The strong spin-orbit coupling and structural diversity in the two-dimensional layered transition metal sulfide MX2 (M for Mo, Nb, W; X for S, Se, Te) give these materials many novel physical properties, For example, the quantum spin Hall effect was observed in WTe2 of the 1Td phase in the minority layer, and the Ising superconductivity was observed in the MoS2 and NbSe2 of the 2H phase of the minority layer. These findings make MX2 materials a hot spot in current condensed matter physics and materials science research.
Usually the upper critical magnetic field of the BCS superconductor will not exceed the Pauli paramagnetic limit, because once this limit is exceeded, the s-wave pairing between the two spin opposite electrons cannot be maintained. However, in the crystal structure of the 2H-MX2 material, the breaking of the in-plane center inversion symmetry leads to the appearance of Ising spin-orbit coupling. At this time, the spin direction of Cooper's centering electrons will be pinned out of the plane, so that the upper critical magnetic field in the plane can far exceed the Pauli limit, reaching tens or even hundreds of Tesla. This superconductivity is called Ising superconductivity, and its unique properties provide a new way for the application of superconductivity in strong magnetic fields. In addition, there are recent theories that the Ising superconductor can be used to construct Majorana fermions, providing a new experimental platform for the research of topological quantum computing.
The team of Lu Li, a researcher at the Solid State Quantum Information and Computing Laboratory of the Institute of Physics, Chinese Academy of Sciences / Beijing National Research Center for Condensed Matter Physics, has been engaged in research on topological states in recent years. Since the end of 2015, Liu Guangtong, a team member and associate researcher, began to pay attention to the use of two-dimensional transition metal sulfides to conduct topological superconductivity research. In collaboration with Zheng Liu, a professor at Nanyang Technological University in Singapore, in recent years, a series of progress has been made in the study of high-quality few-layer MX2 samples, and a "book" on the growth, morphology, structure and physical properties of this material family has been constructed. Pavilion ", related results have been published in Adv. Mat. 29, 1603471 (2017) (selected as cover article), Nat. Commun. 8, 394 (2017) and Nature 556, 355 (2018).
Recently, they have made new progress in the study of the physical properties of a few high-quality MoTe2 samples. A new Ising superconductor electrical property caused by anisotropic spin-orbit coupling was discovered in the MoTe2 sample of Td phase. Different from the found isotropic Ising superconductor, the in-plane supercritical magnetic field (Hc2, ∥) of this new superconductor exhibits significant two-degree symmetry and exceeds the Pauli limit in different directions. This is the first experimental observation of in-plane anisotropic Ising superconductivity. Theoretical calculations show that this phenomenon is due to the special lattice symmetry of the Td phase, which induces a unique spin-orbit coupling g = (gx, gy, gz). On the one hand, the mirror symmetry breaking in the upper direction of the x direction leads to the out-of-plane Ising spin-orbit coupling effect gz, which makes the researchers observe the Hc2, ∥enhancement phenomenon of the Ising superconductor discovered in the past. On the other hand, the out-of-plane mirror symmetry breaking results in the in-plane anisotropy of spin-orbit coupling (gx and gy). This is the reason why the two-degree symmetry in Hc2, ∥plane was observed experimentally. This discovery will help deepen the understanding of the novel superconducting phenomena in transition metal sulfides, and help to promote the application research of related materials superconducting spintronics devices.
This work was completed under the cooperation of many researchers. Liu Dong, a postdoctoral fellow under the guidance of Liu Zheng, prepared a high-quality few-layer MoTe2 sample; Junhao Lin, a professor at Southern University of Science and Technology, used high-resolution transmission electron microscopy (STEM) to characterize a few-layer MoTe2 sample and confirmed its structure It is the Td phase; Liu Guangtong led students Cui Jian and Li Peiling to complete the electrical transport measurement at low temperature and strong magnetic field, and found the two-dimensional superconductivity, the enhancement of the in-plane upper critical field and its two-degree symmetry; Professor Luo Jintuan (KT) Law) and student He Wenyu theoretically gave the superconducting mechanism under the action of anisotropic spin-orbit coupling.
Related research results were published online in the journal Nature Communications (DOI: 10.1038 / s41467-019-09995-0) on May 3. This work was supported by the Ministry of Science and Technology (2016YFA0300600 and 2015CB921101), the National Natural Science Foundation of China (11527806 and 11874406), the Shenzhen Science and Technology Innovation Fund (ZDSYS20170303165926217), and the Singapore National Research Fund (NRF-RF2013-08, MOE Tier 2 MOE2016-T2-2 -153, MOE2015-T2-2-007, A * Star QTE programme), Science Foundation of Japan Society for the Promotion of Science (JP16H06333, P16382), Qiu Cha Foundation and Hong Kong Research Grants Council (C6026-16W, 16309718, 16307117, 16324216, ECS26302118).
Figure 1. Experimental data of the in-plane upper critical magnetic field in a few Td-phase MoTe2 samples. a) Magnetoresistance curves of samples with different thicknesses at 0.3 K. The vertical axis is the reduced magnetoresistance (R / Rn), and the horizontal axis is the reduced in-plane magnetic field (H∥ / Hp). b) The dependence of the upper critical magnetic field (Hc2, ∥ / Hp) in the reduced plane on the thickness of the sample at 0.3 K. c) The phase diagram of reduced temperature (T / Tc) and upper critical magnetic field (Hc2 / Hp) in samples of different thicknesses.
Figure 2. The two-dimensional symmetry of the upper critical magnetic field in the plane of the minority layer Td phase MoTe2 sample. a) Magnetoresistance curves of 3nm thick MoTe2 samples at 0.3 K at different in-plane rotation angles (φ). b) The reduced in-plane upper critical magnetic field (Hc2, ∥ / Hp) at different temperatures in the in-plane rotation angle (φ) measured at 0.07 Tc, 0.35 Tc, 0.6 Tc and 0.95 Tc. c) The dependence of the reduced spin susceptibility (χS / χN) on the reduced temperature (T / Tc) in the x and y planes. d) First-principles calculation results of 1Td-MoTe2 energy band structure. e) In-plane spin texture given by theoretical calculations. f) and g) The theoretically expected phase diagram of the critical magnetic field in the anisotropic in-plane y and x directions.
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