Browsing by Author "Keimer, B"

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    Anomalous spin dynamics and orbital excitations in Mott-insulating titanates
    (Australian Institute of Physics, 2010-02-03) Ulrich, C; Khaliullin, G; Ament, LJP; Ghiringhelli, G; Braicovich, L; Lorenz, T; Tokura, Y; van den Brink, J; Keimer, B
    Spin and orbital degrees of freedom play an important role in the various phenomena of strongly correlated electron systems like unconventional high-temperature superconductivity in cuprates or colossal magnetoresistance in manganates. Our extensive neutron scattering experiments on the cubic perovskite titanates LaTiO3 and YTiO3 lead to the discovery of a highly unusual magnetic ground state which is in contradiction to the standard Goodenough-Kanamori rules, but indicates the presence of strong orbital fluctuations [1-4]. Raman light scattering spectra of LaTiO3 and YTiO3 exhibit unexpected features in the high energy range well above the phonon spectrum [5]. Using momentum dependent resonant inelastic x-ray scattering (RIXS) experiments in combination with theoretical calculations, we were able to identify these excitations as collective orbital excitations (orbital waves termed ‘orbitons’) [6-7].
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    Competing exchange interactions on the verge of a metal-insulator transition in the two-dimensional spiral magnet Sr3Fe2O7
    (Americal Physical Society, 2014-10-03) Kim, JH; Jain, A; Reehuis, M; Khaliullin, G; Peets, DC; Ulrich, C; Park, JT; Faulhaber, E; Hoser, A; Walker, HC; Adroja, DT; Walters, AC; Inosov, DS; Maljuk, A; Keimer, B
    We report a neutron scattering study of the magnetic order and dynamics of the bilayer perovskite Sr 3 Fe 2 O 7 , which exhibits a temperature-driven metal-insulator transition at 340 K. We show that the Fe 4+ moments adopt incommensurate spiral order below T N =115  K and provide a comprehensive description of the corresponding spin-wave excitations. The observed magnetic order and excitation spectra can be well understood in terms of an effective spin Hamiltonian with interactions ranging up to third-nearest-neighbor pairs. The results indicate that the helical magnetism in Sr 3 Fe 2 O 7 results from competition between ferromagnetic double-exchange and antiferromagnetic superexchange interactions whose strengths become comparable near the metal-insulator transition. They thus confirm a decades-old theoretical prediction and provide a firm experimental basis for models of magnetic correlations in strongly correlated metals. © 2014, American Physical Society.
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    Electronic and phononic Raman scattering in detwinned YBa2Cu3O6.95 and Y0.85Ca0.15Ba2Cu3O6.95: s-wave admixture to the d(x)(2)-y(2)-wave order parameter
    (America Physical Society, 2009-08-01) Bakr, M; Schnyder, AP; Klam, L; Manske, D; Lin, CT; Keimer, B; Cardona, M; Ulrich, C
    Inelastic light (Raman) scattering has been used to study electronic excitations and phonon anomalies in detwinned, slightly overdoped YBa2Cu3O6.95 and moderately overdoped Y0.85Ca0.15Ba2Cu3O6.95 single crystals. In both samples modifications of the electronic pair-breaking peaks when interchanging the a and b axis were observed. The lineshapes of several phonon modes involving plane and apical oxygen vibrations exhibit pronounced anisotropies with respect to the incident and scattered light-field configurations. Based on a theoretical model that takes both electronic and phononic contributions to the Raman spectra into account, we attribute the anisotropy of the superconductivity-induced changes in the phonon lineshapes to a small s-wave admixture to the d(x)(2)-y(2) pair wave function. Our theory allows us to disentangle the electronic Raman signal from the phononic part and to identify corresponding interference terms. We argue that the Raman spectra are consistent with an s-wave admixture with an upper limit of 20%. © 2009, American Physical Society.
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    Evolution of the metallic state in LaNiO3/LaAlO3 superlattices measured by Li8β-detected NMR
    (American Physical Society (APS), 2021-11-12) Karner, VL; Chatzichristos, A; Cortie, DL; Fujimoto, D; Kiefl, RF; Levy, CDP; Li, RH; McFadden, RML; Morris, GD; Pearson, MR; Benckiser, E; Boris, AV; Cristiani, G; Logvenov, G; Keimer, B; MacFarlane, WA
    Using ion-implanted Li8 β-detected NMR, we study the evolution of the correlated metallic state of LaNiO3 in a series of LaNiO3/LaAlO3 superlattices as a function of bilayer thickness. Spin-lattice relaxation measurements in an applied field of 6.55T reveal two equal amplitude components: one with metallic (T linear) 1/T1 and a second with a more complex T dependence. The metallic character of the slow relaxing component is only weakly affected by the LaNiO3 thickness, while the fast component is much more sensitive, exhibiting the opposite temperature dependence (increasing toward low T) in the thinnest, most magnetic samples. The origin of this bipartite relaxation is discussed in terms of electronic phase separation. ©2021 American Physical Society.
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    Highly anisotropic anomaly in the dispersion of the copper-oxygen bond-bending phonon in superconducting YBa2Cu3O7 from inelastic neutron scattering
    (American Physical Society, 2011-10-19) Raichle, M; Reznik, D; Lamago, D; Heid, R; Li, Y; Bakr, M; Ulrich, C; Hinkov, V; Hradil, K; Lin, CT; Keimer, B
    Motivated by predictions of a substantial contribution of the "buckling'' vibration of the CuO2 layers to d-wave superconductivity in the cuprates, we have performed an inelastic neutron scattering study of this phonon in an array of untwinned crystals of YBa2Cu3O7. The data reveal a pronounced softening of the phonon at the in-plane wave vector q = (0, 0.3) upon cooling below similar to 105 K, but no corresponding anomaly at q = (0.3,0). Based on the observed in-plane anisotropy, we argue that the electron-phonon interaction responsible for this anomaly supports an electronic instability associated with a uniaxial charge-density modulation and does not mediate d-wave superconductivity. © 2011, American Physical Society.
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    Inelastic neutron scattering in multiferroic materials
    (Australian Institute of Physics, 2012-02-02) Reynolds, NM; Graham, P; Mulders, AM; McIntyre, G; Danilkin, SI; Fujioka, J; Tokura, Y; Keimer, B; Reehuis, M; Ulrich, C
    Magnetism and ferroelectricity are both exciting physical properties and are used in everyday life in sensors and data storage. Multiferroic materials are materials where both properties coexist. They offer a great potential for future technological applications like the increase of data storage capacity or in novel senor applications. The coupling mechanism between both antagonistic effects, electrical polarization and magnetic polarization, is not fully understood yet. The aim of the project is the systematic study of multiferroic materials such as TbMnO3 and related materials by inelastic neutron scattering (INS) in order to obtain a deeper insight into the interplay between the two interacting effects. We have started our investigations with TbVO3, which is isostructural to TbMnO3, but has a collinear antiferromagnetic spin arrangement [1] instead of a cycloidal spin structure [2]. By using inelastic neutron scattering (INS) we have obtained the spin wave dispersion relation and the crystal field excitations of the Tb-ions in TbVO3. These data will be compared with previously obtained data of D. Senff on TbMnO3 [3]. Experiments were performed at the ILL in Grenoble, France and at the research reactor OPAL at ANSTO, Australia.
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    Inelastic neutron scattering in multiferroic materials
    (Australian Institute of Physics, 2012-02-02) Reynolds, NM; Graham, PJ; Mulders, AM; McIntyre, GJ; Dainlkin, SA; Fujioka, J; Tokura, Y; Keimer, B; Reehuis, M; Ulrich, C
    In order to obtain a deeper understanding of the spin interactions between the magnetic moments of the Tb-ions and the Mn-ions in multiferroic TbMnO3, inelastic neutron scattering experiments (at the ILL in Grenoble and the Bragg Institute at ANSTO) are performed on isostructural, non-multiferroic TbVO3. Acoustic and optical magnon branches are identified at energies comparable to the spin wave excitation spectrum of YVO3. In addition, a crystal field excitation arising from the Tb-ions is identified at the energy of 14.9 meV. This is substantially larger than the crystal field excitation at 4.5 meV in TbMnO3.
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    Investigations into the magnetic and crystal field excitations of the orthorhombically distorted perovskites RVO3 (R=Dy, Tb, Pr, Ce)
    (Australian Institute of Physics, 2013-02-06) Reynolds, NM; Rovillain, P; Danilkin, SA; Schmalzl, K; Reehuis, M; Miyasaka, S; Fujioka, F; Tokura, Y; Keimer, B; McIntyre, GJ; Ulrich, C
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    Investigations into the magnetic and crystal field excitations of the orthorhombically distorted perovskites TbVO3 and CeVO3
    (Australian Institute of Physics, 2018-01-30) O'Brien, J; Reynolds, N; Rovillain, P; Danilkin, SA; Schmalzl, K; Reehuis, M; Mole, RA; Miyasaka, S; Fujioka, F; Tokura, Y; Keimer, B; McIntyre, GJ; Ulrich, C
    Inelastic neutron scattering experiments have been performed on a series of vanadates, in particular TbVO3 and CeVO3, to categorise the crystal field and magnetic excitations. The vanadates possess a configuration with corner sharing, distorted VO6 octahedra (space group Pbnm) with a collinear C-type antiferromagnetic structure occurring below Néel temperatures of TN = 110 K and 124 K respectively. Data from neutron scattering experiments reveal a hitherto unobserved shift of crystal field excitation energy in TbVO3 and CeVO3. Point-charge model calculations have confirmed this shift by theoretically calculating the crystal field excitation spectrum. We propose that the mechanism behind the effect is the onset of local magnetism caused by the ordering of the vanadium sublattice at the magnetic phase transition. This magnetic exchange field from the vanadium ions polarises the spins of the rare-earth ions located at the centre of the unit cell. This results in a Zeeman-like splitting of crystal field energy levels. As a result, crystal field transition energies demonstrate a linear shift as a function of internal magnetic field strength.
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    Investigations of the magnetic and crystal field excitations in orthorhombically distorted perovskites RVO3 (R=Dy, Tb, Pr, Ce)
    (Australian Institute of Physics, 2017-01-31) O'Brien, J; Reynolds, NM; Mole, RA; Rovillain, P; Danilkin, SA; Schmalzl, K; Reehuis, M; Miyasaka, S; Fujioka, F; Tokura, Y; Keimer, B; McIntyre, GJ; Ulrich, C
    Inelastic neutron scattering experiments have been performed on a series of vanadates, in particular DyVO3, TbVO3, PrVO3, and CeVO3, to categorise the crystal field and magnetic excitations. The vanadates are isostructural to the multiferroic manganites TbMnO3 and DyMnO3, with corner sharing, Jahn-Teller distorted VO6 octahedra (orthorhombic space group Pbnm). However, they posses a collinear C-type antiferromagnetic structure, instead of an incommensurate spin arrangement as in the manganites. In the vanadates, the antiferromagnetic order sets in below Neel temperatures of TN = 110 K to 124 K [1-5]. Using inelastic neutron scattering on single crystals we were able to determine the crystal field spectrum and spin wave dispersion relations independently. In order to determine the nature of the crystal field excitations of these materials and in order to understand how the magnetic and crystal field excitations influence one another, we have theoretically calculated the crystal field excitation spectrum. The results are compared to the crystal field and spin wave excitations in the multiferroic maganites [6], in order to obtain a deeper understanding of the coupling mechanism between the rare earth elements and the transition metals in RVO3 and RMnO3, respectively.
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    Local metallic and structural properties of the strongly correlated metal LaNiO3 using 8Li β–NMR
    (American Physical Society, 2019-10-07) Karner, VL; Chatzichristos, A; Cortie, DL; Dehn, MH; Foyevtsov, O; Foyevtsova, K; Fujimoto, D; Kiefl, RF; Levy, CDP; Li, R; McFadden, RML; Morris, GD; Pearson, MR; Stachura, M; Ticknor, JO; Cristiani, G; Logvenov, G; Wrobel, F; Keimer, B; Zhang, J; Mitchell, JF; MacFarlane, WA
    We report β-detected NMR of ion-implanted Li8 in a single crystal and thin film of the strongly correlated metal LaNiO3. Spin-lattice relaxation measurements reveal two distinct local environments, both metallic as evident from T-linear Korringa 1/T1 below 200K with slopes comparable to other metals. A small approximately temperature-independent Knight shift of ∼74ppm is observed, yielding a normalized Korringa product characteristic of substantial antiferromagnetic correlations. We find no evidence for a magnetic transition from 4 to 310K. The similarity of these features in the two very different samples indicates that they are intrinsic and unrelated to dilute oxygen vacancies. We attribute the two environments to two distinct but similar crystallographic Li8 sites and not to any form of phase inhomogeneity, but this is inconsistent with the conventional rhombohedral structure of LaNiO3, and also cannot be simply explained by the common alternative orthorhombic or monoclinic distortions. ©2024 American Physical Society.
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    Long-range transfer of electron-phonon coupling in oxide superlattices
    (Australian Institute of Physics, 2013-02-06) Ulrich, C; Driza, AN; Blanco-Canosa, SA; Bakr, M; Soltan, S; Khalid, S; Mustafa, L; Kawashima, K; Christiani, G; Habermeier, HU; Khaliullin, G; Le Tacon, M; Keimer, B
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    Long-range transfer of electron-phonon coupling in oxide superlattices
    (Nature Publishing Group, 2012-08-01) Driza, N; Blanco-Canosa, S; Bakr, M; Soltan, S; Khalid, M; Mustafa, L; Kawashima, K; Christiani, G; Habermeier, HU; Khaliullin, G; Ulrich, C; Le Tacon, M; Keimer, B
    The electron-phonon interaction is of central importance for the electrical and thermal properties of solids, and its influence on superconductivity, colossal magnetoresistance and other many-body phenomena in correlated-electron materials is the subject of intense research at present. However, the non-local nature of the interactions between valence electrons and lattice ions, often compounded by a plethora of vibrational modes, presents formidable challenges for attempts to experimentally control and theoretically describe the physical properties of complex materials. Here we report a Raman scattering study of the lattice dynamics in superlattices of the high-temperature superconductor YBa2Cu3O7 (YBCO) and the colossal-magnetoresistance compound La2/3Ca1/3MnO3 that suggests a new approach to this problem. We find that a rotational mode of the MnO6 octahedra in La2/3Ca1/3MnO3 experiences pronounced superconductivity-induced line-shape anomalies, which scale linearly with the thickness of the YBCO layers over a remarkably long range of several tens of nanometres. The transfer of the electron-phonon coupling between superlattice layers can be understood as a consequence of long-range Coulomb forces in conjunction with an orbital reconstruction at the interface. The superlattice geometry thus provides new opportunities for controlled modification of the electron-phonon interaction in complex materials. © 2012, Nature Publishing Group.
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    Magnetic phase diagram of Sr3Fe2O7-delta
    (American Physical Society, 2013-06-10) Peets, DC; Kim, JH; Dosanjh, P; Reehuis, M; Maljuk, A; Aliouane, N; Ulrich, C; Keimer, B
    Magnetometry, electrical transport, and neutron scattering measurements were performed on single crystals of the Fe4+-containing perovskite-related phase Sr3Fe2O7−δ as a function of oxygen content. Although both the crystal structure and electron configuration of this compound are closely similar to those of well-studied ruthenates and manganates, it exhibits very different physical properties. The fully oxygenated compound (δ=0) exhibits a charge-disproportionation transition at TD=340 K, and an antiferromagnetic transition at TN=115 K. For temperatures T≤TD, the material is a small-gap insulator; the antiferromagnetic order is incommensurate, which implies competing exchange interactions between the Fe4+ moments. The fully deoxygenated compound (δ=1) is highly insulating, and its Fe3+ moments exhibit commensurate antiferromagnetic order below TN∼600 K. Compounds with intermediate δ exhibit different order with lower TN, likely as a consequence of frustrated exchange interactions between Fe3+ and Fe4+ sublattices. A previous proposal that the magnetic transition temperature reaches zero is not supported. © 2013, American Physical Society.
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    Magnetic proximity effect in YBa2Cu3O7/La2/3Ca1/3MnO3 and YBa2Cu3O7/LaMnO3+delta superlattices
    (Amnerican Physical Society, 2012-05-07) Satapathy, DK; Uribe-Laverde, MA; Marozau, I; Malik, VK; Das, S; Wagner, T; Marcelot, C; Stahn, J; Bruck, S; Ruhm, A; Macke, S; Tietze, T; Goering, E; Frano, A; Kim, JH; Wu, M; Benckiser, E; Keimer, B; Devishvili, A; Toperverg, BP; Merz, M; Nagel, P; Schuppler, S; Bernhard, C
    Using neutron reflectometry and resonant x-ray techniques we studied the magnetic proximity effect (MPE) in superlattices composed of superconducting YBa(2)Cu(3)O(7) and ferromagnetic-metallic La(0.67)Ca(0.33)MnO(3) or ferromagnetic-insulating LaMnO(3+delta). We find that the MPE strongly depends on the electronic state of the manganite layers, being pronounced for the ferromagnetic-metallic La(0.67)Ca(0.33)MnO(3) and almost absent for ferromagnetic-insulating LaMnO(3+delta). We also detail the change of the magnetic depth profile due to the MPE and provide evidence for its intrinsic nature. © 2012, American Physical Society.
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    Momentum dependence of orbital excitations in Mott-insulating titanates
    (American Physical Society, 2009-09-04) Ulrich, C; Ament, LJP; Ghiringhelli, G; Braicovich, L; Moretti Sala, M; Pezzotta, N; Schmitt, T; Khaliullin, G; van den Brink, J; Roth, H; Lorenz, T; Keimer, B
    High-resolution resonant inelastic x-ray scattering has been used to determine the momentum dependence of orbital excitations in Mott-insulating LaTiO(3) and YTiO(3) over a wide range of the Brillouin zone. The data are compared to calculations in the framework of lattice-driven and superexchange-driven orbital ordering models. A superexchange model in which the experimentally observed modes are attributed to two-orbiton excitations yields the best description of the data. 2008, American Physical Society.
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    Neutron diffraction study of spin and charge ordering in SrFeO3-delta
    (American Physical Society, 2012-05-22) Reehuis, M; Ulrich, C; Maljuk, A; Niedermayer, C; Ouladdiaf, B; Hoser, A; Hofmann, T; Keimer, B
    We report a comprehensive neutron diffraction study of the crystal structure and magnetic order in a series of single-crystal and powder samples of SrFeO3-delta in the vacancy range 0 <= delta <= 0.23. The data provide detailed insights into the interplay between the oxygen vacancy order and the magnetic structure of this system. In particular, a crystallographic analysis of data on Sr8Fe8O23 revealed a structural transition between the high-temperature tetragonal and a low-temperature monoclinic phase with a critical temperature T = 75 K, which originates from charge ordering on the Fe sublattice and is associated with a metal-insulator transition. Our experiments also revealed a total of seven different magnetic structures of SrFeO3-delta in this range of delta, only two of which namely an incommensurate helix state in SrFeO3 and a commensurate, collinear antiferromagnetic state in Sr4Fe4O11) had been identified previously. We present a detailed refinement of some of the magnetic ordering patterns and discuss the relationship between the magnetotransport properties of SrFeO3-delta samples and their phase composition and magnetic microstructure. © 2012, American Physical Society.
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    Phonon anomalies in pure and underdoped R1-xKxFe2As2 (R=Ba, Sr) investigated by raman light scattering
    (American Physical Society, 2009-08) Rahlenbeck, M; Sun, GL; Sun, DL; Lin, CT; Keimer, B; Ulrich, C
    We present a detailed temperature-dependent Raman light scattering study of optical phonons in Ba1-xKxFe2As2 (x similar to 0.28, superconducting T-c similar to 29 K), Sr1-xKxFe2As2 (x similar to 0.15, T-c similar to 29 K), and nonsuperconducting BaFe2As2 single crystals. In all samples we observe a strong continuous narrowing of the Raman-active Fe and As vibrations upon cooling below the spin-density wave transition T-s. We attribute this effect to the opening of the spin-density wave gap. The electron-phonon linewidths inferred from these data greatly exceed the predictions of ab initio density-functional calculations without spin polarization, which may imply that local magnetic moments survive well above T-s. A first-order structural transition accompanying the spin-density wave transition induces discontinuous jumps in the phonon frequencies. These anomalies are increasingly suppressed for higher potassium concentrations. We also observe subtle phonon anomalies at the superconducting transition temperature T-c with a behavior qualitatively similar to that in the cuprate superconductors. © 2009, American Physical Society
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    Raman light scattering study and microstructural analysis of epitaxial films of the electron-doped superconductor La-2 (-x) Ce (x) CuO4
    (Springer, 2010-06) Rahlenbeck, M; Wagenknecht, M; Tsukada, A; Koelle, D; Kleiner, R; Keimer, B; Ulrich, C
    We present a detailed temperature-dependent Raman light scattering study of optical phonons in molecular-beam-epitaxy-grown films of the electron-doped superconductor La2 -x Ce x CuO4 close to optimal doping (x ~ 0.08, T c = 29 K and x ~ 0.1, T c = 27 K). The main focus of this work is a detailed characterization and microstructural analysis of the films. Based on micro-Raman spectroscopy in combination with X-ray diffraction, energy-dispersive X-ray analysis, and scanning electron microscopy, some of the observed phonon modes can be attributed to micron-sized inclusions of Cu2O. In the slightly underdoped film (x ~ 0.08), both the Cu2O modes and others that can be assigned to the La2 -x Ce x CuO4 matrix show pronounced softening and narrowing upon cooling below T ~ T c . Based on control measurements on commercial Cu2O powders and on a comparison to prior Raman scattering studies of other high-temperature superconductors, we speculate that proximity effects at La2 -x Ce x CuO4/Cu2O interfaces may be responsible for these anomalies. Experiments on the slightly overdoped La2 -x Ce x CuO4 film (x ~ 0.1) did not reveal comparable phonon anomalies. © 2010, Springer.
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    Structural and magnetic phase transitions of the orthovanadates RVO3 (R= Dy, Ho, Er) as seen via neutron diffraction
    (American Physical Society, 2011-02-10) Reehuis, M; Ulrich, C; Prokeš, K; Mat'aš, S; Fujioka, J; Miyasaka, S; Tokura, Y; Keimer, B
    The structural and magnetic phase behavior of RVO3 with R=v Dy, Ho, and Er was studied by single-crystal neutron diffraction. Upon cooling, all three compounds show structural transitions from orthorhombic (space group Pbnm) to monoclinic (p21/b) symmetry due to the onset of orbital order at T= 188–200 K, followed by Néel transitions at T= 110–113 K due to the onset of antiferromagnetic (C-type) order of the vanadium moments. Upon further cooling, additional structural phase transitions occur for DyVO3 and ErVO3 at 60 and 56 K, respectively, where the monoclinic structure changes to an orthorhombic structure with the space group Pbnm, and the magnetic order of the V sublattice changes to a G-type structure. These transition temperatures are reduced compared to the ones previously observed for nonmagnetic R3+ ions due to exchange interactions between the V3+ and R3+ ions. For ErVO3, R-R exchange interactions drive a transition to collinear magnetic order at T= 2.5 K. For HoVO3, the onset of noncollinear, weakly ferromagnetic order of the Ho moments nearly coincides with the structural phase transition from the monoclinic to the low-temperature orthorhombic structure. This transition is characterized by an extended hysteresis between 24 and 36 K. The Dy moments in DyVO3 also exhibit noncollinear, weakly ferromagnetic order upon cooling below 13 K. With increasing temperature, the monoclinic structure of DyVO3 reappears in the temperature range between 13 and 23 K. This reentrant structural transition is associated with a rearrangement of the Dy moments. A group theoretical analysis showed that the observed magnetic states of the R3+ ions are compatible with the lattice structure. The results are discussed in the light of recent data on the magnetic field dependence of the lattice structure and magnetization of DyVO3 and HoVO3. © 2011, American Physical Society