Pascal Quémerais

Melting of a Wigner crystal in an ionic dielectric

Abstract:The melting of a Wigner Crystal of electrons placed into a host polar material is examined as a function of the density and the temperature. When the coupling to the longitudinal optical modes of the host medium is turned on, the WC is progressively transformed into a polaronic Wigner crystal. We estimate the critical density for crystal melting at zero temperature using the Lindeman criterion. We show that above a certain critical value of the Fröhlich electron-phonon coupling, the melting towards a quantum liquid of polarons is not possible, and the insulator-to-metal transition is driven by the ionization of the polarons (polaron dissociation). The phase diagram at finite temperature is obtained by making use of the same Lindeman criterion. Results are also provided in the case of an anisotropic electron band mass, showing that the scenario of polaron dissociation can be relevant in anisotropic compounds such as the superconducting cuprates at rather moderate e-ph couplings.

Polarization catastrophe in the polaronic Wigner crystal

Abstract:We consider a three dimensional Wigner crystal of electrons lying in a host ionic dielectric. Owing to their interaction with the lattice polarization, each localized electron forms a polaron. We study the collective excitations of such a polaronic Wigner crystal at zero temperature, taking into account the quantum fluctuations of the polarization within the Feynman harmonic approximation. We show that, contrary to the ordinary electron crystal, the system undergoes a polarization catastrophe when the density is increased. An optical signature of this instability is derived, whose trend agrees with the experiments carried out in Nd-based cuprates.

Nature of the metal–nonmetal transition in metal–ammonia solutions. I. Solvated electrons at low metal concentrations

Using a theory of polarizable fluids, we extend a variational treatment of an excess electron to the many-electron case corresponding to finite metal concentrations in metal-ammonia solutions (MAS). We evaluate dielectric, optical, and thermodynamical properties of MAS at low metal concentrations. Our semianalytical calculations based on a mean-spherical approximation correlate well with the experimental data on the concentration and temperature dependencies of the dielectric constant and the optical absorption spectrum. The properties are found to be mainly determined by the induced dipolar interactions between localized solvated electrons, which result in the two main effects: the dispersion attractions between the electrons and a sharp increase in the static dielectric constant of the solution. The first effect creates a classical phase separation for the light alkali metal solutes (Li, Na, K) below a critical temperature. The second effect leads to a dielectric instability, i.e., polarization catastrophe, which is the onset of metallization. The locus of the calculated critical concentrations is in a good agreement with the experimental phase diagram of Na-NH(3) solutions. The proposed mechanism of the metal-nonmetal transition is quite general and may occur in systems involving self-trapped quantum quasiparticles.

Near-field optical microscopy with a scanning tunneling microscope

A homemade apertureless near-field optical microscope using a scanning tunneling microscope (STM) is described. The experimental set-up simultaneously provides optical and topographic images of the sample. Technical details and features of the set-up are presented, together with results demonstrating the sub-wavelength resolution achieved as well as its sensitivity to dielectric contrasts. We show that the use of a STM permits to precisely control very small distances between the tip and the sample which is a great advantage to excite localized optical resonances between the tip and the surface.

Herzfeld instability versus Mott transition in metal-ammonia solutions

Abstract Although most metal–insulator transitions in doped insulators are generally viewed as Mott transitions, some systems seem to deviate from this scenario. Alkali metal–ammonia solutions are a brilliant example of this. They reveal a phase separation in the range of metal concentrations where a metal–insulator transition occurs. Using a mean spherical approximation for quantum polarizable fluids, we argue that the origin of the metal–insulator transition in such a system is likely to be similar to that proposed by Herzfeld a long time ago, namely, due to fluctuations of solvated electrons. We also show how the phase separation may appear: the Herzfeld instability of the insulator occurs at a concentration for which the metallic phase is also unstable. As a consequence, the Mott transition cannot occur at low temperature. The proposed scenario may provide a new insight into the metal–insulator transition in condensed-matter physics. To cite this article: G.N. Chuev, P. Quemerais, C. R. Physique 8 (2007).

Comment on “Model of saturated lithium ammonia as a single-component liquid metal” [J. Chem. Phys.124, 074702 (2006)]

We demonstrate in this Comment that the theory of simple metals applied to the saturated Li-NH3 solution in the titled paper [U. Pinsook and S. Hannongbua, J. Chem. Phys.124, 074702 (2006)] should account for the peculiarities of the solution, namely, the high solvent polarizability and different energy scales for ion-ion and electron-electron interactions. Calculations not taking into account these peculiarities contradict the experimental phase diagram of the Li-NH3 solution.

Muon diffusion and electronic magnetism in Y2Ti2O7

We report a muon spin relaxation study in a Y2Ti2O7 single crystal.We observe slow local field fluctuations at low temperature which become faster as the temperature is increased. Our analysis suggests that muon diffusion is present in this system and it is small below 40 K and therefore incoherent. A surprisingly strong electronic magnetic signal is observed with features typical for muons thermally diffusing towards magnetic traps below ≈100 K and released from them above this temperature.We attribute the traps to Ti3+ defects in the diluted limit. Our observations are highly relevant to the persistent spin dynamics debate on R2Ti2O7 pyrochlores and their crystal quality

Microscope spectrometer for light scattering investigations

We describe a setup including a microscope to study volumes of a few mum(3) by static and dynamic light scattering (DLS) in a backscattering configuration. Light scattered by individual objects of micrometric size can be analyzed in the 400-800?nm spectral range. This setup can also be employed to study both diluted and concentrated colloidal solutions by DLS measurements. For diluted solutions we found evidence of the fluctuations of the number of particles in a confocal volume. We discuss their contribution to the autocorrelation function of the scattered intensity measured as a function of time.

Enhancement of Wigner crystallization in quasi-low-dimensional solids

The crystallization of electrons in quasi low-dimensional solids is studied in a model which retains the full three-dimensional nature of the Coulomb interactions. We show that restricting the electron motion to layers (or chains) gives rise to a rich sequence of structural transitions upon varying the particle density. In addition, the concurrence of low-dimensional electron motion and isotropic Coulomb interactions leads to a sizeable stabilization of the Wigner crystal, which could be one of the mechanisms at the origin of the charge ordered phases frequently observed in such compounds.