SCPMA出版最新非常规超导体和Weyl半金属专题

　Science China-Physics Mechanics Astronomy第5期出版“New Unconventional Superconductors and 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. 

Yi Liu, Jin-Ke Bao, Hao-Kun Zuo, Abduweli Ablimit, 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 important probe for the nature of superconducting pairing state. Here we report the impurity 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-purity Cr metal (>99.99%) have an electron mean free path much larger than the superconducting coherence length. For the crystals prepared using impure Cr that contains various non-magnetic impurities, however, the T_c decreases significantly, in accordance with the generalized Abrikosov-Gor’kov pair-breaking theory. This finding supports a non-s-wave superconductivity in K₂Cr₃As₃.

2.

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

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

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

3. 

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

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

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

4. 

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 superconductor Ca₁₀(Pt_4–δAs₈)((Fe_1–xPt_x)₂As₂)₅ single crystal (“10-4-8”, T_c=22 K) was measured down to 80 mK. In a zero field, the residual linear term κ₀/T  is negligible, suggesting the nodeless superconducting gaps in this multiband compound. In the magnetic fields, κ₀/T  increases rapidly, which mimics the multiband superconductor NbSe₂ and LuNi₂B₂C with highly anisotropic gap. Such a field dependence of κ₀/T  is an evidence for the multiple superconducting gaps with quite different magnitudes or highly anisotropic gap. Compared with the London penetration depth results of 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 superconducting gap structure.

5. 

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)

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