Mei-Yin Chou

Professor Emerita

Education

Honors & Awards

Research

The objectives of our research have been to investigate the electronic structure of condensed matter, and to study its effects on the structural and dynamical properties of materials. For systems with translational symmetry, calculations are performed from first principles with no adjustable parameters. This has been made possible by advances in computational methods developed in the past decades. When these first-principles calculations are not feasible in some larger and more complicated systems, model calculations are used to identify the important features. The purposes of these studies are to provide unambiguous explanations for various interesting phenomena observed experimentally in clusters, solids, and surfaces, and to make reliable predictions of new material properties from microscopic quantum theories.

Our theoretical efforts can be classified into two categories: (1) the study of the electronic and dynamical properties of technologically important materials, and (2) the development of new algorithms and calculational methods in studying materials properties using quantum mechanics. Detailed descriptions of specific projects are:

(1) Hydrogen in Metals
Hydrogen is the smallest and lightest interstitial impurity in transition metals. It interacts strongly with the metal host and behaves like a three-dimensional lattice gas with extremely high mobility. The diversity of phenomena exhibited by these systems (such as structural, order-disorder, and metal-semiconductor phase transitions) are closely linked to electronic structure changes induced by the presence of hydrogen. We are interested in examining the fundamental interaction of hydrogen with the metal lattice in this strongly coupled system.

Theoretical studies using the state-of-the-art computational methods have been carried out for hydrogen in yttrium, palladium, and niobium, each with a different lattice structure. The hydrogen-yttrium system is a prototype of rare-earth-metal hydrides. We have thoroughly examined a variety of intriguing properties over the whole composition range. Topics investigated include the nature of hydrogen pairing in the solid solution phase, the mechanism of lattice contraction and the (420)-plane ordering of hydrogen in the cubic phase, and the identification of Peierls distortion in the trihydride which is related to the metal-insulator transition. We have also studied the ordering of hydrogen in close-packed palladium, and in niobium which has a relatively open lattice.

(2) Hydrogen on Metal and Semiconductor Surfaces
Hydrogen on surfaces modifies the physical properties dramatically and has drawn attention from researchers in many different fields. This is the simplest chemisorption system on a surface, with connections to the catalytic synthesis of hydrocarbons and homoepitaxial growth of semiconductors. Interesting features we have studied include the hydrogen ordering and hydrogen induced giant phonon anomaly on surfaces of certain transition metals, and the hydrogen induced change in the surface energy anisotropy on semiconductor surfaces and its effect on stability of different adsorption phases.

(3) Development of Theoretical Techniques
We have also carried out developmental work on computational techniques in studying real materials. A new method to obtain the phonon spectrum has been developed by reconstructing real-space force-constant matrices from the projection along several symmetry directions. The dynamical matrix at any arbitrary wave vector can then be determined and the full phonon spectra can be obtained completely from first principles for the material. We have applied this method to Si and Ge, and obtained excellent phonon dispersions and polarization vectors. The volume-dependent phonon frequencies were then used to investigate thermodynamical properties of silicon, including the abnormal (negative) thermal-expansion coefficient at low temperature.

Recently, we have explored the possibility of using the wavelet basis in self-consistent electronic structure calculations. Wavelet analysis is a newly developed mathematical tool that has found a unique niche in many scientific and engineering fields. This basis set is capable of describing localized features of different scales and at different locations. We have successfully carried out the first implementation of orthonormal wavelets in three-dimensional self-consistent calculations, and demonstrated the feasibility and advantages of this basis in modern electronic-structure calculations.

(4) Quantum Monte-Carlo Study of Real Materials
The electronic many-body problem has been a long-standing challenge in condensed matter physics. Difficulties arise mainly from the number of electrons involved and the special statistical properties required. For lack of feasible theoretical tools to attack the problem directly, most researchers have been working with various simplified models in the past. On the other hand, the widely adopted local-density approximation within the density functional theory maps the many-body problem to a series of one-particle equations. However, it has not been possible to assess the error and to make improvements in a systematic fashion. We have therefore decided to investigate the quantity of central importance in density functional theory, the exact exchange-correlation energy density, using quantum Monte-Carlo techniques. In collaboration with Dr. Richard Needs' group at Cambridge through a NATO grant, we have evaluated the pair correlation function and the exchange-correlation hole in crystalline silicon using a variational many-body wave function obtained by the Monte-Carlo method. Through the coupling-constant integration technique, we have then computed the almost exact exchange-correlation energy density in bulk silicon, and identified the errors in the results obtained from the local density and other approximations.

Papers

Recent Publications:

“Comment on “Determination of Phonon Dispersion from X-Ray Transmission Scattering: The Example of Silicon”, M. Y. Chou, and M. Choi, Phys. Rev. Lett. 84, 3733 (2000).

“Sn/Ge(111) Surface Charge-Density-Wave Phase Transition”, T.E. Kidd, T. Miller, M. Y. Chou, and T.-C. Chiang, Phys. Rev. Lett. 85, 3684-3687 (2000).

“X-Ray Studies of Phonon Softening in TiSe2”, M. Holt, P. Zschack, H. Hong, P. Jemian, M. Y. Chou, and T.-C. Chiang, Phys. Rev. Lett. 86, 3799-3802 (2001).

“Quantum Electronic Stability of Atomically Uniform Films”, D.-A. Luh, T. Miller, J.J. Paggel, M. Y. Chou, and T.-C. Chiang, Science, 292, 1131-1133 (2001).

“Exchange and Correlation in the Si Pseudo Atom: A Quantum Monte Carlo Study”, A. Puzder, M. Y. Chou, and R.Q. Hood, Phys. Rev. A 64, 022501/1-16 (2001).

“A Comparative Study of Density Functional Theories of the Exchange-Correlation Hole and Energy in Si”, A.C. Cancia, M. Y. Chou, and R.Q. Hood, Phys. Rev. B. 64, 115112/1-15 (2001).

“Fermi Surfaces and Energy Gaps in Sn/Ge(111)”, T.-C. Chiang, M. Y. Chou, T.E. Kidd, and T. Miller, J. Phys. Condensed Matter 14, R1-R20 (2002).

“Reply to Comment on Sn/GE(111) Surface Charge-Density-Wave Phase Transition”, T.E. Kidd, T. Miller, M. Y. Chou, and T.-C. Chiang, Phys. Rev. Lett. 88, 189702/1-4 (2002).

“Electron-Hole Coupling and Charge Density Wave Transition in TiSe2”, T.E. Kidd, T. Miller, M. Y. Chou, and T.-C. Chiang, Phys. Rev. Lett. 88, 226402 (2002).

“Atomic-Layer-Resolved Quantum Oscillations in Work Function: Theory and Experiment for Ag/Fe(100)”, J.J. Paggel, C.M. Wei, M. Y. Chou, D.-A. Luh, T. Miller, and T.-C. Chiang, Phys. Rev. B 66, 233403/1-4 (2002).

“Theory of Quantum Size Effects in Pb(111) Films”, C.M. Wei, and M. Y. Chou, Phys. Rev. B 66, 233408/1-4 (2002).

“Alternating Layer and Island Growth of Pb on Si by Spontaneous Quantum Phase Separation”, H. Hong, C.M. Wei, M. Y. Chou, Z. Wu, L. Basile, H. Chen, M. Holt, T.-C. Chiang, Phys. Rev. Lett. 90, 076104 (2003).

“Quasiparticle Band Structure of Cubic YH3 and LaH3", J.A. Alford, M. Y. Chou, E.K. Chang, and S.G. Louie, Phys. Rev. B 67, 125110 (2003).

"Effects of the Substrate on Quantum Well States: A First-Principles Study for Ag/Fe(100) ", C.M. Wei, and M. Y. Chou, Phys. Rev. B 68, 125406 (2003).

“Quantum Confinement and Electronic Properties of Silicon Nanowires”, X. Zhao, C.M. Wei, L. Yang, and M. Y. Chou, Phys. Rev. Lett. 92, 236805/1-4 (2004).

“Thermal Stability and Electronic Structure of Pb Films on Si(111)”, C.M. Upton, C.M. Wei, M. Y. Chou, T. Miller, and T.-C. Chiang, Phys. Rev. Lett. 93, 026802/1-4 (2004).

“Alternative Low-Symmetry Structure for 13-Atom Metal Clusters”, C.M. Chang, and M. Y. Chou, Phys. Rev. Lett. 93, 133401/1-4 (2004).

“First-Principles Study of NaAlH4 and Na3AlH6 Complex Hydrides”, A. Peles, J.A. Alford, Z. Ma, L. Yang, and M. Y. Chou, Phys. Rev. B 70, 165105/1-7 (2004).

“Hydrogeneration-Induced Insulating State in the Intermetallic Compound LaMg2Ni”, K. Yvon, G. Renaudin, C.M. Wei, and M. Y. Chou, Phys. Rev. Lett. 94, 066403/1-4 (2005).

"Reply to Comment on Quantum Confinement and Electronic Properties of Silicon Nanowires," X. Zhao, C. M. Wei, L. Yang, and M. Y. Chou, Phys. Rev. Lett. 94, 219702 (2005).

"Persistent Superconductivity in Ultrathin Pb Films: A Scanning Tunneling Spectroscopy Study," D. Eom, S. Qin, M. Y. Chou, and C.K. Shih, Phys. Rev. Lett. 96, 027005/1-4 (2006).

"Lattice Dynamics and Thermodynamic Properties of NaAlH4," A. Peles and M. Y. Chou, Phys. Rev. B. 73, 184302/1-11 (2006).
"Beyond the local approximation to exchange and correlation: The role of the Laplacian of the density in the energy density of Si," A. C. Cancio and M. Y. Chou, Phys. Rev. B 74, 081202(R)/1-4 (2006).

"Exchange-correlation energy in molecules: A variational quantum Monte Carlo study," C. R. Hsing, M. Y. Chou, and T. K. Lee, Phys. Rev. A 74, 032507/1-10 (2006).

"LaMg2PdH7, a New Complex Metal Hydride Containing Tetrahedral [PdH4]4- Anions," K. Yvon, J.-Ph. Rapin, N. Penin, Z. Ma, and M. Y. Chou, J. Alloys Compd. 446-447, 34-38 (2007).

"Enhanced Electron-Hole Interaction and Optical Absorption in a Silicon Nanowire," L. Yang, C. D. Spataru, S. G. Louie, and M. Y. Chou, Phys. Rev. B (Rapid Communications) 75, 201304/1-4 (2007).

"Band-Structure Contribution to the Quantum Size Effect in Pb(100) Films," C. M. Wei and M. Y. Chou, Phys. Rev. B 75, 195417/1-4 (2007).

"First-Principles Study of Cation and Hydrogen Arrangements in the Li-Mg-N-H Hydrogen Storage System," Y. Wang and M. Y. Chou, Phys. Rev. B 76, 014116/1-6 (2007).

"Size and Orientation Dependence in the Electronic Properties of Silicon Nanowires," J.-A. Yan, L. Yang, and M. Y. Chou, Phys. Rev. B 76, 115319/1-6 (2007).

"Variational Calculation of the Depolarization of the Maximum Density Droplet in Two-Dimensional Quantum Dots," W. Geist and M. Y. Chou, Phys. Rev. B 76, 235306/1-7 (2007).

"Electronic and Vibrational Properties of gamma-AlH3," Y. Wang and M. Y. Chou, Phys. Rev. B 77, 014101/1-8 (2008).

"Phonon dispersions and vibrational properties of monolayer, bilayer, and trilayer graphene," J. A. Yan, W. Y. Ruan, and M. Y. Chou, Phys. Rev. B 77, 125401/1-7 (2008).

"Quantum Confinement Effect in Si/Ge Core-Shell Nanowires," L. Yang, R. N. Musin, X.-Q. Wang, and M. Y. Chou, Phys. Rev. B 77, 195325/1-5 (2008).

"Size- and Strain-Dependent Electronic Structures in H-Passivated Si [112] Nanowires," L. Huang, N. Lu, J. A. Yan, M. Y. Chou, C. Z. Wang, and K. M. Ho, J. Phys. Chem. C 112, 15680–15683 (2008).

"Low-Energy Ordered Structures of Li2Mg(NH)2," Z. Ma and M. Y. Chou, J. Appl. Phys. 104, 093619/1-6 (2008).

"First-Principles Investigation of Sodium and Lithium Alloyed Alanates," Z. Ma and M. Y. Chou, J. Alloys Compd. 479, 679-683 (2009).

"Electron-Phonon Interactions for Optical Phonon Modes in Few-Layer Graphene," J.-A. Yan, W. Y. Ruan, and M. Y. Chou, Phys. Rev. B 79, 115443/1-6 (2009).

"Phase Relations Associated with One-Dimensional Shell Effects in Thin Metal Films," T. Miller, M. Y. Chou, and T.-C. Chiang, Phys. Rev. Lett. 102, 236803/1-4 (2009).

"Path to Wigner Localization in Circular Quantum Dots," L. Zeng, W. Geist, W.Y. Ruan, C.J. Umrigar, and M. Y. Chou, Phys. Rev. B 79, 235334/1-5 (2009).

"Structural and Electronic Properties of Oxidized Graphene," Jia-An Yan, Lede Xian, and M. Y. Chou, Phys. Rev. Lett. 103, 086802/1-4 (2009).

"Effects of Metallic Contacts on Electron Transport through Graphene," S. Barraza-Lopez, M. Vanevic, M. Kindermann, and M. Y. Chou, Phys. Rev. Lett. 104, 076807/1-4 (2010).

"Low-Lying Spectra of Massless Dirac Electrons in Magnetic Dots and Rings," C. M. Lee, R. C. H. Lee, W. Y. Ruan, and M. Y. Chou, Appl. Phys. Lett. 96, 212101/1-3 (2010).

"Quantum Size Effects on the Work Function of Metallic Thin-Film Nanostructures," J. Kim, S. Qin, W. Yao, Q. Niu, M. Y. Chou, and C.-K. Shih, Proc. Natl. Acad. Sci. 79, 12761-12765 (2010).

"Energy Spectra of a Single-Electron Magnetic Dot Using Massless Dirac-Weyl Equation," C. M. Lee, R. C. H. Lee, W. Y. Ruan, and M. Y. Chou, J. Phys.: Condens. Matter 22, 355501/1-4 (2010).

"Theoretical Investigation of Intermediate Phases between Li2NH and LiNH2," Feng Zhang, Yan Wang, and M. Y. Chou, Phys. Rev. B 82, 094112/1-6 (2010).

"Oxidation Functional Groups on Graphene: Structural and Electronic Properties," J.-A. Yan and M. Y. Chou, Phys. Rev. B 82, 125403/1-10 (2010).

"Stability of the Hydrogen-storage Compound Li6Mg(NH)4 from First Principles," by Feng Zhang, Yan Wang, and M. Y. Chou, Phys. Rev. B 83, 012101 (2011).

"Theoretical Study of the Vibrational Properties of NaAlH4 with AlH3 Vacancies,” Feng Zhang, Yan Wang, and M. Y. Chou, Faraday Discussion 151, 243-251 (2011).

"Catalytic Effect of near-Surface Alloying on Hydrogen Interaction on the Aluminum Surface," Y. Wang, F. Zhang, P. Lin, R. Stumpf, and M. Y. Chou, Phys. Rev. B 83, 195419/1-5 (2011).

 "Lattice Vibrational Modes and their Frequency Shifts in Semiconductor Nanowires," L. Yang and M. Y. Chou, Nano Lett. 11, 2618-2621 (2011).

"Enhanced Optical Conductivity Induced by Surface States in ABC-stacked Few-Layer Graphene," J. Yan, W. Ruan, and M. Y. Chou, Phys. Rev. B 83, 245418/1-6 (2011).

 "Effect of Electrostatic Fields and Charge Doping on the Linear Bands in Twisted Graphene Bilayers," L. Xian, S. Barraza-Lopez, and M. Y. Chou, Phys. Rev. B 84, 075425/1-6 (2011).

"Phase Diagram of Graphene Nanoribbons and Band-Gap Bifurcation of Dirac Fermions under Quantum Confinement," Y. Y. Sun, W. Y. Ruan, X. Gao, J. Bang, Y. H. Kim, K. Lee, D. West, X. Liu, T.-L. Chan, M. Y. Chou, and S. B. Zhang, Phys. Rev. B 85, 195464/1-5 (2012).

"Charge Transport through Graphene Junctions with Wetting Metal Leads," S. Barraza-Lopez, M. Kindermann, and M. Y. Chou, Nano Lett. 12, 3424-3430 (2012).

"Fractal Landau-Level Spectra in Twisted Bilayer Graphene," Z. F. Wang, F. Liu, and M. Y. Chou, Nano Lett. 12, 3833- 3838 (2012).
"Optical Phonon Anomaly in Bilayer Graphene with Ultrahigh Carrier Densities," J.-A. Yan, K. Varga, and M. Y. Chou, Phys. Rev. B 86, 035409/1-5 (2012).

"Quantum Monte Carlo Investigations of Chemisorption Energetics on Graphene," C. R. Hsing, C. M. Wei, and M. Y. Chou, J. Phys.: Condensed Matter 24, 395002/1-7 (2012).

"Hydrogen Interaction with the Al Surface Promoted by Subsurface Alloying with Transition Metals," F. Zhang, Y. Wang, and M. Y. Chou, J. Phys. Chem. C 116, 18663-18668 (2012).

"Diffusion of Si and C Atoms on and between Graphene Layers," L. Xian and M. Y. Chou, J. Phys. D (in press).