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Emerging local Kondo screening and spatial coherence in the heavy-fermion metal YbRh2Si2


The entanglement of quantum states is both a central concept in fundamental physics and a potential tool for realizing advanced materials and applications. The quantum superpositions underlying entanglement are at the heart of the intricate interplay of localized spin states and itinerant electronic states that gives rise to the Kondo effect in certain dilute magnetic alloys1. In systems where the density of localized spin states is sufficiently high, they can no longer be treated as non-interacting; if they form a dense periodic array, a Kondo lattice may be established1. Such a Kondo lattice gives rise to the emergence of charge carriers with enhanced effective masses, but the precise nature of the coherent Kondo state responsible for the generation of these heavy fermions remains highly debated1,2,3. Here we use atomic-resolution tunnelling spectroscopy to investigate the low-energy excitations of a generic Kondo lattice system, YbRh2Si2. We find that the hybridization of the conduction electrons with the localized 4f electrons results in a decrease in the tunnelling conductance at the Fermi energy. In addition, we observe unambiguously the crystal-field excitations of the Yb3+ ions. A strongly temperature-dependent peak in the tunnelling conductance is attributed to the Fano resonance4,5 resulting from tunnelling into the coherent heavy-fermion states that emerge at low temperature. Taken together, these features reveal how quantum coherence develops in heavy 4f-electron Kondo lattices. Our results demonstrate the efficiency of real-space electronic structure imaging for the investigation of strong electronic correlations6,7, specifically with respect to coherence phenomena, phase coexistence and quantum criticality.

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Figure 1: Topography of a cleaved YbRh 2 Si 2 single crystal at 4.6 K.
Figure 2: Tunnelling spectroscopy of YbRh2Si2.
Figure 3: Heavy-quasiparticle formation in a Kondo lattice.


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We thank P. Coleman, T. Costi, A. Georges, D. Morr, J. Schmalian, S. Seiro, Q. Si and P. Wölfle for discussions. We are indebted to J. C. Davis for comments on our manuscript. This work is partly supported by the German Research Foundation through DFG Forschergruppe 960.

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Authors and Affiliations



F.S. and S.W. designed the project. S.E. performed measurements, S.E. and S.W. conducted the data analysis. S.K. provided the theoretical framework and non-crossing approximation calculations. G.Z. did the band-structure calculations. C.K. and C.G. synthesized and characterized the materials. S.W., S.K. and F.S. wrote the manuscript.

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Correspondence to S. Wirth.

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The authors declare no competing financial interests.

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Ernst, S., Kirchner, S., Krellner, C. et al. Emerging local Kondo screening and spatial coherence in the heavy-fermion metal YbRh2Si2. Nature 474, 362–366 (2011).

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