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Nai Phuan Ong

Department/Program(s):

Physics

Position: Professor

Title: Eugene Higgins Professor of Physics. Director, Princeton Center for Complex Materials.

Office: 302 Jadwin Hall

Phone: 609-258-4347

Email: npo@princeton.edu

Homepage: http://www.princeton.edu/~npo/

I am currently interested in the following areas:

Dirac Materials and topological insulators: In materials such as graphene, bismuth-based compounds and some organic superconductors, the electrons display a linear energy-momentum "relativistic" dispersion described by the massless Dirac Hamiltonian. My group has explored the nature of the ground state in graphene in high magnetic fields at the charge-neutral point (Dirac Point). Recently, the bismuth-based compounds Bi-Sb, Bi₂Se₃, Bi₂Te₃ have been identified to be topological insulators. Topological insulators have current-carrying massless, Dirac surface states in which the spins are locked perpendicular to the momentum ("helical" like neutrinos). Interesting neutral excitations (Majorana fermions) have been predicted to exist on the surface. My group is currently actively exploring the transport and magnetic properties of these surface states using both chemical doping and electrostatic gating of small crystals to isolate the surface states. The research is in collaboration with the groups led by Bob Cava, Zahid Hasan and Ali Yazdani.

Superconductivity: In the cuprate superconductors, my interests are in exploring the role of phase fluctuations associated with the spontaneous appearance of vortices (quantized flux lines) in limiting the critical temperature T_c. The orbital currents associated with the fluctuating vortices above T_care detected by transport (the Nernst effect) and thermodynamic probes (torque magnetometry). These experiments reveal that Cooper pairing onsets at temperatures high above T_c, but the condensate lacks long-range phase "stiffness" to sustain full flux expulsion (the Meissner effect). We are also interested in the iron-based pnictide superconductors.

Quorum Sensing: Bacteria communicate with each other by emitting and detecting small molecules called auto-inducers (AI) in a process called quorum sensing. For e.g., the marine bacterium V. harveyi regulates the genes for emitting photons (luminescence) based on the local concentration of AIs. Research reveals that the quorum-sensing circuit is highly evolved and sophisticated, with sRNAs playing a vital role. By measuring in vivo the light emitted by proteins tagged with fluoresecent proteins, we are attempting to untangle the regulatory network in a quantitative way. The research involves measuring the integrated response of microcolonies of various mutant strains subject to varying concentrations of AIs. Recently, we measured the absolute number of the master regulator protein LuxR by analyzing the small-number fluctuations in the partitioning of LuxR between the daughter cells during a cell division. This research is done in collaboration with Bonnie Bassler and Ned Wingreen.

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Research in N.P. Ong's Group

Publications

Z. Xu, N.P. Ong, Y. Wang, T. Kakeshita and S. Uchida, "Vortex-like excitations and the onset of superconducting phase fluctuation in underdoped La_2-xSr_xCuO₄ ", Nature 406, 486 (2000).
Yayu Wang, N. P. Ong, Z.A. Xu, T. Kakeshita, S. Uchida, D. A. Bonn, R. Liang and W. N. Hardy, "High field phase diagram of cuprates derived from the Nernst effect", Phys. Rev. Lett. 88, 257003 (2002).
Y. Zhang, R. H. Austin, J. Kraeft, E. C. Cox, and N. P. Ong, "Insulating behavior of lambda-DNA on the micron scale", Phys. Rev. Lett., Phys. Rev. Lett. 89, 198102 (2002).
Yayu Wang, Nyrissa S. Rogado, R. J. Cava, and N. P. Ong, "Spin entropy as the likely source of enhanced thermopower in Na_xCo₂O₄", Nature 423, 425 (2003).
Wei-Li Lee, Satoshi Watauchi, R. J. Cava and N. P. Ong, "Dissipationless anomalous Hall current in the ferromagnetic spinel CuCr₂Se_4-xBr_x ", Science 303, 1647 (2004).
Lu Li, J. G. Checkelsky, S. Komiya, Y. Ando and N. P. Ong, "Low temperature vortex liquid in La_2-xSr_xCuO₄", Nature Physics 3, 311 (2007).
J. G. Checkelsky, L. Li and N. P. Ong, "Zero-energy state in graphene in a high magnetic field", Phys. Rev. Lett. 100, 206801 (2008).
Lu Li, J. G. Checkelsky, Y.S. Hor, C. Uher, A. F. Hebard, R. J. Cava and N. P. Ong, “Phase transitions of Dirac electrons in bismuth”, Science 321, 547 (2008).
Minhyea Lee, W.W. Kang, Y. Onose, Y. Tokura, and N. P. Ong, “Unusual Hall Effect Anomaly in MnSi Under Pressure”, Phys. Rev. Lett. 102, 186601 (2009).
J. G. Checkelsky and N. P. Ong, “Thermopower and Nernst Effect in graphene in a magnetic field”, Phys. Rev. B 80, 081413 (2009).
J. G. Checkelsky, Y. S. Hor, M.-H. Liu, D.-X. Qu, R. J. Cava and N. P. Ong, “Quantum Interference in Macroscopic Crystals of Nonmetallic Bi₂Se₃”, Phys. Rev. Lett. 103 , 246601 (2009).
Shu-Wen Teng, Yufang Wang, Kimberly C. Tu, Tao Long, Pankaj Mehta, Ned S. Wingreen, Bonnie L. Bassler and N. P. Ong, “Measurement of the copy number of the master quorum-sensing regulator of a bacterial cell”, Biophysical Jnl 98 (2010).