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 and theoretical modeling. In addition, I am also developing novel theoretical/numerical methods.
My research interest is the rich electronic/magnetic/optical properties of condensed matter materials, using first-principles quantum many-body theory. Special focus is placed to systems with stronger correlation that renders classical or mean-field treatments inadequate. Examples of existing works include (see publications for more detail):
Historically, one of the traditional emphasis 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 emerging properties of a collection of these particles can no longer be understood simply from the properties of few fundamental particles. In particular, as the energy scale reduces, the dominant physical effects changes: some new effective interactions emerge while other interactions become irrelevant. The rich physics associated with quantum many-body effect is, in my opinion, the most interesting pioneering aspects of modern physics.
This 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, superconductivity, and optical absorption) 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 functional materials.
While the formal frameworks of many-body theories for weakly interacting systems are well developed and 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 developments that enable inclusion of more many-body processes within finite computation resource.
An even more difficult challenge is systems with strong many-body interaction. While numerous computational approximation have been proposed, the NP-hard nature of the problem makes it impossible to have one universal approach to capture the rich physics at low energy. This is truly the playground for the endless possibilities only limited by human intelligence and imagination.
Modern functional materials make heavy use of substitution, doping and vacancies to enable and optimize their functionality. However, the disorder-induced physics induced by these imperfection is largely unclear due to the limitation of current status of first-principles method. A recent development of my group is to overcome this difficulty via two newly developed Wannier function based methods. The first is to unfold the band structure of a super cell (resulting from some broken symmetry) to a larger Brillouin zone of higher symmetry (Phys. Rev. Lett. . 104, 216401 (2010).) The second is a construction of effective Hamiltonian that capture the influence of impurities on the fully self-consistent DFT Hamiltonian (Phys. Rev. Lett. 106, 077005 (2011).) Together, this enables an efficient computation of disorder configuration average spectral function that captures weak and strong localization, and displays mean free path and life time of the coherent propagation.
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)
To understand the low-energy physics that emerges from the many-body nature of the quantum system, the most intuitive approach is to drive the system toward lower energy, by projecting out the high-energy subspace. Such a derivation is performed based on first-principles density functional calculation, followed by a series of canonical transformation in the many-body Hiltbert space, employing symbolic non-commuting operations numerically.
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.
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.
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.
· “Itinerancy enhanced quantum
fluctuation of magnetic moments in iron-based superconductors”
Yu-Ting Tam, Dao-Xin Yao and Wei Ku, accepted by Phys. Rev. Lett.
· “Interpretation of Scanning Tunneling
Quasiparticle Interference and Impurity States in Cuprates”
A. Kreisel, et al., Phys. Rev. Lett. 114, 217002 (2015).
· “Bulk Signatures of Pressure-Induced Band
Inversion and Topological Phase Transitions in Pb1−xSnxSe”
Xiaoxiang Xi, et al., Phys. Rev. Lett. 113, 096401 (2014).
· “Consequences of broken translational
symmetry in FeSexTe1-x”
L. Moreschini, et al., Phys. Rev. Lett. 112, 087602 (2014).
· “First-principles method of
propagation of tightly bound excitons in LiF: Verifying the exciton band
structure with inelastic x-ray scattering”
Chi-Cheng Lee, et al., Phys. Rev. Lett. 111, 157401 (2013)
· “Signatures of a pressure-induced topological
quantum phase transition in BiTeI”
Xiaoxiang Xi, et al., Phys. Rev. Lett. 111, 155701 (2013)
of disordered Ru substitution in BaFe2As2: possible
realization of superdiffusion in real materials”
Limin Wang, et al., Phys. Rev. Lett. 110, 037001 (2013)
transformation of the magnetic excitation spectrum on approaching
superconductivity in Fe1-x(Ni/Cu)xTe0.5Se0.5”
Zhijun Xu, et al., Phys. Rev. Lett. 109, 227002 (2012)
doping and suppression of Fermi surface reconstruction via Fe vacancy disorder
Tom Berlijn, P. J. Hirschfeld, and Wei Ku, Phys. Rev. Lett. 109, 147003 (2012)
magnetism in vacancy-ordered K0.8Fe1.6Se2”
Wei-Guo Yin, Chia-Hui Lin, and Wei Ku, Phys. Rev. B 86, 081106(R) (2012)
transition metal substitutions dope carriers in iron-based superconductors?”
Tom Berlijn, Chai-Hui Lin, William Garber and Wei Ku, Phys. Rev. Lett. 108, 207003 (2012)
of the Heisenberg-Kitaev Model for the Honeycomb Lattice Iridates A2IrO3”
Yogesh Singh, et al., Phys. Rev. Lett. 108, 127203 (2012)
versus Two-Fe Brillouin Zone of Fe-Based Superconductors: Creation of the
Electron Pockets via Translational Symmetry Breaking”
Chia-Hui Lin, Tom Berlijn, Limin Wang, Chi-Cheng Lee, Wei-Guo Yin, and Wei Ku, Phys. Rev. Lett. 107, 257001 (2011)
Superconducting Gap in Underdoped Cuprate Superconductors Within the
Y. Yildirim and Wei Ku, Phys. Rev. X 1, 011011 (2011)
disorder alone destroy the eg’ hole pockets of Na0.3CoO2?”
Tom Berlijn, Dmitri Volja, and Wei Ku, Phys. Rev. Lett. 106, 077005 (2011)
temperature magnetism of Cu-doped ZnO films probed by soft X-ray magnetic
T.S. Herng, et al., Phys. Rev. Lett. 105, 207201 (2010)
unified picture for magnetic correlations in iron-based high-temperature
Wei-Guo Yin, Chi-Cheng Lee, and Wei Ku, Phys. Rev. Lett. 105, 107004 (2010)
Linear Response of TDDFT with LDA+U Functional: Strongly hybridized Frenkel
excitons in Mott insulators”
Chi-Cheng Lee, Hung-Chung Hsueh, and Wei Ku, Phys. Rev. B 82, 081106 (R) (2010)
first-principles band structures”
Wei Ku, Tom Berlijn, and Chi-Cheng Lee, Phys. Rev. Lett. 104, 216401 (2010)
observation of the crystallization of a paired holon state”
A. Rusydi, W. Ku, et al., Phys. Rev. Lett. 105, 026402 (2010)
of covalent bonding on magnetism and the missing neutron intensity in copper
Andrew C Walters, et al., Nature Physics 5, 867 (2010)
Ordering in Half-Doped Manganites: Weak Charge Disproportion and Leading
D. Volja, W.-G. Yin, and Wei Ku, Europhys. Lett. 89 27008 (2010)
Order and Strong Magnetic Anisotropy in the Parent Compounds of Iron-Pnictide
Chi-Cheng Lee, Wei-Guo Yin, and Wei Ku, Phys. Rev. Lett. 103, 267001 (2009)
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
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
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)
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
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
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)
Department of Physics, Brookhaven National Laboratory, Bldg 734
Upton, NY 11973-5000
Last updated: Jan 8, 2014