Dynamical and Quenched Disorder of 2-DIMENSIONAL Charge Stripes in LA5/3SR1/3NIO4

Strontium doped La₂NiO₄ is isostructural with the high T_c cuprates La_2-xSr_xCuO₄ and La_2-xBa_xCuO₄. The hole-doped La₂NiO₄ system is a prototypical system for the understanding of strong electron-phonon coupling, and the resultant effects on material properties. The substitution of La with Sr provides holes, which tend to order at low temperatures into stripes. At doping levels of 1/3 and 1/2 commensurate charge modulations have been observed using electron diffraction. Neutron scattering has been used to show charge and spin ordering are common to many nickelates. Using synchrotron x-ray scattering on a single crystal of La_5/3Sr_1/3NiO₄, we have demonstrated the existence of charge stripes at low temperatures. Satellite reflections were located at positions (h±2ɛ 0 l), (ɛ = 1/3, h= even and l= odd) below the charge ordering transition T_CO. The electronic transition into the charge stripe phase is second order without any corresponding structural transition. Above the transition (T_co ≈ 240 K) critical scattering was observed due to fluctuations into the charge stripe phase. The charge stripes are shown to be two-dimensional in nature, both by measurements of the correlation lengths (ξ_a≈ 150 Å, ξ_b≈300 Å, ξ_c≈25 Å.), and by the critical exponents of the charge stripe melting which show that the system is in the 2D universality class (2β = 0.25). The charge stripe ordering does not develop into long range order indicating that the stripes are disordered and quenched at the lowest temperatures. Detailed measurements of the correlation lengths at higher temperatures, just below the charge stripe melting temperature, display an anisotropic broadening indicative of an order-disorder transition. We ascribe these results to a smectic-nematic phase transition at approximately 225 K. Such electronic quantum liquid-crystal phase transitions have been theoretically predicted before, but these results provide the first experimental evidence for them in charge stripe nickelates.