T. Gog

Argonne National Laboratory, Lemont, Illinois, United States

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Publications (115)404.52 Total impact

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    ABSTRACT: Heisenberg interactions are ubiquitous in magnetic materials and have been prevailing in modeling and designing quantum magnets. Bond-directional interactions offer a novel alternative to Heisenberg exchange and provide the building blocks of the Kitaev model, which has a quantum spin liquid (QSL) as its exact ground state. Honeycomb iridates, A2IrO3 (A=Na,Li), offer potential realizations of the Kitaev model, and their reported magnetic behaviors may be interpreted within the Kitaev framework. However, the extent of their relevance to the Kitaev model remains unclear, as evidence for bond-directional interactions remains indirect or conjectural. Here, we present direct evidence for dominant bond-directional interactions in antiferromagnetic Na2IrO3 and show that they lead to strong magnetic frustration. Diffuse magnetic x-ray scattering reveals broken spin-rotational symmetry even above Neel temperature, with the three spin components exhibiting nano-scale correlations along distinct crystallographic directions. This spin-space and real-space entanglement directly manifests the bond-directional interactions, provides the missing link to Kitaev physics in honeycomb iridates, and establishes a new design strategy toward frustrated magnetism.
    Nature Physics 04/2015; DOI:10.1038/nphys3322 · 20.60 Impact Factor
  • Physical Review Letters 03/2015; 114:126401. · 7.73 Impact Factor
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    ABSTRACT: We combine resonant inelastic x-ray scattering and model calculations in the Kondo lattice compound YbInCu_{4}, a system characterized by a dramatic increase in Kondo temperature and associated valence fluctuations below a first-order valence transition at T≃42 K. The bulk-sensitive, element-specific, and valence-projected charge excitation spectra reveal an unusual quasigap in the Yb-derived state density which drives an instability of the electronic structure and renormalizes the low-energy effective Hamiltonian at the transition. Our results provide long-sought experimental evidence for a link between temperature-driven changes in the low-energy Kondo scale and the higher-energy electronic structure of this system.
    Physical Review Letters 03/2015; 114(12):126401. · 7.73 Impact Factor
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    ABSTRACT: We present a refined and improved study of the influence of screening on the effective fine structure constant of graphene, $\alpha^\ast$, as measured in graphite using inelastic x-ray scattering. This follow-up to our previous study[1] was carried out with two times better momentum resolution, five times better energy resolution, and improved techniques for reducing experimental background. We compare our results to RPA calculations and evaluate the relative importance of interlayer hopping, excitonic corrections, and screening from higher energy excitations. We find that the static, limiting value of $\alpha^\ast$ falls in the range 0.25 to 0.35, which is higher than our previous result of 0.14[1], but still below the value expected from RPA. Graphitic effects from interlayer hopping were found to play a negligible role in the range of momenta studied. Instead, the anomalous screening was found to arise from a combination of excitonic effects in the $\pi \rightarrow \pi^\ast$ continuum and screening from the tightly bound $\sigma$-bands. Both effects appear to fade in the limit of low momentum, suggesting a recovery of RPA in the asymptotic limit, but should have significant influence on low-energy measurements on length scales less than a few nm, such as STM.
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    ABSTRACT: We present observations of the Ni K-edge resonant inelastic X-ray scattering (RIXS) in NiO x thin films showing unipolar resistive switching (RS). The RIXS spectra of RS NiO x thin films can be described in terms of crystal field (dd) and charge transfer (CT) excitations. We found distorted dd excitations in the films' pristine state before electroforming, and identical excitations for high and low resistance states after electroforming. This suggests that the RS property of NiO x thin film is related to defects in pristine NiO x films, and RS occurs in local nanosized spots too small to be detected by RIXS.
    Applied Physics Express 01/2015; 8(2):021101. DOI:10.7567/APEX.8.021101 · 2.57 Impact Factor
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    ABSTRACT: We report an Fe $K$-edge resonant inelastic X-ray scattering (RIXS) study of K$_{0.83}$Fe$_{1.53}$Se$_2$. This material is an insulator, unlike many parent compounds of iron-based superconductors. We found a sharp excitation around 1 eV, which is resonantly enhanced when the incident photon energy is tuned near the pre-edge region of the absorption spectrum. Through momentum dependence measurements we find that this excitation has two contributions; a dispersive $0.8-1$ eV excitation and a momentum-independent excitation at $\sim\!1.15$ eV. Calculations based on a 70 band $dp$ orbital model, using a moderate $U_{\rm eff}\approx 2.5$ eV, indicate that the $\sim\!0.9$ eV feature originates from the correlated Fe 3$d$ electrons, with a dominant $d_{xz}$ and $d_{yz}$ orbital character, while the $\sim\!\!1.15$ eV feature arises from $4p$ interband transitions as a result of the Fe vacancy order. We find that a moderate $U_{\rm eff}$ yields a satisfying agreement with the experimental spectra, suggesting that the electron correlations in the insulating and metallic iron based superconductors are comparable.
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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 07/2014; 5:4453. DOI:10.1038/ncomms5453 · 10.74 Impact Factor
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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. DOI:10.1103/PhysRevLett.112.147201 · 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.
    Physical Review B 02/2014; 89(11). DOI:10.1103/PhysRevB.89.115111 · 3.66 Impact Factor
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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. DOI:10.1103/PhysRevLett.112.056401 · 7.73 Impact Factor
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    ABSTRACT: The dynamical theory of x-ray diffraction and the coupled-wave theory for modeling diffraction of light from periodic structures are two equivalent theories but with incompatibilities, as they were developed independently along two parallel directions in history. Here we reformulate the two theories into a universal Fourier coupled-wave diffraction theory (FCWDT), in which the fundamental coupled-wave equations for almost all practical diffraction geometry can always be written as a straightforward eigenvalue equation that is easily solvable by standard mathematical procedures. Since it removes most of the approximations and complexities in the two conventional theories, the FCWDT is almost rigorous yet simple and, in principle, can be used to compute scattering of electromagnetic waves from any kinds of periodic (nonmagnetic) structures, including x-ray diffraction from crystals and soft x-ray and light diffraction from periodic multilayers, gratings, and photonic crystals.
    Physical Review A 06/2013; 87(6). DOI:10.1103/PhysRevA.87.063828 · 2.99 Impact Factor
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    ABSTRACT: MERIX (Medium Energy Resolution Inelastic X-ray Scattering) is an instrument for Resonant Inelastic X-ray Scattering (RIXS) studies, in the hard X-ray regime, designed to work with photons in the 5–12 keV range, spanning atomic resonances near the K-edges of 3d elements, and the L-edges of 4f and 5d elements. The energy analysis of inelastically scattered photons is performed with segmented spherical crystal analyzers in close to Bragg backscattering geometry. For each resonance (edge) a specially designed analyzer is used, fabricated from Ge, Si, or LiNbO3 crystals. MERIX uses a position sensitive (micro-strip) detector to take snapshots of IXS spectra which are dispersed in space and over an energy range of a few eV with ≃20–40 meV energy resolution. The spectral resolution of the MERIX spectrometer depends on the analyzer and varies from ≃45 meV to ≃170 meV, while the momentum transfer resolution is ≃1–4 nm−1. Samples are illuminated by micro-focused beams of size ≃10 μm × 45 μm, allowing for studies at high-pressure and other extreme conditions. Polarization selectivity is ensured by vertical or horizontal momentum transfer scans. MERIX features ≃100 times higher count-rates compared to previously built RIXS instruments.
    Journal of Electron Spectroscopy and Related Phenomena 06/2013; 188:140-149. DOI:10.1016/j.elspec.2012.09.003 · 1.55 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). DOI:10.1103/PhysRevB.87.220407 · 3.66 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 $(0, 0.4pi) for all doping points regardless of the stripe ordered state, suggesting that the corresponding structure is not directly related to stripe order. Measurements with Ei = 9.003 keV on metallic La(1.7)Sr(0.3)CuO(4) and Bi(1.76)Pb(0.35)Sr(1.89)CuO(6+d) exhibit a dispersive intra-band excitation below 4 eV, similar to that observed in the electron-doped Nd(1.85)Ce(0.15)CuO(4). This is the first observation of a dispersive intra-band excitation in a hole doped system, evidencing that both electron and hole doped systems have a similar dynamical charge correlation function.
    Physical review. B, Condensed matter 02/2013; 87(10). DOI:10.1103/PhysRevB.87.104511 · 3.66 Impact Factor
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    ABSTRACT: The electronic structure of the honeycomb lattice iridates Na2IrO3 and Li2IrO3 has been investigated using resonant inelastic x-ray scattering (RIXS). Crystal-field-split d-d excitations are resolved in the high-resolution RIXS spectra. In particular, the splitting due to noncubic crystal fields, derived from the splitting of jeff=3/2 states, is much smaller than the typical spin-orbit energy scale in iridates, validating the applicability of jeff physics in A2IrO3. We also find excitonic enhancement of the particle-hole excitation gap around 0.4 eV, indicating that the nearest-neighbor Coulomb interaction could be large. These findings suggest that both Na2IrO3 and Li2IrO3 can be described as spin-orbit Mott insulators, similar to the square lattice iridate Sr2IrO4.
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    ABSTRACT: We report a combined experimental and theoretical study of the unusual ferromagnetism in the one-dimensional copper-iridium oxide Sr$_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). DOI:10.1103/PhysRevLett.111.057202 · 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. DOI:10.1107/S0909049512043154 · 3.02 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). DOI:10.1103/PhysRevLett.110.047003 · 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. DOI:10.1103/PhysRevLett.109.157401 · 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. DOI:10.1103/PhysRevLett.109.157402 · 7.73 Impact Factor

Publication Stats

2k Citations
404.52 Total Impact Points

Institutions

  • 2000–2015
    • Argonne National Laboratory
      • Division of Materials Science
      Lemont, Illinois, United States
  • 2014
    • Universität Stuttgart
      • Institute of Theoretical Chemistry
      Stuttgart, Baden-Württemberg, Germany
    • Technische Universität Dresden
      • Department of Physics
      Dresden, Saxony, Germany
  • 2005–2006
    • University of Pennsylvania
      • Department of Chemistry
      Philadelphia, PA, United States
    • Bar Ilan University
      • Department of Physics
      Ramat Gan, Tel Aviv, Israel
    • Stanford University
      • Department of Applied Physics
      Stanford, CA, United States