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Feedback Spin Resonance in Superconducting<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>CeCu</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>Si</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>CeCoIn</mml:mi><mml:mn>5</mml:mn></mml:msub></mml:math>
We show that the recently observed spin resonance modes in heavy-fermion superconductors ${\mathrm{CeCoIn}}_{5}$ and ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$ are magnetic excitons originating from superconducting quasiparticles. The wave vector $\mathbf{Q}$ of the resonance state leads to a powerful criterion for the symmetry and node positions of the unconventional gap function. The detailed analysis of …