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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ABSTRACT: Scanning tunneling microscopy is used to image the additional quasi-particle states generated by quantized vortices in the high critical temperature superconductor Bi2Sr2CaCu2O8+delta. They exhibit a copper-oxygen bond-oriented "checkerboard" pattern, with four unit cell (4a0) periodicity and a approximately 30 angstrom decay length. These electronic modulations may be related to the magnetic field-induced, 8a0 periodic, spin density modulations with decay length of approximately 70 angstroms recently discovered in La1.84Sr0.16CuO4. The proposed explanation is a spin density wave localized surrounding each vortex core. General theoretical principles predict that, in the cuprates, a localized spin modulation of wavelength lambda should be associated with a corresponding electronic modulation of wavelength lambda/2, in good agreement with our observations.
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ABSTRACT: We propose that the enigmatic pseudogap phase of cuprate superconductors is characterized by a hidden broken symmetry of d(x^2-y^2)-type. The transition to this state is rounded by disorder, but in the limit that the disorder is made sufficiently small, the pseudogap crossover should reveal itself to be such a transition. The ordered state breaks time-reversal, translational, and rotational symmetries, but it is invariant under the combination of any two. We discuss these ideas in the context of ten specific experimental properties of the cuprates, and make several predictions, including the existence of an as-yet undetected metal-metal transition under the superconducting dome. Comment: 12 pages of RevTeX, 9 eps figures
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