Surface pinning of fluctuating charge order: An extraordinary surface phase transition

摘要: We study the mean-field theory of charge-density wave (CDW) order in a layered system, including effect long-range Coulomb interaction and screening by uncondensed electrons. particularly focus on conditions necessary for an ``extraordinary'' transition, which surface orders at higher temperature, is more likely to be commensurate, than bulk. interpret recent experiments ${\mathrm{Ca}}_{2\ensuremath{-}x}{\mathrm{Na}}_{x}\mathrm{Cu}{\mathrm{O}}_{2}{\mathrm{Cl}}_{2}$ as indicating presence commensurate CDW that not present More generally, we show poor tends stabilize incommensurate order, possibly explaining why ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ $\mathrm{Nb}{\mathrm{Se}}_{2}$ remains $T\ensuremath{\rightarrow}0$, despite small magnitude incommensurability.