We study underdoped high-Tc superconductors YBa2Cu3O6.5 and YBa2Cu4O8 usingfirst-principles pseudopotential methods with additional Coulomb interactionsat the Cu atoms, and obtain Fermi-surface pocket areas in close agreement withmeasured Shubnikov-de Haas and de Haas-van Alphen oscillations. Withantiferromagnetic order in CuO2 planes, stable in the calculations, small holepockets are formed near the so-called Fermi-arc positions in the Brillouin zonewhich reproduce the low-frequency oscillations. A large electron pocket,necessary for the negative Hall coefficient, is also formed in YBa2Cu3O6.5,giving rise to the high-frequency oscillations as well. Effective masses andspecific heats are also calculated and compared with measurements. Our resultshighlight the important role of magnetic order in the electronic structure ofunderdoped high-Tc superconductors.
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