Zizhao Gan
Peking University
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Featured researches published by Zizhao Gan.
Superconductor Science and Technology | 2009
Chenggang Zhuang; Teng Tan; Yazhou Wang; Shanshan Bai; Xiaobai Ma; Huan Yang; Guohua Zhang; Yusheng He; Hai-Hu Wen; X. X. Xi; Qingrong Feng; Zizhao Gan
Phase pure and clean MgB2 films have been fabricated on different types of single-crystal substrate, including SiC, sapphire, MgO, LaAlO3, SrTiO3, and yttrium-stabilized ZrO2 (YSZ), via the hybrid physical–chemical vapor deposition (HPCVD) technique. Pure MgB2 films with different structural textures are obtained by selecting the substrate and its orientation, such as off-axis epitaxial MgB2 on MgO(211) or textured MgB2 on YSZ(110) substrates. Films show Tc(0) in the range 37–41xa0K, and residual resistivity ρ(42xa0K) between 0.3 and 5xa0µΩxa0cm as the substrate and film thickness are varied. The crystal structures and film morphologies are presented and discussed. Further investigations are focused on c-axis epitaxial films. Epitaxial MgB2 films with thickness about 300xa0nm manifest Tc(0) = 41.2xa0K and residual resistivity ρ(42xa0K) = 0.35xa0µΩxa0cm on SiC(0001) while Tc(0) = 39.5xa0K and ρ(42xa0K) = 0.27xa0µΩxa0cm on c-cut sapphire, reflecting that the films are of high quality and in the clean limit. Low upper critical field and large anisotropy of the upper critical field γ comparable to the results from MgB2 single crystals are also revealed. The smooth surface of the epitaxial films, with root-mean-square (RMS) roughness less than 5xa0nm, makes them well qualified for device applications. Microwave performance measurements are carried out using a coplanar resonator on a 300xa0nm thick film on sapphire substrate and no power dependence of unloaded quality factor in the output power range of −10–25xa0dB is found.
Superconductor Science and Technology | 2008
Chenggang Zhuang; S. Meng; H Yang; Y Jia; Hai-Hu Wen; X. X. Xi; Qingrong Feng; Zizhao Gan
We report a significant enhancement in upper critical field Hc2 and irreversibility field Hirr in carbon-doped MgB2 films fabricated by hot-filament-assisted hybrid physical–chemical vapor deposition (HFA-HPCVD) using methane as the doping source. For the parallel field, a very large temperature derivative value of 3xa0Txa0K−1 near Tc was achieved in a heavily doped film with Tc near 28xa0K. Carbon doping also enhanced flux pinning, resulting in a much higher critical current density in a magnetic field Jc(H) than in undoped samples. The result suggests a more effective doping of carbon into the MgB2 structure and a better connectivity between the carbon-doped MgB2 grains than in previous reports. A clear correlation between the decrease in grain size and the enhancement of Hc2 was observed.
Superconductor Science and Technology | 2007
Chenggang Zhuang; Dan Yao; Fen Li; Kaicheng Zhang; Qingrong Feng; Zizhao Gan
1 µm thick polycrystalline MgB2 films have been grown on commercial niobium sheets using the hybrid physical–chemical vapour deposition technique (HPCVD). Their zero-resistance transition temperature Tc0 ranges from 38 to 39 K and the transition temperature width ΔTc is about 0.1–0.2 K. The films are composed of regular plate-shaped MgB2 crystallites and have lattice constants about the bulk values. Some diffusion regions were observed at the interface between the film and the substrate by a line scanning spectra of energy-dispersive x-ray spectroscopy (EDX) on the cross section, which might enhance the adhesion of the coated layer to the substrate. The critical current densities, Jc, of these films, calculated by the Bean model, are greater than 5 × 106 A cm−2 at 10 K in zero field. Although tiny cracks in the film were created by bending the sample on a curved surface with a radius of 1.4 mm, however, Tc0 of the bent film was not affected and remained about 39 K, which indicates that the sample has a certain ductibility.
Superconductor Science and Technology | 2011
Li-ping Chen; Chen Zhang; Y.Y. Wang; Yue Wang; Qingrong Feng; Zizhao Gan; Junzhi Yang; Xingguo Li
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(BH4)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 ~ 37xa0K, a critical current density of JC(5xa0K, 0xa0T) ~ 5 × 106xa0Axa0cm − 2, and a critical field of HC2(0) ~ 19xa0T. Such a sol–gel technique shows potential in the commercial fabrication of practically used MgB2 films as well as MgB2 wires and tapes.
Superconductor Science and Technology | 2010
Y. Zhu; A. V. Pogrebnyakov; Rudeger H. T. Wilke; Ke Chen; Xiaoxing Xi; Joan M. Redwing; Chenggang Zhuang; Qingrong Feng; Zizhao Gan; R. K. Singh; Y. Shen; Nathan Newman; J. M. Rowell; F Hunte; J Jaroszynski; D. C. Larbalestier; Scott A. Baily; F. F. Balakirev; Paul M. Voyles
MgB2 thin films have superior superconducting properties compared to bulk MgB2 and demonstrate the potential for further improving the performances of MgB2 wires and tapes. Using transmission electron microscopy, we have characterized the microstructure of pure and C-doped MgB2 using various carbon sources grown by hybrid physical?chemical vapor deposition (HPCVD), and cold-grown?annealed film deposited by molecular beam epitaxy (MBE). The MgB2 HPCVD films increase in crystal quality in the order (MeCp)2Mg-sourced films, CH4-sourced films, B(CH3)3-sourced films, pure films, while the Hc2 values of these films follow the opposite order. The cold-grown?annealed MgB2 MBE film contains non-epitaxial ? 10?nm MgB2 grains and MgO nanoparticles. The microstructural origins of electron scattering and flux pinning in both films are discussed. We also show the structure and chemistry of the degraded phase in HPCVD films and its effects on superconducting properties.
Superconductor Science and Technology | 1993
Qingrong Feng; Xing Zhu; S.Q. Feng; Han Zhang; Zizhao Gan
Ag-doped YBa2Cu3O7- delta samples aged in air for 47 months have been examined using structure analysis, resistivity, thermal analysis magnetic measurement, SEM, and TEM (STEM). The experimental results demonstrate that some properties of the samples were hardly changed after being kept in air for a prolonged period of time. On the other hand, the superconductive volume fraction of the sample, the intragranular critical current density Jcg, and the pinning force density Fp were increased. The authors consider that Ag-doping YBa2Cu3O7- delta can effectively prevent samples from degradation.
IEEE Transactions on Applied Superconductivity | 2013
Chen Zhang; Yue Wang; Da Wang; Yan Zhang; Qingrong Feng; Zizhao Gan
MgB2 has been recognized as a candidate material for fabricating superconducting nanowire single photon detectors with the expectation of having an operating temperature of the detector at 10-20 K. Preparation of ultrathin MgB2 films with high superconducting transition temperature TC is the first crucial step to achieve this goal. Here we present the hybrid physical-chemical vapor deposition of MgB2 superconducting films with thicknesses around 10 nm. The films are epitaxially grown on (111) MgO substrates. The TC of the films, including for a 6-nm-thick film, is found to be above 34 K, close to the bulk value of MgB2 (39 K). The normal-state resistivity at 42 K of a 10-nm-thick film is shown to be 3.1 μΩ cm, indicating that the film is relatively clean. The superconducting critical current density JC of the films has also been measured on strip lines patterned in the films. The JC of the 10-nm-thick film reaches 7.7 ×107 A/cm2 at 2 K, so far the highest value reported for MgB2 films of the same thickness. The high TC, low residual resistivity, and high JC of the ultrathin MgB2 films on MgO substrates suggest that these films have the potential in developing MgB2-based superconducting nanowire single photon detectors or other related superconducting devices.
Superconductor Science and Technology | 2009
Y. Zhu; F Hunte; Chenggang Zhuang; Qingrong Feng; Zizhao Gan; Xiaoxing Xi; D. C. Larbalestier; Paul M. Voyles
We report that C-doped MgB2 thin films deposited by hybrid physical–chemical vapor deposition (HPCVD) using CH4 as the carbon source have Hc2(0xa0K)~60xa0T, similar to that of HPCVD films obtained using (MeCp)2Mg for the carbon. Using transmission electron microscopy, we show that in the films doped using CH4 there is a MgB2C2 layer on top of the MgB2 film, which does not degrade the MgB2 connectivity or Jc. We also find a high density of coherent MgO nanoplatelets in the MgB2 which create strain fields which may give rise to strong π-band scattering and the very high Hc2.
AIP Advances | 2015
Chen Zhang; Da Wang; Zhenghao Liu; Yan Zhang; Ping Ma; Qingrong Feng; Yue Wang; Zizhao Gan
High quality superconducting nanowires were fabricated from ultrathin MgB2 films by a focused ion beam milling technique. The precursor MgB2 films in 10 nm thick were grown on MgO substrates by using a hybrid physical-chemical vapor deposition method. The nanowires, in widths of about 300-600 nm and lengths of 1 or 10 μm, showed high superconducting critical temperatures (Tc’s) above 34 K and narrow superconducting transition widths (ΔTc’s) of 1-3 K. The superconducting critical current density Jc of the nanowires was above 5 × 107 A/cm2 at 20 K. The high Tc, narrow ΔTc, and high Jc of the nanowires offered the possibility of making MgB2-based nano-devices such as hot-electron bolometers and superconducting nanowire single-photon detectors with high operating temperatures at 15-20 K.
AIP Advances | 2014
Chen Zhang; Fakuan Tang; Ping Ma; Zizhao Gan
We have developed a four-channel high temperature radio-frequency superconducting quantum interference device (HTcnrf SQUID) in a simple magnetically shielded room (MSR) that can be used to analyze the cardiacnmagnetic field. It is more robust and compact than existing systems. To achieve the high-quality magnetocardiographic signal, we explored new adaptive software gradiometry technology constructed by the first-order axial gradiometer with a baseline of 80mm, which can adjust its performance timely with the surrounding conditions. The magnetic field sensitivity of each channel was less than 100fT/√Hz in the white noise region. Especially, in the analysis of MCG signal data, we proposed the total transient mapping (TTM) technique to visualize current density map (CDM), then we focused to observe the time-varying behavior of excitation propagation and estimated the underlying currents at T wave. According to the clear 3D imaging, isomagnetic field and CDM, the position and distribution of a current source in the heart can be visualized. It is believed that our four-channel HTcnrf SQUID magnetometer based on biomagnetic system is available to detect MCG signals with sufficient signal-to-noise (SNR) ratio. In addition, the CDM showed the macroscopic current activation pattern, in a way, it has established strong underpinnings for researching the cardiac microscopic movement mechanism and opening the way for its use in clinical diagnosis.