Condensed Matter and Statistical Physics


Atomistic, Molecular, and Electronic Structure Simulations


 Work at CMSP in this area focuses on simulations of systems under a variety of different conditions. All of this with a strong interdisciplinary nature, ranging from condensed matter physics to computational chemistry. On the condensed matter side, this includes studying materials under high pressure, the physics of friction and lubrication, multiferroics and surface physics. For soft-matter systems, this includes bulk liquid water, ions and small organic molecules in water, water at interfaces and finally biological systems such as proteins and DNA. Simulation methods include classical and ab-initio molecular dynamics, path-integral molecular dynamics and electronic structure calculations.


Ongoing research

Physics of ultra-high pressure systems

Most condensed matter ceases to exist in the form we know it at ambient conditions and transforms into something very different once brought to the highest pressures achievable today in the laboratory or found in the interiors of planets. Liquids solidify, crystalline solids repeatedly change their lattice structure, compounds often decompose, insulators generally turn to metals, magnetism fades away, etc. Understanding, explaining, predicting this evolution under pressure constitutes an important physics challenge as well as a potential opportunity to create new metastable but useful materials. Recent themes include the formation of non-molecular CO2 phases, the mixing behavior of methane/water fluids at planetary conditions, the role of anharmonicity in LiH, the computational discovery of a new spin liquid phase in O2, and new phenomena in high pressure frictional shear.

Development of polarizable force-fields for the oxides

 Atomistic methods based on the ab-initio description of the electrons guarantee a chemically accurate description of the forces at play but their computational load grows rapidly with the size of the system.
Accurate force-fields parameterized on ab-initio calculations are a promising way to extend the size and time constraints of ab-initio simulations. Extensive work on polarizable models for oxides has shown that accurate potentials can be constructed that reproduce faithfully the ab-initio results on homogeneous bulk phases. Compounds studied so far include SiO2, MgO, MgSiO3, TiO2, and water. The force-field developed for water, for example, is all-atom, dissociable, and capable of describing with reasonable accuracy also the dissociated state.

Acid-Base Chemistry in Water and Interfaces

 One of the most fundamental processes in acid-base chemistry is the ionization of water, a rare event where a water molecule will spontaneously ionize and form a hydronium ion and a hydroxide ion. The microscopic fluctuations that drive this process remains poorly understood. Research in this area is aimed at understanding the molecular mechanisms by which the proton and hydroxide ion diffuse through bulk water systems. How these processes change at hydrophobic interfaces such as the surface of water and protein surfaces and how these properties are related to spectroscopic features of experiments is an active area of research. A particular area of ongoing development is probing the properties of water as a 3D-network which will hopefully help its collective structural and dynamical signatures.


Hydrogen-Bond Networks in Biological Systems

 When protein folding goes bad, aggregated structures of biological matter known as Amyloid plaques can form. These proteins have been implicated in neurodegenerative diseases such as Alzheimers and have rather unusual optical properties such as being able to fluoresce. An active area of ongoing research is to use a combination of both ground and excited state electronic structure calculations to understand the electronic and optical properties of these systems. Classical molecular dynamics combined with enhanced sampling methods like umbrella sampling and metadynamics are used to explore the conformational landscape of these proteins. These systems also provide good model systems to understand classical problems with bio-physical chemistry such as hydrophobic effects.

Chemical Physics of Solvation

 Water is the most ubiquitous solvent for biological matter such as proteins and DNA – it acts a lubricant and maintains the structural integrity of biomolecules. Research in this area is aimed at understanding the intimate coupling between the fundamental interactions of water and the biological systems. The physical chemistry of how organic molecules such amino acids and DNA bases interact with water is at the heart of hydrophobic solvation which drives many processes such as protein folding, drug-ligand binding and protein aggregation. A combination of both computational simulations and statistical mechanical theory is used to examine how these solutes perturb the structural, dynamical, electronic and spectroscopic properties of water.

Multiferroic materials

Multiferroic materials exhibit more than one of the so-called ferroic orders, namely magnetic, ferroelectric, and ferroelastic order. Current research is strongly focusing of magnetoelectric multiferroics that are simultaneously ferromagnetic and ferroelectric, and in which an electric field can be used to modify the magnetic properties. Theoretical predictions of the existence of such materials were substantiated in past years by the growth of epitaxial films of a variety of complex and nanostructured oxides exhibiting magnetoelectric properties. Besides scientific interest in their physical properties, such systems have potential applications in a number of devices exploiting electric-field control of magnetization or magnetic properties, including actuators, switches, and new types of electronic memory, spintronic and high-frequency magnetic devices. Using first-principles density-functional calculations, we investigate the microscopic mechanisms generating magnetoelectric effects in multiferroic layered materials and interface systems and explore ways to chemically and structurally tailor the magnetoelectric couplings for targeted electrically controlled magnetic properties.

Nanostructures and nanostructured phases of graphene on metal surfaces

Graphene is the most widely researched two-dimensional (2D) material. Its many uncommon properties include a semi-metallic character, high electric and thermal conductivity, and unique mechanical properties: graphene membranes have emerged as the strongest materials measured. Together with the high fexibility provided by the nature of sp2 carbon bond, these outstanding properties make graphene films ideal candidates for ultra-thin, ultra strong, impermeable membranes separating different environments. Graphene films can be grown on metal surfaces on which they form a variety of nanostructured physisorbed and/or chemisorbed phases and are able to trap mesoscopic volumes of gas in nanobubbles, demonstrating suitability for application such as gas-storage, or anvil cells for high-pressure reactions inaccessible under ambient conditions. In addition, the large resulting lattice deformation permits to strain-engineer the local electronic and magnetic properties of the film. We presently investigate by means ab-initio computations the formation and properties of nanostructured phases of graphene on metal surfaces and the physics governing the formation and growth of intercalated gas nanobubbles in graphene; this includes in particular rare-gas solidification in nanobubbles at ambient temperature and their growth/ripening with increasing annealing temperature.

Main researchers