Competition or cooperation?

Strongly correlated systems strike a very delicate balance between different types of interactions, whose energy scales are close to each other. Therefore, it is not a surprise, that such materials are often characterized by the coexistence of different types of order: charge, orbital, and spin-density wave, as well as superconducting and magnetic orders. It is natural to assume that these phenomena are competing with each other and, depending on the experimental conditions (as e.g., applied magnetic field, pressure, or temperature), only one type of order to predominate.

Our research group is devoted to the investigation of electronic phase transition in strongly correlated, with the aim of unveiling new phenomena beyond the bulk response. The understanding of these cooperative effects is far from clear and our research group strives to get new insights into their ground through the use of different thermodynamic probes under pressure.

Research topic I: Interplay of magnetism and superconductivity in novel superconductors

The iron pnictide family of superconductors, with transition temperatures as high as 55 K, became the second family of materials capable of achieving high Tc, ending the monopoly of the cuprates in this field. The pnictides are similar to the cuprates in that both families are quasi-2d layered compounds with having an antiferromagnetically ordered ground state in the undoped parent compounds. With doping, the magnetic ordering is suppressed and superconductivity emerges. However, the two families are quite different as well. The pnictides have a multiband electronic structure as opposed to the single band physics of the cuprates. Furthermore, the undoped parent compounds of pnictides are metallic instead of Mott insulators as in the case of the cuprates. For sure, a comparison and contrast between the pnictides and cuprates will help to better understand high Tc superconductivity by identifying the common necessary contributing ingredients. One important issue in the field is e.g. whether the magnetism in the pnictides is driven by local moment physics like it is the case for the cuprates or by itinerant physics as well as to better understand the interplay between the magnetic properties and superconductivity in both iron pnictide and cuprate compounds.

Research topic II: Transition metal dichalcogenides / trichalcogenide (TMD/TMT)

Although great efforts have been made in all known quasi-two dimensional superconductors, including TMDs to high- Tc cuprates and iron based superconductors, the origin and exact boundary of the electronic orderings and superconductivity are still interesting but controversial problems and therefore require further study.

The aim of our research group it to understand the link between superconductivity and charge density wave (CDW) states in TMD materials using thermodynamics and transport measurements under pressures.  We are working on eliminating the grain boundary problem in the high temperature superconductors. We shall do this by changing the chemical or physical doping to these materials.

Research topic III: Studying of the group-IV covalent semiconductors

Despite great efforts in the study of the group-IV covalent semiconductors, there still remain many open questions and unexplained results. The nature of the superconducting coupling mechanism, a characterization of the doping level by a change of lattice parameter, and the origin of the electronic orderings and superconductivity are still debated issues and require further studies to develop a robust understanding.

Pressure is a clean way to tune basic electronic and structural properties without changing the chemistry. To study the BDD films under pressure, we used a diamond anvil cell with silicon oil as a transparent and inert pressure medium.