Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session G28: Focus Session: Spin Ice |
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Sponsoring Units: GMAG DMP Chair: Claudio Castelnovo, Cambridge University Room: 205 |
Tuesday, March 3, 2015 11:15AM - 11:27AM |
G28.00001: Thermal and Magnetic Responses of Spin Ice via SQUID Magnetometry K. W. Schlax, J. A. Moyer, A. Thaler, K. Sendgikoski, G. J. MacDougall, P. Schiffer, D. J. Van Harlingen The spin ice pyrochlore class of frustrated magnets exhibits quasiparticle excitations that behave like Dirac monopoles on the pyrochlore lattice. We are studying single crystals of the spin ice Dy$_2$Ti$_2$O$_7$ using dc SQUID magnetometry to look for thermal fluctuations in the monopole density. Specifically, we are seeking to observe a local excess of monopole magnetic charge induced by the simultaneous application of a temperature gradient, which should create a gradient in the monopole density, and a parallel magnetic field, which drives the flow and counterflow of oppositely-charged monopoles. This configuration should allow us to investigate the distribution, dynamics, and relaxation of the monopole-like excitations in spin ice pyrochlores. [Preview Abstract] |
Tuesday, March 3, 2015 11:27AM - 11:39AM |
G28.00002: Coherent heat conduction of quantum monopoles in Yb$_2$Ti$_2$O$_7$ Takuya Yamashita, Yoshi Tokiwa, Daiki Terazawa, Yusuke Shimoyama, Yukio Yasui, Masafumi Udagawa, Takasada Shibauchi, Yuji Matsuda The rare-earth pyrochlore magnets are realization of spin ice which have macroscopically degenerate ground states. The elementary excitation of classical spin ice is thought to be thermally activated magnetic monopoles with dispersion-less energy gap $\Delta$ $\sim$ 2$J_{zz}$. We have measured the thermal conductivity $\kappa$ of quantum spin ice Yb$_2$Ti$_2$O$_7$ at magnetic field $B$ // [100] and [111]. The field direction dependence of $\kappa$ is consistent with monopole excitations. However, the temperature dependence indicates that the energy gap is at most 0.2 K, which is much smaller than $\Delta$ $\sim$ 4 K. This reduction of gap suggests the band formation of monopole excitations, giving rise to coherent heat conduction of $``$quantum$"$ monopoles. Unlike diffusive monopoles in classical spin ice, the mean free path of these quantum monopoles is extremely long $\sim$ 100 nm. [Preview Abstract] |
Tuesday, March 3, 2015 11:39AM - 12:15PM |
G28.00003: Ordering, thermal excitations and phase transitions in dipolar coupled mono-domain magnet arrays Invited Speaker: Vassilios Kapaklis Magnetism has provided a fertile test bed for physical models, such as the Heisenberg and Ising models. Most of these investigations have focused on solid materials and relate to their atomic properties such as the atomic magnetic moments and their interactions. Recently, advances in nanotechnology have enabled the controlled patterning of nano-sized magnetic particles, which can be arranged in extended lattices. Tailoring the geometry and the magnetic material of these lattices, the magnetic interactions and magnetization reversal energy barriers can be tuned [1,2]. This enables interesting interaction schemes to be examined on adjustable length and energy scales. As a result such nano-magnetic systems represent an ideal playground for the study of physical model systems, being facilitated by direct magnetic imaging techniques [3]. One particularly interesting case is that of systems exhibiting frustration, where competing interactions cannot be simultaneously satisfied. This results in a degeneracy of the ground state and intricate thermodynamic properties [1-4]. An archetypical frustrated physical system is water ice. Similar physics can be mirrored in nano-magnetic arrays, by tuning the arrangement of neighboring magnetic islands, referred to as artificial spin ice. Thermal excitations in such systems resemble magnetic monopoles [4]. In this presentation key concepts related to nano-magnetism and artificial spin ice will be introduced and discussed, along with recent experimental and theoretical developments. [1] V. Kapaklis et al., New J. Phys. 14, 035009 (2012). [2] U. B. Arnalds et al., Appl. Phys. Lett. 105, 042409 (2014). [3] A. Farhan et al., Nature Physics 9, 375 (2013). [4] V. Kapaklis et al., Nature Nanotech 9, 514 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 12:15PM - 12:27PM |
G28.00004: Phase transitions and charge ordering in a square spin ice model with conserved monopole density Yunlong Xie, Xiaohui Zhou, Jun-Ming Liu Artificial spin ices represent a class of highly interested frustrated magnetic systems under intensive investigations for fascinating ground states and thermodynamics/dynamics of spin excitations in recent years. As one of these issues, magnetic charge ordering and the corresponding phase transitions in the two-dimensional system are emerging topics in condensed matter physics. In this work, we investigate all the monopole-ordered phases of the square spin ice model using the conserved monopole density algorithm. In low monopole density ($\rho \sim 0$), the Coulomb potential determines the monopoles' dynamics. We test the Coulomb's law in a two-dimension lattice and justify the monopole dimerization which is quite different from the three-dimensional pyrochlore spin ice. These monopole dimers are charge neutral, and the interactions between them have also been investigated using our algorithm. In the cases of high monopole density ($\rho \sim 1$), the system is similar to the dipolar kagome spin ice model, and our simulation results show that there exists an intermediate phase between the paramagnetic phase and the ordered magnetic phase. Such intermediate phase can be distinguished by the order of magnetic charges. In a cooling process, the system undergoes a two-stage magnetic phase transition before freezing to the long range magnetic ordered phase via a staggered charge ordering. Furthermore, a liquefaction process of monopole dimers can be justified upon the increasing effective internal pressure in the isothermal condition. [Preview Abstract] |
Tuesday, March 3, 2015 12:27PM - 12:39PM |
G28.00005: What is the quantum ground state of dipolar spin ice? Paul McClarty, Olga Sikora, Roderich Moessner, Karlo Penc, Frank Pollmann, Nic Shannon Recent work on Dy2Ti2O7 spin ice has revealed a partial loss of residual entropy deep within the spin ice state [1]. It has been known for some time that the spin ice materials should have either magnetically ordered [2] or quantum spin liquid [3] ground states and this latest work hints at the possibility of determining them experimentally. We study a natural model for the dipolar spin ice materials and map out the entire ground state phase diagram in the presence of quantum tunneling between the ice states [4]. In the classical case, we show that the ground states in our 3D long-range interacting model can be determined from those of a short-range interacting 2D model and, remarkably, in the quantum case, only a very small tunneling coupling compared to the dipolar coupling is necessary to enter the quantum spin liquid state. \\[4pt] [1] D. Pomaranski et al., Nature Physics 9, 353-356 (2013).\\[4pt] [2] R. G. Melko, B. C. den Hertog, and M. J. P. Gingras, Phys. Rev. Lett. 87, 067203 (2001).\\[0pt] [3] M. Hermele, M.P.A. Fisher, and L. Balents, Phys. Rev. B 69, 064404 (2004);A. Banerjee et al., Phys. Rev. Lett. 100, 047208 (2008);N. Shannon et al., Phys. Rev. Lett. 108, 067204 (2012).\\[0pt] [4] P. McClarty et al., arXiv:1410.0451 [Preview Abstract] |
Tuesday, March 3, 2015 12:39PM - 12:51PM |
G28.00006: Demagnetization Effects in Dipolar Systems Patrik Henelius, Mikael Twengstr\"{o}m, Laura Bovo, Michel J.P. Gingras, Steven T. Bramwell The internal magnetic field of a uniformly magnetized body depends in general on the shape of the object. The calculation of this field, and the associated demagnetization factors, is a classical subject in the study of magnetism. Here we revisit the relationship between the demagnetization factor obtained through fluxmetric, magnetometic and bulk susceptibility techniques. Apart from simple uniaxial systems we also consider more complicated systems, such as the dipolar spin ice model on a pyrochlore lattice, where we compare our results to experimental bulk susceptibility measurements performed on a variety of sample shapes. [Preview Abstract] |
Tuesday, March 3, 2015 12:51PM - 1:03PM |
G28.00007: Magnetic Susceptibility of Spin Ice Mikael Twengstr\"{o}m, Laura Bovo, Tom Fennell, Steven T. Bramwell, Oleg A. Petrenko, Michel J. P. Gingras, Patrik Henelius The magnetic susceptibility of a spin ice material is a sensitive probe of the relevant physics in different temperature ranges. At high temperatures, where crystal field excitations dominate the susceptibility, the spin ice picture is not applicable. However, at temperatures below 10 K, the Ising anisotropy is well developed and the dipolar spin ice model (DSIM) can be employed. In this study we present experimental susceptibility data between 0.4 K and 10 K and revisit the DSIM in order to theoretically model this data. We find that the DSIM provides a good semi-quantitative description of both the temperature dependence of the uniform bulk susceptibility and the $\textbf{Q}$-dependent susceptibility measured by neutron scattering. [Preview Abstract] |
Tuesday, March 3, 2015 1:03PM - 1:15PM |
G28.00008: Pseudo-spin model and neutron scattering studies of hydrogen disorder in D$_{2}$O David Jonathan P. Morris, Konrad Siemensmeyer, Bastian Klemke, Jens-Uwe Hoffmann, Illia Glavatskyi, Klaus Seifert, Sergei Isakov, Roderich Moessner, Alan Tennant The crystal structure of water is made up of a regular lattice of oxygen atoms connected by hydrogen bonds with an intermediate hydrogen atom. The hydrogen atom is displaced from the midpoint between nearest neighbor oxygen-oxygen atoms. Therefore the H-bond between neighboring hydrogen and oxygen can either be short or long. These H-bonds are known to obey the famous ``ice rules'' where each oxygen has two neighboring hydrogen sitting on the close site and two neighboring hydrogen sitting on the further site. The ice rules do not specify which hydrogen bonds will be short and which will be long for any particular oxygen, i.e. they do not describe long-range order, merely that there will be two short and two long bonds for a particular oxygen atom allowing for disorder. Here we will present a pseudo-spin model of water ice and diffuse neutron scattering measurements from D$_{2}$O. The displacements of hydrogen away from the midpoint between neighboring oxygen-oxygen are treated as interacting Ising spins ($\sigma=\pm 1$). The diffuse neutron scattering measurements allow us to test the resulting theoretical predictions. [Preview Abstract] |
Tuesday, March 3, 2015 1:15PM - 1:27PM |
G28.00009: Critical dynamics and finite-time scaling in spin ice systems Claudio Castelnovo, James Hamp, Anushya Chandran, Roderich Moessner Spin ice materials such as Dy2Ti2O7 and Ho2Ti2O7 provide a rare instance of emergent gauge symmetry and fractionalisation in three dimensions. Magnetic frustration leads to highly degenerate yet locally constrained ground states. Their elementary excitations carry a fraction of the magnetic moment of the microscopic spin degrees of freedom and can be thought of as magnetic monopoles. One of the distinguishing manifestations of this emergent ``Coulomb phase'' is a liquid-gas phase diagram that appears in an applied magnetic field--a feature that is expected in itinerant charge liquids but unprecedented in localised spin systems. Monopoles act as facilitators to the spin dynamics. At low temperatures they are sparse and dynamics becomes slow, leading to an interplay between emergent topological properties and lattice scale physics in response and equilibration properties. In this work, we investigate the dynamics in spin ice close to the critical end point of the liquid gas phase diagram. Critically divergent length scales give rise to finite time scaling properties that reflect the universal scaling exponents at the critical point. We use our results to obtain these exponents by tuning the approach direction in the field-temperature plane. [Preview Abstract] |
Tuesday, March 3, 2015 1:27PM - 1:39PM |
G28.00010: A Measure of Monopole Inertia in the Quantum Spin Ice Yb$_{2}$Ti$_{2}$O$_{7}$ LiDong Pan, N. J. Laurita, Kate A. Ross, Edwin Kermarrec, Bruce D. Gaulin, N. Peter Armitage We report a time domain terahertz spectroscopy study of quantum spin ice material Yb$_{2}$Ti$_{2}$O$_{7}$. We measure the complex dynamic susceptibility of Yb$_{2}$Ti$_{2}$O$_{7}$ in the temperature range between 1.4K and 20K. The data are consistent with a picture where the emergent magnetic monopoles are the principle degrees of freedom. Among other measures, we observe a zero crossing in the real part of the frequency dependent susceptibility. Such a feature is not possible without introducing inertial effects e.g. a mass dependent term to the equations of motion. Through a comparison of the magnetic spectral weight with numerical data that estimates the low temperature monopole density, we derive a value for the magnetic monopole mass. [Preview Abstract] |
Tuesday, March 3, 2015 1:39PM - 2:15PM |
G28.00011: Real time imaging of magnetic excitations in the spin ice Ho$_{2}$Ti$_{2}$O$_{7}$ Invited Speaker: Ilya Sochnikov Theoretically, a special form of spin frustration in classical spin ices results in emergence of excitations that are directly mapped to magnetic monopoles. Experimentally, many aspects of the energetics of the magnetic excitations in spin ices are still not well understood, in part, because of scarcity of experimental tools that can explicitly and directly test for monopole dynamics. Using scanning Superconducting QUantum Interference Device (SQUID) microscopy we obtain real time images of spontaneous magnetic field fluctuations in the spin ice Ho$_{2}$Ti$_{2}$O$_{7}$. We determine a distribution of activation energies of spontaneous magnetic excitations from the temperature and frequency dependence of the observed field fluctuations. We discuss an agreement of the extracted energy distributions with the expected ones for monopoles excitations. This study opens new horizons for studies of real space and real time magnetic fluctuations and their relations to emergent phenomena in a variety of frustrated magnets. [Preview Abstract] |
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