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 nanomaterials), using stateoftheart 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 firstprinciples quantum manybody theory. Special focus is placed to systems with stronger correlation that renders classical or meanfield 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 manybody 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 manybody effects. In addition to the technical importance of these materials, this makes them playgrounds for physicists to study approaches/approximations for describing the manybody behavior, and to manipulate/synthesize new artificial functional materials.
While the formal frameworks of manybody theories for weakly interacting systems are well developed and manageable within toy models, realistic firstprinciples (parameterfree) 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 manybody processes within finite computation resource.
An even more difficult challenge is systems with strong manybody interaction. While numerous computational approximation have been proposed, the NPhard 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 disorderinduced physics induced by these imperfection is largely unclear due to the limitation of current status of firstprinciples 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 selfconsistent 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 lowenergy Hilbert space is most convenient for theoretical formulation. To this end, a novel approach of constructing multienergy resolved, symmetry respecting Wannier function is developed and applied to many strongly correlated systems. The following are two examples of lowenergy Wannier functions that lead to deep insight of the physics of dichalcogenides and manganites.

Lowenergy Wannier states (WS) of real materials
Left: Gapless excitations in the charge density wave phase of TaSe_{2} 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 spindependence of resonant inelastic Xray spectrum of LaMnO_{3} is explained by the strong charge transfer nature of LaMnO_{3}, which is directly observable from the large hybridization with Op states in the WS. Based on further novel WS analysis, origin of orbital ordering of MnF_{3} and LaMnO_{3} is, surprisingly, mainly electronelectron interaction, rather than the electronphone coupling (JahnTeller effects). (Phys. Rev. Lett. 94, 047203 (2005) & condmat/0509075)
To understand the lowenergy physics that emerges from the manybody nature of the quantum system, the most intuitive approach is to drive the system toward lower energy, by projecting out the highenergy subspace. Such a derivation is performed based on firstprinciples density functional calculation, followed by a series of canonical transformation in the manybody Hiltbert space, employing symbolic noncommuting operations numerically.
One particle Green's function, G, of the electrons in the solid system can be used, to derive quasiparticle 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 selfenergy diagrams, at finite temperature within conserving scheme of the ManyBody Perturbation Theory that guarantees the microscopic conservation laws. This effort is especially important in understanding the role of manybody interactions in systems that deviate from the simple singleparticle picture, as this kind of parameterfree ab initio approach allows an unambiguous assignment for effect of different selfenergy diagrams.
As a natural (but nontrivial) extension of density functional theory, timedependent density functional theory (TDDFT) provides a “shortcut” of obtaining properties related to the timedependent 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 manybody 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 TDDFT. 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. Energyresolved symmetryrespecting Wannier functions are constructed based on approached developed myself. Numerical quantities like transition probability amplitude matrix elements and Physical quantities like density response function, selfenergy, 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 publicdomain 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.
· “What is the valence of Mn in Ga_{1x}Mn_{x}N?”
Ryky Nelson, Tom Berlijn, Juana
Moreno, Mark Jarrell, and Wei Ku, accepted by Phys. Rev. Lett.
· “Itinerancy enhanced quantum
fluctuation of magnetic moments in ironbased superconductors”
YuTing Tam, DaoXin Yao and Wei Ku, Phys. Rev. Lett. 115, 117001 (2015).
· “Interpretation of Scanning Tunneling
Quasiparticle Interference and Impurity States in Cuprates”
A. Kreisel,
et al., Phys. Rev. Lett. 114,
217002 (2015).
· “Bulk Signatures of PressureInduced
Band Inversion and Topological Phase Transitions in Pb_{1−x}Sn_{x}Se”
Xiaoxiang Xi, et al., Phys. Rev. Lett. 113, 096401 (2014).
· “Consequences of broken translational
symmetry in FeSe_{x}Te_{1x}”
L. Moreschini, et al., Phys. Rev. Lett. 112, 087602
(2014).
· “Firstprinciples method of
propagation of tightly bound excitons in LiF:
Verifying the exciton band structure with inelastic xray scattering”
ChiCheng Lee, et al., Phys. Rev. Lett. 111, 157401 (2013)
· “Signatures of a pressureinduced
topological quantum phase transition in BiTeI”
Xiaoxiang Xi, et al., Phys. Rev. Lett. 111, 155701 (2013)
· “Effects
of disordered Ru substitution in BaFe_{2}As_{2}: possible realization
of superdiffusion in real materials”
Limin Wang, et al., Phys. Rev. Lett. 110, 037001 (2013)
· “Temperaturedependent
transformation of the magnetic excitation spectrum on approaching
superconductivity in Fe_{1x(}Ni/Cu)_{x}Te_{0.5}Se_{0.5}”
Zhijun Xu, et al., Phys. Rev. Lett. 109, 227002 (2012)
· “Effective
doping and suppression of Fermi surface reconstruction via Fe vacancy disorder
in K_{x}Fe_{2y}Se_{2}”
Tom Berlijn, P. J. Hirschfeld, and Wei Ku, Phys. Rev. Lett. 109, 147003
(2012)
· “Insulating
magnetism in vacancyordered K_{0.8}Fe_{1.6}Se_{2}”
WeiGuo Yin, ChiaHui Lin, and Wei Ku, Phys. Rev. B 86, 081106(R) (2012)
· “Do
transition metal substitutions dope carriers in ironbased superconductors?”
Tom Berlijn, ChaiHui Lin, William Garber and Wei Ku, Phys. Rev. Lett. 108,
207003 (2012)
· “Relevance
of the HeisenbergKitaev Model for the Honeycomb
Lattice Iridates A_{2}IrO_{3}”
Yogesh Singh, et al., Phys. Rev. Lett. 108, 127203 (2012)
· “OneFe
versus TwoFe Brillouin Zone of FeBased Superconductors: Creation of the
Electron Pockets via Translational Symmetry Breaking”
ChiaHui Lin, Tom Berlijn, Limin Wang, ChiCheng Lee, WeiGuo Yin, and Wei Ku,
Phys. Rev. Lett. 107, 257001 (2011)
· “KineticsDriven
Superconducting Gap in Underdoped Cuprate
Superconductors Within the StrongCoupling Limit”
Y. Yildirim and Wei Ku, Phys. Rev. X 1, 011011 (2011)
· “Can
disorder alone destroy the eg’ hole pockets of Na_{0.3}CoO_{2}?”
Tom Berlijn, Dmitri Volja, and Wei Ku, Phys. Rev.
Lett. 106, 077005 (2011)
· “Room
temperature magnetism of Cudoped ZnO films probed by
soft Xray magnetic circular dichroism”
T.S. Herng, et al., Phys. Rev. Lett. 105,
207201 (2010)
· “A
unified picture for magnetic correlations in ironbased hightemperature superconductors”
WeiGuo Yin, ChiCheng Lee, and Wei Ku, Phys. Rev. Lett. 105, 107004
(2010)
· “Dynamical
Linear Response of TDDFT with LDA+U Functional: Strongly hybridized Frenkel
excitons in Mott insulators”
ChiCheng Lee, HungChung Hsueh, and Wei Ku, Phys. Rev. B 82, 081106 (R)
(2010)
· “Unfolding
firstprinciples band structures”
Wei Ku, Tom Berlijn, and ChiCheng Lee, Phys. Rev. Lett. 104, 216401
(2010)
· “Experimental
observation of the crystallization of a paired holon
state”
A. Rusydi, W. Ku, et al., Phys. Rev. Lett. 105,
026402 (2010)
· “Effect
of covalent bonding on magnetism and the missing neutron intensity in copper
oxide compounds”
Andrew C Walters, et al., Nature Physics 5, 867 (2010)
· “Charge
Ordering in HalfDoped Manganites: Weak Charge
Disproportion and Leading Mechanisms”
D. Volja, W.G. Yin, and Wei Ku, Europhys. Lett. 89
27008 (2010)
· “FerroOrbital
Order and Strong Magnetic Anisotropy in the Parent Compounds of IronPnictide Superconductors”
ChiCheng Lee, WeiGuo Yin, and Wei Ku, Phys. Rev. Lett. 103,
267001 (2009)
· “Dynamical
reconstruction of the exciton in LiF with inelastic
xray scattering”
Peter Abbamonte, Tim Graber, James P. Reed, Serban Smadici, ChenLin Yeh, Abhay Shukla, JeanPascal Rueff, and Wei Ku, PNAS 105, 12159 (2008)
· “Nanoscale Disorder in
CaCu_{3}Ti_{4}O_{12}: 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)
· “Nonresonant Inelastic
XRay Scattering and EnergyResolved Wannier Function Investigation of dd
Excitations in NiO and CoO”
B. C. Larson, Wei Ku, J. Z. Tischler, ChiCheng Lee,
O. D. Restrepo, A. G. Eguiluz,
P. Zschack, and K. D. Finkelstein, Phys. Rev. Lett. 99,
026401 (2007)
· “Orbital ordering in
LaMnO_{3}: Electronlattice versus electronelectron interactions”
W.G. Yin, D. Volja, and Wei Ku,
Phys. Rev. Lett. 96, 116405 (2006)
· “Coexistence of gapless
excitations and commensurate chargedensity wave in the 2Htransition 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 xray 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 La_{4}Ba_{4}Cu_{2}O_{10}: an Ab
Initio Wannier Function Analysis”
Wei Ku, H. Rosner, W. E. Pickett, and R. T.
Scalettar, Phys. Rev. Lett. 89, 167204 (2002)
· “BandGap Problem in
Semiconductors Revisited: Effects of Core States and ManyBody
SelfConsistency”
Wei Ku and A. G. Eguiluz, Phys. Rev. Lett. 89,
126401 (2002)
· “Ab Initio
Investigation of Collective Charge Excitations in MgB_{2}”
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
ManyParticle 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)
Email: weiku@bnl.gov or weiku@mailaps.org Tel: (631)3442684, Fax: (631)3442918
Address:
Department of Physics, Brookhaven National Laboratory, Bldg
734
Upton, NY 119735000
Last updated: Jan 8, 2014