TRR 360 Seminar:

Quantum criticality on compressed lattices

Markus Garst

Quantum criticality on compressed lattices

Prof. Dr. Markus Garst
Karlsruher Institut für Technologie

At a quantum phase transition, the ground state of a system changes as a function of a non-thermal control parameter. In a solid, the associated quantum critical degrees of freedom generically couple to the crystal lattice. This coupling is potentially able to change the universality class of the quantum critical point. We discuss various scenarios where this is realised. The coupling is necessarily non-perturbative when the order parameter couples bilinearly to strain as it is the case for quantum critical endpoints or nematic quantum criticality [1]. If the strain couples only to the square of the order parameter, it is non-perturbative at weak coupling if the effective dimension d+z, with the spatial dimension d and the dynamical exponent z, is smaller than 2/ν, where ν is the correlation length exponent. This criterion is, for example, marginally fulfilled at a Lifshitz transition of a two-dimensional metal like Sr2RuO4 [2]. We discuss in detail the quantum phase transition in the O(N) universality class with dynamical exponent z=1, and we show that non-perturbative effects might also arise if this criterion is not fulfilled but the coupling exceeds a critical threshold [3].

[1] I. Paul and M. Garst, Phys. Rev. Lett. 118, 227601 (2017).
[2] H. M. L. Noad et al. Science 382, 447 (2023)
[3] S. Sarkar, L. Franke, N. Grivas and M. Garst, Phys. Rev. B 108 235126 (2023).