IQIM Postdoctoral and Graduate Student Seminar

Abstract:

The discovery of a fluctuating charge density wave (CDW) in underdoped YBa2Cu3O(6+x) (YBCO) has established that charge ordering proximate to the superconducting dome is a universal feature of the cuprate superconductors(1,2). This order is observed to co-exist and compete with superconductivity, and the relationship of charge ordering to the mechanism of high Tc superconductivity is an important open question.

In this talk I will present a study of competing order in the cuprates using time-domain reflectivity measurements. In these experiments, low-intensity light pulses at 1.5 eV pump the sample, creating high-energy excitations which rapidly thermalize into gap-energy quasiparticles. This non-equilibrium state is then probed by a second pulse, allowing the study of reflectivity dynamics with sub-ps time resolution. In YBCO, I will demonstrate that this quasiparticle excitation couples directly to the amplitude mode of the CDW, launching a coherent oscillation in the reflectivity. This oscillation softens upon cooling through Tc, indicative of competition between superconducting (SC) and CDW order. I will then move on to discuss a detailed study of quasiparticle relaxation dynamics as a function of doping, temperature, and magnetic field in the especially simple cuprate superconductor HgBa2CuO(4+delta) (Hg-1201). We observe cusp-like features in the recombination dynamics near Tc which can be understood in terms of quasiparticles that are of mixed SC and CDW character. Finally, if time permits, I will present evidence for direct excitation via the impulsive-Raman process of an as-yet unidentified collective mode in the electron-doped compound Nd(2-x)CexCuO(4+delta). We associate this mode with the onset of the pseudogap, and compare it to the pseudogap dynamics in the hole-doped cuprates.

1) G. Ghiringelli et. al., Science 337, 821 (2012)

2) J. Chang e. al., Nature Physics 8, 871 (2012)

 - Refreshments follow the seminar on the Bridge patio.