Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session C50: Low-D and Molecular Magnetism IIFocus
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Sponsoring Units: GMAG DMP Chair: Mark Lumsden, ORNL Room: 397 |
Monday, March 13, 2017 2:30PM - 2:42PM |
C50.00001: Coupled antiferromagnetic spin-1/2 chains in green dioptase, Cu$_6[$Si$_6$O$_{18}]\cdot 6$H$_2$O Andrey Podlesnyak, L. M. Anovitz, A. I. Kolesnikov, M. Matsuda, T. R. Prisk, G. Ehlers, S. Toth Gem crystals of natural dioptase with colors ranging from emerald-green to bluish have delighted people since ancient times and still attract attention of mineral collectors around the globe. The crystal structure of green dioptase (space group $R\bar{3}$) consists of corrugated silicate rings Si$_6$O$_{18}$ interconnected by Cu$^{2+}$ ions. Oxygen atoms form axially-elongated octahedral of CuO$_4$(H$_2$O)$_2$. The magnetic ground state of green dioptase remains controversial. We report inelastic neutron scattering measurements of the magnetic excitations of green dioptase Cu$_6[$Si$_6$O$_{18}]\cdot 6$H$_2$O. The observed spectrum contains two magnetic modes and a prominent spin gap that is consistent with the ordered ground state of Cu moments coupled antiferromagnetically in spiral chains along the $c$ axis and ferromagnetically in $ab$ planes on the hexagonal cell. The data are in excellent agreement with a spin-1/2 Hamiltonian that includes AFM nearest-neighbor intra-chain coupling $J_{c} = 10.6(1)$ meV, ferromagnetic inter-chain coupling $J_{ab} = -1.2(1)$ meV and exchange anisotropy $\Delta J_{c} = 0.14(1)$ meV. This appears compatible with reduced N\`{e}el temperature, $T_\mathrm{N}=14.5$ K $\ll J_{c}$, and can be explained by a presence of quantum spin fluctuations. [Preview Abstract] |
Monday, March 13, 2017 2:42PM - 2:54PM |
C50.00002: Dynamic structure factor of the square-lattice S=1/2 Heisenberg antiferromagnet Hui Shao, Yanqi QIn, Ziyang Meng, Anders Sandvik We use a newly improved stochastic analytic continuation (SAC) method for QMC data to study the dynamic spin structure factor of the square-lattice S =1/2 Heisenberg antiferromagnet at low temperature. We present results for the dispersion curve and the spectral weight of the single-magnon excitation, as well as the full multi-magnon continua along high-symmetry cuts through the Brillouin zone. Our study provides a benchmark for neutron scattering experiments. We focus on the high-energy part of the spectrum (close to the magnetic zone boundary) where the continuum is large and there is a dispersion anomaly not captured by spin-wave theory. [Preview Abstract] |
Monday, March 13, 2017 2:54PM - 3:06PM |
C50.00003: Spinon confinement in a quasi-one-dimensional XXZ Heisenberg antiferromagnet. Bella Lake, Anup K. Bera, Fabian H. L. Essler, Laurens Vanderstraeten, Claudius Hubig, Ulrich Schollwock, A. T. M. Nazmul Islam, Astrid Schneidewind, Diana L. Quintero-Castro Half-integer spin Heisenberg chains constitute a key paradigm for quantum number fractionalization: flipping a spin creates a minimum of two elementary spinon excitations. These have been observed in numerous experiments. We report on inelastic neutron scattering experiments on the quasi-one-dimensional anisotropic spin-1/2 Heisenberg antiferromagnet SrCo$_{2}$V$_{2}$O$_{8}$. These reveal a mechanism for temperature-induced spinon confinement, manifesting itself in the formation of sequences of spinon bound states. A theoretical description of this effect is achieved by a combination of analytical and numerical methods. [Preview Abstract] |
Monday, March 13, 2017 3:06PM - 3:18PM |
C50.00004: Antiferromagnetic dynamical spin structure factor in doped Heisenberg chains Kevin Jaegering, Annabelle Bohrdt, Sebastian Eggert, Imke Schneider Inelastic neutron scattering experiments on weakly doped quasi one-dimensional spin chain compounds have found a surprising enhancement of the spectral weight at low energies at the antiferromagnetic point as compared to the pure samples. We now take the weak disorder into account as an effective fragmentation of the spin chains and obtain the momentum-resolved spectral weight for the finite segments at low-lying energies exactly using the density-matrix renormalization group algorithm. The numerical data are compared to bosonization results for scattering wave-vectors $k\approx \pi$, where the impurity contribution to the spin dynamics can be identified in a systematic finite-size scaling analysis. Surprisingly the overall contribution from impurities may either enhance or deplete the dynamical structure factor, depending on wave-vector and spin interaction anisotropy. [Preview Abstract] |
Monday, March 13, 2017 3:18PM - 3:30PM |
C50.00005: The role of frustration, low-dimensionality and low crystallographic symmetry on the magnetic order in $\beta$-TeVO$_{4}$ Dagmar Weickert, N. Harrison, B.L. Scott, M. Jaime, A. Leitmae, I. Heinmaa, R. Stern, O. Janson, H. Berger, H. Rosner, A.A. Tsirlin $\beta$-TeVO$_{4}$ is a model compound to study $S=1/2$ zigzag $J_{1}-J_{2}$ chain behavior in a low symmetry (space group P21/c) environment. Weak coupling between individual chains and significant frustration between $J_{1}$ FM nearest-neighbor and $J_{2}$ AFM next-nearest neighbor interactions leads to competing ground states at low temperature that can be tuned depending on the size and direction of the applied magnetic field[1]. We use specific heat, magnetostriction, thermal expansion and NMR experiments on single crystals to explore the $H-T$ phase diagram in all three directions $a,b,c$. Our study identifies 1st and 2nd order transitions to anisotropic SDW, helical order and a stripe phase in magnetic fields up to 10\,T. Measurements of the magnetostriction and magnetization in capacitor-driven pulsed magnets up to 30\,T and at temperatures down to 0.46\,K reveal an almost isotropic high field phase with perpendicular phase boundaries of currently unknown origin. The high field phase is accompanied by strong magneto caloric effect. Our experimental results are supported by DFT band-structure calculations analyzing the microscopic spin Hamiltonian and quantifying the leading exchange constants. [1] Weickert et al., PRB 94, 064403 (2016). [Preview Abstract] |
Monday, March 13, 2017 3:30PM - 3:42PM |
C50.00006: Deconfined spinons and metamagnetism in the J-Q model Adam Iaizzi, Anders W Sandvik We study the J-Q model on a square lattice of localized $S=1/2$ degrees of freedom. This model augments the standard $S=1/2$ Heisenberg antiferromagnet exchange, $J$, with a four-spin interaction, $Q$, that induces a quantum phase transition from the N\'{e}el antiferromagnet to a valence-bond solid state. At the phase boundary, the elementary excitations are presumably deconfined spinons ($S=1/2$ bosons). Going beyond previous studies [1], we use a magnetic field to produce a macroscopic ground-state density of spinons and find that they produce a linear contribution to the low-temperature specific heat as predicted using a low-energy effective spinon theory [2]. As in the previously studied 1D case [3], at high field there is a metamagnetic transition to saturation driven by the onset of attractive interactions between magnons beyond a minimum value of Q/J. [1] H. Shao, W. Guo, and A. W. Sandvik, Science \textbf{352}, 213 (2016) [2] H. D. Scammell and O. P. Sushkov, Phys. Rev. Lett. \textbf{114}, 055702 (2015). [3] A. Iaizzi, K. Damle, and A. W. Sandvik, arXiv:1603.04359. [Preview Abstract] |
Monday, March 13, 2017 3:42PM - 4:18PM |
C50.00007: Continuous excitations of the triangular-lattice quantum spin liquid candidate YbMgGaO$_4$ Invited Speaker: Martin Mourigal A quantum spin liquid (QSL) is an exotic state of matter in which electrons' spins are quantum entangled over long distances, but do not show magnetic order in the zero-temperature limit. The observation of QSL states is a central aim of experimental physics, because they host collective excitations that transcend our knowledge of quantum matter; however, examples in real materials are scarce. In this talk, I will report neutron-scattering experiments on YbMgGaO$_4$, a recently discovered QSL candidate in which Yb$^{3+}$ ions with effective spin-1/2 occupy a triangular lattice. Our single-crystal measurements reveal a continuum of magnetic excitations -- the essential experimental hallmark of a QSL -- at very low temperature (0.06 K). The origin of this peculiar excitation spectrum is a crucial question, because isotropic nearest-neighbor interactions do not yield a QSL ground state on the triangular lattice. Using measurements in the field-polarized state, we identify antiferromagnetic next-nearest-neighbor interactions, spin-space anisotropies, and chemical disorder between the magnetic layers as key ingredients in YbMgGaO$_4$. \textit{Reference:} J. A. M. Paddison, M. Daum, Z. L. Dun, G. Ehlers, Y. Liu, M. B. Stone, H. D. Zhou, and M. Mourigal, Nature Physics (In press, 2016). arXiv:1607.03231. [Preview Abstract] |
Monday, March 13, 2017 4:18PM - 4:30PM |
C50.00008: New quantum phases in spin ladders with ring exchange and frustration Alexandros Metavitsiadis, Sebastian Eggert The ground state properties of spin-1/2 ladders are studied, emphasizing the role of frustration and ring exchange coupling. We present a unified field theory for ladders with general coupling constants and geometry. Rich phase diagrams can be deduced by using a renormalization group calculation for ladders with in--chain next nearest neighbor interactions and plaquette ring exchange coupling. In addition to established phases such as Haldane, rung singlet, and dimerized phases, we also observe a surprising instability towards an incommensurate phase for weak interchain couplings, which is characterized by an exotic coexistence of self-consistent ferromagnetic and anti-ferromagnetic order parameters. [Preview Abstract] |
Monday, March 13, 2017 4:30PM - 4:42PM |
C50.00009: Spin-orbit interaction driven dimerization in one dimensional frustrated magnets Shang-Shun Zhang, Cristian D. Batista Spin nematic ordering has been proposed to emerge near the saturation of field of a class of frustrated magnets. The experimental observation of this novel phase is challenging for the traditional experimental probes. Nematic spin ordering is expected to induce a local quadrupolar electric moment via the spin-orbit coupling. However, a finite spin-orbit interaction explicitly breaks the U(1) symmetry of global spin rotations down to $Z_2$, which renders the traditional nematic order no longer well-defined. In this work we investigate the relevant effect of spin-orbit interaction on the 1D frustrated $J_1-J_2$ model. The real and the imaginary parts of the nematic order parameter belong to different representations of the discrete symmetry group of the new Hamiltonian. We demonstrate that spin-orbit coupling stabilizes the real component and simultaneously induces bond dimerization in most of the phase diagram. Such a bond dimerization can be observed with X-rays or nuclear magnetic resonance. In addition, an incommensurate bond-density wave (ICBDW) appears for smaller values of $J_2/|J_1|$. The experimental fingerprint of the ICBDW is a double-horn shape of the the NMR line. These conclusions can shed light on the experimental search of this novel phase. [Preview Abstract] |
Monday, March 13, 2017 4:42PM - 4:54PM |
C50.00010: Absence of long range order in SrDy$_2$O$_4$ frustrated magnet due to trapped defects from a dimensionality crossover Nicolas Gauthier, Amy Fennell, Anne-Christine Uldry, Bernard Delley, Romain Sibille, Jonathan White, Christof Niedermayer, Vladimir Pomjakushin, Michel Kenzelmann, Bobby Prevost, Alexandre Desilets-Benoit, Andrea D. Bianchi, Hanna A. Dabkowska, Goran Nilsen, Louis-Pierre Regnault The simultaneous occurence of geometrical frustration and low dimensionality can lead to strongly correlated fluctuating ground states. In the SrLn$_2$O$_4$ compounds, the Ln magnetic ions form one-dimensional (1D) zig-zag chains that have both of these characteristics, offering a playground to study novel states of matter. In SrDy$_2$O$_4$, the two inequivalent Dy$^{3+}$ sites are Ising-like with perpendicular easy-axes, favouring the decoupling of neighbouring zig-zag chains. No long range order is observed down to $T=60$ mK in zero field but diffuse neutron scattering indicates short range correlations that are consistent with those of the 1D Ising zig-zag chain model. AC susceptibility measurements indicate a slowing down of the fluctuations at low temperatures. We attribute this behaviour to the domain walls in the zig-zag chains. Experimental evidence of a dimensionality crossover at low temperatures in SrDy$_2$O$_4$ suggest that the domains walls are trapped because of interchain interactions, precluding long-range order to the lowest temperatures. [Preview Abstract] |
Monday, March 13, 2017 4:54PM - 5:06PM |
C50.00011: Unusual UUDD magnetic chain structure of the spin-1/2 tetragonally distorted spinel GeCu$_{\mathrm{2}}$O$_{\mathrm{4}}$. Tao Zou, Yunqi Cai, Clarina dela Cruz, V. Ovidue Garlea, S.D. Mahanti, Jinguang Cheng, Xianglin Ke GeCu$_{\mathrm{2}}$O$_{\mathrm{4}}$ exhibits a tetragonal spinel structure due to the strong Jahn-Teller distortion associated with Cu$^{\mathrm{2+}}$ ions. We show that its magnetic structure can be described as slabs composed of a pair of layers with orthogonally oriented spin 1/2 Cu chains in the basal ab plane. The spins between the two layers within a slab are collinearly aligned while the spin directions of neighboring slabs are perpendicular to each other. Interestingly, we find that spins along each chain form an unusual up-up-down-down (UUDD) pattern, suggesting a non-negligible nearest-neighbor biquadratic exchange interaction in the effective classical spin Hamiltonian. We hypothesize that spin-orbit coupling and orbital mixing of Cu$^{\mathrm{2+}}$ ions in this system is non-negligible, which calls for future calculations using perturbation theory with extended Hilbert (spin and orbital) space and calculations based on density functional theory including spin-orbit coupling and looking at the global stability of the UUDD state. [Preview Abstract] |
Monday, March 13, 2017 5:06PM - 5:18PM |
C50.00012: Dynamic Correlations in One-Dimensional Quantum Magnets at Finite Temperature B. Fauseweh, G. S. Uhrig In our contribution we investigate the anomalous decoherence effects found previously in theory and experiment of several quantum magnets. A diagrammatic perturbative approach is developed and applied, which incorporates the hard-core bosonic nature of spin excitations. We will discuss in detail, how poles at infinite energy in the approach leaves traces at finite energies. This is important to obtain the correct sum rule for the quasi-particle response.\\ The key result of our contribution is the investigation of two quantum magnets: BaCu$_2$V$_2$O$_8$ and Cu(NO$_3$)$_2 \cdot 2.5$ D$_2$O. The agreement between the experimental data, measured in the time and frequency domain, and the theoretical prediction are exceptional. As a consequence, the anomalous decoherence observed can be traced back to non-trivial scattering processes of the hard-core bosonic excitations. This indicates, that quantum coherence plays a significant role in describing dynamical correlations, even at finite temperature. [Preview Abstract] |
Monday, March 13, 2017 5:18PM - 5:30PM |
C50.00013: $^{29}$Si NMR study of polycrystalline NaTiSi$_{2}$O$_{6}$ Raivo Stern, Riho Rästa, Ivo Heinmaa, Enno Joon, Harlyn J. Silverlein, Christopher Wiebe NaTiSi$_{2}$O$_{6}$ (NTSO) structure consists of quasi 1-D zig-zag chains of edge-sharing slightly distorted TiO$_{6}$ octahedrons. The chains are separated by SiO$_{4}$ tetrahedrons. At high T the distance between magnetic spin-1/2 Ti$^{3+}$ ions in the chain is equal. At T$_c=$210 K the compound undergoes orbital-Peierls transition. As a result, below 210 K TiO$_{6}$ chain becomes dimerized having diamagnetic singlet ground state. Neutron spectroscopy provided singlet-triplet gap value 615(35) K [*], $\mu$SR 2$\Delta=$700(100) K. Our $^{29}$Si magic angle spinning NMR spectra show in paramagnetic region one single resonance with paramagnetic shift $K=$713 ppm at 300 K. The shift slightly increases with decreasing T and has maximum $K=$796 ppm at T= 213 K. Below T$_c$ the resonance transforms into two lines with different paramagnetic shifts. At T = 56 K the spectrum shows 2 sharp lines with diamagnetic chemical shifts -84 and -101 ppm corresponding to 2 different Si sites in the low-T unit cell. T-dependence of $^{29}$Si spin-lattice relaxation $T_{1}$ in 70 K $<$ T $<$ 140 K follows activation type T-behavior with $E_{a}$=300(20) K, which we ascribe to the splitting between the 2 lowest $d$-orbital energy levels.\\ $^{*}$ H. J. Silverstein et al., PRB 90, 140402(R) (2014). [Preview Abstract] |
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