Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session H16: Focus Session: 2D Quantum Magnetism |
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Sponsoring Units: GMAG Chair: Igor Zaliznyak, Brookhaven National Laboratory Room: Colorado Convention Center Korbel 4F |
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H16.00001: Competing Impurities in an Antiferromagnetic Background. Lyudmyla Adamska, Marcello B. Silva Neto, Cristiane Morais Smith We study the order-from-disorder transition and reentrant magnetism in a doped crystal La$_{2-x}$Sr$_{x}$Cu$_{1-z}$Zn$_{z}$O$_{4}$ within the framework of a long-wavelength nonlinear sigma model that properly incorporates the Dzyaloshinskii- Moriya and XY anisotropies. The effect of doping La$_{2}$CuO$_{4}$ with nonmagnetic impurities, such as Zn, is considered according to classical percolation theory, while the effect of the Sr-doped charge carriers is described as a dipolar frustration of the antiferromagnetic order. We derive the expressions for several magnetic, thermodynamic, and spectral properties of the system, such as the N\'{e}el temperature, the spin-stiffness, and the anisotropy gaps, as well as their evolution with both Zn and Sr doping for cases of ballistic and diffusive vacancies. We solve the problem analytically in the limits of low and high temperatures. The connection to several experiments will be discussed [Phys. Rev. B59, R725 (1999), Science 295, 1691 (2002), Phys. Rev. Lett. 93, 027001 (2004)]. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H16.00002: Quantum effects of impurity-induced frustrations in diluted antiferromagnets Shiu Liu, Sasha Chernyshev We show that in an antiferromagnet doped with non-magnetic impurities, e.g. Zn-doped La$_2$CuO$_4$, an impurity can induce substantial frustrating interactions among the next- and next-next-nearest neighbor Cu spins around the impurity site, $J'_{Zn}$ and $J''_{Zn}$, respectively. Such interactions arise from the virtual transitions through the Zn and O orbitals. We study the 2D, square lattice, $S=1/2$ Heisenberg antiferromagnet in which the frustration is induced by the dilution with such impurities. We use the $T$-matrix approach to calculate the quantum effect of such dilution on the antiferromagnetic order parameter, staggered magnetization $M(x)$, as a function of the doping concentration $x$. In the experimentally relevant range of $J'_{Zn}$ and $J''_{Zn}$ we find substantial deviation of our results for $M(x)$ from the non-frustrated site-dilution theories. We argue that the frustration effect explains discrepancies between the experimental data and the non-frustrated site-dilution theory of the copper-oxide plane with Zn impurities. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H16.00003: Frustration in the Hubbard model: a quantum cluster study Andriy Nevidomskyy, Christian Scheiber, David Senechal The role of frustration in the Hubbard model is studied on the square lattice with nearest and next-nearest neighbour hoppings $t$ and $t'$ using the Variational cluster perturbation theory (VCPT, see [1]). We find two phases with long-range magnetic order: the usual antiferromagnet (AF1) phase, stable at small $t'/t,$ and the so-called superantiferromagnetic phase (AF2) for large frustration. These are separated by a phase with no magnetic order. We also find d-wave superconductivity (d$_{x2-y2})$ for small values of $U\le $4$t$ and sufficiently weak frustration. The Mott-Hubbard transition is discussed in this context. We also compare the classical phase diagram obtained from the large-U expansion with that of the frustrated J$_{1}$-J$_{2}$ Heisenberg model. \newline \newline [1] M. Potthoff, M. Aichhorn, C. Dahnken, Phys. Rev. Lett. \textbf{91}, 206402 (2003). [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H16.00004: High Frequency ESR study of the two-dimensional Heisenberg antiferromagnet copper pyrazine perchlorate. Johan van Tol, Saritha Nellutla, Chris Landee Copper pyrazine perchlorate, Cu(Pyrazine)$_{2}$(ClO$_{4})_{2}$, is thought to be an almost prefect 2D Heisenberg antiferromagnet. We have performed single crystal electron magnetic resonance experiments at 120, 240, and 336 GHz on this system. Both the line-width and resonance shift are field-dependant and indicate that the 3D antiferromagnetic ordering temperature increases from 4.2 K at zero field to about 7 K at 12 Tesla. This can be interpreted by a the field-induced XY-behavior, as has been predicted[1]. A. Cuccoli et al., \textit{Phys. Rev. B} \textbf{68}, 060402 (2003). [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H16.00005: Field dependent ordering temperature in copper pyrazine perchlorate, Cu(pz)$_{2}$(ClO$_{4}$)$_{2}$ Christopher Landee, Fan Xiao, Mark Turnbull, N. Tsyrulin, Michel Kenzelmann, Hans Van Tol Copper pyrazine perchlorate is a molecular-based 2D S=1/2 Heisenberg antiferromagnet (QHAF) with a moderate exchange constant (J/k = 17.5 K) and a saturation field of 60 T. The zero-field ordering temperature, as recently determined by muon spin relaxation experiments [1], is 4.3 K corresponding to excellent isolation (J$'$/J $\approx $ 8x10$^{-4})$ between magnetic layers [2]. Recent studies of Cu(pz)$_{2}$(ClO$_{4})_{2}$ in applied fields (specific heat and ESR) show the ordering transition to increase with field by as much as 30{\%} in a field of nine tesla. This effect will be discussed in terms of a field-induced anisotropy crossover model [3]. 1. T. Lancaster, S. J. Blundell et al, submitted for publication. 2. P. Sengupta, A. W. Sandvik, and R. R. P. Singh, Phys. Rev. B \textbf{68}, 094423 (2003). 3. A. Cuccoli et al, Phys. Rev. B \textbf{68}, 060402 (2003). [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H16.00006: Field-induced anisotropy crossover in copper pyrazine perchlorate Cu(pz)$_{2}$(ClO$_{4}$)$_{2}$ Fan Xiao, Christopher Landee, Mark Turnbull The temperature-dependent magnetization of a single crystal of the 2D QHAF copper pyrazine perchlorate was studied at different fields along all three crystalline orientations. The crystal has identical response when the applied field lies within the layer, only showing a low temperature minimum for fields larger than 3 kOe. The temperature of the minimum increases smoothly with the field strength. The effect is explained as a field-induced 2D Heisenberg to 2D XY anisotropy crossover [1]. \newline \newline [1] A. Cuccoli et al, Phys. Rev. B \textbf{68}, 060402 (2003). [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:48AM |
H16.00007: Spontaneous spin-lattice coupling in the geometrically frustrated triangular lattice antiferromagnet CuFeO2 Invited Speaker: We use high-resolution synchrotron x-ray and neutron diffraction to study the geometrically frustrated triangular lattice antiferromagnet CuFeO$_{2}$. On cooling from room temperature, CuFeO$_{2}$ undergoes two antiferromagnetic phase transitions with incommensurate (IC) and commensurate magnetic order at $T_{N1}$=14 K and $T_{N2}$=11 K, respectively. The occurrence of these two magnetic transitions is accompanied by second- and first-order structural phase transitions from hexagonal to monoclinic symmetry. Application of a 6.9 T magnetic field lowers both transition temperatures by 1 K, and induces additional IC structural and magnetic modulations in the temperature region where magnetoelectric behavior has been observed.$^{\S }$ These results show that a strong magneto-elastic coupling is intimately related to the multiferroic effect. It is believed that the IC magnetic structure is noncollinear because this would break the inversion symmetry, as required for the appearance of multiferroic behavior. Because the multiferroic phase appears only in a high magnetic field it is not possible to perform a full crystallographic study to demonstrate this noncollinearity, and an indirect method is being pursued that involves the measurement and characterization of the spin dynamics of this system using neutron scattering.~ This novel approach probes the link between multiferroelectricity and the appearance of noncollinear IC magnetic structures. $^{\S }$Ye et al, Phys. Rev. B 73, 220404 (R) (2006) [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H16.00008: Quantum and Classical Spins on the Distorted Kagome (volborthite) Lattice Fa Wang, Ashvin Vishwanath, Yong Baek Kim The spin-1/2 quantum antiferromagnet on the distorted Kagome lattice (with bonds along one direction stronger than along the other two directions) is realized in the mineral volborthite [F.Bert, et.al. Phys.Rev.Lett. 95, 087203 (2005)]. Here we study properties of antiferromagnetic spin systems on this lattice, in particular how the distortion affects the extreme frustration of the Kagome structure. We first consider ground states of classical O(3) spins on this lattice and show that there is a very large (although probably sub-extensive) number of them. Order-by-disorder effects resulting from thermal or quantum spin waves will be discussed. Finally, we consider approaching the problem directly from the quantum limit via Schwinger boson and fermion mean field theories. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H16.00009: Spin chirality and high-field phase diagram of the kagom\'e lattice antiferromagnet $A$Fe$_{3}$(OH)$_{6}$(SO$_{4}$)$_{2}$ K. Matan, J. S. Helton, D. Grohol, B. M. Bartlett, V. Sikolenko, D. G. Nocera, Y. S. Lee We have used neutron scattering to study spin chirality and high-field spin re-oriention in single crystal and powder samples of the antiferromagnetic kagom\'e lattice compounds, $A$Fe$_{3}$(OH)$_{6}$(SO$_{4}$)$_{2}$ ($A$ = K and Ag). The $\frac{5}{2}$ Fe$^{3+}$ spins on the kagom\'e lattice order three dimensionally for temperatures below the N\'eel temperature, T$_{N}$ = 65 K. Above the N\'eel temperature, we observed the spin chiral order. Neutron measurements reveal critical spin fluctuations above T$_N$, which indicate that the spin-rotational symmetry and the vector chiral symmetry are not broken simultaneously at T$_N$. Below the N\'eel temperature, neutron scattering measurements in high fields show a spin re-orientation transition, which appear to be first-order in nature. Details of the high field phase diagram will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H16.00010: Ground state and excitation properties of the quantum kagom\'{e} system ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ investigated by local probes. Oren Ofer, Amit Keren, Emily Nytko, Matthew Shores, Bart Bartlett, Daniel Nocera, Alex Amato, Chris Baines We present a comprehensive study on the ground state and excitation spectrum of the $S=1/2$, analytically pure and perfect kagom\'{e} system ZnCu$_{3}$% (OH)$_{6}$Cl$_{2}$ using the following measurements: magnetization, muon spin rotation frequency shift $K$, transverse relaxation time $T_{2}^{\ast }$% , and zero field relaxation, and Cl nuclear spin-lattice relaxation $T_{1}$. Using our data we address four questions which are at the heart of the investigation of the quantum kagom\'{e} system: Do $S=1/2$ spins on kagom% \'{e} lattice freeze? Is the ground state magnetic? What is the density of excited states, and is there a gap in the spin energy spectra? Finally, does the lattice distort in order to accommodate spin-Peierls state? We found no sign of singlet formation, no long range order nor spin freezing, and no sign of spin-Peierls transition even at temperatures as low as $60$~mK. The density of states has an $E^{1/4}$ energy dependence with a negligible magnetic gap to excitation. Thus ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ is an exotic magnet with no broken continuous symmetry but gapless excitations. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H16.00011: Spin dynamics of the S=1/2 kagom\'e lattice antiferromagnet ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ J.S. Helton, K. Matan, M.P. Shores, E.A. Nytko, B.M. Bartlett, Y. Yoshida, Y. Takano, A. Suslov, Y. Qiu, J.-H. Chung, D.G. Nocera, Y.S. Lee An important challenge in condensed matter physics is the search for quantum spin liquid states in two dimensional frustrated systems. We have performed thermodynamic and neutron scattering measurements on the $S=1/2$ kagom\'e lattice antiferromagnet ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$. The susceptibility indicates a Curie-Weiss temperature of $\theta_{CW} \simeq -300$~K; however, no magnetic order is observed down to 50~mK. Inelastic neutron scattering reveals a spectrum of low energy spin excitations with no observable gap down to 0.1~meV. The specific heat at low-$T$ follows a power law with exponent $\alpha \leq 1$. These results suggest that an unusual spin-liquid state with essentially gapless excitations is realized in this kagom\'e lattice system. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H16.00012: Theory of spin liquid on the kagome lattice and application to ZnCu$_3$(OH)$_6$Cl$_2$: Projected wavefunction study Ying Ran, Michael Hermele, Patrick A. Lee, Xiao-Gang Wen Recent experiments on ZnCu$_3$(OH)$_6$Cl$_2$ have drawn new attention to the ground state of the spin-1/2 kagome lattice antiferromagnet. We have examined this issue using Gutzwiller projected fermion wavefunctions, and propose that the ground state is described by the projection of fermions hopping on the kagome lattice in a background $\pi$-flux on the hexagons and zero flux on the triangles. This state has gapless Dirac points and is an algebraic spin liquid. Properties of the wavefunction and low-energy excitations will be discussed, with a focus on application to the experiments. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H16.00013: Theory of spin liquid on the kagome lattice and application to ZnCu$_3$(OH)$_6$Cl$_2$ : Quantum field theory Michael Hermele, Ying Ran, Patrick A. Lee, Xiao-Gang Wen We study the properties of an algebraic spin liquid on the kagome lattice, which is suggested by projected wavefunction calculations to be the ground state. Various quantities of relevance to ZnCu$_3$(OH)$_6$Cl$_2$ will be discussed, including specific heat, magnetic susceptibility and spin correlations. The role of magnetic impurities in this spin liquid state and the implications for experiments will also be discussed. [Preview Abstract] |
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