Archives: Superconductivity

Evidences for pressure-induced two-phase superconductivity and mixed structures of NiTe2 and NiTe in type-II Dirac semimetal NiTe2-x (x = 0.38 ± 0.09) single crystals

Bulk NiTe2 is a type-II Dirac semimetal with non-trivial Berry phases associated with the Dirac fermions. Theory suggests that monolayer NiTe2 is a two-gap superconductor, whereas experimental investigation of bulk NiTe1.98 for pressures (P) up to 71.2 GPa do not reveal any superconductivity. Here we report experimental evidences for pressure-induced two-phase superconductivity as well as mixed structures of NiTe2 and NiTe in Te-deficient NiTe2-x (x = 0.38 ± 0.09) single crystals. Hole-dominant multi-band superconductivity with the P3¯m1 hexagonal-symmetry structure of NiTe2 appears at P ≥ 0.5 GPa, whereas electron-dominant single-band superconductivity with the P2/m monoclinic-symmetry structure of NiTe emerges at 14.5 GPa < P < 18.4 GPa. The coexistence of hexagonal and monoclinic structures and two-phase superconductivity is accompanied by a zero Hall coefficient up to ∼ 40 GPa, and the second superconducting phase prevails above 40 GPa, reaching a maximum Tc = 7.8 K and persisting up to 52.8 GPa. Our findings suggest the critical role of Te-vacancies in the occurrence of superconductivity and potentially nontrivial topological properties in NiTe2-x.

https://authors.elsevier.com/a/1cTXE926gyzxH0

Mn-doping induced ferromagnetism and enhanced superconductivity in Bi4−xMnxO4S3 (0.075 ≤ x ≤ 0.15) [Zhenjie Feng et al., Phys. Rev. B 94, 064522 (2016)]

The physical mechanism for high-temperature superconductivity remains one of the most challenging issues in modern condensed matter physics. Interestingly, one of the commonalities among high-temperature superconducting cuprates and iron-based compounds is their layered structures. Even among conventional superconductors, the highest superconducting transition temperature (Tc) has been found in layered magnesium diboride MgB2. Recently, superconductivity with Tc = 4.5 K was discovered in a new superconductor Bi4O4S3. This compound has a layered structure composed of two superconducting BiS2 layers and spacer layers of Bi4O4(SO4)1x, where x indicates the deficiency of (SO4)2 ions at the interlayer sites. Since the discovery of Bi4O4S3, several other BiS2-based superconductors LnO1xF xBiS2 (Ln = La, Ce, Pr, Nd) with the highest Tc ~ 10.6 K have been reported. Both experimental and theoretical studies to date have indicated that the BiS2 layers play the role of the superconducting planes in these sulfide superconductors, similar to the CuO2 planes in the cuprate superconductors and the Fe2An2 (An = P, As, Se, Te) layers in the iron-based superconductors. A major challenge facing this new class of layered superconductors is to optimize Tc by exploring different spacer layers. Additionally, the effects of doping by either nonmagnetic or magnetic elements are important issues for investigation.

In collaboration with Professor Zhenjie Feng’s group at the Shanghai University in China, we demonstrate in this latest publication that Mn-doping in the layered sulfides Bi4O4S3 leads to stable Bi4xMnxO4S3 compounds that exhibit both long-range ferromagnetism and enhanced superconductivity for 0.075  x  0.15, with a record Tc ~ 15 K amongst all BiS2-based superconductors. Based on our experimental investigations of Bi4xMnxO4S3 and comparative studies of related compounds Bi4xCoxO4S3 and Bi4xNixO4S3, we suggest that that the coexistence of superconductivity and ferromagnetism may be attributed to Mn-doping in the spacer Bi2O2 layers away from the superconducting BiS2 layers, whereas the enhancement of Tc may be due to excess electron transfer to BiS2 from the Mn4+/Mn3+-substitutions in Bi2O2. These findings therefore shed new light on feasible pathways to enhance the Tc values of BiS2-based superconductors.
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Mn-doping induced ferromagnetism and enhanced superconductivity in Bi4−xMnxO4S3 (0.075 ≤ x ≤ 0.15)

Mn-doping induced ferromagnetism and enhanced superconductivity in Bi4−xMnxO4S3 (0.075 ≤ x ≤ 0.15) Zhenjie Feng, Xunqing Yin, Yiming Cao, Xianglian Peng, Tian Gao, Chuan Yu, Jingzhe Chen, Baojuan Kang, Bo Lu, Juan Guo, Qing Li, Wei-Shiuan Tseng, Zhongquan Ma, Chao Jing, Shixun Cao, Jincang Zhang, and N.-C. Yeh Phys. Rev. B 94, 064522 (2016). arXiv:1608.04410
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.94.064522

Measurement of a sign-changing two-gap superconducting phase in electron-doped Ba(Fe_{1-x}Co_x)_2As_2 single crystals using scanning tunneling spectroscopy

“Measurement of a sign-changing two-gap superconducting phase in electron-doped Ba(Fe_{1-x}Co_x)_2As_2 single crystals using scanning tunneling spectroscopy” M. L. Teague, G. K. Drayna, G. P. Lockhart, P. Cheng, B. Shen, H.-H. Wen and N.-C. Yeh, Phys. Rev. Lett. 106, 087004 (2011). [arXiv:1007.5086]
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.106.087004

Spectroscopic evidence for competing order-induced pseudogap phenomena and unconventional low-energy excitations in high-Tc cuprate superconductors

“Spectroscopic evidence for competing order-induced pseudogap phenomena and unconventional low-energy excitations in high-Tc cuprate superconductors” N.-C. Yeh, A. D. Beyer, M. L. Teague, S.-P. Lee, S. Tajima and S. I. Lee. Journal of Superconductivity and Novel Magnetism 23, 757 (2010). [Manuscript received the Scientific & Technological Accomplishment Papers Award at the 7th International Conference on “New Theories, Discoveries and Applications of Superconductors and Related Materials” in Beijing, China, May 13 — 16 (2009)] [arxiv:0906.1776]
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