Assistant professor
Theoretical/Computational Condensed Matter Physics
Robeson 119, Department of Physics, Virginia Tech, Blacksburg VA 24061
Tel: 540-231-5533, Fax: 540-231-7511
Education:
Ph.D. Princeton Univ., Condensed Matter Theory 2000
M.S. Korea Univ., High Energy Experiment 1993
B.S. Korea Univ., Physics 1991
Research Experience:
Aug 2002 - July 2005: Postdoctoral research associate at Naval Research Lab, Washington DC
July 2000 - July 2002: Postdoctoral research associate at Florida State Univ., Tallahassee
Group members : Salvador Barraza-Lopez (Postdoc), Michael Avery (undergraduate), Jessica Gorzo (undergraduate)
Teaching : Computational Physics 5794 (Spring 2006)
Research Interests:
My research interests are theoretical and computational studies of electronic, magnetic, and transport properties of various magnetic materials and nanostructures. A few examples are shown below. For these calculations we use a local beowulf linux cluster in the physics department, VTech owned supercomputer System X, the Cornell Nanoscale Facility (CNF) linux cluster, and supercomputers in NCSA.
Electronic and magnetic properties of single-molecule magnets
Single-molecule magnets consist of several transition metal ions surrounded by organic and inorganic atoms. They have typical size of a few nanometers and their magnetic moment can be 32 times larger than the moment of an electron. They have shown quantum tunneling of magnetization and large magnetization reserval barriers at low temperatures. Because of these properties, single-molecule magnets could be used as ultra-high density information storage devices or materials for quantum computation. Below shown are a prototype single-molecule magnet Mn12-acetate (left figure: organe balls are Mn ions, red balls are oxygens, and gray ones are carbons. hydrogen atoms are not shown) synthesized by a Polish chemist, Lis, in 1981.The total ground-state spin of Mn12-acetate is S=10. This molecule was brought attention to physicisits since Sarachik, Friedman, and their collaborators found many steps (almost quantized) in magnetic hysteresis loop measurements in 1996. This can be understood by quantum tunneling of magnetic moment (tunneling between different directions of magnetic moment) schematically shown in the figure (right). In our group we calculate electronic, magnetic, and optical properties of various single-molecule magnets (including cyanide-based single-molecule magnets in collaboration with Prof. Gordon Yee's group at Virginia Tech) using a free finite-system density-functional theory code NRLMOL. NRLMOL has been developed by Dr. Mark Pederson and his collaborators at NRL. The references of the code are as follows: M.R. Pederson and K.A. Jackson, Phys. Rev. B 41, 7453 (1990); K.A. Jackson and M.R. Pederson, Phys. Rev. B 42, 3276 (1990); D. V. Porezag, Ph.D. thesis, Chemnitz Technical Institute, 1997. (for further information on the code and a copy of the code, please contact pederson@dave.nrl.navy.mil).
So far we have calculated the magnetic anisotropy barriers, quantum tunneling rates, intramolecular and intermolecular exchange constants, and excited spin states for Mn12-acetate, Mn4, and Ni4. The calculated magnetic anisotropy and tunneling rate are sensitive to local molecular environments.
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Exact diagonalization of the Heisenberg Hamiltonian (exchange coupling constants were obtained from broken-symmetry configurations within density-functional theory) provides ground-state and a few low-lying excited-state spin multiplets for Mn12-acetate.
The 2nd-order magnetic anisotropy barrier for Mn12-acetate structure changes significantly with the number of extra electrons added to the structure due to the lack of Jahn-Teller distortion.
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Publications:
(1) M. R. Pederson, K. Park, and T. Baruah, "Density-Functional Based Investigation of Molecular Magnets, " Current Trends in Computational Chemistry (2006).
(2) K. Park, E.-C. Yang, and D.N. Hendrickson, "Electronic structure and magnetic anisotropy for nickel-based molecular magnets," J. Appl. Phys. 97, 10M522 (2005).
(3) K. Park, M.R. Pederson, T. Baruah, N. Bernstein, J. Kortus, S.L. Richardson, E. del Barco, A. Kent, S. Hill, and N.S. Dalal, "Incommensurate transverse anisotropy induced by disorder and spin-vibron coupling in Mn12-acetate," J. Appl. Phys. 97, 10M505 (2005).
(4) K. Park and M.R. Pederson, "Effect of extra electrons on the exchange and magnetic anisotropy in the anionic single-molecule magnet Mn12," Phys. Rev. B 70, 054414 (2004).
(5) K. Park, T. Baruah, N. Bernstein, and M.R. Pederson, "Second-order transverse magnetic anisotropy induced by disorders in the single-molecule magnet Mn12," Phys. Rev. B 69, 144426 (2004).
(6) K. Park, M.R. Pederson, and C.S. Hellberg, "Properties of low-lying excited manifolds in the Mn12 acetate," Phys. Rev. B 69, 014416 (2004).
(7) K. Park, M.R. Pederson, and N. Bernstein, "Electronic, Magnetic, and Vibrational Properties of the Molecular magnet Mn4 monomer and dimer," J. Phys. Chem. Sol. 65, 805 (2004).
(8) K. Park, M.R. Pederson, S.L. Richardson, N. Aliaga-Alcalde, and G. Christou, "Density-functional theory calculation of the intermolecular exchange interaction in the magnetic Mn4 dimer, " Phys. Rev. B 68 020405(R) (2003).
(9) K. Park, M.A. Novotny, N.S. Dalal, S. Hill, and P.A. Rikvold, " Role of dipolar exchange interactions in the positions and widths of EPR transitions for the single-molecule magnets Fe8 and Mn12, " Phys. Rev. B 66, 144409 (2002).
(10) K. Park, M.A. Novotny, N.S. Dalal, S. Hill, and P.A. Rikvold, "Effects of D-strain, g-strain, and dipolar interactions on EPR linewidths of the molecular magnets Fe8 and Mn12," Phys. Rev. B 65, 014426 (2002).
Vibrational van der Waals interactions in comparison to electronic contributions
Quantum mechanically electrons localized at nuclei can move instantaneously, which causes a fluctuating dipole moment within an atom. This induces dipole moments in neighboring atoms. The interaction between the fluctuating dipole moment and the induced dipole moments is called van der Waals interaction or dispersion interaction. For a collection of neutral atoms and molecules without permanent dipole moments, the van der Waals interaction plays a major rold in binding atoms and molecules. The van der Waals interactions can be also caused by ionic vibrations. Considering the interaction between the induced dipoles caused by the infrared-active normal modes of a neutral molecule, we derived the formula for the vibrational van der Waals interaction and quantified the interaction, within the density-functional theory formalism, using a screened, self-consistent, vibrational polarizability. We found that the vibrational contributions for dimers examined are substantially smaller than their electronic contributions.
(1) K. Park, M.R. Pederson, and A.Y. Liu, "Comparison of vibrational and electronic contributions to van der Waals interactions," accepted for publication in Phys. Rev. B.
Electronic structure calculations on semiconducting boron-carbide molecular films
Boron-carbide films are interesting because they can be used as small portable neutron detectors but there was not much theoretical understanding on the structure of the films. Depending on types of isomers, some isomers showed n-type semiconducting behaviour, while the other type of isomers showed p-type semiconducting behaviour. To understand this, we concentrate on the B10C2 cluster with three isomers depending on where the two carbon atoms sit on the icosahedral cage. We calculated the electronic structure and infrared (IR) and Raman spectra for different configurations of the B10C2 cluster. This work has been performed in collaboration with Drs. Larry Boyer and Mark Pederson at NRL and with Profs. Dowben and Adenwalla, and Zheng and their collaborators at Nebraska. The figures shown below are calculated and experimental (thick solid curve in the bottom panel is background subtracted experimental curve) Raman spectra for B10C2 clusters.
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Publications:
(1) K. Park, M.R. Pederson, L.L. Boyer, W.N. Mei, R.F. Sabirianov, X.~C. Zeng, S. Bulusu, S. Curran, J. Dewald, E. Day, S. Adenwalla, M. Diaz, L.G. Rosa, S. Balaz, and P.A. Dowben, "Electronic structure and vibrational spectra of B10C2 Based Clusters and Films," Phys. Rev. B 73, 035109 (2006).
Deriving a realistic spin dynamic for magnetic nanoparticles coupled to phonon heat baths
Starting from a microscopic Hamiltonian that couples a spin system with phonon heat baths, we derived a transition probability and applied it to kinetic Monte Carlo simulations in order to understand how the magnetization changes with time upon field reversal at very low temperatures. The process of creating nuclear droplets follows a Poisson distribution. We found that the process highly depends on the underlying dynamic we used in the simulations.
Publications:
(1) G.M. Buendia, P.A. Rikvold, K. Park, and M.A. Novotny, "Low-temperature nucleation in a kinetic Ising model under different stochastic dynamics with local energy barriers," J. Chem. Phys. 121, 4193 (2004).
(2) K. Park, P.A. Rikvold, G.M. Buendia, and M.A. Novotny, "Low-temperature nucleation in a kinetic Ising model with soft stochastic dynamics," Phys. Rev. Lett. 92, 015701 (2004).
(3) K. Park, M.A. Novotny, and P.A. Rikvold, "Scaling analysis of a divergent prefactor in the metastable lifetime of a square-lattice Ising ferromagnet at low temperatures," Phys. Rev. E 66, 056101 (2002).
(4) K. Park and M.A. Novotny, "Dynamic Monte Carlo Simulations for a Square-Lattice Ising Ferromagnet with a Phonon Heat Bath," Comp. Phys. Comm. 147, 737 (2002).