Vortex phases and glassy dynamics in the highly anisotropic superconductor HgBa$_2$CuO$_{4+δ}$

We present an extensive study of vortex dynamics in a high-quality single crystal of HgBa$_2$CuO$_{4+δ}$ (Hg1201), a highly anisotropic superconductor that is a model system for studying the effects of anisotropy. From magnetization $M$ measurements over a wide range of temperatures $T$ and fields $H$, we construct a detailed vortex phase diagram. We find that the temperature-dependent vortex penetration field $H_p(T)$, second magnetization peak $H_{smp}(T)$, and irreversibility field $H_{irr}(T)$ all decay exponentially at low temperatures and exhibit an abrupt change in behavior at high temperatures $T/T_c \gtrsim 0.5$. By measuring the rates of thermally activated vortex motion (creep) $S(T,H)=|d \ln M(T,H) / d \ln t|$, we reveal glassy behavior involving collective creep of bundles of 2D pancake vortices as well as temperature- and time-tuned crossovers from elastic (collective) dynamics to plastic flow. Based on the creep results, we show that the second magnetization peak coincides with the elastic-to-plastic crossover at low $T$, yet the mechanism changes at higher temperatures.

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