The spin pseudogap quantum critical point of La$_{2-x}$Sr$_x$CuO$_4$ overlooked for nearly four decades due to the distribution of the NMR relaxation rate $1/T_1$

by Prof. Takashi Imai (Department of Physics and Astronomy, McMaster University, Canada)

Friday 16 May 2025, 14:00 → 15:00 Europe/Zurich

SZ-WBGB/019

Enhanced quantum fluctuations near the pseudogap quantum critical point (QCP) have long been suspected to be playing the central role in the mechanism of high T_c superconductivity in cuprates. However, the nature of the quantum fluctuations near the QCP and the exact hole concentration x_QCP of the QCP have been controversial. The NMR/NQR relaxation rate 1/T₁ is an ideal probe to tackle this issue, because it is sensitive to both paramagnetic spin fluctuations in the doped Mott insulator regime below x_QCP and the excitations at the Fermi surface of the overdoped Fermi liquid regime. Nonetheless, past NMR/NQR effort to shed light on the quantum criticality of La_2-xSr_xCuO₄ was hampered by the growing distributions in the magnitude of 1/T₁ induced by the electronic inhomogeneity above the optimum doping level x ~ 0.16. In this talk, I will explain how one can deduce the average relaxation rate 1/T₁ along with its density distribution function P(1/T₁) based on the inverse Laplace transform analysis [1-3], and its application to the ⁶³Cu NQR search of the spin pseudogap QCP in the ⁶³Cu isotope enriched samples of La_2-xSr_xCuO₄ [4]. This ILT method, in principle, can replace the stretched exponential fit of the mSR relaxation rate as well [5].

 

[1] P. M. Singer, A. Arsenault et al., PRB 101, 174508 (2020) and references therein.

[2] J. Wang et al., Nature Physics 17, 1109 (2021).

[3] W. Yuan, J. Wang et al., npj Quantum Materials 7, 120 (2022).

[4] C. Janesiripanich, A. Manilal, J. Wang, P. M. Singer and T. Imai, unpublished (2025).

[5] W. A. MacFarlane et al., J. Phys.: Conf. Ser. 2462, 012015 (2023)