Inelastic neutron scattering study of single crystal heavy fermion YbAgGe

Single crystals of the heavy-fermion compound YbAgGe have been studied by neutron scattering. The magnetic ordering occurring below T1 ≈ 0.5 K is characterized by a commensurate propagation vector k=(1/3,0,1/3) and the moments in the basal plane of the hexagonal structure. The dynamic magnetic susceptibility is dominated by quasielastic spin fluctuations with a characteristic energy Γq(T ) of the order of 1 meV. The spins fluctuate predominantly in the basal plane. No spin-wave excitations are observed in the magnetically ordered phase. Below the Kondo temperature, TK ∼ 20 K, Γq(T ) shows a strong q dependence for wave vectors along the c direction, but is q-independent in the basal plane. Γq(T ) shows initially a rapid increase with temperature T at the antiferromagnetic zone center, but follows a standard √ T law for other q values and for T > TK in general. These observations classify YbAgGe as a well-behaved heavy-fermion compound with a particular q-dependence of the antiferromagnetic spin fluctuations, possibly related to the geometrical frustration of the Yb ions. PACS numbers: 71.27.+a, 75.25.+z, 75.40.Gb, 75.30.Mb,