Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session X13: Focus Session: Layered Manganites |
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Sponsoring Units: DMP GMAG Chair: John Mitchell, Argonne National Laboratory Room: Colorado Convention Center Korbel 4C |
Friday, March 9, 2007 8:00AM - 8:12AM |
X13.00001: Nonthermal Photoresponse in Epitaxial Thin Films of (La,Pr).67Ca.33MnO4: Correlation with Non-ohmic Electrical Transport and Magnetoresistance Anthony Davidson III, Rajeswari Kolagani, Grace Yong, Vera Smolyaninova, Mason Overby We have recently observed a non-thermal component of light induced resistance change in the vicinity of the insulator-metal transition, in epitaxial thin films of the CMR manganite material (La,Pr).67Ca.33MnO4 (LPCMO). LPCMO is known to have the co-existence of insulating and metallic regions. On cooling, the metallic regions grow at the expense of the insulating regions, giving rise to a percolative insulator-metal transition. Our results indicate that light may cause electronic changes in the insulating regions thus decreasing the electrical resistance. We will present our studies of the photoresponse and the correlation of the observed non-thermal photoresponse with magnetoresistance as well as current-voltage (I-V) characteristics. I-V measurements show that there is a current induced change in resistance which is not due to the joule heating effects. This effect is only seen in the metal-insulator transition range of the samples, similar to the nonthermal photoresponse, suggesting a common origin for these two phenomena. The effects of magnetic field however are seen to be distinct. A large magnetoresistance is seen at lower temperatures where the light and current induced effects are absent, thus suggesting a very different physical origin for the magnetoresistance. [Preview Abstract] |
Friday, March 9, 2007 8:12AM - 8:24AM |
X13.00002: Evolution of the CO-OO and AF ordering in the single-layer manganite $\rm Pr_{1-x}Ca_{1+x}MnO_4$ near half doping Feng Ye, J. A. Fernandez-Baca, Songxue Chi, Pengcheng Dai, J. W. Lynn, R. Mathieu, Y. Kaneko, Y. Tokura Manganese oxides have attracted considerable attention due to the CMR effect observed in the perovskite manganite $\rm A_{1-x}A'_{x}MnO_3$ near $x=0.3$. A peculiar charge/orbital (CO-OO) accompanied by antiferromagnetic (AF) order occurs when the carrier concentration is close to half doping (x=0.5). To understand the interplay between the charge, lattice and spin degrees of freedom in such insulating state, we used elastic neutron scattering to study the evolution of the CO-OO as well as the AF correlations in the single-layer manganite $\rm Pr_{1-x}Ca_{1+x}MnO_4$ ($x=0.40$, 0.45 and 0.50). Upon cooling, all three samples exhibit long-range CO-OO near 300~K. However, only the $x=0.50$ system displays long-range AF order at low temperatures. The CE-type AF correlations are quickly suppressed and become short-ranged as more $e_g$ electrons are introduced to the $\rm MnO_2$ plane. More interestingly, the CO-OO and AF order associated with Mn$^{3+}$ ions appears at incommensurate positions while the AF order associated with Mn$^{4+}$ ions remains commensurate. Our observations indicate that the orbital physics play an important role in the understanding of the electronic and magnetic properties of doped manganites. [Preview Abstract] |
Friday, March 9, 2007 8:24AM - 8:36AM |
X13.00003: Dimensional Crossover of Antiferromagnetism in Half-Doped La$_{0.5}$Sr$_{1.5}$MnO$_{4}$ Kuo-Sheng Chao, J. Okamoto, D. J. Huang, C. Y. Mou, H. -J. Lin, C. -H. Hsu, Y. Kaneko, R. Mathieu, Y. Tokura, C. T. Chen Like cuprates which exhibits high-temperature superconductivity, half-doped single-layered manganites such as La$_{0.5}$Sr$_{1.5}$MnO$_{4}$ have distinct features of the MnO$_{2}$ plane in the perovskite structure. In addition to charge-orbital order, of particular interest is the antiferromagnetism in half-doped manganites with the so called CE-type antiferromagnetic (AF) structure, which is essentially composed of ferromagnetic zigzag chains antiferromagnetically coupled to one-another. In this talk, we will report critical behavior and dimensional crossover of AF order in La$_{0.5}$Sr$_ {1.5}$MnO$_{4}$, based on measurements of resonant soft-x-ray magnetic scattering. A 2D incommensurate AF order exists at temperatures above the Neel temperature $T_N$. As the temperature cools across $T_N$, the interlayer exchange coupling prevails and the 2D incommensurability collapses to stabilize the 3D AF order. The measurements unravel spin correlations in the classical renormalized region for a non- standard (CE-type) antiferromagnet. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X13.00004: Role of oxygen in the orbital ordered state of La$_{0.5}$Sr$_{1.5}$MnO$_4$ John W. Freeland, Michel van Veenendaal, Ken Gray, Qing'an Li, Hong Zheng, John F. Mitchell In the single layer manganite, La$_{0.5}$Sr$_{1.5}$MnO$_4$ , experimental evidence points clearly to formation of orbital ordering but leaves the question open as to the exact nature of the state. Using oxygen K edge spectroscopy, we find that oxygen holes related to the Mn-O hybridization between O($2p$) and Mn($e_g$) states play an important role in the formation of the ordered groundstate. The large change in the number of $e_g$ related oxygen holes with the formation of the charge/orbital ordered states demonstrates it is not due to a locking in of orbitals which are fluctuating in orientation, but that the disordered state possesses a different orbital occupancy. The change in the number of $e_g$ holes occurs mainly within the ab plane and seems to be related to the crossover from ferromagnetic to anti-ferromagnetic correlations with the onset of the charge/orbital ordered state. This idea is supported by Mn$_4$O$_8$ cluster calculations. Work at Argonne is supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X13.00005: Orbital Ordering in Magnanites Probed with Soft X-Ray Scattering Stuart Wilkins, Natasa Stojic, Thomas Beale, Nadia Bingeli, Peter Hatton, D. Prabhakaran, Andrew Boothroyd, Massimo Altarelli Orbital ordering is important in the understanding of transition metal oxides as the magnetic and transport properties are strongly related to the orbital and charge degrees of freedom. In the case of La$_{0.5}$Sr$_{1.5}$MnO$_{4}$ we will present results using diffraction at the L-edges of Mn, for the orbital order superlattice relfection, and show that while there indeed does exist long range order of the orbitals the dominant process is cooperative Jahn-Teller distortions. By comparing these measurements with theoretical calculations we find a good general agreement showing a dominant orbital ordering of the $d_{x^2 - z^2} / d_{y^2 - z^2}$ type, in contrast to the $d_{3x^{2}-r^{2}}$ / $d_{3y^{2} - r^{2}}$ as previously proposed. In LaSr$_{2}$Mn$_{2}$O$_7$, which is found crystallographically to have virtually no Jahn-Teller distortion of the oxygen octahedra. We will show that even in the absence of Jahn-Teller distortions, this compound is strongly orbitally ordered. Fitting of the Mn $L$-edge resonance spectra demonstrates the presence of orbital ordering of the Mn$^{3+}$ ions within an almost cubic crystal field. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X13.00006: Rethinking the Orbital Ordering Transition: Using Coherent Soft X-ray Scattering to Study Dynamics in the CMR Manganites Joshua Turner, Jessica Thomas, John Hill, Mark Pfeifer, Karine Chesnel, Y. Tokura, Y. Tomioka, Steve Kevan The colossal magnetoresistance (CMR) phenomenon has baffled physicists since its discovery over a decade ago. Central to the puzzle is the short-range orbital ordering that arises in certain hole concentrations of the manganese oxides, even for low temperatures. We have used a Coherent Soft X-ray Scattering (CSXS) technique to resonantly enhance the orbital ordering contrast and measure speckle patterns from the domain structure of the self-assembling Mn d-orbitals. Our dynamics measurements suggest that the orbital domains remain static through the orbital ordering transition temperature -- challenging the previous belief of a mediation through slow, glass-like characteristics. Our experiments force us to rethink the role and nature of the orbitally-ordered state in the manganites, intrinsic to CMR. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X13.00007: Resonant soft x-ray scattering study on antiferromagnetic ordering of LaSr$_2$Mn$_2$O$_7$ J.-S. Lee, J. Koo, H. Jang, K.-T. Ko, H. J. Lee, Y. H. Jeong, K.-B. Lee, J.-H. Park, J.-Y. Kim, Y. Bang, T. Kimura, Y. Tokura Resonant soft x-ray scattering experiments at the Mn $L_{2,3}$- edge and O $K$-edge have been performed to probe the magnetic structure of LaSr$_2$Mn$_2$O$_7$ which is well known as the $A$-type antiferromagnetic (AFM) phase. At the low temperature, strongly resonant intensity of (001) AFM reflection was found. The temperature dependences of AFM resonance at both the Mn $L_{2,3} $- and O $K$-edge were relatively different, in which the order parameter at Mn $L_{2,3}$-edge showed an anomalous transition above N$\acute{\rm e}$el temperature (170 K). This feature could be regarded as the mixed valence state (Mn$^{3+}$ and Mn$^{4+}$) phenomenon, and besides, it could be supported by theoretical calculation and bulk measurement on magnetism. Detailed description will be discussed in presentation. [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X13.00008: Reentrant orbital order and the true ground state of LaSr$_{2}$Mn$_{2}$O$_{7}$. S. Nyborg Ancona, S. Rosenkranz, R. Osborn, K. E. Gray, H. Zheng, Qing'An Li, J. F. Mitchell, Y. Chen, J. Lynn Strongly correlated electron systems, and colossal magnetoresistive materials in particular, exhibit a strong competition among orbital, charge and spin order. The phase diagram of the bilayer manganites, La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$, display interesting features near half doping, $x\approx0.5$, and it has been commonly accepted that CE order at $x=0.5$ is reentrant. Here, we present x-ray and neutron diffraction data of our purified La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ crystals contrasting the conventional wisdom. Our crystals exhibit CE-type orbital and charge order as the low-temperature ground state for $x=0.5$. For small deviations from $x=0.5$, the high temperature CE phase is replaced at low temperatures by an A-type antiferromagnetic phase {\sl without coexistence}. Larger deviations from $x=0.5$ result in a lack of CE-order at any temperature. Thus small compositional variations could explain why others commonly see this reentrance with coexistence. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X13.00009: Exotic Bilayer Manganite Phase Diagram near x=0.6 Hole Doping. Kenneth Gray, Hong Zheng, Qing'An Li, John F. Mitchell, Stine Nyborg Ancona, Stephan Rosenkranz, Ray Osborn Important modifications to the phase diagram of the bilayered manganites have been discovered through combined measurements of conductivity, magnetization and neutron and x-ray diffraction. The previously reported phase diagram for the CE-type orbital and charge order in LaSr$_{2}$Mn$_{2}$O$_{7}$ crystals (near hole doping of x=0.5) is qualitatively reproduced and graphically demonstrated for the bi-stripe (BIS) orbital and charge order in La$_{0.8}$Sr$_{2.2}$Mn$_{2}$O$_{7}$ crystals near x=0.6. Individual crystals, taken along the compositional gradient of a single boule made by the floating-zone technique with nominal composition x=0.61, display a large fraction of the phase diagram. As in the case of x=0.5, we find (1) crystals that enter the BIS phase at 240-300 K and reenter into an in-plane metal, A-type antiferromagnetic phase, and (2) crystals that retain BIS order down to at least 5 K. Surprisingly, the BIS state seems somewhat more stable than the CE state. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X13.00010: Polarized transmission EXAFS study of single crystal La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7 }$ as a function of temperature Jesse Guzman, G. Kurczveil, L. Downward, F. Bridges, J. Mitchell, H. Zheng The temperature dependence (3-300K) of the bilayer Colossal Magneto Resistive (CMR) manganites La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$, x=0.34 and 0.36, was studied using polarized, transmission EXAFS through a thin single crystal, with the x-rays polarization parallel to the c-axis or in the ab-plane. To eliminate Bragg diffraction effects from the single crystal, a 2D sample oscillator setup was used; the results of using this oscillator will be discussed. The data analysis shows a sharp increase in the width $\sigma $ of the pair distribution function (PDF) for the Mn-O peak near the ferromagnetic transition temperature T$_{c}$. This sharp increase in $\sigma ^{2}$ corresponds to a Jahn-Teller-like distortion as the temperature goes through T$_{c}$. Furthermore, a plot of the reduction in $\sigma ^{2}$, $\Delta (\sigma ^{2})$, vs. sample magnetization M shows a linear dependence with a large change in slope at M/M$_{o}\sim $2x, which is consistent with the recently proposed dimeron model proposed by Downward, et al. with regards to the LCMO system. Furthermore, there is some evidence for an increase in disorder below 75K which has not been discussed previously. Finally, another weaker step in $\sigma ^{2}$ at T$^{\ast } \quad \sim $250K, well above T$_{c}$ can be seen, which is comparable to a T$^{\ast }$in recent neutron scattering experiments, proposed to be a spin-liquid/spin-glass phase transition. Support: NSF DMR0301971. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X13.00011: Magnetic momentum density, Fermi surface and directional magnetic Compton profiles in LaSr$_{2}$Mn$_{2}$O$_{7}$ and La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ P. E. Mijnarends, S. Kaprzyk, B. Barbiellini, A. Bansil, Yinwan Li, J.F. Mitchell, P.A. Montano We have carried out first principles, all-electron computations of the magnetic momentum density $\rho_{mag}({\mathbf p})$ and magnetic Compton profiles (MCPs) for momentum transfer along the [100], [001], and [110] directions in LaSr$_2$Mn$_2$O$_7$ and La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ [1]. Parallel measurements of these three MCPs from a single crystal of La$_{1.2}$Sr$_{1.8} $Mn$_2$O$_7$ at 5 K in a magnetic field of 7 T are also reported. Here, we discuss details of the FS-related signatures in the first and higher BZs in the MCPs and show that high resolution magnetic Compton scattering experiments with a momentum resolution of 0.1 a.u. FWHM (full-width-at-half- maximum) or better will be necessary to observe this fine structure. We comment also on the feasibility of using positron annihilation spectroscopy in this connection. Work supported by the USDOE.\\ \mbox{[1]} Yinwan Li, P. A. Montano, J.F. Mitchell, B. Barbiellini, P. E. Mijnarends, S. Kaprzyk, and A. Bansil, Phys. Rev. Lett. 93 (2004) 207206. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X13.00012: Optic phonon anomaly as a precursor to polaron formation in a layered CMR manganite. Dmitry Reznik, Frank Weber, Nadir Aliouane, Dimitri Argyriou, Marcus Braden, Winfried Reichardt We found evidence of a precursor effect to polaron formation in the bond stretching phonons in the bilayer CMR manganite La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ at 10K. Inelastic neutron scattering measurements of these phonons in the XX0 direction show that both the transverse and longitudinal branches broaden abruptly from X=0.15, where they are resolution limited, to X=0.25 where FWHM=13meV. They then narrow again on approach to the zone boundary (X=0.5). The shell model predicts a downward dispersion following the cosine function for the transverse branch and upward dispersion for the longitudinal branch, which is exactly what is observed in undoped cuprates. But in La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ both branches show a steep downward dispersion above x=0.15. The anomalous dispersion and broadening can be understood as a precursor effect to the CMR transition at 125K because they appear at the same wavevectors as the polaron peaks in the same compound observed above 125K near q=(0.27,0.27,0). This behavior is very unusual, because such precursor effects have previously been associated with phase transitions where charge order appears on cooling, not on heating as in CMR manganites. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X13.00013: A local metallic state in globally insulating La$_{1.24}$Sr$_{1.76}$Mn$_{2}$O$_{7}$ well above the metal-insulator transition Zhe Sun, J. Fraser Douglas, Alexei V. Fedorov, Yi-De Chuang, Hong Zheng, John F. Mitchell, D. S. Dessau La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ is a typical colossal magnetoresistive oxide, and it shows a drastic transition from a low-temperature metal to a high-temperature insulator at 120K -160K. The famous CMR ( colossal magnetoresistive) effect usually accompanies the metal-insulator transition. Using angle-resolved photoemission spectroscopy (ARPES) we studied the electronic structure of a bi-layer manganite compound La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ (x=0.38). We found that in the insulating state there remain local metallic regions up to a very high temperature. In these metallic regions, the electronic behavior has minimal change with temperature. Our data indicate that the metal-insulator transition is a new type and an ``emergent'' phenomenon driven by the phase separation and percolation effect. The CMR effect can also be understood in the framework of the phase separation and percolation effect. [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X13.00014: Angle resolved photoemission studies of colossal magnetoresistive bilayer manganites Norman Mannella, Wanli Yang, Kiyohisa Tanaka, Xingjiang Zhou, Jennifer Zheng, John Mitchell, Jan Zaanen, Tom Devereaux, Naoto Nagaosa, Zahid Hussain, Zhi-Xun Shen In this talk, we will discuss the results of some recent angle-resolved photoemission spectroscopy (ARPES) investigations in the colossal magnetoresistive (CMR) bilayer compound La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ (LSMO, x = 0.4) [1]. The temperature dependent evolution of the quasiparticle excitations in LSMO has been found to track remarkably well the DC conductivity, thus accounting for the macroscopic transport properties and the metal to insulator transition.~ Our results indicate that the microscopic mechanism leading to the MIT and the CMR effect in manganites is intrinsically a quantum effect linked to a crossover via the nodal QP collapse from a coherent polaronic conductor in the FM state below T$_{C}$ to a hopping regime with thermally activated single polarons in the paramagnetic state above T$_{C}$.~ The role of the exchange interaction is crucial in controlling the competition between coherence and localization effects. [1] N. Mannella et al., Nature \underline {438}, 474 (2005) [Preview Abstract] |
Friday, March 9, 2007 10:48AM - 11:00AM |
X13.00015: Energy Range of State Conservation for Coulomb Interaction Effects in $La_{1.28}Sr_{1.72}Mn_2O_7$ Daniel Mazur, K. E. Gray, J. F. Zasadzinski, L. Ozyuzer, I. Beloborodov, H. Zheng, J. F. Mitchell Tunneling data on $La_{1.28}Sr_{1.72}Mn_2O_7$ crystals confirm Coulomb interaction effects through the $\sqrt{E}$ dependence of the density of states (DOS). Importantly, the data and analysis at high energy, $E$, show conservation of states: those removed from near $E_F$ are found between $\sim$30 and 70 meV from $E_F$. This quantum correction to the DOS agrees in magnitude with the $\sqrt{T}$ dependence of the bulk conductivity. Combining our results, with published theory and quantum interference data, we find a scattering time and Fermi velocity that agree reasonably well with recent ARPES results. [Preview Abstract] |
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