Session J38: Magnetism Quantum Theory and Computation Studies

Live

Topological phases in magnetic materials are of great current interest in spintronics because of their fundamental significance and practical utility for robust information processing. A particularly interesting system is magnetic-texture-based metamaterials, since they can offer flexible controllability that can benefit from modern spintronic techniques. The collective motion of magnetic texture or soliton is described by Thiele's equation, which results in a wavelike equation in the artificial crystal, and the equation differs from the wave equations of its electronic, photonic, and acoustic counterparts in the following respects: (i) The nonvanishing topological charge induces a gyration term that is analogous to an effective magnetic field acting on a quasiparticle, thus breaking time-reversal symmetry. (ii) The inertial effect is taken into account by a mass term. A non-Newtonian gyration term is included to capture the high-frequency behavior of the magnetic texture and to determine the interaction parameters with high accuracy. (iii) The particle-particle coupling is strongly anisotropic.
Through analytical and numerical calculations, we predict (higher-order) topological insulator states in magnetic-texture (e.g., skyrmion and vortex) based metamaterials, which manifests robust edge and/or corner states at device boundaries and is characterized by the quantized Chern number and the Z_N Berry phase. Note that both fabricating the spin-texture metamaterials of interest and detecting the spatially localized edge modes are already within the reach of current technology. Our findings open up a promising route to realizing (higher-order) topological insulators in magnetic systems and to achieving robust spintronic memories, imaging, and computing.

References: PRApplied 13, 064058 (2020); PRB 101, 184404 (2020); PRL 124, 217204 (2020); PR Research 2, 022028(R) (2020); Nano. Lett. 20, 7566 (2020); PRB 98, 180407(R) (2018); npj Comput. Mater. 5, 107 (2019).   

Live

Motivated by recent progress on synthesizing two-dimensional magnetic van der Waals systems, I will talk about a proposal for detecting the topological Berezinskii-Kosterlitz-Thouless (BKT) phase transition in spin-transport experiments. Here we demonstrate that the spatial correlations of injected spin-currents into a pair of metallic leads can be used to measure the predicted universal jump of 2/π in the ferromagnet spin-stiffness. Our setup provides a simple route to measuring this (up to now elusive) topological phase transition in two-dimensional magnetic systems. It is hoped that this will encourage experimental efforts to investigate critical phenomena beyond the standard Ginzburg-Landau paradigm in low-dimensional magnetic systems with no local order parameter.   

Live

Many materials of intense current interest are layered materials or molecules in which dispersive van der Waals interactions play a central role, for example CrI₃, WTe₂, phosphorene, graphene and and its allotropes. In these materials, the electronic properties depend sensitively on the interlayer separation and stacking. While commonly used density functional theory (DFT) methods work well for covalent bonding, they have difficulties capturing all aspects of van der Waals-bonded systems. The L7 molecular test set was designed specifically to highlight dispersive interactions [1]. We will compare results on L7 from highly accurate diffusion Monte Carlo simulations from those of DFT methods, including bonding energy but also charge densities, and also discuss these results in the context of layered van der Waals-bonded materials.
[1] R. Sedlak, T. Janowski, M. Pitonak, J. Rezac, P. Pulay, and P. Hobza, Journal of chemical theory and computation 9, 3364 (2013).   

Live

We study the response of Luttinger semimetals, such as alpha-Sn, HgSe, HgTe, YPtBi and Pr₂Ir₂O₇, to charge and magnetic impurities. In these materials the strong spin-orbit coupling and quadratic band-touching lead to unusual Friedel and Ruderman-Kittel-Kasuya-Yoshida (RKKY) oscillations, as well as an asymmetry in the response upon electron or hole doping. We also examine the magnetic Pauli and Landau susceptibilities, which differ from those of regular metals. These results motivate the study of Kondo physics in Luttinger semimetals, where novel quantum phases can arise due to spin-orbit coupling.   

Live

Condensed matter systems offer parallels to some of the most important phenomena in high-energy physics, exemplified by the Anderson-Higgs transition in superconductors and magnetic monopoles in spin ice. Here we ask whether similar parallels exist between the fundamental force carrying particles of electromagnetism and gravity and the collective excitations of quantum magnets.

Using a combination of analytic methods and semi-classical molecular dynamics simulation, we first revisit the analogy between photons and the massless spin-1 Goldstone modes of antiferromagnets. We then consider the linearly dispersing, spin-2 Goldstone modes of a quantum magnet with quadrupolar order, arguing that these provide an analogue to gravitational waves. Through explicit simulation, we show how the quadrupole waves of a spin-1 magnet can mimic the gravitational radiation observed from binary mergers of black holes.   

Live

Classical hydrodynamics is a remarkably versatile description of the coarse-grained behavior of many-particle systems once local equilibrium has been established. The form of the hydrodynamical equations is determined by the conserved quantities present in a system. Generically, there is a small number of conserved quantities, which give rise to diffusive transport properties. However, in integrable systems with an extensive number of conserved quantities, more exotic transport properties are possible. In particular, recent work suggests the spin-half Heisenberg chain exhibits Kardar-Parisi-Zhang (KPZ) dynamics at infinite temperature. In this work, we study the dynamical structure factor using a tensor network approach, and show signatures of KPZ survive at finite temperatures. Moreover, we find excellent agreement with neutron scattering experiments on the compound KCuF3, suggesting KPZ physics is present.   

Live

Recent experiments on $\mathrm{Fe_{1/3}NbS_2}$, a bulk antiferromagnet (AFM) with a N{e}el temperature of 42K, have shown that an applied current can reversibly switch the magnetic order, which is read out in the resistivity*. The resistivity changes are thought to be caused by a redistribution of magnetic domains**. To shed light on these findings, we examine the magnetic properties and transport of $\mathrm{Fe_{1/3}NbS_2}$ using density functional theory calculations. We find two near-degenerate magnetic states corresponding to an in-plane stripe and zigzag ordering, in agreement with neutron measurements. We show that the in-plane conductivity for both magnetic orders is anisotropic, consistent with domain repopulation being the basis for the electrical switching. However, the anisotropy with zigzag AFM order is reduced with respect to that of stripe order due to the strong nearest-neighbor exchange.
* Nair et al., \emph{Nature Materials} {\bf19}, 153-157(2020)\newline
**Little et al., \emph{Nature Materials} {\bf19}, 1062-1067(2020)   

Live

The precise manipulation of domain walls in nanowires is an essential condition for the realization of memory devices based on domain walls (DWs) displacement. To achieve the controlled movement of the DWs between well-defined positions, a pinning mechanism is needed. An initial approach is to pin the DWs at artificial constrictions in the nanowire and train of such DWs can be displaced regularly. Another approach is to modify locally the material parameters by modulating for example the anisotropy along the wire. In this presentation, we show that using a multisegmented nanowire, the precise manipulation of transverse DWs can be achieved depending on the segments length and materials parameters. The competition between the variation of anisotropy and demagnetizing energy is crucial to obtain the controlled motion. Furthermore, in some cases, the DW displacement with polarity changing can be obtained even with thermal stability just by manipulating the current pulse shape. This paves the way for practical applications with bit encoding in the DWs polarity.   

Not Participating

Ligated metal-chalcogenide clusters have attracted considerable interest due to the recent developments in the synthesis process and their assembly into solids. In this work, the electronic and magnetic properties of the Fe₆S₈ cluster attached with ligands and the fused superatomic dimer are investigated using the first-principles density functional theory. It is shown that the redox properties of the Fe₆S₈ cluster can be effectively controlled by altering the nature of the attached ligands. Donor ligands such as phosphines reduce the ionization energy of the Fe₆S₈ cluster, whereas the acceptor ligands such as CO increase the electron affinity. Such variation in the redox properties of the Fe₆S₈ cluster is the result of the ligand-induced shift in the cluster’s electronic levels, so the occupation number remains mostly unaffected, leading to only marginal variation in the spin magnetic moment of the cluster. The fusion of two Fe₆S₈ clusters decorated by unbalanced ligands forms a superatomic dimer, which exhibits rare features: a massive dipole moment and a large spin magnetic moment. The resulting superatomic dimer offers an exciting motif for spintronics-related applications.   

Live

The quantum spin Hall effect (QSHE) has been observed in topological insulators using spin-orbit coupling (SOC) as the probe, but it has not yet been observed in a metal. We propose an experiment to measure the QSHE of an electron or hole in a two-dimensional (2D) metal without SOC. Through the inner radius of a 2D metallic Corbino disk lies a long solenoid, and a radial charge current is applied between the inner and outer disk radii, thereby inducing a uniform fixed potential differentce across those radii. By combining changes in the flux in the solenoid and in the potential difference, spontaneous quantized azimuthal currents that can depend upon the particle's spin are generated.   

Live

We study the time-evolution of classical spin-dimer system and nature of the transition from non-chaotic into the chaotic region of phase space. We apply complementary numerical diagnostics including lyapunov spectra and boxcounting of trajectories to quantify the degree of chaoticity and compare with other better known models such as standard maps.   

Live

Transition metal oxides (TMOs) exhibit exotic magnetic properties, often difficult to understand in conventional wisdom. Magnetic insulator Ba2CoO4 has mystified the community, because the CoO4 tetrahedron appears to be completely isolated with the nearest Co atoms far apart (~5 Å), making it difficult to account for long-range magnetic ordering seen experimentally using only Co. By first-principles calculations in bulk Ba2CoO4, we illustrate for the first time that the magnetic moment on oxygen atoms are the origin of the unexpected long-range magnetic ordering and low magnetic dimensionality. We find that the magnetic moment is not only localized on Co atoms, as assumed in all conventional data analysis, but also distributed on its tetrahedrally-coordinated O atoms, making CoO4 the magnetic building block instead of Co alone. Having oxygen contribute to the magnetic moment may be identified as a universal property of magnetic TMOs, which will require a fresh look at conventional models of magnetism in TMOs.   

Live

In clean metallic ferromagnets at asymptotically low temperature, the magnon-exchange contribution to the electrical resistivity is exponentially suppressed due to the exchange gap [1]. The power-law prefactor of the exponential term is difficult to determine because the gap provides an energy scale in addition to the temperature. To solve this problem, we developed a technique for obtaining exact solutions to the linearized Boltzmann equation in the low-temperature limit. Our method dispenses with the various uncontrolled approximations that have been used in all previous solutions of the integral equations for the transport coefficients. We present a rigorous proof of Bloch's T^5 law due to electron-phonon scattering [2] and give the corresponding result for scattering by magnons [3]; the prefactor of the exponential is asymptotically independent of the temperature.

[1] S. Bharadwaj, D. Belitz, and T.R. Kirkpatrick, Phys. Rev. B 89, 134401 (2014).
[2] J. Amarel, D. Belitz, and T.R. Kirkpatrick, arXiv:2001.01148; arXiv:2009.03234
[3] J. Amarel, D. Belitz, and T.R. Kirkpatrick, arXiv:2009.02787