Ultrafast optical spectroscopy of strongly correlated materials and high-temperature superconductors: a non-equilibrium approach

Abstract

In the last two decades non-equilibrium spectroscopies have evolved from avant-garde studies to crucial tools for expanding our understanding of the physics of strongly correlated materials. The possibility of obtaining simultaneously spectroscopic and temporal information has led to insights that are complementary to (and in several cases beyond) those attainable by studying the matter at equilibrium. From this perspective, multiple phase transitions and new orders arising from competing interactions are benchmark examples where the interplay among electrons, lattice and spin dynamics can be disentangled because of the different timescales that characterize the recovery of the initial ground state. For example, the nature of the broken-symmetry phases and of the bosonic excitations that mediate the electronic interactions, eventually leading to superconductivity or other exotic states, can be revealed by observing the sub-picosecond dynamics of impulsively excited states. Furthermore, recent experimental and theoretical developments have made it possible to monitor the time-evolution of both the single-particle and collective excitations under extreme conditions, such as those arising from strong and selective photo-stimulation. These developments are opening the way for new, non-equilibrium phenomena that can eventually be induced and manipulated by short laser pulses. Here, we review the most recent achievements in the experimental and theoretical studies of the non-equilibrium electronic, optical, structural and magnetic properties of correlated materials. The focus will be mainly on the prototypical case of correlated oxides that exhibit unconventional superconductivity or other exotic phases. The discussion will also extend to other topical systems, such as iron-based and organic superconductors, (Formula presented.) and charge-transfer insulators. With this review, the dramatically growing demand for novel experimental tools and theoretical methods, models and concepts, will clearly emerge. In particular, the necessity of extending the actual experimental capabilities and the numerical and analytic tools to microscopically treat the non-equilibrium phenomena beyond the simple phenomenological approaches represents one of the most challenging new frontiers in physics

Towards properties on demand in quantum materials

“…2. At present, state-of-the-art ultrafast methods offer a near-complete characterization of the transient state 152 . Indeed, tr-ARPES provides a direct probe of the transient electronic structure 66,67,120,153 , tabletop and accelerator-based X-ray free-electron lasers readily capture momentary crystal arrangements 118,154 , nonlinear optical methods can directly interrogate the symmetries of quantum phases and their host lattices 30,[155][156][157][158] , while optical and soft X-ray techniques document electronic excitations, in some cases with elemental and orbital specificity 159 .…”

Nonlinear light–Higgs coupling in superconductors beyond BCS: Effects of the retarded phonon-mediated interaction

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14] Specifically, for relatively low frequencies of the laser such as terahertz (THz) and mid-infrared, we can expect to suppress the generation of quasiparticles having high energies that might be quickly transformed into heat through inelastic collisions causing a destruction of quantum coherence. Recent experiments indeed report that a superconducting-like state can be generated from the normal state by such low-energy excitations.…”

Nonequilibrium steady states and transient dynamics of conventional superconductors under phonon driving

“…Hence, we can simplify the problem by assuming t 0 = −∞ and neglecting the left-mixing, right-mixing, and Matsubara components in the KB formalism (which reduces to the Keldysh formalism) and focusing directly on the NESS. 1 In the Keldysh formalism, we focus on the contour Fig. 1(b)] and hence only need to consideř…”

“…Here · indicates the matrix product, I is the 2 × 2 identity matrix, and 1 We assume that at t 0 = −∞ the system is free and decoupled from the baths and that the interactions and the coupling to the baths are adiabatically turned on.…”

“…The prospect of nonequilibrium exploration and control of material properties has caught the interest and attention of broad segments of the condensed matter community [1]. In particular, the resonant excitation of mid-infrared phonon modes has opened a new pathway for manipulating properties such as metallicity [2], magnetism [3], and superconductivity (SC) [4][5][6][7][8][9] and for modifying lattice structures in ways which may not be achievable in equilibrium.…”