Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session A54: Focus Session: Cooperative Phenomena: Spin Waves and Excitations |
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Sponsoring Units: GMAG Room: Mile High Ballroom 1B |
Monday, March 3, 2014 8:00AM - 8:12AM |
A54.00001: Inelastic neutron scattering studies of YFeO$_3$ Steven Hahn, Andrey Podlesnyak, Randy Fishman, Garrett Granroth, Alexander Kolesnikov, Ekaterina Pomjakushina, Kazimierz Conder, Georg Ehlers Spin waves in the the rare earth orthorferrite YFeO$_3$ have been studied by inelastic neutron scattering and analyzed with a full four-sublattice model including contributions from both the weak ferromagnetic and hidden antiferromagnetic orders. Antiferromagnetic (AFM) exchange interactions of $J_1 = -4.23\pm0.08$ meV (nearest-neighbors only) or $J_1 = -4.77\pm0.08 $ meV and $J_2= -0.21\pm0.03$ meV lead to excellent fits for most branches at both low and high energies. An additional branch associated with the hidden antiferromagnetic order was observed. This work paves the way for studies of other materials in this class containing spin reorientation transitions and magnetic rare earth ions. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A54.00002: Magnetic structure and spin excitations in BaMn$_2$Bi$_2$ M.D. Lumsden, S. Calder, B. Saparov, H.B. Cao, J.L. Niedziela, A.S. Sefat, A.D. Christianson The magnetic structure and associated spin wave excitations of the recently synthesized BaMn$_2$Bi$_2$ have been studied using neutron scattering. BaMn$_2$Bi$_2$ exhibits the same ThCr$_2$Si$_2$ crystal structure as the 122 iron superconductors (AFe$_2$As$_2$). Single crystal neutron diffraction reveals that the ordered state below T$_N$ $\sim$ 390 K is consistent with G-type antiferromagnetic order and suggests the presence of a structural phase transition at 100 K. Inelastic neutron scattering reveals anisotropic spin waves characterized by a gap of 16 meV, in-plane excitations with a maximum energy of 55 meV and a c-axis dispersion extending to about 35 meV. The observed magnetic excitations are well described by a J$_1$-J$_2$-J$_c$ Heisenberg Hamiltonian and the relevant exchange interactions are extracted. The results will be compared to other related materials such as BaFe$_2$As$_2$ and BaMn$_2$As$_2$. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A54.00003: Impact of inter-ladder coupling in a coupled spin-1/2 two-leg ladder Tao Hong, K.P. Schmidt, K. Coester, F.F. Awwadi, M.M. Turnbull, Y. Qiu, J.A. Rodriguez, X. Ke, C. Aoyama, Y. Takano, H. Cao, W. Tian, J. Ma, R. Custelcean, H.D. Zhou, M. Matsuda We present the zero-field specific heat and neutron scattering studies on an $S=$1/2 Heisenberg antiferromagnet to understand the nature of its spin Hamiltonian. The system is magnetically ordered below $T_{\mathrm{N}}=$2.0(1) K. The conclusion that the system is best described as coupled two-leg spin-1/2 ladders is supported by the material structure, neutron scattering measurements, and theoretical calculation. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A54.00004: Observation of a gapless linear dispersion at quantum criticality in the Ising chain ferromagnet CoNb$_2$O$_6$ in transverse field Ivelisse Cabrera, Jordan D. Thompson, Radu Coldea, Dharmalingam Prabhakaran, Robert I. Bewley, Tatiana Guidi The Ising chain in transverse field is one of the canonical paradigms for a continuous field-driven quantum phase transition between spontaneous magnetic order and a quantum paramagnet. The mechanism driving this phase transition has long been predicted to involve the closing of the spin gap, or minimum excitation energy, at the quantum critical point, where a characteristic linear dispersion is expected at low energies. We report single-crystal neutron diffraction and inelastic neutron scattering measurements that unveil how the magnetic order and excitations evolve in the very close proximity of the quantum critical point in the quasi-1D Ising chain ferromagnet CoNb$_2$O$_6$. Near criticality, we observe an essentially gapless spectrum with an almost perfectly-linear dispersion along the chain direction. Below the critical field, the frustrated interchain couplings stabilize 3D incommensurate spin-density-wave order, as observed through diffraction measurements. To our knowledge, this is the first time that essentially-gapless, linearly dispersive excitations have been observed in the very close proximity of a transverse field-tuned quantum critical point. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A54.00005: Cooperative spin decoherence in finite spin chains Fernando Delgado, Joaquin Fernandez-Rossier Overcoming the problem of relaxation and decoherence of magnetic nanostructures is one of the mayor goals in magnetic data storage. Although spin chains with as few as 12 magnetic atoms have revealed stability in cryogenic conditions [1], understanding the mechanism leading to these effects is essential for the engineered of stable structures. Here we consider the problem of spin decoherence and relaxation of finite size quantum spin chains due to elastic and spin conserving interactions with an electron gas. Specifically, we consider how the decoherence ($T_2$) and relaxation ($T_1$) times between the two degenerate ground states of a chain of $N$ coupled spins compares with the one of an isolated spin in the same environment. We find that the spin decoherence time of Ising chains can be either enhanced or suppressed depending on the matching between the Fermi wavelength $2\pi/k_F$ and the inter-spin distance $a$. In particular, we find that depending on the values of $k_Fa$, it can show, for certain values that depends on the dimensionality of the electron gas, a cooperative enhancement proportional to $N^2$ of the decoherence, analogous to super radiance decay of atom ensembles, or a suppression. [1] S. Loth et al., Science 335, 196 (2012). [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A54.00006: Magnon Hall effect in the Shastry Sutherland material Judit Romhanyi, R Ganesh We demonstrate that SrCu$_2$(BO$_3$)$_2$ (SCBO), the well known realization of the Shastry Sutherland (SS) model, in fact hosts the magnon Hall effect. The SS model has an exact dimer singlet ground state. However, the material SCBO has small Dzyaloshinskii Moriya (DM) interactions which admix a triplet component into the ground state. The resulting state has small magnetic moments and its lowest excitations are three gapped magnon modes, well described by bond wave theory. An applied magnetic field splits these modes and opens band gaps. Surprisingly, we are left with topological magnon bands with non-zero Chern numbers ($\pm 2$). Thus, SCBO supports protected magnonic edge modes and is a magnetic analogue of the integer quantum Hall effect. Ultimately, this topological character stems from the DM interactions which generate a Lorentz force for magnons. We discuss several interesting consequences and possible experimental probes. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A54.00007: Experimental Determination of Spin Glass Lower Critical Dimension Samaresh Guchhait, Raymond Orbach Zero field cooled (ZFC) measurements on thin film Ge:Mn spin glass can explore the lower critical dimension $d_l$. The correlation length $\xi(t, T)$ is nucleated upon a rapid quench into the spin glass phase, and grows to the thickness of the film, $L$, resulting in a transition for dynamics from $d=3$ to $d=2$ at a crossover time $t_{co}$. Our experiments demonstrate that conventional ZFC dynamics vanish at $t=t_{co}$, but there remain spins within a length scale $\leq L$ for which $d=3$ dynamics remain. Because of the ultrametric distribution of states, the rise of the remaining ZFC magnetization exhibits an exponential time dependence determined by the highest barrier surmounted at $t_{co}$, $\Delta_{\rm max}(t_{co}, T)$. By carefully choosing a temperature region where the dynamics fall within experimental time scales, both regimes are observed. Further, there is a direct relationship between the magnitude of $\xi(t_{co}, T)$ and $\Delta_{\rm max}({t_{co}}, T)$. This relationship is satisfied, determining the parameters controlling the growth of $\xi(t, T)$ without arbitrary parameters. The existence of the crossover establishes that $2 < d_l < 3$ for spin glass dynamics, in agreement with theory for Ising (Franz {\it et al.}) and Heisenberg (Lee and Young) spin glasses. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A54.00008: The``Higgs'' amplitude mode in weak ferromagnetic metals Yi Zhang, Paulo Farinas, Kevin Bedell Using Ferromagnetic Fermi liquid theory, Bedell and Blagoev derived the collective low-energy excitations of a weak ferromagnet. They obtained the well-known magnon (Nambu-Goldstone) mode and found a new gapped mode that was never studied in weak ferromagnetic metals. In this Letter we have identified this mode as the Higgs boson (amplitude mode) of a ferromagnetic metal. This is identified as the Higgs since it can be show that it corresponds to a fluctuation of the amplitude of the order parameter. We use this model to describe the itinerant-electron ferromagnetic material MnSi. By fitting the model with the existing experimental results, we calculate the dynamical structure function and see well-defined peaks contributed from the magnon and the Higgs. From our estimates of the relative intensity of the Higgs amplitude mode we feel that it can be seen in neutron scattering experiments on MnSi. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A54.00009: Magnetic Excitations in MnV$_{2}$O$_{4}$ Studied by Inelastic Neutron Scattering Keisuke Matsuura, Amane Uehara, Yoichi Nii, Nobuyuki Abe, Hajime Sagayama, Taka-hisa Arima, Sungdae Ji, Ryoichi Kajimoto We focus on the dynamical structure of a spin-orbital coupled system MnV$_{2}$O$_{4}$, which crystallizes in the spinel structure. Each V$^{3+}$ ion with the 3d$^{2}$ configuration is surrounded by an oxygen octahedron. The orbital degree of freedom consequently exists in the t$_{\mathrm{2g}}$ states. Below T$_{\mathrm{oo}}=$53K, the t$_{\mathrm{2g}}$ orbitals are arranged in the layered antiferroic way. Simultaneously, non-collinear ferrimagnetic ordering takes place. In this spin-orbital correlated system, in addition to conventional spin waves, orbital waves and spin-orbital coupled excitations are expected to appear. A measurement of inelastic neutron scattering on single crystals of MnV$_{2}$O$_{4}$ was carried out at 5K using a Fermi-chopper type spectrometer 4SEASONS installed at BL01, J-PARC, Japan. The dispersion of the magnetic excitations at 8-9meV have been revealed, which was only rather vaguely observed in the previous study. We have performed spin-wave calculations based on the spin Hamiltonian and compared with the experimental results in order to identify the 8-9meV modes. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A54.00010: deHaas-vanAlphen study of the ungapped Fermi surface in the spin-density-wave system GdSi D.M. Silevitch, Yejun Feng, Nayoon Woo, A.V. Suslov, J.-Q. Yan, T.F. Rosenbaum In the rare earth-intermetallic GdSi, the nested Fermi surface of the itinerant electrons induces strong interactions between local moments at the nesting vector, and the ordered local moments in turn provide the necessary coupling for a spin- density wave to form among the itinerant electrons. We examine the Fermi surface in the magnetically ordered phase through deHaas-vanAlphen magnetization measurements. Ungapped portions of the Fermi surface, consisting of tubular structures and discrete pockets, are found to span less than 5\% of the cross-sectional area of the first Brillouin zone projected along the three principal axes. The effective masses of orbiting electrons in the different regions of the Fermi surface are determined through the temperature dependence of the oscillation amplitudes. We interpret the implications of these results for magnetoresistive properties and responsiveness to pressure. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A54.00011: Hidden one-dimensional order in a three-dimensional metal Yejun Feng, Jiyang Wang, A. Palmer, B. Mihaila, J.-Q. Yan, P.B. Littlewood, T.F. Rosenbaum The rare-earth intermetallic GdSi has a spin-density-wave ground state originating from a cooperative interaction between nested itinerant spins and RKKY exchange-ordered local moments. We probe directly the stability of the SDW under pressure, using non-resonant x-ray magnetic diffraction. The incommensurate antiferromagnetic state remains unchanged up to 16.4 GPa, even though the lattice contracts by 5{\%}! Band structure calculations show that the stability can be attributed to a persistently nested portion of the Fermi surface that grows increasingly one-dimensional under pressure. This cooperatively ordered itinerant and local spin ensemble is expected to provide a stable antiferromagnetic state in thin films, even with large lattice strain and lattice mismatch, and could be suitable for spin-valve and giant magnetoresistance device applications. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A54.00012: Role of domain wall fluctuations in non-Fermi-liquid behavior of metamagnets Vladimir Zyuzin, Alexander Yu. Zyuzin In this paper we study the resistivity temperature dependence of a three-dimensional metamagnet near the metamagnetic phase transition point. The phase transition is characterized by a phase separation of regions with high and low magnetization. We show that, in the case of weak pinning, the spin relaxation time of the domain wall, which separates the two phases, is much larger than that of the volume spin fluctuations. This opens a temperature range where resistivity temperature dependence is determined by scattering of conducting electrons by the domain wall fluctuations. We show that it leads to quasi-linear low temperature dependence of resistivity. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A54.00013: Supersymmetric integrable perturbations on the lattice Liza Huijse, Christian Hagendorf, Thessa Fokkema We study a supersymmetric model that describes the multicritical point where a Kosterlitz-Thouless and Ising transition coincide. The model is integrable at the multicritical point (Fendley, Nienhuis, Schoutens, 2003). We expand this result by identifying a line in parameter space that intersects with the multicritical point for which the model is Bethe Ansatz solvable. We show that this is a lattice realization of a well-known supersymmetric integrable perturbation of the field theory describing the multicritical point. We discuss how supersymmetry manifests itself in the Bethe equations and the consequences of dynamical supersymmetry on scaling functions. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A54.00014: Complex Solitary Wave Dynamics, Pattern Formation, and Chaos in the Gain-Loss Nonlinear Schr\"odinger Equation Justin Anderson, Lincoln Carr, Mingzhong Wu Complex solitary wave dynamics, pattern formation and chaos are numerically studied in the context of spin wave envelopes in magnetic thin film active feedback rings and analogous driven damped nonlinear physical systems. Distinct dynamical behaviors of the gain-loss nonlinear Schr\"odinger equation were numerically identified during a parameter space exploration utilizing over 180\,000 core hours of simulation. Numerically identified dynamical behaviors include: spatially symmetric/asymmetric interactions of solitary wave peaks; dynamical pattern formation and recurrence, intermittency, steady state solutions and chaotically modulating bright soliton trains. Ten new dynamical behaviors, eight demonstrating long lifetimes, are predicted to be observable in experiments. [Preview Abstract] |
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