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134 CHAPTER 6. ORBITAL CURRENTS IN THE CUPRATES
Figure 6.1: Reproduced from Ref. [136]. a) The colors indicate the magnetic moments
intensity versus the hole doping concentration n h . The white dots indicates
the pseudo-gap critical temperature T* obtained from resistivity measurements.
b) the θ 2 phase proposed by Chandra Varma that characterize the pseudo-gap
phase of the cuprates, c) on-site spins lying on the oxygen atoms.
further in the absence of experimental evidence. The discovery of an unusual and
robust regime called the pseudo-gap in superconductors has changed the picture
once more. The pseudo-gap has a density of states looking like a superconducting
gap, but the state itself is not superconducting. This phase is remarkable as
physical properties show anomalies with respect to the behavior expected for a
standard metal and at the same time there is no evidence of broken symmetry
so far. One of the accepted scenarii is that the pseudo-gap state represents
a precursor of the superconducting state. Another scenario would be that an
order parameter associated with the pseudo-gap phase is competing with the
superconducting one. In this context a proposal has been made recently that a
true broken symmetry is the origin of the pseudo-gap [137]. This state would
lead to a breakdown of time reversal symmetry in which the circulating currents
obey translational symmetry.
One of the recent proposal is that the anomalous properties of the cuprates
may be due to quantum critical fluctuations of current patterns formed spontaneously
in the CuO 2 planes. Related to this assumption, a break-through was
realized recently by Bourges et al. [136]: by using polarized elastic neutron diffraction,
they report the signature of an unusual magnetic order in the underdoped
phase of YBa2Cu3O6 + x (YBCO). They argue that this hidden order param-