This website is no longer updated!

-> please go to my new group website:  https://sites.google.com/sportfan.ch/group-cmonney/ .


I have received a grant for SNSF professorship for a new project entitled "Investigating the ultrafast dynamics of Mott correlations".

I have started my own research group at the Physics department of the University of Fribourg on January 2018.


  • I am working since the 1st of January 2018 at the University of Fribourg, Switzerland, as a SNSF professor.

  • I obtained my PhD in physics in 2009.

  • I'm doing experimental physics, with some theory.

  • I'm specialized in spectroscopy on strongly correlated systems.

  • author and co-author of more than 50 papers (in peer-reviewed journals)



Contact information:

Claude Monney
University of Fribourg
Department of Physics
Chemin du Musée 32
1700 Fribourg

Phone: +41 26 300 9163
Email: claude.monney(at)unifr.ch cdw strongly correlated RIXS physics
Last update: 12.01.2018



Diploma in condensed matter physics (theory)

From 2000 to 2005
University of Fribourg, Switzerland
On Quantum Hall Systems, with Prof. C. de Morais-Smith

PhD in condensed matter physics (experiment) 

From 2005 to 2009
University of Neuchâtel, Switzerland
Using angle-resolved photoemission spectroscopy on charge density wave materials, with Prof. P. Aebi
Title: Exciton condensation in 1T-TiSe2, available (in English) at http://doc.rero.ch/record/17529?ln=fr#abstract_fre

Professional employment

From 2009 to 2010

University of Fribourg, Switzerland
Postdoc (continuation of PhD thesis) with Prof. P. Aebi

From 2010 to 2012

Paul Scherrer Institut, Villigen, Switzerland (National Lab)
Using Resonant Inelastic X-ray Scattering on spin chain cuprates, with Dr. T. Schmitt
From August 2012 to October 2014
Fritz-Haber-Institut der MPG, Berlin (Germany)
Time- and angle-resolved photoemission spectroscopy with Prof. Martin Wolf 
Funded by an Advanced researcher grant from the Swiss National Fonds and a postdoctoral research fellowship from the Alexander von Humboldt foundation
From August 2014 to October 2017

Department of physics of the University of Zürich (Switzerland)
Time- and angle-resolved photoemission spectroscopy in the group of Prof. J. Osterwalder.
Funded by an Ambizione research grant from the Swiss National Fonds.

From January 2018

Department of physics of the University of Fribourg(Switzerland)
Time- and angle-resolved photoemission spectroscopy (associate professor).
Funded by a professorship research grant from the Swiss National Fonds.

Research experience

Experimental techniques

Electron spectroscopies: time-resolved and angle-resolved UV photoemission, X-ray photoemission (core-level studies and X-ray photoelectron diffraction), soft X-ray ARPES
Photon spectroscopies: resonant inelastic X-ray scattering, X-ray absorption

Theoretical methods

Green's function formalism (perturbation theory)
Density functional theory (band structure calculations)
Tight-binding calculations (band structure calculations and more)

Research Interests

Strongly correlated electron systems, charge density wave systems and the origin of their phase transition, the excitonic insulator phase as a possible CDW phase, spin chain cuprates investigated by RIXS.


  • publications in 2018

56) S. Mor, M. Herzog, D. Golez, P. Werner, M. Eckstein, N. Katayama, M. Nohara, H. Takagi, T. Mizokawa, C. Monney and J. Staehler: Self-protection Inhibition of the photoinduced structural phase transition in the excitonic insulator Ta2NiSe5, accepted for publication in Phys. Rev. B.

55) C. Monney , A. Schuler, T. Jaouen, M. -L. Mottas, Th. Wolf, M. Merz, M. Muntwiler, L. Castiglioni, P. Aebi, F. Weber and M. Hengsberger: Robustness of the charge-ordered phases in IrTe2 against photoexcitation, accepted for publicaton in Phys. Rev. B.

  • publications in 2017

54) R.O. Kuzian, R. Klingeler, W.E.A. Lorenz, N. Wizent, S. Nishimoto, U. Nitzsche, H. Rosner, D. Milosaylieyic, L. Hozoi, R. Yaday, J. Richter, A. Hauser, J. Geck, R. Hayn, V. Yushankhai, L. Siurakshina, C. Monney, T. Schmitt, G. Roth, T. Ito, H. Yamaguchi, M. Matsuda, S. Johnston, S.-L. Drechsler: Comment on `Oxygen vacancy-induced magnetic moment in edge-sharing CuO2 chains of Li2CuO2' , arXiv:1708.06335.

53) O. Ivashko, N.E. Shaik, X. Lu, C.G. Fatuzzo, M. Dantz, P.G. Freeman, D.E. McNally, D. Destraz, N.B. Christensen, T. Kurosowa, N. Momono, M. Oda, C.E. Matt, C. Monney, H.M. Ronnow, T. Schmitt, J. Chang: Damped spin-excitations in a doped cuprate superconductor with orbital hybridization, Phys. Rev. B 95, 214508, arXiv:1702.02782.

52) N.A. Bogdanov, V. Bisogni, R. Kraus, C. Monney, K. Zhou, T. Schmitt, J. Geck, A. Mitrushchenkov, H. Stoll, J. van den Brink, L. Hozoi: Orbital breathing effects in the computation of x-ray d-ion spectra in solids by ab-initio wave-function-basd methods, J.Phys.: condensed matter 29, 035502 (2017), arXiv:1611.03693.

51) C.N. Nicholson, C. Berthod, M. Puppin, H. Berger, M. Wolf, M. Hoesch, C. Monney, Dimensonal crossover in a charge density wave material probed by ARPES, Phys. Rev. Lett. 118, 206401 (2017), arXiv:1610.05024.

50) M.D. Watson, Y. Feng, C.N. Nicholson, C. Monney, J.M. Riley, H. Iwasawa, K. Refson, V. Sacksteder, D.T. Adroja, J. Zhao, M. Hoesch, Multiband one-dimensional electronic structure and spectroscopic signature of Tomonoga-Luttinger liquid behavior in K2Cr3As2, Phys. Rev. Lett. 118, 097002 (2017), arXiv:1610.04157.

49) S. Mor, M. Herzog, D. Golez, P. Werner, M. Eckstein, N. Katayama, M. Nohara, H. Takagi, T. Mizokawa, C. Monney and J. Staehler, Ultrafast electronic band gap control in an excitonic insulator, accepted for publication in Phys. Rev. Lett. 119, 086401 (2017), arXiv:1608.05586.

  • publications in 2016

48) R. Bertoni, C. Nicholson, L. Waldecker, H. Huebener, C. Monney, U. De Giovannini, M. Puppin, M. Hoesch, E. Springate, R. Chapman, C. Cacho, M. Wolf, A. Rubio, R. Ernstorfer: Generation and evolution of spin-, valley- and layer-polarized excited carriers in inversion-symmetric WSe2, Phys. Rev. Lett. 117, 277201 (2016), arXiv:1606.03218.

47) C. Monney, M. Puppin, C.W. Nicholson, M. Hoesch, R.T. Chapman, E. Springate, H. Berger, A. Magrez, C. Cacho, R. Ernstorfer, M. Wolf: Revealing the role of electrons and phonons in the ultrafast recovery of charge density wave correlations in 1T-TiSe2, Phys. Rev. B 94, 165165 (2016).

46) C.W. Nicholson, C. Monney, R. Carley, B. Frietsch, J. Bowlan, M. Weinelt, M. Wolf: Ultrafast spin density wave transition in Chromium governed by thermalized electron gas, Phys. Rev. Lett. 117, 136801 (2016).

45) C. Monney, V. Bisogni, K.J. Zhou, R. Kraus, V. Strocov, G. Behr, S.-L. Drechsler, H. Rosner, S. Johnston, J. Geck, T. Schmitt: Probing the magnetic and electronic correlations in Li2CuO2 with Zhang-Rice excitons, Phys Rev. B 94, 165118 (2016).

44) C. Monney, T. Schmitt, C. E. Matt, J. Mesot, V. N. Strocov, O. J. Lipscombe, S. M. Hayden, and J. Chang: A resonant inelastic x-ray scattering study of the spin and charge excitations in the overdoped superconductor La1.77Sr0.23CuO4, Phys. Rev. B 93, 075103 (2016). Link here or arXiv here.

43) S. Johnston, C. Monney, V. Bisogni, K. Zhou, R. Kraus, G. Behr, V. N. Strocov, J. Malek, S.-L. Drechsler, J. Geck, T. Schmitt, J. van den Brink: Electron-lattice interactions strongly renormalize the charge transfer energy in the spin-chain cuprate Li2CuO2, Nature Communications 7, 10563 (2016). Link here. ArXiv here: http://arxiv.org/abs/1512.09043.

  • publications in 2015
42) C.W. Nicholson, C. Monney, U. Krieg, C. Tegenkamp, H. Pfnür, K. Horn and M. Wolf: Electronic structure of self-assembled Ag nanowires on Si(557): spectroscopic evidence for dimensionality, New J. Phys. 17, 093025 (2015)

41) N. Mariotti, C. Didiot, E.F. Schwier, C. Monney and P. Aebi: Quasi one-dimensional Ag nanostructures on Si(331)-(12x1), Surf. Sci. 639, 39 (2015).

40) Z. Vydrova, E.F. Schwier, G. Monney, T. Jaouen, E. Razzoli, C. Monney, B. Hildebrand, C. Didiot, H. Berger, T. Schmitt, V. N. Strocov, F. Vanini, and P. Aebi: Three-dimensional momentum-resolved electronic structure of 1T-TiSe2: A combined soft-x-ray photoemission and density functional theory study, Phys. Rev. B 91, 235129 (2015).

39) G. Monney, C. Monney, B. Hildebrand, P. Aebi, H. Beck: Impact of Electron-hole correlations on the electronic structure of 1T-TiSe2, Phys. Rev. Lett. 114, 086402 (2015).

V. Bisogni, K. Wohlfeld, S. Nishimoto, C. Monney, J. Trinckauf, K.J. Zhou, R. Kraus, K. Koepernik, C. Sekar, V. Strocov, B. Buchner, T. Schmitt, J. van den Brink, J. Geck: Orbital control of effective dimensionality: from spin-orbital fractionalization to confinement in the anisotropic ladder system CaCu2O3, Phys. Rev. Lett. 114, 096402 (2015).
  • publications in 2014
37) V. Bisogni, S. Kourtis, C. Monney, K.J. Zhou, R. Kraus, C. Sekar, V. Strocov, B. Buchner, L. Braicovich, T. Schmitt, M. Daghofer J. van den Brink, J. Geck: Femtosecond dynamics of momentum dependent magnetic excitations from resonant inelastic x-ray scattering inCaCu2O3, Phys. Rev. Lett. 112, 147401 (2014).

36) B. Barbiellini, J.N. Hancock, C. Monney, Y. Joly, G. Ghiringhelli, L. Braicovich and T. Schmitt: Inelastic X-ray scattering from valence electrons near absorption edges of FeTe and in TiSe2, Phys. Rev. B 89, 235138 (2014).

35) J.J. Lee, B. Moritz, W.S. Lee, M. Yi, C. Jia, A.P. Sorini, K. Kudo, Y. Koike, K.J. Zhou, C. Monney, V. Strocov, L. Patthey, T. Schmitt, T.P. Devereaux and Z.X. Shen: Charge-orbital-lattice coupling effects in the dd-excitation profile of one-dimensional cuprates, Phys. Rev. B 89, 041104(R) (2014).
  • publications in 2013
34) Monney, C., Uldry, A.-C., Zhou, K.J., Krzton-Maziopa, A., Pomjakushina, E., Strocov, V.N., Delley, B. and Schmitt, T.: Resonant inelastic x-ray scattering at the Fe L3 edge of the one-dimensional chalcogenide BaFe2Se3, Phys. Rev. B. 88, 165103(2013).
33) B. Zenker, H. Fehske, H. Beck, C. Monney and A.R. Bishop, Chiral charge order in 1T-TiSe2: Importance of lattice degrees of freedom,

Phys. Rev. B 88, 075138 (2013).

32) van Schoonveld, M., Suljoti, E., Campos-Cuerva, C., Gosselink, R.W., Van der Eerden, A., Schlappa, J., Zhou, K.J., Monney, C., Schmitt, T., de Groot, F.M.F., Transition Metal Nanoparticle Oxidation in a Chemically Non-Homogenous Environment Revealed by 2p3d Resonant X-ray Emission,

J. Phys. Chem. Lett. 4, 1161 (2013).

31) T. Schmitt, V.N. Strocov, K.J. Zhou, J. Schlappa, C. Monney, U. Flechsig and L. Patthey, High-Resolution Resonant Inelastic X-ray Scattering with Soft X-Rays at the ADRESS beamline of the Swiss Light Source: further instrument development and scientific highlights, J. Electron Spectrosc. Relat. Phenom. (2013).
30) M.P.M Dean, A. J. A. James, R. S. Springell, X. Liu, C. Monney, K. J. Zhou, R. M. Konik, J. S. Wen, Z. J. Xu, G. D. Gu, V. N. Strocov, T. Schmitt, and J. P. Hill, High-energy magnetic excitations in the cuprate Bi2Sr2CaCu2O8 superconductor: Towards a unified description of its electronic and magnetic degrees of freedom, accepted for publication in Phys. Rev. Lett. (2013).
29) K.J. Zhou, Y.B. Huang, C. Monney, X.Dai, V. Strocov, N.L. Huang, Z.G. Chen, C. Zhang, P. Dai, L. Patthey, J. van den Brink, H. Ding and T. Schmitt, Persistent high-energy spin excitations in pnictides superconductors, Nat. Comm. 4, 1470 (2013).
28) C. Monney, V. Bisogni, K.J. Zhou, R. Kraus, V. Strocov, G. Behr, J. Malek, R. Kuzian, S.-L. Drechsler, S. Johnston, A. Revcolevschi, B. Buchner, H. Ronnow, J. van den Brink, J. Geck and T. Schmitt, Determining the Short-Range Spin Correlations in Cuprate Chain Materials with Resonant Inelastic X-ray Scattering, Phys. Rev. Lett. 110, 087403 (2013).
  • publications in 2012

27) M. van Schooneveld, R.W. Gosselink, T.M. Eggenhuisen, M. Al Samarai, C. Monney, K. Zhou, T. Schmitt and F.M.F. de Groot, A multispectroscopic study of 3d orbitals in cobalt carboxylates: the high sensitivity of 2p3d resonant x-ray emission spectroscopy to the ligand field, Ang. Chemie 51, 1 (2012)

26) V.N. Strocov, M. Shi, M. Kobayashi, C. Monney, X. Wang, J. Krempasky, T. Schmitt, L. Patthey, H. Berger and P. Blaha, Three dimensional electron realm in VSe2 by soft-x-ray photoelectron spectroscopy: origin of charge density waves, Phys. Rev. Lett. 109, 086401 (2012).
25) N. Mariotti, C. Didiot, E.F. Schwier, C. Monney, L. Perret-Aebi, C. Battaglia, M.G. Garnier and P. Aebi, Scanning tunneling microscopy at multiple voltage biases of ring-like Ag cluster on Si (111)-7x7, Surface Science 606, 1755 (2012). 
24) M.P.M. Dean, R.S. Springell,  C. Monney, K.J. Zhou, J. Pereiro, I. Bozovic, B. Dalla Piazza, H.M. Ronnow, E. Morenzoni, J. van den Brink, T. Schmitt, J. Hill, Spin excitations in a single La2CuO4 layer.
Nature Materials aop, (2012) | doi:10.1038/nmat3409
23) C. Monney, G. Monney, P. Aebi and H. Beck, Electron-hole instability in 1T-TiSe2.
New Journal of Physics, 14, 075026 (2012).
22) C. Monney, G. Monney, P. Aebi and H. Beck, Electron-hole fluctuation phase in 1T-TiSe2.
Phys. Rev. B 85, 2351050 (2012).
21) C. Monney, K.J. Zhou, H. Cercellier, Z. Vydrova, M.G. Garnier, G. Monney, V.N. Strocov, H. Berger, H. Beck, T. Schmitt and P. Aebi: Mapping of electron-hole excitations in the charge density wave system 1T-TiSe2 using Resonant Inelastic X-ray Scattering.
Phys. Rev. Lett. 109, 047401 (2012).
20) M. Cazzaniga, H. Cercellier, M. Holzmann, C. Monney, P. Aebi. G. Onida and V. Olevano: Ab initio Many-Body effects in 1T-TiSe2: A possible excitonic insulator scenario from GW band-shape renormlization, Phys. Rev. B 85, 195111 (2012).
19) Schlappa, J., Wohlfeld, K., Zhou, K. J., Mourigal, M., Haverkort, M. W., Strocov, V. N., Hozoi, L., Monney, C., Nishimoto, S. , Singh, S., Revcolevschi, A., Caux, J.-S., Patthey, L., Ronnow, H. M., van den Brink, J., Schmitt, T.:  Spin-Orbital Separation in the quasi 1D Mott-insulator Sr2CuO3, Nature (2012).
  • publications in 2011

18) Monney, C., Aebi , P., Beck H.: Exciton condensation driving the periodic lattice distortion of 1T-TiSe2, Physical Review Letters 106, 106404 (2011).
17) Le Tacon, M., Ghiringhelli, G., Chaloupka, J., Sala, M., Hinkov, V., Haverkort, M., Minola, M., Bakr, M., Zhou, K.J., Blanco-Canosa, S., Monney, C., Song, Y., Sun, G., Lin, C., De Luca, G., Salluzzo, M., Khaliullin, G., Schmitt, T., Braicovitch, L., Keimer, B.: Intense paramagnon excitations in a large family of high-temperature superconductors, Nature Physics 7, 725 (2011).
16) Glawion, S. , Heidler, J., Haverkort, M., Duda, L.C., Schmitt, T., Strocov, V., Monney, C., Zhou, K.J., Ruff, A., Sing, M., Claessen, R.: Two-Spinon and Orbital Excitations of the Spin-Peierls System TiOCl, Phys. Rev. Lett. 107, 107402 (2011).
15) Schwier, E.F., Monney, C., Mariotti, N., Vydrova, Z., Garcia-Fernandez, M., Didiot, C., Garnier, M.G., Aebi, P.: Influence of elastic scattering on the measurement of core-level binding energy dispersion in X-ray photoemission spectroscopy, Euro. Phys. Journal B 81, 399 (2011).
  • publications in 2010

14) Battaglia, C., Schwier, E.F., Monney, C., Didiot, C., Mariotti, N., Gaal-Nagy, K., Onida, G., Garnier, M.G., Aebi, P.: Valence band structure of the Si(331)-(12 x 1) surface reconstruction. J. Phys. Condens. Matt. 23, 135003 (2010).



13) Monney, C., Schwier, E.F., Garnier, M.G., Battaglia, C., Mariotti, N., Didiot, C., Cercellier, H., Marcus, J., Berger, H., Titov, A.N., Beck, H., Aebi, P.: Dramatic effective mass reduction driven by a strong potential of competing periodicity, Euro. Phys. Lett. 92, 47003 (2010).
12) Monney, C., Schwier, E.F., Garnier, M.G., Mariotti, N., Didiot, C., Cercellier, H., Marcus, J., Berger, H., Titov, A.N., Beck, H., Aebi P.: Probing the exciton condensate phase in 1T-TiSe2 with photoemission, New Journal of Physics 12, 125019 (2010).
11) Monney, C., Schwier, E.F., Garnier, M.G., Mariotti, N., Didiot, C., Cercellier, H., Marcus, J., Battaglia, C., Berger, H., Titov, A.N., Beck, H., Aebi, P.: Temperature-dependent photoemission on 1T-TiSe2: Interpretation within the exciton condensate phase model, Phys. Rev. B 81, 155104 (2010).
  • publications in 2009

10) Monney, C., Cercellier, H., Clerc, F., Battaglia, C., Schwier, E.F., Didiot, C., Garnier, M.G., Beck, H., Aebi, P., Berger, H., Forro, L., Patthey, L.: Spontaneous exciton condensation in 1T-TiSe2: BCS-like approach, Phys. Rev. B 79, 045116 (2009).
9) Monney, C., Cercellier, H., Battaglia, C., Schwier, E.F., Didiot, C., Garnier, M.G., Beck, H., Aebi, P.: Temperature dependence of the excitonic insulator phase model in 1T-TiSe2, Physica B, 404, 3172 (2009).
8) Battaglia, C., Gaal-Nagy, K., Monney, C., Didiot, C., Schwier, E.F., Garnier, M.G., Onida, G., Aebi. P.: Elementary structural building blocks encountered in silicon surface reconstructions, J. Phys. Cond. Mat. 21, 013001 (2009).
7) Battaglia, C., Gaal-Nagy, K., Monney, C., Didiot, C., Schwier, E.F., Garnier, M.G., Onida, G., Aebi, P.: New structural model for the Si(331)-(12x1) reconstruction, Phys. Rev. Lett. 102, 066102 (2009).
  • publications in 2008

6) Battaglia, C., Cercellier, H., Monney, C., Despont, L., Garnier, M.G., Aebi, P.: Unveiling new systematics in self-assembly of atomic chains on Si(111), J. Phys. Conf. Ser. 100, 052078 (2008).
  • publications in 2007

5) Cercellier, H., Monney, C., Clerc, F., Battaglia, C., Despont, L., Garnier, M.G., Beck, H., Aebi, P., Patthey, L., Berger, H., Forro, L.: Evidence for an excitonic insulator phase in 1T-TiSe2, Phys. Rev. Lett. 99, 146403 (2007).
4) Clerc, F., Battaglia, C., Cercellier, H., Monney, C., Berger, H., Despont, L., Garnier, M.G., Aebi, P.: Fermi surface of layered compounds and bulk charge density wave systems, J. Phys.: Condens. Matter, Special Issue, July (2007).
3) Battaglia, C., Cercellier, H., Despont, L., Monney, C., Prester, M., Berger, H., Forro, L., Garnier, M.G., Aebi, P.: "Non-uniform doping acreoss the Fermi surface of NbS2 intercalates", Eur. Phys. J. B 57, 385 (2007).
2) Battaglia, C., Cercellier, H., Monney, C., Garnier, M.G., Aebi, P.: Stabilization of silicon honeycomb chains by trivalent adsorbates, Eur. Phys. Lett. 77, 36003 (2007).
  • publications in 2006

1) Clerc, F., Battaglia, C., Bovet, M., Despont, L., Monney, C., Cercellier, H., Garnier, M.G., Berger, H., Forro, L., Aebi, P.: “Lattice-distortion-enhanced electron-phonon coupling and Fermi surface nesting of 1T-TaSe2, Phys. Rev. B 74, 155114 (2006).



News (not updated...)

February 2016:

Our new paper about RIXS on the spin chain cuprate Li2CuO2 has just been published in Nature Comm, see the link here. This is the result of a nice collaboration between experiment and theory!

January 2016:

Our new paper about RIXS on the high-temperature superconductor La1.77Sr0.23CuO4 has just been published in PRB. See the link here. We have done a Random Phase Approximation calculations to interpret our RIXS data of the spin and charge excitations observed in this overdoped cuprate.

September 2014: Ambizione research fellowship from SNF accepted!

My proposal submitted to the Swiss National Science Foundation for Science has been accepted. I will pursue my research activities on correlated materials using time-resolved ARPES at the University of Zurich, in the group of Prof. J. Osterwalder.

October 2012: postdoctoral researcher fellowship from AvH accepted.

My project submitted also to the Alexander von Humboldt foundation has been accepted. The fellowship will be combined to that of the SNF to extend my stay in Berlin.

June 2012: advanced researchers fellowship accepted by SNF!
My project submitted to the Swiss national science foundation for an Advanced Researchers fellowhip has been accepted with the best mark. I will move soon to Berlin to start a new collaboration with Prof. Martin Wolf at the Fritz-Haber Institute. My project will be dealing with time-resolved photoemission on charge density wave materials.
June 2012: theoretical paper on the CDW instability accepted PRB
We have studied the origin of the CDW instability in TiSe2 using the Green's formalism in perturbation theory at the level of the Bethe-Salpeter equation (ladder expansion). We show that electron-hole excitations in the Coulomb interaction are responsible for the CDW instability due to the particular case of the Fermi surface topology of TiSe2. More news to come after publication of the article in PRB.
June 2012: first RIXS paper accepted in PRL
We have performed RIXS measurement on a charge density wave materials, TiSe2. We show for the first time that low energy electron-hole excitations can be mapped as a function of the transferred light momentum and allow to extract band stucture information both on the occupied and unoccupied sates. This paper has been accepted for publication in PRL. More news to come after publication of the article.
March 2012: GW calculations have been performed on 1T-TiSe2
 In a recent article (Cazzaniga, PRB 85, 195111(2012)), we have published GW calculation done on TiSe2 showing a strong renormalization of the valence band, resulting in a band shape (Mexican hat like) very similar to what has been measured by ARPES in this material at low temperature. We interpret this renormalization as the consequence of the excitonic contributions, which are already included at the GW level.
Left figure: GW and DFT calculations of the band structure of TiSe2, together with ARPES data. From Cazzaniga, PRB 85, 195111(2012).
February 2012: Discovery of the spin-orbital separation: the orbiton has been unambiguoulsy observed!
Follow this link to the original article on Nature's website.
Due to the unique sensitivity of RIXS to orbital degree of freedom and to its capacity of mapping excitation dispersions thanks to the momentum of x-ray light, spin-orbital separation have been observed unambiguously for the first time (Schlappa, Nature 485, 82 (2012)).
We have measured the one-dimensional cuprate Sr2CuO3. This material was particularly appropriate for this discovery for the following reasons:
  1. its d-orbital states are well separated in energy due to the strong crystal/ligand field and are well localized states in this strongly correlated material.
  2. the corner-sharing geometry of the CuO3 chains results in a high nearest neighbour exchange coupling J.
  3. it is a 1D system.
These fact have the following consequences:
  1. dd-excitations observed in RIXS are well separated from each other and well resolved by RIXS (at Cu L-edge). Furthermore they are sharp.
  2. The dispersions of the orbital excitations are strongly dependent on the exchange coupling J. The larger J, the larger they are and thus the more visible.
  3. In a 1D system, collective excitations observed by RIXS tend to be easier to evidence (sharper). The RIXS spectra are easier to interpret too, because of the dispersions being dependent only on the transferred momentum of light parallel to the direction of the chains. Finally, as explained in the article, the spin-orbital separation pertubs strongly the spin texture and is thus highly energetically unfavorable in 2D.

Left figure: spin-orbital separation in Sr2CuO2, (a) described schematically and (c) as observed in RIXS data. From Schlappa, Nature 485, 82 (2012).



My Research

RIXS on Spin chain cuprate

Using Resonant Inelastic X-ray scattering, I'm studying low dimensional magnetic cuprates. In particular, I'm investigating edge-sharing chain cuprates, like Li2CuO2 and CuGeO3. In these systems, we have discovered peculiar excitations around 3-4 eV energy loss which strongly respond to the magnetic correlations and to temperature. We identified them as Zhang-Rice singlet or triplet excitons, the intensity of which is directly dependent on the nearest neighbour spin configuration.

The processus leading to the creation of such a Zhang-Rice singlet (ZRS) is shown here below. It is created as a final state of the RIXS process at the O K-edge.

We have observed such ZRS in RIXS on Li2CuO2 and CuGeO3 and measured their RIXS intensity as a function of temperature:

Their intensities permit to follow as a function of temperature the development of nearest-neighbour spin-spin correlations in the 1-dimensional materials. This interpretation has been confirmed by state-of-the-art cluster calculations (of the RIXS cross-section).

Follow this link (publication in ) for more details.

We have more recently elucidated why the charge transfer energy in these quasi-1D chain cuprate is so large and what is its nature. The charge transfer energy is the energy needed to excite a charge from the O atom to the Cu atom (or more general from the cations to the ligand anion). This is particularly important in the so-called charge transfer insulators, where this quantity is a direct measure of the energy gap, since for such materials, the on-site Coulomb interaction is much larger (by opposition to Mott-Hubbard insulator, where the on-site Coulomb interaction is smaller than the charge transfer energy).

We have shown in this work that the charge transfer gap in the spin chain cuprate Li2CuO2 is 4.6 eV and has a combined origin: it has a purely electronic part, which amounts to 2.1 eV, and a lattice related part of 2.5 eV. The lattice part is coming from the strong electron-phonon coupling in this material, as well as the poor screening occuring in low dimensional materials. When changing the net charge on the Cu atom with such an excitation, Cu-O bond stretching phonon modes are excited and modulate the Madelung energy of the Cu atoms, modifying in turn the charge transfer energy.
Here on the left are shown some RIXS spectra from Li2CuO2 showing the strong effects of excitations of phonon modes during the RIXS process, in terms of peak broadening and the apparition of satellites due to the excitation of phonon quanta. The charge transfer gap corresponds to the energy distance between the (quasi-)elastic peak and the first charge transfer (CT) peak. The lower panels shown the results of a cluster calculation including both electronic and lattice degrees of freedom. More details online here.

RIXS on a charge density wave materials: first momentum mapping of electron-hole excitations by RIXS

We have used RIXS at the Ti L-edge of TiSe2 to map the electron-hole excitations across the occupied and unoccupied electronic structure of this charge density wave materials. We have been able to show that RIXS can indeed measure the dispersion of such electronic excitations in a momentum-resolved way and permit to access both the occupied and unoccupied part of the electronic structure of a material.

More details at this link (publication).

Charge density wave systems and the exciton condensate phase

Within a perturbative approach using Green's function formalism, we have calculated the influence of charge density wave fluctuations generated by strong electron-hole correlations on the spectral function adapted to the case of TiSe2.

We have calculated the electronic susceptibility coming from electron-hole scattering between the valence and conduction bands and have shown that it can lead to a divergence at 0 energy, meaning an instability of the ground towards a new ground state, which is the excitonic insulator phase.

We have calculated the self-energy for the valence and conduction band coming from these electron-hole correlations in the CDW fluctuations phase. The resulting photoemission intensity maps compare well with the experiment.

See this link (online publication) for more details.

Our ARPES data on 1T-TiSe2 shows strong signatures of the charge density wave (CDW) phase.


Left figure: comparison of the calculated spectral function for the exciton condensate phase model together with measured ARPES data (at L). For more details, see New Journal of Physics 12, 125019 (2010).

 In this transition metal dichalcogenide, a 2x2x2 CDW is clearly evidenced in the electronic sector, occuring together with a periodic lattice distortion (PLD). However, there is here a peculiarity: while the atomic displacements involved in the PLD (see Di Salvo et al., PRB 14, 4321 (1976)), the signature of the CDW in ARPES is very intense. This is opposite to other CDW materials like TaS2, where the backfolded bands in ARPES are very weak.
This lead us to the idea that the CDW transition is triggered by a purely electronic mechanism, possibly resulting in the formation of an exciton condensate at low temperature.
We have calculated the spectral function for a simple system consisting of 2 types of bands, a valence and a conduction band, being coupled together by a Coulomb interaction. The photoemission intensity maps calculated with this model compare well to the experimental data (see Cercellier, PRL 99, 146104 (2007) & Monney, PRB 79, 045116 (2009)), giving support to our approach. However, this is not a proof of the realization of an excitonic insulator phase in the CDW phase: the order parameter used in the spectral function is not specific to such an exotic phase.
We were also curious to see whether the occurence of an excitonic insulator phase could generate the observed PLD. Indeed, in TiSe2, the valence and conduction bands have extrema at different momenta in the Brillouin zone, so that their coupling in our model leads to the formation of excitons with a finite momentum. This will then create a modulated electron density characterized with this momentum, which turns out to be the CDW. The question is then: is the electon-phonon coupling between this CDW and the lattice able to provoke the observed PLD? The answer is yes and has been published in Monney, PRL 106, 106404 (2010).
Right picture: tight-binding fit to DFT calculations used to extract the transfer integral parameters necessary to estimating the electron-phonon coupling between the valence and conduction bands (Monney, PRL 106, 106404 (2010)).