Evidence for linelike vortex liquid phase in ${\mathrm{Tl}}_2{\mathrm{Ba}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ probed by the Josephson plasma resonance

We measured the Josephson plasma resonance (JPR) in optimally doped ${\mathrm{Tl}}_2{\mathrm{Ba}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ thin films using terahertz time-domain spectroscopy in transmission. The temperature and magnetic field dependence of the JPR frequency shows that the c-axis correlations of pancake vortices remain intact at the transition from the vortex solid to the liquid phase. In this respect ${\mathrm{Tl}}_2{\mathrm{Ba}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ films, with anisotropy parameter γ≈150, are similar to the less anisotropic ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ $\left(\gamma \approx 8\right)$ rather than to the most anisotropic ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ single crystals (γ⩾500).