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 E37: Spin Chains and Multimetallic SystemsFocus Live
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Sponsoring Units: GMAG DMP Chair: Wei Tian, Oak Ridge National Lab |
Tuesday, March 16, 2021 8:00AM - 8:36AM Live |
E37.00001: Propagating spinons and magnons in coupled quantum spin chains Invited Speaker: Xianglin Ke In conventional magnets with magnetic long range order (LRO), low-energy excitations are carried by spin waves, represented by massless bosons called magnons with S = 1. However, in one-dimensional (1D) antiferromagnetic quantum spin systems, quantum fluctuations destroy LRO. Their low-energy excitations are spinons, a fractionalized fermion with S = 1/2, instead of magnons. In quasi-1D antiferromagnets with quantum spins, magnetic excitations are carried by either magnons or spinons in different energy regimes: they do not coexist independently, nor could they interact with each other. In this talk, I will present our recent neutron scattering and theoretical studies of a unique quasi-1D quantum spin system, Cu2(OH)3Br, which consists of weakly-coupled, ferromagnetic and antiferromagnetic alternating chains [1]. As a result, this system shows coexistence of two different magnetic quasiparticles: the ferromagnetic chains give rise to conventional magnon excitation, while the antiferromagnetic chains yield spinons. Furthermore, these magnetic quasiparticles interact via weak interchain interactions, leading to gap opening of magnetic excitations and asymmetric spectral weight. |
Tuesday, March 16, 2021 8:36AM - 8:48AM Live |
E37.00002: Excitations of critical quantum spin chains from non-equilibrium classical dynamics St��phane Vinet, Gabriel Longpré, William Witczak-Krempa A critical quantum spin 1/2 chain dual to the non-equilibrium Kawasaki dynamics of an Ising chain coupled to a bath is presented. We give the exact groundstates, and single magnon excitations. Solutions for the two-magnon spectra are derived via a Bethe ansatz scheme. We discuss the corresponding two-magnon continuums and dynamical critical exponents, the latter of which suggest that these chains host multiple dynamics at low energy. |
Tuesday, March 16, 2021 8:48AM - 9:00AM Live |
E37.00003: Evolution of the quantum phase transitions in XXY general spin trimers with applied magnetic field and variable exchange parameters Amelia Brumfield, Jason Haraldsen This study investigates the thermodynamics and magnetic excitations of XXY general spin isosceles quantum trimers with a variable magnetic field and exchange parameters. Using an isotropic Heisenberg Hamiltonian, we analyze how the energy eigenstates, heat capacities, and magnetic susceptibilities evolve in mixed asymmetric systems of with exchange parameters of αJ1 and J2). Our analysis of these states' evolution allows us to map out the quantum phase transitions in these spin systems. From our findings, we hope to provide further insight into the general understanding of molecular magnet systems with an increase in different parameters and useful characterization methods of experimental investigations of molecular magnets and spin clusters. |
Tuesday, March 16, 2021 9:00AM - 9:12AM Live |
E37.00004: Weakly-coupled alternating S=1/2 chains in the distorted honeycomb-lattice compound Na2Cu2TeO6 Shang Gao, Ling-Fang Lin, Andrew May, Binod K Rai, Qiang Zhang, Elbio Dagotto, Andrew Christianson, Matthew Brandon Stone Spin-1/2 chains with alternating antiferromagnetic (AF) and ferromagnetic (FM) couplings exhibit quantum entanglement like the integer-spin Haldane chains and might be similarly utilized for quantum computations. Such alternating AF-FM chains have been proposed to be realized in the distorted honeycomb-lattice compound Na2Cu2TeO6, but ambiguities remain as to the coupling strengths. Here we employ neutron scattering to study the spin dynamics in Na2Cu2TeO6 and accurately determine the coupling strengths through the random phase approximation and density functional theory (DFT) approaches. We find the AF and FM intrachain couplings are the dominant terms in the spin Hamiltonian, while the interchain couplings are AF but perturbative, thus establishing Na2Cu2TeO6 as a weakly-coupled alternating AF-FM chain compound. The hierarchy in the coupling strengths and the alternating signs of the intrachain couplings can be understood through their different exchange paths. |
Tuesday, March 16, 2021 9:12AM - 9:24AM Live |
E37.00005: Magnetization plateau of the S=2 antiferromagnetic Heisenberg chain with anisotropies Toru Sakai, Kiyomi Okamoto, Takashi Tonegawa The magnetization process of the S=2 antiferromagnetic quantum spin chain with the exchange and single-ion anisotropies is investigated, using thenumerical exact diagonalization of finite-size clusters and the level spectroscopy analysis[1]. It is found that a magnetization plateau possibly appears at a half of the saturation magnetization for some suitable anisotropy parameters. The level spectroscopy analysis indicates that the 1/2 magnetization plateau is formed by two different mechanisms, depending on the anisotropy parameters. The phase diagram of the 1/2 plateau states and some typical magnetization curves are also presented. In addition the biquadratic interaction is revealed to enhance the plateau induced by the Haldane mechanism. |
Tuesday, March 16, 2021 9:24AM - 9:36AM Live |
E37.00006: Comparison of spin dynamics in green and black dioptase: from classical chains to quantum fluctuations Andrey Podlesnyak, Stanislav Nikitin, Oleksandr Prokhnenko, Alexander Kolesnikov, Timothy R Prisk, Larry Anovitz Magnetic excitations of a gem crystal green dioptase Cu6[Si6O18].6H2O and fully dehydrated black dioptase Cu6Si6O18 have been studied by inelastic neutron scattering. The intrachain Jc and interchain Jab interactions as well as the anisotropy of the Jc have been determined. The observed spectrum of green dioptase contains two sharp magnetic modes and a prominent spin gap that is consistent with the ordered ground state of Cu moments coupled antiferromagnetically in spiral chains along the c axis and ferromagnetically in ab planes on the hexagonal cell. We found that dehydration has a dual effect on the magnetic interactions: it significantly increases the intrachain exchange interaction Jc, and simultaneously decouples the chains by suppressing Jab. The spin dynamics of green dioptase are dominated by conventional magnon excitations, whereas the continuum of excitations observed in black dioptase indicates the presence of fractionalized spinons. We suggest that black dioptase behaves like a nearly ideal spin-1/2 antiferromagnetic Heisenberg spiral chain with enhanced quantum fluctuations and weak spinon confinement Jab/Jc~0.02. |
Tuesday, March 16, 2021 9:36AM - 9:48AM Live |
E37.00007: Single-ion properties of quasi-1D transverse Ising-critical CoNb2O6 John Ringler, Alexander Kolesnikov, Kate Ross The transverse-field Ising model (TFIM) is the most theoretically tractable model that displays a quantum phase transition. CoNb2O6 is one of the few known real materials to exhibit a quantum critical point in the presence of an applied transverse field. However, despite having long been modeled as a truly Ising-like system, recent theoretical results [1] suggest that the critical behavior observed in CoNb2O6 is due not to the breaking of a global Ising symmetry, but to the breaking of a glide symmetry which is a consequence of its 3D space group Pbcn (60). Additionally, THz spectroscopy measurements have indicated that the magnetism in CoNb2O6 possibly approximates the "twisted" Kitaev chain model [2]. In both cases the model maps onto the TFIM through a unitary transformation, keeping the observed critical behavior within the same Ising universality class. We present the results of both inelastic neutron scattering measurements and electron paramagnetic resonance spectroscopy on CoNb2O6, which explores the single-ion anisotropy present in the local environments of the Co2+ ions in an effort to shed light on these recent developments. |
Tuesday, March 16, 2021 9:48AM - 10:00AM Live |
E37.00008: Giant pressure-enhancement of multiferroicity in CuBr2 from first principles Aleksandar Razpopov, Vladislav Borisov, Bruce Normand, Roser Valenti Type-II multiferroics, in which ferroelectric polarization is induced by inversion-nonsymmetric magnetic order, promise new and efficient applications based on the mutual control of magnetic and electronic properties. Recent experiments show a dramatic increase of the multiferroic Tc in CuBr2 from 73.5K to 162K under a pressure of 4.5GPa, with no evidence that the effect saturates. We provide a quantitative explanation by two-step density-functional-theory calculations that include correlations by GGA+U. First we fix the lattice parameters from XRD experiments performed up to 15GPa and optimize the full atomic structure at each pressure using VASP. Then we estimate 9 key magnetic interactions in each structure by a total-energy mapping analysis within the FPLO framework. Our results show how rising pressure increases the interactions between CuBr2 chains by a factor of 4 up to 4.5GPa, strongly reinforcing their incommensurate spiral magnetism, whose ordering temperature sets Tc. Our results create a microscopic basis for the possibility of engineering room-temperature multiferroicity in suitably strained CuBr2. |
Tuesday, March 16, 2021 10:00AM - 10:12AM Live |
E37.00009: Rashba + exchange” effects in 1D magnets: bismuth nanoribbons as model systems Ivan Naumov, Pratibha Dev The Bi(111) bilayer is a 2D topological insulator and in the form of nanoribbons, it exhibits 1D spin-helical gapless edge states or quantum spin Hall effect (QSHE). In this QSHE phase, the system is non-magnetic. However, our first principles calculations show that Bi zigzag nanoribbons, like their graphene counterparts, can also exist in ferromagnetic and anti-ferromagnetic states, which are (meta)stable. These nanoribbons are useful model systems for studying “Rashba + exchange” effects in other possible 1D magnets. Although the magnetic Bi zigzag nanoribbons do not demonstrate QSHE, they still possess interesting physical properties associated with the interplay of Rashba and spin-orbit coupling. They exhibit a strong 1D Rashba effect, retain helical spin texture, become half-metallic with doping and, can have asymmetric transport properties. |
Tuesday, March 16, 2021 10:12AM - 10:24AM Live |
E37.00010: Competing Magnetic Ground States of Novel Sawtooth Co2+ S = 3/2 Chains in NaCo2(SeO3)2(OH) Duminda Sanjeewa, Jie Xing, Vasile Garlea, Keith Taddei Low-dimensional magnetic materials have drawn continued attention in condensed matter physics, owing to their distinct electronic and magnetic properties. In particular, the oxyanion systems-based transition-metal (M) oxide sublattices that are magnetically isolated by closed-shell nonmagnetic oxyanions (SiO4−4, PO3−4, AsO3−4) show great potential for exploring and characterizing new emergent phenomena. Recently we have synthesized single crystals of NaCo2(SeO3)2(OH) and characterized the crystals structure. NaCo2(SeO3)2(OH) belongs to a rare class of compounds, namely delta-chain or saw tooth type structure in which corner sharing isosceles triangles whose vertices consist of one Co(1) and two Co(2) atoms. The magnetic susceptibility reveals a two competing magnetic transition at TN1 = 11 K and TN2 = 3.2 K. The isothermal magnetization data shown a field induced transition at 1.3 kOe. Neutron powder diffraction was used to determine the magnetic structure. |
Tuesday, March 16, 2021 10:24AM - 10:36AM Live |
E37.00011: Level statistics of a critical quantum spin chain from non-equilibrium classical dynamics Gabriel Longpré, St��phane Vinet, William Witczak-Krempa A critical quantum spin 1/2 chain dual to the non-equilibrium Kawasaki dynamics of an Ising chain coupled to a bath is presented. Its level spacing distributions are analysed in the context of random matrix theory. The implications on the integrability of the system are discussed using the Berry-Tabor conjecture. |
Tuesday, March 16, 2021 10:36AM - 10:48AM Live |
E37.00012: Helical spin structure of 1D chains with hybridized boundaries Nicolas M Vargas, Alexander Baker, Felipe Torres, Roman Pico, Jonathan R.I. Lee, Ignacio Hamad, Paula Abufager, Miguel G Kiwi, Trevor M Willey, Carlos Monton, Ivan Schuller We have synthesized and studied quantitatively the structure and magnetic properties of ultra-short 1D Fe chains subject to various boundary conditions. The length of Fe chains are controlled by using iron phthalocyanine(FePc) thin films and FePc/ metal-free phthalocyanine(H2Pc) superlattices(SLs). Their boundary conditions are controlled by electronic hybridization at the end of the chains. The local bonding environment was determined from element selective XAS, and the angular dependence of Spin and orbital moment was resolved by XMCD. DFT calculations provided a fundamental understanding of the structural and electronic properties of 1D Fe chains subject to different electronic environments. The coercive field increases with the iron chain length with hybridized boundaries, whereas in chains not subject to hybridization the coercive field remains constant. The slow magnetic relaxation of 1D Fe chains at different freezing temperatures was extracted from the magnetic remanence evolution with time. A quantitative, semi-classical model based on the DMI, implies the presence of an unusual Helical spin structure which arises from hybridized boundaries |
Tuesday, March 16, 2021 10:48AM - 11:00AM Live |
E37.00013: Engineering magnetic edge states in zigzag graphene nanoribbons Raymond Blackwell, Fangzhou Zhao, Erin Brooks, Ilya Piskun, Shenkai Wang, Junmian Zhu, Yea-Lee Lee, Steven G Louie, Felix Fischer Zigzag graphene nanoribbons (ZGNRs) are expected to host electronic edge states that are ferromagnetically coupled along the long axis of the ribbon and antiferromagnetically coupled across the width of the ribbon. This unique electronic structure however is often obscured by the innate chemical reactivity of spin-ordered edge states. An unusually strong hybridization on ZGNRs with accessible surface states of the underlying support represents a veritable challenge to their exploration. We describe the bottom-up synthesis of ZGNRs featuring a superlattice of heteroatom dopants along the zigzag edge. Using scanning tunneling spectroscopy (STS), we demonstrate that the substitutional doping of ZGNRs with nitrogen atoms leads to a partial localization of the valence and conduction band edge states and an overall contraction of the quasiparticle band gap. Experimental results reveal that the magnetic edge states of nitrogen doped ZGNRs can readily be decoupled from the metallic growth substrate using a simple tip manipulation protocol. Our work suggests a new strategy to chemically passivate the reactive edges of ZGNRs without sacrificing the emergent spin degree of freedom critical to the exploration and realization of graphene-based spintronics. |
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