Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session U14: Focus Session: Quantum Dynamics in Spin Ice |
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Sponsoring Units: GMAG DMP Chair: Satoru Nakatsuji, University of Tokyo Room: 316 |
Thursday, March 21, 2013 11:15AM - 11:51AM |
U14.00001: Effective S$=$1/2 Hamiltonians and the Quantum Spin Ice Ground State of Yb$_{2}$Ti$_{2}$O$_{7}$ Invited Speaker: Bruce D. Gaulin New neutron scattering instrumentation offers unprecedented opportunities for mapping out the full dispersion and dynamic susceptibility of magnetic materials. In turn, these measurements can be exploited to determine their microscopic spin Hamiltonians in great detail. We've used these techniques to examine the exotic quantum spin ice ground state of Yb$_{2}$Ti$_{2}$O$_{7}$, a pyrochlore magnet, which can be thought of in terms of spins decorating a network of corner-sharing tetrahedra. In this environment, Yb$^{\mathrm{3+}}$displays a ground state crystal field doublet which is very well separated from its excited states, resulting in an effective S$=$1/2 description for the Yb moments. It's positive Curie-Weiss constant of $\sim$ 0.5 K indicates net ferromagnetic interactions and it displays a g-tensor with XY anisotropy. However strong spin orbit effects give rise to an anisotropic exchange Hamiltonian, which can be understood in quantitative detail by modeling time-of-flight neutron scattering in a high field polarized state with spin wave theory using anisotropic exchange. The resulting Hamiltonian shows strong coupling between local z-components of spin, as in spin ice, but also substantial terms that encourage quantum fluctuations. Armed with the microscopic spin Hamiltonian, the mean field phase diagram and a range of physical properties can be calculated and compared with experiment. We see that any possible ordering is strongly suppressed relative to mean field theory by the presence of geometrical frustration, quantum fluctuations, or both; and the low temperature bulk properties are well accounted for by the effective S$=$1/2 Hamiltonian we determine. \\[4pt] [1] K.A. Ross, L. Savary, B.D. Gaulin and L. Balents, Phys. Rev X, 1, 021022, 2011. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U14.00002: Spin Liquid Regimes at Nonzero Temperature in Quantum Spin Ice: Extension to Finite Temperature of the Phase Diagram of Pyrochlore Magnets Lucile Savary, Leon Balents Many quantum spin liquid theories described so far have not yet benefitted of much attention as regards how they should be interpreted at finite temperature. With growing interest in quantum spin liquid phases and increasingly many material candidates, it is becoming all the more imperative to tackle this matter. Here, we address the finite temperature properties of quantum spin ices, for which quantum spin liquid regimes have been predicted. In particular, we extend to finite temperature the two-dimensional phase diagram found in [PRL 108, 037202 (2012)] using an extension of the gauge mean field theory first introduced in the aforementioned paper. We find that the {\sl quantum} spin liquid features of the $U(1)$ QSL and Coulomb Ferromagnet survive at nonzero temperature and that a first order transition to an entropy-dominated classical spin liquid regime, similar to the classical spin ice liquid, occurs at temperatures lower than a na\"{i}ve scaling with the strength of the interactions might predict. We discuss our results in light of recent experiments on Yb$_2$Ti$_2$O$_7$, where features reminiscent of the well-known classical spin ice phase were reported. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U14.00003: Time domain terahertz study of quantum spin ice Yb$_{2}$Ti$_{2}$O$_{7}$ LiDong Pan, Yuan Wan, Chris M. Morris, Kate A. Ross, S.M. Koohpayeh, Bruce D. Gaulin, Oleg Tchernyshyov, N. Peter Armitage We report the time domain terahertz spectroscopy study of the quantum spin ice material Yb$_{2}$Ti$_{2}$O$_{7}$. Temperature and magnetic field dependence of the transmission spectrum was obtained. Several spin resonance absorption peaks are observed in magnetic field. The results are discussed in comparison with the recently proposed theory of the quantum string excitations in this material. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U14.00004: Hidden order in Yb2TI2O7 Robert D'Ortenzio, Hanna Dabkowska, Sarah Dunsiger, Tatsuo Goko, Jan Kycia, Lian Liu, Teresa Medina, Timothy Munsie, David Pomaranski, Kate Ross, Yasutomo Uemura, Travis Williams, Graeme Luke We report low temperature specific heat and positive muon spin rotation ($\mu$-SR) measurements of both polycrystal and single crystal Yb$_2$Ti$_2$O$_7$. Our zero field (ZF) $\mu$-SR shows little spin relaxation temperature dependence in the polycrystal Yb$_2$Ti$_2$O$_7$, contrast to previously reported results. We observe no collinear ferromagnetic order, rather a hidden order ground state where spin fluctuations remain dynamic down to 16 mK. Single crystal Yb$_2$Ti$_2$O$_7$ zero field $\mu$-SR measurements with the crystallographic $<$111$>$ direction parallel to the initial muon polarization show small but measurable temperature dependence. In addition, our transverse field (TF) $\mu$-SR measurements show the spin susceptibility undergoes a distinct change at temperatures corresponding to the magnetic transition measured in the specific heat. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U14.00005: Single crystals of Yb2Ti2O7 grown by the Optical Floating Zone technique: naturally ``stuffed'' pyrochlores? Kate Ross, Thomas Proffen, Hanna Dabkowska, Jeffery Quilliam, Luke Yaraskavitch, Jan Kycia, Bruce Gaulin In the ``quantum spin ice'' pyrochlore material Yb2Ti2O7, Yb3+ ions are coupled to each other via Ising-like ferromagnetic exchange, creating a situation similar to the highly frustrated classical spin ice compounds, but with significant quantum fluctuations. The ground state of the model resides near two exotic and disordered ``quantum spin liquid'' phases. The experimentally observed ground state of Yb2Ti2O7 is, however, controversial in the literature. Most samples, except one crystal which orders ferromagnetically, show disordered states with varying properties. The controversy is likely to be related to the presence of structural defects of an unspecified type that are known to cause sample-dependence of the low temperature specific heat, particularly in the single crystal samples. Using neutron powder diffraction, we investigated one pulverized single crystal of Yb2Ti2O7 grown by the standard Optical Floating Zone method, and found evidence that 2.3\% excess Yb3+ ions reside on the non-magnetic Ti4+ sites, despite perfect stoichiometry of the starting material. This type of defect lattice is known as a ``stuffed'' pyrochlore structure. The effect of the stuffed spins is an open question which can now be investigated in detail. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U14.00006: Yb$_{2}$Sn$_{2}$O$_{7}$: a quantum critical point approaching the ferromagnetic ordering from the quantum spin liquid side Zhiling Dun, Haidong Zhou, Alannah Hallas, Harlyn Silverstein, Yiming Qiu, John Copley, Jason Gardner, Eunsang Choi, Christopher Wiebe The neutron scattering measurements on pyrochlore Yb$_{2}$Sn$_{2}$O$_{7}$ show no long range ordering down to 0.05 K but appearance of diffuse scattering, low energy spin wave excitations, and temperature-independent relaxation time below 2 K, which indicate the system enters a quantum dynamics region with ferromagnetic interactions. The AC susceptibility further shows that Yb$_{2}$Sn$_{2}$O$_{7}$ enters a ``spin freezing'' region below 0.14 K. Our results suggest that Yb$_{2}$Sn$_{2}$O$_{7}$ sits on a quantum critical point by approaching the ferromagnetic ordering from the spin liquid side. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U14.00007: Not so accidental degeneracies: origin of dimensional-reduction in the Quantum Spin Ice Yb$_2$Ti$_2$O$_7$ Ludovic Jaubert, Han Yan, Owen Benton, Nic Shannon Despite being the best-characterised example of a ``quantum spin ice'' [1], Yb$_2$Ti$_2$O$_7$ remains an enigma. One of its most striking, and puzzling, features are the diffuse, rod-like structures seen in quasi-elastic neutron scattering [2]. These suggest that spin fluctuations in Yb$_2$Ti$_2$O$_7$ decouple into independent Kagome planes, even though magnetic ions occupy a fully three-dimensional pyrochlore lattice [3]. Here, we use a combination of lattice gauge theory, spin-wave calculations and Monte Carlo simulation, to show how the dimensional-reduction seen in Yb$_2$Ti$_2$O$_7$ follows from a two-dimensional branch of excitations ``inherited'' from a nearby phase transition. This analysis sheds new light on ground state selection in a wide range of rare-earth pyrochlore oxides, including the model ``order-by-disorder'' system Er$_2$Ti$_2$O$_7$.\\[4pt] [1] Ross, Savary, Gaulin \& Balents, Phys. Rev. X {\bf 1}, 021002 (2011)\\[0pt] [2] Hodges \textit{et al.}, Phys. Rev. Lett. {\bf 88}, 077204 (2002)\\[0pt] [3] Ross \textit{et al.}, Phys. Rev. Lett. {\bf 103}, 227202 (2009) [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U14.00008: Thermodynamic properties of 3-dimensional quantum antiferromagnets Rajiv R.P. Singh, Jaan Oitmaa, Michel J.P. Gingras We present systematic calculations of thermal properties of 3-dimensional quantum antiferromagnets, in the thermodynamic limit, using series expansions. For this purpose, High Temperature Expansions (HTE) are supplemented by Numerical Linked Cluster (NLC) Expansions.\footnote{R. Applegate et al, Phys. Rev. Lett. 109, 097205 (2012); R. R. P. Singh and J. Oitmaa Phys. Rev. B 85, 144414 (2012); R. R. P. Singh and J. Oitmaa Phys. Rev. B 85, 104406 (2012).} These expansions provide essentially exact calculations of thermodynamic properties of the system at (i) all fields at high temperatures and (ii) at all temperatures at high fields. In addition, we show that for classical exchange spin-ice model defined on the pyrochlore lattice, the first order NLC leads to the Pauling approximation, which gives even the zero-field ground state entropy to about one percent accuracy. Thus, these calculations are accurate over a wide parameter range. Results are presented and compared with a variety of experimental systems including pyrochlore materials Yb$_2$Ti$_2$O$_7$ and Er$_2$Ti$_2$O$_7$ and the Hyper Kagome material Na$_4$Ir$_3$O$_8$ [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U14.00009: Seeing the light: Observing photons in quantum spin ice Owen Benton, Olga Sikora, Nic Shannon Spin ice, with its magnetic monopole excitations, is perhaps the best studied example of a classical spin liquid. Quantum mechanical tunnelling between the classical ground states of spin ice leads to an exciting new scenario- a quantum spin liquid ground state with emergent photon excitations [1, 2]. Here we explore how this ``artificial electromagnetism'' would manifest itself in neutron scattering experiments on putative ``quantum spin ice'' materials. Using lattice gauge theory we make explicit predictions for the ghostly, linearly dispersing magnetic excitations which are the ``photons'' of this emergent electromagnetism. We find that ``pinch points,'' which are the signal feature of a classical spin ice, fade away as the system approaches its zero-temperature ground state. The predictions of this field theory are shown to be in excellent quantitative agreement with quantum Monte Carlo simulations at zero temperature~[3].\\[4pt] [1] M. Hermele, M. P. A. Fisher and L. Balents, Phys. Rev. B. {\bf 69}, 064404 (2004).\\[0pt] [2] L. Savary and L. Balents, Phys. Rev. Lett. {\bf 108}, 037202 (2012).\\[0pt] [3] O. Benton, O. Sikora and N. Shannon, Phys. Rev. B. {\bf 86}, 075154, (2012). [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U14.00010: Dynamical spectra of quantum strings in quantum spin ice Wesley Fuhrman, Yuan Wan, Oleg Tchernyshyov String-like excitations in quantum spin-ice are a fascinating manifestation of quantum fluctuations and may be observable in materials such as Yb$_2$Ti$_2$O$_7$ and Pr$_2$Zr$_2$O$_7$. We study quantum spin-ice under external magnetic fields on both the checkerboard and pyrochlore lattice for experimentally relevant conditions. We show that excitations in quantum spin ice may be string-like, and that stronger quantum fluctuations reduce string tension and lead to deconfined monopoles. Additionally, we discuss the crossover from strings to magnons in the high-field regime. We provide predictions for observing strings via inelastic neutron scattering and THz spectroscopy. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U14.00011: Quantum Fluctuations in Spin-Ice-Like Pr$_2$Zr$_2$O$_7$ Jiajia Wen, Kenta Kimura, Satoru Nakatsuji, Collin Broholm, Matthew Stone, Eiji Nishibori, Hiroshi Sawa We report the experimental evidence of spin-ice-like correlation and quantum fluctuation in the rare earth pyrochlore Pr$_{2}$Zr$_{2}$O$_{7}$. Low temperature magnetization together with high energy inelastic neutron scattering spectrum reveal the single ion crystal field ground state of Pr$^{3+}$ is a non-Krammer's doublet with local \textless 111\textgreater\ anisotropy. Heat capacity and magnetic susceptibility data show no evidence of long range ordering down to 50 mk. The magnetic interaction energy scale is estimated from AC magnetic susceptibility data where an activation energy gap of 1.6 K is extracted from T-dependent relaxation time. The wave vector dependence of quasi-elastic neutron scattering at 0.1 K resembles that of exchange spin ice, including well-defined pinch points. This indicates the 2-in 2-out ice rule is satisfied over the time scale set by the instrumental energy resolution. In contrast, inelastic scattering with energy transfer of 0.25 meV does not show pinch pints, which suggests these fluctuations break the ice rule. The spectral weight of the elastic scattering accounts for less than 10{\%} of the total scattering from the ground state doublet, providing evidence for the strong quantum fluctuation. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U14.00012: Universal monopole scaling near transitions from the Coulomb phase Stephen Powell Certain frustrated systems, such as spin ice and dimer models, exhibit a Coulomb phase at low temperatures, with power-law correlations and fractionalized monopole excitations. Applied perturbations (external field, pressure, etc.) can drive a transition to a phase where the monopoles become confined. I will present a general analysis of behavior in the vicinity of such critical points, incorporating the effects of a nonzero density of thermal monopoles. Scaling theory allows one to arrive at universal results for the crossover phenomena, which can be tested in numerics or experiment. I will also present Monte Carlo results that confirm these predictions for two particular transitions in spin ice. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U14.00013: Investigation of the Magnetic Properties in the Pyrochlore Pr$_{2}$Sn$_{2}$O$_{7}$ Elizabeth Green, T. Herrmannsd\"{o}rfer, R. Sch\"{o}nemann, Z. Wang, M. Uhlarz, J. Wosnitza, H.D. Zhou Pyrochloric compounds are best known for their remarkable magnetic properties, particularly the possibility to generate magnetic monopoles excitations at low temperatures. Compared to the A$^{3+}$ ions in the spin ice compounds A$_{2}$Ti$_{2}$O$_{7}$ (where A = Ho or Dy), the Pr$^{3+}$ ions in Pr$_{2}$Sn$_{2}$O$_{7}$ have a smaller magnetic moment (2.6 $\mu_{B}$/Pr [1]). This ultimately leads to quantum fluctuations that suppress the spins' ability to freeze [2]. AC susceptibility measurements were performed on a polycrystalline Pr$_{2}$Sn$_{2}$O$_{7}$ sample to probe its dynamic ground state for temperatures down to 11 mK. Preliminary results indicate a narrow distribution of relaxation rates which, as evidenced by neutron experiments [3], are governed by quantum tunneling between states. In addition, relaxation times extracted from isothermal frequency sweeps were found, within error, to be temperature independent below 1 K. Future measurements include specific heat from which the field-dependence of the magnetic monopole densities may be extracted.\\[4pt] [1] K. Matsuhira et al., J. Phys. Soc. Jpn. \textbf{71}, 1576 (2002)\\[0pt] [2] S. Onoda et al., PRL \textbf{105}, 047201 (2010)\\[0pt] [3] H.D. Zhou et al., PRL \textbf{101}, 227204 (2008) [Preview Abstract] |
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