Browsing by Author "Park, K."

Now showing 1 - 13 of 13
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    Antiferromagnetic coupling of the single-molecule magnet Mn-12 to a ferromagnetic substrate
    Park, K. (American Physical Society, 2011-02-28)
    We investigate magnetic coupling between a monolayer of prototype single-molecule magnets Mn-12 and a ferromagnetic Ni(111) substrate through S, using density-functional theory (DFT) and a DFT+U method. Our DFT and DFT+U calculations show that the Mn-12 molecules favor antiferromagnetic coupling to the Ni substrate, and that they possess magnetic moments deviated from the magnetic moments of isolated Mn-12 molecules. We find that the magnetic easy axis of the Mn-12 on Ni (whole system) is dictated by that of the Ni substrate. The antiferromagnetic coupling is, dominantly, caused by superexchange interactions between the magnetic moments of the Mn and the Ni substrate via the S, C, and O anions. Our findings can be observed from x-ray magnetic circular dichroism or scanning tunneling microscopy.
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    Comparison of vibrational and electronic contributions to van der Waals interactions
    Park, K.; Pederson, M. R.; Liu, A. Y. (American Physical Society, 2006-05)
    The van der Waals interaction can be caused by either ionic vibrations or instantaneous electronic motion relative to the atomic center. In this study, the vibrational contribution to the van der Waals interaction is formulated by considering the interaction between induced dipoles caused by the infrared-active normal modes of a neutral molecule. Using the derived formula, the contribution is quantified, within the density-functional theory formalism, using a screened, i.e., self-consistent, vibrational polarizability. Applications for several neutral nonpolar dimers are presented. It is found that the vibrational contributions for the dimers are substantially smaller than their electronic contributions. The ratio of the vibrational to electronic contributions depends strongly on the ratio of the screened vibrational to electronic polarizabilities and on the ratio of the frequency of the strongest infrared-active mode to an ionization energy.
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    Cooper instability of composite fermions
    Scarola, Vito W.; Park, K.; Jain, J. K. (2000-08-24)
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    Effect of the size distribution of magnetic nanoparticles on metastability in magnetization relaxation
    Yamamoto, Y.; Park, K. (American Physical Society, 2011-09-15)
    We theoretically examine metastability occurring in magnetization relaxation for magnetic nanoparticles with size distributions. An array ofmagnetic nanoparticles is simulated using a spin S = 1 ferromagnetic Blume-Capel model on a square lattice. The particle size distributions give rise to distributions of magnetic anisotropy. Including the distributions, we perform kinetic Monte Carlo simulations of magnetization relaxation at low temperatures for the Blume-Capel model. We compute the average lifetime of the metastable state from the simulations and the absorbing Markov chains method in the low-temperature limit. We also carry out similar simulations and calculations for a constant value of magnetic anisotropy for comparison. Our results suggest that the lifetime of the metastable state is determined by the smallest particle for a given system, and that the lifetime with size distributions obeys a modified Arrhenius-like law, where the energy barrier depends on even temperature and standard deviation of the distributions as well as magnetic field and magnetic anisotropy.
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    Effects of bonding type and interface geometry on coherent transport through the single-molecule magnet Mn-12
    Park, K.; Barraza-Lopez, S.; Garcia-Suarez, V. M.; Ferrer, J. (American Physical Society, 2010-03)
    We examine theoretically coherent electron transport through the single-molecule magnet Mn-12, bridged between Au(111) electrodes, using the nonequilibrium Green's function method and the density-functional theory. We analyze the effects of bonding type, molecular orientation, and geometry relaxation on the electronic properties and charge and spin transport across the single-molecule junction. We consider nine interface geometries leading to five bonding mechanisms and two molecular orientations: (i) Au-C bonding, (ii) Au-Au bonding, (iii) Au-S bonding, (iv) Au-H bonding, and (v) physisorption via van der Waals forces. The two molecular orientations of Mn-12 correspond to the magnetic easy axis of the molecule aligned perpendicular [hereafter denoted as orientation (1)] or parallel [orientation (2)] to the direction of electron transport. We find that the electron transport is carried by the lowest unoccupied molecular orbital (LUMO) level in all the cases that we have simulated. Relaxation of the junction geometries mainly shifts the relevant occupied molecular levels toward the Fermi energy as well as slightly reduces the broadening of the LUMO level. As a result, the current slightly decreases at low bias voltage. Our calculations also show that placing the molecule in the orientation (1) broadens the LUMO level much more than in the orientation (2) due to the internal structure of the Mn-12. Consequently, junctions with the former orientation yield a higher current than those with the latter. Among all of the bonding types considered, the Au-C bonding gives rise to the highest current (about one order of magnitude higher than the Au-S bonding), for a given distance between the electrodes. The current through the junction with other bonding types decreases in the order of Au-Au, Au-S, and Au-H. Importantly, the spin-filtering effect in all the nine geometries stays robust and their ratios of the majority-spin to the minority-spin transmission coefficients are in the range of 10(3)-10(8). The general trend in transport among the different bonding types and molecular orientations obtained from this study may be applicable to other single-molecule magnets.
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    Epitaxial thin films of pyrochlore iridate Bi₂₊ₓIr₂₋ᵧO₇₋δ: structure, defects and transport properties
    Yang, W. C.; Xie, Y. T.; Zhu, W. K.; Park, K.; Chen, A. P.; Losovyj, Y.; Li, Z.; Liu, H. M.; Starr, M.; Acosta, J. A.; Tao, C. G.; Li, N.; Jia, Q. X.; Heremans, Jean J.; Zhang, S. X. (Nature, 2017-08-10)
    While pyrochlore iridate thin films are theoretically predicted to possess a variety of emergent topological properties, experimental verification of these predictions can be obstructed by the challenge in thin film growth. Here we report on the pulsed laser deposition and characterization of thin films of a representative pyrochlore compound Bi₂Ir₂O₇. The films were epitaxially grown on yttriastabilized zirconia substrates and have lattice constants that are a few percent larger than that of the bulk single crystals. The film composition shows a strong dependence on the oxygen partial pressure. Density-functional-theory calculations indicate the existence of BiIr antisite defects, qualitatively consistent with the high Bi: Ir ratio found in the films. Both Ir and Bi have oxidation states that are lower than their nominal values, suggesting the existence of oxygen deficiency. The iridate thin films show a variety of intriguing transport characteristics, including multiple charge carriers, logarithmic dependence of resistance on temperature, antilocalization corrections to conductance due to spin-orbit interactions, and linear positive magnetoresistance.
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    Exchange coupling and contribution of induced orbital angular momentum of low-spin Fe3+ ions to magnetic anisotropy in cyanide-bridged Fe2M2 molecular magnets: Spin-polarized density-functional calculations
    Park, K.; Holmes, S. M. (American Physical Society, 2006-12)
    Electronic structure and intramolecular exchange constants are calculated for three cyanide-bridged molecular magnets, [Tp(star)Fe(III)(CN)(3)M-II(DMF)(4)](2)(OTf)(2)center dot 2DMF (M-II=Mn,Co,Ni) (abbreviated as Fe2Mn2, Fe2Co2, and Fe2Ni2) that have been recently synthesized, within a generalized-gradient approximation in spin-polarized density-functional theory (DFT). Here Tp(star)=[C-3(CH3)(2)HN2](3)BH, OTf=O3SCF3, and DMF=HCON(CH3)(2). Due to strong ligand fields present in the [Tp(star)Fe(III)(CN)(3)](-) units, the Fe3+ ions exhibit a low ground-state spin of S=1/2. Our calculations show that the metal ions in the Fe2Mn2 molecule interact antiferromagnetically via cyanide ligands, while those in the Fe2Co2 and Fe2Ni2 molecule interact ferromagnetically. The calculations also suggest that the smallest gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for Fe2Mn2, Fe2Co2, and Fe2Ni2 are 0.12, 0.03, and 0.33 eV. Based on the calculated electronic structures, the second-order magnetic anisotropy is computed including single-electron spin-orbit coupling within a DFT formalism. In comparison to a prototype single-molecule magnet Mn-12, the three cyanide-bridged molecular magnets are found to bear substantial transverse magnetic anisotropy that becomes 15%-36% of molecular longitudinal anisotropy. Spin-orbit coupling arising from the low-spin Fe3+ and high-spin Co2+ ions induces significant orbital angular momentum that contributes to the total magnetic anisotropy of the three cyanide-bridged molecular magnets. The induced orbital angular momentum is 4-8 times those calculated for Mn-12. The total magnetic anisotropy present in the three molecular magnets is due to competition between the magnetic anisotropy of the Fe3+ and of the M2+ ions. In the Fe2Mn2 and Fe2Ni2 molecules, the anisotropy is primarily due to the Fe3+ ions, while in the Fe2Co2 molecule, the single-ion anisotropy of the Co2+ ions counters the Fe3+ contributions. These results are supported by previously reported magnetic measurements.
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    First-principles study of a single-molecule magnet Mn-12 monolayer on the Au(111) surface
    Barraza-Lopez, S.; Avery, M. C.; Park, K. (American Physical Society, 2007-12)
    The electronic structure of a monolayer of single-molecule magnets Mn-12 on a Au(111) surface is studied using spin-polarized density-functional theory. The Mn-12 molecules are oriented such that the magnetic easy axis is normal to the surface, and the terminating ligands in the Mn-12 are replaced by thiol groups (-SH) where the H atoms are lost upon adsorption onto the surface. This sulfur-terminated Mn-12 molecule has a total magnetic moment of 18 mu(B) in the ground state, in contrast to 20 mu(B) for the standard Mn-12. The Mn-12 molecular orbitals broaden due to the interaction of the molecule with the gold surface and the broadening is of the order of 0.1 eV. It is an order of magnitude less than the single-electron charging energy of the molecule so the molecule is weakly bonded to the surface. Only electrons with majority spin can be transferred from the surface to the sulfur-terminated Mn-12 since the gold Fermi level is well above the majority lowest unoccupied molecular orbital (LUMO) but below the minority LUMO. The amount of the charge transfer is calculated to be 1.23 electrons from a one-dimensional charge density difference between the sulfur-terminated Mn-12 on the gold surface and the isolated sulfur-terminated Mn-12, dominated by the tail in the electronic distribution of the gold surface. A calculation of a level shift upon charging provides 0.28 electrons being transferred. The majority of the charge transfer occurs at the sulfur, carbon, and oxygen atoms close to the surface. The total magnetic moment also changes from 18 mu(B) to 20 mu(B), which is due to rearrangements of the magnetic moments on the sulfur atoms and Mn atoms upon adsorption onto the surface. The magnetic anisotropy barrier is computed including spin-orbit interaction self-consistently in density-functional theory. The barrier for the Mn-12 on the gold surface decreases by 6 K in comparison to that for an isolated Mn-12 molecule.
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    Metastability for the Blume-Capel model with distribution of magnetic anisotropy using different dynamics
    Yamamoto, Y.; Park, K. (American Physical Society, 2013-07-11)
    We investigate the relaxation time of magnetization or the lifetime of the metastable state for a spin S = 1 square-lattice ferromagnetic Blume-Capel model with distribution of magnetic anisotropy (with small variances), using two different dynamics such as Glauber and phonon-assisted dynamics. At each lattice site, the Blume-Capel model allows three spin projections (+1, 0, -1) and a site-dependent magnetic anisotropy parameter. For each dynamic, we examine the low-temperature lifetime in two dynamic regions with different sizes of the critical droplet and at the boundary between the regions, within the single-droplet regime. We compute the average lifetime of the metastable state for a fixed lattice size, using both kinetic Monte Carlo simulations and the absorbing Markov chains method in the zero-temperature limit. We find that for both dynamics the lifetime obeys a modified Arrhenius-like law, where the energy barrier of the metastable state depends on the temperature and standard deviation of the distribution of magnetic anisotropy for a given field and magnetic anisotropy and that an explicit form of this dependence differs in different dynamic regions for different dynamics. Interestingly, the phonon-assisted dynamic prevents transitions between degenerate states, which results in a large increase in the energy barrier at the region boundary compared to that for the Glauber dynamic. However, the introduction of a small distribution of magnetic anisotropy allows the spin system to relax via lower-energy pathways such that the energy barrier greatly decreases. In addition, for the phonon-assisted dynamic, even the prefactor of the lifetime is substantially reduced for a broad distribution of magnetic anisotropy in both regions considered, in contrast to the Glauber dynamic. Our findings show that overall the phonon-assisted dynamic is more significantly affected by the distribution of magnetic anisotropy than the Glauber dynamic.
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    Nanostructure and velocity of field-driven solid-on-solid interfaces moving under a phonon-assisted dynamic
    Buendia, G. M.; Rikvold, P. A.; Kolesik, M.; Park, K.; Novotny, M. A. (American Physical Society, 2007-07)
    The nanoscopic structure and the stationary propagation velocity of (1+1)-dimensional solid-on-solid interfaces in an Ising lattice-gas model, which are driven far from equilibrium by an applied force, such as a magnetic field or a difference in (electro)chemical potential, are studied by an analytic nonlinear-response approximation [P. A. Rikvold and M. Kolesik, J. Stat. Phys. 100, 377 (2000)] together with kinetic Monte Carlo simulations. Here, we consider the case that the system is coupled to a two-dimensional phonon bath. In the resulting dynamic [K. Saito , Phys. Rev. E 61, 2397 (2000); K. Park and M. A. Novotny, Comput. Phys. Commun. 147, 737 (2002)], transitions that conserve the system energy are forbidden, and the effects of the applied force and the interaction energies do not factorize (a so-called hard dynamic). In full agreement with previous general theoretical results, we find that the local interface width changes dramatically with the applied force. However, in contrast with other hard dynamics, this change is nonmonotonic in the driving force. Results are also obtained for the force dependence and anisotropy of the interface velocity, which also show differences in good agreement with the theoretical expectations for the differences between soft and hard dynamics. However, significant differences between theory and simulation are found near two special values of the driving force, where certain transitions allowed by the solid-on-solid model become forbidden by the phonon-assisted dynamic. Our results show that different stochastic interface dynamics that all obey detailed balance and the same conservation laws nevertheless can lead to radically different interface responses to an applied force. Thus, they represent a significant step toward providing a solid physical foundation for kinetic Monte Carlo simulations.
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    Robustness of Topologically Protected Surface States in Layering of Bi₂Te₃ Thin Films
    Park, K.; Heremans, Jean J.; Scarola, Vito W.; Minic, Djordje (American Physical Society, 2010-10-27)
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    Robustness of Topologically Protected Surface States in Layering of Bi₂Te₃ Thin Films
    Park, K.; Heremans, Jean J.; Scarola, Vito W.; Minic, Djordje (American Physical Society, 2010-10-27)
    Bulk Bi2Te3 is known to be a topological insulator. We investigate surface states of Bi2Te3(111) thin films of one to six quintuple layers using density-functional theory including spin-orbit coupling. We construct a method to identify topologically protected surface states of thin film topological insulators. Applying this method to Bi2Te3 thin films, we find that the topological nature of the surface states remains robust with the film thickness and that the films of three or more quintuple layers have topologically nontrivial surface states, which agrees with experiments.
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    Transition rates for a S >= 1 spin model coupled to a d-dimensional phonon bath
    Park, K. (American Physical Society, 2008-03)
    We consider a S >= 1 spin model (or a generalized Blume-Capel model) weakly coupled to a d-dimensional phonon bath and investigate transition rates between different spin configurations. This study is motivated by understanding magnetization relaxation as a function of temperature in diverse magnetic systems such as arrays of magnetic nanoparticles and magnetic molecules. We assume that the magnetization of the spin system relaxes through consecutive emission or absorption of a single phonon. From a weak, linear spin-phonon coupling Hamiltonian, we derive transition rates that would be used to examine dynamic properties of the system in kinetic Monte Carlo simulations. Although the derived phonon-assisted transition rates satisfy detailed balance, in the case of two- and three-dimensional phonon baths, transitions between degenerate states are not allowed. ( This is a major difference of the phonon-assisted transition rates from the Metropolis and Glauber transition rates. ) Thus, if there are no alternative paths along which the spin system can relax, the relaxation time diverges. Otherwise, the system finds other paths, which leads to an increase in the relaxation time and energy barrier. However, when higher-order phonon processes are included in the transition rates, it is found that the system can reach the states which were inaccessible due to the forbidden transitions. As a result, the system recovers some of the dynamic properties obtained using the Glauber transition rate.