Wydarzenia

Seminarium Oddziału Badań Magnetyków

An STM view on strongly correlated electron systems: From heavy fermion metals to topological Kondo insulators

prof. Steffen Wirth

Instytut Maxa Plancka Fizyki Chemicznej Ciał Stałych w Dreźnie

Electronic correlations give rise to a plethora of interesting phenomena and phases. For example, hybridization between 4f and conduction electrons in heavy fermion metals may result in the generation of low-energy scales that can induce quantum criticality and unconventional superconductivity [1]. One of the most important techniques that helped shaping our understanding of nonlocal correlations, both magnetic and superconducting, has been scanning tunneling spectroscopy (STS) with its unique ability to give local, microscopic information that directly relates to the one-particle Green's function. We combine STS with bulk measurements to obtain complementary information on different length scales.

We studied the temperature evolution of hybridization effects and Kondo lattice coherence as observed by STS, focusing on the model heavy fermion metal YbRh₂Si₂ and the intermediate-valence Kondo insulator SmB₆. Investigation of high-quality single crystals of YbRh₂Si₂ allows to study the evolution the single-ion Kondo effect and Kondo lattice coherence upon lowering the temperature [2]. We also show how Kondo coherence connects with quantum criticality [3]. These results by STS are compared to magnetotransport and thermodynamic measurements, as well as to findings on other heavy fermion materials.

Low-temperature in-situ cleaving of SmB₆ single crystals mostly resulted in reconstructed surfaces, while non-reconstructed patches were found less frequently. The different surface terminations give rise to marked differences in the STS results [4]; in our analysis, however, we concentrate on STS of non-reconstructed areas. These spectra confirm the hybridization picture typically considered for this material [5]. At the surface, the Kondo effect is suppressed to lower temperatures as compared to the bulk material [6]. All types of surfaces, reconstructed and non-reconstructed, displayed a finite zero-bias conductance of considerable magnitude confirming the robustness of the metallic surface states.

References:
[1] S. Wirth and F. Steglich, Nature Rev. Mat. 1 (2016) 16051.
[2] S. Ernst et al., Nature 474 (2011) 362.
[3] S. Seiro, Lin Jiao, S. Kirchner, S. Hartmann, S. Friedemann, C. Krellner, C. Geibel, Q. Si, F. Steglich, and S. Wirth, Nature Commun. 9 (2018) 3324.
[4] S. Rößler et al., Phil. Mag. 96 (2016) 3262.
[5] S. Rößler et al., Proc. Natl. Acad. Sci. USA 111 (2014) 4798.
[6] Lin Jiao et al., Nature Commun. 7 (2016) 13762.