Steffen BACKES

Professeur assistant à l'Ecole Polytechnique

Equipe de recherche au CPhT : Matière Condensée

Thématique de recherche :

I graduated from the Goethe University Frankfurt, Germany, in 2017 with a PhD in theoretical solid-state physics, where I was using a combination of Density Functional Theory and Dynamical Mean-Field Theory (DFT+DMFT) methods to study the properties of strongly correlated materials like iron-based superconductors. 
Within these methods it is possible to both tread the itinerant nature of electrons in metallic materials and the localization of the electronic wave function in insulators on the same footing, which allows one to study the interesting properties of systems that cannot be assigned to the two extremes of a good metal and a good insulator.
For example, the electronic interactions can give rise to a greatly enhanced effective electron mass, leading to a significant deviation from the free-electron picture while staying metallic, like in the so called Hund's metals, where the intraatomic exchange coupling plays an important role for their properties.
Currently I am working on combining the GW approximation with dynamical mean-field theory DMFT (GW+DMFT), which can include non-local corrections, electronic screening and long-range exchange effects in addition to DFT+DMFT. As these calculations are computationally expensive, we also work on systematic approximations to the full GW+DMFT scheme in terms of screened-exchange+DMFT, which in combination with effective interactions calculated from constrained random-phase approximations (cRPA) allow for an ab-initio description of correlated materials.
I am also interested in analytic continuation methods, and occasionally I work on porting existing or implementing new algorithms for GPGPU (General-purpose computing on graphics processing units) purposes, which can allow for speed-up factors of more than an order of magnitude compared to CPU implementations.
Tél. +33(0) 1 69 33 42 34
Adresse CPHT, Ecole Polytechnique, 91128 Palaiseau cedex, France
Bureau Aile 0, Piéce 1015


Thèse : Density functional theory and dynamical mean-field theory: A way to model strongly correlated systems [Link]

Publications :

S. Backes, J.-H. Sim, S. Biermann
Non-local correlation effects in fermionic many-body systems: Overcoming the non-causality problem
K. Zantout, S. Backes, R. Valenti
Two-Particle Self-Consistent method for the multi-orbital Hubbard model
K. Zantout, S. Backes, R. Valenti
Effect of non-local correlations on the electronic structure of LiFeAs
Phys. Rev. Lett. 123, 256401 (2019)  
K. Zantout, M. Altmeyer, S. Backes, R. Valenti
Superconductivity in correlated BEDT-TTF molecular conductors: critical temperatures and gap symmetries
Phys. Rev. B 97, 014530 (2018)  
P. Delange, S. Backes, A. van Roekeghem, L. Pourovskii, H. Jiang, S. Biermann
Novel approaches to spectral properties of correlated electron materials: From generalized Kohn-Sham theory to screened exchange dynamical mean field theory
J. Phys. Soc. Jpn. 87, 041003 (2018) 
M. D. Watson, S. Backes, A. A. Haghighirad, M. Hoesch, T. K. Kim, A. I. Coldea, R. Valenti
Formation of Hubbard-like bands as a fingerprint of strong electron-electron interactions in FeSe
Phys. Rev. B. 95 081106 (2017) 
S. Backes, T. C. Roedel, F. Fortuna, E. Frantzeskakis, P. Le Fevre, F. Bertran, M. Kobayashi, R. Yukawa, T. Mitsuhashi, M. Kitamura, K. Horiba, H. Kumigashira, R. Saint-Martin, A. Fouchet, B. Berini, Y. Dumont, A. J. Kim, F. Lechermann, H. O. Jeschke, M. J. Rozenberg, R. Valenti, A. F. Santander-Syro
Hubbard band or oxygen vacancy states in the correlated electron metal SrVO3?
Phys. Rev. B 94, 241110(R) (2016) 
A. Chernenkaya, A. Morherr, S. Backes, W. Popp, S. Witt, X. Kozina, S. A. Nepijko, M. Bolte, K. Medjanik, G. Oehrwall, C. Krellner, M. Baumgarten, H. J. Elmers, G. Schoenhense, H. O. Jeschke, and R. Valenti
Microscopic origin of the charge transfer in single crystals based on thiophene derivatives: A combined NEXAFS and density functional theory approach
J. of Chem. Phys. 145, 3, 10.1063/1.4958659 (2016) 
F. Lechermann, H. O. Jeschke, A. J. Kim, S. Backes, and R. Valenti
Electron dichotomy on the SrTiO3 defect surface augmented by many-body effects
Phys. Rev. B 93, 121103(R) (2016)
S. Backes, H. O. Jeschke, R. Valenti
Microscopic nature of correlations in multi-orbital AFe2As(A=K, Rb, Cs): Hund's coupling versus Coulomb repulsion
Phys. Rev. B 92, 195128 (2015) 
D. Guterding, S. Backes, H. O. Jeschke, R. Valenti
Origin of the superconducting state in the collapsed tetragonal phase of KFe2As2
Phys. Rev. B 91, 140503(R) (2015)
J. Diehl, S. Backes, D. Guterding, H. O. Jeschke, R. Valenti
Correlation effects in the tetragonal and collapsed tetragonal phase of CaFe2As2
Phys. Rev. B 90, 085110 (2014) 
S. Backes, D. Guterding, H. O. Jeschke, R. Valenti
Electronic structure and de Haas-van Alphen frequencies in KFe2As2 within LDA+DMFT
New J. Phys. 16 083025 (2014) 

Thèse : Density functional theory and dynamical mean-field theory: A way to model strongly correlated systems [Link]