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81.
82.
Xi-Zhou Qin 《Frontiers of Physics》2018,13(1):130302
We investigate the quantum dynamics of a driven two-level system under spontaneous emission and its application in clock frequency estimation. By using the Lindblad equation to describe the system, we analytically obtain its exact solutions, which show three different regimes: Rabi oscillation, damped oscillation, and overdamped decay. From the analytical solutions, we explore how the spontaneous emission affects the clock frequency estimation. We find that under a moderate spontaneous emission rate, the transition frequency can still be inferred from the Rabi oscillation. Our results enable potential practical applications in frequency measurement and quantum control under decoherence. 相似文献
83.
Investigation of Zn_(1-x)Cd_xO films bandgap and Zn_(1-x)Cd_xO/ZnO heterojunctions band offset by x-ray photoelectron spectroscopy
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A series of Zn_(1-x)Cd_xO thin films have been fabricated on sapphire by pulsed-laser deposition(PLD), successfully. To investigate the effect of Cd concentration on structural and optical properties of Zn_(1-x)Cd_xO films, x-ray diffraction(XRD),ultraviolet-visible spectroscopy(UV-vis), and x-ray photoelectron spectroscopy(XPS) are employed to characterize the films in detail. The XRD pattern indicates that the Zn_(1-x)Cd_xO thin films have high single-orientation of the c axis. The energy bandgap values of ZnCdO thin films decrease from 3.26 eV to 2.98 eV with the increasing Cd concentration(x)according to the(αhν)~2–hν curve. Furthermore, the band offsets of Zn_(1-x)Cd_xO/ZnO heterojunctions are determinated by XPS, indicating that a type-I alignment takes place at the interface and the value of band offset could be tuned by adjusting the Cd concentration. 相似文献
84.
A rubidium-beam microwave clock, optically pumped by a distributed feedback diode laser, is experimentally investigated. The clock is composed of a physical package, optical systems, and electric servo loops. The physical package realizes the microwave interrogation of a rubidium-atomic beam. The optical systems, equipped with two 780-nm distributed feedback laser diodes, yield light for pumping and detecting. The servo loops control the frequency of a local oscillator with respect to the microwave spectrum. With the experimental systems, the microwave spectrum, which has an amplitude of 4 n A and a line width of 700 Hz, is obtained. Preliminary tests show that the clock short-term frequency stability is 7 × 10~(-11) at 1 s, and 3 × 10~(-12) at 1000 s. These experimental results demonstrate the feasibility of the scheme for a manufactured clock. 相似文献
85.
Filter bank multicarrier (FBMC) systems are active candidates for the physical layer of the fifth generation of wireless communication networks (5G). The combination of FBMC system with the coordinated multipoint (CoMP) has been proposed to improve the performance of mobile terminals (MT) located at the cell edge. Accurate estimation of carrier frequency offset (CFO) and channel has great importance in communication systems. In this paper, a data-aided joint maximum likelihood (ML) CFO and channel estimation is proposed for the downlink of CoMP. Notably choosing the proper training sequences from Hadamard matrix for different base stations (BSs) reduces the complexity of estimator. Additionally joint CFO correction and channel equalization has been studied at the MT. Finally, we evaluate the efficacy of the presented algorithms using simulation. 相似文献
86.
In this paper, the problem of spectrum sensing of OFDM signals for cognitive radios is considered. It is proposed to detect the cyclostationary features introduced in an OFDM signal due to inter-pilot correlation. The performance of the proposed detector is derived and verified in case of AWGN channels. It is observed that the performance of cyclostationary detectors relies on the knowledge of the exact value of the cyclic frequency of the signal of interest. However, an offset in the cyclic frequency may arise due to several reasons. Therefore, for the proposed detector to perform reliably, there is a need to estimate the cyclic frequency offset. The Cramer–Rao bound for the cyclic frequency offset (CFO) estimator is derived, and based on it, two algorithms to estimate and compensate for the CFO are proposed. Simulation results are then used to study the performance of the proposed detection technique under Rayleigh fading both in the presence and the absence of CFO. The performance of the proposed system model is also studied under fast fading, and an alternative test statistic is proposed. 相似文献
87.
Cd_xZn_(1-x)S/Cu_2ZnSnS_4 (CZTS)-based thin film solar cells usually use Cd S as a buffer layer, but due to its smaller band gap(2.4 e V), Cd S film has been replaced with higher band gap materials. The cadmium zinc sulfide(Cd Zn S) ternary compound has a higher band gap than other compounds, which leads to a decrease in window absorption loss. In this paper, the band offsets at Cd_xZn_(1-x)S/Cu_2ZnSnS_4 (CZTS) heterointerface are calculated by the first-principles, densityfunctional and pseudopotential method. The band offsets at Cd_(1-x)Zn_x S/CZTS heterointerface are tuned by controlling the composition of Zn in Cd_(1-x)Zn_x S alloy, the calculated valence band offsets are small, which is consistent with the commonanion rule. The favorable heterointerface of type-I with a moderate barrier height( 0.3 e V) can be obtained by controlling the composition of Zn in Cd_(1-x)Zn_x S alloy between 0.25 and 0.375. 相似文献
88.
Wei-Long Chen Zhi-Jun Wang Chi Feng Wei-Ping Dou Yue Tao Huan Jia Wang-Sheng Wang Shu-Hui Liu Yuan He 《中国物理C(英文版)》2016,40(7):077003-077003
Beam-based BPM offset calibration was carried out for Injector II at the C-ADS demonstration facility at the Institute of Modern Physics(IMP), Chinese Academy of Science(CAS). By using the steering coils integrated in the quadrupoles, the beam orbit can be effectively adjusted and BPM positions recorded at the Medium Energy Beam Transport of the Injector II Linac. The studies were done with a 2 m A, 2.1 Me V proton beam in pulsed mode.During the studies, the "null comparison method" was applied for the calibration. This method is less sensitive to errors compared with the traditional transmission matrix method. In addition, the quadrupole magnet's center can also be calibrated with this method. 相似文献
89.
Band gaps of semiconductor materials are reduced due to band gap narrowing (BGN). Photoluminescence measurements on GaAs and AlGaAs thin films revealed a dependency of incident light intensity, and temperature in BGN in addition to the doping density. As a result, the valence band offset of p-doped GaAs/AlGaAs heterojunctions were reduced under illumination and high temperatures. We present evidence of incident-light-intensity causing barrier reduction at temperature >50 K causing zero valence band offsets in low-barrier heterostructures such as p-GaAs/Al0.01Ga0.99As, in addition to the dark-current increase by thermal excitations, causing the device failure at high temperatures. 相似文献
90.