Amorphization of vortex matter and indication of a reentrant peak effect in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$

The peak effect (PE) has been observed in a twinned crystal of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ for $H\Vert c$ in the low-field range, close to the zero-field superconducting transition temperature $\left[T_c\mathrm{}\right(0\left)\mathrm{}\right].$ A sharp depinning transition succeeds the peak temperature $T_p$ of the PE. The PE phenomenon broadens and its internal structure smoothens out as the field is increased or decreased beyond the interval between 250 and 1000 Oe. Moreover, the PE could not be observed above 10 kOe and below 20 Oe. The locus of the $T_p\mathrm{}\left(H\right)\mathrm{}$ values is indicative of the occurrence of a reentrant characteristic with a noselike feature located at $T_p{\mathrm{}\left(H\right)/T}_c\mathrm{}\left(0\right)\mathrm{}\approx 0.99$ and $H\approx 100\hspace{0.5em}\mathrm{Oe}$ (where the flux line lattice constant $a_0$≈penetration depth $\lambda ).\mathrm{}$ The upper part of the PE curve $(0.5<H<\mathrm{}10\mathrm{}\hspace{0.5em}\mathrm{kOe})$ can be fitted to a melting scenario with the Lindemann number $c_L\approx \mathrm{0.25.}$ The vortex phase diagram near $T_c\mathrm{}\left(0\right)\mathrm{}$ determined from the characteristic features of the PE in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }\left(H\Vert c\right)$ bears close resemblance to that in the $2\mathrm{H}-{\mathrm{NbSe}}_2$ system, in which a reentrant PE had been distinctly observed earlier.