Kondo scaling of $4f$-electron states and the Kondo singlet breakdown in heavy fermions

B. Tegomo Chiogo; M. Tagliavini; D. Wong; C. Schulz; 1 V. Porée; A. Nicolaou; R. Feyerherm; T. Schweitzer; T. Mazet; M. W. Haverkort; A. Chainani; D. Malterre; K. Habicht

arXiv · 2603.29599

Kondo scaling of $4f$-electron states and the Kondo singlet breakdown in heavy fermions

B. Tegomo Chiogo·M. Tagliavini·D. Wong·C. Schulz·1 V. Porée·A. Nicolaou·R. Feyerherm·T. Schweitzer·T. Mazet·M. W. Haverkort·A. Chainani·D. Malterre·K. Habicht

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Abstract

The low-energy spin- and charge-sensitive thermodynamic properties of a broad range of strongly correlated 4f-electron systems follow Kondo scaling, with a characteristic Kondo temperature, $T_K$. While the theory is known for thermodynamic properties and high-energy spectroscopies of Kondo materials, the surface sensitivity of electron spectroscopy limits the extent to which Kondo scaling can be quantitatively verified. In this study, bulk-sensitive photon-in photon-out temperature-dependent resonant inelastic X-ray scattering (RIXS), in combination with single-impurity Anderson model (SIAM) calculations, is used to provide quantitative evidence of low- and high-energy Kondo scaling in CeSi$_2$. RIXS Ce M$_5$-edge spectra show a clear decrease in the occupancy of the $f^0$ state as temperature increases accompanied by an increase of the spectral weight of the $f^1\underline L^1$ state, in good agreement with the SIAM calculations. The results demonstrate the breakdown of the Kondo singlet state, coupled with thermal occupation of the low-lying first-excited magnetic states. The RIXS data reveal a temperature evolution of the $f^n$ spectral weights, which is in stark contrast to that extracted from photoemission and inverse photoemission spectroscopies. This study provides an accurate spectroscopic method to determine the Kondo energy $k_B$$T_K$ that is consistent with thermodynamic measurements, and highlights soft X-ray RIXS as a quantitative bulk probe of low- and high-energy-scale hybridization effects in strongly correlated materials.

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B. Tegomo Chiogo, M. Tagliavini, D. Wong, C. Schulz, 1 V. Porée, A. Nicolaou, R. Feyerherm, T. Schweitzer, T. Mazet, M. W. Haverkort, A. Chainani, D. Malterre, K. Habicht. 2026-03-31. Kondo scaling of $4f$-electron states and the Kondo singlet breakdown in heavy fermions. https://arxiv.org/abs/2603.29599

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