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Institut de minéralogie, de physique des matériaux et de cosmochimie



Extending ab-initio modelling to vibrational electron energy loss spectroscopy

By combining microscopic density functional theory calculations with the electrostatic  formalism necessary to account for the experimental specificities of electron energy loss spectroscopy (EELS) as implemented in the transmission electron microscope, we showed how a quantitative description of the experimental spectra obtained on molecular crystals in the vibrational range of frequencies can be reached.




"First-principles vibrational electron energy loss spectroscopy of β-guanine"

G. Radtke, D. Taverna, M. Lazzeri and E. Balan, PHYSICAL REVIEW LETTERS 119, 027402 (2017)




Probing the quantum thermal fluctuations of nuclei by X-ray absorption spectroscopy and Nuclear Magnetic Resonance

In material sciences, local spectroscopies such as X-ray absorption (XANES) and solid-state nuclear magnetic resonance (NMR) are commonly interpreted using ab-initio calculations performed assuming an equilibrium static lattice. Yet, nuclear motion affects spectra even when reduced to the zero-point motion at 0 K. For instance, the nuclear dynamics generates specific discrepancies between standard calculations and measurements, such as an additional XAS pre-edge peak involving forbidden electronic transitions and inaccurate assignment of NMR resonance peaks. We developed a theoretical framework based on DFT that includes quantum thermal fluctuations in the calculation of the XAS cross-section and NMR parameters (chemical shifts, quadrupolar coupling constants), and allows a good description of the temperature effects observed experimentally.



"Temperature dependence of X-ray absorption and nuclear magnetic resonance spectra: probing quantum vibrations of light elements in oxides."

Nemausat R., Gervais C., Brouder C., Trcera N., Bordage A., Coelho-Diogo C., Florian P., Rakhmatullin A., Errea I., Paulatto L., Lazzeri M. and Cabaret D. PHYSICAL CHEMISTRY CHEMICAL PHYSICS 19, 6246–6256 (2017)




Exchange interaction and the charge-density-wave instability in TiSe2

The exchange interaction is due to the exchange symmetry of the wave function of indistinguishable particles, and hence of quantum mechanical origin. In standard ab-initio calculations based on local approximations to the exchange-correlation energy (e.g. LDA and GGA) this interaction is only partially captured. In this work we show that the fully nonlocal description of exchange is needed to capture the physics of TiSe2. The exchange interaction provides two competing effects, a short range repulsive interaction, important for the electronic band structure, and a long-range attractive electron-hole interaction which cooperates with the electron-phonon interaction to induce the charge-density-wave transition in TiSe2.


"Critical Role of the Exchange Interaction for the Electronic Structure and Charge-Density-Wave Formation in TiSe2"

Hellgren M., Baima J., Bianco R., Calandra M., Mauri F., and Wirtz L., PHYSICAL REVIEW LETTERS 119, 176401 (2017)




Rashba coupling amplification by a staggered crystal field

There has been increasing interest in materials where relativistic effects induce non-trivial spin splittings with promise for spintronics applications. Sizable splittings have been hitherto obtained using heavy elements, where the Rashba spin-orbit coupling is intrinsically strong. This paradigm has been broken by our finding of a large Rashba coupling in the BaNiS2, where nickel is spin-orbit active, despite its low Z number. This unexpected result in the absence of heavy elements demonstrates an effective mechanism of Rashba coupling amplification that may foster spin-orbit band engineering.



"Rashba coupling amplification by a staggered crystal field"

David Santos-Cottin, Michele Casula, Gabriel Lantz, Yannick Klein, Luca Petaccia, Patrick Le Fèvre, François Bertran, Evangelos Papalazarou, Marino Marsi & Andrea Gauzzi Nature Communications, 7, 11258 (2016)



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