Heavy Fermion URu_2-xFe_xSi₂ Electronic Structure And Uu_1-xSb ₂ Magnetic Research

In uranium-based heavy-fermion systems,unusual physical properties,such as unconventional superconductivity,non-Fenni liquid behavior,quantum critical phenomena,tunable magnetic order,and hidden order arise from the subtle interplay between the felectrons and the conduction electrons.Such unusual physical properties have attracted a large number of researchers worldwide and become one of the attractive topics in condensed matter physics.How to understand these novel physical phenomena of uranium-based heavy fermions requires not only theoretical studies,but also a lot of experimental research.However,due to the radioactive and chemical toxicity of metallic uranium and its compounds,a large number of studies have been done on the theoretical region,and there are few domestic experimental studies.Three typical materials of uranium-based heavy fermion materials(URu2Si2,UAu1-xSb2 and UGe2)are selected in this dissertation to study the hidden order problem and complex magnetism in uranium-based heavy fermion materials.The main results and conclusions are as follows:1.High quality URu2Si2 Single crystals and a series of URu2-xFexSi2 Single crystals were grown by the Czochralski method.The effect of Fe doping on crystal structure and physical properties was studied.We present a detailed characterization of different surface terminations in URu2Si2 by angle-resolved photoemission spectroscopy(ARPES),in conjunction with dynamical mean-field theory calculations and scanning tunneling spectroscopy(STM).On this basis we have studied the the temperature evolution of the electronic structure around the X point and do not observe any abrupt change of the electronic structure around the coherence temperature(55 K).2.The electronic structures of hidden order(HO)phase and large moment antiferromagnetic(LMAFM)phase in URu2Si2 have been studied by ARPES and STM experiments.The M-shaped band,HO gap and hybridization gap are observed both in HO phase and LMAFM phase indicating these features show no direct signature of the hidden order parameter.All of them are concomitant features.Our studies also reveal a close relationship between HO phase and LMAFM phase.On this basis we propose the symmetry breaking along c axis in HO phase which is similar to the symmetry breaking in LMAFM phase.Our studies provide a new idea for searching the order parameter of HO transition in the future.3.A series of nonstoichiometric UAu1-xSb2 single crystals were successfully synthesized using a flux method and their physical properties have been comprehensively studied.Evidence for at least three magnetic phases is observed:a high temperature antiferromagnetic phase(59～95 K),a phase with both ferromagnetic and antiferromagnetic components at intermediate temperatures(34～47 K),and a low temperature ferromagnetic phase(below 15 K).With an external field applied along the c axis,two metamagnetic transitions occur:AFM1 to AFM2 and AFM2 to FM.The net magnetization of AFM2 with one third of the saturated value suggests a magnetic structure along the lines of an ’up-up-down’ spin configuration.Furthermore,the magnetic order can be tuned by varying the site occupancies of Au.More occupancies of Au site can suppress the anti ferromagnetic interactions,reducing TN,but induces a ferromagnetic transition at lower temperature.A potential quantum critical point is possible in between x=1 and 0.25.4.High-quality single crystals of UGe2 were grown by Czochralski method and the strong coupling between localized 5f moments and itinerant quasiparticles was studied by conventional magnetic measurements.A detectable small negative magnetic moment(about 0.03 μb)of the itinerant quasiparticles in in the ferromagnetic matrix of localized 5f moments are observed by the thermal remanent magnetization and the field-cooling magnetization in rotating magnetic fields.Our studies indicated an antiferromagnetic(AFM)coupling between localized 5f moments and itinerant quasiparticles.Moreover,the collective behavior of cluster-like spins has been observed below TX,signifying that the unusual short-range order is indeed a cluster-glass state of itinerant quasiparticles.