Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session X3: Magnetic Monopoles and Dirac Strings in Condensed Matter |
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Sponsoring Units: DCMP Chair: Peter Schiffer, The Pennsylvania State University Room: Oregon Ballroom 203 |
Thursday, March 18, 2010 2:30PM - 3:06PM |
X3.00001: Signatures of magnetic monopoles in spin ice Invited Speaker: Following the original proposal of the existence of magnetic monopoles as emergent particles in spin ice, this talk focuses on possible experimental signatures of such quasiparticles in the spin ice compounds. In particular, it presents a theory of the low-temperature heat capacity for the gas of monopoles. It discusses direct detection of monopoles via zero-field NMR measurements, which can act as a local probe for the quasiparticle density. Finally, it shows that both the long- and short-range part of the Coulomb interaction between monopoles leaves characteristic traces in, among other quantitites, neutron scattering cross sections. [Preview Abstract] |
Thursday, March 18, 2010 3:06PM - 3:42PM |
X3.00002: Measurement of the charge and current of magnetic monopoles in spin ice Invited Speaker: The transport of electrically charged quasiparticles (based on electrons, holes or ions) plays a pivotal role in modern technology as well as determining the essential function of biological organisms. In contrast, the transport of magnetic charges has barely been explored experimentally, mainly because magnetic charges are generally considered to be, at most, convenient macroscopic parameters rather than sharply defined quasiparticles. However, the recent proposition of emergent magnetic monopoles in certain materials may change this point of view. Here we address the question of whether these magnetic charges and their associated currents (`magnetricity') can be directly measured in experiment, without recourse to any material-specific theory. By mapping the problem onto Onsager's theory of electrolytes, we show that this is possible, and devise an appropriate method. Then, using muon spin rotation as a convenient local probe, we apply the method to a real material: the spin ice Dy$_2$Ti$_2$O$_7$. Our experimental measurements prove that magnetic charges exist in this material, interact via a Coulomb interaction, and have measurable currents. We further characterise deviations from Ohm's Law, and determine the elementary unit of magnetic charge to be 5~$\mu_{\rm B}{\rm \AA}^{-1}$. We show further results from magnetic susceptibility confirming the Wein effect and that the surface of the crystal behaves like a capacitor, storing charge, with the resultant relaxation described by the dissociation and recombination of charge carriers. The measurement of magnetic charge and observation of magnetic current emphasises the reality of these quantities and establishes an instance of a perfect symmetry between electricity and magnetism. [Preview Abstract] |
Thursday, March 18, 2010 3:42PM - 4:18PM |
X3.00003: Dirac Strings and Magnetic Monopoles in Spin Ice Dy2Ti2O7 Invited Speaker: |
Thursday, March 18, 2010 4:18PM - 4:54PM |
X3.00004: Monopole and Dirac string Dynamics in Spin Ice Invited Speaker: Magnetic frustration in spin ice gives rise to a highly degenerate Coulomb phase ground state with algebraically decaying spin correlations [1], that acts as a vacuum for local \textit{magnetic monopole} excitations [2]. We shall present how the previously unexplained temperature dependance of the time relaxation for a spin ice material Dy$_{2}$Ti$_{2}$O$_{7}$ [3] can be understood in the light of these emergent quasi-particles connected by a network of classical analogues of \textit{Dirac strings} [4]. These experimental data [3] give a clear signature of fractionalised excitations into pairs of opposite magnetic charges, and the dynamical slow down appears as a consequence of the rarefaction of monopoles at low temperature, strengthened by the influence of the Coulomb interactions on the chemical potential of these quasi-particles. \\[4pt] [1] C. Henley Phys. Rev. B \textbf{71}, 014424 (2005). \\[0pt] [2] C. Castelnovo \& al. Nature \textbf{451}, 42-45 (2008). \\[0pt] [3] J. Snyder \& al. Phys. Rev. B \textbf{69}, 064414 (2004). \\[0pt] [4] L.D.C. Jaubert \& P.C.W. Holdsworth, Nat. Phys. \textbf{5}, 258 (2009). [Preview Abstract] |
Thursday, March 18, 2010 4:54PM - 5:30PM |
X3.00005: Induce a magenetic monopole on the surface of topological insualtor Invited Speaker: Topological insulators are new states of matter which are gapped in the bulk but have gapless, topologically protected surface states. In recent years, time reversal invariant topological insulators have been discovered in HgTe quantum wells and bulk crystals Bi2Se3, Bi2Te3 and Bi1-xSbx alloy. The three-dimensional topological insulator is described by a topological term E.B in the effective field theory, leading to a topological magneto-electric effect. As a consequence, an image ``magnetic monopole'' is induced by a point charge near the surface of the topological insulator. Electrons near the surface of topological insulator become ``dyons'' with anionic statistics. If in addition an antiferromagnetic order is established in the topological insulator phase, a topological coupling is induced between spin-wave excitations and photons, leading to the design of a tunable optical modulator. [Preview Abstract] |
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