VORTEX PHASE DIAGRAM OF BI2SRCACU2O8+DELTA : C-AXIS SUPERCONDUCTING CORRELATION IN THE DIFFERENT VORTEX PHASES

1998 
The vortex phase diagram in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ is investigated by studying the linear ac transverse permeability ${\ensuremath{\mu}}_{\ensuremath{\perp}}={\ensuremath{\mu}}_{\ensuremath{\perp}}^{\ensuremath{'}}+{\ensuremath{\mu}}_{\ensuremath{\perp}}^{\ensuremath{''}} {(h}_{\mathrm{ac}}\ensuremath{\perp}H\ensuremath{\Vert}c$ axis) in a wide range of temperature and magnetic fields. At low fields $(Hl300\phantom{\rule{0ex}{0ex}}\mathrm{O}\mathrm{e})$superconducting phase coherence in the $c$ direction shows that the ordered vortex solid state is characterized by a $c$-axis vortex correlation length ${l}_{c}$ limited by the sample thickness $d$. At low temperatures $(Tl20\phantom{\rule{0ex}{0ex}}\mathrm{K})$ point disorder is sufficiently strong and the phase coherent ordered vortex solid becomes metastable. In the temperature range $20lTl42\mathrm{K}$ when the applied field exceeds ${H}_{\mathrm{ent}}(T)$, the ordered vortex state transforms into an entangled state with no indication of reduction of ${l}_{c}.$ At a higher crossover field the linear response becomes Ohmic, ${l}_{c}ld$. At higher temperatures the vortex structure becomes an uncorrelated vortex liquid in all directions.
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