The effect of quantum size confinement on the optical properties of PbSe nanocrystals under exposure to heat and hydrostatic pressure

A study based on photoluminescence and absorption measurements as a function of temperature and pressure for PbSe nanocrystals with sizes in the range 3-13 nm reveals the influence of size quantum confinement on the observed variation. In the case of the temperature variation, the effective bandgap changes from showing a positive rate of change to showing a negative one (for a quantum dot 3 nm in diameter), which can be accounted for by incorporating a linear variation of the carrier effective masses into a simple calculation of the exciton ground state in the quantum dot. In the case of the pressure variation, we observe a clear inverse correlation between the absolute value of the pressure coefficient and the nanocrystal size, a signature of quantum size confinement, with values changing from -76 to -41 meV GPa⁻¹ for quantum dots ranging from 13 to 3 nm in diameter, respectively, clearly smaller in absolute value than the rate for bulk PbSe (-84 meV GPa⁻¹). We used again the hypothesis of a linear variation of the carrier effective masses with pressure in order to fit this experimental variation quantitatively.

Spatial distribution of electron concentration and strain in bulk GaN single crystals - relation to growth mechanism

Micro-infrared reflectivity and micro-Raman scattering have been used to determine free electron density and residual strain distribution in bulk GaN crystals. As-grown samples exhibit significant variation of electron concentration and strain along their surfaces. Increase of electron concentration correlates with the growth direction. The observed inhomogeneities in the properties of bulk GaN crystals can be eliminated by mechanical polishing of the crystal and removing the surface layer or growth figures formed during the sample cooling, which follows its high-pressure, high-temperature growth. Properties of that surface layer can differ from the bulk due to different growth mechanisms and impurity distribution. We suggest that oxygen is a likely candidate for the donor impurity supplying high amount of electrons to the conduction band of GaN.

The pressure-induced phase transition of mechanically alloyed nanocrystalline GaSb

Ga K-edge energy dispersive x-ray absorption spectroscopy and Raman spectroscopy measurements were employed to follow the pressure-induced semiconductor-metal phase transition of nanocrystalline GaSb produced by mechanical alloying up to 26 GPa. The results showed a slight increase of the phase transition pressures for both as-milled (8 GPa) and annealed (10 GPa) GaSb samples, as compared to that for the bulk one. The extended x-ray absorption fine structure analysis of the zinc blende (ZB) pressure domain (<10 GPa) showed that the microscopic compressibility of the bonds in the as-milled/annealed samples is higher/lower than the crystalline bulk modulus (56 GPa). The comparison between x-ray absorption near edge structure regions of the spectra and multiple scattering calculations suggests that the ZB structure evolves to a short-range chemically ordered β-Sn structure for pressures as high as 8 GPa. Raman measurements confirm the semiconductor-metal phase transitions of ZB-GaSb between 8 and 11 GPa for both as-milled and annealed samples, showing that the semiconductor character was not recovered on releasing the pressure down to 3.9 and 1.8 GPa, indicating a very strong hysteresis effect (or even irreversible transitions). The well-known transverse effective charge reduction with pressure was also observed. Furthermore, resonance behaviour is clearly seen for transverse optical phonons and the resonance maxima peak occurs at about 1.2 GPa, corresponding to 2.11 eV in the E(1) scale, smaller by 0.3 eV than the incident photon energy.

Nature of the polyamorphic transition in ice under pressure

We present a neutron diffraction study of the transition between low-density and high-density amorphous ice (LDA and HDA, respectively) under pressure at approximately 0.3 GPa, at 130 K. All the intermediate diffraction patterns can be accurately decomposed into a linear combination of the patterns of pure LDA and HDA. This progressive transformation of one distinct phase to another, with phase coexistence at constant pressure and temperature, gives direct evidence of a classical first-order transition. In situ Raman measurements and visual observation of the reverse transition strongly support these conclusions, which have implications for models of water and the proposed second critical point in the undercooled region of liquid water.

Dynamics of the magnetic and structural alpha-epsilon phase transition in iron

We have studied the high-pressure iron bcc to hcp phase transition by simultaneous x-ray magnetic circular dichroism and x-ray absorption spectroscopy with an x-ray energy dispersive spectrometer. The combination of the two techniques allows us to obtain simultaneously information on both the structure and the magnetic state of iron under pressure. The magnetic and structural transitions simultaneously observed are sharp. Both are of first order in agreement with the theoretical prediction. The pressure domain of the transition observed (2.4+/-0.2 GPa) is narrower than that usually cited in the literature (8 GPa). Our data indicate that the magnetic transition slightly precedes the structural one, suggesting that the origin of the instability of the bcc phase in iron with increasing pressure is to be attributed to the effect of pressure on magnetism as predicted by spin-polarized full-potential total energy calculations.

Two recent developments in XMCD

This paper reports on two new technical developments concerning sample environments for X-ray magnetic circular dichroism (XMCD). The first measurements under high pressures of up to 30 GPa are described. The difficulties of combining the techniques of high pressure and XMCD are commented on. The second development involves the use of a fast-switching magnetic field. A new superconducting device is used to perform XMCD measurements on paramagnetic compounds in magnetic fields of up to 6 T. The small amplitude of the XMCD signal imposes, for a given signal-to-noise ratio, a noise less than a few 10(-5). The signal-to-noise ratio is improved by the use of a series of acquisitions, switching the magnetic field between each acquisition. A very fast switching mechanism has been built based on mechanical rotation of a superconducting coil, with the sample kept in place inside the coil. The XMCD signals at the L(II,III)-edges of paramagnetic rare-earth compounds have been measured at 4.5 K in fields of up to 6 T with a switching time of 11 s.

Anharmonic effects in phonon spectra of GaSe

The different manifestations of the crystalline potential anharmonicity in GaSe were studied. The temperature dependence of one and multiphonon mode energies was obtained from infrared absorption and Raman scattering experiments. The temperature and frequency dependence of multiphonon absorption coefficient were discussed.