Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session A50: Low-D and Molecular Magnetism IFocus
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Sponsoring Units: GMAG DMP Chair: Matthew Stone, Oak Ridge National Laboratory Room: 397 |
Monday, March 13, 2017 8:00AM - 8:12AM |
A50.00001: Magnetic excitations of the spin dimer system PHCC under pressure as seen by Raman scattering Simon Bettler, Gediminas Simutis, Gerard Perren, Dan H\¨uvonen, Severian Gvasaliya, Andrey Zheludev The model spin-1/2 dimer system $(C_4H_{12}N_2)Cu_2Cl_6$ (aka PHCC) has recently been shown to undergo a phase transition to a magnetically ordered state upon applying hydrostatic pressure in both muon spin rotation($\mu^+$SR) and inelastic neutron scattering(INS) experiments. In the $\mu^+$SR experiments a second phase transition from incommensurate to commensurate order was detected at 14 kbar. By contrast, the INS experiments found hardly any difference in the spin dynamics at 9 kbar and 18 kbar. To resolve this discrepancy, we performed Raman scattering experiments on single crystals to study the pressure-dependence of both magnetic and lattice excitations from ambient pressure up to 18~kbar. The three regimes found in $\mu^+$SR could be reproduced. Each regime could be associated with a characteristic peak shape of the magnetic scattering. Increasing pressure leads to a shifting of the magnetic excitations to lower energies up to a pressure of ~15 kbar, where the magnetic peak mode reaches a minimum. Increasing pressure further leads to the magnetic excitations' energy increasing again. Moreover, no indication of a structural phase transition could be found. We conclude that the evolution of spin dynamics in the ordered phase is far more intriguing than expected from INS. [Preview Abstract] |
Monday, March 13, 2017 8:12AM - 8:24AM |
A50.00002: Properties of the random-singlet phase Yu-Rong Shu, Dao-Xin Yao, Chih-Wei Ke, Yu-Cheng Lin, Anders Sandvik We use a strong-disorder renormalization group (SDRG) method and ground-state quantum Monte Carlo (QMC) simulations to study $S=1/2$ spin chains with random couplings, calculating disorder-averaged spin and dimer correlations. The QMC simulations demonstrate logarithmic corrections to the power-law decaying correlations obtained with the SDRG scheme. The same asymptotic forms apply both for systems with standard Heisenberg exchange and for certain multi-spin couplings leading to spontaneous dimerization in the clean system. We show that the logarithmic corrections arise in the valence-bond (singlet pair) basis from a contribution that can not be generated by the SDRG scheme. In the model with multi-spin couplings, where the clean system dimerizes spontaneously, random singlets form between spinons localized at domain walls in the presence of disorder. This amorphous valence-bond solid is asymptotically a random-singlet state and only differs from the random-exchange Heisenberg chain in its short-distance properties.(See also arXiv:1603.04362). [Preview Abstract] |
Monday, March 13, 2017 8:24AM - 8:36AM |
A50.00003: Adiabatic physics of an exchange-coupled spin-dimer system: magnetocaloric effect, zero-point fluctuations, and two-dimensional universal behavior John Singleton, Jamie Brambleby, Paul Goddard, Marcelo Jaime, Tom Lancaster, L Huang, Jochen Wosnitza, Craig Topping, K Carreiro, Hope Tran, Z Manson, Jamie Manson We present the magnetic and thermal properties of the bosonic-superfluid phase in the spin-dimer network Cu(pyz)(gly)ClO$_4$ (pyz = pyrazine; gly = glycinate) using both quasistatic and rapidly-changing pulsed magnetic fields. The entropy derived from heat capacity reveals that the pulsed-field measurements are strongly adiabatic in nature and are responsible for a significant magnetocaloric effect (MCE). In contrast to previous predictions we show that the MCE is not just confined to the critical regions, but occurs for all fields greater than zero at sufficiently low temperatures. We explain the MCE using a model of exchange-coupled dimer spin-states and highlight that failure to take this effect into account inevitably leads to incorrect interpretations of experimental data. In addition, the heat capacity in our material is suggestive of an extraordinary contribution from zero-point fluctuations and appears to indicate universal behavior with different critical exponents at the two field-induced critical points. The data point to a two-dimensional nature of spin excitations in the system. [Preview Abstract] |
Monday, March 13, 2017 8:36AM - 9:12AM |
A50.00004: Defects in low dimensional quantum magnets Invited Speaker: Kirill Povarov Quantum systems are known to be extremely susceptible to defects when confined in one dimension. Spin chains and ladders are no exception to this rule. In this talk I will demonstrate the dramatic effect that a depletion with nonmagnetic impurities has on several such compounds. Due to fundamentally different low-energy degrees of freedom, impurities in spin chains and spin ladders play very different roles. In spin chains, scattering by defects leads to a confinement of low energy quasiparticles already present in the unperturbed system. As a result, a magnetic excitation spectrum acquires a concentration-dependent ``pseudogap'' which bears a universal description~[1]. In contrast, the unperturbed spin ladder has no low-energy excitations of its own. Instead, new local degrees of freedom are released upon the introduction of defects. Strong antiferromagnetic correlations shape them as spatially extended ``spin islands''. Although these spin islands are localized, they strongly interact and thereby give rise to unusual thermodynamic properties and novel collective modes~[2]. In both cases, inelastic neutron scattering allows to see the corresponding defect-induced transformations of the spin excitation spectra \emph{directly}. Good understanding of the clean ground states makes it possible to describe these metamorphoses of gapped and gapless behavior in quantitative ways. ~\\[4pt] [1] G. Simutis, S. Gvasaliya \emph{et al.}; Phys. Rev. Lett. \textbf{111}, 067204 (2013)\\ ~[2] D. Schmidiger, K. Yu. Povarov \emph{et al.}; Phys. Rev. Lett. \textbf{116}, 257203 (2016) [Preview Abstract] |
Monday, March 13, 2017 9:12AM - 9:24AM |
A50.00005: Specific Heat Studies of a New 2D S $=$ 1/2 Heisenberg Antiferromagnet, [Cu(pz)$_{2}$(2-OHpy)$_{2}$](PF$_{6})_{2}$ Christopher Landee, Paul Goddard, Will Blackmore We report on the zero-field specific heat (0.3 -- 300 K) of a highly two-dimensional Heisenberg, S $=$ 1/2 antiferromagnet (2D QHAF), [Cu(pz)$_{2}$(2-OHpy)$_{2}$](PF$_{6})_{2}$ (pz $=$ pyrazine and 2-OHpy $=$ 2-hydroxypyridine). The copper atoms and pyrazine molecules form square layers of pyrazine-bridged copper(II) ions with the pyridone molecules normal to the layers, providing exceptional spacing between layers. The magnetic specific heat of this compound corresponds to an exchange strength J $=$ 6.6 K, in excellent agreement from the value deduced from susceptibility experiments. The N\'{e}el temperature of this compound has been found to be 1.37 K by muon spin relaxation .[1] The critical ratio T$_{N}$/J $=$ 1.37/6.6 $\approx_{\, }$2.1 is the lowest value known for a molecular-based 2D QHAF and indicates a remarkable degree of isolation between the magnetic planes. [1] T. Lancaster, S. Blundell et al, private communication. [Preview Abstract] |
Monday, March 13, 2017 9:24AM - 9:36AM |
A50.00006: Magnetic phase diagram of the randomized two dimensional Heisenberg antiferromagnet (QuinH)$_2$CuCl$_{4x}$Br$_{4(1-x)}\cdot2$H$_2$O Fan Xiao, Rob Williams, Tom Lancaster, Christopher Landee, Mark Turnbull A family of randomized two-dimensional quantum Heisenberg antiferromagnets (2DQHAF) (QuinH)$_2$CuCl$_{4x}$Br$_{4(1-x)}\cdot2$H$_2$O (QuinH=quinolinium) have been synthesized and characterized. In such systems, the original interaction in the square lattice parent compound ($x=0$) is partially replaced by a different exchange strength. Zero-field muon spin relaxation (ZF $\mu^+$SR) experiments have revealed that the magnetic long range ordering can be strongly suppressed by the introduction of the second interaction and the ordering temperature $T_{N}$ drops sharply as $x$ increases. No 3D long range ordered state was observed in the compounds with $x>0.25$ and the system stays disordered down to the lowest accessible temperature. The structure, magnetic properties and the $T_{N}-x$ phase diagram of the family will be presented. [Preview Abstract] |
Monday, March 13, 2017 9:36AM - 9:48AM |
A50.00007: New low-dimensional antiferromagnetic compounds based on Cu-pyz chain Mariusz Kubus, Arianna Lanza, Rebecca Scatena, Leonardo H. R. Dos Santos, Bjorn Wehinger, Nicola Casati, Piero Macchi, Lukas Keller, Christoph Fiolka, Christian Ruegg, Karl W. Kraemer Two new Cu2$+$ coordination complexes, [CuCl(pyz)2](BF4) and [CuBr(pyz)2](BF4) (pyz $=$ pyrazine), were synthesized and their structures determined with single crystal X-ray diffraction. These tetragonal compounds are isostructural and crystallize in space group P4/nbm. The magnetic susceptibility shows a broad maximum around 8 K for both compounds, indicating predominantly two-dimensional (2D) antiferromagnetic interactions localized within the [Cu(pyz)2]2$+$ layers. A fit of a 2D Heisenberg model to the magnetic susceptibility data results in J $=$ 9.6 K for [CuCl(pyz)2](BF4) and 9.1 K for [CuBr(pyz)2](BF4). Towards lower temperature kinks are observed in the magnetic susceptibility at 4 K for the chloride and 3.6 K for the bromide compound, indicating the onset of long-range 3D magnetic order. The magnetic structures were determined by neutron diffraction. Bragg peaks due to long-range 3D magnetic order are observed below TN $=$ 3.9(1) K for the chloride and 3.7(1) K for the bromide compound. The magnetic unit cell is doubled along the c-axis, the moments are antiferromagnetically coupled both in the a-b plane and along the c-axis. [Preview Abstract] |
Monday, March 13, 2017 9:48AM - 10:00AM |
A50.00008: Quantum critical behavior in the 1D Heisenberg linear chain system Rb$_{2}$PbCu(NO$_{2}$)$_{6}$ Michael Hoch, Jin Jung Kweon, Lianyang Dong, Tiglet Besara, Arneil Reyes, Phillip Kuhns, Theo Siegrist The quantum critical region of the phase diagram of the 1D spin ½ Heisenberg linear chain system Rb$_{2}$PbCu(NO$_{2}$)$_{6}$ has been investigatred using $^{87}$Rb NMR measurements on a polycrystalline sample. The low J value (2.6 K) leads to a low value for the saturation field.The $^{87}$Rb frequency shifts and spin-lattice relaxation rates, determined as a function of temperature and applied field, provide information on the transition to the Tomanaga-Luttinger-liquid phase. Scaling behavior in accordance with quantum criticality is examinerd. [Preview Abstract] |
Monday, March 13, 2017 10:00AM - 10:12AM |
A50.00009: Evidence of frustration in the S$=$1/2 square-lattice antiferromagnet Sr$_{\mathrm{2}}$CuTe$_{\mathrm{1-}}_{x}$W$_{x}$O$_{\mathrm{6}}$ Otto Mustonen, Sami Vasala, Elisa Baggio-Saitovitch, Helen Walker, Maarit Karppinen The S$=$1/2 Heisenberg frustrated square-lattice model, or J$_{\mathrm{1}}$-J$_{\mathrm{2}}$ model, describes systems with competing antiferromagnetic interactions. Magnetic order is N\'{e}el type when J$_{\mathrm{1}}\gg $ J$_{\mathrm{2}}$ and columnar when J$_{\mathrm{2}}\gg $ J$_{\mathrm{1}}$. The nature of the ground state in the highly frustrated J$_{\mathrm{2}}$/J$_{\mathrm{1}}\approx $ 0.5 region is under debate with proposals including different valence bond solids and spin liquids. We report experimental evidence of frustration in a tunable J$_{\mathrm{1}}$-J$_{\mathrm{2}}$ model system. Recent neutron scattering experiments by us [1] and ref. [2] have shown the Cu$^{\mathrm{2+}}$ square-lattice double perovskites Sr$_{\mathrm{2}}$CuTeO$_{\mathrm{6}}$ and Sr$_{\mathrm{2}}$CuWO$_{\mathrm{6}}$ to be highly two-dimensional antiferromagnets with J$_{\mathrm{2}}$/J$_{\mathrm{1}}=$ 0.03 (N\'{e}el order) and J$_{\mathrm{2}}$/J$_{\mathrm{1}}=$ 7.92 (columnar order), respectively. We have synthesized the solid solution series Sr$_{\mathrm{2}}$CuTe$_{\mathrm{1-}}_{x}$W$_{x}$O$_{\mathrm{6}}$ 0 $\le $ $x\le $ 1, and report the magnetic properties. Magnetic susceptibility $\chi $(T) shows a broad maximum at T$_{\mathrm{max}}=$ 73 K and 83 K in Sr$_{\mathrm{2}}$CuTeO$_{\mathrm{6}}$ and Sr$_{\mathrm{2}}$CuWO$_{\mathrm{6}}$, respectively. T$_{\mathrm{max}}$ reaches a minimum of $\approx $50 K in the vicinity of $x\approx $ 0.5 coinciding with a maximum in $\chi_{\mathrm{max}}$. This suggests the realization of the highly frustrated J$_{\mathrm{1}}$-J$_{\mathrm{2}}$ antiferromagnet near $x\approx $ 0.5. [1] H. Walker et al., Phys. Rev. B 94 64411. [2] P. Babkevich et al., arXiv:1605.09714. [Preview Abstract] |
Monday, March 13, 2017 10:12AM - 10:24AM |
A50.00010: 1D spin chain of Cu$^{2+}$ in Sr$_3$CuPtO$_6$ with possible Haldane physics Jonathan Leiner, Joosung Oh, Alexander Kolesnikov, Matthew Stone, Manh Duc Le, Sang-Wook Cheong, Je-Geun Park Antiferromagnetic spin chain systems have attracted considerable attention since the discovery of fractional spinon excitations in spin-half chain systems and Haldane gap phases in spin-one chain systems. It has been reported from bulk susceptibility and heat capacity measurements that the magnetic Cu$^{2+}$ ions in Sr$_3$CuPtO$_6$ exhibit S=1/2 Heisenberg spin chain behavior with a substantial amount of AFM interchain coupling. Using the modern time-of-flight inelastic neutron scattering spectrometer SEQUOIA at the SNS, we have probed the magnetic excitation spectrum for a polycrystalline sample of Sr$_3$CuPtO$_6$. Modeling with linear spin wave theory accounts for the major features of the spinwave spectra, including a nondispersive intense magnon band at 8meV. The magnetic excitations broaden considerably as temperature is increased, persisting up to above 100K and displaying a broad transition as previously seen in the susceptibility data. No spin gap is observed in the dispersive spin excitations at low momentum transfer, which we argue is consistent with Haldane physics in an ideal uniform S=1/2 spin-chain system. [Preview Abstract] |
Monday, March 13, 2017 10:24AM - 10:36AM |
A50.00011: Magnetic excitations in the one-dimensional cuprate Sr$_{\mathrm{\mathbf{2}}}$\textbf{CuO}$_{\mathrm{\mathbf{3}}}$\textbf{ probed by resonant inelastic x-ray scattering} Umesh Kumar, Justin Schlappa, Kejin Zhou, Surjeet Singh, Vladimir Strokov, Alexandre Revcolevschi, Henrik Ronnow, Steven Johnston, Thorsten Schmitt We present a resonant inelastic x-ray scattering (RIXS) study of low energy spectra at the oxygen K-edge of Sr$_{\mathrm{2}}$CuO$_{\mathrm{3}}$. The experimental data shows the presence of several low-energy excitations in the quasi-elastic region ($\omega $\textless 0.5 eV), which we associate with magnetic and lattice excitations. We investigate the system using a one dimensional antiferromagnetic Heisenberg chain comprising of copper and oxygen. Using linear spin wave theory, we compute the RIXS cross section in the ultrashort core-hole lifetime approximation to second order and demonstrate the presence of two-magnon excitations in the low-energy region. Phonon and phonon assisted two-magnon excitations are also calculated within our approach. Our analysis establishes that the two-magnon excitations and phonon excitations are present in Sr$_{\mathrm{2}}$CuO$_{\mathrm{3}}$, indicating that this is potentially an ideal system for studying possible spin-lattice coupling. [Preview Abstract] |
Monday, March 13, 2017 10:36AM - 10:48AM |
A50.00012: Observation of spinon spin currents in one-dimensional spin liquid Daichi Hirobe, Masahiro Sato, Takayuki Kawamata, Yuki Shiomi, Ken-ichi Uchida, Ryo Iguchi, Yoji Koike, Sadamichi Maekawa, Eiji Saitoh To date, two types of spin current have been explored experimentally: conduction-electron spin current and spin-wave spin current. Here, we newly present spinon spin current in quantum spin liquid [1]. An archetype of quantum spin liquid is realized in one-dimensional spin-1/2 chains with the spins coupled via antiferromagnetic interaction. Elementary excitation in such a system is known as a spinon. Theories have predicted that the correlation of spinons reaches over a long distance. This suggests that spin current may propagate via one-dimensional spinons even in spin liquid states. In this talk, we report the experimental observation [1] that a spin liquid in a spin-1/2 quantum chain generates and conveys spin current, which is attributed to spinon spin current. This is demonstrated by observing an anisotropic negative spin Seebeck effect along the spin chains in Sr2CuO3. The results show that spin current can flow via quantum fluctuation in spite of the absence of magnetic order, suggesting that a variety of quantum spin systems can be applied to spintronics. [1] D. Hirobe, \textit{et al.} Observation of spinon spin currents. \textit{Nat. Phys.} online publication (DOI: 10.1038/nphys3895). [Preview Abstract] |
Monday, March 13, 2017 10:48AM - 11:00AM |
A50.00013: Steady state of a driven quasi-one-dimensional Mott insulator with spin-orbital separation Tamas Palmai, Robert Konik We consider a periodically driven quasi-one-dimensional Mott insulator with spin-orbital separation. To understand the emerging steady state we employ a combination of Floquet theory and numerical methods based on integrability to treat the time-dependent perturbation of the spin ladder describing the low-energy sector. We discuss our results in connection to pump-probe experiments on the one dimensional spin-orbital chain compound Sr$_2$CuO$_3$. [Preview Abstract] |
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