Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session C21: Magnetic Chains and Kondo EffectsFocus
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Sponsoring Units: GMAG DMP Chair: Christopher Landee, Clark University Room: 320 |
Monday, March 14, 2016 2:30PM - 2:42PM |
C21.00001: Magnetic Behavior of quasi-1D-Ferromagnetic Fe Chains in Metallo-Organic Superlattices C. Monton, A. C. Basaran, I. Valmianski, T. Gredig, D. Altbir, V.. L. Carvalho-Santos, Ivan K. Schuller We report structural and magnetic properties of metallo-organic iron-phthalocyanine (FePc) / metal-free-phthalocyanine (H2Pc) superlattices. H2Pc is a weak diamagnetic molecule in which, instead of a metal ion, two hydrogen atoms occupy the center of the molecule. Due to molecular stacking, the divalent Fe(II) ion of FePc forms quasi one-dimensional (1D) chains. These Fe chains can be oriented either parallel or perpendicular to the substrate based on the choice of the substrate. These quasi-1D chains exhibit two magnetic regimes: ferromagnetic-like order below 5K, and nontraditional paramagnetic order (nonlinear behavior with decreasing saturation intensity with temperature) between 5 and 40 K. We have found that reducing the average Fe chains length from 70 to 7 Fe ions substantially increases the coercive field. We discuss the magnetic behavior of quasi-1D Fe chains as a function of the chains length and we correlate the observed magnetic behavior with structural information obtained from x-ray diffraction and Monte Carlo based micromagnetic simulations. [Preview Abstract] |
Monday, March 14, 2016 2:42PM - 2:54PM |
C21.00002: Spin Liquid Ground State in the Frustrated $J_1$-$J_2$ Zigzag Chain System BaTb$_2$O$_4$ A.A. Aczel, L. Li, V.O. Garlea, J.-Q. Yan, F. Weickert, V.S. Zapf, R. Movshovich, M. Jaime, P.J. Baker, V. Keppens, D. Mandrus We have investigated polycrystalline samples of the zigzag chain system BaTb$_2$O$_4$ with magnetic susceptibility, heat capacity, neutron powder diffraction, and muon spin relaxation ($\mu$SR). No magnetic transitions are observed in the bulk measurements, while neutron diffraction reveals the presence of low-temperature, short-range, intrachain magnetic correlations between Tb$^{3+}$ ions. $\mu$SR indicates that these correlations are dynamic, as no signatures of static magnetism are detected by the technique down to 0.095 K. These combined findings provide strong evidence for a spin liquid ground state in BaTb$_2$O$_4$. [Preview Abstract] |
Monday, March 14, 2016 2:54PM - 3:06PM |
C21.00003: Study of magnetic and magnetocaloric properties of monoclinic and triclinic spin chain CoV$_{2}$O$_{6}$ Moumita Nandi, Prabhat Mandal We have investigated magnetic and magnetocaloric properties of both monoclinic and triclinic phases of CoV$_{2}$O$_{6}$ from magnetization and heat capacity measurements. Conventional and inverse magnetocaloric effects have been observed in both phases of CoV$_{2}$O$_{6}$. For a field change from 0 to 7 T, maximum values of magnetic entropy change and adiabatic temperature change reach 11.8 J kg$^{-1}$ K$^{-1}$ and 9.5 K respectively for monoclinic CoV$_{2}$O$_{6}$ while the corresponding values reach 12.1 J kg$^{-1}$ K$^{-1}$ and 13.1 K for triclinic CoV$_{2}$O$_{6}$. Particularly for triclinic CoV$_{2}$O$_{6}$, the magnetocaloric parameters are quite large in low or moderate field range. Apart from this, we have constructed magnetic phase diagram of monoclinic CoV$_{2}$O$_{6}$ where field-induced complex magnetic phases appear below a certain critical temperature 6 K when external magnetic field is applied along crystallographic easy axis. [Preview Abstract] |
Monday, March 14, 2016 3:06PM - 3:18PM |
C21.00004: Entanglement properties of the bond alternating Heisenberg chain with general integer spins Shohei Miyakoshi, Satoshi Nishimoto, Yukinori Otha Symmetry protected topological (SPT) phases are a gapped phase under a given symmetry. Unless any symmetries that protect the SPT phases are broken, the SPT phases can be distinguished from each other. Recently, it was pointed out that the entanglement spectrum of the many-body state characterizes such SPT phases. In particular, the degeneracy of the entanglement spectrum reflects the corresponding symmetries and edge states of the system. Motivated by recent studies of the SPT phases, we study the bond-alternating Heisenberg model with general integer spins and clarify the entanglement properties of the ground state using the density matrix renormalization group method. In particular, this model has the intermediate phase at $S>1$ due to the bond alternation. The entanglement properties of this phase in the case of $S>2$ have not been studied sufficiently because of the numerical difficulties under an extremely small spin-gap situation. We studied the case of $S=1,2,3$ using the antiperiodic boundary condition. Under the antiperiodic boundary condition, we found that the doubly degenerate spectra which characterize the intermediate phase can be observed in the entanglement spectrum. We will also discuss the effect of the single-ion uniaxial anisotropy. [Preview Abstract] |
Monday, March 14, 2016 3:18PM - 3:30PM |
C21.00005: Finite temperature dynamics of spin-1/2 chains with symmetry breaking interactions Salvatore R. Manmana, Alexander C. Tiegel, Thomas Pruschke, Andreas Honecker I will discuss recent developments for flexible matrix product state (MPS) approaches to calculate finite-temperature spectral functions of low-dimensional strongly correlated quantum systems. The main focus will be on a Liouvillian formulation. The resulting algorithm does not specifically depend on the MPS formulation, but is applicable for any wave function based approach which can provide a purification of the density matrix, opening the way for further developments of numerical methods. Based on MPS results for various spin chains, in particular systems with Dzyaloshinskii-Moriya interactions caused by spin-orbit coupling and dimerized chains, I will discuss how symmetry breaking interactions change the nature of the finite-temperature dynamic spin structure factor obtained in ESR and neutron scattering experiments. [Preview Abstract] |
Monday, March 14, 2016 3:30PM - 3:42PM |
C21.00006: Magnetic Spin Relaxation Probed with Sweep Speed Dependent Coercivity Thomas Gredig, Matthew Byrne The magnetic spin relaxation of finite-length iron chains has been investigated in iron phthalocyanine thin films by means of sweep speed dependence on magnetic coercivity. The Fe(II) ions are embedded in a carbon matrix and molecules self-assemble during vacuum sublimation, so that the Fe(II) cores form well-separated chains of 1.3 nm and tunable chain lengths within the polycrystalline thin film. The average length of the chains is controlled through deposition variables and ranges from 30 nm to 300 nm. The coercivity strongly increases with chain length in this regime. This may be an interesting experimental realization of a low-dimensional finite-sized Ising model. The coercivity dependence on chain length and sweep speed is described with an Ising model based on Glauber dynamics. [Preview Abstract] |
Monday, March 14, 2016 3:42PM - 3:54PM |
C21.00007: \textbf{ANISOTROPIC PHASE DIAGRAM OF THE FRUSTRATED SPIN CHAIN }$\beta $\textbf{-TeVO}$_{\mathbf{4}}$\textbf{.} F. Weickert, M Jaime, N Harrison, B. L. Scott, A. Leitmae, L. Heinmaa, R Stern, O Janson, H. Berger, H Rosner, A. A. Tsirlin We will present experimental as well as theoretical data on $\beta $-TeVO$_{4}$ a candidate for the $J_{1}$-$J_{2}$ chain model with ferromagnetic $J_{1}$ \textasciitilde -18 K and antiferrromagnetic $J_{2}$ \textasciitilde 48 K coupling constants. The $T-H$ magnetic phase diagram is revealed by measurements of the magnetization, specific heat, magnetostriction, and thermal expansion on oriented single crystals at temperatures between 0.5 K and 50 K and in magnetic fields up to 50 T. The high field data were taken in a capacitor bank-driven pulsed magnet at NHMFL -- LANL and complemented with measurements in a superconducting magnet. Our comprehensive study allows for the first time a detailed mapping of the phase diagram in both directions, $H$ ll \textit{ab} and $H$ ll $c$. We find clear evidence for 5 different phases including full polarization of the magnetic moments above 23 T that is only weakly dependent on the crystal orientation. Surprisingly, the phase boundary at the saturation field splits into two distinct lines below 5 K. The magnetic phases occurring at fields below 10 T show significant magnetic anisotropy between $H$ ll \textit{ab} and $H$ ll $c$. The nature of the different phases and regions in $\beta $-TeVO$_{4}$ is still far from being understood, but our results will stimulate further research on this interesting model compound. [Preview Abstract] |
Monday, March 14, 2016 3:54PM - 4:06PM |
C21.00008: \textbf{Unusual features of magnetism in transition-metal-doped phthalocyanines C}$_{\mathrm{\mathbf{32}}}$\textbf{H}$_{\mathrm{\mathbf{16}}}$\textbf{N}$_{\mathrm{\mathbf{8}}}$\textbf{TM (TM }$=$\textbf{ Mn, Fe, Co, Ni, Cu)} Zhengjun Wang, Mohindar S. Seehra Transition-metal-doped phthalocyanines (TMPc), semiconductors with potential optoelectronic applications [1], are planar molecules with the TM atom at the center bound to four N atoms and forming a linear chain along the monoclinic b-axis. Because of this symmetry, the ground states of TMPc often violate the Hunds' rules; e.g. the S $=$ 3/2 state for d$^{\mathrm{5}}$ Mn(II) in $\beta $-MnPc, S $=$ 1/2 state for the d$^{\mathrm{7}}$ Co(II) in $\beta $-CoPc, and S$=$0 for Ni(II) in NiPc. The magnetic properties of TMPc are also affected by the stack angle $\delta $ between the orientation of the molecular plane and the b-axis, $\delta $ being 65\textdegree (45\textdegree ) for $\alpha $ ($\beta )$ phase [2]. For $\beta $-CoPc, our M vs. T data fits well with the Bonner-Fisher model for S $=$ 1/2 AFM Heisenberg linear chain [3] yielding the Co$^{\mathrm{2+}}$-Co$^{\mathrm{2+}}$ exchange constant J/k$_{\mathrm{B}}$ $=$ - 1.5 K. For $\beta $-MnPc , a long-presumed ferromagnet with T$_{\mathrm{C}} \quad \approx $ 9 K [4], our magnetic studies show it to be an Ising chain magnet with Arrhenius magnetic relaxation governed by J/k$_{\mathrm{B}} \quad =$ 2.6 K and the zero-field splitting D/k$_{\mathrm{B}}$ $=$ 8.3 K. In $\beta $-MnPc, the absence of $\lambda $-type peak in specific heat and no peaks in ac susceptibilities near the quoted T$_{\mathrm{C}}$ $\approx $ 9 K confirms the absence of long range order (LRO). Instead we argue that LRO is absent in $\beta $-MnPc as D \textgreater J makes the spins in a chain parallel but canted with respect to spins in neighboring chains. [1]G. Mattioli et al, Phys. Rev. Lett. 101, 126805 (2008); [2]Z. Wang et al, IEEE Trans. Mag. 51, 2700104(2015); [3]J. Bonner {\&} M. Fisher, Phys. Rev. 135, A640 (1964); [4]Y. Taguchi et al, J. Magn. Magn. Mater.301, 1229 (2007). [Preview Abstract] |
Monday, March 14, 2016 4:06PM - 4:18PM |
C21.00009: Heat transport in spin chains with weak spin-phonon coupling Alexander Chernyshev, Alexander Rozhkov We propose that the heat conductivity by 1D spin excitations in $S=1/2$ Heisenberg spin chains can be quantitatively described within the bosonization framework, in which large-momentum scattering of spin excitations is due to optical phonons with the spin-phonon couplings that are well within the physical bounds. Our theory provides an excellent fit to the data from the systematic experimental thermal conductivity studies in the high-quality single-crystalline large-$J$ spin-chain cuprates that have recently become available. Our description of the spin-phonon scattering is also in accord with a physically intuitive picture of phonons playing the role of thermally-populated weak impurities for the fast spin excitations. Our approach stands out from previous considerations that require large coupling constants to explain the data and thus imply a spin-Peierls transition, absent in real materials. [Preview Abstract] |
Monday, March 14, 2016 4:18PM - 4:30PM |
C21.00010: Spin dynamics in critical regime of the spin-$1/2$ XXZ chain Wang Yang, Jianda Wu, Congjun Wu The spin-$1/2$ Heisenberg XXZ chain is one of the most well-studied quantum integrable models. Although its eigenstates and spectrum are solvable through Bethe ansatz, even understanding its zero temperature spin dynamics remains a challenge. In the axial anisotropic regime, by tuning longitudinal magnetic field, the system undergoes a quantum phase transition, entering into the critical regime. Recent experiments provided some evidences for understanding spin dynamics in the critical regime. Here we investigate the spin dynamics in this regime by form factor methods. Our results can be directly compared with experiments on relevant materials. [Preview Abstract] |
Monday, March 14, 2016 4:30PM - 4:42PM |
C21.00011: First principles electron transport simulations in the Kondo regime Ivan Rungger, Milos Radonjic, Wilhelm Appelt, Liviu Chioncel, Andrea Droghetti When magnetic atoms, molecules or thin films are brought into contact with metals the electron-electron interaction leads to the appearance of the correlated Kondo state at low temperatures. In this talk we will present results for the electronic structure and conductance in the Kondo regime of recent STM and break junction experiments for stable radical molecules\footnote{J. Liu et al., J. Am. Chem. Soc. {\bf 135}, 651 (2013); R. Frisenda et al., Nano Lett. {\bf 15}, 3109 (2015).}, which correspond to spin half molecular magnets. We will outline the methodological approach to evaluate the conductance of such systems from first principles, as implemented in the Smeagol electron transport code\footnote{A. Rocha et al., Nature Mater. {\bf 4}, 335 (2005); A. Rocha et al., Phys. Rev. B {\bf 73}, 085414 (2006); I. Rungger et al., Phys. Rev. B {\bf 78}, 035407 (2008)}. The method combines the density functional theory (DFT) with Anderson impurity solvers within the continuum time quantum Monte Carlo (CTQMC) and numerical renormalization group (NRG) approaches. [Preview Abstract] |
Monday, March 14, 2016 4:42PM - 4:54PM |
C21.00012: Cobalt on silicene/ZrB$_{\mathrm{2}}$: an intriguing Kondo system Tobias Gill, Ben Warner, Henning PrĂ¼ser, Antoine Fleurence, Yukiko Yamada-Takamura, Cyrus Hirjibehedin Magnetic atoms placed upon metallic substrates have been used as prototypical systems for the investigation of the fundamentals of atomic-scale magnetism. Often these magnetic impurities undergo the Kondo effect, in which the magnetic moment of the impurity is collectively screened by a cloud of conduction electrons forming a many-body singlet ground state. Here we present results for individual Co adatoms on the silicene/ZrB$_{\mathrm{2}}$ surface. Unlike on metallic surfaces, Co atoms exhibit a distinct energy-dependent change in the spatial distribution of their electronic states when imaged with scanning tunneling microscopy. At low biases around the Fermi level, the Co atoms exhibit a two-lobe structure that is oriented along one of three equivalent directions in the plane and that is revealed by scanning tunneling spectroscopy to result from a Kondo resonance centered upon each lobe. This spatially anisotropic Kondo resonance is reminiscent of the orbital states of magnetic atoms on semiconductor surfaces or of the spatially distributed Kondo resonances seen for magnetic molecules on metallic surfaces, and is a result of the interaction between a magnetic impurity and the unusual electronic structure of the silicene/ZrB$_{\mathrm{2}}$ surface. [Preview Abstract] |
Monday, March 14, 2016 4:54PM - 5:06PM |
C21.00013: ABSTRACT WITHDRAWN |
Monday, March 14, 2016 5:06PM - 5:18PM |
C21.00014: Sub-molecular modulation of a 4f driven Kondo resonance by surface-induced asymmetry Ben Warner, Fadi El Hallak, Nicolae Atodiresei, Philipp Seibt, Henning Pruser, Vasile Caciuc, Michael Waters, Andrew J. Fisher, Stefan Blugel, Joris van Slageren, Cyrus F. Hirjibehedin Coupling between a magnetic impurity and an external bath can give rise to many-body quantum phenomena, including Kondo and Hund's Impurity states in metals, and Yu-Shiba-Rusinov states in superconductors. While advances have been made in probing the magnetic properties of d-shell impurities on surfaces, the confinement of f orbitals makes them much more difficult to access directly. Here we show that a 4f driven Kondo resonance can be modulated spatially by asymmetric coupling between a metallic surface and a molecule containing a 4f-like moment. Strong hybridisation of dysprosium double-decker phthalocyanine (DyPc$_2$) with Cu(001) induces Kondo screening of the central magnetic moment. Misalignment between the symmetry axes of the molecule and the surface induces asymmetry in the molecule's electronic structure, spatially mediating electronic access to the magnetic moment through the Kondo resonance. This work demonstrates the important role that molecular ligands play in mediating electronic and magnetic coupling and in accessing many-body quantum states. [Preview Abstract] |
Monday, March 14, 2016 5:18PM - 5:30PM |
C21.00015: Revealing the Atomic Site-Dependent g Factor within a Single Magnetic Molecule via the Extended Kondo Effect Shixuan Du Control over charge and spin states at the single molecule level is crucial not only for a fundamental understanding of charge and spin interactions but also represents a prerequisite for development of molecular electronics and spintronics. In this talk, I will talk about the extended spin distribution in space beyond the central Mn ion, and onto the non-magnetic constituent atoms of the MnPc molecule. This extended spin distribution results in an extended Kondo effect, which can be explained by spin polarization induced by symmetry breaking of the molecular framework, as confirmed by DFT calculations. Measuring the evolution of the Kondo splitting with applied magnetic fields at different atomic sites, we find a spatial variation of the g-factor within a single molecule for the first time. The existence of atomic site-dependent g-factors can be attributed to specific molecular orbitals distributed over the entire molecule. This work not only open up a new opportunity for quantum information recording, but also provide a new route to explore the internal electronic and spin structure of complex molecules, hard to achieve otherwise. (L.W. Liu et al., Phys. Rev. Lett. 2015, 114, 126601. In collaboration with Liwei Liu, Kai Yang, Yuhang Jiang, Li Gao, Qi Liu, Boqun Song, Wende Xiao, Haitao Zhou, Hongjun Gao in CAS, Min Ouyang in MU, and A.H. Castro Neto in SNU.) [Preview Abstract] |
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