Spin Susceptibility in Underdoped YBa2Cu3O6+x\bf YBa_2Cu_3O_{6+x}

We report a comprehensive polarized and unpolarized neutron scattering study of the evolution of the dynamical spin susceptibility with temperature and doping in three underdoped single crystals of the \YBCO{6+x} high temperature superconductor: \YBCO{6.5} (Tc = 52 K), \YBCO{6.7} (Tc = 67 K), and \YBCO{6.85} (T_c = 87 K). Theoretical implications of these data are discussed, and a critique of recent attempts to relate the spin excitations to the thermodynamics of high temperature superconductors is given.Comment: minor revisions, to appear in PR

Change in Neutron Polarization Induced by a Superconductor Rotated in a Magnetic Field

Neutron depolarization (i.e. the change in the polarization vector projection onto the applied magnetic field) depends on neutron Larmor frequency $\omega_{\rm L}$ in the superconductor and on a frequency $\omega$, $\omega$ being related to the directional variation of the magnetic induction inside the superconductor. Depolarization is small if $\eta = \omega_{\rm L}/\omega \gg 1$. In the opposite limite $\eta \ll 1$, the depolarization is due to neutron spin rotation about the magnetic flux, which, in turn, rigidly rotates with a superconductor. a total spin reversal is possible at $\eta = 1$. Depolarization at a stationary rotation angle in a superconducting plate carrying a transport critical current in a magnetic field is considered as well. Depolarisation is determined by either longitudinal or transversal critical currents depending on the angle between the applied field and the critical current direction