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Abstract

Ultrafast techniques allow an unprecedented look at the electronic and bosonic interactions that govern the macroscopic properties of materials. These processes can now be accessed on their fundamental timescale|the attosecond to picosecond range. Most ultrafast measurements involve using high energy excitation to bring a system out of equilibrium and then probing the subsequent relaxation processes. The development of ultrafast methods to selectively target collective excitations promises to transform ultrafast science into a tool not just of observation, but of precise control over material behavior. Two examples of selective excitation will be explored in this thesis. Both studies involve targeting phonon modes of cuprate materials in order to manipulate their superconducting behavior. However, the pathway by which the light interacts with the phonon and electronic degrees of freedom is distinct in each case. First, in Chapter 4, I present a study of resonant mid-infrared excitation of the lanthanide cuprate La_1.8-xEu_0.2Sr_xCuO₄. This family of compounds exhibits a strong suppression of superconductivity near the doping x = 0.125, the so-called "1/8th anomaly." The suppression appears to be due to competition with charge and spin stripe order. This study expands on a previous investigation that found that superconducting transport appears to be restored by selective excitation of a phonon mode that couples to the electronic charge order. The transient superconducting behavior is characterized in the THz optical response by the appearance of a plasma mode associated with intrinsic Josephson tunneling in the material. Here, I report that the transient plasma mode can be generated up to 65 K, near the charge ordering temperature. Two key observations are extracted from the relaxation dynamics. First, the plasma mode relaxes through a collapse of the carrier coherence length and not the carrier density, consistent with a Josephson plasmon. Second, the relaxation dynamics were found to be quite different above and below the spin ordering temperature TSO, with the regime below TSO showing an anomalous temperature-independent lifetime. These results will be discussed in the context of recent theories that propose the 1/8th regime hosts an intertwined order of superconducting and charge ordered components. The second study, introduced in Chapter 5, attempts to directly target the superconducting condensate of YBa₂Cu₃O_x, rather than aim at destroying a competing order. The superconducting response of cuprates has been found to be sensitive to the position of certain atoms in the unit cell, including the apical oxygen atoms that sit above and below the bilayer of CuO₂ planes in YBa₂Cu₃O_x. Selectively driving a c-axis apical oxygen mode (perpendicular to the planes) results in a stiffening of the Josephson plasma mode associated with Cooper pair tunneling between sets of planes. Above T_c, the same excitation induces a transient plasma mode at frequencies comparable with the Josephson mode. Five compounds were investigated, four in the underdoped regime and one at optimal doping. The transient plasma mode could be induced in all underdoped compounds, with a plasma mode that blue shifts towards optimal doping, tracking the blue shift of the Josephson mode. The midinfrared excitation targets the apical oxygen atoms only at sites that are undoped, therefore the lack of response in the optimally doped compound may be tied to the resonant nature of the excitation. This non-uniform excitation, perhaps along with intrinsic inhomogeneity of the compound, is reflected in an inhomogeneous optical response of the system. The response above T_c is captured quantitatively by an effective medium of a superconductor and the unperturbed bulk. Below T_c, the blue shift of the Josephson plasmon is seen to be inhomogeneous as well, with one component remaining near the equilibrium plasma frequency. The relaxation pathway of the transient mode is explored in Chapter 7. The principal finding is that the relaxation of the transient plasmon is driven by a loss of coherence, characterized by a decrease in carrier mobility, rather than a drop in carrier density as one might expect from quasiparticle excitation. Furthermore, during the relaxation, the transient plasmon splits, with one component centered near the equilibrium Josephson frequency and one component shifted to the blue. This may be related to the generation of (zero field) vortices, in a manner similar to the thermal vortex regime that forms an extended dome above T_c in the cuprate phase diagram in equilibrium. I close this thesis by returning to the topic of research I pursued at the start of my graduate career. Chapter 8 presents a departure from the area of ultrafast science, turning to another (equilibrium) probe of superconductivity, point contact spectroscopy. Point contact spectroscopy (PCS) probes the superconducting order parameter via Andreev reflection and is sensitive to bosonic modes that couple to the quasiparticle spectrum. Recent work has shown that PCS can be used to detect a variety of correlated states that couple to electronic degrees of freedom. The technique has been widely applied to the study of order parameter symmetry, and has proven sensitive to the d-wave order parameter of cuprates and heavy fermion compounds, the multigap s₊₊-wave order of MgB₂, as well as p-wave and anisotropic s-wave symmetries. I will present some work exploring the superconducting state of the pnictide superconductor Sr(Fe_1-xCo_x)₂As₂. The iron-based high temperature superconductors are unique in that multiple Fermi surfaces, with either electron-like or hole-like character, participate in the condensate. They are believed to exhibit a novel pairing mechanism, mediated by spin uctuations and an s-wave order parameter that changes sign between each type of Fermi surface. Detecting this s_+--wave order has become one goal of recent point contact measurements and theoretical developments. I describe the multigap behavior of Sr(Fe_1-xCo_x)₂As₂ and find that the point contact spectra can be well-described by two independent bands, placing restrictions on proposed models of s_+--wave order. Additional modes, detected in the electron-boson spectrum α²F(ω), have been ascribed in literature to possible spin excitations. I show that these modes can in fact be tied to Raman-active phonon modes of the 122 structure.