= Solution
For an ideal long specimen with negligible <demagnetizing field>, the Meissner state has $B=\mu_0(H+M)=0$, so $M=-H$. A <type-I superconductor> retains this response up to its thermodynamic critical field $H_c(T)$ and then enters the normal state, where the small normal magnetic response can be neglected. Nonzero <demagnetizing fields> can create an intermediate state and broaden the measured transition.
A <type-II superconductor> is fully screening only below $H_{c1}(T)$. Between $H_{c1}$ and $H_{c2}$, magnetic flux enters as quantized vortices and the reversible <magnetization> remains diamagnetic but decreases in magnitude towards zero at $H_{c2}$. Above $H_{c2}$ the bulk is normal. Ginzburg–Landau theory distinguishes the types at $\kappa=1/\sqrt2$, not at $\kappa=1$; the later condition in the source merely restricts its estimate.
The $H$–$T$ sketch has two increasing-width regions below $T_c$: Meissner below the lower boundary, mixed <vortex> state between the two boundaries, and normal state above the upper boundary. Both critical fields vanish at $T_c$. The phase-diagram panel of the original figure in <past-exam-of-the-condensed-matter-physics-major-option-course-of-the-university-of-oxford/2018/3/a/solution> shows this topology; its normalized field curves are illustrative, not a fit to the source <magnetization> loop.
Back to article page
