A magnetic quantum phase transition in the electron-doped cuprates

1. A magnetic quantum phase transition in the electron-doped cuprates Richard L. Greene, University of Maryland,DMR 0352735 • The existence of a quantum phase transition (QPT), and its relation to the mechanism of high-temperature superconductivity (SC), is of great current interest. The electron-doped high-Tc superconductors are a good system to study this issue because their low critical magnetic field allows access to the very low temperature normal state. Elastic neutron scattering (NS) measurements have shown evidence of a magnetic QPT, but there is a significant disagreement among these measurements because the uniform oxygen reduction of large crystals is difficult. • We recently found [1] that an angular magnetoresistance (AMR) measurement could track static or quasi-static antiferromagnetism (AFM) by the onset of a four-fold oscillation in electron-doped cuprate thin films (Top Figure). As a result, a coexistence of AFM and SC in the same high quality sample and a magnetic QPT above the optimal doping were observed. This QPT suggests a magnetic origin for the Fermi surface topological reconstruction, which has previously been observed in electron-doped cuprate superconductors. • The QPT found by our AMR measurements occurs at a quite different doping from the one proposed by a recent inelastic neutron scattering result (Bottom Figure). This raises the question of whether our QPT is caused by a long range order AFM, a short range (quasi-static) AFM, or even some other magnetism. A better understanding of this is certainly important since the QPT emerges within the superconducting dome, suggesting the coexistence of SC and AFM and a magnetic origin for the high-Tc superconductivity. • Yu et al., Phys. Rev. B 76, 020503 R ( 2007) Top: A fourfold oscillation of the AMR signals the onset of a static AFM in the electron-doped cuprates Pr2-xCexCuO4. Bottom: The temperature scales of AFM measured by different probes (green line: NS; squares: uSR; triangles: optical; circles: AMR

2. A magnetic quantum phase transition in the electron-doped cuprates Richard L. Greene, University of Maryland,DMR-0352735 Education Societal Impact Two graduate students (Josuha Higgins and Paul Bach) and one postdoc (Weiqiang Yu) contributed to this work. Dr. Yu has accepted a faculty job in China and Dr. Higgins is now a postodoc in England. Mr. Bach continues his graduate research. 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 and electricity generation and distribution.