Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session W27: Focus Session: Spin Glasses and SrCu2(BO3)2 |
Hide Abstracts |
Sponsoring Units: GMAG Chair: Matt Stone, Oak Ridge National Laboratory Room: Morial Convention Center 219 |
Thursday, March 13, 2008 2:30PM - 2:42PM |
W27.00001: Unconstrained Chern-Simons theory for the geometrically frustrated spin compound SrCu$_2$(BO$_3$)$_2$ Cristian Batista, Pinaki Sengupta, Suchitra Sebastian, Neil Harrison We show that an unconstrained Chern-Simons theory -- where the local densities are determined in a self-consistent manner -- correctly reproduces the sequence of magnetization plateaus recently observed in the geometrically frustrated spin compound SrCu$_2$(BO$_3$)$_2$ in an external magnetic field. The theory predicts that at the plateaus, the triplets are arranged in stripe patterns which is consistent with NMR experiments at and close to the 1/8 plateau. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W27.00002: Fractalisation drives crystalline states in the frustrated spin system SrCu$_2$(BO$_3$)$_2$ Suchitra Sebastian, N. Harrison, P. Sengupta, C.D. Batista, S. Francoual, E. Palm, T. Murphy, H.A. Dabkowska, B.D. Gaulin Geometrical frustration in the spin dimer material
SrCu$_{2}$(BO$_{3})_{2}$ leads to a singlet Shastry-Sutherland
groundstate at low magnetic fields, but complex spin
superstructures at higher fields. Our magnetisation measurements
reveal a fine substructure of quantum Hall-like plateaux at all
$1/q$ ratios $2 |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W27.00003: Doped Valence Bond Solid and Superconductivity on the Shastry-Sutherland Lattice Bohm-Jung Yang, Yong Baek Kim, Jaejun Yu, Kwon Park Motivated by recent experiments on SrCu$_2$(BO$_3$)$_2$, we present a theoretical framework for understanding the ground states of the doped Mott insulator on the Shastry-Sutherland lattice. To provide a unified theoretical framework for both the valence bond solid state found in undoped SrCu$_2$(BO$_3$)$_2$ and the doped counterpart in on-going experimental pursuit, we analyzed the $t$-$J$-$V$ model via the bond operator formalism. The interplay between strong dimerization and the nearest-neighbor repulsive interaction leads to different behaviors of the doped holes determining the overall phase diagram. Specifically, if the nearest-neighbor repulsive interaction, $V$, is smaller than a critical value, $V_c$, the hole-pairing within dimers is preferred, resulting in the $S$- wave superconductivity at any non-zero $x$. On the other hand, if $V$ is larger than $V_c$, the density of paired-holes within the same dimers vanishes at finite $x$ and the plaquette $D$-wave superconductivity with a peculiar spatial pattern emerges. Implications to future experiments are discussed. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W27.00004: Critical Properties of the Unconventional Spin-Peierls System TiOBr J.P. Clancy, B.D. Gaulin, F.C. Chou TiOBr is one of only three inorganic compounds which have been shown to undergo a spin-Peierls (SP) transition, developing a dimerized singlet ground state at low temperatures. However, unlike conventional SP systems, TiOBr exhibits not one, but two successive phase transitions: a continuous transition to an incommensurate SP state at T$_{c2}$$\sim$48K, followed by a discontinuous transition to a commensurate SP state at T$_{c1}$$\sim$27K. We have performed x-ray diffraction measurements on single crystal TiOBr, and carried out a detailed analysis of the critical exponent $\beta$ near T$_{c2}$. Our results yield a transition temperature of T$_{c2}$=47.85(5)K and an exponent of $\beta$=0.30(3). This value is consistent with conventional 3D Ising-like behavior ($\beta$=0.326), and closely compares to the exponent reported for CuGeO$_{3}$ ($\beta$=0.36(3)) [1], the canonical inorganic SP system. In contrast with measurements performed on isostructural TiOCl [2], we observe no evidence of commensurate dimerization fluctuations in either the incommensurate SP phase (T$_{c1}$$<$T$<$T$_{c2}$) or the so-called pseudogap phase (T$_{c2}$$<$T$<$T*$\sim$100-150K). Furthermore, the incommensurate scattering between T$_{c1}$ and T$_{c2}$ appears to be shifted in Q-space relative to the commensurate scattering below T$_{c1}$. [1] M.D. Lumsden et al., PRL {\bf 76}, 4919 (1996). [2] J.P. Clancy et al., PRB {\bf 75}, 100401(R) (2007). [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W27.00005: Thermal properties and Ehrenfest relation in an ``ideal" spin glass. S.L. Bud'ko, P.C. Canfield, G.M. Schmiedeshoff In order to test whether the Ehrenfest relation is applicable to the spin glass ``transition,'' we report temperature-dependent heat capacity and thermal expansion measurements on an ``ideal'' spin glass material: single grain icosahedral Gd-Mg-Zn quasicrystal and its non-magnetic counterpart, Y-Mg-Zn. Spin-glass state related signatures are clearly seen in the Gd-Mg-Zn data sets. Directly measured pressure dependence of the freezing temperature in Gd-Mg-Zn is compared to its evaluation through the Ehrenfest relation. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W27.00006: Anomalous field-sweep rate dependence of magnetization process of a spin-glass system Fe$_{x}$Mn$_{1-x}$TiO$_{3}$. N. Miura, H. Aruga Katori, A. Ito, M. Schoenhart, R. Groessinger, N. V. Kozlova, K. Doerr A mixed compound Fe$_{0.50}$Mn$_{0.50}$TiO$_{3}$ is a typical ising spin-glass. When a magnetic field is applied to the spin-glass state with a rather fast sweep rate 0.2 $<$ \textit{dB/dt} $<$ 1 T/s by a steady field magnet, the sample shows step-like jumps in the magnetization, whereas in a pulse magnet with an even faster sweep rate above 700 T/s, it shows a smooth metamagnetic transition. The field of the step-like jump is decreased as the sweep rate is increased, which is contrary to any relaxation phenomena with a slow response time. In the case of smooth metamagnetic transition at faster sweep rates, the transition field is increased as the sweep rate is increased. In this study, we investigated the magnetization process in the intermediate range of field sweep rate between 1.0 and 124 T/s using an ultra-long pulse magnet. It was found that as the sweep rate is increased, the jump behavior changes over to the metamagnetic behavior at around 2.5 T/s, where the transition field takes the minimum. The observed results are suggestive of some locally grown non-equilibrium spin alignment. The jump behavior is discussed in connection with similar phenomena in perovskite manganites. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W27.00007: The Free Energy Landscape of a Two-Dimensional Spin Glass from a Hard-Spin Mean-Field Treatment Michael Mihalco, Susan McKay The two-dimensional Ising antiferromagnet on a triangular lattice shows no ordered phase at finite temperatures until frustration is relieved, often accomplished through dilution (Grest and Gabl PRL 43, Netz and Berker PRL 66) or by the addition of randomly placed ferromagnetic bonds. Using the second approach, we have created a two-dimensional spin-glass phase with tunable frustration and explored its free energy landscape using a hard-spin mean-field approach (Netz and Berker PRL 66). The Monte Carlo implementation of this method quickly provides a self-consistent solution of site magnetizations, and we have used the overlaps and internal energies of various solutions to explore the energy landscape as a function of temperature and the number of frustrated plaquettes. Results indicate a multiplicity of solutions and a rich structure of overlaps, which are sensitive to the temperature and level of frustration. This method shows clearly the distinctions between the spin-glass phase and the bordering ferromagnetic ordering that occurs once the ferromagnetic interactions dominate. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W27.00008: Field-Driven Hysteresis of the d=3 Ising Spin Glass: Hard-Spin Mean-Field Theory Burcu Y\"ucesoy, A. Nihat Berker Hysteresis loops are obtained in the Ising spin-glass phase in $d=3$, using frustration-conserving hard-spin mean-field theory.[1] The system is driven by a time-dependent random magnetic field $H_Q$ that is conjugate to the spin-glass order $Q$, yielding a field-driven first-order phase transition through the spin-glass phase. The hysteresis loop area $A$ of the $Q-H_Q$ curve scales with respect to the sweep rate $h$ of magnetic field as $A-A_{0}$ $\sim $ $h^{b}$. In the spin-glass and random-bond ferromagnetic phases, the sweep-rate scaling exponent $b$ changes with temperature $T$, but appears not to change with antiferromagnetic bond concentration $p$. By contrast, in the pure ferromagnetic phase, $b$ does not depend on $T$ and has a sharply different value than in the two other phases. \newline \noindent [1] B. Y\"ucesoy and A.N. Berker, Phys. Rev. B {\bf 76}, 014417 (2007). [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W27.00009: Quantum Induced Asymmetric Phase Diagrams of Spin-Glass Systems C. Nadir Kaplan, A. Nihat Berker The spin-1/2 quantum Heisenberg spin-glass system is studied in all spatial dimensions $d$ by renormalization-group theory.[1] Strongly asymmetric phase diagrams in temperature and antiferromagnetic bond probability $p$ are obtained in dimensions $d\geq3$. The asymmetry at high temperatures approaching the pure ferromagnetic and antiferromagnetic systems disappears as $d$ is increased. However, the asymmetry at low but finite temperatures remains in all dimensions, with the antiferromagnetic phase receding to the ferromagnetic phase. A finite-temperature second-order phase boundary directly between the ferromagnetic and antiferromagnetic phases occurs in $d\geq6$, resulting in a new multicritical point at its meeting with the boundaries to the paramagnetic phase. In $d=3,4,5$, a paramagnetic phase reaching zero temperature intervenes asymmetrically between the ferromagnetic and reentrant antiferromagnetic phases. There is no spin-glass phase in any dimension. \newline \noindent [1] C.N. Kaplan and A.N. Berker, arXiv:0709.3589v1 [cond-mat.dis-nn], Phys. Rev. Lett., in press. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W27.00010: Spin phonon induced colinear order and magnetization plateaus in triangular and kagome antiferromagnets. Applications to CuFeO$_2$ Fa Wang, Ashvin Vishwanath Coupling between spin and lattice degrees of freedom are important in geometrically frustrated magnets where they can lead to degeneracy lifting and novel orders. We show that moderate spin-lattice couplings in triangular and Kagome antiferromagnets can induce complex colinear magnetic orders. When classical Heisenberg spins on the triangular lattice are coupled to Einstein phonons, a rich variety of phases emerge, including the experimentally observed four sublattice state and the five sublattice 1/5th magnetization plateau state seen in the magneto-electric material CuFeO$_2$. In addition we predict magnetization plateaus at 1/3, 3/7, 1/2, 3/5 and 5/7 at these couplings. Strong spin-lattice couplings induce a striped colinear state, seen in $\alpha$-NaFeO$_2$ and MnBr$_2$. On the Kagome lattice, moderate spin-lattice couplings induce colinear order, but an extensive degeneracy remains. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W27.00011: Jahn-Teller coupling and magnetic ground state in vanadium spinels$^1$ Gia-Wei Chern, Oleg Tchernyshyov The interplay of orbital, lattice, and spin degrees of freedom in vanadium spinels has attracted much interest among researchers. The V$^{3+}$ ion has two electrons occupying three degenerate $t_{2g}$ orbitals and is thus Jahn-Teller active. It also has a total spin $S=1$ in accordance with Hund's rules. Moreover, the V$^{3+}$ ions sitting on the $B$-site of spinel form a pyrochlore lattice, the interactions between these localized spin and orbital degrees of freedom are thus geometrically frustrated [1]. Here we present a theoretical model for the ground states of vanadium spinels. We view all of the vanadates (Cd, Zn, Mg on the one hand and Mn on the other) within the same model in which the influence of Mn is simulated by a magnetic field. In the case of MnV$_2$O$_4$, our calculation yields a ground state with antiferro-orbital ordering accompanied by a tetragonal structural distortion with lattice constants $a=b>c$. In addition, the V spins develop an orthogonal antiferromagnetic order in the $ab$ plane on top of a ferromagnetic moment along the $c$ axis. The results are consistent with a recent experimental characterization of MnV$_2$O$_4$ [2]. In particular, we will discuss the important role played by cooperative Jahn-Teller interaction and spin-orbital coupling in stabilizing the orthogonal spin configuration. [1] O. Tchernyshyov, Phys. Rev. Lett., {\bf 93}, 157206 (2004). [2] V. O. Garlea {\em et al.}, cond-mat/0711.1844. $\,^1$NSF Grant No. DMR-0348679 [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W27.00012: AC Susceptibility and Heat Capacity Studies of Geometrically Frustrated Pyrochlores Daniel Antonio, Andrew Cornelius, Jason Gardner Materials with a geometrically frustrated magnetic pyrochlore lattice have been of interest due to their unusual ground states. Tb$_{2}$Ti$_{2}$O$_{7}$ is known for going to a spin liquid ground state and does not transition to a long-range ordered state down to at least 50 mK [1]. Ho$_{2}$Ti$_{2}$O$_{7}$ has a macroscopically degenerate spin ice ground state which resembles that of the proton ordering in water ice [2]. Heat Capacity measurements of Tb$_{2}$Ti$_{2-x}$Sn$_{x}$O$_{7}$ were done from 300 K to 0.36 K and AC magnetic susceptibility measurements of Ho$_{2-x}$Y$_{x}$Ti$_{2}$O$_{7}$ and $_{2-x}$La$_{x}$Ti$_{2}$O$_{7}$were done for frequencies from 10 Hz to 10 kHz down to 1.8 K, both in magnetic fields up to 9 T. These experiments were performed to further understand the factors leading to their unusual behavior and the effects of introducing disorder through doping with nonmagnetic elements. Determination of the effect of an external field on the hyperfine crystal field at the Tb sites was done. In addition, unusual behavior in the ac susceptibility of the Ho samples at lower temperatures was observed. [1] J.S. Gardner \emph{et al.}, Phys. Rev. Lett. \textbf{82}, 1012 (1999) [2] S.T. Bramwell and M.J.P. Gingras, Science \textbf{294}, 1495 (2001) [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W27.00013: Local studies of AC demagnetization in a model frustrated magnet Jie Li, Xianglin Ke, Cristiano Nisoli, Paul Lammert, Vincent Crespi, Peter Schiffer We have studied the process of ac demagnetization using a model system consisting of single-domain ferromagnetic islands arranged on perpendicular square lattices such that the interaction between the islands are frustrated by the geometry of the arrays. The sample is first subjected to an oscillating magnetic field whose magnitude is stepped down by different step sizes and the sample is rotating at the same time. We find that the net moment of the arrays can be readily brought to near zero with the magnetic field step size below a certain threshold value. However the interaction energy of the demagnetized array state continues to decrease linearly with decreasing step size even as the step size approaches zero. We characterize the result in terms of developing local correlations between the island moments, with strong analogies to other athermal magnetic systems. This research has been supported by the Army Research Office. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W27.00014: Dimensional Crossover in ZnMn$_{2}$O$_{4}$ William Ratcliff, Ying Chen, Goran Gasparovic, Yiming Qiu, Qing Huang, Jeffrey Lynn, Sunmog Yeo, Sang Cheong, Paula Piccoli, Arthur Schultz ZnMn$_{2}$O$_{4}$ crystallizes at high temperatures as a cubic spinel. At lower temperatures, it undergoes a Jahn Teller distortion which lowers its symmetry to tetragonal. At lower temperatures (TN\~{ }60 K), the system orders magnetically. Fits to the order parameter, line shape of powder diffraction peaks, and the direct observation of rods of scattering in single crystal diffraction experiments show the system to be two dimensional. This is likely due to the interplay between orbital ordering and frustration. In this talk I discuss the magnetic structure and spin waves of this system. I will also discuss an interesting crossover in the dimensionality of the magnetism in this compound. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700