Visualizing heavy fermions emerging in a quantum critical Kondo lattice

Abstract

In solids containing elements with f orbitals, the interaction between f-electron spins and those of itinerant electrons leads to the development of low-energy fermionic excitations with a heavy effective mass. These excitations are fundamental to the appearance of unconventional superconductivity and non-Fermi-liquid behaviour observed in actinide- and lanthanide-based compounds. Here we use spectroscopic mapping with the scanning tunnelling microscope to detect the emergence of heavy excitations with lowering of temperature in a prototypical family of cerium-based heavy-fermion compounds. We demonstrate the sensitivity of the tunnelling process to the composite nature of these heavy quasiparticles, which arises from quantum entanglement of itinerant conduction and f electrons. Scattering and interference of the composite quasiparticles is used to resolve their energy-momentum structure and to extract their mass enhancement, which develops with decreasing temperature. The lifetime of the emergent heavy quasiparticles reveals signatures of enhanced scattering and their spectral lineshape shows evidence of energy-temperature scaling. These findings demonstrate that proximity to a quantum critical point results in critical damping of the emergent heavy excitation of our Kondo lattice system.Comment: preprint version, 26 pages, 6 figures. Supplementary: 15 pages, 14 figure

Universal Knight shift anomaly in the periodic Anderson model

“…Importantly, our results clearly reveal that the development of the heavy fermion state occurs over a broad temperature range below T * , and also that both the local f-electrons as well as the itinerant quasiparticles contribute significantly to the NMR response over a broad range of hybridization values where non-Fermi liquid behavior has been observed. Further study of the spectral function A(ω), are in progress, and, in particular, whether A(ω) shows any change of behavior at the coherence temperature, as suggested recently by scanning tunneling microscopy [37].…”

Tunneling in Heavy-Fermion Junctions

“…3 Experimentally, one of the main challenges in probing the emergence of the heavy quasiparticles is in the lack of high resolution spectroscopic measurements. Remarkably, this challenge has been overcome in scanning tunneling microscopy measurements [4][5][6][7][8] as well as in the point contact spectroscopy. 9,10,14 Recent tunneling experiments have been convincingly able to trace the formation of the heavy quasiparticles.…”

“…What is more, momentum and energy resolved tunneling spectra visualized not only the formation of the heavy quasiparticles, but also the formation of unconventional superconductivity in a prototypical Kondo lattice heavy-fermion superconductor CeCoIn 5 . 4,7 Formation of the heavy-particles has also been successfully resolved in more itinerant systems, such as 'hidden order' compound URu 2 Si 2 5 and in a best candidate for correlated topological insulator SmB 6 . 8 Asymmetric or Fano lineshape of the differential tunneling conductance is the basic feature observed in tunneling experiments into Kondo lattice systems.…”

“…(10,11), provides more than adequate description of the available experimental data. [4][5][6][7][8] At the same time, for the analysis of tunneling experiments into a superconducting Kondo lattice 7 these terms may actually be important, especially for probing unconventional Cooper pairing mechanisms.…”

Pairing correlations near a Kondo-destruction quantum critical point

“…Distinctive features of this "local quantum criticality" include ω/T scaling in the spin susceptibility and the single-particle spectral function, vanishing of an additional energy scale, and a jump in the Fermi-surface volume. There is mounting experimental evidence for these characteristic properties, e.g., from inelastic neutron-scattering measurements on Au-doped CeCu 6 [10], scanning tunneling spectroscopy on CeCoIn 5 [11], Hall-effect and thermodynamic measurements on YbRh 2 Si 2 [12], and magnetic quantum-oscillation measurements on CeRhIn 5 [13].…”