Single-crystal Growth And Physical Properties Of Several Uranium-based Heavy-electron Materials

Heavy-electron system is a prototype of Kondo lattice in strongly correlated electron systems,which displays a variety of exotic quantum states,such as,unconventional superconducting,magnetic ordering state,non-Fermi liquid behavior and Kondo insulating behavior,due to the competing interactions between Kondo effect and RKKY interaction.As an important platform for research on the strongly associated system,the abnormal behaviors of uranium-base heavy electron materials are need to study continuously.And it can provide a new opportunity to deepen our understanding on the physics of heavy-fermion.In this dissertation,we systematically studied the physical properties of several uranium-based heavy electron compounds,including USb₂,UFe_0.6Sb₂ and U(Ga_0.95Mn_0.05)₃,by measuring the transport,magnetic and thermal properties.We discussed the electronic properties of USb₂ and focused on the origins of the exotic phenomena in the uranium-based heavy electron compounds.High-quality single crystals of USb₂ were grown by the so-called Sb self-flux method.The magnetic susceptibility,resistivity,magnetoresistance and specific heat were measured.The results indicate that,the 5f electrons in USb₂ begin to be coherent at about 260 K.USb₂transforms from paramagnetic into antiferromagnetic state at 203 K and the reconstruction of the Fermi surface occurs.The electronic structure near Fermi surface is further changed through the first hybridization between the localized 5f electrons and the conduction electrons below 113 K.The second hybridization leads to open one energy gap near the Fermi surface below 54 K.The crystal field effects have an important impact on physical properties at lower temperatures.Angle-resolved photoemission experiments have been performed to investigate the electronic structures of USb₂,and its band structure is obtained.The highly dispersive bands located at higher binding energies are attributed to the strong Sb 5p state,the flat bands around E_F are dominated by the U 5f electrons.Multi-kinks in the dispersion of the USb₂ are observed,the kink around X crossing the Fermi level is caused by the self-energy due to intraband scattering processes.And Contribution to self-energy by interband scattering process is primarily responsible for the observed kink in the non-crossing band near?point.Fermi surface nesting occurs at E_F-20 meV.Mutli-gap was observed in USb₂.The gap at M point is opened below the antiferromagnetic transition temperature 203 K due to the formation of the antiferromagnetic order spin density wave.The gap at?point is opened under 113 K due to the hybridization between the localized U 5f electrons and itinerant electrons near E_F.Atomic resolution images of USb₂ and the related characteristics of the cleavage planes are detected by scanning tunneling microscope?STM?at low temperatures.STS on the flat and complete Sb atom surface presents a peak at+6 meV due to hybridization between the localized 5f electrons and conduction electrons.A characteristic peak at-20meV is overlapped by the energy gap around?points and M points observed by ARPES.The characteristic peak at+40 meV is not affected by temperature,which is associated with the electronic state of the Sb atom.Four symmetrical peaks with high strength appear on the large area of U atom surface,corresponding to the two flat bands under the Fermi level observed by ARPES,and are induced by the crystal field effect.The stripe phases along the[110]are observed on the cleaved Sb atom surface,and the stripes continuously laterally shift as a function of the sample bias and tunneling current.The stripe phase is caused by the charge density wave.The width and cycle of the charge density wave are 2×2^1/2a and 7×2^1/2a,respectively.Single crystals of UFe_0.6Sb₂ grown by Sb flux method have been investigated by means of X-ray diffraction,magnetic susceptibility,electrical resistivity and specific heat.UFe_0.6Sb₂ crystallizes in the tetragonal HfCuSi₂-type structure?space group P4/nmm?.UFe_0.6Sb₂ undergoes a ferromagnetic transition at T_C=28 K with an easy axis along the c-axis.The results of the resistivity imply that three pseudogap formed in both directions.One pseudogap formed in the temperature range of 9³0 K originates from states of impurities due to the partial occupancies of the Fe sites,and it can be closed by applying high magnetic field.The magnetic field,however,has little effects on the other two pseudogaps.A large electronic specific heat coefficient?=233 mJ/mol·K² reveals this compound is a strongly correlated electron system.Single crystals of U(Ga_0.95Mn_0.05)₃ were grown by the Ga self-flux method.The ac and dc magnetic susceptibilities,isothermal magnetization and specific-heat data of U(Ga_0.95Mn_0.05)₃ provide conclusive evidence of spin-cluster glass behavior below the characteristic freezing temperature T_f=41.7 K.We deem that the disorder of Mn and Ga atoms in the crystal lattice and frustration of magnetic moments lead to the randomly distribution of spin-clusters below T_f.Fluctuation randomly of magnetic moment among the clusters will cause the spin glass behavior.