Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session Y20: Focus Session: Ruddlesden-popper Phase Manganites |
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Sponsoring Units: GMAG DMP Chair: Jeff Lynn, National Institute of Standards and Technology Room: Baltimore Convention Center 317 |
Friday, March 17, 2006 8:00AM - 8:12AM |
Y20.00001: Nodal quasiparticle in pseudogapped colossal magnetoresistive manganites N. Mannella, W. L. Yang, X. J. Zhou, K. Tanaka, H. Zheng, J. F. Mitchell, J. Zaanen, T. P. Devereaux, N. Nagaosa, Z. Hussain, Z. X. Shen In this talk, the result of a recent angle-resolved photoemission spectroscopy (ARPES) investigation which allowed elucidating the controversial nature of the ferromagnetic metallic groundstate in the prototypical colossal magnetoresistive manganite bilayer compound La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ will be discussed [1]. The distribution of spectral weight in momentum space exhibits a nodal--antinodal dichotomous character. Quasiparticle excitations have been detected for the first time along the nodal direction (i.e. diagonal), and they are found to determine the metallic transport properties of this compound. The weight of the quasiparticle peak diminishes rapidly while crossing over to the antinodal (i.e. parallel to the Mn--O bonds) parallel sections of the Fermi surface, with the spectra strongly resembling those found in heavily underdoped cuprates high temperature superconductors (HTSC) such as Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_{2}$ [2]. This dichotomy between the electronic excitations along the nodal and antinodal directions in momentum space was so far considered a characteristic unique feature of the copper oxide HTSC. These findings therefore cast doubt on the assumption that the pseudogap state in the cuprate HTSC and the nodal-antinodal dichotomy are hallmarks of the superconductivity state. [1] N. Mannella et al., Nature \underline {438}, 474 (2005) [2] K. M Shen et al., Science \underline {307}, 901 (2005). [Preview Abstract] |
Friday, March 17, 2006 8:12AM - 8:24AM |
Y20.00002: Near-Fermi electronic structure of La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ revealed by ARPES Z. Sun, J. F. Douglas, D. S. Dessau, Y. -D. Chuang, A. V. Fedorov, H. Lin, M. Lindroos, S. Sahrakorpi, R. S. Markiewicz, A. Bansil, H. Zheng, J. F. Mitchell, T. Kimura, Y. Tokura Angle-resolved photoemission experiments were performed on the bi-layer manganite La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ as a function of doping and temperature. High resolution and sensitivity allows us to observe the fine details including bi-layer splitting, dispersion kinks, electron-phonon coupling, and pseudogaps. We discuss new insights into the underlying physics of these compounds, as well as a comparison of some of these properties to what is observed in the cuprates. [Preview Abstract] |
Friday, March 17, 2006 8:24AM - 8:36AM |
Y20.00003: First-order metal-insulator transitions in manganites: Are they universal? Kenneth Gray, Qing'An Li, Stine Klausen, Hong Zheng, Stephan Rosenkranz, Ray Osborn, John Mitchell Conductivity data for La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ (x=0.6) show a first-order transition from an orbital/charge-ordered insulator to a metal as the temperature falls below $\sim $160 K. The change in conductivity of $\sim $10000 is 100 times larger than that seen previously in any single-phase manganite in zero field. The metallic low-temperature state is similar to that found at x=0.58, but x=0.58 shows no evidence of orbital/charge order. This result, and further analysis, supports a conclusion that strongly coupled magnetic/conductive transitions are universally of first order. [Preview Abstract] |
Friday, March 17, 2006 8:36AM - 9:12AM |
Y20.00004: Topotactic Reduction of Complex Manganese Oxides Invited Speaker: Mixed-valent manganese (III/IV) perovskite and Ruddlesden-Popper oxides have received considerable attention due to the observation of strong coupling between spin, charge and lattice degrees of freedom in these materials. Little attention however has been focused on the analogous Mn (II/III) oxides due to the difficulty of their synthesis. The topotactic reduction of the Mn(III/IV) perovskites La$_{1-x}$Sr$_{x}$MnO$_{3}$ (0.2 $<$ x $<$ 0.4) results in the formation of the corresponding La$_{1-x}$Sr$_{x}$MnO$_{2.5}$ brownmillerite phases. The x = 0.25 and 0.2 phases adopt structures with an unusual ordered L-R-L-R alternation of twisted chains of Mn(II) tetrahedra which is accompanied by Mn(II)/(III) charge ordering within the remaining MnO$_{6}$ octahedral layers. In contrast the x = 0.4 phase adopts a structure in which the twisted chains of tetrahedra are disordered. Temperature dependant magnetization data collected from similarly reduced n = 2 Ruddlesden-Popper phases of composition YSr$_{2}$Mn$_{2}$O$_{7-x}$ (0 $<$ x $<$ 1.5) indicate strong antiferromagnetic coupling interactions in all samples. Long range magnetic order is suppressed by structural and/or charge disorder in all samples except YSr$_{2}$Mn$_{2}$O$_{5.5}$ which adopts a G-type antiferromagnetic ordering scheme. [Preview Abstract] |
Friday, March 17, 2006 9:12AM - 9:24AM |
Y20.00005: Tunneling evidence for weak localization in layered manganites Daniel Mazur, Ken E. Gray, John F. Zasadzinski, Hong Zheng, John Mitchell Our point contact tunneling experiments on La$_{1.28}$Sr$_{1.72}$Mn$_{2}$O$_{7}$ (bi-layered LSMO x=0.36) at low temperatures reveals a $\surd $V low bias anomaly in the tunneling conductance. This anomally qualitatively matches the $\surd $E predictions of the weak localization effect on the electronic DOS. The data could provide an important corroboration of the weak localization hypothesis, which was concluded previously from low-temperature conductivity and magnetoresistance data. [Preview Abstract] |
Friday, March 17, 2006 9:24AM - 9:36AM |
Y20.00006: Doping Dependence of Polaron Correlations in Bilayer Manganites Raymond Osborn, Stine Klausen, Stephan Rosenkranz, Peter Chupas, John Mitchell It is now well established that colossal magnetoresistance (CMR) in manganite compounds is strongly enhanced by competing interactions close to the magnetic phase transitions. In the paramagnetic phase of the bilayer manganites, diffuse x-ray scattering data has shown evidence of polaron correlations that grow with decreasing temperature and coexist with the critical magnetic fluctuations down to the ferromagnetic phase transition, before collapsing below T$_C$. These correlations, which have a length scale of about 20\AA, are in the form of longitudinal Jahn-Teller stripes within the bilayers, characterized by an incommensurate wavevector. There have been proposals that these nanoscale correlations are evidence of an incipient charge density wave, whose wavevector corresponds to nesting features in the Fermi surface. We report new measurements of the doping dependence of these correlations that show that the wavevector increases from 0.26 at a hole doping of $x=0.32$ to about 0.3 at $x=0.46$, which is inconsistent with the anticipated change in the Fermi surface volume, and will discuss alternate models of the short-range order. [Preview Abstract] |
Friday, March 17, 2006 9:36AM - 9:48AM |
Y20.00007: Magneto-optical investigation of the field-induced transition in bilayer manganese oxide (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ J. Cao, J.T. Haraldsen, R.C. Rai, S. Brown, J.L. Musfeldt, X. Wei, Y.J. Wang, M. Apostu, R. Suryanarayanan, A. Revcolevschi We measured the magneto-optical response of PrLSMO in order to investigate the microscopic aspects of the magnetic field driven paramagnetic insulator to ferromagnetic metal transition. With applied magnetic field, optical weight transfers to lower energy and develops a clear signature of ferromagnetic domains. Mn-O stretching and bending modes soften through the phase transition as well, demonstrating precisely how the lattice is coupled to the transition. We also extract the H-T optical phase diagram and compare it with that from resistivity, magnetization, and magnetostriction to show that the lattice responds more slowly than charge and spin at low temperature. Color rendering allows visualization of the thermochromic and magnetochromic effect. [Preview Abstract] |
Friday, March 17, 2006 9:48AM - 10:00AM |
Y20.00008: Is CE-type orbital order the true ground state for LaSr$_{2}$Mn$_{2}$O$_{7}$? Helmut Claus, Qing'an Li, Hong Zheng, Kenneth Gray, Stine Kausen, Stephan Rosenkranz, Ray Osborn, John Mitchell Previous scattering and conductivity data for La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ (x=0.5) indicate the CE phase, predicted by Goodenough 50 years ago, is only stable between $\sim $100 and 200 K and below $\sim $100 K an A-type antiferromagnet is the ground state. We present scattering, magnetization and transport evidence that the CE phase is the ground state, but only in a very narrow composition range, presumably at exactly x=0.5. The vitally important roles of crystal homogeneity and methodology of transport measurements will be demonstrated for this conclusion as well as for the field of layered manganites in general. [Preview Abstract] |
Friday, March 17, 2006 10:00AM - 10:12AM |
Y20.00009: Polarons and polaron correlations in 40\% doped bilayer manganite. S. N. Klausen, S. Rosenkranz, R. Osborn, P. J. Chupas, J. F. Mitchell Among the compounds exhibiting colossal magnetoresistance, the bilayer manganites, La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$, where $x$ is the hole doping in the MnO$_2$ layers, are advantageous to study because the reduced dimensionality strongly enhances the spin and charge fluctuations. For the intermediate doping regime ($32\%\le x \le42\%$), the magnetoresistance is strongly enhanced near the combined metal-insulator and ferromagnetic transition. Here, we report on our detailed x-ray scattering investigation of the diffuse scattering associated with lattice distortions around localized charges (polarons) as well as short-ranged polaron correlations present in the paramagnetic phase of the $x=40\%$ doped compound, both collapsing at $T_{\mathrm{C}}=112$~K. We have studied in detail the temperature dependence to high temperatures ($T = 50$~K - 500~K) and are able to directly extract a polaron activation energy from the data. [Preview Abstract] |
Friday, March 17, 2006 10:12AM - 10:24AM |
Y20.00010: A Polaronic Picture of Bilayer Colossal Magnetoresistive Manganites in Angle Resolved Photoemission Spectroscopy C. Jozwiak, G.-H. Gweon, J. Graff, S.Y. Zhou, H. Zheng, J. F. Mitchell, A. Lanzara It is becoming generally recognized that the presence of a strong electron-lattice interaction, in the limit of polaronic coupling, is a common feature to transition metal oxides such as cuprate superconductors and colossal magnetoresistive manganites. Angle resolved photoemission spectroscopy (ARPES) is the ideal tool to address and gain insight on the polaronic nature of the electron-lattice interaction through a direct study of the ARPES single particle spectral function. Here we present a detailed momentum dependent study of the ARPES single particle spectral function in double layer manganites La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7, }$and Bi2212 and LSCO cuprates. A direct comparison between the two classes of materials and the peculiar momentum dependence of the ARPES lineshapes, also known as nodal-antinodal dichotomy, will be discussed in terms of a polaronic picture. [Preview Abstract] |
Friday, March 17, 2006 10:24AM - 10:36AM |
Y20.00011: Order/Disorder Effects in Perovskite Manganites B. Dabrowski, S. Kolesnik, O. Chmaissem, E.N. Caspi, J. Mais, J.D. Jorgensen Indirect magnetic interactions between transition metals in perovskites ABO$_{3}$ are commonly described in terms of superexchange and double-exchange interactions that depend on the bond angle B-O-B. By useing the tolerance factor parameterization procedure we have designed and obtained new substituted manganites with randomly-mixed and layer-ordered structures having identical chemical compositions. The order/disorder effects on magnetic properties have been investigated as a function of the bond angle disorder for both the A- and Mn- sites. By comparing ordered (containing planes of the RO$_{2}$ and BaO$_{2})$ and disordered ferromagnetic compounds La$_{1-x}$Ba$_{1+x}$Mn$_{2}$O$_{6}$, we have demonstrated that by decreasing local structural and charge disorder, an increase of T$_{c}$ of over 100 K can be achieved. Similar effects have been demonstrated for Sr$_{2}$MnGaO$_{6-d}$ where transitions from spin-glass seen for the disordered compound to antiferromagnetic behavior seen for the ordered material (containing planes of the MnO$_{2}$ and GaO$_{2-d})$, have been observed. Supported by NSF (DMR-0302617) and U.S. Department of Energy, BES -- Materials Sciences (W-31-109-ENG-38) [Preview Abstract] |
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