-phonon coupling of mass-less Dirac fermions.Comment: 4 pages, 4 figure
Estimating sizes of Type 2 AGN narrow-line regions from multiple survey spectra -- a demonstration
In the Letter, an interesting method is proposed to estimate size of narrow
emission lines regions (NLRs) of a Type-2 AGN SDSS J083823.91+490241.1 (=SDSS
J0838) at a redshift of 0.101, by comparing spectroscopic properties through
the SDSS fiber (MJD=51873) (diameter of 3 arcseconds) and through the eBOSS
fiber (MJD=55277) (diameter of 2 arcseconds). After subtractions of pPXF method
determined host galaxy contributions, the narrow emission lines of SDSS J0838
in the SDSS spectrum and in the eBOSS spectrum can be well measured by Gaussian
functions, leading more than 90\% of [O~{\sc iii}] emissions to be covered by
the eBOSS fiber with diameter of 2 arcseconds. Meanwhile, both none broad
emission components and none-variabilities of ZTF 3years-long g/r-band light
curves can be applied to confirm SDSS J0838 as a Type-2 AGN, indicating few
orientation effects on the projected NLRs size in SDSS J0838. Therefore, upper
limit about 1arcsecond (2250pc) of the NLRs size can be reasonably accepted in
SDSS J0838. Combining with the intrinsic reddening corrected [O~{\sc iii}] line
luminosity, the upper limit of NLRs size in SDSS J0838 well lies within the
99.9999\% confidence bands of the R-L empirical relation for NLRs in AGN.Comment: 6 pages, 5 figures, accepted to be published in MNRA
Fabrication and Electric Field Dependent Transport Measurements of Mesoscopic Graphite Devices
We have developed a unique micromechanical method to extract extremely thin
graphite samples. Graphite crystallites with thicknesses ranging from 10 - 100
nm and lateral size 2 m are extracted from bulk. Mesoscopic
graphite devices are fabricated from these samples for electric field dependent
conductance measurements. Strong conductance modulation as a function of gate
voltage is observed in the thinner crystallite devices. The temperature
dependent resistivity measurements show more boundary scattering contribution
in the thinner graphite samples.Comment: 3 pages, 3 figures included, submitted to Appl. Phys. Let
Energy Band Gap Engineering of Graphene Nanoribbons
We investigate electronic transport in lithographically patterned graphene
ribbon structures where the lateral confinement of charge carriers creates an
energy gap near the charge neutrality point. Individual graphene layers are
contacted with metal electrodes and patterned into ribbons of varying widths
and different crystallographic orientations. The temperature dependent
conductance measurements show larger energy gaps opening for narrower ribbons.
The sizes of these energy gaps are investigated by measuring the conductance in
the non-linear response regime at low temperatures. We find that the energy gap
scales inversely with the ribbon width, thus demonstrating the ability to
engineer the band gap of graphene nanostructures by lithographic processes.Comment: 7 pages including 4 figure
Effect of Forage Brassica on Subsequent Soil Water Content and Yield of Dual-Purpose Winter Wheat in Rainfed Region of Northwestern China
The dual-purpose performance of winter wheat used for both forage and grain production has been explored as an alternative practice for filling the feed gap during winter and spring in agricultural areas of the Loess Plateau. Profit-ability is still restricted however, because of a three month summer fallow period between harvest and planting of the subsequent wheat crop. During this 3 month period 60% of the annual precipitation occurs, limiting the effective use of soil water and compounding the risk of soil erosion. Thus it is important to cultivate some forage crops with higher water consumption, protecting the ecological environment during this period and enlarging the forage resource base through their high forage yield and good quality. A previous study has shown that inclusion of rapeseed into crop rotations could reduce disease in subsequent plantings, leading to an increase in production of the following wheat crop (Brendan and John 2004). Accordingly, dual-purpose winter wheat after forage brassica may be an effective option to meet these requirements. Other studies have shown however, that wheat yield of grazed plots following brassica was reduced by 29% compared with that of grazed plots following fallow, and that average grain yield in grazed plots was reduced by 38% compared with that in ungrazed plots (Kelman and Dove 2007). Additionally, the possibility of severe water stress occurring after forage crop harvest is an important concern. In this study, soil moisture status and forage and grain yield of dual-purpose winter wheat following forage brassica were investigated and compared with those of winter wheat after fallow
Full-range Gate-controlled Terahertz Phase Modulations with Graphene Metasurfaces
Local phase control of electromagnetic wave, the basis of a diverse set of
applications such as hologram imaging, polarization and wave-front
manipulation, is of fundamental importance in photonic research. However, the
bulky, passive phase modulators currently available remain a hurdle for
photonic integration. Here we demonstrate full-range active phase modulations
in the Tera-Hertz (THz) regime, realized by gate-tuned ultra-thin reflective
metasurfaces based on graphene. A one-port resonator model, backed by our
full-wave simulations, reveals the underlying mechanism of our extreme phase
modulations, and points to general strategies for the design of tunable
photonic devices. As a particular example, we demonstrate a gate-tunable THz
polarization modulator based on our graphene metasurface. Our findings pave the
road towards exciting photonic applications based on active phase
manipulations
Optimal Mixed-ADC arrangement for DOA Estimation via CRB using ULA
We consider a mixed analog-to-digital converter (ADC) based architecture for
direction of arrival (DOA) estimation using a uniform linear array (ULA). We
derive the Cram{\'e}r-Rao bound (CRB) of the DOA under the optimal time-varying
threshold, and find that the asymptotic CRB is related to the arrangement of
high-precision and one-bit ADCs for a fixed number of ADCs. Then, a new concept
called ``mixed-precision arrangement" is proposed. It is proven that better
performance for DOA estimation is achieved when high-precision ADCs are
distributed evenly around the edges of the ULA. This result can be extended to
a more general case where the ULA is equipped with various precision ADCs.
Simulation results show the validity of the asymptotic CRB and better
performance under the optimal mixed-precision arrangement.Comment: 5 pages, 3 figures, accepted by ICASSP202
Flat Chern Band From Twisted Bilayer MnBiTe
We construct a continuum model for the Moir\'e superlattice of twisted
bilayer MnBiTe, and study the band structure of the bilayer in both
ferromagnetic (FM) and antiferromagnetic (AFM) phases. We find the system
exhibits highly tunable Chern bands with Chern number up to . We show that a
twist angle of turns the highest valence band into a flat band with
Chern number that is isolated from all other bands in both FM and AFM
phases. This result provides a promising platform for realizing time-reversal
breaking correlated topological phases, such as fractional Chern insulator and
topological superconductor. In addition, our calculation indicates that
the twisted stacking facilitates the emergence of quantum anomalous Hall effect
in MnBiTe.Comment: 7+6 pages, 3+2 figure
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