Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session L53: Multiferroics, Magnetoelectrics, Spin-Electric Coupling, and Ferroelectrics -3Focus Live
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Sponsoring Units: DMP DCOMP GMAG Chair: Kyungwha Park, Virginia Tech |
Wednesday, March 17, 2021 8:00AM - 8:12AM Live |
L53.00001: Spin State Relaxation in Spin Crossover Molecules Haechan Park, Jia Chen, Hai-Ping Cheng Spin crossover (SCO) materials have drawn intense attention lately due to its unique ability to switch from low-spin (LS) to high-spin (HS) state by external stimuli. The potential applications are including but not limited to magnetic information storage or quantum computing. A typical HS state is a metastable state, it tends to have a long lifetime. Therefore, HS to LS relaxation time is a critical quantity in all application areas. In this study, we aim to build a model in a simple and straightforward manner and solve a master equation within the reduced density matrix (RDM) formalism and the golden rule expression. Density Functional Theory is used to obtain the energy and orbitals of each spin state. The spin-orbit coupling matrix elements (SOCMEs), which couple the different spin states, are calculated by a post-scf code, Molsoc0.1, developed by Chiodo et al. Demonstration of the relaxation dynamics with Fe+2(bpy)3 molecule will be given. |
Wednesday, March 17, 2021 8:12AM - 8:24AM Live |
L53.00002: Modeling Harmonic and Anharmonic Spin-Vibron Coupling in the Fe3+ Molecular Magnetic Karma Dema, Mark Pederson The Fe3+ Molecular Magnetic Qubit has been predicted to be stable in two different spin states[1]. Since only one spin state has been clearly identified we have calculated infrared and Raman spectra for both magnetic states. Vibrational effects also influenced the magnetism through the spin-vibron effect which will be reported here. We have performed frozen phonon calculations on the chiral Fe3O(NC5H5)3(O2CC6H5)6 molecular cation and determined magnetic changes as a function of phonon displacement. We displace the Fe atoms from their equilibrium positions by, δ = 0, 0.01,-0.01,0.02,-0.02.. in atomic units, and allow all other atoms to relax. The resulting forces and energies are used to construct and solve a Hamiltonian that describes spin-spin, spin-orbit, and spin-vibron interactions. The forces as a function of displacement are further used to identify other interesting vibronic degrees of freedom. Progress on determining infrared frequencies that are most likely to lead to spin electric coupling will be reported. |
Wednesday, March 17, 2021 8:24AM - 8:36AM Live |
L53.00003: Spin-crossover in Molecules and Solids with a Low-cost, Well-behaved meta-GGA Density Functional Samuel Trickey, Daniel Mejia-Rodriguez, Angel Albavera Mata Recently the SCAN meta-GGA exchange-correlation functional has been revised to eliminate significant numerical instabilities while maintaining constraint satisfaction. The resulting r2SCAN functional [1] is shown to give substantially better spin-crossover (SCO) electronic energies (high spin minus low spin) on a benchmark data set than the original SCAN as well as on some Fe complexes [2]. The deorbitalized counterpart r2SCAN-L [3] is not as good as r2SCAN but almost as good as the original SCAN. A crucial advantage, however, is that r2SCAN-L is much faster than r2SCAN in periodically bounded systems. A combination strategy for balanced treatment of molecular and periodic system spin-crossover therefore is to optimize geometry with r2SCAN-L, then calculate SCO energies with r2SCAN. We present examples. |
Wednesday, March 17, 2021 8:36AM - 9:12AM Live |
L53.00004: Electric field control of spins in piezoelectrics, ferroelectrics, molecules, and on surfaces Invited Speaker: Arzhang Ardavan Magnetic fields are challenging to localise to short length scales because their sources are electrical currents. Conversely, electric fields can be applied using electrostatic gates on scales limited only by lithography. This has important consequences for the design of spin-based information technologies: while the Zeeman interaction with a magnetic field provides a convenient tool for manipulating spins, it is difficult to achieve local control of individual spins on the length scale anticipated for useful quantum technologies. This motivates the study of electric field control of spin Hamiltonians [1]. |
Wednesday, March 17, 2021 9:12AM - 9:24AM Live |
L53.00005: Magnetic Properties of Frustrated Triangular Copper Based Qubits Ian Reyes, Zahra Hooshmand, Mark Pederson Quantum computers offer the possibility of removing inefficiencies derived from classical computers but pose problems of their own as cost, size and modeling are reoccurring issues. Here we focus on the search for appropriate qubits, polymeric building blocks for quantum computers, that can effectively be implemented into such electronic systems. Specifically we computationally create two unique versions of a trinuclear copper(II) complex derived from a [Cu3(saltag)(py)6]ClO4 previously synthesized by Spielberg et al. DFT, through NRLMOL,is used to investigate the magnetic properties and stability of the three systems. Comparisons of energetic properties such as the HOMO-LUMO gaps of the respective structures allow for reinforcement of previously established principles that convey the ideal features of a qubit. Although J values of 3cm-1 and 15cm-1 for both unique structures limit the temperature range for which the resulting qubit could operate, we successfully identify properties such as shifts in magnetic moments and stability, along with conjugation and hybridization that can increase the predictive power for future ideal qubit candidates. |
Wednesday, March 17, 2021 9:24AM - 10:00AM Live |
L53.00006: Competing Exchange Interactions and Multiferroic Behavior of a Molecule-Based Magnet Invited Speaker: Randy Fishman Multiferroic behavior sensitively depends on the microscopic interactions between spins. The molecule-based magnet (NH4)2FeCl5 (H2O) exhibits a complex magnetic-field or pressure versus temperature phase diagram with three multiferroic phases observed by magnetization, neutron diffraction, and Raman spectroscopy measurements. Both low-field phases contain spin cycloids with electric polarization P along the a axis produced by the inverse Dzyalloshinskii-Moriya interaction. Above the spin-reorientation transition at roughly 4 T, the spins form a canted antiferromagnetic state and P rotates to the c axis. The electric polarization at high fields is believed to be caused by p-d orbital hybridization. We evaluate the magnetic interactions in the low-field multiferroic phase by comparing inelastic neutron scattering spectra of a single crystal with a simple Heisenberg model containing five exchange interactions mediated by intermolecular hydrogen and halogen bonds. Two competing exchange interactions in every bc plane produce a cycloid with spins in the ac plane, helicity Si x Sj along the b axis, and ordering wavevector Q = (0, 0, 0.23) r.l.u. along the c axis. Using this wavevector as a constraint, we obtain excellent agreement between the observed and predicted inelastic spectra at zero field. With some small but clear differences, the zero-field exchange and anisotropy parameters also provide excellent agreement with the inelastic neutron-scattering spectra of the high-field phase. The resulting exchange and anisotropy parameters are compared with the predictions of first-principle calculations. |
Wednesday, March 17, 2021 10:00AM - 10:12AM Live |
L53.00007: An umbrella for electron rain: Dependence of magnetization on the direction of alkane bridge Zahra Hooshmand, Alexander Johnson, Karma Dema, Mark Pederson A promising class of molecular magnets emerges from triangular metal-center configurations since most of the structures exhibit an antiferromagnetically ordered ground state and consequent formation of a permanent electric dipole. This raises the possibility for switching between magnetic levels by an external electric field. An example is the Fe3 qubit with low-energy states composed of both S=1/2 and S=5/2 local spins predicted by theory and experiment respectively[1,2].Here, we present calculated results based on density functional theory for switching the magnetism of the Fe3-graphene system by controlling the orientation of a mechanical charge-transfer bridge,CH4 .The total energy and therefore the electronic levels are strongly modified by the orientation of CH4. The analysis of charge transfer and magnetic moment on the Fe atoms as well as on the graphene sheets along with the results of exchange coupling, shows clear evidence that the magnetism can be controlled by flipping the bridging methane and therefore provides a new route to electronically controlling the Fe3 magnetic properties for quantum tasks. [1] Boudalis, A. K. et al.; Chem. Eur. J. 24, 14896–14900 (2018). [2] Johnson, A. I. et al.; J. Chem. Phys. 151, 174105 (2019). |
Wednesday, March 17, 2021 10:12AM - 10:24AM Live |
L53.00008: Electric field control of single spins in oxide ferroelectric hosts Nima Leclerc, Katherine Inzani, Sujit Das, Valentin V. Laguta, Arzhang Ardavan, Nabaraj Pokhrel, Elizabeth A Nowadnick, Ramamoorthy Ramesh, Sinead Griffin Despite the successful scaling of CMOS transistor gate lengths below 10 nm, there has been a corresponding increase in losses as we approach the Boltzmann limit of current control. This problem is a fundamental attribute of devices operating with charge control, but exploiting materials which facilitate the control of single spins opens an avenue to avoid parasitic losses with diminishing feature sizes. We investigate the voltage control of individual spins in magnetically-doped complex oxides as a possible candidate for a CMOS replacement. We show, using first-principles calculations, that electric-field controlled polarization in ferroelectric oxides couples to dopant spins through spin-orbit coupling resulting in an observable magnetization response. Using an in house codebase, we compute the magnetocrystalline anisotropy energy (MCAE) surface and extract the anisotropy constants in Fe and Mn-doped PbTiO3 and Bi2WO6. We predict the spin moment’s dependence on electric-field and present ferroelectric switching pathways that can be used for manipulation. We compare our results with electron paramagnetic resonance measurements of single-spin MCAEs and find they compare favorably with our predictions. |
Wednesday, March 17, 2021 10:24AM - 10:36AM Live |
L53.00009: Magnetoelectric effect arising from a field-induced pseudo Jahn-Teller distortion in a rare-earth magnet Minseong Lee, Qiang Chen, Eun Sang Choi, Qing Huang, Zhe Wang, Langsheng Ling, Zhe Qu, Guohua Wang, Jie Ma, Adam Aczel, Haidong Zhou Magnetoelectric materials are attractive for several applications, including actuators, switches, and magnetic field sensors. Typical mechanisms for achieving a strong magnetoelectric coupling are rooted in transition metal magnetism. In sharp contrast, here we identify CsEr(MoO4)2 as a magnetoelectric material without magnetic transition metal ions, thus ensuring that the Er ions play a key role in achieving this interesting property. Our detailed study includes measurements of the structural, magnetic, and magnetoelectric properties of this material. Bulk characterization and neutron powder diffraction show no evidence for structural phase transitions down to 0.3 K and therefore CsEr(MoO4)2 maintains the room temperature P2/c space group over a wide temperature range without external magnetic field. These same measurements also identify collinear antiferromagnetic ordering of the Er3+ moments below TN=0.87K. Complementary dielectric constant and pyroelectric current measurements reveal that a ferroelectric phase with a maximum polarization P∼0.6nC/cm2 emerges when applying a modest external magnetic field, which indicates that this material has a strong magnetoelectric coupling. |
Wednesday, March 17, 2021 10:36AM - 10:48AM Live |
L53.00010: Pressure-induced ferroelectric reentrance in a molecular magnet Yan Wu, Lei Ding, Na Su, Yongqiang Cheng, Bryan C Chakoumakos, Wei Tian, Young Sun, Jinguang Cheng, Huibo Cao Multiferroics have attracted tremendous research interests with their rich physics and potential in constructing next-generation multifunctional devices. Metal-organic framework molecular magnets have been a recent focus in single phase multiferroics investigations. These materials are usually bonded by intermolecular forces and their magnetic moments interact through H-bond super-exchange couplings. As a result, pressure induced lattice changes would play a significant role in exploring for complex phase transitions in these materials. We investigated the crystal structure, magnetism and ferroelectricity changes of a molecular magnet (NH4)2FeCl5 H2O under pressure. Interesting magnetic transitions correlated with ferroelectricity were observed at systematically increased pressures. The strong couplings between magnetic phase, ferroelectricity and the crystal structure are identified that originate from the reorientation of ammonia under pressure. The interplay of spin, charge and lattice under pressure in this molecular magnet system brings new insights into the study of multiferroics and new routes in designing multi-functional materials. |
Wednesday, March 17, 2021 10:48AM - 11:00AM On Demand |
L53.00011: Methodological Choices for the Spin-Crossover Energy: Mn(taa) as an Example Angel Albavera Mata, Daniel Mejia-Rodriguez, Eric Fonseca, Dianteng Cheng, H-P. Cheng, Samuel Trickey, Richard Hennig Systematic prediction of the spin-crossover (SCO) energy for both an isolated molecule and its condensed aggregates poses a challenge to current density functional approximations, DFAs [1]. With Mn(taa) as the example, we discuss the effects of DFA choice, dispersion corrections, whether and how to use Hubbard U corrections, and technical and procedural sensitivities and subtleties. We show that, with one exception (r2SCAN [2]), no comparatively simple, non-empirical DFA provides predictive accuracy. Systematic procedures to calculate U fail with popular DFAs. User-driven tuning of U becomes a diagnostic of DFA appropriateness. It confirms the suitability of r2SCAN [2]. In conjunction with structural calculation using r2SCAN-L [3], a rapid procedure for assessing SCO becomes possible. |
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