Experimental condensed matter physics; Nanoscale electronic transport; Two-dimensional electrons in semiconductor, oxide hetero-interfaces or graphene-like atomically thin crystals; Topological insulators; Electron correlation effects in transport; Thermoelectric and photovoltaic energy conversion; Nano-electronics and sensors.
Nanoscale devices or structures have provided a versatile platform for physicists to study both the fundamental physics in condensed matter as well as explore new approaches to attack important problems in other areas such as biology and medicine. When electrons are confined in low dimensional semiconductor nanostructures, the interplay between quantum mechanics, Coulomb interaction and disorder effects can drive them into a plethora of exotic quantum state of matter. I am interested in understanding the rich phases of strongly correlated two dimensional (2D) electrons (or holes) in semiconductors through delicate electrical transport and thermodynamic measurements at ultra-low temperatures (down to several mKs). In addition, the comparable size between nanometer scale structures and biological species (DNA, protein etc) opens up opportunities for inventing novel functional devices with unprecedented sensitivity for biological studies or medical diagnostics. In this respect, I am attracted to exploiting nano-electronic devices (e.g. nanowire field-effect transistor) in biological and medical applications.
2D electrons have surprised us with many fascinating phenomena, among which, integer and fractional quantum Hall effects in high magnetic fields are perhaps two most notable examples. Even at zero magnetic field, however, recent experiments on 2D electronic systems revealed an intriguing metallic state and metal-insulator transition (MIT) in ultra-clean samples with low carrier density. This 2D metallic state and MIT have set a challenge to the conventional wisdom of localization and Coulomb interaction effects based on the Fermi liquid theory, a cornerstone of modern condensed matter physics. The uniqueness of the systems showing MIT is their strongly interacting nature (related to the low carrier density). In fact, these systems are so strongly interacting that they are in the vicinity of the formation of Wigner crystal, a quantum solid of electrons. We are employing techniques like electrical transport, magneto-transport and thermodynamic measurements to investigate the poorly understood phase diagram of 2D electronic systems in the strongly interacting regime. The 2D samples we study will be primarily GaAs/AlGaAs quantum wells with ultra- high mobility and low density grown by our collaborators at industry (Bell Labs, Lucent Technologies). But other materials (e.g. carbon) are of interest to us too.
Another major focus of my research lies at the interface between nanoscience and energy- and biotechnology. Besides allowing us to study the rich mesoscopic and correlated electron physics, the small size and reduced dimension of nano-structures also give them technological virtues in solving crucial energy or biomedical problems. The one-dimensional (1D) nature of nanowires, for instance, could possibly render nanowire based thermoelectric devices with much better energy conversion efficiency than existing standard. This is attributed to the drastically modified electronic density of states and the reduced electron-phonon scattering in nanowires. Furthermore, it has been demonstrated that nanowire field-effect- transistors can work as sensors to detect extremely low concentration (fMs) of biological macro-molecules in solution. This is intimately related to the small diameter (~10nms) and high quality of semiconductor nanowires which can be synthesized fairly conveniently in laboratory via ‘bottom up’ approach. Further development and understanding of such sensors would be essential for the emergence of new technologies in disease diagnostics and drug screening. I also have interest in using nano-electronic devices in biophysics research, single molecule study of biomolecular reactions, to give an example. These researches are highly interdisciplinary and can lead to exciting advancements in multiple areas including physics, material science, and biotechnology.
We are looking for talented and motivated students (both undergraduates and graduates) and postdocs to help solve important issues in condensed matter physics and nanoscience. If you are interested in research opportunities in Gao’s group, please feel free to contact Prof. Xuan Gao.
S. Sucharitakul, N. J. Goble, U. R. Kumar, R. Sankar, Z. A. Bogorad, F. C. Chou, Y.T. Chen, X. P. A. Gao, “Intrinsic Electron Mobility Exceeding 1000 cm2/(Vs) in Multilayer InSe FETs”, Nano Letters, 15 (6), 3815-3819 (2015).
T. Q. Ngo, N. J. Goble, A. Posadas, K. J. Kormondy, S. Lu, M. D. McDaniel, D. J. Smith, X. P. A. Gao, A. A. Demkov, and J. G. Ekerdt,”Quasi-two-dimensional Electron Gas at the Interface of gamma-Al2O3/SrTiO3 Heterostructures Grown by Atomic Layer Deposition”, Journal of Applied Physics, 118, 115303 (2015).
N.J. Goble, J.D. Watson, M.J. Manfra and X. P.A. Gao, “Impact of short range scattering on the 2D metallic transport in a correlated 2D Hole System”, Physical Review B, 90, 035310 (2014).
Z.H. Wang, R. L.J. Qiu, C.H. Lee, Z.D. Zhang, X. P.A. Gao, “Ambipolar Surface Conduction in Ternary Topological Insulator Bi2(Te1-xSex)3 Nanoribbons”, ACS Nano 7, 2126-2131 (2013).
Y. Tian, M. R. Sakr, J. M. Kinder, D. Liang, R. L.J. Qiu, M. J. MacDonald, H.-J. Gao and X. P.A. Gao, “One-dimensional quantum confinement modulated thermoelectric properties in InAs nanowires”, Nano Letters 12, 6492-6497 (2012).
R. L.J. Qiu, X. P.A. Gao, L. N. Pfeiffer, K. W. West, “Connecting the reentrant insulating phase and the zero field metal-insulator transition in a 2D hole system”, Physical Review Letters 108, 106404 (2012).
D. Liang, X. P.A. Gao, “Strong tuning of spin orbit interaction in an InAs nanowire by surrounding gate”, Nano Letters12 (6), 3263-3267 (2012).
H. Tang, D. Liang, R. L.J. Qiu and X. P.A. Gao, “Two-Dimensional Transport Induced Linear Magneto-Resistance in Topological Insulator Bi2Se3 Nanoribbons”, ACS Nano 5, 7510-7516 (2011)
B. Spivak, S. V. Kravchenko, S. A. Kivelson, and X. P. A. Gao, “Transport in Strongly Correlated Two-Dimensional Electron Fluids”, Reviews of Modern Physics 82, 1743 (2010).
X. P. A. Gao, G.F. Zheng and C.M. Lieber, “Subthreshold Regime has the Optimal Sensitivity for Nanowire FET Biosensors”, Nano Letters 10, 547 (2010).
J. Du, D. Liang, H. Tang and X.P.A. Gao, “InAs Nanowire Transistor as Gas Sensor and the Response Mechanism”,Nano Letters 9, 4348 (2009).
Rockefeller Building 104C
B.S., South China University of Technology (1998)
Ph.D., Columbia University (2003)