UC Berkeley

In recent years, the study of cuprate superconductors has been dominated by the investigation of normal state properties. Of particular interest is the nature of interactions between superconductivity and other incipient orders which emerge above the superconducting transition temperature, T_c. The discovery of charge density wave (CDW) correlations in YBa₂Cu₃O_6+x (YBCO) and HgBa₂CuO_4+d (Hg-1201) has established that some form of charge order is ubiquitous in the cuprates. In this work, we explore the non-equilibrium dynamics of systems which sit near the boundary between superconductivity and competing orders.

Ultrafast pump-probe spectroscopy is ideally suited to the study of competing order. Exciting the sample with an optical pulse perturbs the system from equilibrium, altering the balance between the co-existing orders. The return to equilibrium is then monitored by a time-delayed probe pulse, revealing multiple decay processes as well as collective excitations. We first apply this technique to Hg-1201, conducting a detailed study of the phase diagram. At temperatures near T_c, the pump pulse induces a non-equilibrium quasiparticle population. At T_c we observe a doping-dependent peak in the relaxation time of these quasiparticles which we associate with a divergence in the coherence time of the fluctuating CDW. Using heterodyne probing in the transient grating geometry, we are able to disentangle the transient reflectivity components associated with superconductivity and the pseudogap, domonstrating competition across the phase diagram. We also discuss the observation of a sharp transition in the nature of the pseudogap signal at ∼ 11% doping.

In YBCO, we explore the temperature and doping dependence of coherent oscillations excited by the pump pulse. We associate these oscillations with the excitation of the CDW amplitude mode, and model their temperature dependence within the framework of a Landau model of competing orders.

We conclude with an investigation of pseudogap dynamics in the electron doped compound Nd_2-xCe_xCuO_4+d as a function of temperature and doping. Near optimal doping, we observe the impulsive excitation of a critically damped mode, with time-temperature scaling consistent with quantum-critical fluctuations. This mode competes with superconductivity in a dynamical fashion, such that the suppression of this mode below T_c can be lifted via photo-evaporation of the superconducting condensate.