Isovector and isoscalar dipole excitations inBe9andBe10studied with antisymmetrized molecular dynamics

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Isovector and isoscalar dipole excitations in $^9$Be and $^{10}$Be are investigated in the framework of antisymmetrized molecular dynamics, in which angular-momentum and parity projections are performed. In the present method, 1p-1h excitations on the ground state and large amplitude $\alpha$-cluster mode are incorporated. The isovector giant dipole resonance (GDR) in $E>20$ MeV shows the two peak structure which is understood by the dipole excitation in the 2$\alpha$ core part with the prolate deformation. Because of valence neutron modes against the $2\alpha$ core, low-energy E1 resonances appear in $E<20$ MeV exhausting about $20\%$ of the Thomas-Reiche-Kuhn sum rule and $10\%$ of the calculated energy-weighted sum. The dipole resonance at $E\sim 15$ MeV in $^{10}$Be can be interpreted as the parity partner of the ground state having a $^6$He+$\alpha$ structure and has the remarkable E1 strength because of coherent contribution of two valence neutrons. The ISD strength for some low-energy resonances are significantly enhanced by the coupling with the $\alpha$-cluster mode. The calculated E1 strength of $^9$Be reasonably describes the global feature of experimental photonuclear cross sections consisting of the low-energy strength in $E<20$ MeV and the GDR in $E>20$ MeV.