Optical excitation of stripe-ordered La2-xBaxCuO4 has been shown to transiently enhance superconducting tunneling between the CuO2 planes. This effect was revealed by a blueshift, or by the appearance of a Josephson Plasma Resonance in the terahertz-frequency optical properties. Here, we show that this photo-induced state can be strengthened by the application of high external magnetic fields oriented along the c-axis. For a 7-Tesla field, we observe up to a ten-fold enhancement in the transient interlayer phase correlation length, accompanied by a two-fold increase in the relaxation time of the photo-induced state. These observations are highly surprising, since static magnetic fields suppress interlayer Josephson tunneling and stabilize stripe order at equilibrium. We interpret our data as an indication that optically-enhanced interlayer coupling inLa2-xBaxCuO4 does not originate from a simple optical melting of stripes, as previously hypothesized. Rather, we speculate that the photo-induced state may emerge from activated tunneling between optically-excited stripes in adjacent planes.Charge and spin ordered phases are found throughout wide regions of the phase diagrams of high-TC cuprates [1,2,3,4,5,6,7]. These orders tend to compete with the superconducting state, reducing its coherence. A well-studied case is that of single-layer compounds, in which the doped holes arrange themselves in one-dimensional charge stripes separated by regions of oppositely phased antiferromagnetic order in the CuO2 planes. In La2-xBaxCuO4 (LBCO), for example, stripes completely suppress superconductivity at x = 1/8 doping [8,9], and coexist with it at lower and higher doping values (see phase diagram in Fig. 1(a)).Recent experimental evidence [10,11] suggests that the individual striped planes may be made up of a spatially modulated superfluid, a so-called "pair-density-wave" (PDW), in which the interlayer Josephson tunneling is frustrated by symmetry (see 90° stacking in inset of Fig. 1(a)) [12,13,14].Pressure and magnetic fields have been used to tune the striped state in LBCO and in related compounds. In particular, hydrostatic pressures of few GPa were shown to increase TC [15] and to partially suppress charge order [16]. On the other hand, magnetic fields H ≲ 10 T were shown to amplify the effect of dynamical layer decoupling, leading to a reduction of interlayer tunneling [17,18] and the stabilization of charge-and spin-order [19,20].More recently, optical excitation with femtosecond laser pulses has emerged as a means to drive the interplay between stripes and superconductivity [21], enhancing one or the other transiently. Excitation of either the in-plane Cu-O stretching mode in nonsuperconducting La1.675Eu0.2Sr0.125CuO4 (LESCO1/8) [22,23], or of La2-xBaxCuO4 with high-energy (1.5 eV) optical pulses [24,25], were both shown to enhance interlayer tunneling [26].

In the presence of electron-phonon coupling, an excitonic insulator harbors two degenerate ground states described by an Ising-type order parameter. Starting from a microscopic Hamiltonian, we derive the equations of motion for the Ising order parameter in the phonon coupled excitonic insulator Ta 2 NiSe 5 and show that it can be controllably reversed on ultrashort timescales using appropriate laser pulse sequences. Using a combination of theory and time-resolved optical reflectivity measurements, we report evidence of such order parameter reversal in Ta 2 NiSe 5 based on the anomalous behavior of its coherently excited orderparameter-coupled phonons. Our Letter expands the field of ultrafast order parameter control beyond spin and charge ordered materials.

The fate of a Mott insulator under strong low frequency optical driving conditions is a fundamental problem in quantum many-body dynamics. Using ultrafast broadband optical spectroscopy, we measured the transient electronic structure and charge dynamics of an off-resonantly pumped Mott insulator Ca 2 RuO 4 . We observe coherent bandwidth renormalization and nonlinear doublon-holon pair production occurring in rapid succession within a sub-100-fs pump pulse duration. By sweeping the electric field amplitude, we demonstrate continuous bandwidth tuning and a Keldysh crossover from a multiphoton absorption to quantum tunneling dominated pair production regime. Our results provide a procedure to control coherent and nonlinear heating processes in Mott insulators, facilitating the discovery of novel outof-equilibrium phenomena in strongly correlated systems.