Xiaoxing Xi Professor of Physics and Materials Science and Engineering Ph.D., Peking University , 1987  Tel: (814)863-5350  Fax: (814)865-3604  xxx4@psu.edu

Condensed Matter Research

Materials Physics of Electronic and Photonic Thin Films

• Research interest
• Recent talks
• Current and past research projects
• Research group
• Selected publications
• My background

News stories about our recent work on MgB₂ thin films.

News stories about our recent work on UV Raman Scattering in BaTiO₃/SrTiO₃ superlattices.

Research interest (Back to the top)

My research interest is in the materials physics underlying the applications of oxide and boride thin films, in particular thin films at the nanoscale. Metal oxides are a class of materials that have a wide variety of novel properties such as superconductivity, ferroelectricity, colossal magneto-resistivity, multiferroicity, etc. Similarly, borides display a variety of interesting magnetic, transport, and structural properties. My research focuses on the understanding of fundamental electrical, optical, and magnetic properties of thin film metal oxides and borides and the effects of structural and interfacial properties on them. Since these properties depend critically on the crystallinity of the materials, fabrication of high quality epitaxial thin films is an important part of my research activities. In my lab, Pulsed Laser Deposition is used to fabricate oxide thin films and heterostructures. A Hybrid Physical-Chemical Vapor Deposition (HPCVD) technique has been developed to deposit epitaxial magnesium diboride and other boride thin films. Thermal evaporation and sputtering are also routinely used in my research.

1. Lattice dynamics in nanoscale ferroelectric and multiferroic thin films and heterostructures. Lattice dynamics, in particular the soft mode behavior, is essential for the properties of ferroelectrics. In our research, we use Raman scattering to measure phonons in ferroelectric and multiferroic thin films and heterostructures, with an emphasis on the nanoscale structures. The past achievements include the discoveries of higher soft-mode frequency in ferroelectric thin films than in bulk crystals, the symmetry breaking in thin films due to defects, and the dramatic effect of epitaxial strain on ferroelectric phase transitions. Our recent groundbreaking work on UV Raman scattering in nanoscaled ferroelectric superlattices shows stunning results of ferroelectricity in one-unit-cell thick layers and tuning of T_c by ~500 K.

2. Multi-band superconductor MgB₂. The multi-band nature of MgB₂ offer unique opportunities to discover new physical phenomena not available in single-band superconductors, and MgB₂ also holds great promises for superconducting digital circuits, for high-field magnets such as in Magnetic Resonance Imaging systems, and for superconducting RF cavities used in accelerators. We have developed the HPCVD technique which deposits in situ high quality epitaxial MgB₂ thin films. The films are very clean and the tensile strain in the films leads to higher-than-bulk T_c values. The HPCVD technique also allows doping of the clean films in a controlled manner, resulting in record-high upper critical field H_c2 of over 60 T. We have made MgB₂ Josephson junctions that operate well above 30 K.

3. Oxide thermoelectric materials. Thermoelectric materials can be used to make energy conversion devices that generate power from thermal sources. In recent years, oxides, in particular layered cobaltates, have been actively studied as a new type of thermoelectric materials. The layered structure in these cobaltates offers a possibility to independently manipulate Seebeck coefficient, conductivity, and thermal conductivity to optimize the figure of merit ZT. Further, these oxides are inherently stable at high temperatures in air, making them very attractive for high temperature applications. We are actively investigating thin film oxide thermoelectric materials with an emphasis on significantly improving the current thermoelectric performances of oxides.

Recent Talks (Back to the top)

• Hybrid Physical-Chemical Vapor Deposition for MgB2 Coated Conductors

• Nanoscale ferroelectricity in BaTiO3/SrTiO3 superlattices probed by ultraviolet Raman spectroscopy

Current and past research projects (Back to the top)

• ONR, "Magnesium diboride Josephson junctions and circuits".

• ACS, "Boride Thin Films and Heterostructures Using Hybrid Physical-Chemical Vapor Deposition"

• ONR, "Integrated HPCVD System for Multifunctional Materials".

• NSF, “NIRT: Strain-Enhanced Nanoscale Ferroelectrics”

• DOE, “Lattice Dynamical Properties of Ferroelectric Thin Films at the Nanoscale”

• BMDO/USAF, "High Q, Tunable Microwave Superconducting Strip-Line Filters"
• NSF, "Hybrid Physical-Chemical Vapor Deposition of MgB₂ Thin Films"
• ONR, "Development of Magnesium Diboride Josephson Junction Technology"
• PSU MRI, "Structural Phase Transitions of Thin Films at the Nanoscale"

• DARPA, "Lattice Dynamics Studies of Loss and Tunability of Ferroelectric Thin Films"

• NSF, "NIRT: Artificially Engineered Nanoscale Ferroelectrics"

• DOE, "Lattice Dynamical Properties of Ferroelectric Thin Films"
• ONR, "Effects of Spin and Charge Fluctuations on HTS Thin Films and Devices"
• ONR, "Strain and Oxygen Stoichiometry Effects on High Temperature Superconductor Thin Films and Devices"
• DARPA, "Ferroelectric Resource Center for FAME"
• DOE, "Vibrational and Electronic Properties of Fullerene and Carbon-Based Clusters"
• NSF, DMR, "Acquisition of a Microwave Network Analyzer System"
• NSF, DMR, "CAREER: Low Loss, Tunable Ferroelectric Thin Films by Pulsed Laser Deposition"
• Naval Research Laboratory, "Dielectric Loss in Ferroelectrics"
• NSF, DMR, "Electrostatic Charging Effects in Perovskite Oxides"

Research Group (Back to the top)

Selected Recent Publications (Back to the top)

Thin film superconductors

• "Large Anisotropic Normal-State Magnetoresistance in Clean MgB₂ Thin Films", Qi Li, B. T. Liu, Y. F. Hu, J. Chen, H. Gao, L. Shan, H. H. Wen, A.V. Pogrebnyakov, J. M. Redwing, and X. X. Xi, Phys. Rev. Lett. 96, 167003 (2006).
• "Planar MgB₂ superconductor-normal metal-superconductor Josephson junctions fabricated using epitaxial MgB₂/TiB₂ bilayers", Ke Chen, Y. Cui, Qi Li, X. X. Xi, Shane A. Cybart, R. C. Dynes, X. Weng, E. C. Dickey, and J. M. Redwing, Appl. Phys. Lett. 88, 222511 (2006).
• "Planar MgB₂ Josephson junctions and series arrays via nanolithography and ion damage", Shane A. Cybart, Ke Chen, Y. Cui, Qi Li, X. X. Xi, and R. C. Dynes, Appl. Phys. Lett. 88, 012509 (2006).
• "A Round Table Discussion on MgB₂ Toward a Wide Market or a Niche Production?—A Summary", Yukikazu Iwasa, David C. Larbalestier, Michiya Okada, Roberto Penco, Michael D. Sumption, and Xiaoxing Xi, IEEE Trans. Appl. Supercond. 16, 1457 (2006).
• "High upper critical field and irreversibility field in MgB₂ coated-conductor fibers", V. Ferrando, P. Orgiani, A. V. Pogrebnyakov, J. Chen, Qi Li, J. M. Redwing, X. X. Xi, J. E. Giencke, Chang-Beom Eom, Qing-Rong Feng, J. B. Betts, and C. H. Mielke,  Appl. Phys. Lett. 87, 252509 (2005).
• "Characterization of off-axis MgB₂ epitaxial thin films for planar junctions", M. Iavarone, G. Karapetrov, A. Menzel, V. Komanicky, H. You, W. K. Kwok, P. Orgiani, V. Ferrando, and X. X. Xi, Appl. Phys. Lett. 87, 242506 (2005).
• "SQUID magnetometer operating at 37 K based on nanobridges in epitaxial MgB₂ thin films", D. Mijatovic, A. Brinkman, D. Veldhuis, H. Hilgenkamp, H. Rogalla, G. Rijnders, D. H. A. Blank, A. V. Pogrebnyakov, J. M. Redwing, S. Y. Xu, Q. Li, and X. X. Xi, Appl. Phys. Lett. 87, 192505 (2005).
• "High-field superconductivity in alloyed MgB₂ thin films", V. Braccini, A. Gurevich, J. E. Giencke, M. C. Jewell, C. B. Eom, D. C. Larbalestier, A. Pogrebnyakov, Y. Cui, B. T. Liu, Y. F. Hu, J. M. Redwing, Qi Li, X. X. Xi, R. K. Singh, R. Gandikota, J. Kim, B. Wilkens, N. Newman, J. Rowell, B. Moeckly, V. Ferrando, C. Tarantini, D. Marré, M. Putti, C. Ferdeghini, R. Vaglio, and E. Haanappel, Phys. Rev. B 71, 012504 (2005).
• "Enhancement of the Superconducting Transition Temperature of MgB₂ by a Strain-Induced Bond-Stretching Mode Softening", A.V. Pogrebnyakov, J.M. Redwing, S. Raghavan, V. Vaithyanathan, D.G. Schlom, S.Y. Xu, Qi Li, D. A. Tenne, A. Soukiassian, X. X. Xi, M. D. Johannes, D. Kasinathan, W. E. Pickett, J. S. Wu, and J. C. H. Spence, Phys. Rev. Lett. 93, 147006 (2004).
• "Properties of MgB₂ thin films with carbon doping", A. V. Pogrebnyakov, X. X. Xi, J. M. Redwing, V. Vaithyanathan, D. G. Schlom, A. Soukiassian, S. B. Mi, C. L. Jia, J. E. Giencke, C. B. Eom, J. Chen, Y. F. Hu, Y. Cui, and Qi Li, Appl. Phys. Lett. 85, 2017 (2004).
• "Strain and High Temperature Superconductivity: Unexpected Results from Direct Electronic Structure Measurements in Thin Films", M. Abrecht, D. Ariosa, D. Cloetta, S. Mitrovic, M. Onellion, X. X. Xi, G. Margaritondo, and D. Pavuna, Phys. Rev. Lett. 91, 057002 (2003).
• "In situ epitaxial MgB₂ thin films for superconducting electronics", Xianghui Zeng, Alexej V. Pogrebnyakov, Armen Kotcharov, James E. Jones, X. X. Xi, Qi Li, James Lettieri, Darrell G. Schlom, Wei Tian, Xiaoqing Pan, Zi-Kui Liu, Nature Materials 1, 35(2002).
• "Thermodynamics of the Mg-B system: Implications for the deposition of MgB₂ thin films", Zi-Kui Liu, D. G. Schlom, Qi Li, and X. X. Xi, Appl. Phys. Lett. 78, 3678 (2001).

Thin film ferroelectrics

• "Probing Nanoscale Ferroelectricity by Ultraviolet Raman Spectroscopy", D. A. Tenne, A. Bruchhausen, N. D. Lanzillotti-Kimura, A. Fainstein, R. S. Katiyar, A. Cantarero, A. Soukiassian, V. Vaithyanathan, J. H. Haeni, W. Tian, D. G. Schlom, K. J. Choi, D. M. Kim, C. B. Eom, H. P. Sun, X. Q. Pan, Y. L. Li, L. Q. Chen, Q. X. Jia, S. M. Nakhmanson, K. M. Rabe, X. X. Xi, Science 313, 1614 (2006).
• "Lattice dynamics in Ba_xSr_1−xTiO₃ single crystals: A Raman study", D. A. Tenne, A. Soukiassian, X. X. Xi, H. Choosuwan, R. Guo, and A. S. Bhalla, Phys. Rev. B 70, 174302 (2004).
• "Absence of low-temperature phase transitions in epitaxial BaTiO₃ thin films", D. A. Tenne, X. X. Xi, Y. L. Li, L. Q. Chen, A. Soukiassian, M. H. Zhu, A. R. James, J. Lettieri, D. G. Schlom, W. Tian, and X. Q. Pan, Phys. Rev. B 69, 174101 (2004).
• "Strain relaxation during in situ growth of SrTiO₃ thin films", Luke S.-J. Peng, X. X. Xi, Brian H. Moeckly, and S. P. Alpay, Appl. Phys. Lett. 83, 4592 (2003).
• "Electric field-induced soft-mode hardening in SrTiO3 films", I. A. Akimov, A. A. Sirenko, A. M. Clark, J.-H. Hao, and X. X. Xi, Phys. Rev. Lett. 84, 4625 (2000).
• "Soft-mode hardening in SrTiO3 thin films", A. A. Sirenko, C. Bernhard, A. Golnik, Anna M. Clark, Jianhua Hao, Weidong Si, and X. X. Xi, Nature, 404, 373 (2000).
• "Observation of the first-order Raman scattering in SrTiO3 thin films", A. A. Sirenko, I. A. Akimov, J. R. Fox, A. M. Clark, Hong-Cheng Li, Weidong Si, and X. X. Xi, Phys. Rev. Lett. 82, 4500 (1999).
• "Near Single Crystal-Level Dielectric Loss and Nonlinearity in Pulsed Laser Deposited SrTiO3 Thin Films", Hong-Cheng Li, Weidong Si, Alexander D. West, and X. X. Xi, Appl. Phys. Lett. 73, 190 (1998).
• "Thickness Dependence of Dielectric Loss in SrTiO3 Thin Films", Hong-Cheng Li, Weidong Si, Alexander D. West, and X. X. Xi, Appl. Phys. Lett. 73, 464 (1998).

Other oxide thin films

• "Structural and transport properties of epitaxial Na_xCoO₂ thin films", A. Venimadhav, A. Soukiassian, D. A. Tenne, Qi Li, X. X. Xi, D. G. Schlom, R. Arroyave, Z. K. Liu, H. P. Sun, Xiaoqing Pan, Minhyea Lee, and N. P. Ong, Appl. Phys. Lett. 87, 172104 (2005).

My Background (Back to the top)

• Temporary Member of Technical Staff, Conductus/STI
• Visiting Professor at Stanford University
• Member Technical Staff at Superconducting Core Technologies, Inc .
• Research Scientist at National Institute of Standards and Technologies
• Assistant Research Scientist at University of Maryland
• Research Associate at Rutgers University/ Bellcore
• Guest scientist at Karlsruhe Nuclear Research Center (now called Forschungszentrum Karlsruhe)
• Ph.D. in Physics from Peking University