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1. Link between spin fluctuations and Cooper pairing in electron-doped cuprate superconductorsRichard L. Greene, University of Maryland College Park, DMR 0653535 High-temperature superconductivity could revolutionize technologies ranging from magnetically-levitated trains to power transmission for renewable energy. However, to achieve such practical applications, one needs superconductors that can work above the room temperature (~300 K) to reduce the cost. So far, the copper oxide (cuprate) superconductors hold the record for the highest superconducting transition temperature (Tc ~ 133 K) among all superconductors. Therefore, to obtain room temperature superconductivity, it is crucial to understand the mechanism of the superconductivity in the cuprate high-Tc superconductors. Since 1958, when the BCS theory was developed, it has been known that in low-Tc superconductorsthe conducting electrons of the material are bound together in “Cooper pairs”, a lower energy state that permits electrical conduction with no resistance. The binding of the Cooper pairs is caused by a coupling between the electrons and the thermal vibrations of the atoms themselves, a complex many-body quantum effect called the electron-phonon interaction. But, the electron-phonon mechanism cannot explain the Cooper pairing found in the high-temperature superconducting (high-Tc) cuprates. So what is the pairing “glue”, or is there even a pairing “glue”, in high-Tc superconductors? By studying the magnetotransport at low temperatures in the electron-doped cuprate La2-xCexCuO4 (LCCO) as a function of Ce doping, we found a direct correlation between an anomalous linear-in-temperature variation of the electrical resistivity in the normal state of superconducting films and the superconducting Tc. The linear-in-temperature resistance was shown to arise from scattering of the electrons from spin fluctuations associated with the antiferromagnetism (shown in Fig. 1). Experiments by others had previously shown that spin fluctuations are present in all samples that are superconducting. Therefore, our work very strongly suggests that an electron-spin fluctuation interaction is the cause of high-Tc superconductivity. This leads to hope that it may be possible to enhance the Tc in other magnetic materials where the pairing of electrons by magnetic excitations (spin fluctuations) should exist. Our experimental transport results, in conjunction with some recent inelastic X-ray scattering experiments, rule out many of the theories that had been previously proposed for high-Tc superconductivity. K. Jin, N. P. Butch, K. Kirshenbaum, J. Paglione, and R. L. Greene, Nature 476, 73 (2011). Temperature-doping phase diagram of La2-xCexCuO4 determined from our transport experiments. The linear-in-T (n = 1) normal-state resistivity regime (red) surrounds the superconducting (SC) dome (yellow), both terminating at xc. Beyond the SC dome, the resistivity is quadratic in temperature (blue) below a Fermi-liquid crossover temperature, TFL. The magnitude of the n =1 resistivity is found to be proportional to the Tc at each doping, 0.1 < x < 0.17.

2. Link between spin fluctuations and Cooper pairing in electron-doped cuprate superconductors Richard L. Greene, University of Maryland College Park, DMR 0653535 Societal Impact Education A postdoc (Kui Jin) contributed to this work. Dr. Kui Jin continues his research at Maryland to better understand cuprate superconductors and also search for new superconductors. This work also involved a collaboration with J. Paglione’s group (N.P.Butch,K. Kirshenbaum), supported by a different NSF grant at the University o f Maryland. An understanding of the mechanism causing high temperature superconductivity may enable the development of new materials that are superconducting above room temperature. This would have a large impact on electronic devices , electricity generation and distribution, and renewable energy.