Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session B3: Phonons and Magnetic Frustration in Pyrochlores |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Leon Balents, University of California, Santa Barbara Room: Colorado Convention Center Korbel 2A-3A |
Monday, March 5, 2007 11:15AM - 11:51AM |
B3.00001: Quenching ground state degeneracy in pyrochlore antiferromagnets Invited Speaker: The classical pyrochlore antiferromagnet (AFM) is considered the ``most'' geometrically frustrated system. Classically, this leads to the absence of any ordering transition at non-zero temperature, even in an applied magnetic field. We describe several mechanisms by which the ground state degeneracy can be split by fluctuations or other effects. We first consider quantum fluctuations, which may lead to the formation of a novel spin-liquid state or complex magnetic ordering. By deriving an effective Hamiltonian, we determine the quantum ground states for different values of spin $s$, resolving some of the ambiguities in existing large-$s$ spin-wave treatments. A quantitative application to recent experiments on the spinel chromites, ACr$_2$O$_4$ (A=Cd,Hg) shows that for the relevant $s=3/2$ the quantum effects are too weak to explain the observed ordering and the existance of a very robust magnetization plateau in a field. We then turn to a model of spin-lattice coupling that explains both the plateau formation and the observed ordering on the plateau. The predictions are confirmed by recent neutron scattering and x-ray scattering experiments (S. H. Lee et al.). The same model applied to zero magnetic field predicts a reduced but still large ground state degeneracy, including the states observed in both the Cd and Hg materials. This is consistent with the dominance of spin-lattice interactions, with weak additional effects determining the low field magnetic ordering. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:27PM |
B3.00002: Heisenberg antiferromagnet on the pyrochlore lattice: order from distortion Invited Speaker: The Heisenberg antiferromagnet on the pyrochlore lattice is an
example
of a highly frustrated system with a large degeneracy of the ground
state. The classical model with nearest-neighbor interactions shows
no signs of magnetic order down to very low temperatures. The
quantum
analog, with short enough spins, was considered a prime candidate for
a quantum-disordered ground state, such as a valence-bond liquid or
solid. At the same time, the large degeneracy makes this magnet
susceptible to a variety of nominally small perturbations. A
spin-lattice coupling leads to a spin-Peierls-like distortion of the
lattice. In contrast to spin chains, the spin-Peierls distortion
in a pyrochlore antiferromagnet occurs for any spin length $S$
remaining robust even in the classical
limit.
A recent experimental characterization of the $S=3/2$ prototype
CdCr$_2$O$_4$ [1] provided a test for the theoretical
model. This antiferromagnetic spinel exhibits a tetragonal lattice
distortion with an elongated unit cell $a=b |
Monday, March 5, 2007 12:27PM - 1:03PM |
B3.00003: Nonlinear optical signatures of the tensor order in Cd$_2$Re$_2$O$_7$ Invited Speaker: The pyrochlore oxide Cd$_2$Re$_2$O$_7$ undergoes a structural phase transition at $T=200$~K with an unusual tensor character.\footnote{J. C. Petersen \textit{et al.}, Nature Phys. \textbf{2}, 605 (2006); C. A. Kendziora \textit{et al.}, PRL \textbf{95}, 125503 (2005); I. A. Sergienko \textit{et al.}, PRL \textbf{92}, 065501 (2004).} The order parameter for this state is two-dimensional, and associated with a phonon mode with $E_u$ symmetry. In magnetically frustrated pyrochlores, magnetoelastic coupling to similar modes can induce fascinating magnetically ordered states;\footnote{Y. Yamashita and K. Ueda, PRL \textbf{85}, 4960 (2000); O. Tchernyshyov \textit{et al.}, PRL \textbf{88}, 067203 (2002) and PRB \textbf{66}, 064403 (2002).} Cd$_2$Re$_2$O$_7$ is nonmagnetic, so it is possible to study the structural instabilities of the pyrochlore lattice in isolation. We have used optical second harmonic generation with polarization sensitivity to resolve an ambiguity in the low-temperature crystal structure, and verify an auxiliary condition on the structure that is implied by the order parameter symmetry. We also show that the temperature-dependence of the order parameter is consistent with thermal occupation of a Goldstone mode that results from the $E_u$ order parameter symmetry. The methodology that we have developed may be applied more widely in characterizing ordered states in matter. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:39PM |
B3.00004: Frustrated and Satisfied Ground States in Pyrochlore Antiferromagnet Tb$_2$Ti$_2$O$_7$ Invited Speaker: The Rare-Earth Titanates have been a playground for the physics of geometrical frustration, as magnetic rare earth sites can be arranged on the pyrochlore lattice, a network of corner-sharing tetrahedra, and different rare earth elements display differing exchange interactions and anisotropies. Tb$_2$Ti$_2$O$_7$ is a particularly enigmatic pyrochlore antiferromagnet. It is reasonably well understood in terms of local Ising-like anisotropy, such that the Tb$^3+$ magnetic moments should point either directly into or out of the tetrahedra. The moments are antiferromagnetically coupled, and theory expects a phase transition to an ordered state at $\sim$ 1 K. However, in the abnsence of an external perturbation, it remains in a spin liquid state to very low temperatures, $<$ 0.05 K. We report new neutron scattering measurements on Tb$_2$Ti$_2$O$_7$ using new time-of-flight techniques at NIST, which show the spin liquid state can be brought to order in the presence of relatively weak [110] magnetic fields. We'll also discuss high resolution x-ray scattering results which show that the lattice is not a passive observe to the physics of frustration, but that it displays fluctuation effects typical of Jahn Teller physics, albeit at very low temperatures. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 2:15PM |
B3.00005: Neutron scattering studies of magnetic pyrochlores Invited Speaker: The pyrochlore antiferromagnets in which spins interact in a network of corner-sharing tetrahedra have macroscopic ground state degeneracy, that leads to exotic low temperature properties. Spinels AB$_2$O$_4$ realize the pyrochlore lattice if the B ions couple antiferromagnetically. This talk will start with a quick review of several novel properties found in spinels, such as the spin liquid state in ZnCr$_2$O$_4$, the 3D spin-Peierls transition in ZnCr$_2$O$_4$, the spin-orbital coupling in ZnV$_2$O $_4$, and the heavy fermionic behaviors in LiV$_2$O$_4$. A discussion will follow on our recent neutron and X-ray scattering works on ACr$_2$O$_4$ (A=Cd, Hg). We will show that the 3D spin-Peierls transition in CdCr$_2$O$_4$ is different from that observed in ZnCr$_2$O$_4$, and that the magnetic field-induced half-magnetization plateau state in HgCr$_2$O$_4$ has the P4$_3$32 symmetry. Our results provide direct tests of theoretical models proposed to understand the complex behaviors of the Heisenberg pyrochlore antiferromagnets. A quantum spin pyrochlore system will also be discussed. [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