News: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.
Looking for a PhD student: if you are interested, please look at this document and contact me directly: claude.monney(at)unifr.ch
Profile:

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 coauthor of more than 50 papers (in peerreviewed journals)
Contact information:
Claude Monney University of Fribourg Department of Physics Chemin du Musée 32 1700 Fribourg Switzerland
Phone: +41 26 300 9163
Email: claude.monney(at)unifr.ch cdw strongly correlated RIXS physics
Last update: 12.01.2018
My CV
Education
Diploma in condensed matter physics (theory)
From 2000 to 2005
University of Fribourg, Switzerland
PhD in condensed matter physics (experiment)
From 2005 to 2009
University of Neuchâtel, Switzerland
Using angleresolved photoemission spectroscopy on charge density wave materials, with Prof. P. Aebi
Professional employment
From 2009 to 2010
University of Fribourg, Switzerland
From 2010 to 2012
Paul Scherrer Institut, Villigen, Switzerland (National Lab)
Using Resonant Inelastic Xray Scattering on spin chain cuprates, with Dr. T. Schmitt
From August 2012 to October 2014
FritzHaberInstitut der MPG, Berlin (Germany)
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) 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 angleresolved photoemission spectroscopy (associate professor). Funded by a professorship research grant from the Swiss National Fonds.
Research experience
Experimental techniques
Electron spectroscopies: timeresolved and angleresolved UV photoemission, Xray photoemission (corelevel studies and Xray photoelectron diffraction), soft Xray ARPES
Photon spectroscopies: resonant inelastic Xray scattering, Xray absorption
Theoretical methods
Green's function formalism (perturbation theory)
Density functional theory (band structure calculations)
Tightbinding 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.
Publications56) S. Mor, M. Herzog, D. Golez, P. Werner, M. Eckstein, N. Katayama, M. Nohara, H. Takagi, T. Mizokawa, C. Monney and J. Staehler: Selfprotection Inhibition of the photoinduced structural phase transition
in the excitonic insulator Ta_{2}NiSe_{5}, 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 chargeordered phases in IrTe_{2} against photoexcitation, accepted for publicaton in Phys. Rev. B.
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 vacancyinduced magnetic moment in edgesharing 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 spinexcitations 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 xray dion spectra in
solids by abinitio wavefunctionbasd 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 onedimensional electronic structure and spectroscopic signature of TomonogaLuttinger 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. 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 layerpolarized excited carriers in
inversionsymmetric 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 1TTiSe2, 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 ZhangRice 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 xray 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: Electronlattice
interactions strongly renormalize the charge transfer energy in the spinchain
cuprate Li2CuO2, Nature Communications 7, 10563 (2016). Link here. ArXiv here: http://arxiv.org/abs/1512.09043.
42) C.W. Nicholson, C. Monney,
U. Krieg, C. Tegenkamp, H. Pfnür, K. Horn and M. Wolf: Electronic
structure of selfassembled 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 onedimensional 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: Threedimensional momentumresolved electronic structure of 1TTiSe2: A combined softxray 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 Electronhole correlations on the electronic structure of 1TTiSe2, Phys. Rev. Lett. 114, 086402 (2015).
38) 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 spinorbital fractionalization to confinement in the anisotropic ladder system CaCu2O3, Phys. Rev. Lett. 114, 096402 (2015).
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 xray 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 Xray 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:
Chargeorbitallattice coupling effects in the ddexcitation profile of
onedimensional cuprates, Phys. Rev. B 89, 041104(R) (2014).
34) Monney, C., Uldry, A.C., Zhou, K.J., KrztonMaziopa, A., Pomjakushina, E., Strocov, V.N., Delley, B. and Schmitt, T.: Resonant inelastic xray scattering at the Fe L3 edge of the onedimensional 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 1TTiSe2: Importance of lattice degrees of freedom,
Phys. Rev. B 88, 075138 (2013).
32) van Schoonveld, M., Suljoti, E., CamposCuerva, 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 NonHomogenous Environment Revealed by 2p3d Resonant Xray 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, HighResolution Resonant Inelastic Xray Scattering with Soft XRays 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, Highenergy 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 highenergy 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 ShortRange Spin Correlations in Cuprate Chain Materials with Resonant Inelastic Xray Scattering, Phys. Rev. Lett. 110, 087403 (2013).
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 xray 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 softxray photoelectron spectroscopy: origin of charge density waves, Phys. Rev. Lett. 109, 086401 (2012).
25) N. Mariotti, C. Didiot, E.F. Schwier, C. Monney, L. PerretAebi, C. Battaglia, M.G. Garnier and P. Aebi, Scanning tunneling microscopy at multiple voltage biases of ringlike 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, Electronhole instability in 1TTiSe2.
New Journal of Physics, 14, 075026 (2012).
22) C. Monney, G. Monney, P. Aebi and H. Beck, Electronhole fluctuation phase in 1TTiSe2.
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 electronhole excitations in the charge density wave system 1TTiSe2 using Resonant Inelastic Xray 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 ManyBody effects in 1TTiSe2: A possible excitonic insulator scenario from GW bandshape 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.: SpinOrbital Separation in the quasi 1D Mottinsulator Sr2CuO3, Nature (2012).
18) Monney, C., Aebi , P., Beck H.: Exciton condensation driving the periodic lattice distortion of 1TTiSe2, 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., BlancoCanosa, 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 hightemperature 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.: TwoSpinon and Orbital Excitations of the SpinPeierls System TiOCl, Phys. Rev. Lett. 107, 107402 (2011).
15) Schwier, E.F., Monney, C., Mariotti, N., Vydrova, Z., GarciaFernandez, M., Didiot, C., Garnier, M.G., Aebi, P.: Influence of elastic scattering on the measurement of corelevel binding energy dispersion in Xray photoemission spectroscopy, Euro. Phys. Journal B 81, 399 (2011).
14) Battaglia, C., Schwier, E.F., Monney, C., Didiot, C., Mariotti, N., GaalNagy, 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).
http://m.iopscience.iop.org/09538984/23/13/135003
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 1TTiSe2 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.: Temperaturedependent photoemission on 1TTiSe2: Interpretation within the exciton condensate phase model, Phys. Rev. B 81, 155104 (2010).
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 1TTiSe2: BCSlike 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 1TTiSe2, Physica B, 404, 3172 (2009).
8) Battaglia, C., GaalNagy, 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., GaalNagy, 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).
6) Battaglia, C., Cercellier, H., Monney, C., Despont, L., Garnier, M.G., Aebi, P.: Unveiling new systematics in selfassembly of atomic chains on Si(111), J. Phys. Conf. Ser. 100, 052078 (2008).
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 1TTiSe2, 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.: "Nonuniform 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).
1) Clerc, F., Battaglia, C., Bovet, M., Despont, L., Monney, C., Cercellier, H., Garnier, M.G., Berger, H., Forro, L., Aebi, P.: “Latticedistortionenhanced electronphonon coupling and Fermi surface nesting of 1TTaSe2, Phys. Rev. B 74, 155114 (2006).
http://prb.aps.org/abstract/PRB/v74/i15/e155114
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 hightemperature 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 timeresolved 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 FritzHaber Institute. My project will be dealing with
timeresolved 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
BetheSalpeter equation (ladder expansion). We show that electronhole
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
electronhole 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 1TTiSe2
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 spinorbital 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 xray light, spinorbital separation have been observed
unambiguously for the first time (Schlappa, Nature 485, 82 (2012)).
We have measured the onedimensional cuprate Sr2CuO3. This material
was particularly appropriate for this discovery for the following
reasons:
 its dorbital states are well separated in energy due to the strong
crystal/ligand field and are well localized states in this strongly
correlated material.
 the cornersharing geometry of the CuO3 chains results in a high nearest neighbour exchange coupling J.
 it is a 1D system.
These fact have the following consequences:
 ddexcitations observed in RIXS are well separated from each other
and well resolved by RIXS (at Cu Ledge). Furthermore they are sharp.
 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.
 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 spinorbital separation
pertubs strongly the spin texture and is thus highly energetically
unfavorable in 2D.
Left figure: spinorbital 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 cuprateUsing Resonant Inelastic Xray scattering, I'm studying low dimensional magnetic cuprates. In particular, I'm investigating edgesharing chain cuprates, like Li2CuO2 and CuGeO3. In these systems, we have discovered peculiar excitations around 34 eV energy loss which strongly respond to the magnetic correlations and to temperature. We identified them as ZhangRice 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 ZhangRice singlet (ZRS) is shown here below. It is created as a final state of the RIXS process at the O Kedge.
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 nearestneighbour spinspin correlations in the 1dimensional materials. This interpretation has been confirmed by stateoftheart cluster calculations (of the RIXS crosssection).
Follow this link (publication in ) for more details.
We have more recently elucidated why the charge transfer energy in these quasi1D 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 socalled charge transfer insulators, where this quantity is a direct measure of the energy gap, since for such materials, the onsite Coulomb interaction is much larger (by opposition to MottHubbard insulator, where the onsite 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 electronphonon 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, CuO 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 electronhole excitations by RIXSWe have used RIXS at the Ti Ledge of TiSe2 to map the electronhole 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 momentumresolved 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 electronhole correlations on the spectral function adapted to the case of TiSe2. We have calculated the electronic susceptibility coming from electronhole 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 selfenergy for the valence and conduction band coming from these electronhole 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 1TTiSe2 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 electonphonon 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: tightbinding fit to DFT calculations used to
extract the transfer integral parameters necessary to estimating the
electronphonon coupling between the valence and conduction bands
(Monney, PRL 106, 106404 (2010)).
Links

