Browsing by Author "Ramesh, R."

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    Dramatically enhanced polarization in (001), (101), and (111) BiFeO3 thin films due to epitiaxial-induced transitions
    Li, Jiefang; Wang, Junling; Wuttig, Manfred; Ramesh, R.; Wang, Naigang; Ruette, Benjamin; Pyatakov, Alexander P.; Zvezdin, Anatoly Konstantinovich; Viehland, Dwight D. (AIP Publishing, 2004-06-01)
    Dramatically enhanced polarization has been found for (001), (101), and (111) films, relative to that of BiFeO3 crystals. The easy axis of spontaneous polarization lies close to (111), for the various oriented films. BiFeO3 films grown on (111) have a rhombohedral structure, identical to that of single crystals; whereas films grown on (101) or (001) are monoclinically distorted from the rhombohedral structure, due to the epitaxial constraint. (C) 2004 American Institute of Physics.
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    Epitaxially induced high temperature (> 900 K) cubic-tetragonal structural phase transition in BaTiO3 thin films
    Bai, Feiming; Zheng, H. M.; Cao, Hu; Cross, L. E.; Ramesh, R.; Li, Jiefang; Viehland, Dwight D. (AIP Publishing, 2004-11-01)
    For (001)(c) oriented BaTiO3 thin films, it has been found that epitaxial constraint can result in a dramatic increase in the temperature of a tetragonal (T) structural phase transition. For 2000-Angstrom-thick films grown directly on SrTiO3 substrates, a T-->cubic (C) phase transition was found on heating at >950 K, where the lattice constant changed smoothly with temperature. It was also found for films of the same thickness that the T-->C transition is nearly restored to that of bulk crystals by the use of a buffer layer, which relaxes the epitaxial constraint. These results provide evidence of an epitaxially induced high temperature structural phase transition in BaTiO3 thin films, where the ferroelectric (internal) and structural (external) aspects of the phase transition are decoupled. (C) 2004 American Institute of Physics.
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    General open and closed queueing networks with blocking: A unified framework for approximation
    Vroblefski, Mark; Ramesh, R.; Zionts, Stanley (INFORMS, 2000)
    In this paper, we develop a unified framework for approximating open and closed queueing networks under any general blocking protocol by extending and generalizing the approximation algorithm for open tandem queues under minimal blocking presented in Di Mascolo et al. (1996). The proposed framework is based on decomposition. We develop decomposition structures and analysis algorithms for any general blocking system using the framework. The proposed algorithms have been extensively tested using simulations as a benchmarking device. The results show that the proposed framework yields robust, reliable, and accurate estimates of system characteristics, such as throughput and Work-in-process inventory in a wide range of system configurations. The computational load is minimal. The unified framework presents a highly useful set of tools of analysis for queueing-system designers to use in evaluating the performance under numerous design alternatives. Directions for future research are presented, with a focus on critical application areas such as packet-switching-network design and cellular manufacturing.
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    Two-phonon coupling to the antiferromagnetic phase transition in multiferroic BiFeO(3)
    Ramirez, Mariola O.; Krishnamurthi, M.; Denev, S.; Kumar, Amit; Yang, Seung-Yeul; Chu, Ying-Hao; Saiz, Eduardo; Seidel, Jan; Pyatakov, Alexander P.; Bush, A.; Viehland, Dwight D.; Orenstein, J.; Ramesh, R.; Gopalan, Venkatraman (AIP Publishing, 2008-01-01)
    A prominent band centered at similar to 1000-1300 cm(-1) and associated with resonant enhancement of two-phonon Raman scattering is reported in multiferroic BiFeO(3) thin films and single crystals. A strong anomaly in this band occurs at the antiferromagnetic Neel temperature, T(N)similar to 375 degrees C. This band is composed of three peaks, assigned to 2A(4), 2E(8), and 2E(9) Raman modes. While all three peaks were found to be sensitive to the antiferromagnetic phase transition, the 2E(8) mode, in particular, nearly disappears at T(N) on heating, indicating a strong spin-two-phonon coupling in BiFeO(3). (c) 2008 American Institute of Physics.