H. Mook, ORNL
“Search for Magnetism in YBCO Superconductors”
H. Mook described his work with P. Bourges and their collaborators at the Laboratoire Léon Brillouin (LLB), Saclay, France. The doping corresponded to O(6.6). The Oxygen chain ordering was striking and corresponded to the Ortho-II state (for details, see the reference below). The large crystal weighing 25g was previously studied for the detection of d-density wave: Mook et al. Phys. Rev. B 66, 144513 (2002). Mook reiterated that the previous experiment did exhibit evidence of d-density wave although the signal was weak. The present experiment was designed to detect circulating currents proposed by C. M. Varma in the same sample.
What makes this effort puzzling is that in an unpublished work done at NIST no evidence for circulating currents was found, while the same sample examined in LLB showed strong evidence of circulating currents in agreement with the experiment of B. Fauqué et al. Phys. Rev. Lett. 96, 197001 (2006). To quote Mook, “If you can see it in France, why can’t you see it here?” The joke apart, there may be some serious reasons for this. The beam at LLB was better, with a higher flux and therefore higher intensity. More importantly, the flipping ratio, crucially important for polarized neutron scattering, was much larger, 70 as opposed to 23.3 in the NIST experiment. According to Mook these factors contributed significantly to the success of the measurement in LLB.
Indeed, the rise of the spin flip intensity at (101) at 200K was very sharp and continued through without the slightest hint of the superconducting transition. However, no such scattering was found at (200). The size of the moment is not yet known but the scattering cross section is 1 mb, probably corresponding to a few hundredths of a Bohr magneton. The real difficulty is that the observed direction of the moments is at an angle of 45 degrees, not along the c-direction.
P. A. Lee suggested that it would be useful to repeat the previous measurements at
(1/2,1/2) at LLB. I completely agree.
The bigger issue involves the elusive signatures of magnetic order in the pseudogap phase of high temperature superconductors. I find it particularly puzzling that the same sample shows two signatures of orbital magnetic order: one that breaks translational symmetry of the lattice and the other that does not. However, it is heartening that competing groups are now collaborating to unravel some of the mysteries of the pseudogap. This is real progress.
Thursday, August 2, 2007
Herb Mook: Search for Magnetism in YBCO Superconductors
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The following is an important email exchange with Herb Mook. The two PRB papers that he is referring to are: Phys. Rev. B 66, 144513 (2002) (already contained in my previous blog) and the second one is Phys. Rev. B 69, 134509 (2004).
Begin forwarded message:
From: Herb A. Mook
Date: August 10, 2007 11:09:48 AM PDT
To: Sudip Chakravarty
Subject: Re: DDW
Sudip
I want to make it clear again that the two PRB papers showing a small broad peak on a sloping background at Pi Pi are absolutely correct. Please make sure everybody knows this. In fact this has been seen at both NIST and at the ILL with polarized beams on two crystals. The peaks are too small to get a very good picture of their width, and thus their absolute intensity, but there is certainly a signal there.
Herb
Dear Herb,
I think that your talk gave everybody the impression that you have discredited your earlier measurements published in PRB. I don't think you said so, but that's what people took away from the talk. In fact, I would like to know if this is truly what you think, and if this is what you are going to write jointly with Bourges. Please let me know, because I want write a comment on the blog and perhaps write a paper on this topic soon.
I still cannot find a good reason why the moments are so canted without invoking spin-orbit coupling both for circulating currents and earlier DDW measurements.
Sudip
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