Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session N39: Structural Phase Transitions |
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Sponsoring Units: DCMP Chair: T. Egami, University of Tennessee Room: LACC 514 |
Wednesday, March 23, 2005 8:00AM - 8:12AM |
N39.00001: Coherent optical and acoustic phonon generation correlated to the charge ordering phase transition in La1-xCaxMnO3 Antoinette Taylor, Daeyoung Lim, Verner Thorsmolle, Richard Averitt, Quanxi Jia, Kenhyuk Ahn, Matthias Graf, Stuart Trugman We have observed coherent optical and acoustic phonon generation, which are strongly coupled to the charge ordering (CO) transition in La$_{1-x}$Ca$_{x}$MnO$_{3 }$(x= 0.5, 0.58) using femtosecond optical pump-probe spectroscopy. Coherent optical phonons, observed at low temperatures, suddenly disappear above the charge ordering temperature T$_{CO}$. We attribute the sudden onset of coherent optical phonons to their enhanced coupling to the photoexcited charge carriers in CO phase. The oscillation frequency for coherent acoustic phonon depends on the probe wavelength, which is consistent with the propagating strain pulse mechanism. The dramatic change of lattice constants across the charge ordering transition explains the overall temperature dependence of the coherent acoustic phonon amplitude. [Preview Abstract] |
Wednesday, March 23, 2005 8:12AM - 8:24AM |
N39.00002: Optically Induced Lattice Dynamics of hexagonal manganite using Ultrafast X-ray Diffraction Hae Ja Lee, J.B. Workman, J.S. Wark, R.D. Averitt, A.J. Taylor, J.P. Roberts, Q. McCulloch, D.E. Hof, D.J. Funk, N. Hur, S.-W. Cheong We have studied the picosecond lattice dynamics of optically pumped hexagonal manganite LuMnO3 using ultrafast x-ray diffraction. The results show a shift and broadening of the diffraction curve due to the stimulated lattice expansion. To understand the transient response of the lattice, the measured time- and angle-resolved diffraction curves are compared with a theoretical calculation based on dynamical diffraction theory modified for the hexagonal crystal structure of LuMnO3. Our simulations reveal that a large coupling coefficient between the a-b plane and the c-axis (c13) is required to the data. We compare this result to our previous coherent phonon studies of LuMnO3 using optical pump-probe spectroscopy. [Preview Abstract] |
Wednesday, March 23, 2005 8:24AM - 8:36AM |
N39.00003: Electron interaction effects on Jahn-Teller instability in LaMnO$_3$ Wei-Guo Yin, Dmitri Volja, Wei Ku We investigate the origin of the orbital ordering in LaMnO$_3$ using the LDA+U method and Wannier functions analysis. We find that electron-electron interaction dramatically facilitates the distortion of the MnO$_6$ octahedra and effectively enhances electron-phonon interaction. In addition, electron-electron interaction plays a significant role in determining real lattice distortion and orbital ordering pattern, beyond the conventional Jahn-Teller picture. Our conclusion agrees with existing experimental data and could be directly verified by future measurements, e.g., soft X-ray scattering. [Preview Abstract] |
Wednesday, March 23, 2005 8:36AM - 8:48AM |
N39.00004: Pseudo-spin of orbital-ordered hybridized $e_g$-states in manganites Wei Ku, Wei-Guo Yin, D. Volja The physics of orbital-ordered $e_g$-states in manganites can be conveniently described with quantum pseudo-spin. In order to properly account for the strong hybridization in real materials, energy-resolved Wannier functions are constructed from first-principles to rigorously define the pseudo-spin in LaMnO$_3$ and MnF$_3$. Our quantitative results show that the orientation of the pseudo-spin (mixing of the hybridized $e_g$- states) deviates significantly from what is expected with the lattice distortion, revealing the important role of electron- electron interaction (super-exchange) that competes with the conventional Jahn-Teller effect in determining the orientation. This conclusion can be experimentally verified ($e.g.$: soft X- ray or NMR), and enables further understanding directly accessible with future measurements. [Preview Abstract] |
Wednesday, March 23, 2005 8:48AM - 9:00AM |
N39.00005: Photo Induced Structure in Spin Crossover Complex Kenichi Kato, Yutaka Moritomo, Masaki Takata, Norimichi Kojima The spin crossover complex, Fe(phen)$_{2}$(NCS)$_{2}$, is widely recognized for the temperature and photo-induced spin-state transition of the Fe$^{2+}$ ions. With the increase of temperature, the Fe$^{2+}$ ions show the transition from the low-spin (LS: S=0) to the high-spin (HS: S=2) state at Tc=180K. In addition, a photo-excitation by a green light induces a similar spin-state transition at low temperature and the HS state is trapped after the irradiation. Our purpose is the structural analysis on the charge density level of the photo-induced phase using synchrotron radiation powder diffraction. The powder diffraction data of the complex were measured at 92K under 532nm laser irradiation by using Large Debye-Scherrer Camera installed at BL02B2, SPring-8. The imaging plate was used as a detector to collect whole powder patterns simultaneously. The N$_{2}$ gas flow type system was used for the low-temperature measurements. The wavelength of the incident X-ray was 1.0Å. The photo-induced diffraction data with a high-counting statistics were successfully obtained. We have analyzed the powder diffraction data of photo-induced state by the MEM/Rietveld method. The difference of bonding nature between the photo-exited HS state and temperature-induced HS state has been revealed. [Preview Abstract] |
Wednesday, March 23, 2005 9:00AM - 9:12AM |
N39.00006: X-ray structural and magnetic studies of the magnetic-martensitic transition in Gd$_5$Si$_{0.5}$Ge$_{3.5}$ Daniel Haskel, Zahirul Islam, Jonathan Lang, George Srajer, Yaroslav Mudryk, Vitalij Pecharsky Intermetallic Gd$_5$Si$_{0.5}$Ge$_{3.5}$ undergoes a martensitic phase transition between two different orthorhombic polymorphs. This transition, which involves large shear displacements of atoms, expands the unit cell volume on warming and causes the dissapearance of ferromagnetic ordering. We report Gd $L_3$ and Ge $K$- edges XMCD and XAFS measurements on this compound as a function of temperature and magnetic field. The results indicate this transition appears to occur through an intermediate phase. This phase affects the magnetic ordering and can be transformed into the low temperature martensitic phase by the application of a magnetic field. [Preview Abstract] |
Wednesday, March 23, 2005 9:12AM - 9:24AM |
N39.00007: Local Structure of La$_{2-x}$Sr$_x$CuO$_4$ by Pulsed Neutron PDF Analysis Jin Nakamura, Thomas Proffen, Takeshi Egami La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) has orthorhombic symmetry at $x$ = 0, and orthorhombicity decreases continuously with $x$ up to $x$ = 0.21, where a structural transition to tetragonal symmetry occurs. But the high-energy oxygen LO phonon softening appears to occur suddenly near $x$ = 0.06 where superconductivity sets in. Through the high-resolution pulsed neutron pair-density function (PDF) analysis we show that the medium-range structure changes significantly near this composition. The PDF was determined using the NPDF of LANSCE, LANL. Because of the high Q resolution of this spectrometer the PDF can be determined up to 30 $nm$, connecting the local structure smoothly to the average structure. We found that the PDF peaks in the short-range up to 1 $nm$ change little with $x$, and those in the long-range over 3 $nm$ follow the average structure. However, in the intermediate range from 1 to 3 $nm$ some PDF peaks change non-linearly around $x$ = 0.06, and appear to correspond to the insulator-superconductor transition. Possible implications of these changes in relation to nano-scale charge inhomogeneity will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 9:24AM - 9:36AM |
N39.00008: Suppression of the gamma-alpha structural phase transition in Ce0.8La0.1Th0.1 by large magnetic fields John Singleton, Fivos Drymiotis, Neil Harrison, Luis Balicas, Alimamy Bangura, Jason Lashley, Chuck Mielke, Zachary Fisk, Albert Migliori, James Smith The $\gamma -\alpha $ transition in Ce$_{0.8}$La$_{0.1}$Th$_{0.1 }$is measured as a function of applied magnetic field using both resistivity and magnetization. The $\gamma -\alpha $ transition temperature decreases with increasing magnetic field, reaching zero temperature at approximately 56 T. The magnetic-field dependence of the transition of the transition temperature may be fitted using a model that invokes the field and temperature dependence of the entropy of the 4f-electron moments of the $\gamma $-phase, suggesting that the volume collapse in cerium and its alloys is primarily driven by entropic considerations. We thank DOE-LDRD-DR20030084, NSF-DMR-0433560, DOE-DE-FG03-03NA00066 and the State of Florida for support of this research. [Preview Abstract] |
Wednesday, March 23, 2005 9:36AM - 9:48AM |
N39.00009: Three-Dimensional Dynamic Loading Simulations of Stress-Strain Response of Shape Memory Materials Rajeev Ahluwalia, Turab Lookman, Avadh Saxena We present 3-D simulations of the microstructure and mechanical response of shape memory materials. The simulations are based on a nonlinear elastic free energy for a cubic to tetragonal transition in terms of the appropriate strain fields. The dynamics is simulated by force balance equations for the displacement fields with a damping term derived from a dissipational function. This approach ensures that the elastic compatibility relations, which play a crucial role in determining the microstructure of shape memory alloys, are naturally satisfied. Stress-strain properties in the pseudoelastic as well as the shape memory regime are investigated using three dimensional strain loading simulations that take into account the microstructural evolution during deformation. The role played by the microstructural evolution on the strain-rate dependence of the stress-strain properties is also demonstrated. [Preview Abstract] |
Wednesday, March 23, 2005 9:48AM - 10:00AM |
N39.00010: Catastrophic Fermi surface reconstruction in the shape-memory alloy AuZn Paul Goddard, John Singleton, Ross McDonald, Neil Harrison, Jason Lashley, Hisatomo Harima, Michi-To Suzuki AuZn undergoes a shape-memory transition at 67~K. An abrupt change in the quantum oscillations is observed at this temperature, indicating a disintegration of the Fermi surface associated with the transition. The de Haas-van Alphen effect is measured above and below the transition, at temperatures up to 100~K, and is in reasonable agreement with band-structure calculations in both the high and low-temperature phases. The measurements are strongly suggestive of an inherent, bulk phase separation at low temperatures. In addition, a Dingle analysis reveals a sharp change in the scattering mechanism at a certain cyclotron radius. Both these observations are indicative of the characteristic microstructure that drives the shape-memory effect. [Preview Abstract] |
Wednesday, March 23, 2005 10:00AM - 10:12AM |
N39.00011: Bilayer sliding mechanism for the zincblende to rocksalt transition in SiC H.T. Stokes, D.M. Hatch, J.J. Dong, J. Gunter, H. Wang, J.P. Lewis We have theoretically investigated the mechanism of the pressure-induced reconstructive zincblende-to-rocksalt phase transition in SiC. Starting with an extensive survey of 925 possible transition pathways, we found that those with the lowest enthalpy barriers all have a common mechanism: bilayer sliding of (111) planes. [Preview Abstract] |
Wednesday, March 23, 2005 10:12AM - 10:24AM |
N39.00012: The charge-density wave mechanism in the 2H transition metal dichalcogenides Ryan Barnett, Anatoli Polkovnikov, Eugene Demler During recent years there has been a renewed interest in the transition metal dichalcogenides (TMDs) due to angle resolved photoemission spectroscopy (ARPES) studies. For instance, a saddle band in 2H-TaSe2 was measured around 10 meV below the Fermi energy, extending over large regions of the Fermi surface [1]. Despite the recent progress, however, the mechanism of the charge-density wave (CDW) has remained elusive and controversial for these materials. In another experiment, an intriguing shift of the peak of the quasiparticle self-energy as a function of temperature was observed in 2H-TaSe2 [2]. In this talk, we will address these recent experiments. Using the available ARPES data, we obtain a model for the band structure of these materials. With this model, we argue that the CDW is driven by Fermi surface nesting, but due to the high level of degeneracies at the zone boundaries, the Fermi surface is not gapped in the conventional way even for the commensurate CDW phase. By considering coupling to an optical phonon, we provide an explanation for the shift in the quasi-particle self-energy. Finally, we ways to extract information of the Fermi surface from STM experiments. [1] R. Liu et al., Phys. Rev. Lett. 80, 5762 (1998). [2] T. Valla et al. Phys. Rev. Lett 85, 4759 (2000). [Preview Abstract] |
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N39.00013: Understanding Hydrogen Bonding and Low-Energy Magnetic Excitations in VOHPO$_4$$\cdot$$\frac{1}{2}$H$_2$O J. Cao, J.T. Haraldsen, J.L. Musfeldt, J.R. Thompson, T. Barnes, M.-H. Whangbo, S. Zvyagin, C.C. Torardi We report the variable temperature vibrational properties of the S=1/2, quasi-one-dimensional quantum Heisenberg antiferromagnet VOHPO$_4$$\cdot$$\frac{1}{2}$H$_2$O. Vibrational splitting points toward a weak local symmetry breaking near 180 K, and the low-temperature redshift of V-O and H-O related modes demonstrates enhanced low-temperature hydrogen bonding. Due to spin-orbit interaction, the singlet to triplet gap also appears in the infrared response. We compare this value to those obtained via magnetic susceptibility, electron-spin resonance, and neutron scattering, and we point out the existence of a spectral feature that supports weak interaction between traditional``isolated V-V dimers.'' Both magnon dispersion calculations and the experimental data suggest $\alpha$=J$^\prime$/J is $\sim$ 7\%. [Preview Abstract] |
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N39.00014: Inelastic light scattering investigation of the pyrochlore superconductor Cd2Re2O7 Christopher Knee, Joakim Holmlund, Jakob Andreasson, Mikael Kall, Lars Borjesson, Sten Eriksson The Structural phase transitions of the pyrochlore superconductor Cd$_{2}$Re$_{2}$O$_{7}$, T$_{c} \quad \approx $ 1.5 K, are investigated by Raman light scattering. The cubic to tetragonal transition at 200 K is characterized by the gradual appearance of a broad phonon mode originating from motion of the oxygen ions that form the apices of the ReO$_{6}$ octahedra. In contrast, the rapid growth of well-defined low frequency modes below the 120 K transition indicates that it is driven by ordering of the Cd ions within the channel voids of the distorted pyrochlore. A physical model describing the consecutive phase transitions in terms of the interplay between the Re-O and Cd-O networks will be presented. [Preview Abstract] |
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