Microwave absorption at low magnetic field in sintered ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$: Freezing effects at low temperature and superconducting-glass model

The Josephson-junction (JJ) model in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ sintered samples is confirmed by means of microwave absorption measurements at low magnetic field (<500 G). An explanation of measures supporting the JJ model is given. At low temperature (<7 K) experimental evidence shows that the array of junctions is driven by the magnetic field in a frozen state. A qualitative interpretation of this effect in the frame of the superconducting-glass picture is presented.