Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Z17: Focus Session: Orbital Order |
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Sponsoring Units: DMP GMAG Chair: Despina Louca, University of Virginia Room: 319 |
Friday, March 22, 2013 11:15AM - 11:27AM |
Z17.00001: Local structure and orbital ordering in YTiO$_3$ Bing Li, Keeseong Park, Shinichiro Yano, Despina Louca, Biao Hu, Jianshi Zhou, John Goodenough YTiO$_3$, with the strongest GdFeO$_3$-typed distortion in RTiO$_3$, is a ferromagnet below $T_C$ of 30 K and many theoretical and experimental studies suggest it is of antiferro-orbital ordering due to magnetic superexchange. Here, the local atomic structure of YTiO$_3$ has been investigated by using elastic (inelastic) neutron scattering and the (dynamic) pair density function analysis from 5 to 350 K. Deviations are observed of the local from the average crystal symmetry and these are attributed to distortions involving the Y and O atoms. In the case of Y, the in-plane $x-y$ displacements result in an antiferrodistortive motion exerting influence on Y-O1 (apical sites of octahedral) bonds seen in the temperature dependence. At the same time, the O ion site in the basal plane of the octahedron is split to two (O2 and O3), giving rise to two unequivalent Ti-O bonds, which results consequently in different tilting of basal plane of octahedra (0.5$^\circ$ larger in O3), A-O covalency and about 2$^\circ$ larger Ti-O-Ti bond angle in O3 sites. These facts may be regarded as the structural evidences on antiferro-orbital ordering in YTiO$_3$ and suggest electron-lattice interaction may play an important role in the orbital ordering, in addition to magnetic superexchange interation. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z17.00002: Quasiparticle mass enhancement and temperature evolution of the electronic structure in ferromagnetic SrRuO$_{3}$ Daniel Shai, Carolina Adamo, Dawei Shen, Charles Brooks, John Harter, Eric Monkman, Bulat Burganov, Darrell Schlom, Kyle Shen We report high-resolution angle-resolved photoemission spectroscopy (ARPES) studies of epitaxial thin films of the correlated $4d$ transition metal oxide ferromagnet SrRuO$_{3}$. The Fermi surface in the ferromagnetic state consists of well-defined Landau quasiparticles, exhibiting strong coupling to low-energy bosonic modes which contributes to the large effective masses observed by transport and thermodynamic measurements. Upon warming the material through its Curie temperature, we observe a substantial decrease in quasiparticle coherence, but negligible changes in the ferromagnetic exchange splitting, suggesting that local moments play an important role in the ferromagnetism in SrRuO$_{3}$. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z17.00003: Temperature-dependent photoluminescence studies of GdTiO$_{3}$ thin films Amit Verma, Santosh Raghavan, Vladimir Protasenko, Susanne Stemmer, Debdeep Jena GdTiO$_{3}$ (GTO), a Mott-insulator, has acquired increased prominence in last few years since the discovery of a 2-dimensional electron gas (2DEG) at its heterojunction with the band-insulator SrTiO$_{3}$. These 2DEGs have very large electron densities ($\sim $3x10$^{14}$ cm$^{-2})$ amounting to half electron per unit cell. To realize many possible applications of this large 2DEG, an understanding of the GTO bandstructure is needed. With this goal in mind, in this work we present photoluminescence (PL) studies of GTO thin films (10nm and 20nm) grown by molecular beam epitaxy (MBE) on (001) LSAT substrates. When pumped with a 325 nm He-Cd laser, we observe a red PL ($\sim $683 nm at 300K) from both GTO thin films at RT. Upon lowering the temperature from 300K to 80K, the PL peak blue shifts by $\sim $0.14eV. Interestingly, the reported activation energy of the resistivity of MBE-grown GTO thin films is also $\sim $0.14eV (Moetakef et al., J. Crystal Growth 355, 166 (2012)). We connect the observed temperature-dependent PL data with the expected electronic bandstructure and electrical resistivity, and explain the sharp transition in the peak that occurs between 120K-200K from $\sim $636nm to $\sim $683nm. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z17.00004: Spin Switching and Magnetization Reversal in Single-Crystal NdFeO$_3$ Shujuan Yuan, Fang Hong, La Chen, Yabin Wang, Wei Ren, Jincang Zhang, Shixun Cao, Gang Cao We report results of our recent study of single-crystal NdFeO$_3$ that features a strong interaction between 3d and 4f electrons, which generates \textit{two distinct magnetic states} ordered at 17 K and 170 K. This study reveals novel magnetic behavior that is highly sensitive to the orientation and history of magnetic field and is characterized by the following: \textbf{(1)} sharply contrasting magnetization, M(T), along the \textbf{\textit{a}} and \textbf{\textit{c}}-axis; \textbf{(2)} an abrupt spin-switching along the \textbf{\textit{a}}-axis via a first-order transition below 17 K when the system is \textit{zero-field-cooled}; and \textbf{(3)} a progressive magnetization reversal when the system is \textit{field-cooled}. Such behavior suggests an exotic ground state driven by an extraordinary coupling between spin, orbit and lattice degrees of freedom. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z17.00005: Itinerant ferromagnetism in the oxygen-deficient EuTiO$_3$: A first-principles investigation Hai-Xia Cao, Hai-Shuang Lu, Tian-Yi Cai, Sheng Ju Effects of oxygen vacancy on the electronic structure and magnetism in the quantum paraelectric EuTiO$_{3}$ were investigated from first-principles. In contrast to antiferromagnetism in the pristine EuTiO$_{3}$, itinerant ferromagnetism was revealed in the oxygen-deficient EuTiO$_{3}$. The origin lies in the spin-polarized Ti 3$d$ states, which mediate a ferromagnetic exchange interaction between almost localized Eu 4$f$ spins. In addition, this ferromagnetic exchange coupling was strengthened via the partial occupation of Eu $5d$ states. These findings not only explain the observation of ferromagnetism in the unstrained EuTiO$_{3}$ thin films, but also demonstrate the potential application of EuTiO$_{3}$ in magnetoelectronics. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z17.00006: Spin and orbital order separation in colossal magnetoresistive transition M.A. Hossain, M.H. Burkhardt, E. Weschke, E. Schierle, M.S. Golden, Y. Tomioka, Y. Tokura, J. St\"{o}hr, H.A. D\"{u}rr Understanding the Colossal magnetoresistive (CMR) process in manganites is one of the grand challenges of modern physics. While the metallic ferromagnetic phase is relatively well understood, the triggering mechanism of the metal-insulator transition is not clear and it is believed that lattice strain in term of polarons play an important role in the mysterious insulating phase. Lattice strain occurs in the charge-orbitally ordered insulating phase via the Jahn-Teller type distortion and therefore, to understand the CMR it is critical to understand the interplay of ferromagnetism and orbital order during the CMR transition itself. In this letter, with high magnetic field dependent Resonant Soft X-ray Scattering measurements, we show that during the CMR process, an insulating antiferromagnetic phase, which is extremely susceptible to magnetic field and temperature, directly competes with metallic ferromagnetism while the robust CE type spin and orbitally ordered regions act as a catalyst to seed these antiferromagnetic regions. This allows us to construct a picture of the competing forces at the heart of CMR. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z17.00007: Charge-ordering transitions without charge differentiation Yundi Quan, Victor Pardo, Warren Pickett The distorted perovskite nickelate system RNiO$_3$ (R=rare earth except La) undergoes a metal-insulator transition (MIT) at a temperature that varies smoothly with the R ionic radius. This MIT is accompanied by structural transition which leads to two inequivalent Ni sites in the cell, and has been explained by charge ordering (CO): charge is transferred between the Ni1 and Ni2 sites in a long-range ordered fashion. Experimental data on core binding energies, ionic radii, and Mossbauer shifts are interpreted in terms of Ni cation charges of 3$\pm \delta$ with, for example, $\delta \approx$ 0.3 for YNiO$_3$. Making use of first principles DFT results and a new approach not invoking integration of the charge density, we find\footnote{Y. Quan, V. Pardo, and W. E. Pickett, Phys. Rev. Lett. (2012, in press)} that the Ni $3d$ occupation is identical (to high accuracy) for the two Ni sites. We also present results for other compounds (La$_2$VCuO$_6$, YNiO$_3$, CaFeO$_3$, AgNiO$_2$, V$_4$O$_7$), all of which have distinct ``charge states'' that have identical $3d$ occupation. This quantitative procedure will be discussed and some implications will be outlined. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z17.00008: Real Space Imaging of Phase Separation in Mangnites Kai Du, Lifeng Yin, Jian Shen Electronic phase separation (EPS) in manganites is generally considered to be responsible for their unusual colossal magneto resistance (CMR) [1-2]. However, the dynamic behavior of EPS and the formation mechanism are still not very clear. Magnetic force microscopy (MFM) is one of the most powerful techniques which enables us to study the magnetic domains and direct image the EPS in real space without damaging the samples. In this work, we use a PPMS (Quantum Design) compatible MFM to study the magnetic domains of La0.7Sr0.3MnO3 (LSMO) thin films on NdGaO3(110) and La0.325Pr0.3Ca0.375MnO3 (LPCMO) thin films on SrTiO3 (100) grown by pulsed laser deposition technique (PLD). The LSMO system shows clear stripe domain pattern [3], while the LPCMO system exhibits large scale domains corresponding to charge-ordered insulating phase and ferromagnetic metallic phase [1]. Their transport properties were studied under a variety of temperatures and magnetic fields. The phase separation in submicron scale and their percolative transport have been confirmed by MFM images and the transport measurement during the imaging.\\[4pt] [1] M. Uehara, Nature (London) 399, 560 (1999)\\[0pt] [2] E. Dagotto, Phys. Rep. 344, 1(2001); and references therein\\[0pt] [3] Y. Jiang, Solid State Communications 150, 2028(2010) [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z17.00009: Effective magnetoelectric tensor of a composite material David Stroud, Mehul Dixit We calculate the effective magnetoelectric coefficient tensor of a composite of two single-phase magnetoelectrics in which effect of strain is unimportant. We obtain exact relations for elements of the effective magnetoelectric coefficient tensor entirely in terms of the elements of the individual components, and the composite geometry. The problem is solved by a decoupling transformation that reduces the problem to finding the effective coefficients in a composite of the {\emph same geometry} but with two {\emph independent, curl-free fields}. The decoupling transformation is found to be identical to that used in the problem of composite thermoelectrics \footnote{D. J. Bergman and O. Levy. Thermoelectric properties of a composite medium. J. Appl. Phys., 70:6821 - 6833, 1991}. Details of the calculation will be presented. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z17.00010: Growth and characterization of high crystalline quality Co$_{2}$FeAl$_{\mathrm{x}}$Si$_{\mathrm{1-x}}$ Heusler alloy films on MgAl$_2$O$_4$(001) substrates Brian Peters, Christian Blum, Patrick Woodward, Sabine Wurmehl, Fengyuan Yang A number of Heusler alloys have been predicted to be half-metallic and are thus ideal candidates for use in spintronics. Co$_{2}$FeAl$_{\mathrm{x}}$Si$_{\mathrm{1-x}}$ has been predicted and shown to have some of the highest Tc, saturation magnetization and lowest magnetic damping constant among Heusler half-metals. Here we outline the growth and characterization of the highest crystalline quality epitaxial Heusler films using a novel off-axis UHV sputtering technique. We grow these films onto a closely lattice matched MgAl$_{2}$O$_{4}$(001) substrate, without the need for a Cr-buffer layer or post annealing, as has been done previously. This eliminates the diffusion of Cr across the interface, thus improving the purity and crystallinity of the films at the interface. X-ray diffraction results demonstrate epitaxial films with distinct Laue oscillations and rocking curves of FWHM as low as 0.0035$^{\circ}$, which demonstrates the highest crystalline quality for Heusler films reported to date. Magnetic measurements show highly square hysteresis loops with a remanence of 95-98{\%}, near ideal saturation magnetization, very small coercivities - between 3-8 Oe, pronounced magnetocrystalline anisotropy. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z17.00011: Small bound polarons for ultrafast holography in dielectric LiNbO$_3$ Mirco Imlau, Hauke Bruening, Holger Badorreck, Andreas Buescher Small bound polarons allow for hologram recording with single laser pulses and exceptional photosensitivity in nominally undoped, thermally reduced LiNbO$_3$ [1]. This new type of recording mechanism is of particular interest for the field of nonlinear and ultrafast photonics because of small bound polaron generation on the fs-scale. In this contribution we present our latest results on the emerging field of small bound polarons for fs-holography in the visible spectral range as well as the successful application of the effect for holographic imaging. The impact of two-photon absorption and nonlinear index of refraction is highlighted by studying the temporal dynamics of the diffraction efficiency as a function of time-delay between hologram recording and probing (-6 fs $< \delta t <$ 6 ns). The analysis of the temporal dynamics supports our recent approach (see Ref. [2]) to explain the build-up of the space-charge field on the sub-ps-time scale in the model of optical absorption of small polarons: optically induced polaron hopping to next-neighboring lattice sites is responsible for a fast and efficient charge transport on the nanoscale.\\[4pt] [1] M.~Imlau et al., Opt. Express 19, 15322 (2011)\\[0pt] [2] H.~Bruening et al. Opt. Express 20, 13326 (2012) [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z17.00012: Atomically resolved data for oxide surface analyzed using a local crystallography analysis method Zheng Gai, Wenzhi Lin, K. Fuchigami, T. Ward, P. Snijders, J. Shen, Stephen Jesse, Sergei Kalinin, Arthur Baddorf The emergent physical phenomena of oxides have attracted increasing scientific attention. Here, we report an approach for studying local surface chemistry and order parameter fields based on a local crystallographic analysis of scanning probe microscopy data of oxide surfaces. We obtained initial estimated atom locations by finding the centroid of the remaining isolated regions of pixels, after applying thresholds to the topographic images. Then we determined the refined positions by automatically fitting each atom individually using a shape function. With the refined locations, we can further derive and quantify properties that are not readily clear in the topographic images. This approach was applied to analyze scanning tunneling microscopy data for the surface of La5/8Ca3/8MnO3 (001) and demonstrated distortion domains with different distortion orientations. These studies provide a new pathway to extract and quantify local properties for scanning probe microscopy images. Research was supported (W.L., S.V.K.) by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. This research was conducted at and supported by (Z.G., S.J., A.P.B.) the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z17.00013: Electronic structure and strain-induced Lifshitz transition in epitaxial Ba$_{2}$RuO$_{4}$ thin films as studied by ARPES Bulat Burganov, Carolina Adamo, Daniel Shai, Andrew Mulder, Masaki Uchida, John Harter, Craig Fennie, Darrell Schlom, Kyle Shen We employ oxide molecular beam epitaxy and \emph{in situ} ARPES to synthesize epitaxial thin films of Ba$_{2}$RuO$_{4}$, which is isostructural and isoelectronic to the unconventional superconductor Sr$_{2}$RuO$_{4}$, and characterize its Fermi surface topology and multiorbital quasiparticle dynamics. Although Ba$_{2}$RuO$_{4}$ cannot be synthesized as bulk single crystals, we epitaxially stabilize thin films on TbScO$_{3}$ or SrTiO$_{3}$ substrates. We report a full parametrization of the band structure and compare our results to first-principles calculations as well as our data on Sr$_{2}$RuO$_{4}$. Unlike in Sr$_{2}$RuO$_{4}$ we do not observe a surface reconstruction in line with our expectations for the larger Ba cations. We use ARPES to demonstrate that the combination of a larger cation radius, together with epitaxial strain, can be employed to drive a Lifshitz transition in the d$_{xy}$-like $\gamma$ band from electron-like in Sr$_{2}$RuO$_{4}$ to hole-like in Ba$_{2}$RuO$_{4}$. The ability to control the Fermi surface topology by epitaxial strain is a promising tool for investigating the role of the near-E$_{F}$ van Hove singularity in superconductivity and magnetism in ruthenates, as well as a general tool for controlling and studying correlated electronic materials. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z17.00014: Conducting states caused by a surface electric dipole in CrN(001) very thin films Antia S. Botana, Victor Pardo, Daniel Baldomir, Peter Blaha The changes in the electronic structure of oxides and other correlated compounds caused by electronic reconstructions at their surface and interfaces has attracted much attention recently. CrN shows a magnetostructural phase transition as a function of temperature and controversial electronic properties. It has been argued recently that, with the onset of antiferromagnetic order, CrN as a bulk is always semiconducting, but very close to a metal-insulator transition [1]. In order to check if a small perturbation in the system could drastically change its conduction properties, we have performed electronic structure calculations for CrN in a thin film geometry within the LDA+U method. For thin films with increasing thickness (4-10 layers) starting with a critical thickness of 10 (cubic symmetry) or 6 layers (orthorhombic) the gap closes and conducting states appear. The appearence of metallic states is connected with a structural relaxation at the surface, where Cr (N) atoms buckle inside (outside) forming an effective surface dipole moment. Being CrN a low-gap system, these electric dipoles at the surface are able to shift the bands around the Fermi level significantly enough to drive those thin films metallic. \\[4pt] [1] A. S. Botana et al. Phys. Rev. B 85, 235118 (2012) [Preview Abstract] |
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