Superconductivity In Layered Transition Metal Chalcogenides

Searching for new superconductors and understanding the mechanism of superconductivity is one of the forefront subjects in condensed matter physics.Transition metal chalcogenides(TM-Cs)have enormous members and abundant physical properties,and and this system exist many superconductors.Therefore it is a good system to search new superconductors and explore the relationship of various ordered states.This dissertation mainly studies two families of TMCs:layered transition metal dichalcogenides(TMDCs)and misfit layered compounds(MLCs),and meticulously investigates the structures and properties of three newly discovered superconductors in these two families:4Ha-Ta1.03Se2 and(SnSe)1.16(NbSe2)n(=1,2).This dissertation consists of five chapters.Chapter 1 is the detailed review of the structures and superconductivity in TMDCs and MLCs,Chapter2 introduces the mainly experimental meth-ods.Chapter3 to Chapter5 are the main works of this dissertation.The main original achievements we acquired are as the following:(1)We successfully synthesized single crystals of 4Ha-Ta1.03Se2 by chemical vapor trans-portation(CVT)with I2 as the transport agent,and first discovered superconductivity with an onset Tc of 2.7 K in this poly type of TaSe2.A charge density wave(CDW)transition is observed around 106 K.A kind of special stacking faults is revealed,which may increase the anisotropy.The ?0Hc2ab(0)/Tc=4.48 is larger than other polytypes,and the estimated Hc2ab(0)is 2.44 times of the Pauli paramagnetic limit,which may result from enhanced SOC.?e-ph=0.69,indicating that it is still in the intermediate coupling range.On the other hand,N(EF)is only 1.53 state/eV f.u.,smaller than that of other doped or intercalated TaSe2,but Tc is almost the same as them.This work indicates that the stack mode of 4Ha polytype could benefit to superconductivity and Hc2 for TaSe2.This is the first detailed report on the physical properties of TaSe2 with 4Ha polytype.(2)We successfully synthesized large single crystals of(SnSe)1.16(NbSe2)by CVT with S-nCl2 as the transport agant,and discovered superconductivity with an onset Tc of 3.4 K.Super-conducting shielding volume fraction is close to 100%,confirming the bulk superconductivity.Hc2(0)is estimated to be 4.53 T and 7.82 T for the out-plane and in-plane directions respectively.Especially for the in-plane direction,the value of Hc2(0)exceeding the Pauli limit Hp slightly.The electron-phonon coupling constant ?e-ph=0.80.Both the specific-heat and Hc2 data suggest that(SnSe)1.16(NbSe2)should be a multi-band BCS superconductor.(3)We found a new MLC with structure of n=2:(SnSe)1.16(NbSe2)2,successfully synthe-sized the single crystals by CVT with SnBr2 as the transport agant,and discovered superconduc-tivity with an onset Tc of 4.9 K.?e-ph?0.96,very close to strong coupling range.Comparing with(SnSe)1.16(NbSe2),the enhanced Tc could be mainly due to the enhanced ?e-ph and the reduced change transfer between layers.Hc2(0)is estimated to be 3.4 T and 15.5 T for the out-plane and in-plane directions respectively.Hc2ab(0)/Hp=1.74,which is a large value in MLCs supercon-ductor.The enhancement of Hc2ab may be related to both multi-band effect and the enhanced 2D characteristic.