Locating the pseudogap closing point in cuprate superconductors: Absence of entrant or reentrant behavior

Abstract

Many current descriptions of the pseudogap in underdoped cuprates envision a doping-dependent transition line ${T}^{*}(p)$ which descends monotonically toward zero just beyond optimal doping. There is much debate as to the location of the terminal point ${p}^{*}$ where ${T}^{*}(p)$ vanishes, whether or not there is a phase transition at ${T}^{*}$ and exactly how ${T}^{*}(p)$ behaves below ${T}_{\mathrm{c}}$ within the superconducting dome. One perspective sees ${T}^{*}(p)$ cutting the dome and continuing to descend monotonically to zero at ${p}^{*}\ensuremath{\approx}0.19$ holes/Cu---referred to here as entrant behavior. Another perspective derived from photoemission studies is that ${T}^{*}(p)$ intersects the dome near $p\ensuremath{\approx}0.23$ holes/Cu then turns back below ${T}_{\mathrm{c}}$, falling to zero again around ${p}^{*}\ensuremath{\approx}0.19$---referred to here as reentrant behavior. By examining field-dependent thermodynamic data for ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ we show that neither entrant nor reentrant behavior is supported. Rather, ${p}^{*}$ sharply delimits the pseudogap regime: For $p<0.19$ the pseudogap is always present, independent of $T$. Similar results are found for ${\mathrm{Y}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$. For both materials, ${T}^{*}(p)$ is not a temperature but a crossover scale, $\ensuremath{\approx}{E}^{*}(p)/2{k}_{B}$, reflecting instead the underlying pseudogap energy ${E}^{*}(p)$ which vanishes as $p\ensuremath{\rightarrow}{p}^{*}\ensuremath{\approx}0.19$.

Topics & Concepts