The ability to control quantum materials using light-wave techniques holds immense promise. Intense, phase-coherent laser pulses in the mid-infrared (MIR), terahertz (THz), and attosecond ranges offer the potential for coherent control on sub-oscillation-cycle timescales. In quantum materials, the strong interplay between electronic, spin, and lattice degrees of freedom gives rise to emergent phenomena. Traditional methods like applying high pressure or magnetic/electric fields offer limited selectivity, affecting multiple properties simultaneously. Ultra-short laser fields provide a non-equilibrium approach to manipulate these properties, potentially accessing states inaccessible via quasi-equilibrium pathways. While high-frequency photoexcitation can be hampered by rapid relaxation, multi-cycle driving with intense THz or MIR fields enables non-adiabatic quantum tuning. These fields can accelerate electrons or control the phase of the many-electron wavefunction, driving systems into metastable phases and controlling phase transitions. However, direct light-spin interactions are often absent, making magnetic property manipulation indirect and slower. Recent experimental findings demonstrating femtosecond and attosecond coherent magnetism suggest the possibility of manipulating magnetic properties during the oscillations of a time-periodic driving electric field. This paper addresses the need for a deeper understanding of driven electron spin dynamics on sub-cycle timescales to harness the potential of currently available intense laser pulses. The central hypothesis is that light-wave-periodic modulation of electronic hopping between sites with noncollinear local spins can control magnetic states and phase transitions.

The paper reviews existing literature on coherent control of quantum materials using ultra-short laser pulses. It highlights the potential of multi-cycle THz/MIR driving for non-adiabatic quantum tuning, citing examples of electron acceleration and wavefunction phase control. The authors note previous experimental observations of femtosecond and attosecond coherent magnetism, suggesting that magnetic properties can be manipulated during oscillations of a time-periodic driving electric field. The limitations of previous work, namely the indirect and slow manipulation of magnetic properties due to the absence of direct light-spin interactions, are emphasized. The need for a better understanding of driven electron spin dynamics on non-dissipative sub-cycle timescales is also highlighted. The authors contrast their approach with quasi-stationary Floquet pictures and focus instead on the non-adiabatic time evolution of the quantum state during cycles of oscillation, before quasi-stationary states are reached. Studies on ultrafast switching, photoinduced phase transitions, and light-wave driven phenomena in various materials (e.g., manganites, cuprates, Dirac semimetals) are referenced as motivation and relevant background.

The authors use a theoretical approach based on quantum kinetic equations of motion for the density matrix of Hubbard quasiparticles. This allows them to model the non-equilibrium behavior of a system with strong electron-magnon coupling. They employ a general Hamiltonian, *H*(t) = *H*_local + *H*_hop(t), where *H*_local represents local interactions and *H*_hop(t) represents electronic hopping, modulated in time by a few-cycle bias laser field. The Hubbard operators are utilized to describe the strong local correlations in the system. The strong spin interactions, including a ferromagnetic interaction between itinerant and localized electron spins, are included in *H*_local. To break symmetry and introduce a preferred magnetization direction, a weak magnetic field is included. Local coordinate systems are used to separate quasi-equilibrium spin directions from quantum spin dynamics. A generalized mean-field approximation is employed to truncate the density matrix hierarchy arising from strong local correlations, restricting electronic motion to the lowest Hubbard band. The time evolution is calculated by solving the quantum kinetic equations of motion for the density matrix, considering both adiabatic and non-adiabatic time modulations of the electronic hopping amplitudes. The effects of different Rabi energies and pulse durations are investigated. The specific model considers a complex charge-exchange (CE)-type antiferromagnetic (AFM) unit cell, relevant to experimental studies of quantum femtosecond magnetism in insulating Pr_0.7Ca_0.3MnO₃ (PCMO) manganites. However, the methodology is presented as a more general strategy applicable beyond this specific unit cell. The numerical simulations are performed on a 4x4x4 lattice, taking into account various local configurations and spin states.

The study reveals three main non-equilibrium effects driven by the light-wave modulation of electronic hopping: (i) a more homogeneous spatial electronic distribution develops due to light-driven quantum transport assisted by quantum canting of the AFM local spin background; (ii) a transient magnetization develops simultaneously with coherent electronic transport, resulting from light-induced modulation of the AFM-ordered core spin background; and (iii) a light-induced change in the total energy minimum towards an undistorted lattice (Q=0) occurs during a few cycles of oscillations. The numerical results demonstrate the coherent time evolution in response to the hopping amplitude time modulation, showing how adiabatic and non-adiabatic modulations lead to different outcomes. The adiabatic modulation shows the system reaching a steady state with spin canting and charge transfer. In the non-adiabatic case, with ultra-short pulses (100 fs), a remarkable time delay in the development of different spin populations is observed, indicating non-instantaneous time evolution of core spins during oscillation cycles. Light-induced core spin populations with m<3/2 develop at both bridge and corner sites, which indicates quantum canting. This enhanced canting enhances electronic delocalization. The dynamics are found to be controlled by the Rabi energy, which is directly related to the electric field strength and lattice constant. When the Rabi energy approaches the intersite energy barrier, efficient quantum transport is observed. The non-adiabatic time evolution during few cycles of oscillations dynamically steers the system to a nonequilibrium quantum state characterized by a nonthermal density matrix and a non-instantaneous change in the total energy landscape. Increasing the Rabi energy shifts the global minimum of the total energy from a finite lattice displacement (Q>0, insulating state) to Q~0 (metallic state). This transition occurs via intermediate states and the development of multiple energy minima, showcasing the nonequilibrium nature of the phase transition. The results show that the light-wave-driven modulation of electronic hopping induces femtosecond spatially dependent core spin time modulation due to the off-diagonal interaction JHSiSj and that increasing the Rabi energy causes a decrease in Sz(t) from its equilibrium value at both bridge and corner sites, indicating a more uniform charge distribution and emergence of femtosecond magnetization in sync with coherent electronic hopping.

The findings demonstrate that light-driven itinerant electron spin and charge excitations interacting strongly with an AFM local spin background can destabilize an equilibrium AFM insulating state and drive it towards a transient metallic state with finite magnetization. This suggests a mechanism for light-induced time-dependent modulation of angular momenta. The results support the concept of synchronized quantum THz tuning and coherent control of electronic and magnetic properties using tunable multicycle THz/MIR electric fields. The study offers a microscopic mechanism for quantum femtosecond/attosecond magnetism driven by the light electric field and spin quantum fluctuations, contrasting with previous explanations based solely on adiabatic or coherent processes. The ability to control coherent electronic transport on sub-cycle timescales opens avenues for THz magneto-electronics, coherent spintronics, and the design of quantum materials far from equilibrium.

This research demonstrates a novel mechanism for light-wave control of correlated materials. By applying time-periodic modulation of electronic hopping, it's possible to dynamically steer an antiferromagnetic insulator into a metallic state with transient magnetization. This work contributes to the understanding of quantum femtosecond/attosecond magnetism and opens new possibilities for THz magneto-electronics and coherent spintronics. Future research could explore the detailed dynamics of the lattice displacements during the transition, investigate a wider range of materials, and explore the potential for designing light-induced switches that affect both spins and the crystal lattice.

The model uses a generalized mean-field approximation, which simplifies the treatment of strong local correlations. The numerical calculations are performed on a relatively small 4x4x4 lattice, limiting the accuracy of the results for certain parameter regimes, particularly around QB~0. The assumption of a simple model for the time-periodic modulation of the hopping amplitudes could be refined in future studies to explore more complex pulse shapes and their effects. The focus is primarily on theoretical calculations, and experimental validation is needed to fully confirm the predictions.