Zhi-Xun Shen, Stanford University, DMR 0604701

1. Polaron Coherence Condensation as a Mechanism for Colossal Magnetoresistive in Layered Manganites Zhi-Xun Shen, Stanford University,DMR 0604701 We have shown with angle-resolved photoemission spectroscopy (ARPES) that the electronic structure in the ferromagnetic metallic (FM) groundstate of the colossal magnetoresistive (CMR) bilayer manganite La1.2Sr1.8Mn2O7 (LSMO) shows striking similarities to that of the pseudogap phase in heavily underdoped cuprates high temperature superconductors (HTSC) [Mannella et al., Nature, 438, 474 (2005) ]. More recently, our ARPES data in LSMO show that, upon lowering the temperature below the Curie point, a coherent polaronic metallic groundstate emerges very rapidly with well defined quasiparticles which track remarkably well the electrical conductivity, consistent with macroscopic transport properties. This phenomenology, which constitutes an unexpected manifestation of collective effect in a single particle spectroscopy like ARPES, provides the first spectroscopic evidence of a kinetic energy driven insulator-to-metal transition as a result of coherent polaron condensation. Our data demonstrate that the insulator-to-metal transition in layered manganites is a coherence-driven effect resulting from the polaron condensation, a phenomenon which exhibits striking analogies to the case of underdoped cuprate high temperature superconductors, where the superconducting transition is controlled by the phase coherence of the Cooper pairs. Together, these cases argue that coherence-driven transitions provide a rationale for the novel high temperature quantum phenomena in doped transition metal oxides. Nature, 438, 474 (2005), and Submitted to Phys. Rev. Lett.

2. Polaron Coherence Condensation as a Mechanism for Colossal Magnetoresistive in Layered Manganites Zhi-Xun Shen, Stanford University, DMR 0604701 Education Postdoc (Dr. N. Mannella) worked on research supported by this NSF Award Polaron Quantum Coherence in Layered Manganites Societal Impact By suggesting that coherence-driven transitions provide a rationale for the novel high temperature quantum phenomena as found in high temperature superconductors and layered manganites, our results are of extreme interest and importance to the scientific community because of the broad implications for the study of strongly correlated transition metal oxides, exotic materials with promising revolutionary technological applications such as high temperature superconductivity, colossal magnetoresistance and large thermoelectric power.