Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Y31: Focus Session: Spin Chains II: Mostly S = 1/2 
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Sponsoring Units: GMAG DMP Chair: Michael Baker, Stanford University Room: 207A 
Friday, March 6, 2015 8:00AM  8:12AM 
Y31.00001: Local spin dynamics near quantum critical line of spin$1/2$ antiferromagnetic Heisenberg XXZ chain with longitudinal magnetic field Wang Yang, Jianda Wu, Congjun Wu The spin$1/2$ antiferromagnetic quantum XXZ chain is one of the most wellstudied integrable model. Although all of its eigenstates and spectrum can be obtained via algebraic Bethe ansatz method, understanding its local dynamics remains a great challenge [1]. In the anisotropic gapped region, while tuning longitudinal field, there is a line of critical fields where the system undergoes quantum phase transitions. Recent experiments on $BaCo_{2}V_{2}O_{8}$ provided some evidences for understanding lowenergy spin dynamics near critical line [2]. In this work, we further calculate local spin dynamics in this region in low frequency limit by form factor methods. Our results can be compared with measurements of NMR relaxation rate. [1] J.S. Caux, H. Konno, M. Sorrell, and R. Weston, Journal of Statistical Mechanics: Theory and Experiment 2012, P01007 (2012) [2] S. Kimura, T. Takeuchi, K. Okunishi, M. Hagiwara, Z. He, K. Kindo, T. Taniyama, and M. Itoh, Physical review letters 100, 057202 (2008) [Preview Abstract] 
Friday, March 6, 2015 8:12AM  8:24AM 
Y31.00002: Spin Currents Carried by Spinons in XXZ Spin Chains: Signatures in Polarized Inelastic Neutron Scattering Leonard Patrick English, HansBenjamin Braun, Jiri Kulda Quantum spin chains serve as a paradigm for exploring truly quantum phenomena, with spinons being the elementary excitations. Motivated by compounds such as CsCoBr$_3$ and CsCoCl$_3$, we focus on the Isinglike antiferromagnetic Heisenberg $XXZ$ model (spin1/2). Here we present theoretical results for the total inelastic scattering cross section of spinpolarized neutrons in the presence of an external magnetic field, which is applied transverse to the Ising direction. In particular, we identify the spin current associated with spinons and their corresponding signatures in the neutron scattering cross section. The presence of a transverse magnetic field no longer allows for reliance on $S^{z}_{{\rm tot}}$ as a conserved quantity, which has traditionally been assumed in this context. As a striking consequence, we find that the spinons carry a nonvanishing spin current, even in the limit of infinitesimal fields. Our results are shown to be in good agreement with experimental neutron scattering data on CsCoBr$_3$. [Preview Abstract] 
Friday, March 6, 2015 8:24AM  8:36AM 
Y31.00003: Finitetemperature scaling at the quantum critical point of the Ising chain in a transverse field Manuel Haelg, Dan Huvonen, Tatiana Guidi, Diana Lucia QuinteroCastro, Martin Boehm, LouisPierre Regnault, Andrey Zheludev Inelastic neutron scattering is used to study the finitetemperature scaling behavior of spin correlations at the quantum critical point in an experimental realization of the onedimensional Ising model in a transverse field. The target compound is the wellcharacterized, anisotropic and bondalternating Heisenberg spin1 chain material NTENP. The validity and the limitations of the dynamic structure factor scaling are tested, discussed and compared to theoretical predictions. For this purpose neutron data have been collected on the threeaxes spectrometers IN14 at ILL and FLEXX at HZB as well as on the time of flight multichopper spectrometer LET at ISIS. In addition to the general statement about quantum criticality and universality, present study also reveals new insight into the properties of the spin chain compound NTENP in particular. [Preview Abstract] 
Friday, March 6, 2015 8:36AM  8:48AM 
Y31.00004: A new magnetic phase diagram for the quasionedimensional (1D) spin chain compound Ca$_{3}$Co$_{2}$O$_{6}$ Hari Srikanth, P. Lampen, N.S. Bingham, M.H. Phan, H.T. Yi, S.W. Cheong The spin chain cobaltite system Ca$_{3}$Co$_{2}$O$_{6}$ combines geometric frustration with intrinsic lowdimensionality, giving rise to complex physical phenomena that continue to attract a great deal of interest. A longwavelength spindensity wave (SDW) has recently been observed in Ca$_{3}$Co$_{2}$O$_{6}$ at zero field, stabilized by a helical exchange pathway among neighboring chains. We establish a new and more comprehensive phase diagram for this exotic system through the evolution of the magnetic entropy change $\Delta $S$_{\mathrm{M}}$ (T,H) associated with the magnetocaloric effect. $\Delta $S$_{\mathrm{M}}$ measurements in a single crystal of Ca$_{3}$Co$_{2}$O$_{6}$ prepared by the flux method demonstrate the suppression of the SDW modulation by small applied magnetic fields (\textless 1T). Metamagnetic transitions to a ferrimagnetic upupdown spin chain arrangement and full ferromagnetic alignment are observed below 25 K. Shortrange ordered (SRO) correlations with an antiferromagnetic character grow in volume as the temperature is lowered below 15 K, resulting in a crossover from $\Delta $S$_{\mathrm{M}}$ (H) \textless 0 to $\Delta $S$_{\mathrm{M}}$ (H) \textgreater 0 at 12 K. Our entropybased analysis reflects current understanding of the role of SDW and SRO phases in Ca$_{3}$Co$_{2}$O$_{6}$, resolves new subfeatures of the ferrimagnetic phase, and extends previous results below the onset of slow dynamics ($\sim$ 10 K). [Preview Abstract] 
Friday, March 6, 2015 8:48AM  9:00AM 
Y31.00005: Pressureinduced structural distortions in copper pyrazine dinitrate Kenneth O'Neal, Judy Cherian, Chris Landee, Mark Turnbull, Zhenxian Liu, Janice Musfeldt The vibrational properties of quasionedimensional Heisenberg antiferromagnet copper pyrazine dinitrate were investigated up to 9 GPa using diamond anvil cell techniques and infrared and Raman spectroscopy. Two structural transitions were discovered, at 0.7 GPa and around 5 GPa. The lower pressure transition involves only the nitrate ligands, revealing enhanced interchain interactions. The higher pressure transition includes modes throughout the spectrum. Importantly, the pyrazine ringrelated modes show an overall lowering of symmetry through this transition. Ring buckling under pressure likely reduces the exchange along the chains since the exchange pathway becomes distorted. A smaller J may therefore lower the magnetic field of the quantum critical transition. This tunable exchange interaction could be utilized in other pyrazine bridged molecular systems to bring the quantum critical behavior into experimentally realizable fields. [Preview Abstract] 
Friday, March 6, 2015 9:00AM  9:12AM 
Y31.00006: Wilson ratio of a TomonagaLuttinger liquid in the onedimensional spin$1/2$ Heisenberg antiferromagnet CuPzN Christopher Aoyama, Yohei Kono, Kristen Marino, Haidong Zhou, Chisa Hotta, Mark Turnbull, Christopher Landee, Toshiro Sakakibara, Yasumasa Takano In the TomonagaLuttinger liquid (TLL) phase of a onedimensional antiferromagnet, the Wilson ratio and the TLL parameter, $K$, are one and the same except for a trivial numerical factor. This equivalence allows the determination of $K$ from magnetic susceptibility and specific heat. We have performed accurate magnetization and specificheat measurements on the quasionedimensional spin1/2 Heisenberg antiferromagnet Cu(C$_4$H$_4$N$_2$)(NO$_3$)$_2$, known as CuPzN, at temperatures between 80 mK and 7.5 K and in magnetic fields up to 14.7 T and, from the data in the TLL regime, have obtained $K$ as a function of the magnetic field. The results are in excellent agreement with a prediction based on the Bethe ansatz. [Preview Abstract] 
Friday, March 6, 2015 9:12AM  9:24AM 
Y31.00007: Magnetic properties of the $S$ = 1/2 antiferromagnetic spinchain $\alpha$Cu$_{2}$V$_{2}$O$_{7}$ Ganatee Gitgeatpong, Yang Zhao, Maxim Avdeev, Ross Piltz, Taku Sato, Kittiwit Matan Magnetic properties of the $S = 1/2$ antiferromagnetic spinchain, $\alpha$Cu$_{2}$V$_{2}$O$_{7}$, have been studied using magnetization and neutron scattering measurements on powder and singlecrystal samples. Magnetic susceptibility reveals a CurieWeiss temperature of $\Theta$ = 73.2(9) K with a magnetic phase transition at $T_{N}$ = 33 K while the BonnerFisher fit to the magnetic susceptibility for $T > T_{N}$ with magnetic field perpendicular to the crystallographic $a$axis yields the intrachain coupling of $\left J\right /k$ = 46.0(2) K. Small ferromagnetism below $T_{N}$ is due to spincanting caused by DzyaloshinskiiMoriya interactions. Analysis of the neutron diffraction data reveals that the Cu$^{2+}$ spins are coupled antiferromagnetically along zigzag chains, which run alternately along [011] and [011] directions. The ordered moment of 0.925(3) $\mu_{B}$ is predominantly along the $a$axis. Our recent inelastic neutron scattering, which reveals atypical magnetic excitations centered at commensurate wave vectors (0, $\pm$0.25, 0) around the magnetic zone center, will also be discussed. [Preview Abstract] 
Friday, March 6, 2015 9:24AM  9:36AM 
Y31.00008: Elastic constants and ultrasound attenuation in the spinliquid phase of Cs$_2$CuCl$_4$ Simon Streib, Peter Kopietz, Pham Thanh Cong, Bernd Wolf, Michael Lang, Natalija van Well, Franz Ritter, Wolf Assmus The spin excitations in the spinliquid phase of the anisotropic triangular lattice quantum antiferromagnet Cs$_2$CuCl$_4$ have been shown to propagate dominantly along the crystallographic $b$axis. To test this dimensional reduction scenario, we have performed ultrasound experiments in the spinliquid phase of Cs$_2$CuCl$_4$ probing the elastic constant $c_{22}$ and the sound attenuation along the $b$axis as a function of an external magnetic field along the $a$axis. We show that our data can be quantitatively explained within the framework of a nearest neighbor spin$1/2$ Heisenberg chain, where fermions are introduced via the JordanWigner transformation and the spinphonon interaction arises from the usual exchangestriction mechanism. [Preview Abstract] 
Friday, March 6, 2015 9:36AM  9:48AM 
Y31.00009: Magnetic Behavior of 1DFerromagnetic Fe Chains in FePc/H$_{2}$Pc Organic Superlattices Carlos Monton, Thomas Gredig, Ali Basaran, Ilya Valmianski, Ivan Schuller This work reports on the structural and magnetic properties of ironphthalocyanine (FePc)/ metalfreephthalocyanine (H$_{2}$Pc) superlattices. FePc has a divalent Fe$^{2+}$ ion in the center of the molecule that forms quasi onedimensional (1D) chains when the flat molecules are stacked. These 1D chains exhibit two magnetic regimes; ferromagnetic order below 5K due to interchain interactions, and paramagnetic order between 5K and 25K due to short order intrachain interactions. H$_{2}$Pc is a nonmagnetic molecule in which, instead of a metal ion, two hydrogen atoms occupy the center of the molecule. We have grown FePc/H$_{2}$Pc superlattices, in which we controlled the alignment of the Fe chains (i.e. perpendicular or parallel to the substrate) by the growth conditions and through the choice of substrate. Additionally we controlled the Fe chain lengths by the thickness of the FePc layer. We have found that reducing the Fe chains length from 70 to 5 atoms increases substantially the coercive field. We will correlate the observed magnetic behavior with structural information obtained from xray diffraction. [Preview Abstract] 
Friday, March 6, 2015 9:48AM  10:00AM 
Y31.00010: ABSTRACT WITHDRAWN 
Friday, March 6, 2015 10:00AM  10:12AM 
Y31.00011: Investigation of magnetic structure on (C$_{5}$H$_{12}$N)CuBr$_{3}$ system on the basis of DFT study and orbital interaction Changhoon Lee, Jisook Hong, Ji Hoon Shim The (C$_{5}$H$_{12}$N)CuBr$_{3}$ compound crystallizes in the monoclinic group C2/c. Magnetic susceptibility data down to 1.8 K can be well fitted for the antiferromagnetic spin1/2 chain, giving the intrachain magnetic coupling constant J$_{\mathrm{intra}} \approx $ $$17 K. At zero field, (pipH)CuBr3 shows 3D order below T$_{\mathrm{N}}$ $=$ 1.68 K. Calculated by the meanfield theory, the interchain coupling constant J$_{\mathrm{inter}} = $0.91 K is obtained and the ordered magnetic moment is about 0.23 $\mu_{\mathrm{B}}$. However, the interchain interaction should be strong unlike experimental observation. From the analysis of local structure, the J$_{\mathrm{inter}}$ spin dimer show the possibility of good orbital overlap via CuO...OCu path in which angle for CuO...OCu is 161$^{\circ}$ indicating strong interchain interaction via CuO...OCu path. The magnetic structure of (C$_{5}$H$_{12}$N)CuBr$_{3}$ system in terms of orbital interaction could anticipated by twoleg spin ladder which such spin ladders interact ferromagnetically to form ladder. In this study, we evaluated spin exchange interactions of (pipH)CuBr$_{3}$ based on DFT calculations to find the magnetic structure of this system. As a consequence, the J$_{\mathrm{inter}}$ interaction is strong and the magnetic structure of this system, indeed, is described by twoleg spin ladder. [Preview Abstract] 
Friday, March 6, 2015 10:12AM  10:24AM 
Y31.00012: Magnetic Relaxation and Coercivity of Finitesize Single Chain Magnets Thomas Gredig, Matthew Byrne, Alessandro Vindigni The magnetic coercivity of hysteresis loops for iron phthalocyanine thin films depends on the iron chain length and the measurement sweep speed below 5 K. The average onedimensional (1D) iron chain length in samples is controlled during deposition. These 1D iron chains can be tuned over one order of magnitude with the shortest chain having 100 elements. We show that the coercivity strongly increases with the average length of the iron chains, which selfassemble parallel to the substrate surface. Magnetic relaxation and sweep speed data suggest spin dynamics play an important role. Implementing Glauber dynamics with a finitesized 1D Ising model provides qualitative agreement with experimental data. This suggests that iron phthalocyanine thin films act as single chain magnets and provide a solid test system for tunable finitesized magnetic chains. [Preview Abstract] 
Friday, March 6, 2015 10:24AM  10:36AM 
Y31.00013: Orbital ordering and magnetic dimensionalities in the porbital spin1/2 CsO$_{2}$ and Cs$_{4}$O$_{6}$ Denis Arcon The materials containing magnetic O$_{2}^{}$ anions, i.e., alkali superoxides, $A$O$_{2}$ (A $=$ Na, K, Rb, Cs), and alkali sesquioxides, $A_{4}$O$_{6}$ ($A = $ Rb, Cs), exhibit two key features that make them appealing for investigation of the coupling between lattice, orbital and spin physics as an alternative to the more established $d$orbital materials. First, the O$_{2}^{}$ dumbbells can easily reorient down to the low temperatures, thereby modulating the overlaps of $p$ orbitals. And second, as the $S = $ 1/2 spin is localized in a pair of $p$derived $\pi \ast $ orbitals, their original degeneracy can be removed by the cooperative tilting of O$_{2}^{}$ dumbbells. Here we report on our studies of CsO$_{2}$ and Cs$_{4}$O$_{6}$ using $^{133}$Cs nuclear magnetic resonance and electron paramagnetic resonance techniques. In CsO$_{2}$ we find the structural phase transition occurring at 61 K on cooling associated with the freezing out of the O$_{2}^{}$ librations. The transition also includes $\pi \ast $ orbital ordering that is responsible for the quasionedimensional lowtemperature magnetism. Clear signs of the spin TomonagaLuttinger liquid state are found from the spinlattice relaxation and spin susceptibility data. On the other hand, the mixed valence Cs$_{4}$O$_{6}$ shows much more complex phase diagram with several transitions depending on the exact cooling protocol. [Preview Abstract] 
(Author Not Attending)

Y31.00014: Conjugated molecules as amplifiers of anisotropic magnetoresistance in molecular junctions David Rakhmilevitch, Soumyajit Sarkar, Ora Bitton, Leeor Kronik, Oren Tal The simplest way to manipulate spin transport at the atomic scale is based on the anisotropic magnetoresistance (AMR) effect which refers to the dependence of current through a ferromagnetic element on the direction of its magnetization. However the resulting change in resistance is limited to 15\%, making AMR an unlikely candidate for spintronic applications. In this respect, molecules adsorbed on ferromagnetic surfaces, were shown to modify local spin properties and therefore may facilitate in enhancing AMR effect at the atomic scale. Here we demonstrate a 210\% AMR in a single molecule junction based on a benzene molecule suspended between two nickel (Ni) electrodes. These results are in strike contrast with the AMR for bulk Ni (2\%) or atomic Ni junctions (10\%) measured on our devices. In addition, we take advantage of the electromechanical sensitivity of molecular junctions to show the measured AMR can be effectively tuned by elongating the junction. These results are explained by abinitio calculations in the context of selective orbital hybridization. Our findings pave the way for simple and highlyeffective control of spin transport at the atomic scale, promoting the feasibility of singlemolecule spintronics. [Preview Abstract] 
Friday, March 6, 2015 10:48AM  11:00AM 
Y31.00015: Anderson transition in onedimension using Wegner's Flow equations Paraj Bhattacharjee, Victor Quito, David Pekker, Gil Refael We study the Anderson transition in onedimensional random singleparticle Hamiltonians with longrange hoppings decaying in a powerlaw. Explicitly, we consider the single particle tightbinding model in the spin representation with disorder both in the fields and hoppings. It has been shown by Mirlin et. al. that this model shows an extendedtolocalized transition as a function of the powerlaw exponent with a critical multifractal regime when the decay exponent is equal to one. We generalize the flow equation technique, first introduced by Wegner, to the disordered system and use it to study the model and elucidate the character of this transition. This method allows us to efficiently compute the eigenvalues and local observables. We follow, analytically and numerically, the flow of the coupling distributions for the different exponents as a function of the flowtime and look for signatures of the two distinct phases as well as a characterization of the critical point. [Preview Abstract] 
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