Coupling of a high-energy excitation to superconducting quasiparticles in a cuprate from coherent charge fluctuation spectroscopy

Abstract

Dynamical information on spin degrees of freedom of proteins or solids can be obtained by NMR and electron spin resonance. A technique with similar versatility for charge degrees of freedom and their ultrafast correlations could move the understanding of systems like unconventional superconductors forward. By perturbing the superconducting state in a high-T c cuprate, using a femtosecond laser pulse, we generate coherent oscillations of the Cooper pair condensate that can be described by an NMR/electron spin resonance formalism. The oscillations are detected by transient broad-band reflectivity and are found to resonate at the typical scale of Mott physics (2.6 eV), suggesting the existence of a nonretarded contribution to the pairing interaction, as in unconventional (non-Migdal-Eliashberg) theories.A ccording to Bardeen-Cooper-Schrieffer (BCS) theory (1), superconductivity requires that electrons bind in Cooper pairs and condense collectively in a macroscopic quantum state. In conventional superconductors, the observation of a shift in the superconductivity transition temperature upon isotope substitution (2, 3) was an experimental breakthrough leading to the conclusion that lattice vibrations (phonons) act as a glue among electrons, promoting the required pairing. Since the discovery of high-temperature superconductivity in cuprates in 1986 (4), the observation of an analogous fingerprint of the glue involved in the pairing mechanism, if any, has been lacking.A fertile route to obtain information on excitations in solids and their coupling to electrons is pump-probe spectroscopy (5). Typically, the sample is illuminated by an ultrashort laser pulse lasting a few tens of femtoseconds and carrying 1.5 eV photons. This "pump" pulse creates an out-of-equilibrium distribution of particle-hole excitations that decays to states within a few hundreds of millielectronvolts of the chemical potential (6) in the pulse duration timescale. There, phase space restrictions slow down the dynamics (7) and the subsequent evolution can be studied in real time by a probe pulse. The dynamical response of the system can be observed with a temporal resolution comparable to the timescale of relevant processes in the material, like the pairs breaking, their recombination, or the electron-phonon coupling time.For example, the photoinduced quenching of the superconducting order parameter and its subsequent recovery were followed by recording the temporal evolution of the gap amplitude in the optical spectrum of different cuprates (8-11). Remarkably, it was found that the energy needed to suppress the superconducting state in these materials is several times larger than the condensation energy (9, 10), in contrast to what happens in conventional superconductors where it is of the same order (9, 11). Optical studies also provided insights on the relaxation dynamics of the excited quasiparticles (11-14) and on the optical spectral weight transfers associated with the carriers' kinetic energy changes across the photoinduced phase transition...

Signatures of nonadiabatic BCS state dynamics in pump-probe conductivity

“…Pump-probe spectroscopy has the unique capability to resolve coherent oscillations as well as relaxation processes in the time domain, thereby yielding important information on the intrinsic time and energy scales of the superconductor. [4][5][6] We hope that the findings of this paper will stimulate further time-resolved measurements of superconductors in the nonadiabatic regime.…”

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

“…III, one frequently encounters a momentum integral of a function of k [Eq. (8)] multiplied by some of˙ k ,¨ k , ... k , and…”

“…[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.…”

Stripes in cuprate superconductors: Excitations and dynamic dichotomy

“…It is also possible that coupling to these modes contributes significantly to the pairing. It is worth to remark that new developments in time resolved spectroscopy show [169] that the high energy excitations below the CT band do not participate in the pairing but excitations at the CT band do. The region below 1.6eV has not been measured yet.…”