Research presentation

Materials with an extraordinary behavior

The study of the extraordinary characteristics of certain materials and exploration of the nanoworld are two major research topics in the laboratory.
Take two insulating oxides. Put them in intimate contact. Surprise: at the interface of these two insulators, the material becomes superconducting ! Oxides often exhibit extraordinary properties that are difficult to explain, such as high critical temperature superconductivity, which have a great potential for applications. Fundamental physics still has a bright future ahead to elucidate such phenomena. "This is a major interest of researchers working in the laboratory. The other is a dive into the nanoscale world with the study of the fascinating properties of structures that involve hundreds or thousands of atoms only," says Jérôme Lesueur, director of LPEM. The laboratory now counts about 55 people, including 23 researchers and teacher-researchers. Its activities are organized around three main themes:

• 1) nanophysics, nanostructures and nanomaterials ;
  
• 2) les systèmes électroniques corrélés et de basse dimension ;
  
• 3) l’instrumentation.
  

Promises of the nanoworld

The synthesis of semiconductor nanocrystals is a significant part of this activity. It consists in producing small crystals of different semiconductors, ranging in size between 2 and 30 nm. At this scale, semiconductors exhibit unique electronic and optical properties which can be used in many applications.
Controlling the synthesis of these nanocrystals is crucial since if their properties are obviously dependent on their composition they are also very dependent on their size and shape. An important research result from the lab shows this characteristic. The laboratory has achieved and patented the synthesis of nanocrystals in the form of large area "platelets", which very small thickness is controlled to the monolayer. These crystals have very specific electronic properties. Their emission spectrum is considerably thinner than that of a spherical nanocrystal (8 nm linewidth instead of 20 to 30 nm). In addition, within this platelet, the charges recombine much faster. Two characteristics that make them very interesting candidates for laser applications.
Another very significant research result: semiconductor nanocrystals with thick shells .... Explanation: nanocrystals, excited optically or electrically behave like single photon emitters that can be stably observed for very long periods of time. This could be very useful in biological research or quantum cryptography. The only problem is that these nanocrystals undergo "blinking", which the scientific community has been trying to understand and eliminate for the last twenty years. By covering the spherical crystals with a nano "shell" of 5-15 nm thickness, the LPEM researchers were able to suppress this unwanted blinking.
The laboratory is also heavily involved in the implementation and study of nanostructures: thin layers of materials are finely crafted to make nanowires, nanocontacts..., which then have new properties. Researchers have developed superconducting Josephson nanojunctions that will perhaps be used for tomorrow’s electronics. They also conducted pioneering work on heat transfer at the nanoscale, a major challenge for the miniaturization of electronic devices.

The behavior of oxides and semi-metals in the spotlight

Solid state physics has a long history. However, there are still many areas to explore, especially the world of the oxides or semi-metals that exhibit "strange", or at least unexplained behavior. Take for example transition metals oxides. In contrast to metals, they possess electrons which interact strongly with each other (they are "correlated"). As a result, they organize collectively and form new superconducting, insulating or magnetic phases for example, cooperate or compete on nanometric distances. These power struggles appear for example when studying - one of the lab strengths - superconducting fluctuations, ie how superconducting electrons survive in a "hostile" environment. This competition also shows in the appearance of a massive resistance when a magnetic field is applied in manganites, or "multiferroic" materials for which electric and magnetic phases are coupled. For these studies, the LPEM uses a variety of creative and sophisticated tools. Very significative results have recently been obtained in this field. Beyond these fundamental advances, technological prospects lie ahead. Other highlight results of the LPEM: through the Nernst effect, laboratory researchers have unraveled the mysteries of graphite or other seemingly simple elements such as bismuth, when subjected to a very strong magnetic field (tens of teslas !). In the so-called "ultra-quantum" limit, their behavior departs from traditional metals to resemble unidimensional wires or infinitely thin surfaces. This is amazing physics that opens entirely new fields of research!

A very original instrumentation

Instrumentation is an important activity of the laboratory. Its scope is very broad, from the development of a unique method to detect charges in a semiconductor with submicron resolution, to the participation in the giant interferometer project Virgo, designed to study gravitational waves. The laboratory is also developing new methods of high resolution 3D fluorescence microscopy to probe biological processes.
To characterize materials, optical experiments to detect heat transfer have been developed in the laboratory: mirage effect experience and time-resolved thermoreflectance. Finally, the original "near field" instruments developed in the laboratory can observe the matter at the nanoscale. They measure the local electromagnetic field or the magnetization, heat transfers, or even the electrical conduction of nanowires, nanodots or molecules, etc..