Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session H21: 1D Gapped AntiferromagnetsFocus
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Sponsoring Units: GMAG DMP Chair: Vivien Zapf, NHMFL-LANL Room: 320 |
Tuesday, March 15, 2016 2:30PM - 3:06PM |
H21.00001: Spectroscopic Investigation of the Origin of Magnetic Bistability in Molecular Nanomagnets Invited Speaker: Joris van Slageren Molecular nanomagnets (MNMs) are coordination complexes consisting of one of more transition metal and/or f-element ions bridged and surrounded by organic ligands. Some of these can be magnetized in a magnetic field, and remain magnetized after the field is switched off. Because of this, MNMs have been proposed for magnetic data storage applications, where up to 1000 times higher data densities than currently possible can be obtained. Other MNMs were shown to display quantum coherence, and, as a consequence, are suitable as quantum bits. Quantum bits are the building blocks of a quantum computer, which will be able to carry out calculations that will never be possible with a conventional computer. The magnetic bistability of MNMs originates from the magnetic anisotropy of the magnetic ions, which creates an energy barrier between up and down orientations of the magnetic moment. Currently, most work in the area focuses on complexes of either lanthanide ions or low-coordinate transition metal ions. Synthetic chemical efforts have led to a large number of novel materials, but the rate of improvement has been slow. Therefore a better understanding of the origin of the magnetic anisotropy is clearly necessary. To this end we have applied a wide range of advanced spectroscopic techniques, ranging from different electron spin resonance techniques at frequencies up to the terahertz domain to optical techniques, including luminescence and magnetic circular dichroism spectroscopy. We will discuss two examples, one from the area of lanthanide MNMs [1], one a transition metal MNM (unpublished). [1] . Rechkemmer, J.E. Fischer, R. Marx, M. D\"{o}rfel, P. Neugebauer, S. Horvath, M. Gysler, T. Brock-Nannestad, W. Frey, M.F. Reid, J. van Slageren*, "Comprehensive Spectroscopic Determination of the Crystal Field Splitting in an Erbium Single-Ion Magnet", J. Am. Chem. Soc., 137, 13114--13120 (2015). [Preview Abstract] |
Tuesday, March 15, 2016 3:06PM - 3:18PM |
H21.00002: Conditions for the appearance of boundary modes in topological phases of Heisenberg spin ladders Neil Robinson, Alexander Atland, Reinhold Egger, Nkilas Gergs, Robert Konik, Wei Li, Dirk Schuricht, Alexei Tsvelik, Andreas Weichselbaum We consider the problem of delineating the necessary conditions for the appearance of boundary modes in extended $SU(2)$ Heisenberg spin ladders. Specifically, we study Heisenberg ladders with rung exchange, $J_\perp$, and ring exchange, $J_X$, that admit a field theoretic description in terms of Majorana fermions in the continuum limit. In this description there are four Majorana fermions, arranged in a triplet and a singlet. This suggests there are four distinct phases, corresponding to the configurations of the signs of the triplet $m_t$ and singlet $m_s$ masses. We label these phases as: Haldane ($m_t>0,m_s<0$), rung singlet ($m_t<0,m_s>0$), VBS$_+$ ($m_t,m_s>0$) and VBS$_-$ ($m_t,m_s<0$). Topologically, we find two of these phases support gapless boundary modes: the Haldane phase (the triplet forms a spin-$1/2$ degree of freedom at the ends of the ladder) and the VBS$_+$ phase, where all the Majorana fermions have gapless boundary modes. The absence of a gapless boundary mode in the rung singlet phase is surprising; we find that the singlet mode can become gapless if open boundary conditions are replaced with a continuous change in lattice parameters. We suggest a symmetry-allowed modification to the low-energy effective theory which may be responsible for this behavior. [Preview Abstract] |
Tuesday, March 15, 2016 3:18PM - 3:30PM |
H21.00003: The pressure effects on the antiferromagentic orders in iron-based ladder compounds BaFe$_2$S$_3$ Songxue Chi, Yoshiya Uwatoko, Yasuyuki Hirata, Kenya Ohgushi The ladder compounds have recently become a new test ground for the studies on Fe-based superconductors. The building block for such materials, the two-leg Fe ladder surrounded by edge-sharing chalcogen tetrahedra, has provided a quasi-one-dimensional channel for the remaining critical issues in this field. Recently, superconductivity was successfully induced by pressure in one of such compounds, BaFe$_2$S$_3$. The knowledge of the pressure effect on its antiferromagnetic order is crucial in understanding the superconductivity in the low-dimensional system. I will present the results of our neutron diffraction studies on the evolution of the magnetic phase under hydraulic pressure in single crystalline BaFe$_2$S$_3$. [Preview Abstract] |
Tuesday, March 15, 2016 3:30PM - 3:42PM |
H21.00004: Neutron scattering study of magnetic structure in triangle spin tube CsCrF$_4$ Masato Hagihala, Maxim Avdeev, Hirotaka Manaka, Takatsugu Masuda Triangle spin tube viewed from tube direction is topologically equivalent to kagomé lattice. The rung ($J_1$) and inter-tube ($J_2$) interactions on triangle spin tube correspond respectively to the next nearest neighbor and the nearest neighbor interactions on kagomé lattice. In the case of $J_1 > 0$ (Antiferromagnetic) and $ J_1 >> |J_2|$, the ground state is $q = 0, 120^\circ$ structure with $J_2 > 0$ or Cuboc state that represented multi-$q$ ($q = 2\pi(1/2~0)$ and two symmetric-equivalent vectors) with $J_2 < 0$ [1]. CsCrF$_4$ is a perfect triangle spin tube material with antiferromagnetic intra-tube and rung interactions [2]. Neutron diffraction measurement revealed magnetic long-range order at $T = 1.5~\rm{K}$. Contrary to the expectation, the magnetic structure was determined $q = 2\pi(1/2~0~1/2)$, $120^\circ$ structure by Rietveld refinement. We also confirmed that this structure was stabilized by Dzyaloshinskii -Moriya interaction and small anisotropy that obeyed the three-fold symmetry at Cr sites by calculation. [1] H. Ishikawa et al., JPSJ 83, 043703 (2014). [2] H. Manaka et al., JPSJ 78, 093701 (2009). [Preview Abstract] |
Tuesday, March 15, 2016 3:42PM - 3:54PM |
H21.00005: Dielectric effects at a magnetic Bose-Einstein condensation Kirill Povarov, Aaron Reichert, Erik Wulf, Andrey Zheludev In the presence of magnetoelectric coupling one can expect non-trivial dielectric properties at a magnetic quantum phase transition. A ``toy model'' here is a spin spiral undergoing a field-induced transition into a quantum-disordered phase. In the incommensurate phase the in-plane spin rotational symmetry is protected, making the analogy between the magnetic long-range ordering and BEC exact, but the spin spiral may also host an electric polarization complicating the picture. We have experimentally studied this transition in the spin tube material Sul-Cu$_2$Cl$_4$ [1] to understand if it can be described as a magnetic BEC. We have found that indeed it can. Dielectric spectroscopy results combined with calorimetric measurements, clearly show the absence of polarization fluctuations in the disordered phase down to the very critical point. At the same time the ordered phase shows a huge nonlinearity in dielectric permittivity even for small electric fields. The phase boundary shows beautiful consistency with the 3D BEC universality class. We conclude, that although magnetoelectric coupling does not alter the nature of the transition, it gives rise to complex magnetoelectric effects in the helimagnetically ordered phase.\\[4pt] [1] K. Povarov et al.; Phys. Rev. B 92 140410 (2015) [Preview Abstract] |
Tuesday, March 15, 2016 3:54PM - 4:06PM |
H21.00006: Optical spin excitations in quantum spin ladders Gediminas Simutis, Severian Gvasaliya, Fan Xiao, Christopher Landee, Andrey Zheludev We present a Raman spectroscopy study of magnetic excitations in quantum spin ladders. We start with a strong-rung ladder Cu(Qnx)(Cl$_{1-x}$Br$_x$)$_2$. It has recently attracted attention due to proposal that the ratio of leg to rung exchange can be varied continuously by substituting Br for Cl. We have measured the Raman spectra for the hole doping series and report on the scattering from two magnons [1]. We extract the onset and cutoff of the scattering for the whole series and compare it to the estimates from previous bulk measurements as well as numerical calculations. We find that the magnetic spectrum indeed varies continuously as the halogen ions are exchanged. The general behavior is found to be consistent with expectations, however small systematic deviations persist. The difference can potentially be explained by the existence of three-dimensional coupling, however more systematic computational studies are needed to ascertain the origin of the inconsistencies. Having established the analysis using the strong rung case, we then turn our attention to other ladder systems. Unusual magnetic signal is found in a strong leg spin ladder, which is discussed in terms of selection rules and an unexpected energy scale. [1] G. Simutis {\it et al.} arXiv:1510.06360 [Preview Abstract] |
Tuesday, March 15, 2016 4:06PM - 4:18PM |
H21.00007: ESR modes in a Strong-Leg Ladder in the Tomonaga-Luttinger Liquid Phase S. Zvyagin, M. Ozerov, M. Maksymenko, J. Wosnitza, A. Honecker, C.P. Landee, M. Turnbull, S.C. Furuya, T. Giamarchi Magnetic excitations in the strong-leg quantum spin ladder compound (C$_7$H$_{10}$N)$_2$CuBr$_4$ (known as DIMPY) in the field-induced Tomonaga-Luttinger spin liquid phase are studied by means of high-field electron spin resonance (ESR) spectroscopy. The presence of a gapped ESR mode with unusual non-linear frequency-field dependence is revealed experimentally. Using a combination of analytic and exact diagonalization methods, we compute the dynamical structure factor and identify this mode with longitudinal excitations in the antisymmetric channel. We argue that these excitations constitute a fingerprint of the spin dynamics in a strong-leg spin-1/2 Heisenberg antiferromagnetic ladder and owe its ESR observability to the uniform Dzyaloshinskii-Moriya interaction. [Preview Abstract] |
Tuesday, March 15, 2016 4:18PM - 4:30PM |
H21.00008: Field-induced spontaneous magnon decay in a spin-1/2 coupled two-leg ladder antiferromagnet C9H18N2CuBr4 with small Ising anisotropy Tao Hong, Y. Qiu, D. A. Tennant, K. Coester, K. P. Schmidt, F. F. Awwadi, M. M. Turnbull We present the high-resolution neutron scattering study in magnetic fields applied perpendicular to an easy-axis (Ising type) on the $S=$1/2 coupled two-leg ladder antiferromagnet (dimethylammonium)(3,5-dimethylpyridinium)CuBr$_{\mathrm{4}}$. At finite fields, the magnetic structure becomes noncollinear canted and the observed intriguing spontaneous magnon decays over a large region of Brillouin Zone in the excitation spectra can be well explained by the mechanism where the process of one-magnon decays into two-magnon continuum is kinematically allowed [1]. [1] M. E. Zhitomirsky and A. L. Chernyshev, Rev. Mod. Phys. \textbf{85}, 219 (2013). [Preview Abstract] |
Tuesday, March 15, 2016 4:30PM - 4:42PM |
H21.00009: Local structure of spin Peierls compound TiPO$_{\mathrm{4}}$: $^{\mathrm{47/49}}$Ti and $^{\mathrm{31}}$P NMR study Raivo Stern, Ivo Heinmaa, Alexander Leitmäe, Enno Joon, Alexander Tsirlin, Reinhard Kremer, Robert Glaum TiPO4 structure is made of slightly corrugated TiO2 ribbon chains of edge-sharing TiO6 octahedra. The almost perfect 1D spin \textonehalf Ti3$+$ chains are well separated by PO4 tetrahedra. By magnetic susceptibility and MAS-NMR measurements [1] it was shown that TiPO4 has nonmagnetic singlet ground state with remarkably high Spin-Peierls (SP) transition temperature. The high-T magnetic susceptibility of TiPO4 follows well that of a S$=$1/2 Heisenberg chain with very strong nearest-neighbor AF spin-exchange coupling constant of J$=$965K. On cooling TiPO4 shows two successive phase transitions at 111K and 74K, with incommensurate (IC) SP phase between them. We studied local structure and dynamics in TiPO4 single crystal using 47/49Ti and 31P NMR in the temperature range 40K to 300K, and determined the principal values and orientation of the magnetic shift tensors for 31P and 47,49Ti nuclei. Since 47,49Ti (S$=$5/2 and S$=$7/2, respectively) have quadrupolar moments, we also found the principal axis values and orientations of the electric field gradient (efg) tensor in SP phase and at 295K. In SP phase the structure contains 2 magnetically inequivalent P sites and only one Ti site. From the T-dependence of the relaxation rate of 31P and 47Ti nuclei we determined activation energy Ea $=$ 550 K for spin excitations in SP phase. [Preview Abstract] |
Tuesday, March 15, 2016 4:42PM - 4:54PM |
H21.00010: Pressure-Induced Order in the Gapped Quantum Magnet DTN Alexandra Mannig, Johannes Moeller, Andrey Zheludev, V. Ovidiu Garlea, Clarina dela Cruz, Zurab Guguchia, Rustem Khasanov, Elvezio Morenzoni We present muon-spin relaxation, neutron diffraction and magnetic susceptibility data under applied hydrostatic pressure on the organometallic $S=1$ quantum magnet $NiCl_{2}\cdot4[SC(NH_{2})_{2}]$. The material consists of weakly coupled antiferromagnetic chains and has a spin gap resulting from a large single-ion anisotropy. Our muon spin rotation experiments provide local field dependencies on temperature as well as pressure and allow for the mapping of a detailed phase diagram up to 22$\,$kbar. Thus, we demonstrate that the compound may be driven through two subsequent pressure-induced transitions into apparently distinct magnetically ordered phases. Neutron diffraction and susceptibility measurements support those results and show the potential of low-pressure transitions to be investigated by various techniques. [Preview Abstract] |
Tuesday, March 15, 2016 4:54PM - 5:06PM |
H21.00011: Unusual Magnetic-Pressure Response of an $S=1$ Antiferromagnetic Linear-Chain near the $D/J \approx 1$ Critical Point. M. K. Peprah, P. A. Quintero, J. S. Xia, J. M. P\'erez, M. W. Meisel, A. Garcia, S. E. Brown, J. L. Manson An $S = 1$ chain, [Ni(HF$_2$)(3-Clpy)$_4$]BF$_4$ (py = pyridine), has been identified to have nearest-neighbor antiferromagnetic interaction $J/k_B = 4.86$~K and single-ion anisotropy $D/k_B = 4.3$~K, while avoiding long-range order to 25~mK.\footnote{J.L.~Manson \emph{et al.}, Inorg.~Chem. 51 (2012) 7520.} With $D/J = 0.88$, this system is close to the $D/J \approx 1$ gapless quantum critical point between the Haldane and Large-$D$ phases. The magnetization was studied at 50~mK $\leq T \leq~1$~K and with $B \leq 10$~T.\footnote{J.-S.~Xia \emph{et al.}, arxiv.1409.5971 (2014).} Using a magnetometer equipped with a pressure cell, the low-field (0.1~T), high temperature ($T \geq 2$~K) magnetic susceptibility was studied to 1.47~GPa. These data suggest the response at ambient pressure$^2$ changes between 0.24~GPa and 0.35~GPa. These studies are being extended by $^1$H NMR experiments capable of varying the pressure and of spanning from 300~K to below 100~mK. [Preview Abstract] |
Tuesday, March 15, 2016 5:06PM - 5:18PM |
H21.00012: High-pressure neutron scattering of Prussian blue analogue magnets Daniel Pajerowski Pressure sensitive magnetism is known to be useful in sensors, and while applications tend to use metallic alloys, molecule based magnets (MBMs) have been shown to have large inverse magnetostrictive (IMS) response. A promising group of MBMs are the Prussian blue analogues (PBAs), in which magnetic ordering can be tuned by external stimuli such as light, electric field, and pressure. Previously, high pressure neutron scattering of nickel hexacyanochromate hydrate has shown direct evidence for isomerization of the cyanide linkage with applied pressure. Other probes have suggested a similar effect in iron hexacyanochromate hydrate, although there has yet to be direct crystallographic evidence. Neutron diffraction is sensitive to organic elements, even while in the presence of metals, and we have performed experiments above 1 GPa to look for linkage isomerism in iron hexacyanochromate. These results are supported by bulk probes and calculations. [Preview Abstract] |
Tuesday, March 15, 2016 5:18PM - 5:30PM |
H21.00013: Changes in the unoccupied electronic structure of the spin crossover molecule [Co(dpzca)$_{2}$] Yang Liu, Xin Zhang, Axel Enders, Peter Dowben, Jian Luo, Jian Zhang, Alpha N’Diaye We have investigated the changes in the unoccupied electronic structure of the spin crossover molecule - [Co(dpzca)$_{2}$] using X-ray absorption spectroscopy (XAS) and have compared the results with magnetometry (SQUID) measurements. The studies of the variable temperature of the electronic structure of this cobalt complex with symmetric pyrazine imide ligands, -(2-pyrazylcarbonyl)-2-pyrazinecarboxamide, i.e. [Co(dpzca)$_{2}$], are consistent with density functional theory (DFT). The temperature dependence of the occupancy of the high-spin state and low-spin state molecular orbital states, the unoccupied e$_{g}$/t$_{2g}$ ratio from XAS and high spin state to low spin state ratio from molecular magnetic susceptibility $\chi _{M}$T indicates that the low spin state is not a zero spin state, but simply a lower moment state that would occur below the spin crossover transition of [Co(dpzca)$_{2}$]. [Preview Abstract] |
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