Browsing by Author "Wu, Z."
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Item Chemical Doping and High-Pressure Studies of Layered β-PdBi₂ Single Crystals(American Physical Society, 2015-11-04) Zhao, K.; Lv, Bing; Xue, Y. -Y; Zhu, X. -Y; Deng, L. Z.; Wu, Z.; Chu, C. W.; Lv, BingWe have systematically grown large single crystals of the layered compounds β-PdBi₂, and both the hole-doped PdBi₂₋ₓPb₂ and the electron-doped NaₓPdBi₂, and studied their magnetic and transport properties. Hall effect measurements on PdBi₂, PdBi₁₈Pb₀₂, and Na₀.₀₅₇PdBi₂ show that the charge transport is dominated by electrons in all of the samples. The electron concentration is substantially reduced upon Pb doping in PdBi₂₋ₓPbₓ and increased upon Na intercalation in NaₓPdBi₂, indicating effective hole doping by Pb and electron doping by Na. We observed a monotonic decrease of the superconducting transition temperature (T_c) from 5.4 K in undoped PdBi₂ to less than 2 K for x > 0.35 in hole-doped PdBi₂₋ₓPbₓ. Meanwhile, a rapid decrease of T_c with Na intercalation is also observed in the electron-doped NaₓPdBi₂, which is in disagreement with the theoretical expectation. In addition, both the magnetoresistance and Hall resistance further reveal evidence for a possible spin excitation associated with Fermi surface reconstruction at ∼50 K in the Na-intercalated PdBi₂ sample. The complete phase diagram is thus established from hole doping to electron doping. Meanwhile, a high-pressure study of the undoped PdBi₂ shows that the T_c is linearly suppressed under pressure with a dT_c/dP coefficient of -0.28 K/GPa.Item Evidence for Defect-Induced Superconductivity up to 49 K in (Ca₁₋ₓ)Fe₂As₂(Amer Physical Soc, 2016-02-12) Deng, L. Z.; Lv, Bing; Zhao, K.; Wei, F. Y.; Xue, Y. Y.; Wu, Z.; Chu, C. W.; Lv, BingTo explore the origin of the unusual nonbulk superconductivity with a T_c up to 49 K reported in the rare-earth-doped CaFe₂As₂, the chemical composition, magnetization, specific heat, resistivity, and annealing effect are systematically investigated on nominal Ca₁₋ₓ Rₓ)Fe₂As₂ single crystals with different x and R = La, Ce, Pr, and Nd. All display a doping-independent T_c once superconductivity is induced, a doping-dependent low field superconducting volume fraction ʄ, and a large magnetic anisotropy η the superconducting state, suggesting a rather inhomogeneous superconducting state in an otherwise microscale homogenous superconductor. The wavelength dispersive spectroscopy and specific heat show the presence of defects that are closely related to ʄ, regardless of the R involved. The magnetism further reveals that the defects are mainly superparamagnetic clusters for R = Ce, Pr, and Nd with strong intercluster interactions, implying that defects are locally self-organized. Annealing at 500 ⁰C, without varying the doping level x, suppresses ʄ profoundly but not the T_c. The above observations provide evidence for the crucial role of defects in the occurrence of the unusually high T_c ~ to 49K in Ca₁₋ₓ Rₓ Fe₂As₂ and are consistent with the interface-enhanced superconductivity recently proposed.Item Low-Temperature Microstructural Studies on Superconducting CaFe₂As₂(Nature Publishing Group, 2019-04-23) Huyan, S.; Deng, L. Z.; Wu, Z.; Zhao, K.; Sun, J. Y.; Wu, L. J.; Zhao, Y. Y.; Yuan, H. M.; Gooch, M.; Lv, Bing; Zhu, Y.; Chen, S.; Chu, C. W.; 0000-0002-9491-5177 (Lv, B); Lv, BingUndoped CaFe₂As₂ (Ca122) can be stabilized in two slightly different non-superconducting tetragonal phases, PI and PII, through thermal treatments. Upon proper annealing, superconductivity with a T_{c} up to 25 K emerges in the samples with an admixture of PI and PII phases. Systematic low-temperature X-ray diffraction studies were conducted on undoped Ca122 samples annealed at 350 °C over different time periods. In addition to the diffraction peaks associated with the single-phase aggregation of PI and PII, a broad intermediate peak that shifts with annealing time was observed in the superconducting samples only. Our simulation of phase distribution suggests that the extra peak is associated with the admixture of PI and PII on the nanometer scale. High-resolution transmission electron microscopy confirms the existence of these nano-scale phase admixtures in the superconducting samples. These experimental results and simulation analyses lend further support for our conclusion that interfacial inducement is the most reasonable explanation for the emergence of superconductivity in undoped Ca122 single crystals. ©2019, The Author(s).Item Odd-Integer Quantum Hall States and Giant Spin Susceptibility in P -Type Few-Layer WSe₂(American Physical Society, 2017-02-10) Xu, S.; Shen, J.; Long, G.; Wu, Z.; Bao, Zhi-qiang; Liu, Cheng-Cheng; Xiao, X.; Han, T.; Lin, J.; Wu, Y.; Lu, H.; Hou, J.; An, L.; Wang, Y.; Cai, Y.; Ho, K. M.; He, Y.; Lortz, R.; Zhang, Fan; Wang, N.; Bao, Zhi-qiang; Liu, Cheng-Cheng; Zhang, FanWe fabricate high-mobility p-type few-layer WSe₂ field-effect transistors and surprisingly observe a series of quantum Hall (QH) states following an unconventional sequence predominated by odd-integer states under a moderate strength magnetic field. By tilting the magnetic field, we discover Landau level crossing effects at ultralow coincident angles, revealing that the Zeeman energy is about 3 times as large as the cyclotron energy near the valence band top at the Γ valley. This result implies the significant roles played by the exchange interactions in p-type few-layer WSe₂, in which itinerant or QH ferromagnetism likely occurs. Evidently, the Γ valley of few-layer WSe₂ offers a unique platform with unusually heavy hole carriers and a substantially enhanced g factor for exploring strongly correlated phenomena.