Comments on the observation of quantum oscillations in cuprates.pdf

1 Comments on the observation of quantum oscillations in cuprates H.D. Drew Center for Nanophysics and Advanced Materials, Department of Physics, University of Maryland, College Park, Maryland 20742, USA. Recently the observation of quantum oscillations was reported in YBCO (ortho-II YBa2Cu3O6.5 and YBa2Cu4O8) with periods in 1/H indicative of Fermi surface pockets much smaller than the large hole pocket in optimally doped cuprates observed by ARPES.1,2 This is an important development in cuprate physics. If their interpretation is correct, the existence of small Fermi pockets in these materials will have profound implications on the structure of the pseudogap state. It heralds a new era in which cuprate Fermiology can be used as a powerful tool to critically address the different ideas about the exotic properties of these materials and, of course, the mechanism for superconductivity. These measurements were made in high magnetic fields on oxygen ordered single crystals of YBCO to achieve the carrier long lifetimes τ and high cyclotron frequency ωC needed to achieve sufficiently large ωCτ for oscillations to be observable. This requirement may limit the applicability of the technique to YBCO at a few doping levels using the materials and magnetic fields currently available. These requirements are not so severe with other techniques that also have given evidence, more indirectly, for small Fermi pockets in both electron and hole doped cuprates. These include ARPES,3,4 DC magneto-transport and Hall effect measured at infrared (IR) frequencies.5-8 Nevertheless, the observation of quantum oscillations is compelling and promises to put Fermiology on a firm footing and decisively on the agenda for the cuprates. In this comment I address some of the issues associated with this new development. Evidence for small Fermi pocket has also been seen in ARPES measurements on underdoped cuprates in the form of “Fermi arcs”.3,4 Th