Title Manipulating Electron and Nuclear Spins Dynamics Using Solid Surfaces : A Theoretical Study Author(s) ARGUELLES, ELVIS FLAVIANO Citation Issue Date Text Version ETD URL https://doi.org/10.18910/67135 DOI 10.18910/67135 rights Note Osaka University Knowledge Archive : OUKA https://ir.library.osaka-u.ac.jp/ Osaka University Doctoral Dissertation Manipulating Electron and Nuclear Spins Dynamics Using Solid Surfaces: A Theoretical Study (固体表面を用いた電子・核スピンダイナ ミクス制御に関する理論的研究 ) Elvis F. Arguelles July, 2017 Graduate School of Engineering, Osaka University Osaka University Doctoral Dissertation Manipulating Electron and Nuclear Spins Dynamics Using Solid Surfaces: A Theoretical Study by Elvis F. Arguelles Athesissubmittedinpartialfulfilmentoftherequirements for the degree of Doctor of Philosophy in the Division of Precision Science and Applied Physics Graduate School of Engineering July 2017 Declaration of Authorship I, Elvis F. Arguelles, declare that this thesis titled, ’Manipulating Electron and Nu- clear Spins Dynamics Using Solid Surfaces: A Theoretical Study’ and the work presented in it are my own. I confirm that: ! This work was done wholly or mainly while in candidature for a research degree at this University. ! Where any part of this thesis has previously been submitted for a degree or any other qualification at this University or any other institution, this has been clearly stated. ! Where I have consulted the published work of others, this is always clearly at- tributed. ! Where I have quoted from the work of others, the source is always given. With the exception of such quotations, this thesis is entirely my own work. ! I have acknowledged all main sources of help. ! Where the thesis is based on work done by myself jointly with others, I have made clear exactly what was done by others and what I have contributed myself. Signed: Date: i OSAKA UNIVERSITY Abstract Doctor of Philosophy Manipulating Electron and Nuclear Spins Dynamics Using Solid Surfaces: A Theoretical Study by Elvis F. Arguelles Solid surfaces have been serving as the so-called ”playground of physics” since the de- velopment of ultra high vacuum environment and advanced experimental techniques for their syntheses and characterizations. Significant discoveries have been unraveled ranging from strange electronic states that are only observable in their presence, to the better understanding of gas molecules-surfaces reactions, all of which have immensely contributed to the progress of surface science. In the past decade, there have been considerable interests in the behavior of electron spins in solid surfaces and their ma- nipulations in the next generation electronic devices have given birth to the field of spintronics. Similarly, the effects of solid surfaces on the electronic and nuclear spins of molecules involved in many heterogenous catalytic reactions have been gaining recogni- tion among surface physicists and chemists. This imply that one can tune the physical properties of solid surfaces by intelligent materials design to influence and manipulate the electronic and nuclear spin states. Fundamental to such design strategies is how to take advantage of the system’s symmetry and its breaking processes. Broken symme- tries can lead to drastic changes in the electronic, magnetic, and structural properties of solids and gas molecules which in principle can be used to manipulate the behavior of electron and nuclear spin states. In this dissertation, we presented the manipulations of electrons and nuclei spins using symmetry-driven designs of solid surfaces by considering two case studies using a combination of first principles calculations based on the density functional theory (DFT) and model Hamiltonian calculations. In the first case study, we introduced a new way of designing dilute magnetic semiconductors by taking advantage of the layered structures of an oxide crystal in introducing magnetic and non-magnetic impurities. By means of intelligent materials design computations we showed how the local symmetry breaking introduced by impurities at the interstitial regions changes the crystal fields of its surroundings in solids and induce electron spin polarization at the surface layer. We also found that the formation energies of interstitial impurities in a semiconducting lead oxide (PbO) surface is significantly lower compared to their sub- situtional counterparts due to their unique bonding mechannism with the host crystal ii iii atoms. This bonding results in splitting of the d-orbitals of the impurity atoms that induce spin polarization and magnetic moments in the surface. We also found ferro- magnetic spin ordering that persists above room temperature which is stabilized by the exchange of electrons between impurities through oxide atoms. Further, we have shown that magnetic moments are strongly enhanced by hole doping which can be attributed to the emptying of the minority spin states, giving rise to stronger spin-polarization in the crystal. In the second case study, we demonstrated from quantum dynamics calcula- tions how the introduction of steps on solid surfaces breaks the rotational symmetry of adsorbing H2 that leads to the lifting of the angular momentum levels degeneracies and nuclear spin states transition. We showed that H2 adsorption on stepped Pd surface is governed by a highly anisotropic potential energy surface (PES) that favors adsorption of parallel- over perpendicular-oriented molecule. By solving the quantum motion of H2 in this anisotropic PES, we found that the two lowest energy states correspond to para and ortho nuclear spin isomers, respectively. Upon comparing the gas and the adsorbed phase rotational energy levels, we found that ortho-H2 have higher desorption energy compared to para-H2, implying the separation of the nuclear spin isomers by adsorption on stepped metal surface. We further showed that the presence of steps on metal surfaces promotes H2 nuclear spin transition by enhancing the surface-molecule hybridization. We have concluded that the key factor to faster ortho-para conversion is to bring H2 to the so-called molecular chemisorbed state where there is a significant overlap of surface and molecule wave functions. This is only realized by introducing steps on surfaces. The results of this dissertation have significant direct impacts on the design and development of spintronics materials as well as important contributions to the realization of hydrogen-based society. Contents Declaration of Authorship i Abstract ii Contents iv List of Figures vii List of Tables x Abbreviations xi 1 Spin: Electrons and Nuclei 1 2 Spin in Spintronics and Catalysis 3 2.1 Electron spin states and spintronics ...................... 3 2.2 Spin in Catalysis ................................ 5 2.3 Intelligent design of solid surfaces: a means to manipulate electron and nuclear spins .................................. 7 3 Surface magnetism in αPbO: A case study 11 3.1 Interstitial impurity-induced spin-polarization in αPbO . 11 3.1.1 Background ............................... 11 3.1.1.1 Interstitial impurities in bulk lead oxides . 12 3.1.2 Electronic properties of pristine PbO surface . 13 3.1.3 Surface spin polarization induced by magnetic interstitial impurities 14 3.1.4 Summary and Conculsions ....................... 19 3.2 Surface magnetism induced by magnetic interstitials in αPbO . 20 3.2.1 Background ............................... 20 3.2.2 Electronic and magnetic properties of αPbO with magnetic inter- stitials .................................. 21 3.2.3 Magnetic order and exchange mechanism . 21 3.2.4 Curie temperature ........................... 22 3.2.5 Effects of carrier concentration .................... 24 3.3 Summary and Conclusions ........................... 26 4H2 nuclear spin dynamics on stepped Pd surface: A case study 27 4.1 Model and Theory ............................... 27 iv Contents v 4.1.1 Isolated H2 ............................... 27 4.1.1.1 Hamiltonian ......................... 27 4.1.1.2 Rotational states ....................... 28 4.1.1.3 Correlation between rotational and nuclear spin states . 30 4.1.2 Rotational states of H2 adsorbing on a solid surface . 31 4.1.2.1 H2 hindered rotation when U = U (Xe,Ye,Ze,re,θ,φ) . 32 4.1.2.2 H2 hindered rotation when U = U (Xe,Ye,Z,re,θ,φ) . 33 4.1.3 Nuclear spin isomers conversion of H2 on a solid surface . 34 4.2 Bound nuclear spin states of H2 on Pd(210) . 38 4.2.1 Background ............................... 38 4.2.1.1 H2 adsorption on stepped metal surfaces . 39 4.2.1.2 Nuclear spin states of H2 on stepped metal surfaces . 40 4.2.2 Hindered rotation at H2Pd(210) equilibrium bonding distance . 41 4.2.2.1 Potential energy curves and anisotropic potential . 42 4.2.2.2 H2 nuclear spin isomers separation . 45 4.2.3 Dynamics of nuclear spins of H2 in a 2D PES . 47 4.2.3.1 Hamiltonian ......................... 47 4.2.3.2 2D Potential energy surface: U2D (Z,θ) . 48 4.2.3.3 Nuclear spin bound states . 49 4.2.3.4 Comparison with experiment . 53 4.2.4 Isotope effects .............................. 54 4.2.5 Summary and Conclusions ....................... 55 4.3 H2 nuclear spin isomer conversion on Pd(210) . 56 4.3.1 Model .................................. 56 4.3.2 Transition probability ......................... 57 4.3.3 Summary and Conclusions ....................... 59 5 Summary, Conclusions and Outlook 61 A Magnetism in Density Functional Theory (DFT) 63 A.1 Collinear DFT ................................. 63 A.2 Mapping DFT calculations to Heisenberg Model . 65 B DFT calculations details 68 B.1 DFT calculation details of interstitial impurities in PbO surface . 68 B.1.1 Bulk PbO ................................ 69 B.2 DFT calculations of H2/Pd(210) ....................... 70 B.2.1 H2 dissociative adsorption on a clean Pd(210) . 71 B.2.2 H2 molecular adsorption on Pd(210) . 73 Awards 75 List of Publications 76 Contents vi List of Conference Presentations 77 Bibliography 80 List of Figures 1.1 Schematic diagram of DOS of magnetic and non-magnetic materials. The blue and red shades correspond to up and down spin polarized DOS de- noted by G(E ) and G(E ), respectively. The up and down DOS are filled ↑ ↓ up the Fermi level EF .