Point-contact Spectroscopy Study Of Heavy-fermion Systems URu₂Si₂ And CeOs₂Al₁₀

As a prototypical strongly-correlated elctron system,heavy fermions have been one of the most important research subjects in condensed matter physics.Due to the interactions between the f and conduction electrons,they have displaed rich electronic phase diagrams and physical properties,such as superconductivity,non fermion liquid behavior,quantum critical behaviors and so on.Point-contact spectroscopy(PCS)is an importance tool to study the electronic structure near Fermi level and has its special advantages to investigate the properties of heavy fermions,which requires furture careful explorations.In the thesis,we are focused to apply point-contact spectroscopy to study the gap structure of two heavy fermion systems,URu2Si2 and CeOs2Al10.The details are as following:The first chapter is the introduction,which mainly introduces the basic ideas in heavy fermion systems,such as RKKY and Kondo interactions,quantum criticality and Kondo semiconductor.The second chapter is an overview of our experimental methods,including the fundamental principles and experimental setups on PCS and its application on superductors and heavy fermions.In the third chapter,we have applied PCS on heavy fermion URu2Si2 with hidden order.Our careful analysis on experimental results show that there are two sets of double-peak structures in the point contact spectroscopy on URu2Si2 at low temperatures:one is a symmetric double-peak structure located at about ±7meV,which may be associated with the c-f hybridization gap,while the other asymmetric double peaks are located at +2 and-3.5 meV probably associated with the hidden order gap.Above the hidden order transition temperature THO,the point-contact conductance curves show the a zero-bias conductance peak,corresponding to the c-f hybridization.When the temperature drops below THO,the zero-bias hybridization peak splits into two symmetrical peaks due to the dramatically reduced carrier density and scatterings in the hidden order state,which coexist with the two asymmetric peak for the hidden order gap.Considering varying weights of the two sets of peak structures at different point-contacts,the conductance spectra will show different shapes for different point-contacts.In the fourth chapter,the properties of Kondo semiconductor CeOS2Al10 are studied by point-contact spectroscopy.At low temperatures,CeOs2Al10 and CeRu2Al10 both exhibit an abnormal antiferromagnetic phase transition,which can not be explained by a simple local RKKY picture.The point-contact conductance curves at low temperature show two sets of gap structures:One is the a symmetrical peak structure in the high bias region and the peak positions follow a BCS mean field behavior as a function of temperature.It is ascribed to the AFM gap in CeOS2Al10 and shows a moderate anisotropy along different axes.The other is the symmetrical gap structure in the low bias region,which appears below 20 K and should originate from a new hybridization gap below 20 K.Our PCS results strongly support the itinerant origin of the antiferromagnetism in CeOs2Al10.