Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Y31: Focus Session: Neutron Scattering |
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Sponsoring Units: GMAG Chair: Jason Lashley, Los Alamos National Laboratory Room: 335 |
Friday, March 20, 2009 8:00AM - 8:12AM |
Y31.00001: Anomalous spin-waves in triangular lattice antiferromagnets Mike Zhitomirsky, Sasha Chernyshev The distinct features of spin-wave excitations in the triangular-lattice antiferromagnet are (i) finite lifetime at zero temperature due to spontaneous two-magnon decays, (ii) strong renormalization of magnon energies $\varepsilon_{\bf k}$ with respect to the harmonic result, and (iii) logarithmic singularities in the decay rate $\Gamma_{\bf k}$. Detailed quantitative results are obtained for the magnon spectrum of the spin-1/2 model using both the on-shell and off-shell solutions of the Dyson's equation. In the low-energy limit magnons remain well-defined excitations but with anomalous decay rates. At high energies, magnons are heavily damped with decay rates reaching $(2\Gamma_{\bf k}/\varepsilon_{\bf k})\sim 0.3$. The on-shell solution shows logarithmic singularities in $\Gamma_{\bf k}$ with the concomitant jump-like discontinuities in Re[$\varepsilon_{\bf k}$] along certain contours in the momentum space. Such singularities are even more prominent in the magnon spectral function $A({\bf k},\omega)$. Although the off-shell solution removes such log-singularities, the decay rates remain strongly enhanced. We also discuss the role of higher-order corrections and show that such singularities may lead to complete disappearance of the spectrum in the vicinity of certain ${\bf k}$-points. We conclude that magnon decays and singularities must be prominent in a wide class of noncollinear antiferromagnets. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y31.00002: Two Dimensional Correlations and Field Induced Order in the Pyrochlore Ferromagnet Yb$_{2}$Ti$_{2}$O$_{7}$ Kate Ross, J.P.C. Ruff, B.D. Gaulin, C.P. Adams, J.S. Gardner, H.A. Dabkowska, Y. Qiu, J.R.D. Copley The rare earth pyrochlore magnets are of intense current interest due to their often unconventional magnetic behaviour. The pyrochlore lattice, which is occupied by the magnetic ions in such materials, is the 3D archetype for geometric frustration. We have studied the frustrated pyrochlore magnet, Yb$_{2}$Ti$_{2} $O$_{7}$, in single crystal form using time-of-flight neutron scattering. Our study confirms the presence of diffuse rods of scattering, which indicate unexpected 2D magnetic correlations in this cubic system. The diffuse scattering is measured both above and below a previously reported first order transition at T$_c$ = 240mK. Although rod- like scattering persists below T$_c$, three dimensional correlations develop, indicating a build-up of interplane correlations. Yet Yb$_{2}$Ti$_{2}$O$_{7}$ continues to resist long range order down to $\sim$50mK. We have discovered, however, that the application of a small magnetic field along the [110] direction readily induces a long range ordered magnetic state, as evidenced by well defined spin wave excitations. We will present a magnetic phase diagram for Yb$_ {2}$Ti$_{2}$O$_{7}$ based on the results of this study. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y31.00003: Beyond Simple Bilayers in the Triangular Lattice Dimer compound Ba$_{3}$Mn$_{2}$O$_{8}$ Matthew Stone, Mark Lumsden, Sung Chang, Eric Samulon, Cristian Batista, Kirrily Rule, Eric Ressouche, Beatrice Grenier, Ian Fisher We present single crystal inelastic neutron scattering measurements of the $S=1$ dimerized quasi-twodimensional antiferromagnet Ba$_{3}$Mn$_{2}$O$_{8}$. The singlet-triplet dispersion reveals nearest-neighbor and nextnearest- neighbor ferromagnetic interactions between adjacent bilayers that compete against each other. Although the interbilayer exchange is comparable to the intrabilayer exchange, this additional frustration reduces the effective coupling along the $c$ axis and leads to a quasi-two-dimensional behavior. We also present magnetic field-dependent neutron scattering measurements examining long range order and excitations within the field-induced ordered phases in Ba$_{3}$Mn$_{2}$O$_{8}$. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 9:12AM |
Y31.00004: Spin waves and quantum criticality in the frustrated XY pyrochlore antiferromagnet Er$_2$Ti$_2$O$_7$ Invited Speaker: Jacob Ruff Geometrically frustrated magnetism in the three dimensional pyrochlore lattice often gives rise to unconventional magnetic states at low temperatures. The effective elimination of leading energetic terms via geometric cancellation allows weak perturbations to ultimately determine the behaviour of a magnetic material. In this talk, I will describe the manifold of low energy states in the antiferromagnetic XY pyrochlore Er$_2$Ti$_2$O$_7$, as measured by the time-of-flight neutron scattering technique. In zero applied magnetic field, the ground state is purported to be selected either by unusual anisotropic interactions, or via thermal and/or quantum fluctuations in an example of order-by-disorder [1,2]. Recently, we have shown that this state consists of coexisting short and long range orders[3], and that both orders can be destroyed by the application of modest magnetic fields [3]. This disordering of the magnetic system involves a seemingly continuous quantum critical point at $\mu_{0}H_c \sim$ 1.6 Tesla [3]. The properties of the induced quantum paramagnetic state, and the dependence of these properties on the direction of applied field will be discussed. I will also make comparisons with a recent theoretical treatment of Er$_2$Ti$_2$O$_7$ [2]. \\[4pt] [1] Champion et al. Phys. Rev. B. 68, 020401 (2003)\\[0pt] [2] McClarty et al. arXiv:0810.2483v2\\[0pt] [3] Ruff et al. Phys. Rev. Lett. 101, 147205 (2008) [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y31.00005: Recent neutron scattering results from Gd-based pyrochlore oxides Jason Gardner In my presentation I will present recent results that have determined the spin-spin correlations in the geometrically frustrated magnets Gd$_{2}$Sn$_{2}$O$_{7}$ and Gd$_{2}$Ti$_{2}$O$_{7}$. This will include polarised neutron diffraction, inelastic neutron scattering and neutron spin echo data. One sample of particular interest is Gd$_{2}$Sn$_{2}$O$_{7}$ which is believed to be a good approximation to a Heisenberg antiferromagnet on a pyrochlore lattice with exchange and dipole-dipole interactions. Theoretically such a system is expected to enter long range ordered ground state known as the ``Palmer Chalker'' state [1]. We show conclusively, through neutron scattering data, that the system indeed enters an ordered state with the Palmer-Chalker spin configuration below T$_{c}$ = 1 K [2-3]. Within this state we have also observed long range collective spin dynamics, spin waves. This work has been performed in collaboration with many research groups including G. Ehlers (SNS), R. Stewart (ISIS). \\[0pt] [1] S. E. Palmer and J. T. Chalker, Phys. Rev. B 62, 488 (2000). \\[0pt] [2] J. R. Stewart, G. Ehlers, A. S. Wills, S. T. Bramwell, and J. S. Gardner, J. Phys.: Condens. Matter 16, L321 (2004). \\[0pt] [3] J R Stewart, J S Gardner, Y. Qiu and G Ehlers, Phys. Rev. B. 78, 132410 (2008) [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y31.00006: Electric Control of Spin Chirality in Multiferroic Ni3V2O8 Ivelisse Cabrera, Michel Kenzelmann, Gavin Lawes, Oksana Zaharko, Collin Broholm We discuss electric control of spin chirality in the magnetically frustrated multiferroic Ni$_3$V$_2$O$_8$ through polarized magnetic neutron diffraction. Cooling to the cycloidal magneto-electric phase in an electric field $\mathbf{E}$ causes the incommensurate Bragg reflections to become neutron spin polarizing, the sense of neutron polarization reversing with $\mathbf{E}$. Comprehensive polarized neutron diffraction measurements establish the chiral nature of the long range ordered spin configuration and its response to $\mathbf{E}$. Concomitant evolution of chiral and ferroelectric domains is observed by comparing polarized neutron diffraction data to pyroelectric current measurements and hysteresis under different poling conditions [1]. \newline \newline [1] I. Cabrera, M. Kenzelmann, G. Lawes, Y. Chen, W. C. Chen, R. Erwin, T. R. Gentile, J. Leao, J. W. Lynn, N. Rogado, R. J. Cava, and C. Broholm, (unpublished), 2008. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y31.00007: CEF groundstate of the frustrated quantum magnets SrRE$_2$O$_4$ (RE= Dy, Ho) A. Desilets-Benoit, A. D. Bianchi, V. Pomjakushin, B. R. Hansen, M. Kenzelmann, R. J. Cava We have measured the crystalline electric field (CEF) niveaus of the magnetic ions in the frustrated quantum magnets SrRE$_2$O$_4$ with RE = Dy and Ho by inelastic neutron scattering. SrRE$_2$O$_4$ crystallizes in a $pnam$ structure, which as four in-equivalent rare earth sites, leading to a large degree of geometrical frustration. Fitting a CEF level scheme to the experimental data has allowed us to determine the CEF ground state of this system. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:24AM |
Y31.00008: Dynamics of excitations in a one-dimensional Bose liquid Invited Speaker: Maxim Khodas The dynamical structure factor $S(q,\omega)$ of interacting 1D liquid is studied. This quantity has become experimentally accessible in the recent experiments on cold atoms and neutron scattering off spin chains. We find power law non-analyticities $S(q,\omega) \propto \left(\omega - \epsilon_{1,2}(q) \right)^{-\mu(q)}$ at the kinematical thresholds characterized by the momentum dependent exponents evauated in a broad range of parameters. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y31.00009: Magnetic properties of LuFe$_{2}$O$_{4+\delta }$ Fan Wang, Jungho Kim, G. Xu, S.M. Shapiro, G.D. Gu, Y. Lee, Y.-J. Kim LuFe$_{2}$O$_{4+\delta }$ (LFO) has been drawing much attention as a potential multiferroic compound. We show that the oxygen stoichiometry plays an important role in determining the magnetic properties of LFO. The sample with excess oxygen shows two magnetic transitions at 236K and 228K. The transition at 236K is a paramagnetic to ferrimagnetic transition, and below 228 K the system enters a spin glass phase. The DC magnetization shows strong time and history dependence, while the AC susceptibility exhibits dynamic scaling behaviour similar to that of canonical spin glass systems. This spin glass behaviour seems to disappear in the more stoichiometric sample. In this second sample, long range ferrimagnetic ordering has been observed with neutron scattering experiments. Our x-ray scattering on single crystal and x-ray powder diffraction experiments both show there is a structural change around 170K, which seems to be responsible for an observed low field anomaly. When a small magnetic field is applied, magnetization is only turned on during heating the sample. With large applied field, a metamagnetic transition was observed. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y31.00010: Phonon renormalization of the N\`{e}el transition in KCuF$_{3}$ J.C.T. Lee, S. Yuan, A. Rusydi, S. Smadici, S.L. Cooper, E. Fradkin, P. Abbamonte Critical magnetic fluctuations in the one-dimensional antiferromagnet KCuF$_3$, in the form of diffuse scattering around the magnetic (001) Bragg peak, have been studied with resonant soft x-ray scattering. Using x-rays near the Cu L$_{3}$ edge to exploit the 2\textit{p} $\to$ 3\textit{d} dipole transition, the (001) was directly observed at temperatures ranging from 23K to above the transition temperature (T$_{N}$ $\approx$ 43K). Notably, the phase transition exhibits hysteresis, with T$_{N}$ sensitive to whether the sample was cooled or heated prior to measurement. This suggests that the phase transition is weakly first order, as might be expected by a transition renormalized by phonons. The temperature dependence of the coherence length and the diffuse scattering, as well as the role played by Jahn-Teller phonons in the transition are discussed. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y31.00011: The influence of magnetic field on Cu(tn)Cl$_{2}$ - two-dimensional quantum magnet with N\'{e}el ground state. A. Orendacova, M. Orendac, L. Sedlakova, A. Feher, K. Siemensmeyer, A. Buchsteiner, J. S. Xia, L. Yin, M.W. Meisel AC susceptibility and neutron elastic scattering experiments were performed on Cu(tn)Cl$_{2}$ [1] to explore the origin of a field induced anomaly previously observed in specific heat below 1 K and in finite fields up to 7 T [2]. Isothermal AC susceptibility scans, made down to 40 mK and in fields up to 10 T, confirmed and extended the B vs. T phase diagram obtained from the specific heat data. Neutron spectra of a polycrystalline sample studied down to 0.5 K in B = 0 and 4 T did not reveal any magnetic peaks expected below 1 K and indicated a slight change of (200) nuclear peak intensity induced by the magnetic field. The possible reasons of the absence of a phase transition to ordered state will be discussed. [1] V. Zelenak \textit{et al.}, Inorg. Chem. \underline {45} (2006) 1774. [2] A. Orendacova \textit{et al.}, in preparation. [Preview Abstract] |
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