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98 results on '"Feng, Qingrong"'

1. Schatten Capped p Regularization for Robust Principle Component Analysis

Author

Yang, Lan, Zhang, Bin, Feng, Qingrong, Liu, Xinling, Wang, Jianjun, Goos, Gerhard, Founding Editor, Hartmanis, Juris, Founding Editor, Bertino, Elisa, Editorial Board Member, Gao, Wen, Editorial Board Member, Steffen, Bernhard, Editorial Board Member, Yung, Moti, Editorial Board Member, Sheng, Bin, editor, Bi, Lei, editor, Kim, Jinman, editor, Magnenat-Thalmann, Nadia, editor, and Thalmann, Daniel, editor

Published
2024
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5. Influence of Intrinsic Electronic Properties on Light Transmission through Subwavelength Holes on Gold and MgB2 Thin Films

Author

Fang, Xu, Zhuang, Chenggang, Wen, Zhenchao, Han, Xiufeng, Feng, Qingrong, Xi, X. X., Nori, Franco, Xie, Xincheng, Niu, Qian, and Qiu, Xianggang

Subjects
Condensed Matter - Superconductivity, Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Materials Science
Abstract

We show how intrinsic material properties modify light transmission through subwavelength hole arrays on thin metallic films in the THz regime. We compare the temperature-dependent transmittance of Au films and MgB$_{2}$ films. The experimental data is consistent with analytical calculations, and is attributed to the temperature change of the conductivity of both films. The transmission versus conductivity is interpreted within the open resonator model when taking the skin depth into consideration. We also show that the efficiency of this temperature control depends on the ratio of the transmission peak frequency to the superconducting energy gap in MgB$_{2}$ films., Comment: 6 pages, 6 figures

Published
2011
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6. Ultrathin MgB2 films fabricated on Al2O3 substrate by hybrid physical-chemical vapor deposition with high Tc and Jc

Author

Zhang, Yuhao, Lin, Zhiyuan, Dai, Qian, Li, Dongyao, Wang, Yinbo, Zhang, Yan, Wang, Yue, and Feng, Qingrong

Subjects
Condensed Matter - Superconductivity
Abstract

Ultrathin MgB2 superconducting films with a thickness down to 7.5 nm are epitaxially grown on (0001) Al2O3 substrate by hybrid physical-chemical vapor deposition method. The films are phase-pure, oxidation-free and continuous. The 7.5 nm thin film shows a Tc(0) of 34 K, which is so far the highest Tc(0) reported in MgB2 with the same thickness. The critical current density of ultrathin MgB2 films below 10 nm is demonstrated for the first time as Jc ~ 10^6 A cm^{-2} for the above 7.5 nm sample at 16 K. Our results reveal the excellent superconducting properties of ultrathin MgB2 films with thicknesses between 7.5 and 40 nm on Al2O3 substrate., Comment: 7 pages, 4 figures, 2 tables

Published
2011
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7. A sol-gel method for growing superconducting MgB2 films

Author

Chen, Liping, Zhang, Chen, Wang, Yinbo, Wang, Yue, Feng, Qingrong, Gan, Zizhao, Yang, Junzhi, and Li, Xingguo

Subjects
Condensed Matter - Superconductivity
Abstract

In this paper we report a new sol-gel method for the fabrication of MgB2 films. Polycrystalline MgB2 films were prepared by spin-coating a precursor solution of Mg(BH_4)_2 diethyl ether on (001)Al2O3 substrates followed with annealing in Mg vapor. In comparison with the MgB2 films grown by other techniques, our films show medium qualities including a superconducting transition temperature of Tc ~ 37 K, a critical current density of Jc(5 K, 0 T) ~ 5 {\times} 10^6 A cm^{-2}, and a critical field of H_{c2}(0) ~ 19 T. Such a sol-gel technique shows potential in the commercial fabrication of practically used MgB2 films as well as MgB2 wires and tapes., Comment: 8 pages, 5 figures

Published
2011
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8. Fully Band Resolved Scattering Rate in MgB2 Revealed by Nonlinear Hall Effect and Magnetoresistance Measurements

Author

Yang, Huan, Liu, Yi, Zhuang, Chenggang, Shi, Junren, Yao, Yugui, Massidda, Sandro, Monni, Marco, Jia, Ying, Xi, Xiaoxing, Li, Qi, Liu, Zi-Kui, Feng, Qingrong, and Wen, Hai-Hu

Subjects
Condensed Matter - Superconductivity
Abstract

We have measured the normal state temperature dependence of the Hall effect and magnetoresistance in epitaxial MgB2 thin films with variable disorders characterized by the residual resistance ratio RRR ranging from 4.0 to 33.3. A strong nonlinearity of the Hall effect and magnetoresistance have been found in clean samples, and they decrease gradually with the increase of disorders or temperature. By fitting the data to the theoretical model based on the Boltzmann equation and ab initio calculations for a four-band system, for the first time, we derived the scattering rates of these four bands at different temperatures and magnitude of disorders. Our method provides a unique way to derive these important parameters in multiband systems., Comment: 4 pages, 4 figures

Published
2008
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9. Scaling of Hall Resistivity in the Mixed State of MgB2 Films

Author

Yang, Huan, Jia, Ying, Shan, Lei, Zhuang, Chenggang, Xi, X. X., Li, Qi, Liu, Zikui, Feng, Qingrong, and Wen, Hai-Hu

Subjects
Condensed Matter - Superconductivity
Abstract

The longitudinal resistivity (rho_{xx}) and transverse resistivity (rho_{xy}) of MgB2 thin films in the mixed state were studied in detail. We found that the temperature dependencies of rho_{xx} and \rho_{xy} at a fixed magnetic field (H) satisfy the scaling law of $\rho_{xy}=A\rho_{xx}^\beta$, where the exponent beta varies around 2.0 for different fields. In the low field region (below 1T), beta maintains a constant value of 2.0 due to the weak pinning strength of the vortices, mainly from the superfluid of the pi band. When H>1T, beta drops abruptly to its lowest value at about 2T because of the proliferation of quasiparticles from the pi-band and, hence, the motion of the vortices from the superfluid of the sigma-band dominates the dissipation. As the field is increased further, the vortex pinning strength is weakened and beta increases monotonically towards 2.0 at a high field. All the results presented here are in good agreement with the expectation of the vortex physics of a multi-band superconductor.

Published
2008

21. Low-temperature sintering to fabricate MgB2 by using Mg(BH4)2 as precursor.

Author

Huang, Zigeng, Cai, Xinwei, Xi, Dan, Luo, Wenhao, Guo, Chen, Feng, Qingrong, Nie, Ruijuan, Wang, Furen, and Gan, Zizhao

Subjects
SINTERING, CRITICAL currents, FLUX pinning, GRAIN size, MAGNETIC fields, BALL mills
Abstract

Low-temperature sintering to fabricate MgB2 is considered a promising method because it reduces the formation of MgO and reduces the grain size, thereby increasing the critical current density. Chemical doping, ball milling of precursor and modifying the Mg source in precursor for low-temperature sintering have all been carefully investigated. However, the threshold of sintering temperature have been kept near 500 °C. A new in-situ method to low-temperature sintering using Mg(BH4)2 as a precursor is presented in this paper. Mg(BH4)2 decomposes into highly reactive nanoscale Mg and B in a molar ratio of 1:2 without impurities. In this method, only 2 h of sintering at 400 °C is required to completely react Mg with B to produce MgB2, and after 54 h sintering, the sample has comparable JC to the normal MgB2 from solid-state reaction method sintered at 800 °C. The sample sintered at 500 °C give better JC than normal MgB2 under both high and low magnetic field. [ABSTRACT FROM AUTHOR]

Published
2021
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22. Fabrication of Large-area MgB2 Films on Copper Substrates

Author

Guo, Xin, Feng, Qingrong, Liu, Kexin, Ni, Zhimao, and Yang, Can

Subjects
Fundamental SRF R&D, Accelerator Physics
Abstract

Magnesium diboride (MgB2) is a promising candidate material for SRF cavities because of its higher transition temperature and critical field compared with niobium. To meet the demand of RF test devices, the fabrication of large-area MgB2 films on metal substrates is needed. In this work, MgB2 films with 50-mm diameter were fabri-cated on Cu substrates by using an improved HPCVD setups at Peking University. The transition temperatures of MgB2 film on Cu substrate and with Mo buffer layer on Cu substrate are 36.2 K and 36.5 K, respectively. The fabrication processes, surface morphology, superconduct-ing properties of these large-area MgB2 films are present-ed., Proceedings of the 18th Int. Conf. on RF Superconductivity, SRF2017, Lanzhou, China

Published
2018
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25. Magnetic Shielding Capability of MgB2 Cup Made by Liquid Infiltration Process

Author

Huang, Zigeng, primary, Luo, Wenhao, additional, Cai, Xinwei, additional, Guo, Zhengshan, additional, Pan, Xifeng, additional, Xi, Dan, additional, Wu, Qianhong, additional, Niu, Ruirui, additional, Yang, Can, additional, Li, Yulong, additional, Linghu, Kehuang, additional, Gan, Zizhao, additional, Nie, Ruijuan, additional, Feng, Qingrong, additional, and Wang, Furen, additional

Published
2019
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35. The spatiotemporal responses of <italic>Populus euphratica</italic> to global warming in Chinese oases between 1960 and 2015.

Author

Zhang, Wenxia, Liu, Puxing, Feng, Qingrong, Wang, Tianguang, and Wang, Tianqiang

Abstract

Daily average temperature data from 48 meteorological stations in Chinese oases that are within the distribution area of Populus euphratica were analyzed to determine the spatiotemporal responses of this tree to climate change. Specifically, the start and end date as well as the number of days that comprised the growing season were analyzed with a multi-year trend line and using the Mann-Kendall mutation test, inverse distance weighted interpolation (IDW) in the software ArcGIS, a Morlet wavelet power spectrum, and correlation analysis. The results of this study show that, over the last 56 years, the start date of the P. euphratica growing season has advanced, while the end date has been postponed, and the number of days that comprise the growing season have gradually increased. The changing trend rates recovered in this analysis for these three time slices are -1.34 d/10 a, 1.33 d/10a, and 2.66 d/10a (α ⩾ 0.001), respectively. Data show that while spatial disparity is extremely significant, it is nevertheless the case that along a southwest-to-northeast transect of Chinese oases, the later the start date of the P. euphratica season, the sooner the end data and the shorter the growing season. Mutations points in start and end date, as well as for the growing season overall were observed in 2001, 1989, and 1996, respectively, and the data presented in this paper show that, in particular, the date of this end of this period is most sensitive to climate warming. Growing season cycles for P. euphratica are between 3.56 years and 7.14 years, consistent with the periodicity of El Niño events, while a start date cycle between 3.56 years and 4.28 years is consistent with atmospheric circulation cyclicity. The causal analysis presented in this paper shows that the Asian polar vortex area index (APVAI), the Qinghai- Tibet Plateau index (TPI), the westerly circulation index (WCI), and carbon dioxide emissions (CDE) are the main factors influencing spatiotemporal changes in the growth of P. euphratica, the effect of latitude during the growing season is more significant than altitude, and the start date of the growing season is more significantly influenced by these factors than end date. In addition, data show that the start date, end date, and length of the growing season are all significantly correlated with their average corresponding monthly temperature (correlation coefficients are -0.875, 0.770, and 0.897; α⩾0.001). Thus, if the average temperature in March increases by 1°C, the start date of the growing season will advance by 2.21 days, while if the average temperature in October increases by the same margin then the seasonal end date will be delayed by 2.76 days. Similarly, if the average temperature between March and October increases by 1°C, the growing season will be extended by 7.78 days. The results of this study corroborate the fact that changes in the P. euphratica growing are sensitive to regional warming and are thus of considerable theoretical significance to our understanding of the responses of Chinese vegetation to climate change as well as to ecological restoration. [ABSTRACT FROM AUTHOR]

Published
2018
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37. Measurement of laser-induced voltages of superconducting Bi-Sr-Ca-Cu-O

Author

Wang Guowen, Wang Lanping, Lin Jinlong, Tang Yonggui, Cai Jiye, and Feng Qingrong

Subjects
chemistry.chemical_classification, Superconductivity, Materials science, High-temperature superconductivity, Analytical chemistry, Zero-point energy, Photovoltaic effect, Laser, law.invention, chemistry, law, Single crystal, Inorganic compound, Stoichiometry
Abstract

We report an experimental investigation of laser-induced voltages across point-contact junctions of Pt tips and superconductiong Bi compounds. It has been observed that the laser-induced voltages of all 2:2:2:3 granular samples and 2:2:1:2 single-crystal samples are converted with increasing temperature from positive to negative sign, passing through a zero point. The signs for 2:2:1:2 granular samples and n-type Nd 1.85 Ce 0.15 CuO 4+y sample are always positive

Published
1992

38. Ge Film Contacts to Radio Frequency Sputtered Y-Ba-Cu-O Superconductor Film

Author

Feng Qingrong, Tong Litai, Chen Hai-Ying, Wang Gui-Zhi, Dai Yuandong, Qian Yi, Qiao Yong-Ping, Zhang Bei, Liu Zun-Xiao, Lan Jian, and Xing Miaoxiang

Subjects
Superconductivity, Materials science, business.industry, Contact system, General Physics and Astronomy, Optoelectronics, Radio frequency, Crystallite, business, Epitaxy, Layer (electronics), Deposition (law)
Abstract

The semiconductor-superconductor contact system of Ge/Y-Ba-Cu-O (YBCO)/SrTiO3 has been successfully fabricated by two steps of deposition. The YBCO film in the contact of Ge/YBCO did not lose its superconductivity and its Tc (on set) did not degrade as well. The top layer of Ge was polycrystalline with preferred orientation corresponding to the epitaxial structure of the underlayer of YBCO.

Published
1991

40. The spatiotemporal responses of Populus euphraticato global warming in Chinese oases between 1960 and 2015

Author

Zhang, Wenxia, Liu, Puxing, Feng, Qingrong, Wang, Tianguang, and Wang, Tianqiang

Abstract

Daily average temperature data from 48 meteorological stations in Chinese oases that are within the distribution area of Populus euphraticawere analyzed to determine the spatiotemporal responses of this tree to climate change. Specifically, the start and end date as well as the number of days that comprised the growing season were analyzed with a multi-year trend line and using the Mann-Kendall mutation test, inverse distance weighted interpolation (IDW) in the software ArcGIS, a Morlet wavelet power spectrum, and correlation analysis. The results of this study show that, over the last 56 years, the start date of the P. euphraticagrowing season has advanced, while the end date has been postponed, and the number of days that comprise the growing season have gradually increased. The changing trend rates recovered in this analysis for these three time slices are –1.34 d/10 a, 1.33 d/10a, and 2.66 d/10a (α ⩾ 0.001), respectively. Data show that while spatial disparity is extremely significant, it is nevertheless the case that along a southwest-to-northeast transect of Chinese oases, the later the start date of the P. euphraticaseason, the sooner the end data and the shorter the growing season. Mutations points in start and end date, as well as for the growing season overall were observed in 2001, 1989, and 1996, respectively, and the data presented in this paper show that, in particular, the date of this end of this period is most sensitive to climate warming. Growing season cycles for P. euphraticaare between 3.56 years and 7.14 years, consistent with the periodicity of El Niño events, while a start date cycle between 3.56 years and 4.28 years is consistent with atmospheric circulation cyclicity. The causal analysis presented in this paper shows that the Asian polar vortex area index (APVAI), the Qinghai- Tibet Plateau index (TPI), the westerly circulation index (WCI), and carbon dioxide emissions (CDE) are the main factors influencing spatiotemporal changes in the growth of P. euphratica, the effect of latitude during the growing season is more significant than altitude, and the start date of the growing season is more significantly influenced by these factors than end date. In addition, data show that the start date, end date, and length of the growing season are all significantly correlated with their average corresponding monthly temperature (correlation coefficients are –0.875, 0.770, and 0.897; α⩾0.001). Thus, if the average temperature in March increases by 1°C, the start date of the growing season will advance by 2.21 days, while if the average temperature in October increases by the same margin then the seasonal end date will be delayed by 2.76 days. Similarly, if the average temperature between March and October increases by 1°C, the growing season will be extended by 7.78 days. The results of this study corroborate the fact that changes in the P. euphraticagrowing are sensitive to regional warming and are thus of considerable theoretical significance to our understanding of the responses of Chinese vegetation to climate change as well as to ecological restoration.

Published
2018
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41. Thermally Activated Flux Motion in 2223 Phase High Temperature Superconductors

Author

Feng Xiaoqi, Feng Qingrong, Zhang Han, Li Chuan-Yi, Chen Jun, Yin Dao-Le, Li Gang, and Zhu Xing

Subjects
High-temperature superconductivity, Materials science, Secondary phase, Condensed matter physics, Volume (thermodynamics), law, Phase (matter), General Physics and Astronomy, Thermodynamics, Flux motion, Scaling, Isothermal process, law.invention
Abstract

Isothermal V-I characteristics of Bi1.8Pb0.2Sr2Ca2Cu3Oy high temperature superconductors (HTSCs) have been systematically studied. Below the irreversibility temperature Tr(H), two important features were observed: 1) Both positive and negative curvatures can coexist in the same isothermal V-I curve, so it is different from the scaling behavior V ∝ exp(-(J0/J)μ). 2) The activation volume of flux motion is found temperature dependent as (1-T/Tr(H))-2ν with ν about 0.5. This behavior supports the existence of a two-dimensional secondary phase transition below Hc2(T) in this Bi-based HTSCs.

Published
1992

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