Novel Quantum States in Strongly Correlated Electron Systems

Advisor: Jeroen Custers (MFF CUNI)

Funding: Fully funded.

Website: http://www.kfkl.cz/ and https://mgml.eu/

Contact: custers@mag.mff.cuni.cz

Strongly correlated electron systems are a vibrant frontier in modern condensed matter physics. The almost boundless number of possible materials and complexity of the theory of electrons in solids make this both an experimentally and theoretically exciting and challenging research field. Particularly, the concept of quantum criticality referring to a second order phase transition driven solely by quantum fluctuations resulted in a range of breakthroughs. Novel quantum phases were discovered, including electronic nematicity [1,2], unconventional superconductivity [3,4] and more recently, effects of nontrivial electronic topology further enrich this landscape [5,6]. It was soon recognized that the quantum critical theory provides a holographic description of the quantum theory of black holes [7]. In more simple words, a quantum critical point effectively behaves as a cosmic black hole. This opens up the exciting possibility of experimentally testing quantum gravity ideas in the laboratory.

Heavy fermion compounds are the canonical system for the in-depth study of quantum criticality. These materials are the most remarkable manifestation of strongly electron correlations. They contain rare-earth or actinide ions, forming a matrix of localized magnetic moments and are inherent close to a zero-temperature magnetic instability - a so-called quantum critical point (QCP). The active physics of these materials results from the competition between the on-site Kondo interaction quenching the localized moment and intersite Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction between these moments via the conduction electrons. Depending on which of the interactions prevails the ground state of the heavy fermion material either is non-magnetic or magnetically ordered. The competition is governed by two parameters, the hybridization of the 4f or 5f electrons and the geometrical frustration [8] which both can be modified by chemical doping, exerting pressure or applying a magnetic field and hence permitting a continuous tuning of the heavy fermion material to its QCP and beyond.

The focus of the current research project will be exploring new quantum states in heavy fermion and related materials by means of transport and thermodynamic experiments at extreme low temperatures. The work involves sample synthesis, designing/adjusting/construction of experimental setups, programming (Python) and data analysis. The work will be conducted at DCMP including the support of MGML and in a broad international network of collaborations.

Literature:

[1] S. Licciardello et al., Nature 567, 213 (2019) DOI: 10.1038/s41586-019-0923-y
[2] Jie Wu et al., Proc. Natl. Acad. Sci. U.S.A 117, 10654 (2020) DOI: 10.1073/pnas.1921713117
[3] J. Prokleška et al., Phys. Rev. B 92, 161114(R) (2015) DOI: 10.1103/PhysRevB.92.161114
[4] W.S. Lee et al., Nat. Phys. (2020). DOI: 10.1038/s41567-020-0993-7
[5] D.J. Kim et al, Nature Mater. 13, 466 (2014) DOI: 10.1038/nmat3913
[6] S.K. Kushwaha et al., Nat. Commun. 10, 5487 (2019) DOI: 10.1038/s41467-019-13421-w
[7] S.A. Hartnoll, A. Lucas and S. Sachdev, arXiv:1612.07324v3 [hep-th] (2016)
[8] J. Custers et al, Nature Mater. 11, 189 (2012). DOI: 10.1038/nmat3214