Physics of quantum materials

The Q-MAT center of CESAM explores the behavior of condensed matter systems, both experimentally and theoretically. Quantum effects appear when materials are structured at the nanometer scale, or sollicited on ultrashort time scales. This can happen artificially, e.g. through engineering of interfaces between semiconductors or insulators, or naturally in materials such as transition metal dichalcogenides or graphene

Experiment

EBL-1Q-MAT has expertise and infrastructure for the synthesis, growth, and characterization of nanoscale systems, comprising semiconductor devices and solar cells. Patterning technology is also used to study mesoscale effects in superconductors and their boundaries with normal and magnetic materials.

 

Theory

nickelates Breathing 250x250Q-MAT is also a reference for theoretical simulations of materials properties, in particular for oxides, thermoelectrics, and disordered/phase change materials. We develop and use software based on classical, semi-classical, and quantum atomistic approaches based on Density Functional Theory. The center comprises several core developers of the ABINIT software package (www.abinit.org) for DFT and beyond (GW, TDDFT, DFPT…). These give us access to properties such as electric and thermal transport, or optical properties and spectroscopy.

A specific strength of Q-MAT is accurate multiscale simulation of mesoscopic properties, using second principles models : model interactions for large scale calculations which are fit to accurate first principles (quantum ab initio) data. In this framework, we parametrize Tight Binding Hamiltonians, interatomic potentials and force constants, and Landau and Heisenberg models. These give us the ability to explore mesoscopic effects in grains and domains, and the thermodynamic properties of realistic systems.

  • Semiconductors and functional insulators : Q-MAT has wide ranging expertise in standard semiconductors (synthesis, characterization, modelling at different levels) and also novel materials used in devices (oxides, fluorides, chalcogenides…). We study functional properties such as piezoelectricity, multiferroicity, solar cell and nanoscale device performance, or optical responses.
  • Superconductors : Mesoscale effects in the structuring and defects of classical superconducting materials are studied experimentally within Q-MAT, at the frontier of classical and quantum time and length scales.
  • Phase change and amorphous materials : Phase change materials demonstrate strong changes in resistivity and reflectivity when switched from their amorphous to their crystalline state. These are the central components in rewritable DVDs and in new computer memory cells where a current pulse can switch the amorphization. Q-MAT studies these systems using ab initio simulation techniques to reveal the relation between structure, aging, and optical properties.
  • Thermoelectrics : Q-MAT has extensive experience with thermal and electrical transport, exploiting anisotropic effects, phase space limitations, and band engineering to optimize the thermoelectric figure of merit in bulk and nanostructured materials.