Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session X32: Cooperative Phenomena (incl. Spin Structures, Spin Waves, Phase Transitions) II |
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Sponsoring Units: GMAG Chair: Stephan Rosenkranz, Argonne National Laboratory Room: 336 |
Thursday, March 19, 2009 2:30PM - 2:42PM |
X32.00001: A Spin-Orbital Singlet and Quantum Critical Point on the Diamond Lattice: $FeSc_2S_4$ Gang Chen, Leon Balents, Andreas Schnyder We present a theory of spin and orbital physics in the A-site spinel compound $FeSc_2S_4$, which experimentally exhibits a broad ``spin-orbital liquid''(SOS) regime. A spin-orbital Hamiltonian is derived from a combination of microscopic consideration and symmetry analysis. We demonstrate a keen competition between spin-orbit interactions, which favor formation of a local ``Spin-Orbital Singlet'', and exchange, which favors magnetic and orbital ordering. Separating the SOS from the ordered state is a quantum critical point (QCP). We argue that $FeSc_2S_4$ is close to this QCP on the SOS side. The full phase diagram of the model includes a commensurate-incommensurate transition within the ordered phase. A variety of comparison to and suggestion for experiments are discussed. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X32.00002: Quantum Critical Phenomena near Stoner Transition in Two Coupled Quantum Dots with Spin-Orbit Coupling Oleksandr Zelyak, Ganpathy Murthy We consider a system of two coupled quantum dots. Both the dots and connecting region are assumed to be in universal crossover regimes between Gaussian orthogonal and symplectic ensembles. Using a diagrammatic approach appropriate for energy separations much larger than the level spacing, we obtain the ensemble-averaged one- and two-particle Green's functions. The diffusion and Cooperon parts of the two-particle Green's function are described by separate scaling functions. We then use this information to investigate a model of interacting system in which one dot has Stoner exchange interaction, while the other is non-interacting but contains spin-orbit coupling. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X32.00003: Mean-Field Critical Behavior in the Sr$_{1-x}$(Ca$_{0.5}$Ba$_{0.5}$)$_{x}$RuO$_{3}$ (0 $\le $ x $\le$ 0.35) Jinguang Cheng, Jianshi Zhou, John Goodenough Orthorhombic SrRuO$_{3}$ is a metallic ferromagnet with T$_{c} \quad \approx$ 160 K where mean-field (MF) critical behavior has been observed. [1] Recently, we have shown that cubic BaRuO$_{3}$ belongs to the 3D Heisenberg universality class with short-range exchange interactions. [2] The partial chemical substitution by either smaller Ca$^{2+}$ or larger Ba$^{2+}$ in Sr$_{1-x}$A$_{x}$RuO$_{3}$ which changes the bond angle $<$Ru-O-Ru$>$ and introduces an A-cation size variance $\sigma ^{2}$ $\equiv \quad <$r$_{A}^{2}> \quad - \quad <$r$_{A}>^{2 }$, causes a clear deviation from the MF behavior. In order to distinguish effects of $<$Ru-O-Ru$>$ versus $\sigma ^{2}$, we have synthesized Sr$_{1-x}$(Ca$_{0.5}$Ba$_{0.5})_{x}$RuO$_{3 }$(0 $\le $ x $\le $ 0.35) under 1000\r{ }C and 10 GPa in a Walker-type multianvil; these samples have the same average $<$Ru-O-Ru$>$ as that of SrRuO$_{3}$, but a different $\sigma ^{2}$. All samples exhibit perfect MF critical behaviors, which indicates that the peculiar bond angle $<$Ru-O-Ru$> \quad \approx $ 163\r{ } plays an essential role in determining the MF critical behavior of SrRuO$_{3}$.\\[0pt] [1] D. Kim, \textit{et al}., Phys. Rev. B \textbf{67}, 100406 (2003).\\[0pt] [2] J.-S. Zhou, \textit{et al}., Phys. Rev. Lett. \textbf{101}, 077206 (2008). [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X32.00004: Cross-over of universality class in the Ising chain frustrated by long-range interactions Alessandro Vindigni, Fabio Cinti, Oliver Portmann, Danilo Pescia We investigate a spin chain in which the ferromagnetic nearest-neighbor exchange interaction $J$ competes with a long-range antiferromagnetic interaction of strength $g$ decaying spatially as $\frac{1}{r^{\alpha}}$. For $\alpha$ smaller than a certain threshold $\hat{\alpha}$ (with $\hat{\alpha}\left(\frac{J}{g}\right)>2$), the long-range interaction is able to avoid the global phase separation -- the uniformly magnetized state favored by the exchange interaction -- even at $T=0$. The ground state then consists of an ordered sequence of segments with equal length and alternating magnetization, resulting in a superlattice of magnetic domains. A memory of this periodic spin profile is retained at finite $T$ in the two-point correlation function, which oscillates as well but with a temperature-dependent period. Such an oscillation is then exponentially damped over a spatial scale, the correlation length, which diverges asymptotically, roughly, as the inverse of $T$. This suggests that the long-range interaction drives the Ising chain to acquire a universality class consistent with an underlying continuous symmetry. The $e^{\frac{\Delta}{T}}$-temperature dependence of the correlation length and the uniform ferromagnetic ground state, characteristic of the $g=0$ discrete Ising symmetry, are recovered for $\alpha >\hat{\alpha}$. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X32.00005: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X32.00006: Nature of topological quantum phase transition in chiral spin liquid Suk Bum Chung, Hong Yao, Eun-Ah Kim, Steven Kivelson How to best characterize and detect topological order, which is not associated with any local broken symmetry is one of central questions in the field of topological phases. While the ground state degeneracy that depends on the topology of the manifold the system is defined in has been a successful theoretical indicator of topological order, this concept is applicable only at $T=0$ and not accessible experimentally. Another important indicator has been topological entanglement entropy. However, topological entanglement entropy at $T=0$ can be the same for two distinct topological phases. Here we study an exactly solvable model first introduced in Ref.[1], motivated by the fact that the existence of topological quantum phase transition is known and the full spectrum is available. We examine the nature of Abelian to non-Abelian topological quantum phase transition by studying the expectation value of global flux which shows an abrupt jump at the critical point. We discuss the phase diagram of this quantum phase transition in terms of the global flux and entanglement entropy and discuss to what extent the existence of topologically ordered ground state with non-Abelian excitations is revealed at finite temperature. [1] H. Yao and S.A. Kivelson, PRL ${\bf 99}$ 247203 (2007). [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X32.00007: Scaling in modulated systems and reentrance of order Oliver Portmann, Alessandro Vindigni, Danilo Pescia Ultrathin ferromagnetic iron films exhibit a peculiar reentrance of order.\footnote{O. Portmann, A. Vaterlaus, and D. Pescia, Nature {\bf 422}, 701 (2003).} A less symmetric pattern (stripes) that is present at lower temperatures reoccurs at higher temperatures after a more symmetric intermediate state (labyrinth). We obtain a good qualitative understanding of the system by analytically reducing this problem in two spatial dimensions to an effectively one-dimensional problem that retains important properties of the original system even in the presence of small deviations from mono-dimensionally modulated order. As revealed by a scaling analysis, this system is characterized by a highly anomalous temperature dependence of an elastic constant. This finding is corroborated by mean-field calculations. By means of the scaling analysis, we can relate this experimentally inaccessible elastic constant to experimentally measurable quantities. Comparison with experiment suggests that the driving force for the reentrance of order is indeed the strongly anomalous behavior of this elastic constant. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X32.00008: Generalized Onsager cavity field method for magnets with local spin fluctuations James Glasbrenner, Aleksander Wysocki, Kirill Belashchenko The Onsager cavity field method is extended to magnets described by a classical spin fluctuation model which interpolates between the limits of localized and itinerant magnetism and captures the qualitative features of itinerant thermodynamics. We find that \textit{both} the interatomic exchange coupling and the on-site interaction are renormalized by short-range order. In the localized (Heisenberg) limit Onsager's approximation is recovered, but in itinerant systems it is essential to include both corrections, for in this case a simple subtraction of only the Onsager reaction field leads to poor results. The generalized Onsager method is compared with the results obtained through mean-field and Monte Carlo methods. It is found that for close-packed lattices with nearest-neighbor exchange there is excellent agreement between the generalized Onsager method and Monte Carlo for any degree of itinerancy, and offers a significant improvement over the mean-field approximation in predicting the Curie temperature [1]. [1]A. L. Wysocki, J. K. Glasbrenner, and K. D. Belashchenko, Phys. Rev. B \textbf{78}, 184419 (2008) [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X32.00009: Spin Rotation Technique for Non-Collinear Magnetic Systems: Application to the Generalized Villain Model J.T. Haraldsen, R.S. Fishman This work develops a new generalized technique for determining the static and dynamic properties of any non-collinear magnetic system. By rotating the spin operators in the local spin reference frame, we evaluate the zeroth, first, and second order terms in a Holstein-Primakoff expansion, and through a Green's functions approach, we determine the structure factor intensities for the spin-wave frequencies. To demonstrate this technique, we examine the spin-wave dynamics of the generalized Villain model with a varying interchain interaction. The new interchain coupling expands the overall phase diagram with the realization of two separate spin configurations. The rotational Holstein-Primakoff expansion provides both analytical and numerical results for the spin dynamics and intensities of these phases. Research sponsored by the Division of Materials Sciences and Engineering, U.S. Department of Energy under contract with UT-Battelle, LLC. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X32.00010: Edge States of Quantum Antiferromagnets Jos\'e Hoyos, S. Chandrasekharan, H. U. Baranger We investigate the edge states of spin systems in the Affleck-Kennedy-Lieb-Tasaki (AKLT) phase. Edges of correlated systems may show novel lower dimensional physics (as in quantum Hall edge states) and have recently garnered increasing experimental attention. Here we study spin-1 systems using the directed-loop quantum Monte Carlo technique. Depending on the configuration and parameters, even though the bulk system has a spin (Haldane) gap, the edge states can be gapless and described by an effective Luttinger liquid. We focus on the behavior of the edge states when the bulk undergoes a quantum phase transition from the AKLT to the N\'eel phase. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X32.00011: Magnetic Properties of a Canted Antiferromagnet Mn(N$_{3}$)$_{ 2}$(4,4'-bpy) Youcef Hamida, Dusan Danilovic, C.L. Lin, Tan Yuen, Kunhao Li, Jing Li Results of magnetic susceptibility \textit{$\chi $}($T)$, isothermal magnetization $M(H)$, and heat capacity $C(T)$ measurements on a manganese complex with mixed ligands Mn(N$_{3})_{2}$(4,4'-bpy) (4,4'-bpy = 4,4'-bipyridine) are reported. The crystal structure of this three-dimensional manganese azide network is tetragonal, with lattice parameters $a=b$ = 8.1 {\AA} and $c$ = 16.7 {\AA}. Fitting the high $T$ data of \textit{$\chi $}($T)$ on powder samples resulted an effective moment \textit{$\mu $}$_{eff}$ = 4.9 \textit{$\mu $}$_{B}$ for the Mn$^{2+}$ moments, and a strong antiferromagnetic interaction of \textit{$\theta $} = - 120 K. An antiferromagnetic transition with rather high transition temperature of 39 K was observed in the $M(T)$/$H$ data of Mn(N$_{3})_{2}$(4,4'-bpy), and large non-compensated component in \textit{$\chi $}($T)$ below T$_{N}$ was seen. The result of $M(H)$ measures showed that Mn(N$_{3})_{2}$(4,4'-bpy) behaves like a ferrimagnet below T$_{N}$, with a small coercive field of $H_{coe}$ = 150 G at 1.8 K. A sizable anomaly was observed in $C(T)$ data, and this confirms the long-range magnetic phase transition and the T$_{N}$. The magnetic behavior of this compound is discussed in terms of a strong Mn-Mn coupling through this unique network with end-to-end azido bridges, and compared with the magnetic behavior of its iron isostructural analogue Fe(N$_{3})_{2}$(4,4'-bpy). [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X32.00012: Magnetic switching and phase competition in the multiferroic antiferromagnet Mn$_{1-x}$Fe$_{x}$WO$_{4}$ Feng Ye, Y. Ren, J.A. Fernandez-Baca, H.A. Mook, J.W. Lynn, R.P. Chaudhury, Y.-Q. Wang, B. Lorenz, C.W. Chu Elastic neutron scattering is used to study the spin correlations in the multiferroic Mn$_{1-x}$Fe$_{x}$WO$_{4}$ with $x$=0.035, 0.05, and 0.10. The noncollinear incommensurate (ICM) magnetic structure associated with the ferroelectric (FE) phase in pure MnWO4 is suppressed at $x$=0.035 and completely absent at $x$=0.10. The ICM spin order and FE phase can be restored by applying a magnetic field along the spin easy axis. The low-$T$ commensurate magnetic structure extends in both $H/T$ with increasing Fe concentration. The systematic evolution of the magnetic and electric properties indicates that the noncollinear ICM spin order results from competing magnetic interactions and its stabilization can be tuned by the internal ($x)$ or external magnetic-field perturbations. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X32.00013: The ferromagnetic transition in LiHoF$_4$ Anders Biltmo, Patrik Henelius The rare-earth magnetic compound LiHoF$_4$ is considered to be one of the best realizations of a long-range dipolar Ising model. Several experimental studies have been carried out probing the magnetic properties of the material in the context of classical, as well as quantum, phase transitions. In this work we revisit the effective model for the non-dilute material using numerical simulations. We examine the accuracy of the model in relation to experiments and consider the logarithmic corrections derived in renormalization group theory. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X32.00014: Examination of the magnetic structure of Cs$_{2}$CuCl$_{4}$ by first principles DFT calculations Changhoon Lee, Jinhee Kang, Mike Whangbo The spin-1/2 Cu$^{2+}$ ions of Cs2CuCl4 have a 3D arrangement, but the magnetic properties of Cs2CuCl4 are mainly described by a quasi-2D triangular antiferromagnetic layer model. To understand why the 3D arrangement of (CuCl4)$^{2-}$ ions leads to a 2D magnetic behavior, we evaluated the various spin exchange interactions between adjacent (CuCl4)$^{2-}$ ions by performing DFT calculations. Our results show that the 6p orbitals of Cs$^{+}$ participate in the spin exchange interaction through the Cl?Cs?Cl bridges if the two (CuCl4)$^{2-}$ ions have a symmetric arrangement and if the Cl?Cs?Cl bridges are symmetric, and that the frustrated 2D triangular antiferromagnetism originates from this selective participation of the Cs 6p orbitals in the spin exchange interactions. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X32.00015: Examination of the Coupled Magnetic-Structural Phase Transition in Gadolinium-Silicon-Germanium Magnetocaloric alloys at temperatures well above Tc Ravi Hadimani, Eugene Melikhov, John Snyder, David Jiles The first order phase transition in Gd$_{5}$(Si$_{x}$Ge$_{1-x})_{4}$ from monoclinic phase to orthorhombic phase was investigated from 296-300 K at magnetic fields of up to 9~Tesla. The rate of change of transition temperature with magnetic field was found to be 4.9~K/ Tesla in the field range up to 2.5~Tesla. This linear rate of change of transition temperature with field persisted even at higher magnetic fields of up to 9~Tesla.. Measurements were made on single crystal Gd$_{5}$Si$_{1.95}$Ge$_{2.05}$ and Gd$_{5}$Si$_{2}$Ge$_{2}$ using a high field VSM and a SQUID magnetometer. The single crystal samples were prepared by the Bridgman method at Ames Laboratory US DoE. The first order phase transition temperatures of single crystal samples at nearly zero field were determined to be 264~K and 267~K respectively. The magnetic field required to induce the first order phase transition at 289~K for the single crystal Gd$_{5}$Si$_{1.95}$Ge$_{2.05}$ was 4.8~Tesla and at 300~K it was 8~Tesla. For the single crystal Gd$_{5}$Si$_{2}$Ge$_{2}$ sample, the magnetic field required to induce the first order transition at 289~K was 4.5~Tesla and at 300~K it was 8.4~Tesla. The magnetic field required to induce the first order phase transition increased linearly with the difference T-Tc. [Preview Abstract] |
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