Intercalation induced superconductivity in MoS2 , black phosphorus and Bi2Se3

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Abstract

Intercalation is known to be an efficient method for tuning the band structure of layered materials to bring out superconductivity, without significantly altering the crystal structure of the host material. Graphite intercalation compounds and intercalated transitional metal dichalcogenides (TMDs) are two most studied representatives. This thesis presents an experimental study of several new superconductors obtained by intercalation of layered materials, including MoS2, black phosphorus and a topological insulator Bi2Se3. Polymorphism is an essential feature of MoS2. While, superconductivity in doped 2H-MoS2 has been extensively studied. Superconductivity in its 1T and 1T' counterparts has been neither observed, nor even predicted theoretically. In this thesis, we have investigated potassium (K)-intercalated MoS2 and found that doping with K induces both structural and superconducting phase transitions. We demonstrate that all three phases of MoS2 - 2H, 1T and 1T'- become superconducting as a result of intercalation, with critical temperature Tc of 6.9 K, 2.8 K and 4.6 K, respectively. Black phosphorus has been 'rediscovered' in the last few years due to its layered structure and unique electronic properties. This thesis describes successful intercalation of black phosphorus with several alkali metals (Li, K, Rb, Cs) and alkaline earth metal Ca, with all five compounds showing superconductivity. Importantly, and very unexpectedly, the found superconductivity of intercalated black phosphorus is independent of the intercalant, with all five compounds having exactly the same superconducting characteristics (Tc, critical fields, anisotropy). We suggest that the superconductivity is due to heavily doped phosphorene layers, with alkali metal atoms acting mainly as charge reservoirs. Superconducting topological insulators, such as Bi2Se3, are regarded as the most promising candidates for topological superconductivity. However, the nature of superconductivity in doped Bi2Se3, such as CuxBi2Se3, SrxBi2Se3 and NbxBi2Se3, remains controversial and so far no convincing evidence of topological superconductivity has been reported for these materials. In this thesis, we report superconductivity in a new family of superconductors derived from Bi2Se3, by intercalation with K, Rb and Cs metals. All three superconductors exhibit qualitatively identical but highly anomalous behaviour of magnetisation, with several new features consistent with the properties of topological superconductors. Specifically, the new materials exhibit a highly unusual extra diamagnetic screening in the Meissner state and two coexisting superconducting phase, including surface superconductivity that we attribute to heavily doped surface states of the original topological insulator (Bi2Se3). This work provides a new platform in the study of the interplay between the topological and superconducting orders. In conclusion, superconductivity has been induced in MoS2, black phosphorus and Bi2Se3 through alkali or alkaline earth metal intercalation. The study of these new superconducting materials has been summarised in the thesis for the degree of Doctor of Philosophy at the University of Manchester, titled "Intercalation induced superconductivity in MoS2, black phosphorus and Bi2Se3". Renyan Zhang, June 10, 2017

Keyword(s)

Bi2Se3; MoS2; black phosphorus; intercalation; superconductivity

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