Interlayer magnetotransport in the overdoped cuprateTl2Ba2CuO6+x: Quantum critical point and its downslide in an applied magnetic field

Abstract

Fundamental explanations of high-temperature (high-${T}_{c}$) superconductivity must account for the profound differences in the properties of the ``normal'' (nonsuperconducting) state at the two extremes of charge doping: heavy and light. On the light doping side, its properties clearly violate the standard Fermi-liquid theory of metals. The key to the nature of superconducting pairing lies in understanding the transition to a conventional behavior on the overdoped side. We report a convergence of the pseudogap energy scale and the boundary that separates unconventional from a conventional metal in the zero-temperature limit, both boundaries framing a V-shaped area of ``strange metal'' state in the temperature-doping phase space. By accessing the low-temperature regions of the phase diagram via a high-field interlayer magnetotransport in heavily doped ${\text{Tl}}_{2}{\text{Ba}}_{2}{\text{CuO}}_{6+x}$, we show that the pseudogap boundary has the hallmarks of a quantum phase transition with a zero entropy jump. The critical doping (linkage) point consistently downshifts with magnetic field in unison with the suppression of ${T}_{c}$, suggesting that quantum critical fluctuations that destabilize the pseudogap are connected to the superconductivity with high-${T}_{c}$.