ABSTRACT
The properties of cuprate high-temperature superconductors are largely shaped by competing phases whose nature is often a mystery. Chiefly among them is the pseudogap phase, which sets in at a doping p* that is material-dependent. What determines p* is currently an open question. Here we show that the pseudogap cannot open on an electron-like Fermi surface, and can only exist below the doping p FS at which the large Fermi surface goes from hole-like to electron-like, so that p* ≤ p FS. We derive this result from high-magnetic-field transport measurements in La1.6-x Nd0.4Sr x CuO4 under pressure, which reveal a large and unexpected shift of p* with pressure, driven by a corresponding shift in p FS. This necessary condition for pseudogap formation, imposed by details of the Fermi surface, is a strong constraint for theories of the pseudogap phase. Our finding that p* can be tuned with a modest pressure opens a new route for experimental studies of the pseudogap.
ABSTRACT
An unusual, non-metallic resistivity of the 111 iron-pnictide compound FeCrAs is shown to be relatively unchanged under pressures of up to 17 GPa. Combined with our previous finding that this non-metallic behaviour persists from at least 80 mK to 800 K, this shows that the non-metallic phase is exceptionally robust. Antiferromagnetic order, with a Néel temperature TN â¼ 125 K at ambient pressure, is suppressed by pressure at a rate of 7.0 ± 0.4 K GPa(-1), falling to â¼50 K at 10 GPa. We conclude that the formation of a spin-density-wave gap at TN does not play an important role in the non-metallic resistivity of FeCrAs at low temperatures.
