Surface superconducting states in a polycrystalline $\mathrm{Mg}{\mathrm{B}}_2$ sample

We report results of a dc magnetic and ac linear low-frequency study of a polycrystalline $\mathrm{Mg}{\mathrm{B}}_2$ sample. ac susceptibility measurements at low frequencies, performed under dc fields parallel to the sample surface, provide clear evidence for surface superconducting states in $\mathrm{Mg}{\mathrm{B}}_2$.

Figure 1

(Color online) (a) Magnetization curve $M\left(H_0\right)$ in high magnetic fields. $\nabla$, theoretical calculations for two-band model. Upper inset, magnetization loop after zero field cooling at $T=34\mathrm{K}$. Lower inset, $H_{\mathit{irr}}\left(T\right)$. (b) Magnetic field dependence of ${\chi }_{\mathit{dc}}\equiv M\left(H_0\right)∕H_0$ for the virgin magnetization curve, ${\chi }^{″}$ and ${\chi }^{\prime }$ at $\omega ∕2\pi =1065\mathrm{Hz}$.

Figure 2

(Color online) Frequency dependence of ${\chi }^{″}$ (a) and ${\chi }^{\prime }$ (b) at $T=34\mathrm{K}$ for different magnetic fields. Identical symbols designate the same dc fields in both figures.

Figure 3

(Color online) Field dependence of the frequency $F_m$ at which losses exhibit a maximum in the curve ${\chi }^{″}$ versus frequency.

Figure 4

(Color online) The ratio $H_{c3}∕H_{c2}$ as a function of polar angle $\theta$ for $\phi =0$, 30°, 45°, and 90°. Insets: (a) angular dependence of $H_{c2}\left(\theta \right)$; (b) $H_{c3}∕H_{c2}\left(\phi \right)$ for $\theta ={90}^{\circ }$.

Figure 5

(Color online) Magnetic field dependence of ${\chi }^{\prime }$ and ${\chi }^{″}$ at $T=34\mathrm{K}$ in high magnetic fields. $H^{*}$ is shown for $\omega ∕2\pi =1065\mathrm{Hz}$. The inset shows fraction of grains in SSSs, $x$, as a function of $H_0∕H_{c3}^c$.