The experimental determination of the phonon dispersion at high pressure constitutes an important ingredient for the characterisation of the physical properties of materials at extreme conditions. It gives access to valuable quantitative information concerning elasticity, thermodynamic properties, and the dynamics of phase instabilities. Furthermore, the experimental data provide important tests for the accuracy of theoretical lattice dynamical models. Among these the most advanced ones are ab initio quantum mechanical calculations, using density functional perturbation theory. Critical inputs are the appropriate choice of the potential (all-electron or pseudopotential approaches) and the correct description of the exchange-correlation term. If a good agreement with the experimental phonon dispersion is observed, these calculations can then be used with increased confidence to describe the physical properties at very high pressures beyond the reach of current experimental methods.
Here we present experimental and theoretical results on MgO, a prototype oxide due to its simple structure and the large stability field (in pressure and temperature) of the NaCl structure. MgO is furthermore an important ceramic for industrial applications, and of great interest for Earth sciences, since it is a major mineral phase of the Earth's lower mantle. A doubly polished single crystal of MgO of (100) orientation, 30 x 50 µm size and a thickness of 20 µm was loaded in a diamond-anvil cell with He as pressure transmitting medium. The IXS experiment was performed on beamline ID28 with an overall energy resolution of 3 meV. Theoretical phonon dispersion curves were calculated using density-functional perturbation theory using the pseudopotential plane wave code ABINIT [1]. Details on the calculations can be found elsewhere.
Here we present experimental and theoretical results on MgO, a prototype oxide due to its simple structure and the large stability field (in pressure and temperature) of the NaCl structure. MgO is furthermore an important ceramic for industrial applications, and of great interest for Earth sciences, since it is a major mineral phase of the Earth's lower mantle. A doubly polished single crystal of MgO of (100) orientation, 30 x 50 µm size and a thickness of 20 µm was loaded in a diamond-anvil cell with He as pressure transmitting medium. The IXS experiment was performed on beamline ID28 with an overall energy resolution of 3 meV. Theoretical phonon dispersion curves were calculated using density-functional perturbation theory using the pseudopotential plane wave code ABINIT [1]. Details on the calculations can be found elsewhere.
The fact that characteristic features in the phonon dispersion are well reproduced by calculations gives confidence that ab initio predictions of thermodynamic properties of MgO at high pressure will be accurate. The determination of a thermodynamic property at high pressure requires experimental determination of the thermal expansion and bulk modulus, which are recast into an equation of state (EOS). Such EOS data are very few and when available, the data usually requires large extrapolation. Thermodynamic properties at high pressure may be calculated from a combination of calorimetric data at 1 bar and the volume integral with changing pressure and temperature [3]. Using the available thermodynamic data we obtain CV = 30 (+/-5) and S = 20.68 (+/-1) Jmol-1K-1. From the calculated phonon density-of-states at 35 GPa we determine CV = 31.71 and S = 20.04 Jmol-1K-1. The two data sets match within the errors of experimental data.
In summary, we demonstrate the ability of modern theory to reproduce experimental data on lattice dynamics of an inorganic compound at very high pressure. Expanding such tests to other, more complex systems could be beneficial for the development of both theory and experiment. These tests, validating the approximations done in the calculations, will allow the reliable determination of the thermodynamic properties of materials at high pressure, which are otherwise extremely difficult to assess by experimental methods.
Ultrafast carrier and lattice dynamics in semiconductor and metal nanocrystals
This articule presents an experimental study of the time-resolved optical response of three different nanoscale systems: CdSe and PbSe quantum dots, and silver triangular nanoplates. The first part of the thesis is devoted to the understanding of the effects of quantum confinement on carrier-carrier interaction in a "model" system: CdSe quantum dots. This issue is addressed by investigating the evolution of the early-time fluorescence spectra of quantum dots of different sizes and lattice structure. The experiment is performed using a femtosecond photoluminescence up-conversion technique, with polychromatic detection. The transient photoluminescence spectra reveal the emission from short-lived multiexciton states. By combining a detailed spectral and kinetic analysis, it is possible to: (i) evaluate the binding energies of these states and therefore acquire insight on the strength of multi-particle interactions, (ii) understand how these interactions affect the lifetime of multiexciton states and, (iii) infer their mechanisms of formation upon optical excitation. We find that confinement-enhanced Coulomb interaction between carriers leads to large binding energies (> 20 meV) and activates efficient Auger-type recombination. This last mechanism points to somewhat different carrier interactions with respect to bulk semiconductors. Surprisingly, we observe that "tailoring" the lattice structure of the quantum dot does not significantly affect the spectral and dynamic properties of multiexciton states. The second part of the thesis addresses the effects of quantum confinement in semiconductor nanocrystals from a slightly different point of view, by investigating PbSe quantum dots. This material is supposed to exhibit a mirror-like, sparse, energetic structure due to extreme quantum confinement which should profoundly alter the carrier relaxation dynamics. We analyze the inter- and intra-band relaxation by combining several techniques. In order to characterize the evolution of the particles luminescence from the nanosecond to the femtosecond range, we perform time-correlated single photon counting and femtosecond near-infrared photoluminescence up-conversion measurements. The results are compared with near infrared, broadband transient absorption measurements. Overall, we observe extremely fast intraband relaxation times, on sub-ps time scales, slightly increasing with decreasing dot size. From our analysis we can estimate a weak electron-phonon coupling between excited states, and we observe that surface mediated relaxation does not play a relevant role in this system. The third part of this work concerns the investigation of the time-resolved optical response of silver triangular nanoplates. The optical response provides fundamental information about the relaxation mechanisms of plasmons, electrons and phonons in metal nanocrystals, and access to the mechanical properties of metal nanoparticles. The anisotropy of the system under study is found to influence the physical properties: we observe for the first time two different excitation mechanisms of mechanical vibrations. In order to disentangle homogenous and inhomogeneous contributions, we present a model which takes into account a realistic distribution of particle size and shape, and which is able to capture the relevant dynamics in these complex systems.
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