Bulletin of the American Physical Society
2013 Annual Fall Meeting of the APS Prairie Section
Volume 58, Number 15
Thursday–Saturday, November 7–9, 2013; Columbia, Missouri
Session B2: Condensed Matter Physics I |
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Chair: Tom Heitmann, University of Missouri Research Reactor Room: Memorial Union Stotler I&II |
Friday, November 8, 2013 8:30AM - 9:06AM |
B2.00001: Tb$_{2}$Mo$_{2}$O$_{7}$: Spin glass, spin ice and possible candidate for magnetic monopoles exploration Invited Speaker: Deepak Singh The frustrated pyrochlore compound Tb$_{2}$Mo$_{2}$O$_{7}$ stands at an interesting crossroads, being a representative of both the Mo-based family $R_{2}$Mo$_{2}$O$_{7}$ ($R =$ rare earth) and other Tb-based compounds Tb$_{2}X_{2}$O$_{7}$ ($X =$ metal). As a function of the R-site radius, R$_{2}$Mo$_{2}$O$_{7}$ compounds exhibit a metal-insulator transition between the ferromagnetic metal states and the spin glass insulators. Tb$_{2}$Mo$_{2}$O$_{7}$ exhibits the spin-glass behavior, $T_{G}$ $\sim$ 24 K, despite the apparent lack of chemical disorder. This compound crystallizes in a cubic space group in which both the Tb and Mo atoms form three-dimensional networks of corner-sharing tetrahedra. Thus, each magnetic ion resides on a highly frustrated pyrochlore lattice. Neutron scattering measurements on single crystal specimens of Tb$_{2}$Mo$_{2}$O$_{7}$ revealed the short-ranged spin arrangements resembling the ``spin ice'' structure with Tb moments slightly tilted off the local $<$111$>$ -direction. Detailed analysis of a.c. and nonlinear susceptibilities suggest that Tb$_{2}$Mo$_{2}$O$_{7}$ is not sufficiently frozen below glass transition, rather finite spin dynamics persists to the lowest measurement temperature. Such nonconventional glassy behavior is also reflected in thermodynamic scaling of the nonlinear susceptibilities. In addition to the spin ice configuration and a nonconventional spin glass transition, Tb$_{2}$Mo$_{2}$O$_{7}$ also exhibits \textbf{Q}-independent temperature dependent background. Similar experimental observations in an isostructural pyrochlore, Ho$_{2}$Ti$_{2}$O$_{7}$, were identified as distinct signatures of Dirac's effective magnetic monopoles. Thus Tb$_{2}$Mo$_{2}$O$_{7}$ provides a new frontier to extend this noble quest. [Preview Abstract] |
Friday, November 8, 2013 9:06AM - 9:18AM |
B2.00002: Structural and Magnetic Transitions in Ca$_{10}$(Fe$_{1.996}$Pt$_{0.004}$As$_2$)$_5$(Pt$_{3}$As$_8$) studied by neutron and x-ray diffraction Aashish Sapkota, Andreas Kreyssig, Gregory Tucker, Mehmet Ramazanoglu, Douglas Robinson, Ni Ni, Alan Goldman, Robert McQueeney Ca$_{10}$(Fe$_{1.996}$Pt$_{0.004}$As$_{2}$)$_{5}$(Pt$_{3}$As$_{8}$) compound is a member of the Fe-based high-temperature superconductor family. Recent work showed that instead of being tetragonal as most of the pnictide superconductors are, this compound exhibits only a pseudo-tetragonal structure. We studied the structure and magnetic properties of Ca$_{10}$(Fe$_{1.996}$Pt$_{0.004}$As$_{2}$)$_{5}$(Pt$_{3}$As$_{8}$) single crystal by x-ray and neutron diffraction at the station 6-ID-D, Advanced Photon Source, Argonne, and at the instrument HB-1A, High-Flux Isotope Reactor, Oak Ridge, respectively. We found a lattice distortion from pseudo-tetragonal to pseudo-orthorhombic below T$_{\mathrm{s}} \quad =$ 110 K and stripe-like antiferromagnetic order below T$_{\mathrm{N}} \quad =$ 96 K. Both phase transitions are 2$^{\mathrm{nd}}$ order in nature. Though the structure is pseudo-tetragonal with a complex superstructure rather than being common tetragonal, the magnetic order and lattice distortion are similar to most other pnictide superconductors demonstrating these ordering phenomena extremely robust against deviations from simple structure motifs and against chemical disorder. [Preview Abstract] |
Friday, November 8, 2013 9:18AM - 9:30AM |
B2.00003: Magnetic Order in CeGe$_{1.76}$ studied by neutron diffraction on single crystals W. Jayasekara, W. Tian, A. Kreyssig, S.L. Bud'ko, P.C. Canfield, R.J. McQueeney, A.I. Goldman The CeGe$_{1.76}$ compound shows a very interesting magnetic behavior with a rich set of features in magnetization measurements indicative for several antiferromagnetic and ferromagnetic transitions at low temperatures. In this report we will present our recent neutron diffraction studies on a CeGe$_{1.76}$ single crystal performed at the instrument HB-1A, High-Flux Isotope Reactor, Oak Ridge. We revealed a complex magnetic phase diagram: Below approx. 7 K, the magnetic Ce moments order in an antiferromagnetic structure with an incommensurate propagation along the c direction. The major component of the ordered moment is aligned along the b direction. Detailed temperature-dependent scans along the c direction show a lock-in to a commensurate antiferromagnetic order around 5.5 K connected with a squaring up of the ordered moments indicated by the occurrence of higher harmonic satellite Bragg peaks. Below 4 K additional Bragg peaks occur indicative of a second coexisting type of magnetic order with small moments aligned along the a or c direction. Further experiments are necessary to conclude the determination of this complex magnetic ordering in the CeGe$_{1.76}$ compound. [Preview Abstract] |
Friday, November 8, 2013 9:30AM - 9:42AM |
B2.00004: On the wetting, phase transitions, and diffusion of water on supported bilayer lipid membranes Zachary Buck, Andrew Miskowiec, Mia Brown, Mengjun Bai, Jason Cooley, Renee Jiji, Haskell Taub, Flemming Hansen, Helmut Kaiser, Madhusudan Tyagi, Souleymane Diallo, Eugene Mamontov, Kenneth Herwig Temperature-dependent elastic incoherent neutron scattering shows qualitatively different freezing behavior for water associated with single bilayers of the charge-neutral DMPC (dimyristoylphosphocholine) lipid and for the anionic DMPG (dimyristoylphosphoglycerol) bilayer membrane supported on a silicon substrate. While water in the vicinity of the neutral DMPC membrane shows a major freezing transition slightly below the bulk freezing point, water near DMPG is characterized by continuous freezing to lower temperatures. Water remains mobile in the DMPG system down to 210 K in contrast to water associated with the DMPC membrane, which freezes completely at 255 K. We suggest that this behavior may be related to a film-like water structure in the DMPG case owing to the hydrophilic nature of the substrate, while most of the water in the DMPC system is bulk-like and dewets from the hydrophobic surface. Analysis of the quasielastic spectra of the DMPC system yields a diffusion constant of the membrane-associated water that decreases in a step-like fashion on cooling, indicating a second freezing transition below the one attributed to bulk-like water. [Preview Abstract] |
Friday, November 8, 2013 9:42AM - 9:54AM |
B2.00005: Magnetic diffraction at MURR and possible magnetic ordering in magnetoelectric HoAl$_{3}$(BO$_{3})_{4}$ Tom Heitmann, Qiang Zhang, K.C. Liang, L.M. Bezmaternykh, V.L. Temerov, B. Lorenz, David Vaknin We report on single-crystal diffraction studies of HoAl$_{3}$(BO$_{3}$)$_{4}$ we have performed to unravel its magnetic properties. HoAl$_{3}$(BO$_{3}$)$_{4}$ is among a number of compounds with the trigonal huntite crystal structure (R32, No. 155) that display magnetoelectric (ME) coupling and is notable as having one of the strongest ME effects [Liang et a. PRB 83, 180417(R) (2001)]. To test for any putative magnetic ordering, we have performed single-crystal neutron diffraction measurements on flux grown HoAl$_{3}$(BO$_{3}$)$_{4}$ crystals. We report preliminary results on the appearance of a crystallographic reflection that is forbidden in the huntite structure at a temperature that is consistent with the appearance of the ME effect. We associate this reflection with the emergence of antiferromagnetic ordering of the Ho$^{3+}$ moments that establishes a broken time-reversal symmetry, the prerequisite condition for the ME effect. These experiments will be placed in the context of the burgeoning collaborative effort between MURR and the neutron scattering group at the Ames Laboratory. [Preview Abstract] |
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