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 SpinOrbital 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 Asite spinel compound $FeSc_2S_4$, which experimentally exhibits a broad ``spinorbital liquid''(SOS) regime. A spinorbital Hamiltonian is derived from a combination of microscopic consideration and symmetry analysis. We demonstrate a keen competition between spinorbit interactions, which favor formation of a local ``SpinOrbital 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 commensurateincommensurate 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 SpinOrbit 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 ensembleaveraged one and twoparticle Green's functions. The diffusion and Cooperon parts of the twoparticle 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 noninteracting but contains spinorbit coupling. [Preview Abstract] 
Thursday, March 19, 2009 2:54PM  3:06PM 
X32.00003: MeanField Critical Behavior in the Sr$_{1x}$(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 meanfield (MF) critical behavior has been observed. [1] Recently, we have shown that cubic BaRuO$_{3}$ belongs to the 3D Heisenberg universality class with shortrange exchange interactions. [2] The partial chemical substitution by either smaller Ca$^{2+}$ or larger Ba$^{2+}$ in Sr$_{1x}$A$_{x}$RuO$_{3}$ which changes the bond angle $<$RuORu$>$ and introduces an Acation 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 $<$RuORu$>$ versus $\sigma ^{2}$, we have synthesized Sr$_{1x}$(Ca$_{0.5}$Ba$_{0.5})_{x}$RuO$_{3 }$(0 $\le $ x $\le $ 0.35) under 1000\r{ }C and 10 GPa in a Walkertype multianvil; these samples have the same average $<$RuORu$>$ as that of SrRuO$_{3}$, but a different $\sigma ^{2}$. All samples exhibit perfect MF critical behaviors, which indicates that the peculiar bond angle $<$RuORu$> \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: Crossover of universality class in the Ising chain frustrated by longrange interactions Alessandro Vindigni, Fabio Cinti, Oliver Portmann, Danilo Pescia We investigate a spin chain in which the ferromagnetic nearestneighbor exchange interaction $J$ competes with a longrange 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 longrange 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 twopoint correlation function, which oscillates as well but with a temperaturedependent 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 longrange 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] 

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, EunAh 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 nonAbelian 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 nonAbelian 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 onedimensional problem that retains important properties of the original system even in the presence of small deviations from monodimensionally 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 meanfield 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 onsite interaction are renormalized by shortrange 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 meanfield and Monte Carlo methods. It is found that for closepacked lattices with nearestneighbor 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 meanfield 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 NonCollinear 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 noncollinear magnetic system. By rotating the spin operators in the local spin reference frame, we evaluate the zeroth, first, and second order terms in a HolsteinPrimakoff expansion, and through a Green's functions approach, we determine the structure factor intensities for the spinwave frequencies. To demonstrate this technique, we examine the spinwave 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 HolsteinPrimakoff 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 UTBattelle, 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 AffleckKennedyLiebTasaki (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 spin1 systems using the directedloop 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 threedimensional 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 noncompensated 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 longrange magnetic phase transition and the T$_{N}$. The magnetic behavior of this compound is discussed in terms of a strong MnMn coupling through this unique network with endtoend 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$_{1x}$Fe$_{x}$WO$_{4}$ Feng Ye, Y. Ren, J.A. FernandezBaca, 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$_{1x}$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 magneticfield perturbations. [Preview Abstract] 
Thursday, March 19, 2009 4:54PM  5:06PM 
X32.00013: The ferromagnetic transition in LiHoF$_4$ Anders Biltmo, Patrik Henelius The rareearth magnetic compound LiHoF$_4$ is considered to be one of the best realizations of a longrange 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 nondilute 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 spin1/2 Cu$^{2+}$ ions of Cs2CuCl4 have a 3D arrangement, but the magnetic properties of Cs2CuCl4 are mainly described by a quasi2D 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 MagneticStructural Phase Transition in GadoliniumSiliconGermanium 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$_{1x})_{4}$ from monoclinic phase to orthorhombic phase was investigated from 296300 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 TTc. [Preview Abstract] 
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