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.

Visualizing Heavy Fermions in a Quantum Critical Kondo Lattice

The paper "Visualizing Heavy Fermions Emerging in a Quantum Critical Kondo Lattice," authored by Pegor Aynajian, Eduardo H. da Silva Neto, and colleagues, presents a detailed examination of heavy fermionic excitations in Kondo lattice systems through advanced spectroscopic mapping using a scanning tunneling microscope (STM). The research focuses on Ce-based heavy fermion compounds, particularly the CeIn115 family, to explore the emergence and behavior of composite heavy quasiparticles near quantum critical points (QCPs).

In materials comprising elements with f-orbitals, interactions between f-electron spins and itinerant conduction electrons lead to emergent low-energy quasiparticles with substantially enhanced effective mass. These heavy fermion excitations are critical for understanding phenomena like unconventional superconductivity and non-Fermi liquid behavior in actinide- and lanthanide-based compounds. However, the precise nature of these electronic states close to quantum phase transitions remains inadequately characterized due to limitations in spectroscopic measurements.

The authors employ STM spectroscopic mapping to visualize the formation of these heavy fermions, highlighting their composite nature stemming from the hybridization of conduction electrons with localized f-electrons. This entanglement is demonstrated effectively through scattering and interference patterns, which reveal significant mass enhancement and distinctive energy-momentum structures that evolve with decreasing temperature.

Experimental results show that upon cooling, the tunneling spectra on Ce-Co-In surfaces display marked changes, evidencing hybridization gaps—both direct (2v) and indirect (Δh). These alterations affirm the sensitivity of the tunneling process to the relative contributions of light conduction and heavy f-like components, captured through model calculations. Spectroscopy on different atomic layers further accentuates the composite character of the excitations, with the f-like tunneling chiefly detected on Co layers as opposed to Ce-In surfaces.

Moreover, quasiparticle interference (QPI) mapping reveals substantial changes in the scattering patterns, indicative of the quasiparticles' energy-dependent evolution near the Fermi surface. These patterns provide direct probing into mass enhancement, showcasing a heavy effective mass of approximately 30 times the bare electron mass, correlating well with prior quantum oscillation studies.

The paper's findings imply that strong scattering and energy-temperature scaling near the QCP are signatures of critical damping rather than typical Fermi liquid behavior. Notably, the spectroscopic data suggest the presence of quantum critical fluctuations affecting quasiparticle lifetimes, further confirmed by the linear energy-temperature scaling observed in the STM spectra.

Implications of this study are manifold, both theoretically and practically. Understanding heavy fermion dynamics deepens insight into electron correlations and quantum criticality, providing a basis for investigating the interplay between quantum fluctuations and superconductivity in condensed matter systems. Future directions may explore this intersection at temperatures beneath current experimental capabilities to unravel the precise mechanisms underlying unconventional superconductivity.

Overall, this research contributes significant advancements in visualizing heavy fermion systems, offering novel approaches to discerning their composite nature and dynamic properties near critical transitions, reinforcing the essential role of tunneling spectroscopy in modern condensed matter physics.