Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S25: Strongly Correlated Oxides, Superlattices and Synthesis |
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Sponsoring Units: DMP DCMP Chair: Zenji Hiroi, University of Tokyo Room: LACC 501A |
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S25.00001: Orbital selective gap opening in Ca$_{3}$Ru$_{2}$O$_{7}$ observed by ARPES Felix Baumberger, Naoki Kikugawa, Nicholas Ingle, Worawat Meevasana, Kyle Shen, Donghui Lu, Suman Hussain, Andrea Damasceli, Andrew Mackenzie, Yoshiteru Maeno, Zhi-Xun Shen We present a high--resolution photoemission study of the bilayered ruthenate Ca$_{3}$Ru$_{2}$O$_{7}$, which is right on the verge of a Mott--Hubbard metal-insulator transition. The low carrier density, inferred from transport measurements is manifested in a very small quasiparticle weight, and a characteristic broadening of the valence band states. Coincident with the first order structural phase transition at 48 K, we observe the orbital selective opening of a weak, mean--field like gap. This sheds new light on the mysterious low--temperature metallic state of Ca$_{3}$Ru$_{2}$O$_{7}$, and on the interplay of charge, lattice, and orbital degrees of freedom. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S25.00002: Multi-$q$ crystal and magnetic structure in TbMnO$_{3}$: Evidence for a Soliton-lattice N. Aliouane, D.N. Argyriou, S. Landsgesell, C.J. Milne, J. Strempfer, W. Caliebe In TbMnO$_{3}$, Mn-spins order with a sinusoidal antiferromagnetic (AF) propagation wave vector $Q_{Mn}$=[0,k+/-q,l] ($q\sim0.288b^{\ast}$) at T$_{N}$(Mn)=41K. The propagation vector $Q_{Mn}$ varies with temperature on cooling until T$_{Lock}$(Mn) $\sim$30K, which coincides with a ferroelectric transition. In addition to $Q_{Mn}$ reflections we find magnetic reflections at 3$Q_{Mn}$. Our X-ray measurements show that the magneto-elastic coupling gives rise to a structural modulation at twice the magnetic wavevector (2$Q_{Mn}$). Field cooling the sample under a magnetic field oriented along the a-direction with H$>$9T shows that all magnetic wavevectors for Mn and Tb collapse to a single $q$ structure with $Q$=[0,1/4,0], an up-up, down-down phase, and coincide with anomalies in the polarization. We argue that the temperature and field dependence of the magnetic and superlattice reflections are consistent with a soliton formalism which predicts a stable commensurate single q=1/4 phase [1]. [1] Kimura {\sl et al.}, $PRB$ 68, 60403(2003). [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S25.00003: Incommensurate to commensurate transition under magnetic field in TbMnO$_{3}$ Dimitri N. Argyriou, N. Aliouane, S. Landsgesell, C.J. Milne, J. Strempfer, I. Zegkinoglou, M. von Zimmermann We have studied the evolution of the incommensurate satellite peaks of TbMnO$_{3}$ by neutron and x-ray single crystal diffraction with field H$\| b$ and H$\| c$. For H$\| b$ and a temperature of T=3K, Tb undergoes the first order phase transition characterized by a change of the magnetic wavevector from $q_{Tb}\sim$0.44 to 0.33 r.l.u. at a field of 1.75T. X-ray diffraction measurements show that the structural counterpart for the magnetic Tb reflection follows a similar dependence with 2$q_{Tb}$=0.82 r.l.u. changing to 0.66 r.l.u.. At H=6T and T=3K, the structural superlattice reflections for both the Mn and Tb wavevectors collapse to a single $q$-commensurate structure with propagation vector $Q$=[0,1/2,l]. This transition coincides with anomalies in the polarization P$\| c$. For fields of up to 4T applied along the c-direction no new magnetic phases arise, except for the shift of the antiferromagnetism and locking transitions. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S25.00004: Optical-Pump Mid-Infrared Probe Study of Quasiparticle Dynamics in (La,Pr,Ca)MnO3 Hae Ja Lee, R.P. Prasankumar, R.D. Averitt, D.J. Funk, A.J. Taylor, S.-W. Cheong We have investigated the temperature dependence of the quasiparticle dynamics in single crystal (La,Pr,Ca)MnO3 (LPCMO) using ultrafast optical spectroscopy. As a function of temperature, distinct changes in the dynamics occur upon crossing through the charge ordering and ferromagnetic transition temperatures. We will discuss the origin of the quasiparticle dynamics in light of the phase separation into submicrometer-sized mixtures of ferromagnetic metallic and charge-ordered insulating domains that occur in LPCMO. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S25.00005: Properties of Nonstoichiometric SrRu$_{1-v}$O$_{3}$\\Perovskites B. Dabrowski, S. Kolesnik, O. Chmaissem, J. Mais, M. Avdeev, J.D. Jorgensen Annealing of stoichiometric SrRuO$_{3}$ perovskites in high-pressure oxygen of 600 atm. near 1100$^{\circ}$C produces SrRu$_{1-v}$O$_{3}$ compounds with vacancies on the Ru-sites. The creation of Ru vacancies rapidly suppresses the ferromagnetic ordering temperature, $T_{C}$, from 163 K to 45 K with increase of v $\approx $ 0.09. The resistivity shows a metallic behavior near room temperature with progressively more insulating behavior at low temperatures for increasing v. All samples display clear metallic-like decrease of the resistivity and negative magneto-resistance right below $T_{C}$. Structural changes that accompany creation of Ru-site vacancies indicate reduced charge screening caused by the Ru-vacancies that offsets expected decrease of the average interatomic distance Ru--O. The $b$ and $c$ lattice parameters and the unit cell volume are virtually temperature independent for the stoichiometric material below $T_{c}$. We show that this previously reported invar-effect originates from freezing of the octahedral tilting about the [001] axis that can be observed for both stoichiometric and Ru-deficient samples. Spontaneous magnetostriction has the largest effect on the $b$ axis of the orthorhombic \textit{Pbnm} crystal structure. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S25.00006: Bulk modulus anomaly in RCoO3 (R=La, Pr, and Nd) J.-S. Zhou, J.-Q. Yan, J.B. Goodenough The RCoO$_{3}$ family undergoes a transition from the low-spin state to a higher spin state with increasing temperature. The onset temperature for this thermally driven, progressive transition is 35 K for LaCoO$_{3}$; the onset temperature increases to 200K for PrCoO$_{3}$ and 300K for NdCoO$_{3}$. At room temperature the population of the low-spin state increases as the ionic size of rare earth reduces from La to Nd. High pressure stabilizes the low-spin state by enlarging the crystal field splitting. The structural study with more pressure sampling points in this work has given a more accurate V-P relationship for the RCoO$_{3}$ family. A linear fitting to the V-P relationship instead of the Birch-Murnaghan (B-M) equation with a parameter B'=4 as default has been obtained for LaCoO$_{3}$. The bulk modulus B is even lower than that reported previously. In contrast, NdCoO$_{3}$ shows a regular B by fitting the V-P curve with the B-M equation. The V-P curve of PrCoO$_{3} $ is irregular and has been explained in terms of both a pressure- induced spin-state transition and a structural transition from an orthorhombic to a rhombohedral phase. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S25.00007: Ferroelectricity in one unit-cell period oxide superlattices T.W. Noh, S.S.A. Seo, J.H. Lee, J. Yu, H.N. Lee We present electric properties of one unit-cell period superlattices composed of CaTiO$_{3 }$(CTO), SrTiO$_{3}$ (STO), and BaTiO$_{3}$ (BTO) perovskites, in which the structural symmetry and lattice misfit strain can be systematically varied without changing the chemical valence states. The one unit-cell period CTO/BTO, BTO/STO, and CTO/STO superlattices were grown by high oxygen pressure pulsed laser deposition on atomically flat SrRuO$_{3}$ conducting oxide grown on STO (001) substrates. CTO/BTO and BTO/STO showed ferroelectricity in room temperature, while CTO/STO showed paraelectric behavior. Such spontaneous electric polarization was an unexpected result, because all TiO$_{6}$ octahedron was not in the same structural condition with ferroelectric BTO, but was sandwiched by CaO (SrO) and BaO layers in these superlattices. By performing first principle calculations, ferroelectric ground states can be found in the distorted TiO$_{6}$. Moreover, the ferroelectricity was described as the collective displacement of the titanium-oxygen-titanium ions, which is different from that of bulk ferroelectric material. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S25.00008: Tailoring of ferromagnetic/ferroelectric superlattices for multiferroic properties W. Prellier, P. Murugavel, D. Saurel, Ch. Simon, B. Raveau Superlattices composed of ferromagnetic Pr$_{0.85}$Ca$_{0.15}$MnO$_{3}$ insulating layers and ferroelectric Ba$_{0.6}$Sr$_{0.4}$TiO$_{3}$ layers were fabricated on (100)-SrTiO$_{3}$ substrates by pulsed-laser deposition. The magnetotransport properties were measured with the current perpendicular to the plane geometry. An increase in magnetoresistance (MR), with no significant low field effect, was observed as the number of ferroelectric layer increases even up to 9 unit cells. This observed large MR cannot be explained by simple interfacial ferromagnetism or by the tunneling magnetoresistance. The capacitance and resistive parts of the samples were also analyzed from the complex impedance measurements, performed on the samples using a special experimental set-up. The superlattice with larger ferroelectric thickness shows unique characteristics which are not present in the parent ferromagnetic thin film and both ferromagnetic and ferroelectric transitions which is an evidence for the coexistence of both the properties. The high magnetoresistance (40{\%} at 80K) shown by the superlattice can be attributed to the coupling between ferromagnetic and ferroelectric layers, i.e, to the magnetoelectric effect. Various examples with different materials will also be presented to confirm these results. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S25.00009: Multi-scale Modeling of Relaxor Ferroelectrics Aravind Asthagiri, Narayani Choudhury, Wu Zhigang, Ronald Cohen The origin of the high piezoelectric response observed in complex solid solution perovskites like PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$-PbTiO$_ {3}$ (PMN-PT) is still not well understood. We have taken a multi-scale approach to examine PMN and PMN-PT by developing a shell potential model by fitting to extensive first-principles data that can then be used in molecular dynamics (MD) simulations. For PMN we have performed both total energy calculations with LAPW and linear response calculations using ABINIT to obtain the phonon spectra of ordered 1:2 supercells along the [111] and [001] directions. For both ordered structures, we find small energy differences between the polar ferroelectric and non-polar antiferroelectric structures and large LO-TO splittings. For the [111] structure, we find the ground state to be triclinic. The results above and similar results for PbTiO$_{3} $ have been used to fit a shell model potential for PMN and PT. We will report the temperature and applied field behavior of disordered PMN and the phase diagram of PMN-PT obtained from MD simulations and compare to existing experimental data. Preliminary results for PMN indicate that we obtain qualitatively similar Raman spectra and bulk modulus to experiment. Work supported by the Office of Naval Research (contract number N000149710052) and the Carnegie Institution of Washington. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S25.00010: First-principles study of symmetry lowering in relaxed BaTiO$_3$/SrTiO$_3$ superlattices Karen Johnston, Xiangyang Huang, Jeffrey B. Neaton, Karin M. Rabe The crystal structure and local spontaneous polarization of (BaTiO$_3$)$_m$/(SrTiO$_3$)$_n$ superlattices is calculated using a first-principles density functional theory method. The in-plane lattice constant is 1\% larger than the SrTiO$_3$ substrate to imitate the relaxed superlattice structure and the symmetry is lowered to monoclinic space group $Cm$ which allows polarization to develop along the [110] and [001] directions. The polarization component in the [110] direction is found to develop only in the SrTiO$_3$ layers and falls to zero in the BaTiO$_3$ layers, whereas the polarization in the [001] direction is approximately uniform throughout the superlattice. These findings are consistent with recent experimental data and first-principles results for epitaxially strained BT and ST. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S25.00011: Rare-earth doped chalcogenide thin films for optoelectronic applications by laser ablation Prabhat Dwivedi, T. Allen, T.J. Clement, Y.Y. Tusi, C.J. Haugen, R.G. Decorby, J.N. McMullin, S.O. Kasap Chalcogenide glasses have been extensively studied as host media for rare-earth (RE) doped photonic devices due to potential application as optical amplifiers for optical telecommunication. However, fabrication of RE doped homogeneous thin films of chalcogenide glass systems is a difficult task. Doping high amounts of RE atoms (1 to 2 at{\%}) using conventional preparation methods such as glass quenching or physical vapor deposition techniques often results in physical or chemical clustering of the RE atoms in the glass matrix also. In this paper, we report the deposition and properties of RE doped chalcogenide films fabricated by pulsed laser deposition (PLD), using 15 ns KrF laser pulses at various laser energy densities and substrate temperatures. We examined the effects of changing the substrate temperature during deposition on the optical constants and photoluminescence. The thermal stability was examined using a temperature modulated differential scaning calorimetry (TMDSC) measurements. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S25.00012: Fingering Instability in a Growing Elastic Gel Ariel Balter, Allan Bower, Jay Tang It has been experimentally observed that when an actin gel is made to grow around a small ($\sim$1 $\mu$m) bead, the gel may finger, i.e. grow a small number of stable protrusions. The case of one finger represents the case of ``comet tail'' motility. We present at theory for this process as an instability driven by the interplay of surface energy and elastic energy at the outer edge of the growing gel. In our theory, the number of fingers is selected by the bead size, the growth rate, the elastic modulus and the surface energy, so direct comparisons to experiment can be made. We have also simulated the process with a finite element code. Although we present this as an explanation for the morphology of actin gels around beads, in particular the formation of actin ``comet tails,'' we suggest this as a generic process which may emerge whenever an elastic material is growing away from a curved surface and the elastic material has a stress-dependant chemical stability. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S25.00013: Methylamine Intercalation Rates in Lead Iodide M. Gallegos, B. Magness, W. Tikkanen, H. Goldwhite, T. Berhe, C. C. Coleman The intercalation and deintercalation rates and some optical spectral changes of methylamine guests intercalated into layer structured lead iodide hosts are reported. Vapor diffused purified lead iodide was used to make powder, thick film and thin film samples. Host films of 200 to 500nm were evaporated on quartz crystals. Degassed host samples were exposed to guest gas pressures ranging from 5 to 200 Torr in an isolated glass system. Mass changes were determined by quartz spring and crystal frequency change methods. The final intercalation of methylamine into lead iodide at low guest pressures reaches saturation in 3 hours. This results in an uptake of 1.3:1 mole ratio of guest to host. However, a plateau in uptake occurs at a 0.64:1 guest to host mole ratio within ten seconds of exposure depending upon the pressure of the guest. Deintercalation obtained by pumping takes 15 days suggesting the intercalated state is fairly stable. There are indications of a plateau in the deintercalation data at the 0.64:1 mole ratio as well further suggesting the possibility of a second stage in this system. Optical data were obtained using thin film host samples, which produced a 0.46 eV increase of the optical band edge energy upon intercalation. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S25.00014: Electronic structure of nano-sized iron oxide particles measured by scanning tunneling- and photoelectron spectroscopy Marcus Preisinger, Michael Krispin, Torsten Rudolf, Siegfried Horn We have investigated the electronic structure of nano-sized iron oxide by scanning tunnelling microscopy (STM) and spectroscopy (STS) as well as by photoelectron spectroscopy (PES). Nano particles were produced by thermal treatment of Ferritin molecules containing a self-assembled core of iron oxide. Depending on the thermal treatment we were able to prepare different phases of iron oxide nanoparticles resembling $\gamma$-Fe$_{2}$O$_{3}$, $\alpha$-Fe$_{2}$O$_{3}$, and a phase which apparently contains both $\gamma$-Fe$_{2}$O$_{3}$ and $\alpha$-Fe$_{2}$O$_{3}$. Changes to the electronic structure of these materials were studied under reducing conditions. We show that the surface band gap of the electronic excitation spectrum can differ from that of bulk material and is dominated by surface effects. [Preview Abstract] |
Wednesday, March 23, 2005 5:18PM - 5:30PM |
S25.00015: Structural and Electronic Properties of (HAlO)$_n$ Clusters Yi Dong, Michael Springborg HAlO is a nanostructured material that can be used as a substrate for organized structures of organic molecules. However, except for the fact that HAlO is stoichiometric, very little is known about its structural and electronic properties. Using two different unbiased methods for structure optimization (our own {\it Aufbau} method as well as Genetic Algorithms) together with a parameterized density-functional method in calculating the total energy and the electronic properties for a given structure, we have optimized the structure of (HAlO)$_n$ clusters with $n$ up to 26. We shall briefly outline our theoretical approach and subsequently present the results for isolated (HAlO)$_n$ clusters together with those for interacting (HAlO)$_n$ clusters and for layers of HAlO. [Preview Abstract] |
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