Publications by authors named "Claude Monney"

13 Publications

  • Page 1 of 1

Ultrafast Electronic Band Gap Control in an Excitonic Insulator.

Phys Rev Lett 2017 Aug 23;119(8):086401. Epub 2017 Aug 23.

Fritz-Haber-Institut der MPG, Faradayweg 4-6, 14195 Berlin, Germany.

We report on the nonequilibrium dynamics of the electronic structure of the layered semiconductor Ta_{2}NiSe_{5} investigated by time- and angle-resolved photoelectron spectroscopy. We show that below the critical excitation density of F_{C}=0.2  mJ cm^{-2}, the band gap narrows transiently, while it is enhanced above F_{C}. Hartree-Fock calculations reveal that this effect can be explained by the presence of the low-temperature excitonic insulator phase of Ta_{2}NiSe_{5}, whose order parameter is connected to the gap size. This work demonstrates the ability to manipulate the band gap of Ta_{2}NiSe_{5} with light on the femtosecond time scale.
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http://dx.doi.org/10.1103/PhysRevLett.119.086401DOI Listing
August 2017

Orbital breathing effects in the computation of x-ray d-ion spectra in solids by ab initio wave-function-based methods.

J Phys Condens Matter 2017 Jan 21;29(3):035502. Epub 2016 Nov 21.

Institute for Theoretical Solid State Physics, IFW Dresden, Helmholtzstr. 20, 01069 Dresden, Germany. Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany.

In existing theoretical approaches to core-level excitations of transition-metal ions in solids relaxation and polarization effects due to the inner core hole are often ignored or described phenomenologically. Here we set up an ab initio computational scheme that explicitly accounts for such physics in the calculation of x-ray absorption and resonant inelastic x-ray scattering spectra. Good agreement is found with experimental transition-metal L-edge data for the strongly correlated d cuprate LiCuO, for which we determine the absolute scattering intensities. The newly developed methodology opens the way for the investigation of even more complex d electronic structures of group VI B to VIII B correlated oxide compounds.
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http://dx.doi.org/10.1088/1361-648X/29/3/035502DOI Listing
January 2017

Electron-lattice interactions strongly renormalize the charge-transfer energy in the spin-chain cuprate Li2CuO2.

Nat Commun 2016 Feb 17;7:10563. Epub 2016 Feb 17.

Leibniz Institute for Solid State and Materials Research, IFW Dresden, Helmholtzstrasse 20, D-01171 Dresden, Germany.

Strongly correlated insulators are broadly divided into two classes: Mott-Hubbard insulators, where the insulating gap is driven by the Coulomb repulsion U on the transition-metal cation, and charge-transfer insulators, where the gap is driven by the charge-transfer energy Δ between the cation and the ligand anions. The relative magnitudes of U and Δ determine which class a material belongs to, and subsequently the nature of its low-energy excitations. These energy scales are typically understood through the local chemistry of the active ions. Here we show that the situation is more complex in the low-dimensional charge-transfer insulator Li2CuO2, where Δ has a large non-electronic component. Combining resonant inelastic X-ray scattering with detailed modelling, we determine how the elementary lattice, charge, spin and orbital excitations are entangled in this material. This results in a large lattice-driven renormalization of Δ, which significantly reshapes the fundamental electronic properties of Li2CuO2.
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http://dx.doi.org/10.1038/ncomms10563DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4757783PMC
February 2016

Orbital control of effective dimensionality: from spin-orbital fractionalization to confinement in the anisotropic ladder system CaCu(2)O(3).

Phys Rev Lett 2015 Mar 4;114(9):096402. Epub 2015 Mar 4.

IFW Dresden, Helmholtzstrasse 20, 01069 Dresden, Germany.

Fractionalization of an electronic quasiparticle into spin, charge, and orbital parts is a fundamental and characteristic property of interacting electrons in one dimension. However, real materials are never strictly one dimensional and the fractionalization phenomena are hard to observe. Here we studied the spin and orbital excitations of the anisotropic ladder material CaCu_{2}O_{3}, whose electronic structure is not one dimensional. Combining high-resolution resonant inelastic x-ray scattering experiments with theoretical model calculations, we show that (i) spin-orbital fractionalization occurs in CaCu_{2}O_{3} along the leg direction x through the xz orbital channel as in a 1D system, and (ii) no fractionalization is observed for the xy orbital, which extends in both leg and rung direction, contrary to a 1D system. We conclude that the directional character of the orbital hopping can select different degrees of dimensionality. Using additional model calculations, we show that spin-orbital separation is generally far more robust than the spin-charge separation. This is not only due to the already mentioned selection realized by the orbital hopping, but also due to the fact that spinons are faster than the orbitons.
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http://dx.doi.org/10.1103/PhysRevLett.114.096402DOI Listing
March 2015

Femtosecond dynamics of momentum-dependent magnetic excitations from resonant inelastic X-ray scattering in CaCu2O3.

Phys Rev Lett 2014 Apr 7;112(14):147401. Epub 2014 Apr 7.

Leibniz Institute for Solid State and Materials Research IFW Dresden, 01069 Dresden, Germany.

Taking spinon excitations in the quantum antiferromagnet CaCu2O3 as an example, we demonstrate that femtosecond dynamics of magnetic electronic excitations can be probed by direct resonant inelastic x-ray scattering (RIXS). To this end, we isolate the contributions of single and double spin-flip excitations in experimental RIXS spectra, identify the physical mechanisms that cause them, and determine their respective time scales. By comparing theory and experiment, we find that double spin flips need a finite amount of time to be generated, rendering them sensitive to the core-hole lifetime, whereas single spin flips are, to a very good approximation, independent of it. This shows that RIXS can grant access to time-domain dynamics of excitations and illustrates how RIXS experiments can distinguish between excitations in correlated electron systems based on their different time dependence.
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http://dx.doi.org/10.1103/PhysRevLett.112.147401DOI Listing
April 2014

Transition-Metal Nanoparticle Oxidation in a Chemically Nonhomogenous Environment Revealed by 2p3d Resonant X-ray Emission.

J Phys Chem Lett 2013 Apr 25;4(7):1161-6. Epub 2013 Mar 25.

‡Paul Scherrer Institut (PSI), Swiss Light Source, CH-5232 Villigen, Switzerland.

X-ray absorption spectroscopy (XAS) is often employed in fields such as catalysis to determine whether transition-metal nanoparticles are oxidized. Here we show 2p3/2 XAS and 2p3d resonant X-ray emission spectroscopy (RXES) data of oleate-coated cobalt nanoparticles with average diameters of 4.0, 4.2, 5.0, 8.4, and 15.2 nm. Two particle batches were exposed to air for different periods of time, whereas the others were measured as synthesized. In the colloidal nanoparticles, the cobalt sites can have different chemical environments (metallic/oxidized/surface-coordinated), and it is shown that most XAS data cannot distinguish whether the nanoparticles are oxidized or surface-coated. In contrast, the high-energy resolution RXES spectra reveal whether more than the first metal layer is oxidized based on the unique energetic separation of spectral features related to the formal metal (X-ray fluorescence) or to a metal oxide (d-d excitations). This is the first demonstration of metal 2p3d RXES as a novel surface science tool.
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http://dx.doi.org/10.1021/jz4002696DOI Listing
April 2013

Determining the short-range spin correlations in the spin-chain Li2CuO2 and CuGeO3 compounds using resonant inelastic x-ray scattering.

Phys Rev Lett 2013 Feb 20;110(8):087403. Epub 2013 Feb 20.

Research Department Synchrotron Radiation and Nanotechnology, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland.

We report a high-resolution resonant inelastic soft x-ray scattering study of the quantum magnetic spin-chain materials Li(2)CuO(2) and CuGeO(3). By tuning the incoming photon energy to the oxygen K edge, a strong excitation around 3.5 eV energy loss is clearly resolved for both materials. Comparing the experimental data to many-body calculations, we identify this excitation as a Zhang-Rice singlet exciton on neighboring CuO(4) plaquettes. We demonstrate that the strong temperature dependence of the inelastic scattering related to this high-energy exciton enables us to probe short-range spin correlations on the 1 meV scale with outstanding sensitivity.
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http://dx.doi.org/10.1103/PhysRevLett.110.087403DOI Listing
February 2013

Persistent high-energy spin excitations in iron-pnictide superconductors.

Nat Commun 2013 ;4:1470

Paul Scherrer Institut, Swiss Light Source, Villigen PSI CH-5232, Switzerland.

Motivated by the premise that superconductivity in iron-based superconductors is unconventional and mediated by spin fluctuations, an intense research effort has been focused on characterizing the spin-excitation spectrum in the magnetically ordered parent phases of the Fe pnictides and chalcogenides. For these undoped materials, it is well established that the spin-excitation spectrum consists of sharp, highly dispersive magnons. The fate of these high-energy magnetic modes upon sizable doping with holes is hitherto unresolved. Here we demonstrate, using resonant inelastic X-ray scattering, that optimally hole-doped superconducting Ba(0.6)K(0.4)Fe(2)As(2) retains well-defined, dispersive high-energy modes of magnetic origin. These paramagnon modes are softer than, though as intense as, the magnons of undoped antiferromagnetic BaFe(2)As(2). The persistence of spin excitations well into the superconducting phase suggests that the spin fluctuations in Fe-pnictide superconductors originate from a distinctly correlated spin state. This connects Fe pnictides to cuprates, for which, in spite of fundamental electronic structure differences, similar paramagnons are present.
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http://dx.doi.org/10.1038/ncomms2428DOI Listing
June 2013

A multispectroscopic study of 3d orbitals in cobalt carboxylates: the high sensitivity of 2p3d resonant X-ray emission spectroscopy to the ligand field.

Angew Chem Int Ed Engl 2013 Jan 6;52(4):1170-4. Epub 2012 Dec 6.

Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands.

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http://dx.doi.org/10.1002/anie.201204855DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3564409PMC
January 2013

Three-dimensional electron realm in VSe2 by soft-x-ray photoelectron spectroscopy: origin of charge-density waves.

Phys Rev Lett 2012 Aug 20;109(8):086401. Epub 2012 Aug 20.

Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen-PSI, Switzerland.

The resolution of angle-resolved photoelectron spectroscopy (ARPES) in three-dimensional (3D) momentum k is fundamentally limited by ill defined surface-perpendicular wave vector k(perpendicular) associated with the finite photoelectron mean free path. Pushing ARPES into the soft-x-ray energy region sharpens the k(perpendicular) definition, allowing accurate electronic structure investigations in 3D materials. We apply soft-x-ray ARPES to explore the 3D electron realm in a paradigm transition metal dichalcogenide VSe2. Essential to break through the dramatic loss of the valence band photoexcitation cross section at soft-x-ray energies is the advanced photon flux performance of our synchrotron instrumentation. By virtue of the sharp 3D momentum definition, the soft-x-ray ARPES experimental band structure and Fermi surface of VSe2 show a textbook clarity. We identify pronounced 3D warping of the Fermi surface and show that its concomitant nesting acts as the precursor for the exotic 3D charge-density waves in VSe2. Our results demonstrate the immense potential of soft-x-ray ARPES to explore details of 3D electronic structure.
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http://dx.doi.org/10.1103/PhysRevLett.109.086401DOI Listing
August 2012

Valence band structure of the Si(331)-(12 × 1) surface reconstruction.

J Phys Condens Matter 2011 Apr 14;23(13):135003. Epub 2011 Mar 14.

Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering (IMT), Neuchâtel, Switzerland.

Using angle-resolved photoelectron spectroscopy we investigate the electronic valence band structure of the Si(331)-(12 × 1) surface reconstruction for which we recently proposed a structural model containing silicon pentamers as elementary structural building blocks. We find that this surface, reported to be metallic in a previous study, shows a clear band gap at the Fermi energy, indicating semiconducting behavior. An occupied surface state, presumably containing several spectral components, is found centered at - 0.6 eV exhibiting a flat energy dispersion. These results are confirmed by scanning tunneling spectroscopy and are consistent with recent first-principles calculations for our structural model.
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http://dx.doi.org/10.1088/0953-8984/23/13/135003DOI Listing
April 2011

New structural model for the si(331)-(12x1) surface reconstruction.

Phys Rev Lett 2009 Feb 9;102(6):066102. Epub 2009 Feb 9.

Institut de Physique, Université de Neuchâtel, 2000 Neuchâtel, Switzerland.

A new structural model for the Si(331)-(12x1) surface reconstruction is proposed. Based on scanning tunneling microscopy images of unprecedented resolution, low-energy electron diffraction data, and first-principles total-energy calculations, we demonstrate that the reconstructed Si(331) surface shares the same elementary building blocks as the Si(110)-(16x2) surface, establishing the pentamer as a universal building block for complex silicon surface reconstructions.
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http://dx.doi.org/10.1103/PhysRevLett.102.066102DOI Listing
February 2009

Elementary structural building blocks encountered in silicon surface reconstructions.

J Phys Condens Matter 2009 Jan 1;21(1):013001. Epub 2008 Dec 1.

Institut de Physique, Université de Neuchâtel, 2000 Neuchâtel, Switzerland.

Driven by the reduction of dangling bonds and the minimization of surface stress, reconstruction of silicon surfaces leads to a striking diversity of outcomes. Despite this variety even very elaborate structures are generally comprised of a small number of structural building blocks. We here identify important elementary building blocks and discuss their integration into the structural models as well as their impact on the electronic structure of the surface.
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http://dx.doi.org/10.1088/0953-8984/21/1/013001DOI Listing
January 2009
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