Xiaoxing Xi

Laura H. Carnell Professor of Physics
Barton Hall, Room A-116
1900 N. 13th Street
Philadelphia, PA 19122



Ph.D., Peking University, 1987 



Materials Physics of Electronic and Photonic Thin Films


Research interest

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 Tc 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 Tc values. The HPCVD technique also allows doping of the clean films in a controlled manner, resulting in record-high upper critical field Hc2 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

News stories about our work on MgB2 thin films.

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