Theoretical/Computational Condensed Matter Physics
Robeson 119, Department of Physics, Virginia Tech, Blacksburg VA 24061
Tel: 540-231-5533, Fax: 540-231-7511
Ph.D. Princeton Univ., Condensed Matter Theory 2000
M.S. Korea Univ., High Energy Experiment 1993
B.S. Korea Univ., Physics 1991
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
Current group members: Kristen Brown, Yoh Yamamoto (graduate students), and Yu Wang (postdoctoral associate)
Former group members: Michael Avery, Emalee Popoff, Daniel Flisek, Elizabeth Lowry, Jessica Gorzo, Salvador Barraza-Lopez (postdoc)
Teaching: Computational Physics (Spring 2006-2010), Quantum and Solid State Physics (Fall 2006, Fall 2007), Electricity and Magnetism (graduate level, Fall 2008 and Fall 2009)
Funding: Jeffress Memorial Fund, National Science Foundation
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, magnetic, and electron transport 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 physicists 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 are interested in understanding electronic, magnetic, and electron transport properties of single-molecule magnets Mn12 and Fe4 and other spin-orbit coupled nanostructures. For electronic and magnetic properties, we use density-functional theory code NRLMOL (developed by M.R. Pederson and his collaborators),
SIESTA, and VASP. For electron transport properties, we use SIESTA-interfaced SMEAGOL.
The calculated magnetic anisotropy and tunneling rate are sensitive to local molecular environments.
Schematic top and side views of a Mn12 molecule adsorbed onto a gold slab via a thiol group. The three different Mn ions regarding the symmetry are denoted as I, II, and III. The side view of the six gold monolayers with 36 surface atoms per monolayer are shown. In supercell calculations, 10 angstrom of vertical vacuum is added on the top of the Mn12 molecule. Cited from Ref..
(Left) Charge density difference between the whole structure (Mn12 adsorbed on Au(111) via two S atoms) and an isolated Mn12 (S-terminated,geometry 2) vs the z coordinate. A, B, and C denote the locations of the S atoms, the bonding C atoms, and the Mn ions, respectively. (Right) Spin magnetic moment difference between the whole structure and geometry 2. Cited from Ref..
Majority-spin (spin-up) and minority-spin (spin-down) density of states integrated between -0.23 and 0.06 eV relative to the Fermi level with some isosurface criterion for a Mn12 molecule bridged between Au(111) electrodes. This clearly shows a spin-filtering effect. Cited from Ref..
Current-voltage characteristics for six interface geometries: majority-spin contribution (red box),minority-spin contribution (blue star). The six interface geometries are Geo 1:Au-(SC3H6)2-hollow, Geo 1:Au-(C3H6)2, Geo 1:Au-(AuC3H6)2, Geo 2:Au-S4, Geo 2:phys-(CH3)4, and Geo 2:Au-H-no-linker. In Geo 1, the magnetic easy axis of the Mn12 is normal to the transport direction. In Geo 2, the magnetic easy axis of the Mn12 is parallel to the transport direction. Cited from Ref..
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.
Hybrid structures including topological insulators
Work in progress...
(1) Kyungwha Park, J. J. Heremans, V. W. Scarola, and Djordje Minic, "Robustness of topologically protected surface states in layering of Bi2Te3 thin films," arXiv:1005.3476, Phys. Rev. Lett. 105, 186801 (2010).
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.
(1) Kyungwha Park, "Transition rates for a S ≥ 1 spin model coupled to a d-dimensional phonon bath," Phys. Rev. B 77, 104420 (2008).
(2) G.M. Buendía, P.A. Rikvold, M. Kolesik, K. Park, and M.A. Novotny, " Nanostructure and velocity of field-driven solid-on-solid interfaces moving under a phonon-assisted dynamic," Phys. Rev. B 76, 045422 (2007).
(3) 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).
(4) 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).
(5) 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).
(6) 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).