Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Z53: Novel Magnetism in Low Dimensional SystemsFocus Recordings Available

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Sponsoring Units: GMAG DMP Chair: Omar Chmaissem, Argonne National Laboratory Room: McCormick Place W475B 
Friday, March 18, 2022 11:30AM  12:06PM 
Z53.00001: From the transversefield Ising chain to the quantum E8 integrable model: Theoretical progress and experimental realization Invited Speaker: Jianda Wu Exotic thermodynamics and excitations can emerge in the vicinity of a quantum phase transition. In the talk, I will first detailedly discuss the very unique quantum criticality for the Grüneisen ratio in the transverse field Ising chain (TFIC) [1], which then serves as a smoking gun to identify the underlying TFIC universality observed in quasionedimensional (1D) antiferromagnetic material BaCo_{2}V_{2}O_{8} with transverse field applied along [110] direction [2]. From systematic quantum critical analysis for the effective model of the material [3], we confirm SrCo_{2}V_{2}O_{8} with field applied along [100] direction can also accommodate the TFIC universality with much weaker magnetic field [4]. Furthermore, when the quantum critical point (QCP) of the TFIC is perturbed by a longitudinal magnetic field, it was predicted that its massive excitations are precisely described by the exceptional E_{8} Lie algebra. Here we show an unambiguous experimental realization of the massive E_{8} phase in the material BaCo_{2}V_{2}O_{8}, via NMR and inelastic neutron scattering measurements, and detailed theoretical analysis [5, 6, 7]. The large separation between the masked 1D and 3D QCPs of the system allows us to identify, for the first time, the full 8 singleparticle E_{8} excitations and various multiE_{8}particle states in the spin excitation spectrum. Our results open new experimental and theoretical route for exploring the dynamics of quantum integrable systems and physics beyond integrability, and thus bridge key physics in condensed matter and statistical field theory. 
Friday, March 18, 2022 12:06PM  12:18PM 
Z53.00002: Density functional theory calculations of singlemolecule magnets Co_{3}(SALPN)_{2}(O_{2}CCH_{3})_{2}⋅R_{2} Shuanglong Liu, JieXiang Yu, Silas Hoffman, ChristiAnna Brantley, Gautam D Stroscio, Ryan G Hadt, George Christou, Xiaoguang Zhang, HaiPing Cheng In search of candidate magnetic molecular systems that emulate Majorana zero modes, we perform density functional theory (DFT) calculations for Co trimers Co_{3}(SALPN)_{2}(O_{2}CCH_{3})_{2}⋅R_{2}, where R is an OCH_{2}, OCHNH_{2}, or OCHN(CH_{3})_{2} solvent molecule. The three Co atoms form a onedimensional chain, and each Co atom exhibits axial spin with S=3/2. We extract the exchange coupling constant and the local axial and rhombic zerofield splitting (ZFS) parameters based on DFT total energies. According to our calculations, the ratio between the exchange coupling constant and the local axial ZFS parameter is 7.3 for the end Co atoms and 5.7 for the center Co atom; the local magnetic easy axis of the center Co atom differs from that of an end Co atom by 42.9 degrees. These results provide a starting point for mapping the spin3/2 Hamiltonian to an effective spin1/2 Hamiltonian, which indicates the existence of Majorana zero modes. We will also analyze the dorbital occupation matrix and discuss the necessity to control it for obtaining reliable results. 
Friday, March 18, 2022 12:18PM  12:30PM 
Z53.00003: Ultralow Thickness Titanium Nitride Thin Films for Spintronic Devices Manosi Roy, Dhananjay Kumar TiN thin films were deposited using a pulsed laser deposition method in the thickness range of 945 nm. Temperaturedependent resistivity measurement showed a metaltoinsulator (MI) the transition of the TiN thin films below 15 nm. This value represents a critical thickness at which the properties of TiN films are strikingly different from the bulk TiN material. The thicknessdependent MI transition was free from structural phase transition and can be attributed to the localization of 3d^{1} electrons in the valence band. This localization takes place due to the separation in the energy states larger than the gaps between the valence and conduction bands. The analysis of transport resistivity data demonstrated that the Arrhenius law governs the transport properties of TiN films in the 300350 K range, while the transport properties are governed by a thicknessdependent variable range hopping mechanism below 300 K. Both metallic and semiconducting TiN films showed ferromagnetic behavior at room temperature. There was a marked correlation between coercivity (H_{c}) and saturation magnetization (M_{s}) with TiN film thickness. At room temperature, H_{c} increased from 66 to 134 Oe as the thickness decreased from 45 nm to 9 nm. The M_{s} at room temperature was also highest for the 9 nm sample (2.8 emu/g). The observed magnetic behavior of TiN films due to the localized 3d^{1} electron. These electrons become more localized with the decrease in film thickness due to the sizedependent enhanced separation of energy states. 
Friday, March 18, 2022 12:30PM  12:42PM 
Z53.00004: Avoided Quantum Critical Point in La_{5}(Co_{1x}Ni_{x})_{2}Ge_{3} Atreyee Das, Tyler J Slade, Sergey L Budko, Paul C Canfield La_{5}Co_{2}Ge_{3 }is an itinerant ferromagnetic system that undergoes a second order phase transition at 3.8K with a lowfield saturated moment of about 0.1 μ_{B}/Co^{1}, making it a promising candidate for tuning the transition to even lower temperatures with a nonthermal tuning parameter like pressure, chemical substitution, or magnetic field. Under applied pressure, the FM transition in La_{5}Co_{2}Ge_{3} is weakly suppressed to 3 K at 1.7 GPa, upon which a new possibly magnetic, phase emerges upto 5.12GPa^{2}. The new phase is presumed to be antiferromagnetic, meaning the FM QCP appears to be avoided in favor of an AFM ground state. Here we further study the La_{5}Co_{2}Ge_{3 }system using Ni substitution. We synthesize single crystals of La_{5}(Co_{1x}Ni_{x})_{2}Ge_{3} with x = 0.01–0.10 and construct a Tx phase diagram from magnetization, transport, and thermodynamic measurements. Details of this diagram will be discussed and compared to the earlier TP work. 
Friday, March 18, 2022 12:42PM  12:54PM 
Z53.00005: The temperaturepressure phase diagram of La_{5}(Co_{1x}Ni_{x})_{2}Ge_{3}; x = 0.03 Shuyuan Huyan, Atreyee Das, Sergey L Budko, Paul C Canfield La_{5}Co_{2}Ge_{3 }is an itinerant ferromagnet (FM) system that undergoes a second order phase transition at 3.8 K with a lowfield saturated moment of about 0.1 μ_{B}/Co^{1}, suggesting that the magnetic order is could be fragile. Under applied hydrostatic pressure, the FM quantum critical point in undoped La_{5}Co_{2}Ge_{3} is avoided by the emergence of a different, likely antiferromagnetically ordered, state^{2}. Our recent studies on La_{5}(Co_{1x}Ni_{x})_{2}Ge_{3} with x = 0.01  0.10 show that the system can be tuned from FM ground state to a different, antiferromagnetic, ground state with a clear and welldefined signature in magnetization as well as resistivity. We now investigate the high pressure electrical properties of La_{5}(Co_{1x}Ni_{x})_{2}Ge_{3} with x = 0.03, where the sample has just crossed into the clean AFM ground state, to study the evolution of the AFM ordering in this system. The temperaturepressure magnetic phase diagram of La_{5}(Co_{1x}Ni_{x})_{2}Ge_{3}; x = 0.03 is constructed. Details of this diagram will be discussed and compared to the TP work in the undoped system. 
Friday, March 18, 2022 12:54PM  1:06PM 
Z53.00006: Probing Intermolecular Magnetic Interactions in a Metal Complex by Inelastic Neutron Scattering Joseph Prescott, Alexandria N Bone, Andrey Podlesnyak, ZiLing Xue, Jason T Haraldsen Molecular magnetic materials, such as singlemolecule magnets (SMMs) composed of a single paramagnetic metal center complexed by organic ligands, are of intense interest for their potential applications as traditional bits in highdensity data storage devices or as qubits in quantum computers. It is generally assumed that these paramagnetic centers behave independently of one another. However, spinspin couplings among the molecules are known to contribute to magnetic relaxation. Few techniques exist to directly probe the intermolecular magnetic interactions. We have utilized inelastic neutron scattering (INS) to reveal these interactions in a fieldinduced singlemolecule magnet, Mn(TPP)Cl (TPP^{2 }= mesotetraphenylporphyrinate) as an oscillatory Qdependence of the magnetic transition. We analyze the INS results using a combination of Heisenberg spinspin exchange for intermolecular spin interactions in combination with singleion excitations. 
Friday, March 18, 2022 1:06PM  1:18PM 
Z53.00007: Machine Learning Study of the Magnetic Ordering in Twodimensional Materials Carlos Mera Acosta, Elton Ogoshi de Melo, Gustavo M Dalpian The advent of twodimensional (2D) materials opened new arenas for magnetic compounds, even when classical theories discourage their examination. The recent experimental discovery of 2D magnetic materials has inspired fundamental questions about the physical mechanism behind the magnetic ordering. Despite the huge interest raised by 2D magnetic materials, there are no a priori rules or trends allowing the prediction and understanding of this class of functional compounds. Here, we perform a machine learning study to predict and understand the tendency in the space of atomic species and 2D structures for a material to be classified as i) magnetic or nonmagnetic using a random forest algorithm coupled to the SHAP method for model interpretability; and ii) ferromagnetic or antiferromagnetic based on the SISSO method. The ML models (i.e., a materials map that is function of the composition, atomic properties, and crystal symmetry) provide an accuracy higher than 85%. We find classification rules discriminating the existence of magnetism as well as the magnetic ordering type in terms of the strength of the spinorbit coupling, the type of transition metal, the indirect bonding among transitions metals, and atomic properties of the constituent atoms, indicating new routes for experimental exploration. 
Friday, March 18, 2022 1:18PM  1:30PM 
Z53.00008: Magnetic and transport properties of singlecrystalline EuPd_{3}Si_{2} shivani sharma, masoud mardani, Keke Feng, Kaya Wei, Ryan Baumbach, David J Singh, Theo Siegrist More than a hundred ternary borides, gallides and silicides are known which crystallize with a great variety of structure types that can be derived from the hexagonal CaCu_{5} type (hexagonal, P6/mmm). However, no ternary silicide is reported being formed with Pd and Eu. We have synthesized single crystal and powder samples of the new phase EuPd_{3}Si_{2} and investigated the crystallographic structure, magnetization, specific heat, and resistivity as a function of temperature and magnetic field. Single crystal data confirms that EuPd_{3}Si_{2} crystallizes in the orthorhombic space group Imma (pseudohexagonal) with lattice parameters a = 7.1463(3), b = 10.0711(4), c = 5.7469 (2) Å. Longrange ferromagnetic order at T_{C} = 78 K and a possible spin rearrangement at T* = 5 K appear as distinctive features in all physical properties investigated. The electronic band structure calculations using the PBE+U method within the density functional theory approach corroborate the orthorhombic structure and ferromagnetic ordering of Eu^{2+} spins. The band structure calculations also show that the Pd derived dband polarization is along the Eu f polarization, resulting in an enhancement of the spin moment in ordered states. 
Friday, March 18, 2022 1:30PM  1:42PM 
Z53.00009: SpinMediated Response in 2D Honeycomb Magnets Caitlin Carnahan, Di Xiao Twodimensional honeycomb ferromagnets that include a second nearest neighbor DzyaloshinskiiMoriya interaction reproduce the Haldane model and are therefore expected to exhibit a nontrivial magnon band topology. It has been shown that such a system gives rise to a thermal Hall effect via magnon carriers. Likewise, chiral honeycomb antiferromagnets may generate a spin Nernst effect in response to the application of a temperature gradient. Recent theoretical studies modeling these spinmediated responses in honeycomb magnets have been restricted to the linear spin wave limit – and are therefore only valid at low temperatures – or otherwise neglect to incorporate higherorder spin fluctuations which are crucial in the vicinity of the critical temperature. In this talk, we will describe our approach to model the thermal Hall effect and spin Nernst effect in honeycomb magnets, particularly near and beyond the critical temperature. We will also show that spinmediated response bolstered by increasing thermal fluctuations can be particularly robust at high temperatures, a result that is unexpected in the traditional magnon picture. 
Friday, March 18, 2022 1:42PM  1:54PM 
Z53.00010: Spin dynamics of Dy_{2} molecules deposited onto microSQUID sensors Fernando M Luis, Diego Gella, Maria Carmen Pallares, Veronica Velasco, Ana Repolles, Mark D Jenkins, David Aguila, Olivier Roubeau, Anabel Lostao, Leoni Barrios, Javier Sese, Dietmar Drung, Thomas Schurig, Maria Jose MartinezPerez, Guillem Aromi We report the results of ac susceptibility measurements performed down to very low temperatures (T > 13 mK) on thin layers of asymmetric Dy_{2} molecular coordination complexes that have been proposed as candidates for the realization of 2qubit quantum gates. The molecules are integrated into a μSQUID susceptometer by means of Dip Pen Nanolithography. Frequencydependent susceptibility data measured on 5 and 20 molecular layers thick films are compared with similar results obtained for bulk polycrystalline samples. The results show that the magnetic anisotropy, exchange interactions and spin tunneling rates of Dy_{2} units largely remain intact at the surface. Lownuclearity lanthanide magnetic clusters might then provide suitable building blocks for the development of a scalable hybrid quantum architecture. 
Friday, March 18, 2022 1:54PM  2:06PM 
Z53.00011: Magnetoelastic coupling in αRuCl_{3} and possible technological applications of αRuCl_{3}/graphene heterostructures Sananda Biswas, David Kaib, Kira Riedl, Stephen Winter, Jesse Balgley, Jackson Butler, Erik A Henriksen, Bernd Wolf, Michael Lang, Roser Valenti In this talk, I will focus on the firstprinciples results on the magnetoelastic coupling in α−RuCl_{3} and the dependence of the magnetic coupling constants on strain effects. Different magnetic interactions are found to respond unequally to the variations in the lattice, with the Kitaev interaction being the most sensitive. We will discuss how uniaxial strain perpendicular to the honeycomb planes reorganizes the relative coupling strengths, strongly enhancing the Kitaev interaction while simultaneously weakening the other anisotropic exchanges under compression. Uniaxial strain may therefore pose a fruitful route to experimentally tune α−RuCl_{3} nearer to Kitaev limit. I will also discuss the corresponding scenario when the strain changes from uniaxial to homogeneous. 
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