Strongly correlated quantum condensed matter
We are a theoretical research group in the field of quantum condensed matter physics.
When we say "Quantum condensed matter" we mean we are interested in situations where quantum phenomena emerges at macroscopic scales (e.g. Avogadro number of particles...). A perfect example is superconductivity, which is clearly a quantum phenomena that manifests itself on scales relevant to our everyday life. However, it is not the only one. We are also intrigued by magnetic conductors, magnetic insulators, correlated insulators (that are not magnetic!), emergent lower dimensional states, quantum liquid crystals, ....
We mainly focus on fundamental open questions. These questions can be motivated by puzzling experiments or driven by well posed theoretical reasoning. Below, we specify a few of the problems that have recently swept us away:
1. Superconductivity at low density
Recently a new family of superconductors has been discovered. This family consists of topological materials and semimetals, which become superconducting when they are doped with a small number of carriers. Their phenomenology makes them promising candidates for topological superconductivity (for example the most prominent candidate is the topological insulator Bi2Se3), but why they become superconducting at such low density remains a mystery. At low density the density of states is so small that the standard BCS attractive interaction between electrons becomes ineffective. I believe the solution to this puzzle is a key aspect in understanding these superconductors. To find out more click here.
Bismuth
2. Dynamics of quantum information in many body systems
Imagine an isolated system of many interacting particles, which is initially prepared in some non-generic state, far from equilibrium (e.g. a finite box of atoms, where initially all atoms are squeezed into one corner). Thermodynamics and statistical physics are founded on the idea that eventually, after enough time, such a system will reach equilibrium where the atoms fill the box uniformally, and forget their initial conditions. Despite the importance of this process to the emergence of thermodynamics, the quantum mechanical description of this concept is not completely understood. I am interested in understanding the new aspects of this problem that arise from the quantum perspective. For example, how does quantum information and quantum operators propagate during such an evolution process? How is this propagation affected by disorder, conserved quantities and the nature of the evolution? When does thermalization breakdown and how? etc... To continue reading click here
3. Strontium titanate never ceases to amaze
Strontium titanate (chemical formula SrTiO3) is truly an amazing material. Let me quickly review some of its specs. It is an insualtor which is naturally on the verge of a ferroelectric instability (understood to remain paraelectric only due to quantum fluctuations in the positions of its ions!). As a result it has a huge dielectric constant (around 20,000) and a nearly "gapless" optical phonon mode (a phonon of frequency around 16 cm^-1 ~ 2meV). To make things even more interesting it can be doped with carriers (by a number of techniques) leading to metallic state at a ridiculously low density. As if that was not enough, this metal becomes superconducting below a temperature of roughly half a Kelvin. Finally, it exhibits a huge temperature dependent resistivity, with the following brahvior
R ~ R0 + A T^2 + ...
At 10 or 20 K this resistivity can already grow by 10 folds compared to its zero temperature limit, (despite its tiny Fermi surface). The huge temperature dependence of the resistivity and superconductivity, which are probably related in some way, are most puzzling and are far from being understood. Also, a ferroelectric critical point can be tuned by atom substitution and pressure, and thus the interplay of all this mess with the QCP can be studied.