  A quantum spin liquid (QSL) is a novel phase of matter with long-range
entanglement where localized spins are highly correlated with the vanishing of
magnetic order. Such exotic quantum states provide the opportunities to develop
new theoretical frameworks for many-body physics and have the potential
application in realizing robust quantum computations. Here we show that a
gapless QSL can naturally emerge from a deconfined quantum critical point
(DQCP), which is originally proposed to describe Landau forbidden continuous
phase transition between antiferromagnetic (AFM) and valence-bond solid (VBS)
phases. Via large-scale tensor network simulations of a square-lattice spin-1/2
frustrated Heisenberg model, both QSL state and DQCP-type AFM-VBS transition
are observed. With tuning coupling constants, the AFM-VBS transition vanishes
and instead, a gapless QSL phase gradually develops in between. Remarkably,
along the phase boundaries of AFM-QSL and QSL-VBS transitions, we always
observe the same correlation length exponents $\nu\approx 1.0$, which is
intrinsically different from the one of the DQCP-type transition, indicating
new types of universality classes. Our results explicitly demonstrate a new
scenario for understanding the emergence of gapless QSL from an underlying
DQCP. The discovered QSL phase survives in a large region of tuning parameters
and we expect its experimental realizations in solid state materials or quantum
simulators.
