Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J43: Low Dimensional Magnetism: General Topics |
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Sponsoring Units: GMAG Chair: Yasu Takano, University of Florida Room: Convention Center 150C |
Tuesday, March 22, 2005 11:15AM - 11:27AM |
J43.00001: Proton NMR study of the local magnetic field and fluctuations in a single crystal of the quasi-2D organic conductor $\lambda$-(BETS)$_{2}$FeCl$_{4}$ Predrag Ranin, Guoqing Wu, W.G. Clark, L.K. Montgomery, L. Balicas Measurements of the proton NMR spectrum and spin-lattice relaxation rate 1/T$_{1}$ in a single, 3 $\mu$g crystal of the quasi-2D organic conductor $\lambda$-(BETS)$_{2} $FeCl$_{4}$ at a magnetic field B$_{0}$ = 9 T $\parallel$ $a-c$ plane over the temperature ($T$) range 2-180 K are reported. They probe the static local field and its fluctuations in both the antiferromagneic insulator (AFI) and paramagnetic metal (PM) phases. As $T$ is decreased, there is an increase in the shift and the overall width of the NMR spectrum and a jump in these features at the PM-AFI transition ($\sim$4 K). A reasonable fit to these properties in the PM phase is obtained using the dipole field of non- interacting Fe$^{+3}$ ions. These features show that the proton spectrum and 1/T$_{1}$ are dominated by the Fe$^{+3}$ spins. The work at UCLA was supported by NSF Grants DMR-0334869 (WGC) and DMR-0203806 (SEB) and that at Indiana by the Petroleum Research Fund ACS-PRF-33912-ACI. [Preview Abstract] |
Tuesday, March 22, 2005 11:27AM - 11:39AM |
J43.00002: Far-infrared and high field electron spin resonance study of magnetic and phonon excitations in the two dimensional cuprate Na$_5$RbCu$_4$(AsO$_4$)$_4$Cl$_2$ Urmas Nagel, D. H{\"u}vonen, T. R{\~o}{\~o}m, S. Zvjagin, J. Krzystek, S.-J. Hwu, M. Ulutagay-Kartin, X. Mo, W. Queen, J. A. Clayhold Na$_{5}$RbCu$_{4}$(AsO$_{4})$Cl$_{2}$ is a novel low dimensional magnetic material, where the magnetic properties are determined by layers Cu$_{4}$O$_{4}$ tetramers [S.-J.~Hwu \textit{et al}., J. Am. Chem. Soc.,\textbf{ 124},~12404 (2002)]. There is a second order phase transition at 17K, that likely involves the onset of antiferromagnetic order [J.A. Clayhold \textit{et al}., Phys. Rev. B, \textbf{66}, 052403 (2002)]. We have measured the far-infrared spectra and high field electron spin resonance spectra at low temperatures. A triplet excitation is observed at 12.68 cm$^{-1}$. The magnetic field dependent triplet components with the electron g-factor $g_{b}$=1.44 are split in zero field by 0.75 cm$^{-1}$. Several temperature dependent resonances are also observed. The results are being analyzed in the framework of a four-tetramer model. [Preview Abstract] |
Tuesday, March 22, 2005 11:39AM - 11:51AM |
J43.00003: Neutron Scattering Study on a S=1/2 Quasi-two-dimensional Magnet (CuCl)LaNb$_2$O$_7$ Noriaki Oba, Taro Kitano, Hiroshi Kageyama, Masakazu Nishi, Kazuma Hirota, Satoshi Nagai, Liliana Viciu, Jhon B. Wiley, Jun Yasuda, Yoichi Baba, Yoshitami Ajiro, Kazuyoshi Yoshimura Magnetic properties of polycrystalline (CuCl)LaNb$_2$O$_7$ were studied by magnetic susceptibility and neutron scattering measurements. Magnetic susceptibility exhibits a broad maximum around 16 K, followed by a rapid decrease with decreasing temperature. Powder neutron diffraction measurement provides no evidence of magnetic ordering down to 1.5 K. Furthermore inelastic neutron scattering measurement shows magnetic excitation at around 2.1 meV. These experimental results suggest that this compound has a spin-singlet ground state which is separated from the excited state with a finite-energy gap. [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:03PM |
J43.00004: Magnegtic Phase Diagram and Specific Heat of the Quasi-Two-Dimensional S=1/2 Antiferromagnet Cs$_2$CuBr$_4$ Yasu Takano, Hiroyuki Tsujii, Costel Rotundu, Bohdan Andraka, Toshio Ono, Hidekazu Tanaka Cs$_2$CuBr$_4$ is an excellent laboratory model for the S=1/2 Heisenberg antiferromagnet on a triangular lattice, with a small anisotropy. In a magnetic field, the most distingushing feature of this antiferromagnet is a magnetization plateau, which occurs at 1/3 of the saturation magnetization. We have determined the magnetic phase diagram of Cs$_2$CuBr$_4$ for magnetic fields up to 20 T, using specific-heat measurements and the magnetocaloric effect. We find that the ordering temperature in the plateau region is slightly higher than in the surrounding regions of the phase diagram. The transition to the plateau phase is first order, if it is from the adjacent ordered phase at either lower or higher magnetic fields, whereas the transition from the high-temperature disordered phase is second order. Surprisingly, the temperature dependence of the specific heat indicates that the excitations in the plateau phase are gapless. [Preview Abstract] |
Tuesday, March 22, 2005 12:03PM - 12:15PM |
J43.00005: High field magnetization, specific heat and NMR of the two dimensional tetramer-cuprate Na5RbCu4(AsO4)4Cl2 Raivo Stern, A. Kriisa, I. Heinmaa, E. Joon, P. Kuhns, A. Harter, A. Reyes, W. Moulton, M. Jaime, N. Harrison, J. A. Clayhold, S.-J Hwu, M. Kartin-Ulutagay, X. Mo, W. Queen Complex novel 2D compound Na$_{5}$RbCu$_{4}$(AsO$_{4})$Cl$_{2}$ is a unique magnetic material, which contains layers of Cu$_{4}$O$_{4}$ tetramers [S.-J.~Hwu \textit{et al}., J. Am. Chem. Soc.,\textbf{ 124},~12404 (2002)]. The ground state following a second order low entropy phase transition at 15(1) K is antiferromagnetically ordered one [J.A. Clayhold \textit{et al}., Phys. Rev. B, \textbf{66}, 052403 (2002)]. We have measured the $^{87}$Rb and $^{35}$Cl NMR spectra and high field magnetization and specific heat at low temperatures. We discuss the field dependence of T$_{N}$, hyperfine couplings at various nuclear sites, and characteristics of the ordered state as seen from Rb and Cl nuclear sites. [Preview Abstract] |
Tuesday, March 22, 2005 12:15PM - 12:27PM |
J43.00006: Laser-controlled local magnetic field using semiconductor quantum rings Yuriy Pershin, Carlo Piermarocchi We analize theoretically the dynamics of $N$ electrons localized in a semiconductor quantum ring under a train of phase-locked infrared laser pulses. The pulse sequence is designed to control the total angular momentum of the electrons. The quantum ring can be put in metastable states characterized by a persistent current much stronger than the one generated by an Aharonov-Bohm flux. The local magnetic field created by these currents can be used for a selective quantum control of single spins in semiconductor systems. [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 12:39PM |
J43.00007: Spin-gap state and the structural phase transition in delta-phase NH$_4$V$_4$O$_{10}$ Natasha A. Chernova, Samuel T. Lutta, Peter Y. Zavalij, M. Stanley Whittingham The magnetic properties of the delta-phase vanadium oxide NH$_{4}$V$_{4}$O$_{10}$ are studied using a SQUID Magnetometer. Temperature dependence of the magnetic susceptibility reveals a broad maximum at 200 K and a drop of susceptibility at 120 K consistent with a spin-gap behavior. The magnetic network in NH$_{4}$V$_{4}$O$_{10}$ can be described as a system of spin 1/2 Heisenberg two-leg ladders with antiferromagnetic exchange. However, a theoretical temperature dependence of the susceptibility produced by this model does not fit the experimental data, as the measured susceptibility drop is much steeper than that predicted by the model. Low-temperature X-ray studies have shown that the transition to the spin-gap state is accompanied by an increase of V-V leg distance, and decrease of V-V rung distance in the ladders. Therefore, the rung exchange interaction in the low-temperature phase becomes much stronger leading to a spin-gap state and the sharp drop of the susceptibility. Magnetic properties of other delta-phase vanadium oxides, Li$_{x}$V$_{4}$O$_{10}$ and Li$_{0.6}$(C$_{2}$H$_{8}$N)$_{0.4}$V$_{4}$O$_{10}$, are also studied and explained by linear chain Heisenberg model. The relation between the magnetic and structural parameters of the compounds is discussed. This work was supported by NSF DMR 0313963. [Preview Abstract] |
Tuesday, March 22, 2005 12:39PM - 12:51PM |
J43.00008: The XYZ-model in a magnetic field Ute L\"ow We present a numerical study of the one-dimensional XYZ-model in a magnetic field. In particular we discuss DMRG- and Lanczos results for the gaps, the staggered and uniform magnetisation, the phase diagram and the quantum critical point of the model. We also consider thermodynamic properties. [Preview Abstract] |
Tuesday, March 22, 2005 12:51PM - 1:03PM |
J43.00009: The observation of lattice distortion in TiOBr by using synchrotron X-rays diffraction Masaichiro Mizumaki, Tomoyuki Sasaki, Kenichi Kato, Yasuo Watanabe, Yoshiki Nishihata, Masaki Takata, Jun Akimitsu TiOBr has the same structure of TiOCl, and the physical properties of TiOBr were very similar to that of TiOCl. These systems attract attention as the new candidate of spin-Peierls transition accompanying orbital ordering. We performed the specific heat measurement, magnetization measurement and $\mu $SR measurement, and checked that it was the candidate of a spin-Peierls transition. However, lattice modification of TiOBr has not been observed directly before. Therefore, in this study, the X-rays diffraction measurement was performed at BL46XU of SPring-8, Japan. It succeeded in observation of the super-lattice spot by lattice modification of TiOBr, and succeeded also in observation of the temperature change. Furthermore, we investigate the superlattice structure. [Preview Abstract] |
Tuesday, March 22, 2005 1:03PM - 1:15PM |
J43.00010: Generic Chern Numbers for a Degenerate Multiplet: For a Characterization of Topological Orders Yasuhiro Hatsugai Chern numbers for a multiplet which is a set of low lying states near the ground states are defined and an explicit expression is obtained in a gauge dependent form. We allow intrinsic degeneracies within the multiplet where a well-known standard procedure does not work. As an example, we give expressions for a spin Hall conductance for unitary superconductors with equal spin pairing. Generic topological orders will be treated in this manner particularly with nontrivial topological degeneracies.\footnote{Y. Hatsugai, J. Phys. Soc. Jpn, 2604, (2004), cond-mat/0405551}. It can be useful for a characterizaton of some class of low dimensional quantum (spin) liquids with topological degeneracies. \footnote{Y. Hatsugai, preprint} [Preview Abstract] |
Tuesday, March 22, 2005 1:15PM - 1:27PM |
J43.00011: Magnetochromism in the Quasi-2D Heisenberg Antiferromagnet, K$_2$V$_3$O$_8$ R.C. Rai, J. Cao, J.L. Musfeldt, B.C. Sales, D. Mandrus, X. Wei We present the optical and magneto-optical properties (0 - 32 T) of the S = 1/2 quasi-two-dimensional Heisenberg antiferromagnet, K$_2$V$_3$O$_8$. A large magnetochromic effect, centered at $\sim$2.6 eV, is observed at 4.2 K, and it is attributed to a change in the V$^{4+}$ on-site excitation. An additional very sharp magneto-optical feature near $\sim$1.2 eV may be associated with a vibronic excitation. Fine structure allows us to identify the coupling phonon. These field-induced color changes point toward a 12 T transition involving local distortions. \newline \newline *Work at the University of Tennessee is supported by the Materials Science Division, Basic Energy Sciences, U.S. Department of Energy (Grant DE-FG0201ER45885). Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725. [Preview Abstract] |
Tuesday, March 22, 2005 1:27PM - 1:39PM |
J43.00012: NMR studies of incommensurate quantum antiferromagnetic state of LiCuVO$_{4}$ A. P. Reyes, R. Smith, T. Caldwell, R. Achey, A. Prokofiev, W. Assmus, G. Teitel'baum We report $^{51}$V NMR measurements in the linear spin-chain compound LiCuVO$_{4}$ single crystals. High temperature $^{51}$V spectra exhibit a classic quadrupole-split line expected for $I$ = 7/2 nuclei (splitting disappears below 50K). Linewidth anomalies correlated with characteristic features of the resonance shift were observed near 25K, 6K, and 2K. This is attributed to increased spin correlations leading to 3D antiferromagnetic order at low temperatures. The Knight shift tracks susceptibility giving a transfered hyperfine field of A$_{hf }$ = 6.5 kOe/$\mu _{B}$ on the V nuclei. Below 2K for a field parallel to crystal $c$-axis the spectra exhibit a broad two-peak feature which is absent for field along the $b$-axis. The moment orientation determined from the spectra suggests an incommensurate AF modulation along the $b$-axis in agreement with recent neutron scattering data.$^{1}$ Spin dynamics through relaxation measurements will be discussed. $^{1}$ B. J. Gibson, \textit{et al.,} Physica B350, e253 (2004). [Preview Abstract] |
Tuesday, March 22, 2005 1:39PM - 1:51PM |
J43.00013: Magnetic interactions and spin waves in LiCu$_2$O$_2$ A. Zheludev, T. Masuda, B. Roessli, A. Bush, M. Markina, A. Vasiliev Magnetic excitations in the spiral magnet LiCu$_2$O$_2$ are studied by thermal and cold neutron scattering techniques. The spin models are discussed based on coupled ladder model including four exchange parameters; rung interaction $J_1$, nearest neighbor inchain interaction $J_2$, next-nearest neighbor inchain interaction $J_4$, and inter-ladder coupling $J_ {\perp}$. The magnetic dispersion suggests that antiferromagnetic $J_1$, {\it ferromagnetic $J_2$}, and antiferromagnetic $J_4$ induce frustration in this compound. Weak inter-ladder coupling was also observed. This work was carried out under DOE Contract No. DE-AC05-00OR22725. [Preview Abstract] |
Tuesday, March 22, 2005 1:51PM - 2:03PM |
J43.00014: Magnetic excitations in weakly coupled classical and quantum spin systems T. Masuda, A. Zheludev, S. Imai, K. Uchinokura, S. Park Magnetic excitations in the ordered state of Cu$_2$Fe$_2$Ge$_4$O$_{13}$ is studied by inelastic neutron scattering technique. Low energy excitations up to 10 meV is well explained by coupled classical chains of Fe$^{3+}$. Checkerboard like intensity patterns in wide reciprocal space is reproduced by spin-wave theory including magnetic non-Bravis lattice. The distinct narrow-band excitation at 25 meV is identified as quantum dimer excitation of Cu$^{2+}$. The cooperative ordering at low temperature is a result of weakly coupling between classical and quantum entities in this compound. This work was carried out under DOE Contract No. DE-AC05-00OR22725. [Preview Abstract] |
Tuesday, March 22, 2005 2:03PM - 2:15PM |
J43.00015: Symmetry and light coupling to magnetic excitations in SrCu2(BO3)2 Adrian Gozar, Girsh Blumberg, Brian Dennis, Hiroshi Kageyama We present a T = 3 K Raman scattering study in the spin-dimer compound SrCu$_{2}$(BO$_{3})_{2}$. The symmetry of the elementary triplets and magnetic bound states in the 0 - 60 cm$^{-1}$ energy range are experimentally determined. A 4-spin cluster approximation is able to reproduce the symmetry and the anisotropic dispersions in external magnetic fields of the spin gap modes. We propose a light coupling mechanism induced by intra-dimer antisymmetric interactions and discuss the observation of a distinct Raman resonance behavior of the elementary and composite magnetic modes. [Preview Abstract] |
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J43.00016: Magnetic field effect on the spin-wave gaps in the real antiferromagnets Sasha Chernyshev An unusual dependence of the excitation spectrum of a 2D antiferromagnet with a weak Dzyaloshinskii-Moriya interaction $D$ on the external magnetic field $H$ is studied. While the ${\bf k}=0$ gap follows closely the ``uniform precession'' behavior, $\Delta_0\sim H$, the gap at the antiferromagnetic ordering vector $\Delta_{{\bf Q}_{AF}}$ evolves from $\sim D$ at $H=0$ to $\propto \sqrt{D}$ at $H\sim H_s/2$ and then to $\propto D^{2/3}$ at $H=H_s$, where $H_s$ is the saturation field. In small fields this gap shows a non-analytic behavior $\Delta_{{\bf Q}_{AF}}\sim \sqrt{H}$. These results are directly applicable to the 2D AF K$_2$V$_3$O$_8$. The mutual effect of the magnetic field and DM interaction on the quantum fluctuations is also studied. [Preview Abstract] |
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