pdf, 9 MiB - Infoscience - EPFL
pdf, 9 MiB - Infoscience - EPFL
pdf, 9 MiB - Infoscience - EPFL
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134 CHAPTER 6. ORBITAL CURRENTS IN THE CUPRATES<br />
Figure 6.1: Reproduced from Ref. [136]. a) The colors indicate the magnetic moments<br />
intensity versus the hole doping concentration n h . The white dots indicates<br />
the pseudo-gap critical temperature T* obtained from resistivity measurements.<br />
b) the θ 2 phase proposed by Chandra Varma that characterize the pseudo-gap<br />
phase of the cuprates, c) on-site spins lying on the oxygen atoms.<br />
further in the absence of experimental evidence. The discovery of an unusual and<br />
robust regime called the pseudo-gap in superconductors has changed the picture<br />
once more. The pseudo-gap has a density of states looking like a superconducting<br />
gap, but the state itself is not superconducting. This phase is remarkable as<br />
physical properties show anomalies with respect to the behavior expected for a<br />
standard metal and at the same time there is no evidence of broken symmetry<br />
so far. One of the accepted scenarii is that the pseudo-gap state represents<br />
a precursor of the superconducting state. Another scenario would be that an<br />
order parameter associated with the pseudo-gap phase is competing with the<br />
superconducting one. In this context a proposal has been made recently that a<br />
true broken symmetry is the origin of the pseudo-gap [137]. This state would<br />
lead to a breakdown of time reversal symmetry in which the circulating currents<br />
obey translational symmetry.<br />
One of the recent proposal is that the anomalous properties of the cuprates<br />
may be due to quantum critical fluctuations of current patterns formed spontaneously<br />
in the CuO 2 planes. Related to this assumption, a break-through was<br />
realized recently by Bourges et al. [136]: by using polarized elastic neutron diffraction,<br />
they report the signature of an unusual magnetic order in the underdoped<br />
phase of YBa2Cu3O6 + x (YBCO). They argue that this hidden order param-