Photoexcitation Spectroscopy of Insulating Cuprates

Synopsis

The ultrafast optical response of noble metals and insulating cuprates have been studied and characterized to better understand the lifecycle of photoexcitations in these two drastically different materials systems. Through the use of a tunable two-colour optical pump-probe system combined with THz photoconductivity measurements we are able to resolve questions around the nature of ultrafast response in these materials and estimate physical parameters, including the electron-phonon coupling in Au and Cu, the Auger recombination coefficient in YBa2Cu3O6, and the effect of carriers on the charge-transfer gap in La2CuO4 and Sr2CuO2Cl2.

In the noble metals, where electrons thermalize on timescales shorter than our laser pulses, we use the systemic ability to tune the energy of the pump photons to expand upon past measurements. We show that the two-temperature model may be applied under a wide variety of conditions, demonstrate the importance of considering the depth dependence of the photoexcitation profile in tunable pump-probe experiments, and find mathematical simplifications for the fluence dependence of the relaxation time of the photoexcitation system. We estimate the electron-phonon coupling g = (23 ± 1) PWm−3 K−1 for Au and g = (120 ± 10) PWm−3 K−1 for Cu.

Single-crystal and thin-film samples of the insulating cuprates, Sr2CuO2Cl2, La2CuO4, and YBa2Cu3O6, were studied over a variety of pump-probe conditions. Prior measurements of insulating cuprates showed seemingly contradictory behaviour, where the magnitude of the response had a sublinear fluence dependence but the dynamics of the response were fluenceindependent. Through a series of experiments we identify a simple model which can explain both behaviours through strong Auger recombination combined with Shockley-Read-Hall (SRH) recombination. For YBa2Cu3O6 we find that the nonlinear response depends primarily on the carrier density throughout our measurement time window, and we estimate an Auger coefficient of Ch = 7.8 × 10−27 cm6/s, several orders of magnitude larger than in conventional semiconductors with comparable gap energies. For Sr2CuO2Cl2 and La2CuO4 we find that the nonlinear response also depends primarily on carrier density at early times, but that as time evolves contributions emerge from the bosons given off as the carriers relax.

Keywords: Condensed Matter; Ultrafast Optics; Cuprates; Correlated Insulators; Recombination; Pump-probe spectroscopy