Condensed matter physics is one of the widest and most active branches of physics. It deals with the properties of condensed phases of matter, e.g., solids, liquids, and superconductors, typically using the rules of quantum mechanics, statistical physics, and electromagnetism. In the past it has given us everything from the components that make up your computer to ultrafast levitating trains, and now it holds the promise of unlocking new types of nanotechnology with applications still barely touched by imagination. The Division of Condensed Matter Theory is mainly working on two material systems that have emerged in recent years as a major focus of the condensed matter physics community:
- graphene, a two-dimensional carbon layer exhibiting a number of fantastic properties, and related two-dimensional material;
- nanophotonic structured materials, such as metamaterials, photonic crystals, and plasmonic systems, which provides new optical properties by structuring materials on a nanoscale instead of on an atomic scale.
Nanomechanics and thermal transport in graphene-related materials (Andreas Isacsson)
Our research is mainly concerned with nanomechanics and thermal transport in graphene and related materials (GRMs). Being genuine 2D crystalline membranes, GRMs hold uniqe positions in contemporary condensed matter and nanotechnology research. In our research, the flexural (out-of-plane) vibrational modes hold a central position. They are both the main degrees of freedom in NEMS and can at the same time be dominant carriers of heat. For this reason, our current focus is turning towards the interplay between flexural motion and the propagation of heat in GRMs. This opens for the possibility to utilize GRMs in phononic applications, e.g. thermal rectifiers as well as in novel thermomechanical measurement schemes. As an aside, we are also starting to take a more active interest in the rheology in aqueous suspensions of 2D-materials. The methods we rely on are analytical calculations and simple numerics along with more extensive molecular dynamics simulations.
Graphene plasmonics (Philippe Tassin, Jari Kinaret, and Peter Apell)
Graphene is an extraordinary material for plasmonics for many reasons: Firstly, owing to the two-dimensional nature of electron motion in graphene, the plasmon dispersion at long wave lengths starts at a very low frequency compared to plasma frequencies in 3D metals; secondly, the electron density in graphene, and hence the plasmon frequency, can be varied over several orders of magnitude; and thirdly, the linear spectrum of non-interacting electrons in graphene implies qualitative differences relative to more conventional systems with parabolic dispersion. Combined these features open the route to widely tunable plasmonics with a very strong concentration of electromagnetic energy. We explore both fundamental aspects of graphene plasmons, such as nonlocal and nonlinear effects, and applications as different types of sensors and novel components for terahertz electronics.
Microscopic modelling of 2D materials (Ermin Malic)
We develop microscopic modelling of ultrafast phenomena in atomically thin 2D nanomaterials including graphene and transition metal dichalcogenides (TMDs). These materials exhibit fascinating properties that are interesting both for fundamental science as well as for technological applications. In particular, we perform quantum mechanical calculations of time- and energy-resolved non-equilibrium dynamics of electrons, excitons, phonons, and photons in these materials. The driving force of our research is to unravel elementary processes behind ultrafast phenomena and to exploit the gained insights to propose novel concepts for technological devices. The explored scientific questions cover fundamental physics including strong many-particle phenomena (such as carrier-carrier, carrier-phonon, and carrier-photon interactions) and application-oriented research aiming at the design of future nanoelectronic devices (such as photo-emitting, amplifying, and detecting devices).
Electromagnetic structured media (Philippe Tassin)
We design electromagnetic metamaterials and structured media and we study their physics using theoretical and computational tools. Electromagnetic structured media are man-made materials that can manipulate electromagnetic radiation—from microwaves, over terahertz waves, to visible light—in ways that are impossible with natural materials. In metamaterials, this is achieved by replacing atoms by small electric circuits as the basic constituents for the interaction of electromagnetic radiation with matter. Metamaterials have the potential to create devices that can exert precise and advanced control over electromagnetic waves.
Electromagnetic waves are conceivably the most widespread concept of physics in our modern society—from a fundamental as well as an applied point of view. Examples range from the use of microwave cavities for the acceleration of elementary particles to numerous industrial, medical and consumer applications such radar technology, mobile phone communication, terahertz imaging for security and medical applications, and optical display technology, just to name a few. Many of these scientific accomplishments are based on our ability to control the propagation and other properties of electromagnetic radiation by interaction with matter. Even though natural materials provide substantial means to control light, they exhibit important shortcomings. For instance, we can typically manipulate only the electric component of electromagnetic waves, whereas the magnetic component is out of reach. Metamaterials and other electromagnetic structured media can manipulate waves in ways that go beyond what natural materials can do. In this way, they may lead to improved technology and new applications, e.g., in microscopy, nano-optomechanical systems, spectroscopy, terahertz devices, etc.
Graphene-based NEMS (Andreas Isacsson and Leonid Gorelik)
Just as shortening a violin string to create a higher pitch, diminishing the size of any mechanical system will increase its frequency. In a nanomechanical (NEM) resonator, the characteristic length scales are measured from nanometers up to a few micrometers while frequencies range from 100 MHz up into the GHz regime. While the equations describing a NEM-resonator are homologous to those of its macroscopic counterpart, their miniscule size leads to an increased sensitivity to external perturbations and fluctuations. For some applications these perturbations are intentional, such as in ultrasensitive mass sensing where the frequency shift of a resonant more due to the adsorption of a single molecule can be measured. For fundamental research, the high sensitivity to force and charge allows the study of quantum fluctuations and coherence effects on mechanical systems.
For obtaining high sensitivity, it is advantageous if the mechanical resonator itself has a low mass, a large resonant frequency, and low dissipation. Graphene is in this respect a promising material, which in addition literally has added a new dimension to NEM-resonator, facilitating the fabrication of atomically thin 2D membrane resonators. We study theoretical aspects of graphene resonators for fundamental and applied implementations. In particular, we take an interest in understanding the consequences of the strong geometric nonlinearities present in these systems as well as the coupling between mechanical degrees of freedom and electronic degrees of freedom. In graphene, the latter is distinct from previously studied material in that the charge carriers have a relativistic behavior due to the linear dispersion near the Fermi-level.