Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session F18: Frustrated Magnetism: Pyrochlore and Spin IceFocus
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Sponsoring Units: GMAG DMP Chair: Kate Ross, Colorado State University Room: 317 |
Tuesday, March 15, 2016 11:15AM - 11:27AM |
F18.00001: Diffuse neutron scattering of Dy$_2$Ti$_2$O$_7$ revisited Mikael Twengstr\"{o}m, Martin Ruminy, Juan Carlos Andresen, Marek Bartkowiak, Sean Giblin, Steven T. Bramwell, Michel J. P. Gingras, Tom Fennell, Patrik Henelius Neutron scattering is a sensitive probe of correlations in condensed matter physics, and measurements of spin correlations by diffuse neutron scattering is one of the foremost methods of constraining the Hamiltonian of spin ice materials such as Dy$_2$Ti$_2$O$_7$. Recent investigations of spin ices have highlighted the possibility of very slow equilibration at low temperature and/or structural defects in samples, effects which were not taken into account in the original parametrizations of the Hamiltonian. Hence, we have in this study performed a new set of diffuse magnetic neutron scattering experiments on an oxygen-annealed single crystal of the spin ice $^{162}$Dy$_2$Ti$_2$O$_7$. Compensation coils and an in-situ AC-susceptometer were used to control the zero magnetic field at the sample position as well as determining the thermal equilibrium of the magnetic spins respectively. In addition, we performed large--scale Monte Carlo simulations as to make a statistical fit of the dipolar spin ice model to the measured structure factor in the temperature range [0.65, 2] K, where the spin ice physics starts to develop. This will enable a most carefully controlled determination of the Hamiltonian of Dy$_2$Ti$_2$O$_7$. [Preview Abstract] |
Tuesday, March 15, 2016 11:27AM - 11:39AM |
F18.00002: Radio-frequency magnetic susceptibility of spin ice crystals Dy\(_{2}\)Ti\(_{2}\)O\(_{7}\) using tunnel diode resonator Serafim Teknowijoyo, Kyuil Cho, Makariy A. Tanatar, Ruslan Prozorov, Robert J. Cava, Jason W. Krizan Spin ice compound, Dy$_2$Ti$_2$O$_7$, has shown complex frequency - dependent magnetic behavior at low temperatures. While the DC measurements show conventional paramagnetic behavior, finite frequency susceptibility shows two regimes, - complex kagom\'e ice behavior at around 2 K and spin collective behavior above 10 K, depending on the frequency. Conventional AC susceptometry is limited to frequencies in a kHz range, but to get an insight into the possible Arrhenius activated behavior and characteristic relaxation times, higher frequencies are desired. We used self-oscillating tunnel-diode resonator (TDR) to probe magnetic susceptibility at 14.6 MHz, in the presence of a DC magnetic field and down to 50 mK. We found an unusual non-monotonic field dependence of the lower transition temperature, most likely associated with different spin configurations in a kagom\'e ice and an activated behavior of the upper transition, which has now shifted to 50 K range. This work was supported by the U.S. DOE BES MSED and was performed at the Ames Laboratory, Iowa State University under contract DE-AC02-07CH11358. The work at Princeton university was supported by DOE BES grant number DE-FG02-08ER46544. [Preview Abstract] |
Tuesday, March 15, 2016 11:39AM - 11:51AM |
F18.00003: Possible observation of photon excitations in the quantum spin-ice Pr$_{\mathrm{2}}$Zr$_{\mathrm{2}}$O$_{\mathrm{7}}$ Yoshifumi Tokiwa, Takuya Yamashita, Daiki Terazawa, Takahito Terashima, Kenta Kimura, Mario Halim, Satoru Nakatsuji, Yuji Matsuda It has been theoretically shown that the ground state of spin-ice system with quantum fluctuations can be quantum spin liquid, where new elementary excitations, photon, emerge [1]. In the rare-earth pyrochlore, Pr$_{\mathrm{2}}$Zr$_{\mathrm{2}}$O$_{\mathrm{7}}$, which contains spin-ice correlations with significant quantum fluctuations, the absence of magnetic ordering even at very low temperature suggests formation of quantum spin liquid state [2]. In order to examine the emergence of new exotic excitations, we have performed low-temperature thermal conductivity ($\kappa )$ measurements of Pr$_{\mathrm{2}}$Zr$_{\mathrm{2}}$O$_{\mathrm{7}}$. Interestingly, our data of $\kappa $/T shows a steep increase with decreasing temperature below 0.2K. Since the monopole density is negligibly small at such low temperature, the steep increase possibly indicates emergence of new elementary excitations. Anomalous magnetic-field dependence of $\kappa $/T observed below 0.2K further supports this possibility. [1] M. Hermele et al., Phys. Rev. B 69, 064404 (2004). [2] K. Kimura et al., Nature Commun. 4, 1934 (2013). [Preview Abstract] |
Tuesday, March 15, 2016 11:51AM - 12:03PM |
F18.00004: Thermal transport measurements of spin ice materials William Toews, Jennifer Reid, Rafeal Nadas, Stefan Kycia, Timothy Munsie, Hanna Dabkowska, Bruce Gaulin, Robert Hill Extensive thermal conductivity measurements have been conducted on several rare-earth titanate materials. We report the consequences of crystalline quality and magnetic impurities on the mobility and dynamics of delocalized magnetic excitations. Detailed x-ray diffraction measurements have also been conducted on these samples to accurately characterize the sample quality. Differences between the various materials measured are also discussed. [Preview Abstract] |
Tuesday, March 15, 2016 12:03PM - 12:15PM |
F18.00005: \textbf{Atomic Structure Study of the Quantum Spin-ice Pyrochlore Yb}$_{\mathrm{\mathbf{2}}}$\textbf{Ti}$_{\mathrm{\mathbf{2}}}$\textbf{O}$_{\mathrm{\mathbf{7}}}$ Ali Mostaed, Geetha Balakrishnan, Martin Lees, Richard Beanland The quantum spin-ice candidate \textit{Yb}$_{2}$\textit{Ti}$_{2}O_{7}$ (\textit{YTO}) lies on the boundary between a number of competing magnetic ground states. Features in the low-temperature specific heat capacity are found to vary in sharpness and temperature depending on materials processing. It has been suggested that these changes in the magnetic ground state could be influenced by several factors, including the degree of cation stuffing, changes in oxygen occupancy and/or vacancies. In the present work, the structures of three different \textit{YTO} samples, grown by the optical floating zone technique and that exhibit quite different heat capacity behaviour, have been studied by annular dark field scanning transmission microscopy (ADF-STEM). We show that the detailed intensity distribution around the visible atomic columns is sensitive to the presence of nearby atoms of low atomic number (in this case oxygen), even though they are not directly visible in the images. To the best of our knowledge, this is the first time that oxygen columns with a distance of \textasciitilde 30 pm have been distinguished in ADF-STEM images. Furthermore, by comparing atomic columns with different configurations of nearby oxygen atoms, we are able to distinguish between the different \textit{YTO} samples Finally, the ADF data for the crystal that exhibits no specific heat anomaly shows signs of the substitution of \textit{Yb} atoms on \textit{Ti} sites, supporting the view that the magnetic ground state of \textit{YTO} is extremely sensitive to disorder. [Preview Abstract] |
Tuesday, March 15, 2016 12:15PM - 12:27PM |
F18.00006: Impact of Stoichiometry of Yb$_{\mathrm{2+}}_{x}$Ti$_{\mathrm{2-}}_{x}$O$_{\mathrm{7-}}_{x}_{\mathrm{/2}}$ on its Structure and Physical Properties Kathryn Arpino, Benjamin Trump, Tyrel McQueen, Collin Broholm, Seyed Koohpayeh The rare-earth pyrochlores ($R_{\mathrm{2}}M_{\mathrm{2}}$O$_{\mathrm{7}})$ are topic of intense study in the field of magnetism as an ideal host for geometric frustration including spin-liquid and spin-ice behaviour. Specifically, $R_{\mathrm{2}}$Ti$_{\mathrm{2}}$O$_{\mathrm{7\thinspace }}$has proved a rich playground: compounds $R \quad =$ Ho and Dy are classical spin ices, $R \quad =$ Tb has been shown to be a spin liquid at low temperatures, and $R \quad =$ Yb is a candidate quantum spin ice. This system is attractive for the large anisotropic magnetic properties of rare earth ions, Ti$^{\mathrm{4+}}$'s lack of magnetic moment which isolates the magnetic ordering of $R^{\mathrm{3+}}$, and the comparative ease of making single crystals via the floating zone technique. This talk will present the structure and physical properties of a Yb$_{\mathrm{2+}}_{x}$Ti$_{\mathrm{2-}}_{x}$O$_{\mathrm{7-}}_{x}_{\mathrm{/2}}$ series including both the pure and stuffed samples. The series shows a dramatic change in the low-temperature (50-200 mK) specific heat signature upon doping away from the pure compound. Understanding the magnetic and physical properties of the off-stoichiometric series sheds light both on the magnetic ordering of the ideal spin ice candidate compound as well as aids in evaluating the quality and stoichiometry of a measured sample. In this vein, proper single-crystal growth conditions in order to ensure single crystals of appropriate stoichiometry will also be discussed. [Preview Abstract] |
Tuesday, March 15, 2016 12:27PM - 12:39PM |
F18.00007: Magnetic monopole condensation transition out of quantum spin ice: application to Pr2Ir2O7 and Yb2Ti2O7 Gang Chen We study the proximate magnetic orders and the related quantum phase transition out of quantum spin ice (QSI). We apply the electromagnetic duality of the compact quantum electrodynamics to analyze the condensation of the magnetic monopoles for QSI. The monopole condensation transition represents a unconventional quantum criticality with unusual scaling laws. The magnetic monopole condensation leads to the magnetic states that belong to the “2-in 2-out” spin ice manifold and generically have an enlarged magnetic unit cell. We demonstrate that the antiferromagnetic state with the ordering wavevector Q = 2p(001) is proximate to QSI while the ferromagnetic state with the ordering wavevector Q = (000) is not proximate to QSI. This implies that if there exists a direct transition from QSI to the ferromagnetic state, the transition must be strongly first order. We apply the theory to the puzzling experiments on two pyrochlore systems Pr2Ir2O7 and Yb2Ti2O7. [Preview Abstract] |
Tuesday, March 15, 2016 12:39PM - 12:51PM |
F18.00008: How quantum are classical spin ices? Michel J P Gingras, Jeffrey G Rau The pyrochlore spin ice compounds Dy$_2$Ti$_2$O$_7$ and Ho$_2$Ti$_2$O$_7$ are well described by classical Ising models down to low temperatures. Given the empirical success of this description, the question of the importance of quantum effects in these materials has been mostly ignored. We argue that the common wisdom that the strictly Ising moments of non-interacting Dy$^{3+}$ and Ho$^{3+}$ ions imply Ising interactions is too naive and that a more complex argument is needed to explain the close agreement between the classical Ising model theory and experiments. By considering a microscopic picture of the interactions in rare-earth oxides, we show that the high-rank multipolar interactions needed to induce quantum effects in these two materials are generated only very weakly by superexchange. Using this framework, we formulate an estimate of the scale of quantum effects in Dy$_2$Ti$_2$O$_7$ and Ho$_2$Ti$_2$O$_7$, finding it to be well below experimentally relevant temperatures. [Preview Abstract] |
Tuesday, March 15, 2016 12:51PM - 1:03PM |
F18.00009: Disorder-Induced Quantum Spin Liquid in Spin Ice Pyrochlores Lucile Savary, Leon Balents We discuss disorder in spin ice materials, and in particular in compounds with non-Kramers magnetic ions. We show that in the minimal relevant model, disorder succeeds in inducing a long-range entangled Coulombic quantum spin liquid phase. The phase diagram also contains an analog of the Mott glass state, envisioned in dirty boson systems with particle-hole symmetry. We discuss the relevance of our results to the material Pr$_2$Zr$_2$O$_7$, and how these ideas might be applied to convert a classical spin ice to a quantum one. [Preview Abstract] |
Tuesday, March 15, 2016 1:03PM - 1:15PM |
F18.00010: Low-temperature Spin-Ice State of Quantum Heisenberg Magnets on Pyrochlore Lattice Yuan Huang, Kun Chen, Youjin Deng, Nikolay Prokof'ev, Boris Svistunov We establish that the isotropic spin-1/2 Heisenberg antiferromagnet on pyrochlore lattice enters a spin-ice state at low, but finite, temperature. Our conclusions are based on results of the bold diagrammatic Monte Carlo simulations that demonstrate good convergence of the skeleton series down to temperature T $=$ J/6. The ``smoking gun'' identification of the spin-ice state is done through a remarkably accurate microscopic correspondence for static spin-spin correlation function between the quantum Heisenberg and classical Heisenberg/Ising models at all accessible temperatures. In particular, at T/J $=$ 1/6, the momentum dependence shows a characteristic bow-tie pattern with pinch points. By numerical analytical continuation method, we also obtain the dynamic structure factor at real frequencies, showing a diffusive spinon dynamics at pinch points and spin wave continuum along the nodal lines.? [Preview Abstract] |
Tuesday, March 15, 2016 1:15PM - 1:27PM |
F18.00011: Antiferromagnetic order in the pyrochlores R$_{2}$Ge$_{2}$O$_{7}$ (R = Er, Yb) Zhiling Dun, Xiang Li, Rafael Freitas, Everton Arrighi, Clarina Cruz, Minseong Lee, Eun Sang Choi, Huibo Cao, Harlyn Silverstein, Chris Wiebe, Jinguang Chen, Haidong Zhou Elastic neutron scattering, ac susceptibility, and specific heat experiments on the pyrochlores Er$_{2}$Ge$_{2}$O$_{7}$ and Yb$_{2}$Ge$_{2}$O$_{7}$ show that both systems are antiferromagnetically ordered in the $\Gamma_5$ manifold. The ground state is a $\psi_{3}$ phase for the Er sample and a $\psi_{2}$ or $\psi_{3}$ phase for the Yb sample, which suggests ``Order by Disorder"(ObD) physics. Furthermore, we unify the various magnetic ground states of all known R$_{2}$X$_{2}$O$_{7}$ (R = Er, Yb, X = Sn, Ti, Ge) compounds through the enlarged XY type exchange interaction $J_{\pm}$ under chemical pressure. The mechanism for this evolution is discussed in terms of the phase diagram proposed in the theoretical study [Wong et al., Phys. Rev. B 88, 144402, (2013)]. [Preview Abstract] |
Tuesday, March 15, 2016 1:27PM - 1:39PM |
F18.00012: Theory for magnetic excitations in quantum spin ice Shigeki Onoda, Trinanjan Datta Magnetic excitations in magnetic rare-earth pyrochlore oxides called quantum spin ice (QSI) systems such as Yb$_2$Ti$_2$O$_7$, Pr$_2$Zr$_2$O$_7$, and Tb$_2$Ti$_2$O$_7$ have attracted great interest for possible observations of the quantum dynamics of spin ice monopoles and emergent photon excitations. However, their spectral properties remain open especially for cases relevant to experimental systems. Here, we develop a theoretical framework that incorporates gauge fluctuations into a modified gauge mean-field approach, so that it reproduces key features of recent quantum Monte-Carlo results on the double broad specific heat in the simplest QSI model and can describe a continuous growth of a coherence in gauge-field correlations on cooling down to Coulomb-phase ground states. Using this new approach, we provide a theory for magnetic neutron-scattering spectra. It is found that spin-flip exchange interactions produce dispersive QSI monopole excitations which create a particle-hole continuum neutron-scattering spectrum. Gauge fluctuations give multi-particle contributions to the spectrum, which will be possibly detected in Higgs phases. [Preview Abstract] |
Tuesday, March 15, 2016 1:39PM - 1:51PM |
F18.00013: Ubiquitous Magnetic Excitations in the Ytterbium Pyrochlores Alannah Hallas, Jonathan Gaudet, Nicholas Butch, Makoto Tachibana, Rafael Freitas, Chris Wiebe, Graeme Luke, Bruce Gaulin The ytterbium pyrochlores, Yb$_{\mathrm{2}}$B$_{\mathrm{2}}$O$_{\mathrm{7}}$ (B $=$ Sn, Ti, Ge) are well described in terms of S$_{\mathrm{eff}}=$1/2 quantum spins with local XY anisotropy, decorating the cubic pyrochlore lattice and interacting via anisotropic exchange. While structurally only the non-magnetic B-site cation, and hence, primarily the lattice parameter, is changing across the series Yb$_{\mathrm{2}}$B$_{\mathrm{2}}$O$_{\mathrm{7}}$ (B $=$ Sn, Ti, Ge), a range of magnetic behavior is observed. The low temperature magnetism in Yb$_{\mathrm{2}}$Ti$_{\mathrm{2}}$O$_{\mathrm{7}}$ and Yb$_{\mathrm{2}}$Sn$_{\mathrm{2}}$O$_{\mathrm{7}}$ has ferromagnetic character. Conversely, Yb$_{\mathrm{2}}$Ge$_{\mathrm{2}}$O$_{\mathrm{7}}$ displays an antiferromagnetically ordered Neel state at low temperatures. We present a comparative analysis of the spin dynamic properties of these three systems using inelastic neutron scattering. While the static properties of the ytterbium pyrochlores are distinct from one another, we find a ubiquitous character to the spin dynamics. The inelastic scattering for each of these ytterbium pyrochlores show a gapless continuum of spin excitations, that tends to resemble over-damped ferromagnetic spin waves at low Q. Furthermore, the specific heat for each of these materials follows a common form with a broad, high-temperature anomaly followed by a sharp low-temperature anomaly. We find that the dynamic properties correlate strongly with the broad specific heat anomaly but remain unchanged across the sharp, low temperature specific heat anomaly. [Preview Abstract] |
Tuesday, March 15, 2016 1:51PM - 2:03PM |
F18.00014: Ferromagnetic correlations in Yb$_2$Ti$_2$O$_7$ as revealed by small angle neutron scattering techniques Connor Buhariwalla, Qianli Ma, Lisa Debeer-Schmitt, Hanna Dabkowska, Bruce Gaulin We report low temperature SANS measurements on frustrated S$_\text{eff}$=1/2 XY pyrochlore magnet Yb$_2$Ti$_2$O$_7$[1]. The ground state of this material has been proposed as a realization of a quantum spin ice; however, the low temperature phase behaviour has been complicated by sample dependencies believed to be related to weak ``stuffing" [2]. Our SANS study focuses on the low Q structure of elastic ``rods'' of magnetic scattering which extend from Q=0 along the 111 direction. Using a single crystal sample, we characterize the low Q ($<$0.2\AA$^{-1}$) temperature dependence of this structured diffuse scattering intensity to T=30mK, passing through the enigmatic heat capacity anomaly near Tc=200mK. The temperature dependence of this diffuse scattering near Q=0 is largely consistent with that measured previously near 111 [3]. [1] K.A. Ross et al., Phys. Rev. X 1, 021002 (2011) [2] K.A. Ross et al., Phys. Rev. B 86, 174424 (2012) [3] K.A. Ross et al., Phys. Rev. B 84, 174442 (2011) [Preview Abstract] |
Tuesday, March 15, 2016 2:03PM - 2:15PM |
F18.00015: Extended spin ice Jeffrey G. Rau, Michel J. P. Gingras We introduce a new classical spin liquid on the pyrochlore lattice which we call `extended spin ice'. The ground state manifold of this model is extensive and characterized by a set of local rules that extend the conventional 2-in/2-out spin ice condition. This includes the ice states in addition to a complex set of tree structures built from 3-in/1-out, 3-out/1-in and all-in/all-out tetrahedra. Under local dynamics this model freezes heterogeneously at low temperatures into a `spin slush', with extremely slow relaxation for some spins while other spin clusters fluctuate quickly. In addition to this dynamical heterogeneity, distinctive spherical patterns in the spin correlations serve as a further signature. Possible applications to materials as well the effects of transverse quantum exchange will be discussed. [Preview Abstract] |
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