First-principles calculations of spectroscopic signatures | NHR-Verein: Wir sind Hochleistungsrechnen

The project studies the spectroscopic signatures of molecular clusters and ferroelectric solid solutions with extreme non-linear optical properties. It examines how atomic and electronic structure, chemical composition, and their interactions influence these signatures. Using first-principles modeling, atomistic calculations are performed within the density functional theory (DFT) framework and advanced methods like hybrid-DFT, time-dependent DFT, and many-body perturbation theory. Prototypical systems such as adamantane- or cubane-shaped clusters and crystalline solids are investigated to identify the prerequisites for optical non-linearities, guiding the synthesis of new compounds with tailored optical properties.

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Introduction
The project aims to link experimental and theoretical observations of physical systems by calculating spectroscopic signatures of solids and molecules from first principles, using only basic laws of physics without empirical parameters. This approach allows for the explanation of microscopic physical mechanisms and verification of proposed models. The process involves two steps: modeling the electronic ground state using density functional theory (DFT) and then calculating structural and electronic excitations. The study covers systems from 0D clusters to 3D bulk materials, with a strong connection to experimental data, enabling a combined approach that advances materials science.

Methods
DFT computes the electronic ground-state energy and charge density of atomic configurations. The Kohn-Sham (KS) equations are solved iteratively until the total energy change is below a threshold. Efficient algorithms, such as the periodic unit cell approach and matrix diagonalization schemes like Davidson block diagonalization, are used. The number of plane waves and k-points is determined by the convergence of macroscopic quantities.

Results
Subproject A focuses on LiNbO3-LiTaO3 solid solutions, using DFT as implemented in VASP and QuantumEspresso. The nonlinear optical response and heat capacity were calculated, revealing the origin of strong nonlinear optical coefficients. Ferroelectric domain walls were simulated as strained materials, aligning with X-ray investigations. The calculated heat capacity matched experimental results from Prof. S. Ganschow's group, providing insights into structural phase transitions. Collaborations with Prof. Lukas Eng explored strained cells, aiding in Raman spectroscopy calibration and explaining lattice dynamics. The reduced formation energy of lattice defects in strained samples explains defect accumulation at domain walls.
Subproject B investigates adamantane-based molecular clusters and their optical properties. Ground state structures were calculated, and the effects of chemical modifications and habitus on the optical response were explored. Collaborations with Prof. D. Mollenhauer studied cluster dimers, trimers, and tetramers. Calculations demonstrated white light generation from clusters forming glasses, a significant step toward practical applications. Understanding cluster interactions allows for predicting the habitus of molecular compounds, crucial for white light generation. The calculations align with experimental results from collaborators, advancing the understanding of these molecules.
Ongoing work/outlook
Within subproject A, much effort is currently dedicated to the atomistic simulation of ferroelectric domain walls, which represent 2D conducting paths in otherwise insulating materials. Understanding the origin of the conductivity is of central relevance toward applications in nanoelectronics.
Within subproject B, the current goal is to understand the microscopic mechanisms leading to the nonlinear optical response. An analytic scheme to correlate electronic transitions and resulting optical response is being developed, which allows to univocally determine the prerequisites for optical nonlinearities.

References
[1] U. Bashir, K. Böttcher, D. Klimm, S. Ganschow, F. Bernhardt, S. Sanna, M. Rüsing, L. M. Eng, and M. Bickermann, Solid Solutions of Lithium Niobate and Lithium Tantalate: Crystal Growth and the Ferroelectric Transition, Ferroelectrics 613, 250 (2023)
[2] C. Kofahl, L. Dörrer, B. Muscutt, S. Sanna, S. Hurskyy, U. Yakhnevych, Y. Suhak, H. Fritze, S. Ganschow, and H. Schmidt, Li Self-Diffusion and Ion Conductivity in Congruent LiNbO3 and LiTaO3 Single Crystals, Phys. Rev. M 7, 033403 (2023)
[3] E. Singh, M. N. Pionteck, S. Reitzig, M. Lange, M. Rüsing, L. M. Eng, and S. Sanna, Vibrational properties of LiNbO3 and LiTaO3 under uniaxial stress, Phys. Rev. Mat. 7, 024420 (2023)
[4] C. Dues, M. J. Müller, S. Chatterjee, C. Attaccalite, and S. Sanna, Nonlinear optical response of ferroelectric oxides: First-principles calculations within the time and frequency domains, Phys. Rev. Mat. 6, 065202 (2022)
[5] M. J. Müller, F. Ziese, J. Belz, F. Hoppe, S. Gowrishankar, B. Bernhardt, S. Schwan, D. Mollenhauer, P. Schreiner, K. Volz, S. Sanna, and S. Chatterjee, Octavespanning emission across the visible spectrum from single crystalline 1,3,5,7-tetrakis-(pmethoxyphenyl)adamantane, Opt. Mat. Express 12, 3517 (2022)
[6] I. Rojas-Léon, J. Christmann, S. Schwan, F. Ziese, S. Sanna, D. Mollenhauer, N. W. Rosemann, and S. Dehnen, Cluster-Glass for Low-Cost White-Light Emission, Adv. Mat. 34, 2203351 (2022)
[7] K. Eberheim, C. Dues, C. Attaccalite, M. J. Müller, S. Schwan, D. Mollenhauer, S. Chatterjee, and S. Sanna, Tetraphenyl Tetrel Molecules and Molecular Crystals: From Structural Properties to Nonlinear Optics, J. Phys. Chem. C 126, 3713 (2022)
[8] M. Denk, S. Chandola, E. Speiser, J. Plaickner, S. Sanna, P. Zeppenfeld, N. Esser, Surface Resonant Raman Scattering from Cu(110), Phys. Rev. Lett. 128, 216101 (2022)
[9] S. Chandola, S. Sanna, C. Hogan, E. Speiser, J. Plaickner, N. Esser, Adsorbateinduced modifications in the optical response of Si(553)-Au nanowires, Phys. Stat. Sol. B Rapid Research Letters 16, 2200002 (2022)
[10] P. Yogi, J. Koch, S. Sanna, and H. Pfnür, Electronic phase transitions in quasi-onedimensional atomic chains: Au wires on Si(553), Phys. Rev. B 105, 235407 (2022)