Article

Particle–hole symmetry breaking in the pseudogap state of Bi2201

Nature Physics (Impact Factor: 19.35). 06/2010; DOI: 10.1038/nphys1632
Source: arXiv

ABSTRACT 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; refs 2, 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 broadening-indicative 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.

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