Collective excitation continuum in the antiferromagnetic heavy-fermion system U₂Zn₁₇

Our best knowledge of magnetic interactions and dynamics stems from experiments and theories on long-range ordered magnets. Protected by symmetry-breaking, magnetic excitations form spin waves with a dispersion relation that reflects the underlying interactions. In this talk, I will discuss the unusual spin dynamics in antiferromagnetic heavy-fermion metal U₂Zn₁₇, in which the canonical scenario breaks down due to magnetic frustration and/or charge fluctuations. Our inelastic neutron scattering experiments, covering three decades of energy scale, documented no signatures of magnons but only continuum of spin fluctuations in the antiferromagnetic state below T_N = 9.7K. The excitation continuum persists in the paramagnetic state up to at least T = 2T_N and carries a distinguished transverse nature as evidenced by its polarization-dependence. With no gap observed down to 0.1meV, the collective excitation demonstrates a correlation length of 13 Å at the low-energy limit, which is isotropic in three-dimension up to 3meV. Moving up in energy, correlation in the a-b plane gradually fades and vanishes above 5meV, while that along the c-axis persists at 20meV (~20T_N) up to the second nearest-neighbor in the c-direction. The excitation continuum, collective and transverse in nature with no signatures of magnons and prevailing at energy and temperature much higher than T_N, are indicative of U₂Zn₁₇ as a one-dimensional spin system whose low-energy dynamics are strongly modified by either frustrated two-dimensional interactions or the large density state of heavy fermions.