ScienceChina-Physics Mechanics Astronomy第5期出版“New Unconventional Superconductorsand Weyl Semimetal”专题，南京大学闻海虎教授带您走进最新的超导体研究。

　　非常规超导电性和拓扑量子现象是当前凝聚态物理的两个前沿方向，本期专题涵盖了这两个方向的最新进展。1911年，荷兰的Kamerling Onnes研究组在Hg中发现超导电性，但直到1957年Bardeen-Cooper-Schrieffer （BCS）提出电子-声子耦合机制，超导电性才正式揭开神秘的面纱。

　　BCS理论认为费米面附近的两个电子，如果动量大小相等，方向相反，且自旋相反，就可通过交换声子而形成束缚态，这样的两个电子称为库伯电子对。库伯电子对会凝聚成低能态，宏观上表现为相位相干和超导态的出现。

 　　但是，BCS理论具有一定的局限性，难以解释很多非常规的超导体机制，比如铜氧化物、铁砷族和铁硒族化合物超导体，等等。

　　本专题包含4篇新型非常规超导体（K₂Cr₃As₃、高压下的MnP、(Li_1–xFe_x)OHFeSe和Ca₁₀(Pt_4–δAs₈)((Fe_1–xPt_x)₂As₂)₅）以及1篇Weyl半金属（TaP）的研究论文，多角度揭示超导奥秘：

1. Effect of impurity scattering onsuperconductivity in K₂Cr₃As₃

Yi Liu, Jin-Ke Bao, Hao-Kun Zuo, AbduweliAblimit, Zhang-Tu Tang, Chun-Mu Feng, Zeng-Wei Zhu, and Guang-Han Cao

Sci. China-Phys. Mech. Astron. 59, 657402 (2016)

Impurity scattering in a superconductor may serve as an importantprobe for the nature of superconducting pairing state. Here we report theimpurity effect on superconducting transition temperature T_c in the newly discovered Cr-based superconductor K₂Cr₃As₃.The resistivity measurements show that the crystals prepared using high-purityCr metal (>99.99%) have an electron mean free path much larger than thesuperconducting coherence length. For the crystals prepared using impure Crthat contains various non-magnetic impurities, however, the T_c decreases significantly,in accordance with the generalized Abrikosov-Gor’kov pair-breaking theory. Thisfinding supports a non-s-wavesuperconductivity in K₂Cr₃As₃.

2.³¹P NMR study of magnetic phasetransitions of MnP single crystal under 2 GPa pressure

GuoZhi Fan, Bo Zhao, Wei Wu, Ping Zheng, andJianLin Luo

Sci. China-Phys. Mech. Astron. 59, 657403 (2016)

Superconductivity on the border of thelong-range magnetic order has been discovered in MnP under high pressures. Inorder to investigate the nature of the magnetic properties adjacent to thesuperconducting state, we performed zero-field ³¹P NMR for MnPsingle crystal under ambient and hydrostatic pressure of 2 GPa, respectively.Radio frequency power level was used to determine whether NMR signal originatesfrom a helical state or not. When 2 GPa pressure was applied, the signal fromhelical state exists even above 160 K, while that from the ferromagnetic phasewas not observed. Our NMR results indicate that the magnetic phase which isadjacent to the superconducting state is in a helical magnetic structure.

3. Robust superconductivity andtransport properties in (Li_1–xFe_x)OHFeSe single crystals

Hai Lin, Jie Xing, XiYu Zhu, Huan Yang, andHai-Hu Wen

Sci. China-Phys. Mech. Astron. 59, 657404 (2016)

The recently discovered (Li_1–xFe_x)OHFeSesuperconductor with T_cabout 40 K provides a good platform for investigating the magnetization andelectrical transport properties of FeSe-based superconductors. By using ahydrothermal ion-exchange method, we have successfully grown crystals of (Li_1–xFe_x)OHFeSe. X-ray diffraction on the sample shows thesingle crystalline PbO-type structure with the c-axis preferential orientation.Magnetic susceptibility and resistive measurements show an onsetsuperconducting transition at around T_c=38.3K. Using the magnetization hysteresis loops and Bean critical state model, alarge critical current J_sis observed in low temperature region. The critical current density issuppressed exponentially with increasing magnetic field. Temperaturedependencies of resistivity under various currents and fields are measured,revealing a robust superconducting current density and bulk superconductivity.

4. Nodeless superconducting gaps in Ca₁₀(Pt_4–δAs₈)((Fe_1–xPt_x)₂As₂)₅ probed byquasiparticle heat transport

Xun Qiu, LanPo He, XiaoChen Hong, Zhen Zhang,Jian Pan, XiaoPing Shen, DongLai Feng, and ShiYan Li

Sci. China-Phys. Mech. Astron. 59, 657405 (2016)

The in-plane thermal conductivity of the iron-based superconductorCa₁₀(Pt_4–δAs₈)((Fe_1–xPt_x)₂As₂)₅ single crystal(“10-4-8”, T_c=22 K) wasmeasured down to 80 mK. In a zero field, the residual linear term κ₀/T is negligible, suggesting the nodeless superconducting gaps inthis multiband compound. In the magnetic fields, κ₀/T increasesrapidly, which mimics the multiband superconductor NbSe₂ and LuNi₂B₂Cwith highly anisotropic gap. Such a field dependence of κ₀/T is anevidence for the multiple superconducting gaps with quite different magnitudesor highly anisotropic gap. Compared with the London penetration depth resultsof the Ca₁₀(Pt_4–δAs₈)((Fe_1–xPt_x)₂As₂)₅ (“10-3-8”)compound, the 10-4-8 and 10-3-8 compounds may have a similar superconductinggap structure.

5. Large unsaturated positive andnegative magnetoresistance in Weyl semimetal TaP

JianHua Du, HangDong Wang, Qing Chen, QianHui Mao, RajwaliKhan, BinJie Xu, YuXing Zhou, YanNan Zhang, JinHu Yang, Bin Chen, ChunMu Feng,and MingHu Fang

Sci. China-Phys. Mech. Astron. 59, 657406 (2016)

Aftersuccessfully growing single-crystal TaP, we measured its longitudinalresistivity (ρ_xx) and Hallresistivity (ρ_yx) atmagnetic fields up to 9 T in the temperature range of 2-300 K. At 8 T, themagnetoresistance (MR) reached 3.28×10⁵% at 2 K, 176% at 300 K.Neither value appeared saturated. We confirmed that TaP is a hole-electroncompensated semimetal with a low carrier concentration and high hole mobilityof μ_h=3.71×10⁵cm²/V s, and found that a magnetic-field-induced metal-insulatortransition occurs at room temperature. Remarkably, because a magnetic field (H) was applied in parallel to theelectric field (E), a negative MR dueto a chiral anomaly was observed and reached –3000%at 9 T without any sign of saturation, either, which is in contrast to otherWeyl semimetals (WSMs). The analysis of the Shubnikov-de Haas (SdH)oscillations superimposed on the MR revealed that a nontrivial Berry’s phasewith a strong offset of 0.3958, which is the characteristic feature of chargecarriers enclosing a Weyl node. These results indicate that TaP is a promisingcandidate not only for revealing fundamental physics of the WSM state but alsofor some novel applications.

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