Superconducting Properties Of Alkali-metal Intercalated Para-terphenyl And Organobismuth Compounds

The search for new superconducting compounds with high critical transition temperatures(T_c)has long been the very heart of scientific research in the fields of condensed matter physics and materials science.After more than one hundred years of development,lots of superconducting materials have been discovered,including:elements and alloys,hydrogen-rich systems,transition-metal dichalcogenides,copper oxides,iron pnictides and chalcogenides,and organic superconductors,etc.Organic superconductors usually possess low-dimensional structures,abundant magnetic transitions,and novel ground states.Thus,organic materials are ideal platforms to understand the interplay of electron-electron and electron-lattice interactions,as well as the novel quantum effects in low-dimensional systems.Organic materials are believed to be potential candidates of superconductors with high T_c.In this thesis,alkali-metal(potassium,rubidium)doped para-terphenyl and organobismuth compounds are synthesized by solid phase reaction.The crystal structures and superconducting properties of studied samples are characterized by X-ray diffraction,Raman spectra,magnetic susceptibility and resistivity measurements.The main research works are as follows:1.By doping potassium into p-terphenyl consisting of C and H elements with three phenyl rings connected by single C-C bond in para position,we find that this material can have several superconducting phases with T_c￠s ranging from 4 K to 123 K.Evidence for superconductivity is provided from the dc and ac magnetic susceptibility,resistivity,and Raman spectroscopy measurements.X-ray diffraction patterns and Raman scattering spectra reveal the most probable crystal structure and composition of these phases.The differences of both the carrier concentration and electron-phonon interaction strength are proposed to account for the T_c variation among the synthesized compounds.2.We develop a two-step synthesis method—ultrasound treatment and low temperature annealing to explore superconductivity in potassium-doped triphenylbismuth(TPB), which is composed of one bismuth atom and three phenyl rings.The combination of dc and ac magnetic measurements reveals that one hundred percent of synthesized samples exhibit superconductivity at 3.5 K and/or 7.2 K at ambient pressure.The magnetization hysteresis loops provide a strong piece of evidence of type-II superconductors.It is found that the doped materials crystallize into the triclinic P1 structure,with a mole ratio of 4:1 between potassium and triphenylbismuth.Both the calculated electronic structure and measured Raman spectra indicate that superconductivity is realized by transferring electrons from the K-4s to C-2p orbital.3.Rubidium(Rb)intercalated triphenylbismuth,tri-p-tolylbismuthine(p-TTB),and tri-o-tolylbismuthine(o-TTB)are grown by the two-step synthesis method.For Rb-doped TPB samples,a spin canted magnetic ordering with an irreversible temperature at around 35 K is observed in some synthesized samples,and the other samples show a coexistence of such magnetic ordering and superconductivity with T_c～4 K.Combined with the XRD pattern and theoretical calculation of Rb-doped TPB,it is identified that the 4 K superconducting phase in studied samples crystallizes into the triclinic P1 structure,with a mole ratio of 3:1 between rubidium and triphenylbismuth.Moreover,the calculated electronic structure indicates that the superconductivity originates from the charge transfer effect.For Rb-doped p-TTB and o-TTB,the spontaneous magnetic order at 35 K is always in coexistence with a superconducting phase(T_c～4 K)in all studied samples.Moreover,compared to the Rb-doped TPB,the superconducting shielding fraction is improved by one order of magnitude in Rb intercalated p-TTB and o-TTB.There exists a possible competition between the magnetic ordering and the superconducting phase based on the magnetic study of Rb-doped p-TTB.In addition,both the high-pressure Raman spectra and resistivity measurements show a pressure-induced second superconducting phase with T_c～3.5 K at 1.5 GPa for Rb-doped p-TTB.Our research works open two new electron-acceptor families in organic superconductors:metal-doped aromatic hydrocarbons with phenyl rings connected by single C-C bond and alkali metal-doped organobismuth compounds.Our results are very helpful to understand the structure,magnetism,electrical transport,and optical properties,as well as high-pressure behavior of organic superconductors,particularly alkali-metal doped aromatic hydrocarbon superconductors.