Zhi-Xun Shen, Stanford University - DMR- 0304981

1. Nodal Quasiparticles in Colossal Magnetoresistive Manganites Zhi-Xun Shen, Stanford University -DMR- 0304981 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). The FM phase is a polaronic metal, albeit with a strong anisotropic distribution of spectral weight in momentum space exhibiting a nodal–antinodal dichotomy. Quasiparticle excitations (QP) have been detected for the first time along the nodal direction (i.e. diagonal), and they are found to determine the metallic transport properties in the FM phase. Since this nodal-antinodal dichotomy in momentum space was so far considered a characteristic unique feature of the copper oxide HTSC, these findings cast doubt on the assumption that the pseudogap state and the nodal-antinodal dichotomy in the copper oxides HTSC are hallmarks of the superconductivity state. Furthermore, we found that the temperature dependent evolution of the nodal QP in LSMO tracks remarkably well the DC conductivity, thus accounting for the macroscopic transport properties in LSMO. Our results indicate that the microscopic mechanism leading to the CMR effect in manganites is intrinsically a quantum phase transition which is kinetic energy driven and linked to a crossover from a small polaron hopping regime in the paramagnetic state to a coherent polaronic conductor in the FM state. Polaron Quantum Coherence Nature, 438, 474 (2005)

2. Nodal Quasiparticles in Colossal Magnetoresistive Manganites Zhi-Xun Shen, Stanford University -DMR- 0304981 Education Postdoc (Dr. N. Mannella) and students (Tanja Cuk and Ruihua He) worked on research supported by this NSF Award Societal Impact By discovering that two of the most important classes of materials in condensed matter physics (i.e. the high temperature superconductors and the colossal magnetoresistive manganites) are very similar in regards to their electronic excitations, 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 and colossal magnetoresistance.