Research presentation

Introduction

Research activities at LPEM mainly focus on properties of new and sometimes exotic materials, starting from solid state chemistry to condensed matter physics, mixing basic research and applications. Indeed, this research is rich and broad, and one cannot detail the whole activity of the LPEM laboratory in a few lines. However, three main fundamental lines emerge :

• Nanomaterials, nanostructures and nanophysics.
  Tailoring materials at nanoscale provides new and exciting properties, and possibly applications. Bottom-up and top-down strategies are used at LPEM to design nanomaterials and nanostructures. A great variety of experimental probes are used to investigate their behavior and to draw a physical landscape of the nanoworld, including optics, superconductivity, magnetism, thermal properties ... Applications for energy, biology, medicine are on their way.
• Strongly correlated and low dimensionality electronic systems
  LPEM researchers investigate new materials and phases whose properties strongly depart from the usual Landau theory of metals, such as high Tc superconductors, manganites, low dimensional system, some heavy fermions and semi-metals ... Strong electronic correlations and fluctuations often play a major role in these systems. Their experimental studies through electronic, heat and entropy transport measurement and spectroscopies (optics, NMR, STM ...) is a major research focus at LPEM.
• Instrumentation
  LPEM develops original devices and systems on a wide range of domains, from super-resolution and thermoreflectance microscopes to metrology apparatus for the VIRGO collaboration, including near-field probes and submicronic charge localization.
  

Indeed, research goes beyond this simple scheme, and cross-work gives very interesting and original results.

Nanomaterials, nanostructures and nanophysics

Quantum dots
Synthesis of high quality colloidal Quantum Dots (QD) based on II-VI materials is an emerging key activity LPEM. The quantum confinement of wave functions in nanometer size materials and the adjustable optical properties which follow from can be used on a large scale, thanks to the monodisperse colloidal solutions obtained by the organometallic synthesis invented by Bawendi in the nineties. Breakthroughs in the control of the shape and the optical properties of semi-conducting QD have been recently obtained at LPEM. The main results are the following :

• The blinking of the QD has been drastically reduced. Due to uncontrolled surface states, fluorescence of QD is sometimes blocked, and they “blink”. Core-shell structures have been grown with a thick (5nm) CdS shell on top of a CdSe core, which display a very low blinking. A detailed analysis of the blinking statistics showed that the Auger process is reduced due to the delocalization of the electron in the thick shell, which is in favor of the radiative recombination, and therefore the light emitting configuration.
• Nanoplatelets of CdSe controlled at the atomic level. A new kind of colloidal QD has been discovered at LPEM, namely CdSe nanoplatelets with a very large aspect ratio (more than 20) whose thickness can be controlled at the atomic scale. As a consequence, the fluorescence lines can be adjusted very precisely and are extremely narrow (10 nm). A startup company has been set-up based on patents related to such materials.
  In addition, in-vivo cell imaging with colloidal QD has been put forward, thanks to a new surface chemistry which allows better cell penetration, and to infra-red emitting QD based on the cadmium free CuInS2/ZnS.

Nanostructures and nanophysics
In addition to this bottom-up approach, nanostructures are made at LPEM by use of advanced e-beam lithography to address issues of interest. Two main lines have been followed :

• Nano-plasmonics and thermal properties at a nanoscale. A near-field microscope using a fluorescent nanoparticle is used to locally probe the evanescent electromagnetic field (EMF) of surface plasmons. On the contrary to other techniques, this probe is sensitive to all the components of the EM : precise mapping of the EMF in quantitative agreement with calculations in the three directions have been observed in two-slits or step-induced plasmon interference patterns for instance. This probe has been also use to observe heat confinement and transfer in electrically heated nanostructures (Ti stripes on SiO2/Si) with a resolution better than 200 nm.
• Josephson high Tc nanojunctions. A new technique has been used to make reliable Josephson Junctions (JJ) of the High Tc superconductor YBCO. Local disorder at a scale of 20 nm is introduced by selective ion irradiation to create a weak link which behaves as a JJ. This reproducible and scalable technique opens the way for making devices like SQUIDs, IR and THz photon detectors, and to superconducting digital electronics in the 30-50 K temperature range.

Strongly correlated and low dimensionality electronic systems

Superconducting fluctuations
The transition to the superconducting state is characteristic of both the normal and the superconducting properties of a material and its study is therefore an important element to understand superconductors physics. LPEM researchers have made key experiments on high Tc superconductors (HTSc) to study the transition from the pseudo-gap state, marked a by charge and spin density of states densities, to the superconducting one. Resistivity vs temperature measurements supported by theoretical calculations in underdoped cuprates showed that standard gaussian fluctuations dominate. Pseudo-Josephson experiments directly probed the fluctuating Cooper pairs and drew the same conclusions, that is that the pseudogap phase is not a precursor state of superconductivity.
It has been shown that the best probe of superconducting fluctuations in dirty superconductors is the Nernst effect, whose contribution dominates the normal one up to 30 times Tc in disordered metals like NbSi for instance. This discovery was an important contribution to the debate of the origin of a giant Nernst effect measured in some HTSc.

Semi-metals in the ultra-quantum limit
In semi-metals like bismuth for instance, the carrier density is low and one can reach the quantum limit (beyond the first Landau level) in moderate magnetic field. Thermoelectric response have been found to be a very sensitive probe of the Fermi surface in such extreme conditions. Among results, let’s emphasize the discovery of Nernst peaks at fractional Landau numbers in Bismuth, and the evidence of key role of the dimensionality in Nernst quantum oscillations in semi-metals.

Strongly correlated systems
Many experiments have been devoted to this fundamental subject in condensed matter physics in the recent years at LPEM, mainly on oxides but also on some heavy fermions compounds. NMR experiments on calcium based manganites evidenced magnetic polarons surprisingly far in the paramagnetic phase. Optical spectroscopy on BiFeO3 showed a coupling between polar phonons and spin excitation, which could explain its multiferroic (magnetic and ferroelectric) behavior. Superconductivity has been discovered at the interface between a band insulator SrTiO3 and a Mott insulator LaTiO3.

Instrumentation

Localization of particles and charges
New imaging techniques using structured illumination have been developed at LPEM, with allows enhanced resolution (a factor of 2 beyond the diffraction limit) with a commercial microscope, but also 3D reconstruction ; position of 100 nm beads can be detected with a precision of 50 nm even at a deepness of more than 1.5 mm. This is very important for cell imaging using QD for instance.
Charges in insulators can be detected even in complex structures using pressure wave propagation and electro-acoustic methods. Pioneer work at LPEM has been made with unsurpassed resolution.

Heat transfer in electronic devices
Thermoreflectance measurements under microscope have been developed to characterize heat dissipation in working devices. Resistor lines and VCSEL lasers under running conditions (up to 1 MHz) have been studied, with a spatial resolution of 200 nm.

VIRGO experiment
LPEM is involved in the development of advanced instrumentation for the VIRGO experiment (direct gravitational waves detection). Benchmark tests for cavity mirrors quality have been set-up and conducted, together with a laser-based stabilization of the mirrors positioning system.

Conclusion

Research activity at LPEM presents a wide variety of aspects, from the most fundamental questions studied by measurements in extreme conditions to the realization of devices and systems for tomorrow applications. The great majority of the results are published in international journals, and a very high proportion in top level ones. LPEM researchers are greatly involved in international collaborations. They are strongly supported by funding agencies like ANR, CNano, CNRS, DGA, regional funds ... through competitive invitations to tender. Young and talented researchers recently joined LPEM, with innovative and exciting projects.