Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session J46: Single Molecule MagnetsFocus
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Sponsoring Units: GMAG Chair: Selvan Demir, Michigan State Univ Room: 708 |
Tuesday, March 3, 2020 2:30PM - 3:06PM |
J46.00001: The power of typicality applied to magnetic molecules and low-dimensional quantum spin systems Invited Speaker: Juergen Schnack Molecular or low-dimensional quantum spin systems often prevent an exact calculation of their magnetic properties due to a prohibitively large size of the related Hilbert spaces. Typicality-based approaches such as the finite-temperature Lanczos method allow to investigate rather large |
Tuesday, March 3, 2020 3:06PM - 3:18PM |
J46.00002: Effect of substrate on characteristics of the Mn3 dimer Zahra Hooshmand, Rainier Berkley, Talat S. Rahman Single-molecule magnets (SMMs) are considered candidates for next generation information technology. These molecules possess high spin and their ground states can be tuned. The Mn3 dimer is such an example in which the Mn3 triangles are connected via linkers to form either the ferromagnetic (FM) or the antiferromagnetic (AFM) ground state. One challenge for SMMs is finding a suitable substrate that keeps their magnetic properties intact. We present results of our spin-polarized density functional theory calculations of the adsorption and interactions of Mn3 dimers on graphene and monolayer hexagonal boron nitride (h-BN). We show that while graphene is benign, h-BN has interesting interaction with the SMMs. We compare the effect of these two substrate on the magnetic properties and electronic structure of the Mn3 dimers. We also show that charge corrections need to be accounted for in the calculations for reliable description of the isolated as well as supported Mn3 dimers. Results will be compared with available and ongoing experimental results. |
Tuesday, March 3, 2020 3:18PM - 3:30PM |
J46.00003: A DFT Study of Single-Molecule Magnets (Mn3 Dimers) Rainier Berkley, Zahra Hooshmand, Jie-Xiang Yu, Hai-Ping Cheng, Talat Rahman Single-molecule magnets (SMMs) have become of increasing interest for magnetic technology. This is due to their many quantum phenomena and the ability to modify their structure and magnetic properties. It is thus important that SMM properties be characterized; thus, two configurations of the Mn3 dimer ferromagnetic (FM) and antiferromagnetic (AFM) have been studied via Density Functional Theory (DFT). Our calculations for the total spin of the FM (S = 12) and AFM (S = 0) configurations of the isolated dimers, agrees with the experimental spin results per monomer (S = 6)1 only when the dimers are charged (+2) (similar to molecules in solvent). This charge issue brings about long range jellium effects which have to be corrected in DFT. Our results shed insight into the differences between the magnetic anisotropy, magnetic coupling constants and the spin for both the neutral and the charged (+2) versions of the FM and AFM dimer as a comparison. The application of the Makov-Payne method, for charge corrections, results in very small changes to the magnetic anisotropy and the magnetic coupling constants for both charged version of the Mn3 dimers. |
Tuesday, March 3, 2020 3:30PM - 3:42PM |
J46.00004: Atomic-clock transition behavior in a Cr7Mn molecular nanomagnet Gajadhar Joshi, Ilija Nikolov, Guanchu Chen, Daniel Sava, Grigore Timco, Richard Winpenny, Jonathan Friedman A clock transition (CT) occurs at an avoided level crossing where the transition frequency is independent on the magnetic field. Enhancement of T2 has been observed at clock transitions in some molecular nanomagnets, suggesting these systems as viable spin qubits [1,2]. The study of such systems can elucidate the mechanisms of decoherence since spin-spin interactions are suppressed at CT. A CT is observed in the S = 1 molecular magnet Cr7Mn with pulsed electron-spin resonance. The Carr-Purcell Meiboom-Gill (CPMG) pulse sequence in dilute samples of Cr7Mn increases T2 more than an order of magnitude over that obtained with a Hahn echo sequence, indicating that a significant decoherence mechanism arises from fluctuations that are slower than the delay between the CPMG pulses (~1 us). We also observe electron spin echo envelope modulation (ESEEM) at fields slightly away from the zero-field avoided crossing. Remarkably, CPMG provides a significant enhancement to T2 when the pulse period matches the ESEEM period, suggesting that CPMG dynamically decouples the molecular and the nuclear spins. |
Tuesday, March 3, 2020 3:42PM - 4:18PM |
J46.00005: Chemical Approaches to Quantum Information Science Invited Speaker: Danna Freedman Synthetic chemistry enables us to build up systems from the bottom up with angstrom scale precision. We harness this precise synthetic control towards the emerging challenge of quantum information science (QIS), in which we are creating and understanding qubits, the smallest unit of a QIS system. Our approach enables fundamental insight into the factors that contribute to electronic spin decoherence, the creation of qubits with millisecond coherence times, and the creation of arrays of qubits. These results and others will be described. |
Tuesday, March 3, 2020 4:18PM - 4:30PM |
J46.00006: The muon-fluorine interaction: a model quantum system for exploring decoherence Stephen Blundell, John Wilkinson, Franz Lang, Tom Lancaster In non-magnetic fluorides, an implanted muon will stop very close to the highly electronegative fluoride ion, or very often stop between two of them. The dipolar interaction between the fluorine nuclei spins and the muon spin gives rise to a characteristic signature which characterises the state. The remaining fluorine nuclei are more distant and usually ignored, since their coupling to the muon is weaker. We show that taking them properly into account allows one to model the data in greater quantitative detail, understand the stopping site more accurately, and explore how the quantum information held by the state decoheres into its environment. |
Tuesday, March 3, 2020 4:30PM - 4:42PM |
J46.00007: Decoherence of Molecular Qubits: Insights From Quantum Many-Body Simulations Jia Chen, Cong Hu, John Stanton, Hai-Ping Cheng, Xiaoguang Zhang Quantum properties of magnetic molecules will drive their applications in quantum information science. Electron spin decoherence time in such molecules depends strongly on molecular structure. A synthetic study of a series of vanadyl molecular qubits by Danna Freedman's group found that the decoherence time decreases as size of molecule increases.1 To explain this counterintuitive experimental result and provide insights for decoherence in molecular qubits in general, we combine ab-initio electronic structure calculations and quantum many-body simulation to study hyperfine interactions between electron and hydrogen nuclear spins and their effects on electron spin decoherence. We show that, for an isolated molecule, decoherence is always incomplete, but the residual coherence decreases as the size of the molecule increases. This result explains the experimentally observed diffusion barrier for decoherence, also suggests a quantitative approach to connect molecular structure and diffusion barrier. |
Tuesday, March 3, 2020 4:42PM - 4:54PM |
J46.00008: Counterintuitive Control of Molecular Magnetic Relaxation via Chemically Tunable Electron-Spin Baths Ian Moseley, Joseph Zadrozny Magnetic molecules are critical components in current and next generation applications in molecular imaging and information processing. Toward functionality in these applications, long spin-lattice and spin-spin relaxation times are desired. However, most magnetic molecules display fast relaxation times when in the presence of a magnetic environment. Bioimaging and information storage environments are nuclear and electron spin-rich. Hence, we need to understand how to design long relaxation times in highly magnetic environments. This presentation will detail our efforts toward that knowledge by studying the spin-bath-to-molecule interaction via molecular chemistry and magnetic analysis. Specifically, we will discuss the response of the relaxation times in a set of cobalt-containing molecules to a chemically tunable magnetic environment as determined by electron spin resonance and alternating current magnetic susceptibility. In the long term, these fundamental studies will deliver design principles for molecules with long relaxation times in magnetically chaotic environments. |
Tuesday, March 3, 2020 4:54PM - 5:06PM |
J46.00009: Investigating the Magnetic Properties of the Giant Mn84 Torus Dian-Teng Chen, Ashlyn Hale, George Christou, Hai-Ping Cheng The giant single-molecule magnet Mn84 has a shape of torus of eighty-four Mn3+ ions (S=2). As the manganese atoms in Mn84 are all bridged by O2- or MeO- groups, strong pairwise exchange interactions between these manganese atoms are expected. Even though the magnetic susceptibility of this system has been measured experimentally, its electronic-magnetic structure and spin-spin couplings remain unknown. In this work, we investigate these interactions using first-principles calculations, from which a Heisenberg model is developed. The exchange coupling constants are extracted by fitting to the total energies of different spin configurations of Mn84. In addition, we compute the magnetic anisotropy energy by including spin-orbital couplings. |
Tuesday, March 3, 2020 5:06PM - 5:18PM |
J46.00010: Adjustable coupling and in-situ variable frequency probe with loop-gap resonators for cw and pulse electron paramagnetic resonance spectroscopy up to X-band Gajadhar Joshi, James Kubasek, Ilija Nikolov, Brendan Sheehan, Thomaz de Andrade Costa, R. A. Allāo Cassaro, Jonathan Friedman In standard electron paramagnetic resonance (EPR) spectroscopy, the frequency of an experiment is set and the spectrum is acquired using magnetic field as the independent variable. There are cases in which it is desirable instead to fix the field and tune the frequency such as when studying atomic-clock transitions at avoided level crossings in molecular nanomagnets and other spin qubits [1,2,3]. We have designed and tested an adjustable frequency and variable coupling EPR probe with loop-gap resonators (LGRs) that works at a temperature down to 1.8 K [4]. The frequency is tuned by adjusting the height of a dielectric piece of sapphire inserted into the gap of an LGR; coupling of the microwave antenna is varied with the height of the antenna above the LGR. We demonstrate the operation of our probe with continuous wave EPR by mapping out avoided crossings for the Ni4 single-molecule magnet to determine the tunnel splittings with high precision. |
Tuesday, March 3, 2020 5:18PM - 5:30PM |
J46.00011: A Very-Low-Cost Flexible Electron Spin Resonance Spectrometer for Molecular Nanomagnet Experiments Charles Collett, Jonathan Friedman Electron spin resonance (ESR) spectroscopy is a useful tool for exploring and manipulating a wide variety of systems, including molecular nanomagnets. Commercial spectrometers offer high sensitivity in specific frequency bands, but are both expensive and limited in their experimental flexibility. We have developed an ESR spectrometer based on a cheap and readily-available field-programmable gate array (FPGA), which, when combined with loop-gap resonators (LGRs), enables spectroscopy on a budget at a wide range of frequencies. I will present the spectrometer design along with benchmark results demonstrating its sensitivity and flexibility with two different combinations of cryostat and magnet, and discuss future experiments on molecular nanomagnets. |
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