Near-Threshold Dipole Strength in ^{10}Be with Isoscalar Character

Isoscalar dipole transitions are a distinctive fingerprint of cluster structures. A 1^{-} resonance at 7.27(10) MeV, located just below the α-emission threshold, has been observed in the deuteron inelastic scattering reactions off ^{10}Be. The deformation lengths of the excited states in ^{10}Be below 9 MeV have been inferred from the differential cross sections using coupled channel calculations. This observed 1^{-} resonance has isoscalar characteristics and exhausts approximately 5%-15% of the isoscalar dipole energy-weighted sum rule, providing evidence for pronounced α cluster structure in ^{10}Be. The Gamow coupled channel approach supports this interpretation and suggests the near-threshold effect might be playing an important role in this excitation energy domain. The α+α+n+n four-body calculation reproduces the observed enhanced dipole strength, implying that the four-body cluster structure is essential to describe the 1^{-} states in ^{10}Be.