Session K42: New Developments on Strongly Correlated Topology

Quantum oscillations in small-gap insulators

In recent years it has become understood that quantum oscillations of the magnetization as a function of magnetic field, long recognized as a phenomenon intrinsic to metals, can also manifest in insulating systems. Theory has shown that in certain narrow-gap band insulators, quantum oscillations can appear with a frequency set by the area traced out by the minimum gap in momentum space. I shall provide an overview of the theories of quantum oscillations in simple band insulators of this type, and discuss the relevance of these theories to experimental measurements on novel materials. In particular, I shall focus on situations where the insulating gap is generated by electron-electron interactions.   

Weyl-Kondo semimetal: from heavy fermions to flat band systems

The interplay between topology and interactions has garnered great interest in the study of quantum matter. For metallic systems, the notion of Weyl Kondo semimetal has emerged as a rare example of gapless topological states driven by strong correlations [1,2]. Here we report two lines of recent work along this general direction. First, we introduce a general design principle, with the correlation-induced emergent excitations experiencing symmetry constraints and producing gapless topological states [3]. We also show that d-electron-based systems with geometry-induced flat bands can serve as new platforms for Weyl-Kondo semimetal [4,5]; this follows from a representation of the d-electron systems in terms of compact and extended molecular orbitals. Second, we show that flat-band systems can undergo a continuous selective transition [5] of the molecular orbitals and yields the associated quantum criticality. In this case, our findings connect with recent advances on heavy fermion quantum criticality [6], provide the first theoretical understanding of the observed strange metallicity in line-graph systems, and predict a phase diagram that has been supported by some very recent experiments on kagome metals [7].

[1] H.-H. Lai, S. E. Grefe, S. Paschen, and Q. Si, PNAS 115, 93 (2018); Phys. Rev. B 101, 075138 (2020)

[2] S. Dzsaber et al., PNAS 118, e2013386118 (2021); Phys. Rev. Lett. 118, 246601 (2017)

[3] L. Chen et al, Nat. Phys. 18, 1341 (2022); H. Hu et al., arXiv:2109.13224

[4] L. Chen et al, arXiv:2212.08017. H. Hu et al., Sci. Adv. 9, eadg0028 (2023)

[5] L. Chen et al, arXiv: 2307.09431

[6] H. Hu, A. Cai, L. Chen et al., arXiv:2109.13224; Q. Si et al, Nature 413, 804 (2001)

[7] Y. Liu et al., arXiv:2309.13514   

Interplay of Weyl-Kondo physics and quantum criticality in the Kondo semimetal CeRu4Sn6

Correlation-driven gapless topological phases are a vastly unexplored field of great current interest. In a joint effort of experiment [1,2] and theory [3], the Weyl-Kondo semimetal state in the heavy fermion material Ce₃Bi₄Pd₃ was recently established as a prime example. To advance the field and understand the stabilization mechanisms of such phases, it is crucial to identify other candidate materials. One strategy – coined by tentative evidence for quantum criticality found in Ce₃Bi₄Pd₃ [1] – is to search for novel topological phases in the vicinity of quantum critical points. The noncentrosymmetric Kondo semimetal CeRu₄Sn₆ was theoretically proposed to host type I and type II Weyl-nodes [4] and shows quantum critical behavior without any form of parameter tuning [5]. This makes it a unique platform to investigate the role of quantum critical fluctuations in the formation mechanism of Weyl-Kondo semimetals and is therefore the focus of our experimental investigations. We present magnetotransport and specific heat measurements on CeRu₄Sn₆ under hydrostatic pressure and discuss signatures of Weyl-Kondo physics and how they evolve in the p-B-T phase diagram. Intriguingly, a dome of Weyl-Kondo semimetal phase appears to nucleate out of the ambient pressure and zero field quantum critical regime [6].

[1] S. Dzsaber et al., Phys. Rev. Lett., 118, 246601 (2017)

[2] S. Dzsaber et al., Proc. Natl. Acad. Sci. U.S.A. 118, e2013386118 (2021)

[3] H.-H. Lai et al., Proc. Natl. Acad. Sci. U.S.A. 115, 93 (2018)

[4] Y. Xu et al., Phys. Rev. X 7, 011027 (2017)

[5] W. T. Fuhrman et al., Sci. Adv. 7, eabf9134 (2021)

[6] D. M. Kirschbaum et al., in preparation (2023)