Professor Birgeneau's research is primarily concerned with the phases and phase transition behavior of novel states of matter. These include one and two dimensional quantum magnets, highly disordered magnets, lamellar CuO2 high temperature superconductors, and Fe pnictide and chalcogenide superconductors. He uses primarily neutron and x-ray scattering techniques to probe these systems. The neutron and x-ray scattering experiments are carried out at national facilities located in Berkeley, Stanford, Maryland, Tennessee, Canada, England, Germany and Japan. His group has also implemented state-of-the-art materials growth and characterization facilities at LBL and on campus.
The physics of highly correlated electronic materials is controlled by both quantum effects and many body electron-electron interactions. This means that both the microscopic and macroscopic properties differ dramatically from those which one would deduce using traditional one-electron techniques. The most spectacular manifestation of quantum many body behavior is high temperature superconductivity which is found in a number of doped lamellar CuO2 ceramic materials. We are pursuing a variety of strategies to elucidate the fundamental physics of high temperature superconductors with an emphasis on the interplay between microscopic antiferromagnetic spin fluctuations and the superconductivity. We are also studying related low dimensional magnetic systems in which quantum and/or frustration effects produce behavior which is fundamentally different from that manifested by the equivalent classical system.
Two decades after the discovery of the CuO2 high temperature superconductors, quite unexpectedly, an entirely new class of superconductors based on sheets of FeAs or Fe(Se/Te) has been discovered. The phase diagrams of these new superconducting systems have many similarities to that of the copper oxide superconductors but there also are some essential differences. For example, the copper oxide parent materials are invariably antiferromagnetic Mott insulators whereas for the Fe-based materials the parent materials vary from being antiferromagnetic semimetals to antiferromagnetic narrow band gap semiconductors. In the Fe systems the structural and magnetic transitions are intimately connected to each other whereas in the copper oxides the structural transition is benign. This new field is at the stage where materials discovery, materials fabrication and characterization are playing the dominant role. Accordingly, our group is focused on growing large single crystals of Fe pnictide and chalcogenide superconductors across the entire phase diagrams and characterizing the materials using bulk property measurements together with neutron and synchrotron x-ray scattering techniques.
R.J. Birgeneau, C. Stock, J.M. Tranquada, and K. Yamada, Magnetic Neutron Scattering in Hole-doped Cuprate Superconductors, J. Phys. Soc. Jpn., 75, 111003, (2006)
S.D. Wilson, Z. Yamani, C.R. Rotundu, B. Freelon, E. Bourret-Courchesne, and R.J. Birgeneau, Neutron Diffraction Study of the Magnetic and Structural Phase Transitions in BaFe2As2, Phys. Rev. B, 79, 184519 (2009)
J. Wen, G. Xu, G. Gu, J.M. Tranquada, and R.J. Birgeneau, Interplay between Magnetism and Superconductivity in Iron-Chalcogenide Superconductors: Crystal Growth and Characterization, Rep. Prog. Phys. 74, 124503 (2011)
J. Zhao, H. Cao, E. Bourret-Courchesne, D.-H. Lee and R.J. Birgeneau, Neutron Diffraction Measurements of an Antiferromagnetic Semiconducting Phase in the Vicinity of the High Temperature Superconducting State of KxFe2-xSe2, Phys. Rev. Lett. 109, 267003 (2012)
S.P. Bayrakci, D.A. Tennant, Ph. Leininger, T. Keller, M.C.R. Gibson, S.D. Wilson, R.J. Birgeneau, and B. Keimer, Lifetimes of Antiferromagnetic Magnons in Two and Three Dimensions: Experiment, Theory, and Numerics, Phys. Rev. Lett. 111, 017204, (2013)