Ferromagnetism and the Kondo effect are crucial for 3d electrons to become spin-separated and heavy at the same time.

Heavy fermion materials gain high electronic masses and expand Fermi surfaces when the high-temperature localized f electrons become itinerant and hybridize with the conduction band at low temperatures. However, despite the common application of this model, direct microscopic verification remains lacking. Here we report high-resolution angle-resolved photoemission spec-1 arXiv:1610.06724v1 [cond-mat.str-el]

Angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) have become indispensable tools in the study of correlated quantum materials. Both probe complementary aspects of the single-particle excitation spectrum. Taken together, ARPES and STM have the potential to explore properties of the electronic Green function, a central object of many-body theory. In this article, we explicate this potential with a focus on heavy-electron quantum criticality, especially the role of Kondo destruction. We discuss how to probe the Kondo destruction effect across the quantum critical point using ARPES and STM measurements. We place particular emphasis on the question of how to distinguish between the signatures of the initial onset of hybridization-gap formation, which is the "high-energy" physics to be expected in all heavy-electron systems, and those of Kondo destruction, which characterizes the low-energy physics and, hence, the nature of quantum criticality. We survey recent progress and possible challenges in the experimental investigations, compare the STM and ARPES spectra for several quantum critical heavy-electron compounds, and outline the prospects for further advances. CONTENTS I. Introduction 2 II. Quantum criticality 3 A. High-energy excitations, temperature evolution and mass enhancement 5 B. Isothermal evolution at low temperatures 6 C. Further considerations 6 D. Summary of Section II 6 III. ARPES, STM, and the single-particle Green function 6 A. The one-particle Green function 7 B. ARPES and STM 7 C. Probing quantum criticality in the Kondo lattice 8 D. Summary of Section III 9 IV. Quantum criticality in YbRh 2 Si 2 9 Summary of Section IV 11 V. The Cerium-based 115 family: photoemission vs. tunneling spectroscopy 11 A. CeIrIn 5 11 B. CeCoIn 5 12 C. CeRhIn 5 14 D. Further considerations 15 E. Summary of Section V 16 VI. Progress, challenges and prospects 16 A. High energy Kondo features 16 B. Isothermal evolution at low temperatures 17 C. Outlook 17 VII. Conclusion 19 Acknowledgements 19 References 20

A key issue in heavy fermion research is how subtle changes in the hybridization between the 4f (5f) and conduction electrons can result in fundamentally different ground states. CeRhIn_{5} stands out as a particularly notable example: when replacing Rh with either Co or Ir, antiferromagnetism gives way to superconductivity. In this photoemission study of CeRhIn_{5}, we demonstrate that the use of resonant angle-resolved photoemission spectroscopy with polarized light allows us to extract detailed information on the 4f crystal field states and details on the 4f and conduction electron hybridization, which together determine the ground state. We directly observe weakly dispersive Kondo resonances of f electrons and identify two of the three Ce 4f_{5/2}^{1} crystal-electric-field levels and band-dependent hybridization, which signals that the hybridization occurs primarily between the Ce 4f states in the CeIn_{3} layer and two more three-dimensional bands composed of the Rh 4d and In 5p orbitals in the RhIn_{2} layer. Our results allow us to connect the properties observed at elevated temperatures with the unusual low-temperature properties of this enigmatic heavy fermion compound.