Wei Ku       

About me

I am a physicist with the condensed matter theory group (condensed matter physics and materials science department) in Brookhaven National Laboratory.  My current research interest includes study of electronic, optical, and magnetic properties of strongly correlated systems (including correlated nano-materials), using state-of-the-art computational approaches.  In addition, I am also developing novel theoretical/numerical methods to investigate systems with strong many-body interactions.

About my Research   (complete record in my CV, also in printable PDF format)

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Topics

My research interest is realistic understanding of the rich electronic/magnetic/optical properties of condensed matter, using first-principles quantum many-body theory.  Special focus is placed to systems with stronger “quantum correlation” that renders classical or mean-field treatments inadequate.  Examples of existing works include (see publications for more detail):

·        correlated transition metal nanotubes

·        orbital, spin, and charge ordering of manganites

·        low energy excitations of high-Tc cuprates

·        electron and phonon mechanisms of charge density wave and charge order

·        complex quantum magnetism of novel cuprates and Cu-O chains

·        quasi-particle excitations in semiconductors and insulators (GW)

·        quasi-particle methods for correlated metals and their oxides

·        dynamical collective excitations and optical/dielectric properties (e.g.: plasmon and magnon and their lifetime)

·        interplay between extended and localized excitations in systems with shallow core states

·        multi-energy-resolved Wannier functions

Why Interesting?

One of the main streams of research in the history of physics is to search for the basic laws that dominant the behavior of the “fundamental particles”.  One naïve hope of we physicists is that knowing such would be enough to describe/predict properties of systems of interest.  As the search of ultimate unified theory keeps going, which unavoidably makes the fundamental particles smaller and smaller, a new consideration starts to become apparent.  That is, the number of such particles in real systems of interest becomes unmanageably large, and rich, intriguing properties of a collection of these particles can no longer be understood simply from the properties of few fundamental particles.

This new difficulty is easily illustrated with condensed matter systems, in which the fundamental particles (~10^23 electrons and protons) and their (electromagnetic) interactions are well understood, but almost all the important properties (magnetism, semiconductivity, superconductivity, and optical excitations) cannot be quantitatively understood without incorporating the quantum many-body effects.  In addition to the technical importance of these materials, this makes them playgrounds for physicists to study approaches/approximations for describing the many-body behavior, and to manipulate/synthesize new artificial materials.

Main Challenges

While the formal frameworks of many-body theories are well developed, and are normally manageable within toy models, realistic first-principles (parameter-free) implementations of these theories are still far from being mature, mainly due to the heavy amount of computation, and also some unexplored theoretical “recipes”.  Thus, one of the main challenges is to find clean physical/numerical approximations feasible for the systems of interest, as well as new algorithmic development that enables inclusion of more many-body processes within finite computation resource.

Approaches   (this session is constantly under construction)

·        Multi-Energy-Resolved Wannier Functions:

For strongly correlated systems, a local picture taking into account only low-energy Hilbert space is most convenient for theoretical formulation.  To this end, a novel approach of constructing multi-energy resolved, symmetry respecting Wannier function is developed and applied to many strongly correlated systems.  The following are two examples of low-energy Wannier functions that lead to deep insight of the physics of dichalcogenides and manganites.


Low-energy Wannier states (WS) of real materials

Left: Gapless excitations in the charge density wave phase of TaSe2 is explained with the unique geometric effects derived naturally from the phase interference of the WS.  The hyrdization of ag and eg' symmetry essential to the understanding is clearly observed.  (Phys. Rev. Lett. 96, 026406 (2006))

Right: Unexpectedly strong spin-dependence of resonant inelastic X-ray spectrum of LaMnO3 is explained by the strong charge transfer nature of LaMnO3, which is directly observable from the large hybridization with O-p states in the WS.  Based on further novel WS analysis, origin of orbital ordering of MnF3 and LaMnO3 is, surprisingly, mainly electron-electron interaction, rather than the electron-phone coupling (Jahn-Teller effects).  (Phys. Rev. Lett. 94, 047203 (2005) & cond-mat/0509075)

·        Conserving Finite Temperature Many-Body Perturbation Theory – Quasi-Particle Excitation (one-particle propagator):

One particle Green's function, G, of the electrons in the solid system can be used, to derive quasi-particle properties and the thermal dynamical quantities (and be compared with angular resolve photoemission spectra (see the comment on PRL, for example.))  With the continuous improvement of the computation capability and algorithms, it becomes possible to calculate G, with a careful choice of self-energy diagrams, at finite temperature within conserving scheme of the Many-Body Perturbation Theory that guarantees the microscopic conservation laws.  This effort is especially important in understanding the role of many-body interactions in systems that deviate from the simple single-particle picture, as this kind of parameter-free ab initio approach allows an unambiguous assignment for effect of different self-energy diagrams.

·        Time-Dependent Density Functional Theory - Dynamical Linear Response (two-particle propagator):

As a natural (but non-trivial) extension of density functional theory, time-dependent density functional theory (TD-DFT) provides a “shortcut” of obtaining properties related to the time-dependent density.  In the linear response regime, the dynamical charge/magnetic susceptibility are rigorously shown to satisfy integral equations with a two point kernel (instead of the four point one in the standard many-body perturbation theory.)

The linear response function (or dynamical structure factor, S) gives valuable information about the dynamical electronic/magnetic excitations and screening processes in materials, and it can be directly compared with experiments like EELS, IXS, dielectric function, optical conductivity, reflectivity measurement, and inelastic neutron scattering.  These quantities are calculated based on my all electron, full potential implementation of TD-DFT.  Direct comparison between theoretical spectra and the experimental ones helps to understand the underlying physical mechanisms behind the structures in the spectra, and guide the development of improved theoretical treatment.

Computation

The ground state data is prepared by first running the all electron, full potential, FLAPW DFT package WIEN, followed by extraction of all electron wave functions.  Energy-resolved symmetry-respecting Wannier functions are constructed based on approached developed myself.  Numerical quantities like transition probability amplitude matrix elements and Physical quantities like density response function, self-energy, Green's function and Wannier function are then calculated using my own codes.

All the codes developed to perform calculations are written with C++ with some existing Fortran 77 subroutines.  Listed here are some public-domain libraries that I find useful:

Most of the calculations are performed on the local PC cluster of my group running LINUX with MPICH implementation.  Some older calculations were performed on IBM SP machines (yes, the one that beats human in chess games) at NERSC, and UTK managed by JICS, as well as the cluster in UC Davis.  I have also helped building the PC cluster (see pictures) for the Solid State Division of Oak Ridge National Laboratory.

Academic Honors

  • Lawrence Fellowship in Lawrence Livermore National Lab (2003)  (offer declined due to my moving to Long Island.)
  • Joe Fowler & Jerry Marion Outstanding Graduate Student Award in Department of Physics, University of Tennessee, Knoxville, USA (1998)
  • Department Head's Award in Department of Physics, TamKang Universy, Taiwan ROC (1988)

Selected Invited Presentations (see my CV for a complete list, also in printable PDF format)

·        “Tuning in-plane behavior of high-Tc cuprates via apical atoms: New theoretical findings on the material dependence”
Sanibel Symposium (St. Simons Island, Georgia, February 2008)

·        “Utilizing the short wavelength of X0-ray to study low-energy local excitations: q-dependence of the spectral weights and dispersions”
Workshop on “Inelastic X-ray Scattering at NSLS-II” (NSLS-II, January 2008)

·        “Recent First-Principles Studies of Strongly Correlated Systems: Gapless CDW, orbital/charge ordering and superconducting pair suppression”
International workshop on “Novel Methods for Electronic Structure Calculations” (La Plata, Argentina, November 2007)

·        “Tuning in-plane behavior of high-Tc cuprates via apical atoms: New theoretical findings on the material dependence”
LEHTSC2007 “International Symposium on Lattice Effects in Cuprate High Temperature Superconductors – Spin, phonon or third way?” (Tsukuba, Japan, October 2007)

·        “Local excitations in strongly correlated multi-orbital systems: effective kinetic effects in one-and two-particle channels”
CMSN workshop (Davis, September 2007)

·        “Symmetry Respecting Wannier Functions and Their Applications in Strongly Correlated Systems: New Development of First-Principles Many-Body Down-Folding Approach”
CECAM workshop “Maximally Localized Wannier Functions:Concepts, Applications, and Beyond” (Lyon, June 2007)

·        “Tuning Hold Mobility, Concentration, and Repulsion in High-Tc Cuprates via Apical Atoms: new theoretical findings on the material dependence”
DFLFS3 (Port Jefferson, May 2007)

·        “Tuning Hold Mobility, Concentration, and Repulsion in High-Tc Cuprates via Apical Atoms”
CMSN workshop (Denvor, March 2007)

·        “Recent First-Principles Studies of Strongly Correlated Systems: Gapless CDW, orbital/charge ordering and others”
LLNL international workshop on “Correlated Electrons in Matter” (Half Moon Bay, December 2006)

·        “Bridging first-principles methods and many-body models”
OPCA5 (Taipei, Taiwan, Jun 2006)

·        “First-Principles Many-Body Theories of Excitation and Strongly Correlated Systems”
special summer school, NCKU (Tainan, ROC, Jun 2006)

·        “Probing local excitations with angular dependence of large-q non-resonant IXS:  Sensitivity to weak electronic symmetry breaking in NiO and CoO”
APS user meeting, ANL (Chicago, May 2006)

·        “Applications of Wannier Functions and Derivation of Effective Hamiltonian of Strongly Correlated Systems”
CMSN workshop (Chicago, September 2005)

·        “Energy-Resolved Wannier States with Assigned Local Symmetry : Recent Development & Applications”
CMSN workshop (Oak Ridge, September 2004)

·        “First-Principles Methods of Quasi-Particle and Electron-Hole Excitations”
International Workshop on Computational Materials Physics (Taipei, Taiwan, November 2003)

·        “Magnetic Coupling in Insulating Quasi-1D Cu-O Spin Chains:  Toward Fully First-Principles Approaches for Strong Correlation”
Workshop on Advanced Material Science (Tamsui, Taiwan, November 2003)

·        “Magnetic Coupling in Insulating Quasi-1D Cu-O Spin Chains:  Toward Fully First-Principles Approaches for Strong Correlation”
National Center of Theoretical Sciences (Hsinchu, Taiwan, November 2003)

·        “Simple Construction of Energy-Resolved Wannier States with Assigned Local Symmetry”
CMSN workshop (Knoxville, November 2003)

·        “Quasi-Particle Excitation in Semiconductors: All-Electron Conserving GW scheme”
ES2003 - Fifteenth Annual Workshop on Recent Developments in Electronic Structure Methods (Minneapolis, May 2003)

·        “Wannier Function Study of Insulating Ferromagnetism”
APS March Meeting (Austin, March 2003)

·        “Dynamical Electronic Excitations in Real Materials: Perspective of First-Principles Many-Body Theories”
Lawrence Livermore National Laboratory (Livermore, February 2003)

·        “Wannier State Analysis of Insulating Ferromagnetism in La4Ba2Cu2O10”
ESCM - Electronic Structure and Computational Magnetism (Washington DC, July 2002)

·        “Electronic Excitations in Metals and Semiconductors: Ab Initio Studies of Realistic Many-Body systems”
Solid State Division, Oak Ridge National Laboratory (Oak Ridge, September 2000)

·        “Non-uniform Time Axis Technique and All-electron Self-consistent GWA for Si band gap”
CECAM - Excited states and electronic spectra (Lyon, July 2000)

Representative Publications (see my CV for a complete list, also in printable PDF format)

·        “Strong hybridization of Frankel excitons in Mott insulators: a novel Wannier function perspective”
C.-C. Lee, H.-C. Hsueh, and Wei Ku, in preparation

·        “Charge Ordering in Half-Doped Manganites: Weak Charge Disproportion and Leading Mechanisms”
D. Volja, W.-G. Yin, and Wei Ku, submitted to Euro Phys. Lett.

·        “Microscopic Mechanism of Tuning Local Electron Pair Interaction in High-Tc Superconducting Cuprates via Apical Atoms”
W.-G. Yin and Wei Ku, submitted to Phys. Rev. Lett.

·        “Dynamical reconstruction of the exciton in LiF with inelastic x-ray scattering”
Peter Abbamonte, Tim Graber, James P. Reed, Serban Smadici, Chen-Lin Yeh, Abhay Shukla, Jean-Pascal Rueff, and Wei Ku, PNAS 105, 12159 (2008)

·        “Nanoscale Disorder in CaCu3Ti4O12: A New Route to the Enhanced Dielectric Response”
Y. Zhu, J. C. Zheng, L. Wu, A. I. Frenkel, J. Hanson, P. Northrup, and W. Ku, Phys. Rev. Lett. 99, 037602 (2007)

·        “Non-resonant Inelastic X-Ray Scattering and Energy-Resolved Wannier Function Investigation of d-d Excitations in NiO and CoO”
B. C. Larson, Wei Ku, J. Z. Tischler, Chi-Cheng Lee, O. D. Restrepo, A. G. Eguiluz, P. Zschack, and K. D. Finkelstein, Phys. Rev. Lett. 99, 026401 (2007)

·        “Orbital ordering in LaMnO3: Electron-lattice versus electron-electron interactions”
W.-G. Yin, D. Volja, and Wei Ku, Phys. Rev. Lett. 96, 116405 (2006)

·        “Coexistence of gapless excitations and commensurate charge-density wave in the 2H-transition metal dichalcogenides”
R. L. Barnett, A. P., E. Demler, W.-G. Yin, and Wei Ku, Phys. Rev. Lett. 96, 026406 (2006)

·        “Magnetic correlations in manganites probed by resonant inelastic x-ray scattering”
S. Grenier, J. P. Hill, Wei Ku, V. Kiryukhin, V. Oudovenko, Y.-J. Kim, K. J. Thomas, S.-W. Cheong, Y. Tokura, Y. Tomioka, D. Casa, and T. Gog, Phys. Rev. Lett. 94, 047203 (2005)

·        “Insulating Ferromagnetism in La4Ba4Cu2O10: an Ab Initio Wannier Function Analysis”
Wei Ku, H. Rosner, W. E. Pickett, and R. T. Scalettar, Phys. Rev. Lett. 89, 167204 (2002)

·        “Band-Gap Problem in Semiconductors Revisited: Effects of Core States and Many-Body Self-Consistency”
Wei Ku and A. G. Eguiluz, Phys. Rev. Lett. 89, 126401 (2002)

·         “Ab Initio Investigation of Collective Charge Excitations in MgB2”
Wei Ku, W. E. Pickett, R. T. Scalettar, and A. G. Eguiluz, Phys. Rev. Lett. 88, 057001 (2002)

·        “Electronic Excitations in Metals and Semiconductors: Ab Initio Studies of Realistic Many-Particle Systems”
Wei Ku, thesis, University of Tennessee, Knoxville (2000)

·        “Comment on 'Why is the bandwidth of sodium observed to be narrower in photoemission experiments?' ”
Wei Ku, A. G. Eguiluz, and W. E. Plummer, Phys. Rev. Lett. 85, 2410 (2000)

·        “Plasmon Lifetime in K: A Case Study of Correlated Electrons in Solids Amenable to Ab Initio Theory”
Wei Ku and A. G. Eguiluz, Phys. Rev. Lett. 82, 2350 (1999)

·        “Crucial Role of the Crystal Potential in Magnetism of Edge-Sharing Cu-O Chains and its Interplay with the Bond Angle”
H. Rosner, Wei Ku, R. T. Scalettar, W. E. Pickett, S.-L. Drechsler, J. Malek, R. Neudert, M. Knupfer, J. Fink, and H. Eschrig, to be submitted to Phys. Rev. Lett.

·        “Anomalous Loss Functions of Zn and Cd:  Dynamical d-Threshold and Coherent Electron-Hole Response”
Wei Ku, and Adolfo G. Eguiluz, to be submitted to Phys. Rev. Lett.

Contact

E-mail: weiku@bnl.gov or weiku@mailaps.org    Tel: (631)344-2684,   Fax: (631)344-2918

Address:
Department of Physics, Brookhaven National Laboratory, Bldg 510
Upton, NY 11973-5000

Last updated: 4/14/2008