Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Q31: Focus Session: Spinels, Pyrochlores and General |
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Sponsoring Units: GMAG Chair: Gavin Lawes, Wayne State University Room: 335 |
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q31.00001: Magnetic and Transport Properties of Tb and Y Molybdate Pyrochlores R.P. Guertin, E.-S. Choi, C. Wiebe, H. Zhou The transport, magnetic, and magnetotransport properties of single crystal Tb$_{2}$Mo$_{2}$O$_{7}$ and Y$_{2}$Mo$_{2}$O$_{7}$ are reported. The Mo cation carries a small magnetic moment ($<$1 \'{E}$_{B})$, but neither system shows long-range magnetic order due geometrical frustration. Short-range ($\sim $5 A) correlations [1] cause Mo magnetization irreversibility below T$_{irr}\sim $24 K - a spin glass-like anomaly. The pressure dependence of T$_{irr}$ for Tb$_{2}$Mo$_{2}$O$_{7}$ is strongly negative (-0.24 K/kbar), consistent with similar pressure studies in (Tb$_{1-x}$La$_{x})_{2}$Mo$_{2}$O$_{7}$ [2]. High dc field (33 T) isothermal magnetization shows Tb$_{2}$Mo$_{2}$O$_{7 }$M(H) is not fully saturated due to crystal field splitting and nearly linear Y$_{2}$Mo$_{2}$O$_{7}$ M(H) attains only $\sim $0.4 \'{E}$_{B}$/f.u, A metal-insulator transition (MIT) in Tb$_{2}$Mo$_{2}$O$_{7}$ occurs at T$\sim $50 K, with resistivity \'{E}{\oe}(T) rising $>$10X by T=10 K. The temperature dependence of the Hall resistivity, thermopower, and magnetoresistance are consistent with the MIT. The buildup of the magnetic clusters and the MIT occur at substantially the same temperature, and is discussed in terms of the electronic localization of the charge carriers.\\[0pt] [1] B. D. Gaulin et al, Phys. Rev. Lett. 69, 3244 (1992).\\[0pt] [2] A. Apetrei et al, Phys. Rev. Lett. 97, 206401 (2006). [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q31.00002: Mott transition between a spin-liquid insulator and a metal in three dimensions Daniel Podolsky, Arun Paramekanti, Yong Baek Kim, T. Senthil We study a bandwidth controlled Mott metal-insulator transition (MIT) between a Fermi liquid metal and a quantum spin-liquid insulator at half-filling in three dimensions (3D). Using a slave rotor approach, and incorporating gauge field fluctuations, we find a continuous MIT and discuss the finite temperature crossovers around this critical point. We show that the specific heat $C \sim T \ln \ln (1/T)$ at the MIT and argue that the electrical transport on the metallic side near the transition should exhibit a `conductivity minimum' as a function of temperature. A possible candidate to test these predictions is the 3D spin liquid insulator Na$_4$Ir$_3$O$_8$ which exhibits a pressure-tuned transition into a metallic phase. We also present the electron spectral function of Na$_4$Ir$_3$O$_8$ at the transition. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q31.00003: Transport and ordering properties of geometrically frustrated metallic pyrochlore magnets. Ka-Ming Tam, Michel Gingras Motivated by recent experiments on the Pr$_{2}$Ir$_{2}$O$_{7}$ metallic pyrochlore material, we study the ordering of the localized 4f Pr moments coupled to the itinerant 5d electrons from Ir. A Monte Carlo method is used to study the condition for spin-ice ordering, and the accompanying change of transport property due to spin ordering. We find that the effects from hybridization between the Pr moments and the Ir electrons should be included to describe the metallic spin liquid phase at low temperatures. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q31.00004: Lattice vibration modes in CdCr2O4 J.-H. Kim, S.-H. Lee, M. Matsuda, H. Ueda, Y. Ueda, J.-H. Chung, S. Tsutsui, A. Baron In geometric frustration magnets, spin-lattice coupling can play an important role in lifting the magnetic frustration. In order to understand the mechanism, we have performed inelastic synchrotron x-ray measurements on a single crystal of a frustrated magnet CdCr2O4 that undergoes such a three-dimensional spin-Peierls phase transition at T{\_}N = 7.8 K. Our data taken above and below T{\_}N could be well explained by the rigid ion model, which led to a full identification of the lattice vibration modes in CdCr2O4. A phonon anomaly that might be associated with the transition, however, was not observed. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q31.00005: Crystal and magnetic structures of the three-dimensional spin-Peierls state of ZnCr2O4 Sungdae Ji, S.-H. Lee, C. Broholm, W. Ratcliff II, S.-W. Cheong, P. Zschack The geometrically frustrated spinel ZnCr$_2$O$_4$ undergoes a three-dimensional spin-Peierls phase transition at T$_N$ = 12.5 K from a cubic paramagnetic state to a tetragonal Neel state. The exact nature of the lattice distortion, and the one-to-one-correspondence between the distortion and the magnetic ground state has been a long standing issue. To unveil the mystery, we have performed synchrotron X-ray diffraction measurements on a single crystal of this compound and neutron diffraction measurements on a powder sample. Our detailed analysis of the single crystal X-ray data shows that below T$_N$ the symmetry of the crystal structure is lowered from the cubic $Fd\bar3m$ to the tetragonal $I\bar4m2$ due to formation of a complex pattern of Cr-Cr clustering. The relation between the distorted crystal structure and the magnetic structure will also be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q31.00006: Magnetic Correlations in YBaCo$_{4}$O$_{7}$: A Frustrated Magnet with Novel Trigonal Bipyramidal Chains John Mitchell, Pascal Manuel, Laurent Chapon, Paolo Radaelli, Hong Zheng Novel structural motifs generate opportunities to explore unique geometrically frustrated groundstates and their properties. One such new material,YBaCo$_{4}$O$_{7}$ (Y114), is closely related to the pyrochlore lattice, differing only in the stacking sequence of triangular layers. This alternative stacking sequence leads to chains of corner sharing trigonal bipyramids with magnetic ion vertices, a novel motif among geometrically frustrated lattices. We have studied Y114 using single crystal neutron diffraction above its ordering temperature. Strong magnetic diffuse scattering can be understood using Monte-Carlo simulations, and a simple nearest-neighbor model explains the magnetic structure. Along the c-axis, long-range correlations arising from the corner-sharing bipyramids create a quasi one-dimensional order at finite temperature. In contrast, the spin-spin correlation function decays rapidly in the ab-plane, following a unique short-range configuration that enforces an$ L$=0 for the bipyramid. Approaches to suppressing long-range order in the system will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q31.00007: Specific heat of gadolinium garnets Jeffrey Quilliam, Shuchao Meng, Linton Corruccini, Oleg Petrenko, Michel Gingras, Jan Kycia Specific heat measurements on two different geometrically frustrated, Heisenberg, garnet lattices will be presented. The specific heat of an isotopically pure, single crystal sample of Gd$_3$Ga$_5$O$_{12}$, or GGG, is found to be consistent with previous measurements of the specific heat of GGG \footnote{Schiffer et al. Phys. Rev. Lett. 74, 2379 (1995).} and shows no signs of a standard ordering anomaly despite sharp Bragg peaks that were seen in neutron diffraction experiments. A first measurement of the specific heat of polycrystalline Gd$_3$Li$_2$Te$_3$O$_{12}$, in contrast, shows a sharp first-order phase transition at 240 mK. We will discuss possible explanations for such diverse behavior in very similar systems. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q31.00008: Long-range antiferromagnetic interactions in ZnFe$_{2}$O$_{4}$ and CdFe$_{2}$O$_{4}$ Ching Cheng, Ching-Sheng Liu, Hsueh Fang Yeh For the first time, the Fe-Fe interactions in the well known geometrically frustrated antiferromagets of Zinc and Cadmium ferrite are determined quantitatively by considering structures of different collinear magnetic distributions using first-principles methods. Both the generalized gradient approximation (GGA) and GGA plus the onsite Coulomb interaction (GGA+U) are considered for the exchange-correlation energy functional. The interactions up to third neighbors are determined to be all antiferromagnetic regardless of which approximation scheme (GGA or GGA+U) is used. Surprisingly, the third neighbor interaction is much stronger than the second-neighbor one. The magnetic distributions have prominent effects on energies as well as density of states, including band gaps, for both normal and inverse spinel structures of these two materials. The possible magnetic distributions coincident with neutron scattering results are also investigated. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:27PM |
Q31.00009: Frustrated and Quantum Antiferromagnetism on the Diamond Sublattice of A-site Magnetic Spinels Invited Speaker: Leon Balents Spinel crystals, with the chemical formula AB$_2$X$_4$, in which only the A atom is magnetic, realize antiferromagnetism on a diamond sublattice. We first discuss examples, such as CoAl$_2$O$_4$ and MnSc$_2$S$_4$, which exhibit ``bond frustration'' due to the competing effects of first and second neighbor interactions. This is well modeled by a classical Heisenberg Hamiltonian, which leads to a remarkable ground state degeneracy of coplanar spirals, in which the wavevector of the spiral can lie anywhere on a ``spiral surface'' in momentum space. We describe how thermal fluctuations lead to a broad spin liquid regime, with unique properties, and magnetic ordering at low temperatures. We next discuss the intriguing case of FeSc$_2$S$_4$, in which orbital degeneracy leads to a persistant ``spin orbital liquid'' down to the lowest temperatures. We argue that this material is in the vicinity of an unusual quantum critical point driven by a competition between exchange and spin-orbit interactions. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q31.00010: Phase competition and large residual entropy in the pyrochlore double-exchange system Yukitoshi Motome, Nobuo Furukawa Strong interplay between spin and charge degrees of freedom gives rise to fascinating phenomena, in particular when the system undergoes severe geometrical frustration, such as anomalous magneto-transport and heavy-mass behavior. We investigate this intriguing problem in the double-exchange model on the frustrated pyrochlore lattice by employing an unbiased Mote Carlo simulation. We find that as increasing the antiferromagnetic superexchange between localized moments, the ferromagnetic metallic state stabilized by the double-exchange mechanism becomes unstable, and is taken over by a paramagnetic metal in which spin correlations are strongly suppressed by the frustration. In the critical region, the system exhibits a peculiar electronic and magnetic state; the competing interactions cancel with each other, resulting in an almost temperature independent state. This indicates a large entropy remaining at low temperatures, which potentially leads to heavy mass behavior or some nontrivial symmetry breaking at a lower temperature. We also discuss the relevance to experiments in itinerant pyrochlore materials. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q31.00011: Magnetic Bose condensation vs. magnon localization in a model magnet with site dilution Tommaso Roscilde, Stephan Haas, Rong Yu We report on the theoretical field-temperature phase diagram of anisotropic coupled S=1 chains with site dilution, modeling the magnetic behavior of doped NiCl$_2$-\emph{tetrakis} thiourea (DTN). In absence of doping, this compound clearly displays field-induced Bose-Einstein condensation of magnons [V. Zapf \emph{et al.}, Phys. Rev. Lett. {\bf 98}, 047205 (2007)], as revealed by the mean-field scaling of the field-induced ordering temperature, $T_c \sim |H-H_c|^{\phi}$ with $\phi=2/3$. The critical field $H_c$ corresponds to a $T=0$ quantum phase transition (QPT) between a spin gap phase and a gapless ordered phase. Here we show that site dilution opens a novel gapless spin-liquid phase close to the ordering transition, corresponding to a \emph{Bose glass} phase of localized magnons. Disorder leads to a radical change in the universality class of the QPT (which turns into a \emph{quantum percolation} transition), and in the critical temperature scaling, which exhibits a novel universal exponent $\phi\approx 1.2$. A crossover to mean-field scaling of $T_c$ at finite temperature is observed, and explained via a scenario of \emph{thermal percolation} of magnons. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q31.00012: Effect of chemical substitution on quantum spin ladders Tao Hong, Andrey Zheludev, Hirotaka Manaka, Yiming Qiu Unlike in any other quantum spin ladder system, the magnitude of the spin gap in IPA-CuCl$_{3}$ [1] can be tuned by chemical doping. We describe inelastic neutron scattering measurements on powder samples of IPA-Cu(Br$_{x}$Cl$_{1-x})_{3}$ for a different values of x. Br-doping directly affects the key interactions involving Cu-Cl-Cl-Cu superexchange pathway and induces bond randomness [2]. In the pure system the spin gap is readily visible. At 3{\%} and 7{\%} Br-doping, the gap models are weakened and presumably broadened, while additional scattering emerges at low energies. The gap excitations totally disappear by x=10{\%}. [1] T. Masuda, A. Zheludev, H. Manaka, L.-P. Regnault, J.-H. Chung, Y. Qiu, Phys. Rev. Lett. 96, 047210 (2006). [2] H. Manaka, I. Yamada, H. Mitamura and T. Goto, Phys. Rev. B \textbf{66}, 064402 (2002); H. Manaka \textit{et al.}, Phys. Rev. Lett. \textbf{101}, 077204 (2008). [Preview Abstract] |
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