Bastien Michon (Laboratoire de Physique des Solides – CNRS, Université Paris-Saclay)
Title: An infrared optical study across the pseudogap critical point of cuprates: quantum criticality, scaling laws and link to superconductivity
Institut Néel, Room E424 (Salle Louis Weil)
Abstract: The nature of the pseudogap state in the phase diagram of cuprates has remained an enigma since their discovery in 1986. New experimental breakthroughs have recently revealed important signatures at the pseudogap critical point p*, where the pseudogap state vanishes in doping at zero temperature:
1) Transport measurements put in evidence a dramatic drop in the carrier density n – from 1+p to p – caused by a Fermi-surface reconstruction inside the pseudogap state [1-3].
2) Normal state specific heat shows a strong effective mass m* enhancement and an unusual logarithmic divergence in temperature at p*, resulting from a quantum critical point localized at p* [4][5].
Quantum criticality was already suspected in hole doped cuprates by a linear resistivity ρ ∝ T [6] and power law dependence in the infrared optical conductivity σ(ω,T) ∝ ω-ν [7], with ω the photon energy and ν the critical exponent. However, the critical exponents observed in optics ν < 1 seem to be in direct contradiction with the linear in temperature resistivity and logarithmic specific heat related to an exponent ν = 1.
In this work, we study the pseudogap critical point p* with low temperature infrared spectroscopy in La2-xSrxCuO4 and La1.8-xEu0.2SrxCuO4 cuprates. Through the extended Drude model on σ(ω,T), we extract the spectral weight K, the optical scattering rate τ(ω,T) and effective mass ratio m*/m(ω,T) at different doping contents across p*. With a theoretical model, we are able to compare three experimental probes: resistivity, specific heat and infrared spectroscopy by using only few parameters such as the critical exponent ν. In particular, we can describe the experimental data within reasonable error bars and explain the apparent contradictions between these three probes [8].
In addition, the study of the spectral weight K as a function of doping content confirms the aforementioned behaviors concerning the carrier density n and the effective mass m* across p*, and reveals an intriguing linear relation between the spectral weight associated to the electronic correlations and the superconducting critical temperature Tc [9].
[1] S. Badoux, W. Tabis, F. Laliberté et al., Nature 531, 210 (2016).
[2] C. Collignon, S. Badoux, S.A.A. Afshar, B. Michon et al., Phys. Rev. B 95, 224517 (2017).
[3] B. Michon, A. Ataei, P. Bourgeois-Hope et al., Phys. Rev. X 8, 041010 (2018).
[4] B. Michon, C. Girod, S. Badoux et al., Nature 567, 218 (2019).
[5] C. Girod, D. LeBœuf, A. Demuer et al., Phys. Rev. B 103, 214506 (2021).
[6] R.A. Cooper, Y. Wang, B. Vignolle et al., Science 323, 603 (2009).
[7] D. van der Marel, H.J.A. Molegraaf, J. Zaanen et al., Nature 425, 271 (2003).
[8] B. Michon, C. Berthod, C.W. Rischau et al., Nat. Comm. 14, 3033 (2023).
[9] B. Michon, A.B. Kuzmenko, M.K. Tran et al., Phys. Rev. R 3, 043125 (2021)
