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Theory of the Electron Spin Resonance in the Heavy Fermion Metal β-YbAlB4

The heavy fermion metal β-YbAlB4 exhibits a bulk room temperature conduction electron ESR signal which evolves into an Ising-anisotropic f-electron signal exhibiting hyperfine features at low temperatures. We develop a theory for this phenomenon based on the development of resonant scattering off a periodic array of Kondo centers. We show that the hyperfine structure arises from the scattering off the Yb atoms with non zero nuclear spin, while the constancy of the ESR intensity is a consequence of the presence of CEF excitations of the order of the hybridization strength.