Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Q1: Gapless Spin Liquids |
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Sponsoring Units: DCMP Chair: Claudio Castelnovo, University of Oxford Room: Ballroom A1 |
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q1.00001: Thermal-transport Studies of Quantum Spin Liquids Invited Speaker: Quantum spins, coupling antiferromagnetically on a 2D triangular lattice, cannot simultaneously satisfy all interactions. This frustrated situation is expected to give rise to mysterious fluid-like states of spins without long- range order, so called quantum spin liquid (QSL). The ground state of QSL and its exotic phenomena, such as fractionalized excitation with an artificial gauge field, have been extensively discussed for decades, yet to be identified by lack of any real materials. This is why the recent discoveries of materials possessing an ideal 2D triangular lattice have spurred a great deal of interest. To understand the nature of QSL, knowledge of the low-lying excitation, particularly the presence/absence of an excitation gap, is of primary importance. We employ thermal transport measurements on newly discovered QSL candidates, $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3 $ and EtMe$_3$Sb[Pd(dmit)$_2$]$_2$, and report that the two organic insulators possess different QSLs characterized by different elementary excitations. In $\kappa$-(BEDT-TTF)$_2 $Cu$_2$(CN)$_3$ [1], heat transport is thermally activated in low temperatures, suggesting presence of a spin gap in this QSL. In stark contrast, in EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ [2], a sizable temperature-linear term of thermal conductivity is clearly resolved in the zero-temperature limit, showing gapless excitation with long mean free path ($\sim$1,000 lattice distances), analogous to excitations near the Fermi surface in normal metals. These results are consistent with theoretical suggestions including 2D gapless spinons with a Fermi surface. This work was done in collaboration with N. Nakata, Y. Senshu, M. Nagata, Y. Kasahara, S. Fujimoto, T. Shibauchi, Y. Matsuda, T. Sasaki, N. Yoneyama, N. Kobayashi, H. M. Yamamoto and R. Kato. \\[4pt] [1] M. Yamashita \textit{et al.}, Nature Physics \textbf{5}, 44- 47 (2009). \newline [2] M. Yamashita \textit{et al.}, Science \textbf{328}, 1246 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:27PM |
Q1.00002: Spin liquids, spin-orbit coupling, and band topology Invited Speaker: Much of the search for quantum spin liquids has focused on spin-rotationally invariant model Hamiltonians, appropriate for weakly spin-orbit coupled materials. With strong spin-orbit interactions, distinct theoretical approaches are required, and novel spin liquid states may occur. Some such candidate states will be discussed, along with the mechanisms leading to them. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 1:03PM |
Q1.00003: Time-reversal symmetry breaking and spontaneous Hall effect without magnetic dipole order in Pr$_2$Ir$_2$O$_7$ Invited Speaker: An electric current flowing through a conductor in a magnetic field produces a transverse voltage drop known as the Hall effect. In the absence of the field, this effect also appears in ferromagnets in a plane normal to its spontaneous magnetization vector owing to the spin-orbit coupling. Generally, it may also detect a nontrivial order parameter breaking the time-reversal symmetry on a macroscopic scale, for example, scalar spin chirality. In this talk, we present our recent results in the study of the frustrated magnetism and Hall transport of the metallic pyrochlore magnet Pr$_2$Ir$_2$O$_7$.\footnote{S. Nakatsuji, Y. Machida, Y. Maeno, T. Tayama, T. Sakakibara, J. v. Duijn, L. Balicas, J. N. Millican, R. T. Macaluso, and Julia Y. Chan, \textit{Phys. Rev. Lett.} \textbf{96}, 087204 (2006).}$^,$\footnote {Y. Machida, S. Nakatsuji, Y. Maeno, T. Tayama, T. Sakakibara, and S. Onoda, \textit{Phys. Rev. Lett.} \textbf{98}, 057203 (2007).} Strikingly, a spontaneous Hall effect is observed in the absence of both an external magnetic field and conventional magnetic long-range order.\footnote{Y. Machida, S. Nakatsuji, S. Onoda, T. Tayama, and T. Sakakibara, \textit{Nature} \textbf{463}, 210 (2010).} This strongly suggests the existence of a chiral spin liquid, a spin-liquid phase breaking the time-reversal symmetry. Both our measurements indicate that spin-ice correlations in the liquid phase lead to a non-coplanar spin texture forming a uniform but hidden order parameter: the spin chirality. Interesting phenomena seen under high field will also be discussed. This is the work performed in collaboration with Y. Machida, Y. Ohta, T. Sakakibara, T. Tayama, Y. Uwatoko (ISSP, Univ. of Tokyo), S. Onoda (Riken, Tokyo), L. Balicas (NHMFL), D. E. MacLaughlin (UC, Riverside) and C. Broholm (JHU). [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:39PM |
Q1.00004: Fractional spin textures in the frustrated magnet SCGO Invited Speaker: Spin liquids are remarkable states of matter that do not order magnetically even at very low temperatures and show collective phenomena like emergent gauge fields and topological order. Impurities can potentially reveal the underlying correlations in such states that appear deceptively featureless in their ground state properties. We consider the archetypal frustrated antiferromagnet SrCr$_{9p}$Ga$_{12-9p}$O$_{19}$ (SCGO) in which Ga ions act as non-magnetic impurities in the magnetic lattice composed of Cr$^{3+}$ S=3/2 spins for disordered $p<1$ samples. We demonstrate that a spin in direct proximity to a pair of vacancies is cloaked by a spatially extended spin texture that encodes the correlations of the parent spin liquid. In this spin liquid regime, our analytic theory predicts that the combined object has a magnetic response identical to a classical spin of length S/2=3/4, which dominates over the small intrinsic susceptibility of the pure system. We calculate the full texture on the lattice in the spin liquid regime and check that it agrees well with Monte-Carlo simulations. This fractional-spin texture leaves an unmistakable imprint on the measured $^{71}$Ga nuclear magnetic resonance (NMR) lineshapes, which we compute using Monte-Carlo simulations and compare with experimental data. We also study the long-ranged interactions between these spin textures at low temperatures to gain a better understanding of the case of finite dilution in the parent spin liquid. [Preview Abstract] |
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