Professor Mark Pederson

Chemical Theory Center Seminar
Professor Mark Pederson
Professor and Dr. C. Sharp Cook Chair in Physics
University of Texas at El Paso
Host: Professor Ilja Siepmann

Abstract

Triangular Molecular Magnets: Chemical Models for Protons, Qubits, or Quantum Sensors?

The size range and deceptive complexity albeit behavioral simplicity of molecular magnets attracts physical scientists from many disciplines and challenges them to understand how 50-100 nuclei and 200-1000 electrons can exhibit such simple collective behavior. For example, quantum tunneling of magnetization, which occurs in broken-spin-symmetry magnetic molecules illustrates the power of density-functional-based pictures for predicting both the  magnetic strength of molecules and the magnetic fields at which quantum tunneling occurs. Alternatively, the spin-electric effect^[1-4] explicitly challenges the notion that single-determinantal theories can describe the physics leading this phenomenon. However, the large molecular size resists quantitative quantum chemical explanations and a combination of model Hamiltonians with density-functional treatments are the optimal means for exploring these intrinsically multi-configurational problems. I will review previous work on the Cu₃ molecular magnet and show how the combination of broken symmetry density-functional theory, with simple self-interaction corrections and spin-orbit inclusion, can be used to derive three-spin Heisenberg Hamiltonians that describe the Dzyaloshinskii-Moriya induced splitting of degenerate low-energy Kramer doublets into S=1/2 chiral and anti-chiral pairs. The resulting energy level diagrams will be compared to that of a three-quark system.

The second half of this talk features the Fe₃O(NC₅H₅)₃(O₂CC₆H₅)₆ molecule^[4] that is the first possible spin-electric system based upon spin 5/2.centers. As a curiosity, I discuss the rather unusual point-group symmetry, which includes a set of rotation matrices that are hauntingly similar to those that  appear in elementary particle wavefunctions. We call the generator of these rotations matrices (above) R_Q^[2]. Using standard density-functional methods we show that the spin-electric behavior of this molecule could be more interesting due to energetically competitive reference states with high and low local spins (S=5/2 vs. S=1/2) on the Fe³⁺ ions. We provide spectroscopies to deduce the presence of both states and note that similar multiferroic behavior exists in the Mn₃ molecular magnet^[3]. Rationale for use of a new version to self-interaction corrections, FLOSIC, to improve quantitative predictions, especially in lanthanide systems and periodic systems will be included.^[5]

1. M.F. Islam, J.F. Nossa, C.M. Canali & MRP, First-principles study of spin-electric coupling in a Cu₃ single molecular magnet, PRB 82 155446 (2010).
2. A.I. Johnson, M.F. Islam, C.M. Canali & MRP, A Multiferroic molecular magnetic qubit, JCP. 151, 174105 (2019).
3. Z. Hooshmand & MRP, Control of spin-ordered Mn3 Qubits: A density-functional study, Physical Chemistry and Chemical Physics (2020) DOI: 10.1039/D0CP04455E
4. A.K. Boudalis, J. Robert & P. Turek, 1st demonstration of magnetoelectric coupling in a polynuclear molecular nanomagnet via EPR studies Fe₃O(O2CPh)₆(Py)₃ClO₄, Chem. Eur. J 24 14896-14900 (2018). 
5. MRP, A. Ruzsinszky and J.P. Perdew, Communication: Self-Interaction correction with unitary invariance in density functional theory, J.Chem. Phys. 140 121105 (2014)

Research

Professor Pederson's research is in chemical physics, condensed-matter physics, and computational physics. He has continuously concentrated on next-generation computing paradigms for quantum mechanics. His pioneering work demonstrated the quantitative computational prediction of quantum tunneling of magnetization (QTM) and spin-electric effects in molecular magnets. Both of these different collective phenomena arise from the spin of the electron. Quantitatively understanding conditions that allows for such coherent phenomena, is necessary from the standpoint of spin-Qubit design in quantum information science and may also unlock the mysteries of bio-navigation. He is currently attempting to link the fields of molecular magnetism and photocatalytic water splitting by demonstrating that variations in QTM, in reacting systems, can be used to spectroscopically sense conversion of water into oxygen and hydrogen without pumping energy into the system. Professor Pederson is the primary author of a computer code, the Naval Research Laboratory Molecular Orbital Library (NRLMOL), that describes how nanoscale systems interact with electromagnetic radiation. The opportunity to concentrate on developing this code over a long period has enabled these unique computational investigations and predictions.