Weber F, Aliouane N, Zheng H, Mitchell J F, Argyriou D N, Reznik D
Forschungszentrum Karlsruhe, Institut für Festkörperphysik, POB 3640, D-76021 Karlsruhe, Germany.
Nat Mater. 2009 Oct;8(10):798-802. doi: 10.1038/nmat2513. Epub 2009 Aug 16.
Charge carriers in low-doped semiconductors may distort the atomic lattice around them and through this interaction form so-called small polarons. High carrier concentrations on the other hand can lead to short-range ordered polarons (large polarons) and even to a long-range charge and orbital order. These ordered systems should be insulating with a large electrical resistivity. However, recently a polaronic pseudogap was found in a metallic phase of La(2-2x)Sr(1+2x)Mn(2)O(7) (ref. 7). This layered manganite is famous for colossal magnetoresistance associated with a phase transition from this low-temperature metallic phase to a high-temperature insulating phase. Broad charge-order peaks due to large polarons in the insulating phase disappear when La(2-2x)Sr(1+2x)Mn(2)O(7) becomes metallic. Investigating how polaronic features survive in the metallic phase, here we report the results of inelastic neutron scattering measurements showing that inside the metallic phase polarons remain as fluctuations that strongly broaden and soften certain phonons near the wavevectors where the charge-order peaks appeared in the insulating phase. Our findings imply that polaronic signatures in metals may generally come from a competing insulating charge-ordered phase. Our findings are highly relevant to cuprate superconductors with both a pseudogap and a similar phonon effect associated with a competing stripe order.
低掺杂半导体中的载流子可能会使周围的原子晶格发生畸变，并通过这种相互作用形成所谓的小极化子。另一方面，高载流子浓度会导致短程有序极化子（大极化子），甚至导致长程电荷和轨道有序。这些有序系统应该是具有高电阻率的绝缘体。然而，最近在La(2 - 2x)Sr(1 + 2x)Mn(2)O(7)的金属相中发现了极化子赝能隙（参考文献7）。这种层状锰氧化物以与从低温金属相到高温绝缘相的相变相关的巨磁阻而闻名。当La(2 - 2x)Sr(1 + 2x)Mn(2)O(7)变为金属相时，绝缘相中由大极化子引起的宽电荷有序峰消失。为了研究极化子特征在金属相中是如何存在的，我们在此报告非弹性中子散射测量结果，结果表明在金属相内部，极化子以涨落的形式存在，这些涨落强烈展宽并软化了绝缘相中电荷有序峰出现的波矢附近的某些声子。我们的发现意味着金属中的极化子特征可能通常来自竞争的绝缘电荷有序相。我们的发现与同时具有赝能隙和与竞争条纹序相关的类似声子效应的铜酸盐超导体高度相关。
