| Searching for new high-temperature superconductors is always one of the important topic-s in condensed matter physics.And so far,the known superconductors fall into two categories:one is the conventional superconductors with a phonon-mediated.In 2001,the discovery of su-perconductivity(SC)at 39 K in MgB2,and more recently,the realization of room-temperature SC at high pressure in the hydrogen-containing H3S,LaH10,and C-H-S compounds further motivate the research on the electronic structure and SC of the compounds containing light elements,such as H or B.The other is unconventional high-temperature superconductors,such as copper oxides and iron-arsenic(selenium)compounds.They have a common characteristic:for an antiferromag-netic parent,in which the magnetic order is suppressed by applying pressure or using a chemical doping(introducing charge carriers),at a certain critical pressure or a doping concentration,the system exhibits high-temperature SC,suggesting that the superconducting pairing may originate from spin fluctuations.Based on these progresses,three kinds of compounds were chosen in my thesis:Mo5GeB2 containing the light element B,La0.85Co1.68As2 with the same structure as Fe-based superconductors and low dimensional Cr2Te3 containing magnetic Cr(3d4)atoms,their SC and critical behaviors of ferromagnetic transition were studied systematically respectively.The following results were obtained:(1):Based on the successful synthesis of Mo5GeB2 single phase sample,by combination of the calculation of electronic structure,and the observations of resistivity,magnetization and spe-cific heat,it was found that Mo5GeB2 is a type ? superconductor with a critical temperature of Tc=5.4 K,a lower critical field Hc1=207 Oe,an upper critical field Hc2=5112 Oe,a Ginzburg-Landau(GL)parameter kGL(0)=4.2,an electron-phonon coupling constant ?ph=0.57 and a specif-ic heat jump ?Cel/?nTc?1.33,indicating that it is a weak-coupling,single-gap phonon-mediated superconductor.The results of electronic band calculation show that the density of states(N(EF))at Fermi level(EF)in Mo5GeB2 is similar to that of the isostructural Mo5SiB2(Tc=5.8 K),but smaller than that of Mo5PB2(Tc=9.2 K),further confirming that the superconducting critical tem-perature Tc in this system is determined by N(EF).It is worth pointing out that the electronic band near the EF mainly originates from the 4d orbital of Mo rather than the 2p orbital of B,which may be the reason why this system does not achieve the high-temperature SC as in MgB2.(2):Based on the successful growth of La0.85Co1.68As2 single crystal,by the systematic obser-vations of structure and magnetization,it was found that La0.85Co1.68As2 is an itinerant ferromagnet with Curie temperature Tc=99 K.The magnetization data near Tc are carefully analyzed by using the scaling law theory and found that the 3D-Heisenberg model can describe the critical behavior of the second order phase transition very well,indicating that the long range ferromagnetic order is of three dimensional characteristics,and its exchange interaction is characterized by a short range.(3):Cr2Te3 single crystal was grown by a self-flux method,by the systematic observations of structure,composition analysis,as well as resistivity,magnetization,it was found that Cr2Te3 is an itinerant ferromagnet with Curie temperature Tc=172 K.The magnetization data near Tc are carefully analyzed by using the scaling law theory as well and found that 3D-Ising model can describe the critical behavior of the second order phase transition very well.These results are helpful for the understanding of the SC in the B-containing compounds and the magnetic properties of compounds containing Co(3d7)and Cr(3d4),providing meaningful information for further searching for new Co and Cr based superconductors. |
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