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تسجيل مستخدم جديدThe magnetic phase diagram of underdoped YBa2Cu3Oy inferred from torque magnetization and thermal conductivity
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Strong evidence for charge-density correlation in the underdoped phase of the cuprate YBa2Cu3Oy was obtained by nuclear magnetic resonance (NMR) and resonant x-ray scatter- ing. The fluctuations were found to be enhanced in strong magnetic fields. Recently, 3D (three dimensional) charge-density wave (CDW) formation with long-range order (LRO) was observed by x-ray diffraction in H >15 T. To elucidate how the CDW transition impacts the pair condensate, we have used torque magnetization to 45 T and thermal conductivity $kappa_{xx}$ to construct the magnetic phase diagram in untwinned crystals with hole density p = 0.11. We show that the 3D CDW transitions appear as sharp features in the susceptibility and $kappa_{xx}$ at the fields HK and Hp, which define phase boundaries in agreement with spectroscopic techniques. From measurements of the melting field Hm(T) of the vortex solid, we obtain evidence for two vortex solid states below 8 K. At 0.5 K, the pair condensate appears to adjust to the 3D CDW by a sharp transition at 24 T between two vortex solids with very different shear moduli. At even higher H (42 T) the second vortex solid melts to a vortex liquid which survives to fields well above 45 T. de Haas-van Alphen oscillations appear at fields 24-28 T, below the lower bound for the upper critical field Hc2.
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We have measured temperature and magnetic field dependences of the thermal conductivity along the c-axis, kc, and that along the [110] direction, k110, of CuB2O4 single crystals in zero field and magnetic fields along the c-axis and along the [110] d
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The experimentally measured phase diagram of cuprate superconductors in the temperature-applied magnetic field plane illuminates key issues in understanding the physics of these materials. At low temperature, the superconducting state gives way to a
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Vortices in a type-II superconductor form a lattice structure that melts when the thermal displacement of the vortices is an appreciable fraction of the distance between vortices. In an anisotropic high-Tc superconductor, such as YBa2Cu3Oy, the magne
tic field value where this melting occurs can be much lower than the mean-field critical field Hc2. We examine this melting transition in YBa2Cu3Oy with oxygen content y from 6.45 to 6.92, and fit the data to a theory of vortex-lattice melting. The quality of the fits indicates that the transition to a resistive state is indeed the vortex lattice melting transition, with the shape of the melting curves being consistent with the known change in penetration depth anisotropy from underdoped to optimally doped YBa2Cu3Oy. From the fits we extract Hc2(T = 0) as a function of hole doping. The unusual doping dependence of Hc2(T =0) points to some form of electronic order competing with superconductivity around 0.12 hole doping.
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