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We measure magnetic quantum oscillations in the underdoped cuprates YBa$_2$Cu$_3$O$_{6+x}$ with $x=0.61$, 0.69, using fields of up to 85 T. The quantum-oscillation frequencies and effective masses obtained suggest that the Fermi energy in the cuprates has a maximum at $papprox 0.11-0.12$. On either side, the effective mass may diverge, possibly due to phase transitions associated with the T=0 limit of the metal-insulator crossover (low-$p$ side), and the postulated topological transition from small to large Fermi surface close to optimal doping (high $p$ side).
Neutron Scattering measurements for YBa$_2$Cu$_3$O$_{6.6}$ have identified small magnetic moments that increase in strength as the temperature is reduced below $T^ast$ and further increase below $T_c$. An analysis of the data shows the moments are an
Polarized and unpolarized neutron diffraction has been used to search for magnetic order in YBa$_2$Cu$_3$O$_{6+x}$ superconductors. Most of the measurements were made on a high quality crystal of YBa$_2$Cu$_3$O$_{6.6}$. It is shown that this crystal
In this paper we explore whether the quantum oscillation signals recently observed in ortho-II YBa$_2$Cu$_3$O$_{6.5}$ may be explained by conventional density functional band-structure theory. Our calculations show that the Fermi surface of YBa$_2$Cu
The de Haas-van Alphen effect was observed in the underdoped cuprate YBa$_2$Cu$_3$O$_{6.5}$ via a torque technique in pulsed magnetic fields up to 59 T. Above an irreversibility field of $sim$30 T, the magnetization exhibits clear quantum oscillation
The superconductor-to-insulator transition (SIT) induced by means such as external magnetic fields, disorder or spatial confinement is a vivid illustration of a quantum phase transition dramatically affecting the superconducting order parameter. In p