Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session Y6: Magnetic Complex Oxides IIFocus

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Sponsoring Units: GMAG DMP Chair: Antia Botana, Argonne National Laboratory Room: 302 
Friday, March 18, 2016 11:15AM  11:27AM 
Y6.00001: The extreme quantum limit in lightlydoped SrTiO$_{3}$ Anand Bhattacharya, Brian Skinner, Guru Khalsa, Alexey Suslov When a three dimensional electron gas is placed in a sufficiently strong magnetic field, it is said to be in the quantum limit when the cyclotron energy $\hbar \omega_{c}$ \textgreater $\varepsilon_{F}$ \textgreater \textgreater kT, and all of the electrons occupy the lowest Landau level. Achieving this limit in a material requires a small Fermi energy relative to the applied magnetic field, and a weak disorder potential such that magnetic freezeout is avoided. We present an experimental study of lightlydoped single crystals of SrTiO$_{3}$, which remain good bulk conductors in temperatures down to 25 mK and magnetic fields up to 45 T. Our measurements probe deep into the quantum limit, where $\hbar \omega _{c}$ \textgreater \textgreater $\varepsilon_{F}$ and theory has long predicted that electronelectron interactions can drive the system into a charge density wave or Wigner crystal like state. A number of interesting features arise in electrical transport in this regime, including a striking reentrant nonlinearity in the currentvoltage characteristics. We discuss these features in the context of possible correlated electron states, and present a picture based on magnetic field induced puddling of electrons in a disorder potential landscape. [Preview Abstract] 
Friday, March 18, 2016 11:27AM  11:39AM 
Y6.00002: Phononinduced ultrafast band gap control in LaTiO{\$}\textunderscore 3{\$} Mingqiang Gu, James M. Rondinelli We propose a route for ultrafast band gap engineering in correlated transition metal oxides by using optically driven phonons. We show that the {\$}$\backslash $Gamma{\$}point electron band energies can be deterministically tuned in the nonequilibrium state. Taking the Mott insulator LaTiO$_{\mathrm{3}}$ as an example, we show that such phononassisted processes dynamically induce an indirecttodirect band gap transition or even a metaltoinsulator transition, depending on the electron correlation strength. We explain the origin of the dynamical band structure control and also establish its generality by examining related oxides. Lastly, we describe experimental routes to realize the band structure control with impulsive stimulated Raman scattering. [Preview Abstract] 
Friday, March 18, 2016 11:39AM  11:51AM 
Y6.00003: Tunning the magnetic ordering in EuTiO$_3$ through doping Zhigang Gui, Anderson Janotti EuTiO$_3$ (ETO) is a complex oxide that displays strong spinlattice coupling, large magnetoelectric effects, and undergoes a series of structural and magnetic phase transitions when subjected to pressure or epitaxial strain. ETO adopts a cubic structure and is paramagnetic at high temperatures, while at very low temperatures it transforms to an antiferrodistortive tetragonal structure with a Gtype antiferromagnetic (AFM) ordering. Several approaches have been presented to tune the magnetic ordering from the Gtype antiferromagnetism to the Ftype ferromagnetism, often relying on external pressure or epitaxial strain. Doping through substitution of trivalent species on the europium sites or creation of oxygen vacancies have also been proposed to lead to ferromagnetism. However, the fundamental mechanism by which excess electrons from impurities or defects lead to ferromagnetic ordering is unclear. In this study, we explore the effects of doping on the magnetic ordering in EuTiO$_3$ through firstprinciples calculations. We show how itinerant carriers in the Ti$d$derived conductionband states interact with europium $f$ states, inducing an alignment of the large moments on the europium ions. The effects of doping of different types of magnetic ordering are considered, a [Preview Abstract] 
Friday, March 18, 2016 11:51AM  12:03PM 
Y6.00004: Point defects, impurities, and small hole polarons in GdTiO$_3$ Lars Bjaalie, Anderson Janotti, Karthik Krishnaswamy, Chris G. Van de Walle GdTiO$_3$(GTO) has become the focus of great interest because of its use in complexoxide heterostructures that display twodimensional electron gases with unprecedented high densities. GTO is a Mott insulator, with a band gap arising within the partially filled Ti 3$d$ band due to strong electronelectron interactions. GTO often displays hole conductivity, likely attributed to defects or impurities, yet the cause of this unintentional conductivity has not yet been explored. We therefore used density functional theory with a hybrid functional to study their electronic structure. Among native defects, the cation vacancies have the lowest formation energies in oxygenrich conditions, and oxygen vacancies have the lowest formation energy in oxygenpoor conditions. Among the impurities, r$_\mathrm{Gd}$, H$_i$ and C$_\mathrm{O}$ have the lowest formation energies. The defects and impurities are intrinsically stable only in a single ``natural'' charge state, to which various numbers of hole polarons can be bound, which explains the frequent observation of $p$type hopping conductivity in the rareearth titanates. These small hole polarons also lead to optical absorption and act as electron traps in devices. \\ \\ Work supported by NSF and by the LEAST Center. [Preview Abstract] 
Friday, March 18, 2016 12:03PM  12:15PM 
Y6.00005: Lowenergy dispersion of dynamic charge stripes in La$_{1.75}$Sr$_{0.25}$NiO$_{4}$ observed with inelastic neutron scattering Ruidan Zhong, John Tranquada, Genda Gu, Dmitry Reznik, Barry Winn The dynamic stripe correlations have been the subject of intense research, owing to the possible links with highT$_{c}$ superconductivity. In light of a recently published, direct observation of chargestripe fluctuations in La$_{2x}$Sr$_{x}$NiO$_{4}$ using inelastic neutron scattering\footnote{S. Anissimova, $et$ $al.$, Nat. Commun. 5, 3467 (2014)}, we did a followup neutron experiment on a $x$=0.25 sample to characterize the lowenergy dispersion of these dynamic charge stripes using the HYSPEC instrument at the Spallation Neutron Source. The scattering signals are collected in the vicinity of a chargeorder peak with a large wave vector (4.4, 3, 0), where dynamic spinstripe correlations are negligible. Mapping the lowenergy chargestripe fluctuations in a wide temperature range, we observe a finite dispersion along the stripemodulation direction at T$\geq$160K where the charge stripes become disordered, while the steep dispersion in the orthogonal direction is not resolved. [Preview Abstract] 
Friday, March 18, 2016 12:15PM  12:27PM 
Y6.00006: Ultrafast Dynamics of the Symmetry Breaking in Chargeordered La$_{\mathrm{1.75}}$Sr$_{\mathrm{0.25}}$NiO$_{\mathrm{4}}$ Single Crystals. Giacomo Coslovich, Alexander F. Kemper, Sascha Behl, Bernhard Huber, Hans A. Bechtel, Takao Sasagawa, Michael C. Martin, Robert A. Kaindl We report equilibrium and ultrafast optical pumpTHz probe spectroscopy of the stripephase rareearth nickelate compound La$_{\mathrm{1.75}}$Sr$_{\mathrm{0.25}}$NiO$_{\mathrm{4}}$, unveiling the ultrafast dynamics of the crystal symmetry breaking and of local electronic arrangements. At low temperatures the folding of finite momenta vibrations due to symmetry breaking lead to the appearance of new IRactive resonances, particularly around the phonon bending mode frequency ($\approx $11 THz). Ultrafast experiments in the multiTHz spectral range show sharp THz reflectivity modulations associated with the phonon zonefolding dynamics, while the background conductivity is reminiscent of the opening of the midIR pseudogap. We combine experimental data with DFT calculations of the phonon dispersion to reveal the distinct dynamics of the LO and TO phonon modes at finite momenta. This work provides new insight in the role of polar electronphonon coupling and symmetry breaking in chargeordered systems. [Preview Abstract] 
Friday, March 18, 2016 12:27PM  12:39PM 
Y6.00007: Single crystal preparation and longrange charge fluctuations in the squareplanar nickelate La4Ni3O8. Junjie Zhang, YuSheng Chen, Hong Zheng, Daniel Phelan, John Mitchell Since the discovery of highTc superconductivity in cuprates, intensive effort has been focused on a search for superconductivity in related materials, with particular attention on nickelates. Bulk nickelates containing squareplanar coordinated Ni$+$ are of interest because Ni1$+$ is isoelectronic with Cu2$+$, the building block of highTc cuprates. Here we report the first single crystal synthesis of La4Ni3O8, a layered nickelate containing squareplanar coordinated Ni$^{\mathrm{+}}$ with crystallographic and electronic structure related to that of cuprates. Magnetic susceptibility, resistivity, and heat capacity measurements confirm the reported phase transition at \textasciitilde 105 K[1]. Longrange charge fluctuations with q\textasciitilde (1/3, 1/3, L) was observed for the first time through synchrotron Xray single crystal diffraction. Our results challenge the current understanding of the origin of the phase transition. Availability of bulk La4Ni3O8 single crystals is also of significant importance for unraveling its ambiguous groundstate magnetic structure, the spin state of the Ni ion, and potential for superconductivity in nickelates involving Ni$+$ in a squareplanar coordination. [1] Poltavets, V. V. et al. PRL 2010, 104, 206403. [Preview Abstract] 
Friday, March 18, 2016 12:39PM  12:51PM 
Y6.00008: Cooperative phonon effects in the metalinsulator transitions of manganite and nickelate perovskites Richard T. Brierley, Gian G. Guzmán Verri, Peter B. Littlewood Metalinsulator transitions in manganite and nickelate perovskites depend on the competition between the electron kinetic energy, which favors the metallic phase, and the electronphonon coupling and Coulomb interaction, which favor localization. The size of the Asite cation controls the relative rotation of the octahedral structural units of the perovskite in the range of $0$  $15^\circ$. This is accompanied by changes in the metalinsulator transition temperature from $0$  $600\text{K}$. This effect is commonly attributed to modification in the electron bandwidth from changes in orbital overlap.\\\\ Although previous theoretical studies of these materials include the electronphonon interaction, they typically do not consider cooperative phonon effects. Using a phenomenological model of the perovskite structure, we show that the longrange anisotropic forces arising from intersite phonon interactions are modulated by changes in the octahedral rotation. We demonstrate using statistical mechanical calculations that these changes in the strain interaction can capture the variation in transition temperature with tolerance factor observed in both the manganites and nickelates. [Preview Abstract] 
Friday, March 18, 2016 12:51PM  1:03PM 
Y6.00009: Direct proof of static charge stripe correlations in La$_{2x}$Ba$_x$CuO$_4$ X M Chen, V Thampy, C Mazzoli, A Barbour, G Gu, J P Hill, J M Tranquada, M P M Dean, S B Wilkins The nature of charge stripe order in the cuprates, and in particular whether the stripes are static or dynamic, is a key issue in understanding the relationship between stripes and superconductivity. In La$_{2x}$Ba$_x$CuO$_4$ (LBCO) a low temperature structural distortion is widely believed to pin stripes into fixed, static domains, but such an assertion has never been directly verified. We performed resonant soft xray photon correlation spectroscopy (XPCS) to probe the charge order Bragg peak of 1/8 doped LBCO. At low temperatures, we observe timeindependent xray speckle patterns persisting for more than three hours, proving the static nature of the stripes and we go on to discuss how stripe order melts with increasing temperature. Our results demonstrate that the combination of XPCS with diffraction limited light sources such as the National Synchrotron Light Source II can probe the dynamics of even subtle order parameters such as stripes in the cuprates. [Preview Abstract] 
Friday, March 18, 2016 1:03PM  1:15PM 
Y6.00010: Quasistatic magnetoelectric quadrupoles as the order parameter for the pseudogap phase in cuprate superconductors Michael Fechner, Merlin J. A. Fierz, Florian Th\"ole, Urs Staub, Nicola A. Spaldin A characteristic of ferroic materials is the emergence of a temporally static finite expectation value of an order parameter. Here, we introduce a new mechanism [1] for ferroic order, in which a nonzero quasistatic magnetoelectric quadrupolar order appears due the coupling of fluctuating spin magnetic dipole moments and polar optical phonons. Using firstprinciples calculations within the LSDA$+U$ method of density functional theory, we calculate the magnitude of the effect for the prototypical cuprate superconductor, HgBa$_2$CuO$_{4+\delta}$. We show that our proposed mechanism is consistent, to our knowledge, with many experimental observations for the onset of the pseudogap phase and therefore propose the quasistatic magnetoelectric quadrupole as a possible pseudogap order parameter. Finally, we show that our mechanism embraces some key aspects of previous theoretical models, in particular the description of the pseudogap phase in terms of orbital currents. [1] M. Fechner, M. J. A. Fierz, F. Th\"ole, U. Staub, and N. A. Spaldin, arXiv 1510.04844, (2015). [Preview Abstract] 
Friday, March 18, 2016 1:15PM  1:27PM 
Y6.00011: Quantum oscillations in a bilayer with broken mirror symmetry: a minimal model for YBa$_2$Cu$_3$O$_{6 + \delta}$ Akash Maharaj, Yi Zhang, Brad Ramshaw, Steven Kivelson Using an exact numerical solution and semiclassical analysis, we investigate quantum oscillations (QOs) in a model of a bilayer system with an anisotropic (elliptical) electron pocket in each plane. Key features of QO experiments in the high temperature superconducting cuprate YBCO can be reproduced by such a model, in particular the pattern of oscillation frequencies (which reflect ``magnetic breakdown'' between the two pockets) and the polar and azimuthal angular dependence of the oscillation amplitudes. However, the requisite magnetic breakdown is possible only under the assumption that the horizontal mirror plane symmetry is spontaneously broken and that the bilayer tunneling, $t_\perp$, is substantially renormalized from its `bare' value. Under the assumption that $t_{\perp} = \tilde{Z}t_\perp^{(0)}$, where $\tilde{Z}$ is a measure of the quasiparticle weight, this suggests that $\tilde{Z} \lesssim 1/20$. Detailed comparisons with new YBa$_2$Cu$_3$O$_{6.58}$ QO data, taken over a very broad range of magnetic field, confirm specific predictions made by the breakdown scenario. [Preview Abstract] 
Friday, March 18, 2016 1:27PM  1:39PM 
Y6.00012: Controlling Spin Ordering in RareEarth Perovskite Vanadates Nicholas Wagner, James Rondinelli We investigate the role and influence of local structure distortions on the antiferromagnetic spin ordering temperatures for large $A$site radii $R$VO3 perovskites ($R$=YbLa) using a combination of data analytics (DA) and density functional theory (DFT). First, mode crystallography is used to parameterize the structural phase space. Next, we identify the important local structural features that correlate strongly with the N\'eel temperatures ($T_N$) using Pearson correlation coefficients. From this data, we then formulate a regression model using gradient boosted decision trees (GBDT) that returns the relative importance of each feature in predicting $T_N$. Our analysis indicates that the amplitude of the subtle JahnTeller active mode, which leads to variations in the VO bond lengths and angles, could be used as an effective structural control parameter to modify the spin ordering temperature. We then validate this datadriven structureproperty relationship in artificial vanadate structures using $T_N$ based on both our GBDT model and a model Hamiltonian using DFT energies. This combined approach allows us to gauge the accuracy of existing physical models for the antiferromagnetic ordering in vanadates and opens possible strategies to design materials with targeted $T_N$. [Preview Abstract] 
Friday, March 18, 2016 1:39PM  1:51PM 
Y6.00013: Giant Magnetocaloric Effect in the Doubleperovskite Gd2NiMnO6 Jae Young Moon, Mi Kyung Kim, Dong Kun Oh, Sang Hyup Oh, Nara Lee, Young Jai Choi We have synthesized single crystal of Gd$_{\mathrm{2}}$NiMnO$_{\mathrm{6}}$(GNMO) by the Biflux method and investigated magnetocaloric effect in them by magnetic measurements. Magnetic susceptibility of GNMO increases smoothly as temperature decrease and ferromagnetic order occurs below 135 K, and additional anomaly show at low temperature, indicative of the onset of Gd$^{\mathrm{3+}}$ spin arrangement. At the temperature, magnetic entropy change, $\Delta $S$_{\mathrm{M}}$, with the field changes of 09 T, calculated from isothermal M(H) data using Maxwell relation, exhibits sharp peak. This peak is gigantic and cryogenic, these make GNMO promising cryogenic magnetic refrigerant materials. [Preview Abstract] 
Friday, March 18, 2016 1:51PM  2:03PM 
Y6.00014: ABSTRACT WITHDRAWN 
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