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Résumé |
In the recent tour-de-force experiments [1], the group of Kapitulnik at
Stanford observed spontaneous time-reversal symmetry breaking in $\rm
Sr_2RuO_4$ and in underdoped cuprates by measuring the optical polar Kerr
effect. In this talk, I discuss theoretical challenges [2] for a theoretical
calculation of the polar Kerr angle for $\rm Sr_2RuO_4$ in the $p_x+ip_y$
superconducting state, which represents the Copper pairing with a non-zero
angular momentum $L_z=1$. Then, I present a scenario of the chiral
$d_{xy}+id_{x2-y2}$ density wave [3], which we proposed to explain the
time-reversal symmetry breaking in underdoped YBCO. This state describes
staggered electron currents circulating around the plaquettes of a square
lattice, with the symmetry between the plaquettes broken in a checkerboard
manner.
[1] J. Xia et al., PRL 97, 167002 (2006); 100, 127002 (2008).
[2] V.M. Yakovenko, PRL 98, 087003 (2007); R. M. Lutchyn, P. Nagornykh, and
V.M. Yakovenko, PRB 77, 144516 (2008); in progress.
[3] S. Tewari, C. Zhang, V.M. Yakovenko, and S. Das Sarma, PRL 100, 217004
(2008); arXiv:0803.3220.
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