Study On Superconductivity Of Transition-metal Chalcogenides

The research of superconductivity has always been one of the most noteworthy topics in the condensed matter physics for the vast potential applications and huge scientific significance of superconductors. No matter high-Tc cuprates or iron based superconductors, they all have a layered structure. It is an effective way to search new superconductors in layered compounds. In transition-metal chalcogenides there are very rich layered compounds which provide us a perfect platform to search superconductivity and investigate the interplay of superconductivity and other physical properties. In this thesis we mainly studied the superconductivity in several transition-metal chalcogenides. We found that the superconductivity in these compounds can be tuned by doping or element intercalation. This thesis can be divided into four chapters. And the main contents of each chapter are as follows.In chapter one, we introduced three different superconducting systems, including BiS2 based superconductors, superconductivity induced by application of pressure or element intercalation in topological insulators and superconductivity in transition-metal dichalcogenides.In chapter two, the effects of bivalent Pb doping and trivalent Sm doping on the superconductivity of Bi4O4S3 superconductor have been investigated by a systematical study on the crystal structure, magnetic and transport properties of Bi4-xSmxO4S3 (0≤ x≤0.16) and Bi4-xPbxO4S3 (0≤x≤0.1) samples. It is found that the bivalent Pb doping leads to more severe depression on the superconductivity comparing to the isovalent Sm doping, as the superconducting transition temperature and upper critical field decreases more severely in Pb doped samples. We notice that the Sm 6p orbitals make no contribution to the electron conduction, which means that Sm dopants act as the impurity scatterers. The substitution of Bi by Pb introduces hole-type charge carriers, which give rise to the significantly decrease of electron-type charge carriers. Our results might be helpful in better understanding of the superconducting properties of Bi4O4S3.In chapter three, our research is motivated by the difficulty encountered in searching topological superconductors. Currently research on the intercalation of Bi2Se3 is limited to CuxBi2Se3, and doping in Bi2Se3 is limited to Cu, Cr and Ca. There is no final confirmation about whether CuxBi2Se3 is topological superconductor or not. And the shielding volume fraction in CuxBi2Se3 is only 40%. so there is disagreement about the bulk superconductivity in CuxBi2Se3. In order to search new possible topological superconductors, we intercalated various metal elements in Bi2Se3 and found that Sr intercalation and Nb intercalation can successfully induce superconductivity in Bi2Se3.We obtained high quality SrxBi2Se3 single crystals which shows a superconducting transition at 2.5 K by intercalation of Sr in Bi2Se3 through solid state reaction method. Its shielding volume fraction reaches up to 91.5% and remains unchanged after been exposed in air for four months. Quantum oscillation experiments under high magnetic fields give evidence of the existence of topological surface states. SrxBi2Se3 provide a stable material base with a high shielding volume fraction. Besides SrxBi2Se3 we also found that superconductivity can be induced by Nb intercalation in Bi2Se3. The Nb0.25Bi2Se3 single crystal shows a superconducting transition at 3.2 K with shielding volume fraction of 20%. With increasing Se substituted by Te, the superconducting transition temperature and shielding volume fraction reduces significantly. We deduce that our Nb0.25Bi2Se3 samples is inhomogeneous. The areas with good morphology is not superconducting as Nb is not incorporated into the lattice. But the Nb rich areas is superconducting which shows a polyporous morphology. The crystallinity of Nbo.25Bi2Se3 is improved by Te doping. The obtained samples are mainly flat single crystals and the polyporous areas is reduced.In chapter four, we have successfully synthesized superconducting CuxTaSe2(x=0.05,0.15) and Cu0.15TaSe2-xSx(x=0,0.5,1,1.5) single crystals through a chemical vapor deposition method. The Tc of 2H-TaSe2 can be enhanced by 17 times through copper intercalation. The increase of S concentration caused a further enhancement of superconductivity and a decline of shielding volume fraction in Cu0.15TaSe2-xSx. The greatly enhancement of superconductivity by simultaneous intercalation of copper and the substitution of Se by S in the Cu0.15TaSe2-xSx compound is probably due to the combination effects of the suppression of the CDW state, an increase of coherence length and the suppression of superconducting fluctuation.