Change of carrier density at the pseudogap critical point of a cuprate superconductor

Low-temperature measurements of the Hall effect in cuprate materials in which superconductivity is suppressed by high magnetic fields show that the pseudogap is not related to the charge ordering that has been seen at intermediate doping levels, but is instead linked to the antiferromagnetic Mott insulator at low doping. Bridging the pseudogap phase The possible origin of the enigmatic 'pseudogap' phase in the high-temperature superconductors comes into sharper focus in light of some new low-temperature Hall measurements at magnetic fields high enough to suppress the confounding effects of superconductivity. Louis Taillefer and colleagues are able to show that the psudogap is not, as some have suspected, related to the charge-ordering that has been seen at intermediate doping levels, but is instead linked to the Mott insulator state at low doping. The pseudogap is a partial gap in the electronic density of states that opens in the normal (non-superconducting) state of cuprate superconductors and whose origin is a long-standing puzzle. Its connection to the Mott insulator phase at low doping (hole concentration, p ) remains ambiguous 1 and its relation to the charge order 2 , 3 , 4 that reconstructs the Fermi surface 5 , 6 at intermediate doping is still unclear 7 , 8 , 9 , 10 . Here we use measurements of the Hall coefficient in magnetic fields up to 88 tesla to show that Fermi-surface reconstruction by charge order in the cuprate YBa 2 Cu 3 O y ends sharply at a critical doping p = 0.16 that is distinctly lower than the pseudogap critical point p * = 0.19 (ref. 11 ). This shows that the pseudogap and charge order are separate phenomena. We find that the change in carrier density n from n = 1 + p in the conventional metal at high doping (ref. 12 ) to n = p at low doping (ref. 13 ) starts at the pseudogap critical point. This shows that the pseudogap and the antiferromagnetic Mott insulator are linked.