Michael Herzinger
Interkalationsverbindungen von NbSe2 und SnSe2: Modellsysteme für niederdimensionale Supraleiter
Betreuer: Prof. Dr. Wolfgang Scherer [Chemische Physik und Materialwissenschaften]
Datum der mündlichen Prüfung: 23.07.2013
259 Seiten, deutsch
Quasi-two-dimensional (2D) metal dichalcogenides have received considerable research interest since their complex anisotropic electronic properties can be controlled by the intercalation of donor species. Although layered dichalcogenides have been studied by many aspects of chemical and physical properties, their two-dimensional character is only poorly understood. The present work deals with the layer-shaped dichalcogenides SnSe2 and NbSe2. The host-material SnSe2 was synthesized by chemical transport with Iodine as transport agent in sealed quartz ampoules. The intercalation of the semiconducting layered single crystals SnSe2 with the organometallic compound cobaltocene (CoCp2) leads to superconductivity up to T = 8K. Ex-situ intercalation studies show an intercalation-mechanism outgoing from the host material 2H-SnSe2 in a stage-2 phase which goes over in a stage-1 phase for higher intercalation degrees. In addition, SnSe2{CoCp2}x show remarkable low-temperature properties e.g. the coexistence of superconductivity and magnetism in dependence of the staging and cobaltocene-content of the material. Starting from an intercalation degree of 17%25 CoCp2 long range ordered magnetism (with increasing saturation magnetization) was observed in 18R-SnSe2{CoCp2}x. Furthermore SnSe2{CoCp2}x show an extremely sensitive superconducting pinning behavior in very small magnetic fields partially below B < 0.1mT. For investigation of these fields, a new low magnetic field extension for SQUID magnetometers (%93TinyBee%94) was taken into use. With the upgrade of the SQUID magnetometer, a significant improvement of the magnetic field accuracy better than 1%B5T will be achieved. Temperature dependent resistivity studies in various magnetic fields exhibit a highly anisotropic superconducting behavior. This anisotropy decreases with increasing CoCp2-content. A phase diagram was developed in dependence of the degree of intercalation over the whole range of intercalation between 0%25 and 33%25. For comparison of the low-temperature character of SnSe2{CoCp2}x, another layer-shaped superconductor NbSe2 was intercalated with CoCp2. The layered high-kappa s-wave superconductor 2H-NbSe2 belongs to the most prominent low-dimensional materials studied during the past fifty years. After the discovery of the high temperature superconductor MgB2, a benchmark system for multi-band superconductivity, NbSe2 experienced a renascence of research activities. Especially, since it represents a well-suited candidate for probing the multi-band model in a quasi-two-dimensional superconductor, due to the negligible vortex pinning in NbSe2 single crystals. In order to enhance the anisotropic character we intercalated high quality 2H-NbSe2 single crystals with the organometallic donor molecule cobaltocene, leading to an expansion of the lattice parameter in c direction from 12.53%C5 to 23.81%C5. While the intercalation of organic compounds (which usually act as electron donors) reduces the superconducting transition temperature Tc from 7.1K in 2H-NbSe2 to temperatures below Tc < 3K. An opposite behavior is observed in 2H-NbSe2{CoCp2}0.26 with Tc = 7.35K. Furthermore, the strong increase of the upper critical magnetic field Bc2 ≈ 18.5T in comparison to the native parent compound (Bc2(NbSe2) ≈ 14,5T) indicates a more pronounced anisotropic behavior. Resistivity, susceptibility and specific heat studies parallel and perpendicular to the NbSe2-layers of 2H-NbSe2{CoCp2}0.26 reveal a field-dependent reentrant superconductivity, also observed in the native parent NbSe2. Both intercalated materials NbSe2{CoCp2}x and SnSe2{CoCp2}x are good candidates for further theoretical investigation of the low dimensional superconductivity. The experimental results of the layered materials presented in this thesis will contribute to a better understanding of the low dimensional superconducting behavior.