Enhancement of superexchange pairing in the periodically driven Hubbard model

Abstract

Recent experiments performed on cuprates and alkali-doped fullerides have demonstrated that key signatures of superconductivity can be induced above the equilibrium critical temperature by optical modulation. These observations in disparate physical systems may indicate a general underlying mechanism. Multiple theories have been proposed, but these either consider specific features, such as competing instabilities, or focus on conventional BCS-type superconductivity. Here we show that periodic driving can enhance electron pairing in strongly correlated systems. Focusing on the strongly repulsive limit of the doped Hubbard model, we investigate in-gap, spatially inhomogeneous, on-site modulations. We demonstrate that such modulations substantially reduce electronic hopping, while simultaneously sustaining superexchange interactions and pair hopping via driving-induced virtual charge excitations. We calculate real-time dynamics for the one-dimensional case, starting from zero- and finite-temperature initial states, and we show that enhanced singlet-pair correlations emerge quickly and robustly in the out-of-equilibrium many-body state. Our results reveal a fundamental pairing mechanism that might underpin optically induced superconductivity in some strongly correlated quantum materials

Theory of parametrically amplified electron-phonon superconductivity

“…We note that photo-induced enhancement of superconductivity has also been observed in high-Tc cuprates [64] along with several theoretical proposals for explaining these experiments [20,[65][66][67][68][69]. Cuprate superconductors are considerably more complicated than conventional electron-phonon superconductors that we considered in this paper.…”

Controlling the Floquet state population and observing micromotion in a periodically driven two-body quantum system

“…For example, Floquet engineering has been used to create topological states [7,11], which may lead to new phases of matter in the presence of strong interactions [50][51][52][53]. Additionally, the near-resonant modulation shown in this work has been theoretically demonstrated to generate density-dependent hopping, which significantly alters the properties of many-body phases [16][17][18], and could be applied to enhance anti-ferromagnetic interactions in the Hubbard model, or even probe regimes of magnetic order not accessible within this model [16,54]. Determining the relevant time-scales of the dynamical processes will contribute to the understanding of the scope and limitations of ultrafast optical manipulation of magnetic order [55].…”

“…To investigate the interplay between interactions and modulation, we select a driving frequency ω/2π = 2 kHz which can be comparable to U . In this regime, the periodic drive has been predicted to generate density-dependent tunneling processes [16][17][18]42]. At this lower frequency, the micromotion at the time scale of the periodic drive becomes visible.…”

“…This is particularly the case for many interesting schemes which were realised by driving at low frequencies [4,7] or even close to a characteristic energy scale of the underlying static Hamiltonian. Indeed, driving near-resonantly to the band structure was used to modify kinetic terms in the Hamiltonian [8][9][10][11][12][13][14][15], and modulating close to the interaction energy was proposed to engineer novel interaction terms [16][17][18][19][20]. For all these schemes, the periodic drive strongly couples the static eigenstates, which makes the full control of the population of the different Floquet states and the analysis of their exact time evolution demanding.…”

“…The atomic state is given by |Ψ = i |ψ i , where 1 ≤ i ≤ N D with N D the number of doubly occupied dimers and |ψ i the atomic state on dimer i. We characterize |Ψ by measuring either the ensemble average of the singlet fraction We begin the experiment by characterizing the change in the Floquet state originating from the undriven ground state for a drive frequency ω/2π = 8 kHz, larger than both the tunneling amplitude t/h = 548 (18) Hz and the strength of the on-site interaction |U |/h. This specific frequency is selected to avoid resonant coupling to higher bands of the optical lattice (the first excited band is h × 26(3) kHz higher in energy) [13].…”