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Complex magnetism and novel spin excitations in Ce-based heavy fermion metals

Shan Wu (Johns Hopkins University)

In heavy fermion system, competition between the Kondo screening effect and RKKY interaction epitomize the strongly correlated electron problem. It results in a non-magnetic Kondo-single state at high coupling constant J where 4f electrons immersed into Fermi sea and a magnetically ordered state at low J where 4f spins form a localized band. Since both interactions depend on coupling constant, the ground state could be tuned by hydrostatic pressure, chemical substitution etc. Such competitions can provide interesting phenomena such as quantum criticality, unconventional superconductivity and complex AFM ordering in the extreme Neel limits, involving different characters of 4f electrons.
I will present two Ce-based compounds CeNiAsO and CeCoGe3, which display long-wave length magnetism, incommensurate and commensurate. Firstly, I will present microscopic details across double transitions on a Kondo lattice heavy fermion CeNiAsO near quantum criticality via neutron scattering and zero field muon spin relaxation experiments. CeNiAsO is isotructural to the 1111 Fe-based superconductors but exhibits rare-earth based heavy fermion magnetism near a pressure driven critical point at 6.5 kbar. At ambient pressure CeNiAsO has two phase transitions at TN1 ~ 9.3K and TN2 ~ 7.3K. T < TN1 reveals an incommensurate uniaxial spin density wave state. TN2 is an incommensurate to commensurate transition where spins also re-orientate within the ab plane to form a non-collinear spin structure. In inelastic neutron scattering measurements, broad crystal field like excitations provide evidence of an easy-plane spin system, consistent with the ordered structure.
Secondly, I will present neutron scattering study on single crystal heavy fermion CeCoGe3. It is non-centrosymmetric tetragonal structure with Ce occupying the basic frame of body-centered lattice. Three thermal phase transitions are identified by susceptibility and resistivity, which shows highly anisotropy easy-axis magnetism. The measurements reveal complex square-wave type magnetic ground state in two lower temperature regions. Inelastic neutrons exhibit an unusual magnetic excitation spectrum with propagating only in ab plane not along c. The energy width of magnetic excitations is beyond the instrumental resolutions and top of excitations are damped into a continuum to a higher energy scale.

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