T. Gog

Universität Stuttgart, Stuttgart, Baden-Württemberg, Germany

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Publications (110)339.25 Total impact

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    ABSTRACT: We report x-ray resonant magnetic scattering and resonant inelastic x-ray scattering studies of epitaxially strained Sr2IrO4 thin films. The films were grown on SrTiO3 and (LaAlO3)0.3(Sr2AlTaO6)0.7 substrates, under slight tensile and compressive strains, respectively. Although the films develop a magnetic structure reminiscent of bulk Sr2IrO4, the magnetic correlations are extremely anisotropic, with in-plane correlation lengths significantly longer than the out-of-plane correlation lengths. In addition, the compressive (tensile) strain serves to suppress (enhance) the magnetic ordering temperature TN, while raising (lowering) the energy of the zone-boundary magnon. Quantum chemical calculations show that the tuning of magnetic energy scales can be understood in terms of strain-induced changes in bond lengths.
    Physical Review Letters 04/2014; 112(14):147201. · 7.73 Impact Factor
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    ABSTRACT: In the search for topological phases in correlated electron systems, materials with 5d transition-metal ions, in particular the iridium-based pyrochlores A2Ir2O7, provide fertile grounds. Several topological states have been predicted but the actual realization of such states is believed to critically depend on the strength of local potentials arising from distortions of the IrO6 cages. We test this hypothesis by measuring with resonant inelastic x-ray scattering the electronic level splittings in the A =Y, Eu systems, which we show to agree very well with ab initio quantum chemistry electronic-structure calculations for the series of materials with A =Sm, Eu, Lu, and Y. We find, however, that the primary source for quenching the spin-orbit interaction is not a distortion of the IrO6 octahedra but longer-range lattice anisotropies which inevitably break the local cubic symmetry.
    02/2014; 89(11).
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    ABSTRACT: Vanadium sesquioxide, V2O3, is a prototypical metal-to-insulator system where, in temperature-dependent studies, the transition always coincides with a corundum-to-monoclinic structural transition. As a function of pressure, V2O3 follows the expected behavior of increased metallicity due to a larger bandwidth for pressures up to 12.5 GPa. Surprisingly, for higher pressures when the structure becomes unstable, the resistance starts to increase. Around 32.5 GPa at 300 K, we observe a novel pressure-induced corundum-to-monoclinic transition between two metallic phases, showing that the structural phase transition can be decoupled from the metal-insulator transition. Using x-ray Raman scattering, we find that screening effects, which are strong in the corundum phase, become weakened at high pressures. Theoretical calculations indicate that this can be related to a decrease in coherent quasiparticle strength, suggesting that the high-pressure phase is likely a critical correlated metal, on the verge of Mott-insulating behavior.
    Physical Review Letters 02/2014; 112(5):056401. · 7.73 Impact Factor
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    ABSTRACT: In condensed matter systems, out of a large number of interacting degrees of freedom emerge weakly coupled quasiparticles (QPs), in terms of which most physical properties are described. The lack of identification of such QPs is a major barrier for understanding myriad exotic properties of correlated electrons, such as unconventional superconductivity and non-Fermi liquid behaviours. Here we report the observation of a composite particle in a quasi-two-dimensional spin-1/2 antiferromagnet Sr2IrO4-an exciton dressed with magnons-that propagates with the canonical characteristics of a QP: a finite QP residue and a lifetime longer than the hopping time scale. The dynamics of this charge-neutral excitation mirrors the fundamental process of the analogous one-hole propagation in the background of spins-1/2, and reveals the same intrinsic dynamics that is obscured for a single, charged-hole doped into two-dimensional cuprates.
    Nature Communications 01/2014; 5:4453. · 10.74 Impact Factor
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    ABSTRACT: The low energy excitations in Na2IrO3 have been investigated using resonant inelastic x-ray scattering (RIXS). A magnetic excitation branch can be resolved, whose dispersion reaches a maximum energy of about 35 meV at the \Gamma-point. The momentum dependence of the excitation energy is much larger along the \Gamma-X direction compared to that along the \Gamma-Y direction. The observed dispersion relation is consistent with a recent theoretical prediction based on Heisenberg-Kitaev model. At high temperatures, we find large contributions from lattice vibrational modes to our RIXS spectra, suggesting that a strong electron-lattice coupling is present in Na2IrO3.
    Physical review. B, Condensed matter 04/2013; 87(22). · 3.77 Impact Factor
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    ABSTRACT: We have performed resonant inelastic x-ray scattering (RIXS) near the Cu-K edge on cuprate superconductors La(2-x)Sr(x)CuO(4), La(2-x)Ba(x)CuO(4), La(2-x)Sr(x)Cu(1-y)Fe(y)O(4) and Bi(1.76)Pb(0.35)Sr(1.89)CuO(6+d), covering underdoped to heavily overdoped regime and focusing on charge excitations inside the charge-transfer gap. RIXS measurements of the 214 systems with Ei = 8.993 keV have revealed that the RIXS intensity at 1 eV energy transfer has a minimum at (0,0) and maxima at (0.4pi, 0) and $(0, 0.4pi) for all doping points regardless of the stripe ordered state, suggesting that the corresponding structure is not directly related to stripe order. Measurements with Ei = 9.003 keV on metallic La(1.7)Sr(0.3)CuO(4) and Bi(1.76)Pb(0.35)Sr(1.89)CuO(6+d) exhibit a dispersive intra-band excitation below 4 eV, similar to that observed in the electron-doped Nd(1.85)Ce(0.15)CuO(4). This is the first observation of a dispersive intra-band excitation in a hole doped system, evidencing that both electron and hole doped systems have a similar dynamical charge correlation function.
    Physical review. B, Condensed matter 02/2013; 87(10). · 3.77 Impact Factor
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    ABSTRACT: The electronic structure of the honeycomb lattice iridates Na2IrO3 and Li2IrO3 has been investigated using resonant inelastic x-ray scattering (RIXS). Crystal-field-split d-d excitations are resolved in the high-resolution RIXS spectra. In particular, the splitting due to noncubic crystal fields, derived from the splitting of jeff=3/2 states, is much smaller than the typical spin-orbit energy scale in iridates, validating the applicability of jeff physics in A2IrO3. We also find excitonic enhancement of the particle-hole excitation gap around 0.4 eV, indicating that the nearest-neighbor Coulomb interaction could be large. These findings suggest that both Na2IrO3 and Li2IrO3 can be described as spin-orbit Mott insulators, similar to the square lattice iridate Sr2IrO4.
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    ABSTRACT: We report a combined experimental and theoretical study of the unusual ferromagnetism in the one-dimensional copper-iridium oxide Sr$_3$CuIrO$_6$. Utilizing Ir $L_3$ edge resonant inelastic x-ray scattering, we reveal a large gap magnetic excitation spectrum. We find that it is caused by an unusual exchange anisotropy generating mechanism, namely, strong ferromagnetic anisotropy arising from antiferromagnetic superexchange, driven by the alternating strong and weak spin-orbit coupling on the $5d$ Ir and 3d Cu magnetic ions, respectively. From symmetry consideration, this novel mechanism is generally present in systems with edge-sharing Cu$^{2+}$O$_4$ plaquettes and Ir$^{4+}$O$_6$ octahedra. Our results point to unusual magnetic behavior to be expected in mixed 3d-5d transition-metal compounds via exchange pathways that are absent in pure 3d or 5d compounds.
    Physical Review Letters 02/2013; 111(5). · 7.73 Impact Factor
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    ABSTRACT: Resonant inelastic X-ray scattering (RIXS) experiments require special sets of near-backscattering spherical diced analyzers and high-resolution monochromators for every distinct absorption-edge energy and emission line. For the purpose of aiding the design and planning of efficient RIXS experiments, comprehensive lists of suitable analyzer reflections for silicon, germanium, α-quartz, sapphire and lithium niobate crystals were compiled for a multitude of absorption edges and emission lines. Analyzers made from lithium niobate, sapphire or α-quartz offer many choices of reflections with intrinsic resolutions currently unattainable from silicon or germanium. In some cases these materials offer higher intensities at comparable resolutions. While lithium niobate, sapphire or α-quartz analyzers are still in an early stage of development, the present compilation can serve as a computational basis for assessing expected and actual performance. With regard to high-resolution monochromators, bandpass and throughput calculations for combinations of double-crystal, high-heat-load and near-backscattering high-resolution channel-cuts were assembled. The compilation of these analyzer and monochromator data is publicly available on a website.
    Journal of Synchrotron Radiation 01/2013; 20(Pt 1):74-9. · 2.19 Impact Factor
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    ABSTRACT: We report a Fe K\beta x-ray emission spectroscopy study of local magnetic moments in the rare-earth doped iron pnictide Ca_{1-x}RE_xFe_2As_2 (RE=La, Pr, and Nd). In all samples studied the size of the Fe local moment is found to decrease significantly with temperature and goes from ~0.9 \mu_B at T = 300 K to ~0.45 \mu_B at T = 70 K. In the collapsed tetragonal (cT) phase of Nd- and Pr-doped samples (T<70K) the local moment is quenched, while the moment remains unchanged for the La-doped sample, which does not show lattice collapse. Our results show that Ca_{1-x}RE_xFe_2As_2 (RE= Pr and Nd) exhibits a spin-state transition and provide direct evidence for a non-magnetic Fe^{2+} ion in the cT-phase, as predicted by Yildirim. We argue that the gradual change of the the spin-state over a wide temperature range reveals the importance of multiorbital physics, in particular the competition between the crystal field split Fe 3d orbitals and the Hund's rule coupling.
    Physical Review Letters 12/2012; 110(4). · 7.73 Impact Factor
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    ABSTRACT: The electronic structure of Sr_{3}CuIrO_{6}, a model system for the 5d Ir ion in an octahedral environment, is studied through a combination of resonant inelastic x-ray scattering and theoretical calculations. Resonant inelastic x-ray scattering spectra at the Ir L_{3} edge reveal an Ir t_{2g} manifold that is split into three levels, in contrast to the expectations of the strong spin-orbit-coupling limit. Effective Hamiltonian and ab inito quantum chemistry calculations find a strikingly large noncubic crystal field splitting comparable to the spin-orbit coupling, which results in a strong mixing of the j_{eff}=1/2 and j_{eff}=3/2 states and modifies the isotropic wave functions on which many theoretical models are based.
    Physical Review Letters 10/2012; 109(15):157401. · 7.73 Impact Factor
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    ABSTRACT: Using resonant inelastic x-ray scattering, we observe in the bilayer iridate Sr_{3}Ir_{2}O_{7}, a spin-orbit coupling driven magnetic insulator with a small charge gap, a magnon gap of ≈92  meV for both acoustic and optical branches. This exceptionally large magnon gap exceeds the total magnon bandwidth of ≈70  meV and implies a marked departure from the Heisenberg model, in stark contrast to the case of the single-layer iridate Sr_{2}IrO_{4}. Analyzing the origin of these observations, we find that the giant magnon gap results from bond-directional pseudodipolar interactions that are strongly enhanced near the metal-insulator transition boundary. This suggests that novel magnetism, such as that inspired by the Kitaev model built on the pseudodipolar interactions, may emerge in small charge-gap iridates.
    Physical Review Letters 10/2012; 109(15):157402. · 7.73 Impact Factor
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    ABSTRACT: We demonstrate a strong dependence of high-resolution 1s3p (or K β) resonant x-ray emission spectroscopy (RXES) on local spin correlations at high pressure. We show that the pre-edge region in K β RXES of CoO can be separated into a local quadrupolar contribution and nonlocal dipolar intensity, which arises from the mixing of cobalt 4p states with neighboring cobalt 3d orbitals. For pressures of 0–12 GPa, we observe a twofold increase in nonlocal contributions with respect to local contributions. For pressures greater than 12 GPa, the ratio remains nearly constant. The observed pressure dependence and strong intensity changes cannot be explained by a conventional picture of bond-length shortening and is ascribed to increased intersite 4p-3d mixing due to changes in magnetic ordering. These results are supported by theoretical calculations.
    Physical Review B 09/2012; · 3.66 Impact Factor
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    ABSTRACT: We used resonant inelastic x-ray scattering to reveal the nature of magnetic interactions in Sr2IrO4, a 5d transition-metal oxide with a spin-orbit entangled ground state and J(eff)=1/2 magnetic moments. The magnon dispersion in Sr2IrO4 is well-described by an antiferromagnetic Heisenberg model with an effective spin one-half on a square lattice, which renders the low-energy effective physics of Sr2IrO4 much akin to that in superconducting cuprates. This point is further supported by the observation of exciton modes in Sr2IrO4, whose dispersion is strongly renormalized by magnons, which can be understood by analogy to hole propagation in the background of antiferromagnetically ordered spins in the cuprates.
    Physical Review Letters 04/2012; 108(17):177003. · 7.73 Impact Factor
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    ABSTRACT: We report the first observation by momentum-resolved resonant inelastic x-ray scattering of charge excitations in an iron-based superconductor and its parent compound, PrFeAsO0.7 and PrFeAsO, respectively, with two main results. First, using calculations based on a 16-band dp model, we show that the energy of the lowest-lying excitations, identified as dd interband transitions of dominant xz,yz orbital character, exhibits a dramatic dependence on electron correlation. This enables us to estimate the Coulomb repulsion U and Hund's coupling J, and to highlight the role played by J in these peculiar orbital-dependent electron correlation effects. Second, we show that short-range antiferromagnetic correlations, which are a prerequisite to the occurrence of these excitations at the Γ point, are still present in the superconducting state.
    Physical Review B 01/2012; 86(11). · 3.66 Impact Factor
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    ABSTRACT: Resonant inelastic x-ray scattering (RIXS) is a powerful technique for studying electronic excitations in correlated electron systems. Current RIXS spectrometers measure the changes in energy and momentum of the photons scattered by the sample. A powerful extension of the RIXS technique is the measurement of the polarization state of the scattered photons which contains information about the symmetry of the excitations. This long-desired addition has been elusive because of significant technical challenges. This paper reports the development of a new diffraction-based polarization analyzer which discriminates between linear polarization components of the scattered photons. The double concave surface of the polarization analyzer was designed as a good compromise between energy resolution and throughput. Such a device was fabricated using highly oriented pyrolytic graphite for measurements at the Cu K-edge incident energy. Preliminary measurements on a CuGeO(3) sample are presented.
    The Review of scientific instruments 11/2011; 82(11):113108. · 1.52 Impact Factor
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    ABSTRACT: We used resonant inelastic x-ray scattering to reveal the nature of magnetic interactions in Sr2IrO4, a 5d transition-metal oxide with a spin-orbit entangled ground state and Jeff=1/2 magnetic momemts, referred to as 'isospins'. The magnon dispersion in Sr2IrO4 is well described by an antiferromagnetic Heisenberg model with isospin one-half moments on a square lattice, which renders the low-energy effective physics of Sr2IrO4 much akin to that in superconducting cuprates. This is further supported by the observation of exciton modes in Sr2IrO4 whose dispersion is strongly renormalized by magnons, which can be understood by analogy to the hole propagation in the background of antiferromagnetically ordered spins in the cuprates.
    10/2011;
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    ABSTRACT: The electronic structure of CuIr{sub 2}S{sub 4} is investigated using various bulk-sensitive x-ray spectroscopic methods near the Ir L{sub 3} edge: resonant inelastic x-ray scattering (RIXS), x-ray absorption spectroscopy in the partial fluorescence yield mode, and resonant x-ray emission spectroscopy. A strong RIXS signal (0.75 eV) resulting from a charge-density-wave gap opening is observed below the metal-insulator transition temperature of 230 K. The resultant modification of electronic structure is consistent with the density functional theory prediction. In the spin- and charge-dimer disordered phase induced by x-ray irradiation below 50 K, we find that a broad peak around 0.4 eV appears in the RIXS spectrum.
    Physical Review B 09/2011; 84(12). · 3.66 Impact Factor
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    ABSTRACT: We report a Fe Kβ x-ray emission spectroscopy study of local magnetic moments in various iron-based superconductors in their paramagnetic phases. Local magnetic moments are found in all samples studied: PrFeAsO, Ba(Fe,Co)2As2, LiFeAs, Fe1+x(Te,Se), and A2Fe4Se5 (where A=K, Rb, and Cs). The moment size is independent of temperature or carrier concentration but varies significantly across different families. Specifically, all iron pnictide samples have local moments of about 1μB/Fe, while FeTe and K2Fe4Se5 families have much larger local moments of ∼2μB/Fe and ∼3.3μB/Fe, respectively. Our results illustrate the importance of multiorbital physics in describing magnetism of these compounds.
    Physical Review B 07/2011; 84(10). · 3.66 Impact Factor
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    ABSTRACT: We present Ir L3-edge resonant inelastic x-ray scattering (RIXS) spectra and resonant x-ray emission spectra (RXES) on the thiospinel, CuIr2S4, which has been attracting much interest due to intriguing metal-insulator transitions. At room temperature CuIr2S4 is metallic, but goes through a metal insulator transition at TMI ˜ 226 K due to the formation of charge order (CO) of Ir^3+ and Ir^4+ together with spin dimerization between Ir^4+ ions. By exposing the sample to x-ray below T = 50 K, the crystal symmetry goes from triclinic to tetragonal, accompanied by reduced resistivity. The RIXS signal was dominated by a broad and strong feature around 3 eV, arising from t2g to eg transition, but we were able to observe a clear signature of opening of the insulating gap across the metal-insulator transition. In addition, we also found that this gap is partially filled in the irradiation-induced phase. The emission spectra reveals the existence of an excited Ir-5d t2g state, which is hidden in the Ir L3-edge XAS of CuIr2S4. The result indicates that the electronic reconstruction that takes place in the irradiation-induced phase comes from the Ir^3+ while the Ir^4+ dimers are unchanged.
    03/2011;

Publication Stats

1k Citations
339.25 Total Impact Points

Institutions

  • 2014
    • Universität Stuttgart
      • Institute of Theoretical Chemistry
      Stuttgart, Baden-Württemberg, Germany
    • Technische Universität Dresden
      • Department of Physics
      Dresden, Saxony, Germany
  • 2001–2014
    • Argonne National Laboratory
      Lemont, Illinois, United States
  • 2004–2010
    • University of Pennsylvania
      • Department of Chemistry
      Philadelphia, PA, United States
    • Sandia National Laboratories
      Albuquerque, New Mexico, United States
  • 2001–2008
    • Brookhaven National Laboratory
      • • Condensed Matter Physics & Materials Science Department
      • • Physics Department
      New York City, NY, United States
  • 2005
    • Norwegian University of Science and Technology
      • Department of Physics
      Trondheim, Sor-Trondelag Fylke, Norway
    • Northern Illinois University
      • Department of Physics
      DeKalb, IL, United States
    • Stanford University
      • Department of Applied Physics
      Stanford, CA, United States
    • Bar Ilan University
      • Department of Physics
      Ramat Gan, Tel Aviv, Israel
  • 2004–2005
    • Rutgers, The State University of New Jersey
      • Department Physics and Astronomy
      New Brunswick, NJ, United States