Observation of Gap-like Features in Superconductors and Heavy Fermions via Quasiparticle Scattering and Tunneling Spectroscopies, and Electronic Transport

Electron spectroscopies are powerful tools to reveal Fermi surface gapping in intermetallic systems. In particular, planar tunneling spectroscopy (PTS) and quasiparticle scattering spectroscopy (QPS, also known as point-contact spectroscopy) respectively detect the spectral density and the scattering of quasiparticles around the Fermi surface. An introduction of these two techniques as the major measurement methods in our lab is presented in Chapter. 1, along with their application on superconductors. In particular, a model adopted to analyze the QPS conductance of heavy fermions is also discussed in this chapter. Chapter. 2 is a brief experimental survey of the heavy fermion superconductor URu₂Si₂. It is known for a second-order phase transition at THO = 17:5 K. Despite three decades of intensive research, the nature of this ordering is still not determined. The initial thought of antiferromagnetism was ruled out because the detected magnetic moment by neutron scattering is too small to account for the large entropy loss upon transition. Ever since, this mysterious order was named the hidden order (HO). A key question to finding the HO order parameter is whether it arises from localized or itinerant electrons. Many gap-like behaviors with different gap sizes were observed via various methods. Some of them report the observation of gapping in the Fermi surface such as optical conductivity and angle-resolved photoemission spectroscopy. Theories following this itinerant picture usually associate the HO to the hybridization process at higher temperature. Other experiments, such as inelastic neutron scattering and Raman spectroscopy, report the observation of collective excitations emerging at THO and thus, relate this ordering to localized electrons. Theories following the localized picture typically consider a multipolar ordering. Attempting to end this debate and to further connect the various gaps detected in the HO and related phases induced by tuning of quantum critical parameters, we have investigated URu₂Si₂-ᵪPᵪ and URu₂-ᵧFeySi₂ via a combined study of QPS and resistivity. In Chapter. 4, we show that no abrupt change in the QPS conductance spectra is observed upon the phase transitions either induced by temperature or substitution. This suggests that the widely considered Fermi surface gapping scenario is not likely, i.e. the localized, not itinerant, electrons must be the key in the HO ordering. Furthermore, we have proposed a novel interpretation for the electrical resistivity, based on scattering off gapped bosonic excitations specific to the HO, that connects multiple gap-like behaviors consistently.