  We study the spin liquid candidate of the spin-$1/2$ $J_1$-$J_2$ Heisenberg
antiferromagnet on the triangular lattice by means of density matrix
renormalization group (DMRG) simulations. By applying an external Aharonov-Bohm
flux insertion in an infinitely long cylinder, we find unambiguous evidence for
gapless $U(1)$ Dirac spin liquid behavior. The flux insertion overcomes the
finite size restriction for energy gaps and clearly shows gapless behavior at
the expected wave-vectors. Using the DMRG transfer matrix, the low-lying
excitation spectrum can be extracted, which shows characteristic Dirac cone
structures of both spinon-bilinear and monopole excitations. Finally, we
confirm that the entanglement entropy follows the predicted universal response
under the flux insertion.
