Subjecting a many-body localized system to a time-periodic drive generically leads to delocalization and a transition to ergodic behavior if the drive is sufficiently strong or of sufficiently low frequency. Here we show that a specific drive can have an opposite effect, taking a static delocalized system into the many-body localized phase. We demonstrate this effect using a one-dimensional system of interacting hard-core bosons subject to an oscillating linear potential. The system is weakly disordered, and is ergodic absent the driving. The time-periodic linear potential leads to a suppression of the effective static hopping amplitude, increasing the relative strengths of disorder and interactions. Using numerical simulations, we find a transition into the many-body localized phase above a critical driving frequency and in a range of driving amplitudes. Our findings highlight the potential of driving schemes exploiting the coherent destruction of tunneling for engineering long-lived Floquet phases. DOI: 10.1103/PhysRevB.96.020201Introduction. A key obstacle in the search for new nonequilibrium quantum phases of matter is the tendency of closed quantum many-body systems to indefinitely absorb energy from a time-periodic driving field. Thus, in the long time limit, such systems generically reach a featureless infinite-temperature-like state with no memory of their initial conditions [1][2][3][4][5][6][7][8]. Interestingly, this infinite temperature fate can be avoided by the addition of disorder [9][10][11][12][13]. Sufficiently strong disorder added to a clean interacting system may lead to a many-body localized (MBL) phase [14-18] which does not allow transport of energy and particles. The MBL phase can persist in the presence of a weak, high-frequency drive [9][10][11][12][13]. Periodically driven systems in the MBL phase retain memory of their initial conditions for arbitrarily long times. Thus, they can support nonequilibrium quantum phases of matter, including some which are unique to the nonequilibrium setting [19][20][21][22][23][24][25][26][27][28][29][30].Generically, subjecting an MBL system to a periodic drive increases the localization length [9][10][11]. If the driving is done at sufficiently low frequencies or high amplitudes, it may even cause the system to exit the MBL phase. This delocalization effect is caused by transitions such as photon-assisted hopping, which are mediated by the periodic drive. These transitions conserve energy only modulohω, and can therefore lead to new many-body resonances which destabilize localization.An oscillating linear potential (henceforth an ac electric field) has a more subtle effect, as it can effectively suppress the hopping amplitude between adjacent lattice sites. This effect, called dynamical localization [34] or coherent destruction of tunneling [35], has been implemented in cold atoms [36][37][38], and can be used, for example, to induce a transition from a superfluid to a Mott insulator [39,40]. In noninteracting systems, dynamical localization can be e...