Particle-hole symmetry breaking in the pseudogap state of Bi2201

In conventional superconductors, a gap exists in the energy absorption spectrum only below the transition temperature (Tc), corresponding to the price to pay in energy for breaking a Cooper pair of electrons and creating two excited states. In high-Tc cuprate superconductors above Tc but below a temperature T*, an energy gap called the pseudogap exists, and is controversially attributed either to pre-formed superconducting pairs, which would show particle-hole symmetry, or to competing phases that would typically break it. Scanning tunnelling microscopy (STM) studies suggest that the pseudogap stems from lattice translational symmetry breaking and is associated with a different characteristic spectrum for adding or removing electrons (particle-hole asymmetry; refs2, 3). However, no signature of either energy or spatial symmetry breaking of the pseudogap has previously been observed by angle-resolved photoemission spectroscopy (ARPES). Here we report ARPES data from Bi2201, which reveal both particle-hole symmetry breaking and pronounced spectral broadeningindicative of spatial symmetry breaking without long-range order at the opening of the pseudogap. Our finding supports the STM proposal that the pseudogap state is a broken-symmetry state that is distinct from homogeneous superconductivity.