Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session Y32: Focus Session: Theory and Simulations of Magnetism II |
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Sponsoring Units: DCOMP DMP GMAG Chair: Peter Dederichs, IFF Research Center Juelich Room: Morial Convention Center 225 |
Friday, March 14, 2008 11:15AM - 11:51AM |
Y32.00001: Homochiral magnetic structures at surfaces Invited Speaker: Electrons propagating in the vicinity of inversion asymmetric environments such as surfaces, interfaces, ultrathin films or nanostructures can give rise to an important antisymmetric exchange interaction, known as Dzyaloshinskii-Moriya (DM) interaction. Although this interaction, favoring spatially rotating spin structures, is in principle known for about 50 years, its consequences for the magnetic structure in low-dimensional magnets remained nearly unexplored and has been basically overlooked the past 20 years. Theoretical models considering isotropic exchange, magnetic anisotropy and the DM interaction display a rich phase diagram of complex magnetic phases on different length scales depending on the strength of the different contributions. Today, it is unknown how large is the strength of the DM interaction. Is this just a small perturbation to the collinear uniaxial ferro- or antiferromagnetic state, determined by exchange and magnetic anisotropy or is it strong enough to create new phases which had been overlooked? Surprisingly little first-principles calculations deal with the DM interaction. There might be several reasons for this: The investigation requires the treatment of non-collinear magnetism together with spin-orbit interactions of large magnetic structures in low-symmetry situations. We developed a perturbative strategy implemented into the FLAPW code {\tt FLEUR} which can cope with this challenge. We show by first- principles calculations based on the vector-spin density formulation of the density- functional theory (DFT) that that there are circumstances whre the DM interaction is indeed sufficiently strong to compete with the interactions that favor collinear spin alignment causing magnetic phases of unique handedness e.g.homochiral magnetic phases such as a left rotation cycloidal spiral in Mn on W(110) [M.~Bode {\em et al.}, Nature {\bf 447}, 190 (2007)] or favoring magnetic domain-walls with unique turning sense. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y32.00002: First-principles Calculation of Magnetic Anisotropy of a Single Atom on a Surface Chiung-Yuan Lin, Barbara Jones Recent progress on scanning tunneling microscopy has made it possible to position (in atomic-scale accuracy) and probe single magnetic atom on material surfaces. Targeting the fabrication of a single-atom data storage bit, we perform first-principles calculations of single magnetic atoms (Fe and Mn) on a surface. Structure relaxation determines the binding structures of the magnetic atoms to its surroundings. Charge analysis indicates that the magnetic atoms form polar covalent bonds with the surface. Spin density is found to spread up to 4{\AA} from the magnetic atom, which is qualitatively similar to that reported in DFT calculations of molecular magnets. Total energies with spin-orbit interaction included are calculated in different spin orientations, and are compared to the anisotropy axes measured in the experiments. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y32.00003: First-principles study on the surface half-metallicity of CaC in the zinc-blende structure Kailun Yao, Guoying Gao, Zuli Liu We investigate the electronic structure and the surface half-metallicity of CaC in the zinc-blende structure by using the first-principles full-potential linearized augmented plane-wave (FPLAPW) method. It is found that the (1 1 0) surface preserves the half-metallic character of the bulk, while in the case of the (0 0 1) surfaces including the Ca-terminated and C-terminated surfaces the surface states destroy the half-metallicity. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y32.00004: Finite Hubbard clusters with large spin polarization Erik Nielsen, R.N. Bhatt A generalized Hubbard model can be used to characterize hydrogenic impurities in semiconductors. It has been shown that the ground state spin of such impurity clusters is very sensitive to a cluster's electron number and geometry [1]. An understanding of how these factors affect cluster magnetization is particularly relevant in light of the current ability to position phosphorus donors with nanometer accuracy within bulk silicon [2]. We present numerical results for two-dimensional clusters showing the effect of geometry and electron-hole asymmetry present in real systems of hydrogenic donors. We also consider the robustness of high-spin cluster ground states to perturbations of site position, and discuss the general features of clusters found to possess a high-spin ground state, in particular the fully spin-polarized state.\newline [1] Erik Nielsen and R. N. Bhatt, Phys. Rev. B 76, 161202 (2007). \newline [2] J. L. O'Brien et al., Phys. Rev. B 64, 161401 (2001). [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 1:03PM |
Y32.00005: Magnetic ground state of small nanoparticles: Cr trimers on Au(111) Invited Speaker: The development of scanning tunneling microscopy (STM) and the ability to build clusters with well-controlled structures permit the study of various effects induced by local interactions within magnetic nanoclusters. However, for a clear interpretation of experimental results, first principles studies are often required, even when they are sometimes too demanding. In this talk we show a new way to map the energy of supported magnetic nanoparticles obtained from first principles calculations onto a classical spin Hamiltonian. The half-filled valence configuration of Cr yields a large magnetic moment and strong antiferromagnetic inter-atomic bonding leads in turn to magnetic frustration and complex spin phenomena. The simplest system exhibiting such properties is a trimer. The electronic structure of the Cr trimers are calculated by means of a fully relativistic Green's function embedding method. The relativistic treatment of the electronic structure leads to a proper account of spin-orbit coupling giving rise to tensorial exchange interactions and magnetic anisotropies influencing the formation of non-collinear ground states. In additon, we show that the inclusion of fourth-order terms into the spin-model largely enhance the accuracy of the mapping. The magnetic ground-state of the trimers are found as the solution of the Landau-Lifshitz-Gilbert equations. In case of an equilateral Cr trimer we show that the Dzaloshinsky-Moriya interactions lift the degeneracy of the 120$^\circ$ N\'eel states with different chirality. For the linear and the isosceles Cr trimers we obtain collinear antiferromagnetic ground states. We also address the issue of choosing the reference state inherent to methods based on the magnetic force theorem in the context to the equilateral Cr trimer. This freedom of the method might cause an ambiguity in determining the magnetic ground state of systems exhibiting metastable states close to the ground state. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y32.00006: A Numerical Method to Study Highly Correlated Nanostructures: The Logarithmic Discretization Embedded Cluster Approximation E. Anda, G. Chiappe, C. Busser, M. Davidovich, G. Martins, F. H-Meisner, E. Dagotto A numerical algorithm to study transport properties of highly correlated local structures is proposed. The method, dubbed the Logarithmic Discretization Embedded Cluster Approximation (LDECA), consists of diagonalizing a finite cluster containing the many-body terms of the Hamiltonian and embedding it into the rest of the system, combined with Wilson's ideas of a logarithmic discretization of the representation of the Hamiltonian. LDECA's rapid convergence eliminates finite-size effects commonly present in the embedding cluster approximation (ECA) method. The physics associated with both one embedded dot and a string of two dots side-coupled to leads is discussed. In the former case, our results accurately agree with Bethe ansatz (BA) data, while in the latter, the results are framed in the conceptual background of a two-stage Kondo problem. A diagrammatic expansion provides the theoretical foundation for the method. It is argued that LDECA allows for the study of complex problems that are beyond the reach of currently available numerical methods. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y32.00007: Coulomb and spin-orbit effects in quantum dot molecules under harmonic fields Lilia Meza-Montes, Arezky H. Rodriguez, Sergio E. Ulloa The time evolution of a two-electron quantum dot molecule under strong harmonic electric fields is studied. The wave function is determined in terms of the single-electron orbitals using the Floquet approach. We pay particular attention to the evolution of the spin states of the system, as the surface inversion asymmetry (Rashba-type) and bulk inversion asymmetry (Dresselhaus-type) spin-orbit effects are known to introduce spin mixing. We also study the role of a perpendicular magnetic field, which is shown to have dramatic effects on the dynamics. We present an analysis of the physical behavior of the system in terms of the quasi-energy spectrum, and study the time evolution of the occupation probabilities of the dots. Conditions for singlet-triplet mixing, similar to the spin-flips observed in the single-electron case, are analyzed. These results are relevant for applications in spin-controlled devices. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y32.00008: Electron Transport in Nanogranular Ferromagnets Igor Beloborodov, Andreas Glatz, Valerii Vinokur I will discuss electronic transport properties of ferromagnetic nanoparticle arrays and nanodomain materials near the Curie temperature in the limit of weak coupling between the grains. The conductivity is calculated in the Ohmic and non-Ohmic regimes and the magnetoresistance jump in the resistivity at the transition temperature is estimated. The results are applicable for many emerging materials, including artificially self-assembled nanoparticle arrays and a certain class of manganites, where localization effects within the clusters can be neglected. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y32.00009: Inverse band structure optimization of (InAs)/(GaAs) (001) nanostructures for thermophotovoltaics Paulo Piquini, Peter Graf, Alex Zunger Thermophotovoltaic materials converting black-body thermal radiation to electricity often require conversion efficiency for materials with direct band gaps of 0.6 eV. Random In$_{0.53}$Ga$_{0.47}$As alloy lattice $matched$ to InP have a gap around 0.76 eV, too big for this application. Therefore, difficult to grow lattice-$mismatched$ In-rich InGaAs alloys have been attempted in the past. Here we suggest to use (InAs)$_n$/(GaAs)$_m$ {\it ordered superlattices} (rather than random), lattice $matched$ to InP substrates. Using empirical pseudopotential calculations and genetic algorithm methods we look for the sequence of InAs and GaAs {\it pure} layers that simultaneously lead to a target band gap of 0.6 eV and has a minimum in-plane stress (strain balance condition). Further, since for (InAs)$_n$ layers with n$>$5 the two-dimensional growth is unstable and SK quantum dots are seen to form, we restrict the value for the period of the InAs layers to be always lower than 5. [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y32.00010: Spin-blockade in a Hubbard chain with spin-dependent impurities Chang-Qin Wu, Yao Yao We investigate the spin/charge transport in a one-dimensional strongly-correlated system using adaptive time-dependent density matrix renormalization group method. The model we consider is a non-half-filled Hubbard chain with spin-dependent impurities, which is found to display the blockade of spin current while little influence on charge current. We have considered (1) the spread of a wave packet of both spin and charge in the Hubbard chain and (2) the spin and charge currents induced by a spin-dependent voltage bias that is applied to the ideal leads attached at the ends of this Hubbard chain. It is found that the spin-charge separation plays a key role in the spin-blockade and a large on-site repulsion U is required for more effective utilization of this phenomenon in some spin-related devices, like spin memory. [Preview Abstract] |
Friday, March 14, 2008 2:03PM - 2:15PM |
Y32.00011: Signature of hyperfine interaction through current properties in quantum dots. Ernesto Cota, Fernando Rojas, Sergio E. Ulloa Several experiments have been carried out to observe and control spin properties of electrons in quantum dots subject to the hyperfine interaction due to nuclear spins. In this work, we study the manifestation of the hyperfine interaction through current calculations in one and two quantum dots. We use the density matrix master equation approach in the stationary regime taking into account an external magnetic field and the nuclear magnetic field in the quasistatic approximation characterized by the statistical fluctuations of the components of the nuclear field. As a first step, we study the case of a single quantum dot with one orbital. We study in detail the effects on the current of the hyperfine interaction, temperature and external magnetic field and we find that it is possible to obtain information on the hyperfine interaction directly from the differential conductance. We extend the model to study a double quantum dot with one and two electrons, including tunneling and exchange interactions, where the signature of the hyperfine interaction is more involved. [Preview Abstract] |
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