Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session P50: Spin Chains and QuasiLowD Molecular MagnetsFocus

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Sponsoring Units: GMAG DMP Chair: Christopher Landee, Clark University Room: 397 
Wednesday, March 15, 2017 2:30PM  2:42PM 
P50.00001: NMR relaxation rate in quasi onedimensional antiferromagnets Sylvain Capponi, Maxime Dupont, Nicolas Laflorencie, Pinaki Sengupta, Hui Shao, Anders W. Sandvik We compare results of different numerical approaches to compute the NMR relaxation rate $1/T_1$ in quasi onedimensional (1d) antiferromagnets. In the purely 1d regime, recent numerical simulations using DMRG have provided the full crossover behavior from classical regime at high temperature to universal TomonagaLuttinger liquid at lowenergy (in the gapless case) or activated behavior (in the gapped case).~\footnote{M. Dupont, S. Capponi, N. Laflorencie, Phys. Rev. B {\bf 94}, 144409 (2016).} For quasi 1d models, we can use meanfield approaches to reduce the problem to a 1d one that can be studied using DMRG. But in some cases, we can also simulate the full microscopic model using quantum MonteCarlo techniques. This allows to compute dynamical correlations in imaginary time and we will discuss recent advances to perform stochastic analytic continuation to get real frequency spectra. Finally, we connect our results to experiments on various quasi 1d materials. [Preview Abstract] 
Wednesday, March 15, 2017 2:42PM  2:54PM 
P50.00002: Determining singleion and spatialexchange anisotropies in a $S=$1 quantum antiferromagnet Jamie Manson, Jamie Brambleby, Paul Goddard, Matthew Stone, Roger Johnson, Pascal Manuel, Jacqueline Villa, Craig Brown, Vivien Zapf, Saul Lapidus, Rebecca Scatena, Piero Macchi, Yusheng Chen, LaiChin Wu, John Singleton The magnetic groundstate of the Q1D $S=$ 1 antiferromagnetic (AFM) chain is sensitive to the singleion anisotropy ($D)$ and the relative strength of intra ($J)$ and interchain ($J$') exchange interactions. The ratios $D$/$J$ and $J'$/$J$ dictate the material's placement on the phase diagram for which three competing phases are known to theoretically exist: Haldane, \textit{XY} and quantum paramagnetic. We have identified [Ni(HF$_{\mathrm{2}})$(pyz)$_{\mathrm{2}}$]SbF$_{\mathrm{6}}$ as a candidate in which to explore proximity to these phases. Combining neutron scattering (elastic and inelastic) and highfield magnetization we obtained the ground state Hamiltonian and phase diagram for a powdered sample. Longrange \textit{XY}AFM order ($D$ \textgreater 0) occurs below $T_{\mathrm{N}}=$ 12.2 K and independent simulations of inelastic neutron scattering and $M(H)$ data show excellent consistency for the parameters; $D=$ 13.3 K, $J=$ 10.4 K (NiFHFNi) and $J'=$ 1.4 K (NipyzNi). [Preview Abstract] 
Wednesday, March 15, 2017 2:54PM  3:06PM 
P50.00003: Distinguishing weak and strong symmetryprotected topological phases in deformed AKLT states Kevin Beach, Keola Wierschem The socalled ``strange correlator'' can be used as a tool to detect symmetryprotected topological order in one and twodimensional quantum systems. It takes the form of a spinspin correlation function, computed as a mixed overlap between the state of interest and a trivial local product state. The strange correlator is computed in the featureless bulk but is sensitive to the system's edge modes and its topological character. I will discuss how one can directly evaluate the strange correlator within the valence bond loop gas framework and give some numerical examples of applications to various AffleckKennedyLiebTasaki (AKLT) states. This approach reproduces the predicted evenodd effect in the spin value $S$ for the linearchain and squarelattice AKLT states (in accordance with the Haldane conjecture). Moreover, changes in the strange correlator's asymptotic form in response to deformation of the wave function reveal the robustness of the protecting symmetries and allow for their categorization as either weak or strong. [Preview Abstract] 
Wednesday, March 15, 2017 3:06PM  3:18PM 
P50.00004: Prediction for spin Fano factor generated by biased quantum spin chains Joshua Aftergood, So Takei We theoretically study noise in the spin current injected into a normal metal from a 1D Heisenberg spin$1/2$ antiferromagnet. We consider the noise generated in two separate scenarios: first by inducing an overpopulation of one chiral mode relative to the other in the spin chain at uniform temperature, and second by elevating the temperature of the spin chain relative to the metal, i.e., by way of the spin Seebeck effect. We compute excess noise in the normal metal generated by the coupling to the spin chain and predict the spin Fano factor, defined as the noise in the spin current normalized by the average spin current, for both scenarios. [Preview Abstract] 
Wednesday, March 15, 2017 3:18PM  3:30PM 
P50.00005: Hubbard to Heisenberg crossover behavior in the magnetic excitation spectrum of strongly correlated quasione dimensional systems Alberto Nocera, Niravkumar D. Patel, Jaime FernandezBaca, Elbio Dagotto, Gonzalo Alvarez Magnetic excitations of quasione dimensional strongly correlated materials are often described in terms of model Hamiltonians where only spin degrees of freedom are taken into account typically the Heisenberg Hamiltonian, which represents the main paradigm for quantum magnetism. This is due to the presence of strong electronic correlations, which energetically suppress the possibility of double occupation of the outer shell orbitals and the electronic mobility. In this talk, using the density matrix renormalization group method, we calculate the dynamical spin structure factor of the Hubbard model at half electronic filling on a chain and on a ladder geometry, and compare the results with those obtained using the Heisenberg model. We show the surprising result that even at intermediate values of the Hubbard repulsion $U$, the spectra qualitatively closely resemble the Heisenberg result. However, even in these cases electronic itinerancy effects cannot be neglected to understand quantitatively the magnetic excitation spectra. We discuss the implications of the results for neutron scattering experiments and we propose criteria to deduce the actual value of $U$ from experimental information, even when Heisenberg models appear to describe the data accurately [Preview Abstract] 
Wednesday, March 15, 2017 3:30PM  3:42PM 
P50.00006: Emergent symmetries in disordered quantum spin systems Victor Quito, Pedro Lopes, Jose Hoyos, Eduardo Miranda A common theme in low or highenergy physics is the fact that a state may have a symmetry lower than its underlying Lagrangian. Behind this feature is the phenomenon of spontaneous symmetry breaking. A more exotic situation corresponds to a lowenergy state whose symmetry is greater than its underlying Lagrangian. Recently, we have proved rigorously that strong disorder can lead to emergent symmetries, with no fine tuning required. We will show a generic route to emergent symmetries induced by strong disorder, as well as several examples of rich phase diagrams. Strongly disordered spin1 chains with manifest SU(2) symmetry show two phases with emergent SU(3) symmetry. SpinS chains with S>1 also show emergent SU(N) phases but with less richness. Finally, we show that generic SO(N) symmetricchains accommodate two different phases with emergent SU(N) symmetries. [Preview Abstract] 
Wednesday, March 15, 2017 3:42PM  3:54PM 
P50.00007: z = 2 Quantum Criticality in Heisenberg Spin Chains near Saturation Dominic Blosser, Kirill Povarov, David Schmidiger, Robert Bewley, Emanuele Coira, Thierry Giamarchi, Andrey Zheludev We study quantum critical dynamics in Heisenberg spin chains near saturation. Using inelastic neutron scattering we investigate the prototypical Heisenberg spin chain compound K$_2$CuSO$_4$Cl$_2$. The full excitation spectra are measured at different temperatures near the critical point. This data we compare to finite temperature density matrix renormalization group calculations (TDMRG). In addition, specific heat and magnetization data show remarkable universal scaling behaviour near the quantum critical point. [Preview Abstract] 
Wednesday, March 15, 2017 3:54PM  4:06PM 
P50.00008: High pressure Xray study of the lattice dimerization in the S$=$1/2 chain quantum magnet TiOCl. Costel R. Rotundu, Jiajia Wen, Wei He, Yejun Feng, Yongseong Choi, Daniel Haskel, Young S. Lee TiOCl, composed of S$=$1/2 chains of Ti ions, is a good realization of a spinPeierls system. Considerable effort has been put into doping the material with the goal of achieving metallization. Interestingly, high pressure resistivity shows a dramatic decrease of its insulating behavior, which may coincide with the emergence of charge density wave order. Therefore, high pressure of the structure with xrays are important in further exploring the phase diagram. Upon cooling under normal pressure conditions, TiOCl exhibits a transition to an incommensurate nearly dimerized state at Tc2 $=$ 92 K and to a commensurate dimerized state at Tc1 $=$ 66 K. Here we map its complex temperature vs. pressure phase diagram with xrays on a single crystal in a high pressure diamond anvil down to 6 K in temperature and to pressures up to approximately 14.5 GPa. [Preview Abstract] 
Wednesday, March 15, 2017 4:06PM  4:42PM 
P50.00009: Fractional and hidden magnetic excitations in felectron metal Yb2Pt2Pb Invited Speaker: Igor Zaliznyak Quantum states with fractionalized excitations such as spinons in onedimensional chains are commonly viewed as belonging to the domain of S=1/2 spin systems. However, recent experiments on the quantum antiferromagnet Yb2Pt2Pb, part of a large family of R2T2X (R=rare earth, T=transition metal, X=main group) materials spectacularly disqualify this opinion [1,2,3]. The results show that spinons can also emerge in an felectron system with strong spinorbit coupling, where magnetism is mainly associated with large and anisotropic orbital moment. Here, the competition of several highenergy interactions – Coulomb repulsion, spinorbit coupling, crystal field, and the peculiar crystal structure, which combines low dimensionality and geometrical frustration, lead to the emergence, at low energy, of an effective spin1/2, purely quantum Hamiltonian. Consequently, it produces unusual spinliquid states and fractional excitations enabled by the inherently quantum mechanical nature of the moments [1,2]. The emergent quantum spins bear the unique birthmark of their unusual origin in that they only lead to measurable longitudinal magnetic fluctuations, while the transverse excitations such as spin waves remain invisible to scattering experiments. Similarly “hidden” would be transverse magnetic ordering, although it would have visible excitations. The rich magnetic phase diagram of Yb2Pt2Pb is suggestive of the existence of hiddenorder phases [1,2], while the recent experiments indeed reveal the “dark magnon”, a hidden excitation in the saturated ferromagnetic (FM) phase of Yb2Pt2Pb [2,3]. Unlike copperbased spin1/2 chains, where the magnon in the FM state accounts for the full spectral weight of the zerofield spinon continuum, in the spinorbital chains in Yb2Pt2Pb it is 100 times, or more weaker. It thus presents an example of “dark magnon matter”, whose Hamiltonian is that of the effective spin1/2 chain, but whose coupling to magnetic field, the physical probe at our disposal, is vanishingly small. References [1] W. Miiller, L.S. Wu, M. S. Kim, T. Orvis, J. W. Simonson, M. Gamaza, D. E. McNally, C. S. Nelson, G. Ehlers, A. Podlesnyak, J.S. Helton, Y. Zhao, Y. Qiu, J. R. D. Copley, J. W. Lynn, I. A. Zaliznyak, M. C. Aronson. Phys. Rev. B 93, 104419 (2016). [2] L.S. Wu, W. J. Gannon, I. A. Zaliznyak, A. M. Tsvelik, M. Brockmann, J.S. Caux, M. S. Kim, Y. Qiu, J. R. D. Copley, G. Ehlers, A. Podlesnyak, M. C. Aronson. Science, 352, 1206 (2016). [3] I. A. Zaliznyak, L.S. Wu, A. T. Savici, G. Ehlers, W. J. Gannon, M. Aronson, unpublished (2016). [Preview Abstract] 
Wednesday, March 15, 2017 4:42PM  4:54PM 
P50.00010: Topological transition caused by reconstruction of zeromode Majorana fermions in XYZ spin chain Takanori Sugimoto, Sayed Akbar Jafari, Takami Tohyama Kitaev chain model has attracted much attention as a possible playground of topologicallyprotected quantum computation (TPQC), based on zeroenergy modes of Majorana fermions (ZM2Fs). This model can be realized in a 1/2spin chain compound, which has an anisotropic XY exchange interaction between nearestneighbor sites. However, real materials also have a nonzero Ising interaction necessarily. Here, we theoretically study effects of the Ising interaction in a XYZ spin chain, where a fully anisotropic exchange is introduced in neighboring bonds. The existence of ZM2Fs can be clarified with energy gaps of ground states between different $Z_2\times Z_2 \times Z_2$ sectors, defined by $Q^\alpha=\prod_i (2S_i^\alpha)=\pm 1$ where $\alpha=x,y,z$. By calculating the energy gaps with variational matrixproduct state method, we find a topological transition, where a ZM2F in $Q^\alpha$ sectors changes into that in other $Q^\beta$ sectors. Thus, we conclude that the transition originates from a reconstruction, namely a global $SU(2)$ rotation, of the ZM2F. Our results are helpful not only for understanding effects of possible Ising interactions in real compounds but also for important knowledge on stability of TPQCs. [Preview Abstract] 
Wednesday, March 15, 2017 4:54PM  5:06PM 
P50.00011: Magnons in a honeycomb ferromagnet Saikat Banerjee The original discovery of the Dirac electron dispersion in graphene led naturally to the question of Dirac cone stability with respect to interactions, and the Coulomb interaction between electrons was shown to induce a logarithmic renormalization of the Dirac dispersion. With the rapid expansion of the list of Dirac fermion compounds, the concept of bosonic Dirac materials has emerged. At the single particle level, these materials closely resemble the fermionic counterparts. However, the changed particle statistics affects the stability of Dirac cones differently. Here we study the effect of interactions focusing on the honeycomb ferromagnet  where the quasiparticles are magnetic spin waves (magnons). We demonstrate that magnonmagnon interactions lead to a significant renormalization of the bare band structure. We also address the question of the edge and surface states for a finite system. We applied these results to ferromagnetic CrBr$_3$, where the Cr$^{3+}$ atoms are arranged in weakly coupled honeycomb layers. Our theory qualitatively accounts for the unexplained anomalies in neutron scattering data from 40 years ago for CrBr$_3$ and hereby expand the theory of ferromagnets beyond the standard Dyson theory. [Preview Abstract] 
Wednesday, March 15, 2017 5:06PM  5:18PM 
P50.00012: Direct observation of the Higgs amplitude mode in a twodimensional quantum antiferromagnet near the quantum critical point Tao Hong, M. Matsumoto, Y. Qiu, W. C. Chen, T. R. Gentile, S. Watson, F. F. Awwadi, M. M. Turnbull, S. E. Dissanayake, H. Agrawal, R. ToftPetersen, B. Klemke, K. Coester, K. P. Schmidt, D. A. Tennant The emergence of lowenergy excitations in the spontaneous symmetrybreaking quantum phase transitions can be characterized by fluctuations of phase and amplitude of the order parameter. The phase oscillations correspond to the massless NambuGoldstone (or transverse) modes whereas the massive amplitude (or longitudinal) mode, analogous to the Higgs boson in particle physics, is prone to decay into a pair of lowenergy NambuGoldstone modes in low dimensions, which makes it experimentally difficult to detect Here, using inelastic neutron scattering and applying the bondoperator theory, we directly and unambiguously identify the Higgs amplitude mode in a two dimensional $S=$1/2 quantum antiferromagnet C$_{\mathrm{9}}$H$_{\mathrm{18}}$N$_{\mathrm{2}}$CuBr$_{\mathrm{4}}$ near a quantum critical point in two dimensions. Owing to an anisotropic energy gap of the transverse spin excitation, it kinematically prevents such decay and the Higgs amplitude mode acquires an infinite life time. [Preview Abstract] 
Wednesday, March 15, 2017 5:18PM  5:30PM 
P50.00013: B vs. T phase diagram of (ND$_{\mathbf{4}}${)}$_{\mathbf{2}}${[FeCl}$_{\mathbf{5}}${(D}$_{\mathbf{2}}${O)] studied by neutron diffraction and magnetization} Wei Tian, Huibo Cao, Jiaqiang Yan, Amanda Clune, Kendall Hughey, John Singleton, Janice Musfeldt, Brian Sales, Jaime FernandezBaca (NH$_{4})_{2}$[FeCl$_{5}$(H$_{2}$O)] was recently discovered to exhibit magnetically induced multiferroicity with a rich magnetic field versus temperature (B vs. T) phase diagram. In a previous zerofield study we reported the coexistence of both incommensurate and commensurate ``electronic soft'' phases in (NH$_{4})_{2}$[FeCl$_{5}$(H$_{2}$O)] due to strong magnetic frustration and spinlattice coupling. Here we report neutron diffraction and magnetization studies under applied magnetic field on deuterated (ND$_{4})_{2}$[FeCl$_{5}$(D$_{2}$O)] single crystals. Our measurements show that the presence of the coexistence of competing phases has profound effects to the macroscopic properties of this molecular compound. Our results reveal new novel phases in the B vs. T phase diagram of (NH$_{4})_{2}$[FeCl$_{5}$(H$_{2}$O)]. [Preview Abstract] 
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