Ordering and Superconductivity: Scanning Tunneling Spectroscopy of Unconventional Superconductors

Abstract

In this thesis I present scanning tunneling microscopy (STM) studies that probe the interplay between ordering phenomena and superconductivity in two classes of materials: heavy fermions (CeCoIn5 and URu2Si2) and cuprates (Bi2Sr2CaCu2O8+x (Bi-2212)). In CeCoIn5, these measurements detect the emergence of heavy quasiparticles with lowering of temperature, as well as their composite nature. Interference of these quasiparticles allow their energy-momentum structure to be resolved as a function of temperature. Analysis of the tunneling spectra reveal signatures of energy-temperature

scaling associated with a quantum critical point. In URu2Si2, spatially resolved spectroscopy is used to examine the electronic states that emerge from the uranium f-states. As the temperature is lowered, the spectrum develops into a Fano resonance which characterizes the onset of heavy-fermion coherence below 120 K. At T = 17.5 K, URu2Si2 is known to undergo a second-order phase transition into a phase with a hidden order parameter. Tunneling spectroscopy identies a bias-asymmetric

energy gap with a mean-eld temperature dependence that develops in the hidden order state. In Bi-2212, combined STM and resonant elastic x-ray scattering (REXS) measurements are used to establish the formation of charge ordering. Depending on the hole concentration, the ordering in Bi-2212 occurs with the same period as those found in Y-based or La-based cuprates. This charge ordering also displays the analogous competition with superconductivity found in the other compounds. Together these results indicate the universality of charge organization competing with superconductivity across different families of cuprates. Finally, investigations of the infuence of anisotropic tip structures on STM conductance maps establish, with a model calculation, the presence of a tunneling interference eect within an STM junction that induces an <italic>artificial</italic> nematic signal. This effect is experimentally confirmed on different correlated electron systems, including Bi-2212, and is shown to be a sensitive probe of changes in the band structure of the sample.