Robert J. Birgeneau
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| Research Interests 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, liquid crystals and liquid crystal-nanosilica gels, highly disordered magnets and lamellar CuO2 high temperature 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. Current Projects 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. Thermotropic liquid crystals are encountered commonly in everyday life in various display devices ranging from LCD flat screen televisions to Casio watches. These devices are based on the “nematic” phase of the liquid crystal in which the anisotropic molecules are directionally aligned but positionally disordered as in an ordinary liquid. However, many liquid crystals exhibit “smectic” phases which are characterized by various types of novel one and two dimensional positional order. These smectic phases turn out to be of fundamental interest in statistical physics. Our group is now exploring the fundamental properties of a completely new and quite interesting phase of matter—liquid crystal silica gels. These gels are created by artfully combining smectic liquid crystals with silica nanospheres. The silica nanospheres hydrogen bond to form chains which crosslink to form a dilute three dimensional gels with the liquid crystal molecules in the interstices. We are using synchrotron x-ray techniques to explore the nature of the ordering in such systems. The attendant physics turns out to be remarkably subtle. The basic theory is analogous to supersymmetric field theories in elementary particle physics. These studies may involve graduate students in physics, EECS, and materials science as well as undergraduates. C. Stock, W.J.L. Buyers, R.A. Cowley, P.S. Clegg, R. Coldea, C.D. Frost, R. Liang, D. Peets, D. Bonn, W.N. Hardy, R.J. Birgeneau, “From Incommensurate to Dispersive Spin-fluctuations: The High-energy Inelastic Spectrum in Superconducting YBa2Cu3O6.5,” Phys. Rev. B. 71, 024522, (2005), S. Larochelle, M. Ramazanoglu and R.J. Birgeneau, Effects of Disorder on a Smectic A-Nematic Phase Transition, Phys. Rev. E, 73, 060702, (2006), 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. Wakimoto, K. Yamada, J.M. Tranquada, C.D. Frost, R.J. Birgeneau, and H. Zhang, Disappearance of Antiferromagnetic Spin Excitations in Overdoped La2-xSrxCuO4, Phys. Rev. Lett. 98, 247003, (2007)
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