Anomalous ac magnetic susceptibility of high-temperature $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ superconductors

An anomalous effect in the low-field ac susceptibility of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ superconductors with strong frequency dependence is presented. This phenomenon appears in samples whose intergranular currents flow with low values, and its existence is analyzed in terms of the presense of Josephson vortices and their thermally activated creep of Kim-Anderson type with small pinning potential. Its existence highlights a possible problem in the research and development of high-temperature superconductors; some materials with high $J_c$ defined by $E=E_c={10}^{-4}\mathrm{V}∕\mathrm{m}$ may still have a large resistivity at lower $E$.

Figure 1

Measured perpendicular ${\chi }^{\prime }$ and ${\chi }^{″}$ at $77\mathrm{K}$ as functions of ${\mu }_0{H}_m$ and $f$ for YBCO films deposited on $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ single crystal with (a) ${\mathrm{Ba}}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3$ and (b) Au as buffer layer. The dashed lines are calculated from the Bean model for thin disks (Ref. 9).

Figure 2

Measured axial ${\chi }^{\prime }$ and ${\chi }^{″}$ at $77\mathrm{K}$ as functions of ${\mu }_0{H}_m$ and $f$ for a sintered YBCO disk after (a) zero-field cooling and (b) high-field premagnetization.

Figure 3

Calculated ${\chi }^{\prime }$ and ${\chi }^{″}$ as functions of $H_m∕J_{c}a$ at $f=100–3000\mathrm{Hz}$ for a superconducting cylinder of radius $a=5\mathrm{mm}$ with Kim-Anderson type $E\left(J\right)=(E_{c}J∕\mid J\mid )\exp \left[(U_0∕kT)(\mid J\mid ∕J_c-1)\right]$ and ${\rho }_0=E(J_c∕10)∕(J_c∕10)$ at $\mid J\mid <J_c∕10$, where $E_c={10}^{-4}\mathrm{V}∕\mathrm{m}$, $J_c={10}^4\mathrm{A}∕{\mathrm{m}}^2$, and $U_0∕kT=5$ (a) and 2 (b). Arrows indicate the direction of increasing $f$. The $E∕E_c$ as a function of $J∕J_c$ used in case (a) is shown in the inset.