The coexistence of issing superconductivity and quantum Griffith singularity has been observed in a large area monolayer niobium disselenide crystal

Recently, professor wang and JiShuai hua, an associate professor at tsinghua university, member of qi-kun xue, Chen, a professor at Beijing university of Eva, a researcher at Feng Ji researcher, hefei strong magnetic field center Tian Mingliang researcher of Chinese Academy of Sciences, Dr XiChuanYing hai-wen liu and Beijing normal university associate professor and others, by molecular beam epitaxy of expiration of the double-layer graphene 6 h – SiC (0001) substrate successful preparation of large area above (mm) atomic level high quality single layer of transition metal sulfides NbSe2 film (0.6 nm thick only), on the basis of amorphous Se protective layer for the coverage, Furthermore, the physical properties of non-in-situ electrical transport are systematically studied.
Dimensional layered transition metal sulfide (TMDs) has potential application value in nano-electronics, optoelectronics, spintronics and other fields, so it is praised as beyond graphene materials, and has become a research hotspot in the international frontier field. In previous reports, TMD flakes (transverse size of micron) were mainly obtained by mechanical stripping technology, which attracted much attention due to its simple process and economy. However, practical applications tend to be higher quality and larger area thin film single crystal materials. Therefore, the preparation of ultra-thin transition metal sulfides with high quality and macroscopic area is of great significance for future device applications.
For the field of two-dimensional superconductivity (Nat. Rev. Mater. 2,16094,2016), quantum metals, quantum Griffith singularity (QGS) and ultra-high parallel critical magnetic field behavior named after issing superconductivity have become three important topics. In 2015, for the first time, wang jian’s research group and collaborators from the quantum material Science center of Peking University observed a quantum phase transition affected by disorder in a two-dimensional superconducting system (Ga film with 3 atomic layers thick), and its dynamic critical exponential divergence at the phase transition point. This behavior is known as quantum Griffith singularity (Science 350,542,2015; Science 350,509,2015). Subsequently, the research group further confirmed the quantum Griffith singularity in LaAlO3/SrTiO3(110) interface superconductivity (PRB 94,144517,2016). Issing superconductivity refers to the fact that the spin of the superconducting cooper pair is held in place by an effective zeeman magnetic field, thus exhibiting a very strong in-plane critical magnetic field (far beyond its Pauli paramagnetic limit). Issing superconductivity has been discovered and reported in grid-regulated transition group metal sulfide MoS2 and NbSe2 wafers.
It is found that the initial superconducting critical transition temperature of the monolayer NbSe2 film exceeds 6 K and the zero resistance temperature of up to 2.40 K, which is higher than the superconducting transition temperature of the monolayer NbSe2 obtained by early mechanical stripping and the monolayer NbSe2 grown by molecular beam epitaxy. At the same time, the transport measurement results under strong magnetic field and extremely low temperature directly confirmed that the parallel characteristic critical field Bc//(T = 0) is more than 5 times of the paramagnetic limit field, which conforms to the Zeeman protected issing superconductivity mechanism (the previous evidence of issing superconductivity in NbSe2 sheet requires the theoretical fitting of experimental data for extrapolation under lower temperature and higher magnetic field). In addition, the measurement of electrical transport under the extremely low temperature vertical magnetic field indicates that the quantum critical point of the monolayer NbSe2 film near absolute zero exhibits the quantum Griffith singularity. This is the first time both issing superconductivity and quantum Griffith singularity have been observed in the same system.
This work was supported by the national major scientific research program, the national natural science foundation of China, the China higher education doctoral research foundation, the open research foundation of the state key laboratory of low-dimensional quantum physics of tsinghua university, the project of the open pulsed strong magnetic field of huazhong university of science and technology, and the science foundation of China university of petroleum (Beijing).
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