10,311 research outputs found
Active optical clock based on four-level quantum system
Active optical clock, a new conception of atomic clock, has been proposed
recently. In this report, we propose a scheme of active optical clock based on
four-level quantum system. The final accuracy and stability of two-level
quantum system are limited by second-order Doppler shift of thermal atomic
beam. To three-level quantum system, they are mainly limited by light shift of
pumping laser field. These limitations can be avoided effectively by applying
the scheme proposed here. Rubidium atom four-level quantum system, as a typical
example, is discussed in this paper. The population inversion between
and states can be built up at a time scale of s.
With the mechanism of active optical clock, in which the cavity mode linewidth
is much wider than that of the laser gain profile, it can output a laser with
quantum-limited linewidth narrower than 1 Hz in theory. An experimental
configuration is designed to realize this active optical clock.Comment: 5 page
Superfluidity in a Three-flavor Fermi Gas with SU(3) Symmetry
We investigate the superfluidity and the associated Nambu-Goldstone modes in
a three-flavor atomic Fermi gas with SU(3) global symmetry. The s-wave pairing
occurs in flavor anti-triplet channel due to the Pauli principle, and the
superfluid state contains both gapped and gapless fermionic excitations.
Corresponding to the spontaneous breaking of the SU(3) symmetry to a SU(2)
symmetry with five broken generators, there are only three Nambu-Goldstone
modes, one is with linear dispersion law and two are with quadratic dispersion
law. The other two expected Nambu-Goldstone modes become massive with a mass
gap of the order of the fermion energy gap in a wide coupling range. The
abnormal number of Nambu-Goldstone modes, the quadratic dispersion law and the
mass gap have significant effect on the low temperature thermodynamics of the
matter.Comment: 9 pages, 2 figures, published versio
The 7-channel FIR HCN Interferometer on J-TEXT Tokamak
A seven-channel far-infrared hydrogen cyanide (HCN) laser interferometer has
been established aiming to provide the line integrated plasma density for the
J-TEXT experimental scenarios. A continuous wave glow discharge HCN laser
designed with a cavity length 3.4 m is used as the laser source with a
wavelength of 337 {\mu}m and an output power up to 100 mW. The system is
configured as a Mach-Zehnder type interferometer. Phase modulation is achieved
by a rotating grating, with a modulation frequency of 10 kHz which corresponds
to the temporal resolution of 0.1 ms. The beat signal is detected by TGS
detector. The phase shift induced by the plasma is derived by the comparator
with a phase sensitivity of 0.06 fringe. The experimental results measured by
the J-TEXT interferometer are presented in details. In addition, the inversed
electron density profile done by a conventional approach is also given. The
kinematic viscosity of dimethyl silicone and vibration control is key issues
for the system performance. The laser power stability under different kinematic
viscosity of silicone oil is presented. A visible improvement of measured
result on vibration reduction is shown in the paper.Comment: conference (15th-International Symposium on Laser-Aided Plasma
Diagnostics
A virtual crystal plasticity simulation tool for micro-forming
AbstractThe trend of increasing miniaturization of varied products and devices with a wide range of applications necessitates the forming of metallic parts with dimensions at the micron scale. In micro-forming, the stress and deformation are highly anisotropic. Hence, conventional macro-mechanics models fail to capture the important features, such as necking and bending resulting from strain localization. In this paper, a virtual integrated micro-mechanics simulation tool is presented, that was developed within the framework of Crystal Plasticity (CP) theory. With this tool, a polycrystalline Finite Element (FE) model was produced by introducing grain size, orientations and distribution patterns using VGRAIN software. ABAQUS software was used and the CP constitutive equations were implemented through a user-defined material subroutine, VUMAT. Typical micro-forming processes simulated include tension, extrusion and hydro-forming to demonstrate the effectiveness of the integrated simulation system. Finally, a map is proposed that establishes bounds of appropriate usage for different modeling techniques, namely a macromechanics plasticity model and a micro-mechanics crystal plasticity model, which will be useful to engineers in the metal forming industry in choosing suitable simulation tools
Transmission over EHF mobile satellite channels
Land mobile satellite communications at Ka-band (30/20 GHz) are attracting an increasing interest among researchers because of the frequency band availability and the possibility of small earth station designs. However, communications at the Ka-band pose significant challenges in the system designs due to severe channel impairments. Because only very limited experimental data for mobile applications at Ka-band is available, this paper studies the channel characteristics based on experimental data at L-band (1.6/1.5 GHz) and the use of frequency scaling. The land mobile satellite communication channel at Ka-band is modelled as log-normal Rayleigh fading channel. The first and second-order statistics of the fading channel are studied. The performance of a coherent BPSK system over the fading channel at L-band and K-band is evaluated theoretically and validated by computer simulations. Conclusions on the communication channel characteristics and system performance at L-band and Ka-band are presented
Enhancement of Transition Temperature in FexSe0.5Te0.5 Film via Iron Vacancies
The effects of iron deficiency in FexSe0.5Te0.5 thin films (0.8<x<1) on
superconductivity and electronic properties have been studied. A significant
enhancement of the superconducting transition temperature (TC) up to 21K was
observed in the most Fe deficient film (x=0.8). Based on the observed and
simulated structural variation results, there is a high possibility that Fe
vacancies can be formed in the FexSe0.5Te0.5 films. The enhancement of TC shows
a strong relationship with the lattice strain effect induced by Fe vacancies.
Importantly, the presence of Fe vacancies alters the charge carrier population
by introducing electron charge carriers, with the Fe deficient film showing
more metallic behavior than the defect-free film. Our study provides a means to
enhance the superconductivity and tune the charge carriers via Fe vacancy, with
no reliance on chemical doping.Comment: 15 pages, 4 figure
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