Abstract
Abstract Macroscopic systems generally exhibit collective behavior. More surprisingly, such coherent motions survive in systems with a small number of participants, such as metallic clusters or nuclei. Of particular interest are the collective vibrational modes. In metallic clusters, the so-called surface plasmons have been observed and interpreted as the collective vibration of the electrons against the ions. Also, the nucleus has been known, for a long time, to exhibit a large variety of collective vibrations which are usually called phonons (1). In particular, the giant dipole resonance corresponds to a collective motion of the protons against the neutrons. The monopole vibration is a compression mode analogous to the zero sound in Fermi liquids, and the giant quadrupole resonance is a surface vibration which resembles the wave at the interface of two liquids. The giant resonances are understood as the first oscillator quantum of the collective vibrations. Until recently, the second and higher quanta, the so-called multiphonon states built with giant resonances, remained unobserved. Therefore, the observation of multiple excitations of a giant resonance was an important missing piece in the puzzle of collective excitations. It appears obvious today that the non-existence of multiple excitations would have gravely undermined our understanding of giant resonances, but this aspect was completely neglected twenty years ago (2).
Published Version
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