Quantum electric dipoles: Water molecules in nano-cages of crystals

• Applicants: Professor Dr. Hans Peter Büchler; Professor Dr. Martin Dressel; Professorin Maria Fyta, Ph.D.
• Subject Area: Experimental Condensed Matter Physics ; Theoretical Condensed Matter Physics
• Term: from 2018 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 397476507

We investigate quantum electric dipoles realized by H2O molecules confined to nanocages of crystals. Since the isolated molecules do not form hydrogen bonds, they constitute an ideal model of a quantum electric dipole lattice. Tunneling within the nanocage smears out the positions of the protons. We plan to study the energy levels of nano-confined water molecules in dielectric frameworks with different geometries. We use broad-band optical spectroscopy to investigate the rotational and vibrational modes, the interaction with the cage walls and the doping atoms in the structure. Quantum-mechanical simulations within the DFT approach, as well as MD model the systems and develop a deep understanding of how quantum tunneling and fluctuations affect the properties and behavior of the confined electric dipoles. We address the dipolar interaction between the H2O molecules that eventually leads to long-range order and the occurrence of an incipient ferroelectric state of water in beryl. We explore how the anisotropic dipole-dipole coupling forms a 3D cooperative many-body state from the delocalized H2O. Here, low-frequency optical and dielectric investigations are the superior methods. Starting with the prototype beryl, varying the filling can modify the length of dipolar chains; the coupling can be enhanced by pressure. The anisotropic interaction is reflected in the polarization-dependent optical and dielectric response. We study the influence of an external electric field and the possibility of domain wall motion. We then go beyond beryl with its particular triangular ab-lattice and investigate other nano-porous crystals, and, consequently, altered couplings between the confined H2O molecules. This modifies the dipolar interaction strength and anisotropy within the ab-plane. When the c-axis coupling is dominant, chains of electric dipoles are realized; if this coupling becomes inferior, the system may be treated as two-dimensional with a geometry imposed by the crystal lattice. The interplay of quantum tunneling, fluctuation, and frustration among the coupled electric dipoles could provide the possibility to realize a quantum electric dipole liquid and glasses.From the theoretical side, we tackle the problem numerically and analytically. We model water confined in various crystal types and calculate the optical spectra for different filling. We then turn to the influence of temperature, pressure, and electric fields on the thermodynamic properties and incipient ferroelectricity of nano-confined water in beryl. In parallel we analyze the many-body phenomena of the interacting dipole moments including their quantum rotation. We want to understand the phase diagram within mean-field theory and the competition between the slow decay of the dipolar interaction and their anisotropy. These mean-field studies are accompanied by suitable numerical simulations with the goal to link the predictions of the model system to the experimental observations.

• DFG Programme: Research Grants
• Major Instrumentation: Impedanz Analysator
• Instrumentation Group: 6100 Verstärker und Bausteine der analogen Meßtechnik (außer 670-679)