Analysis of volume dependence of Grüneisen ratio

Various models on volume dependence of the Grüneisen ratio have been analyzed in the present study. The Sharma model [Mod. Phys. Lett. B 22/31 (2008) 3113] is found to be similar to that used by Nie [Phys. Stat. Sol. (b) 219 (2004) 241] on the basis of approximation made by Jeanloz [J. Geophys. Res. 94 (1989) 5929]. The Nie expression is amended in a manner so that the resulted expression follows the constraint of high pressure thermodynamics in the limit of infinite pressure. The newly developed relationship is applied successfully on materials for which experimental data are accessible such as epsilon-iron, NaCl, Li, Na, and K.     

Volume and pressure dependence of Grüneisen parameter γ for solids at high temperatures

The present paper simply proposes a computing model for the volume and pressure dependence of the Grüneisen parameter γ at high temperatures. Applying it to ε-Fe, NaCl, Li, Na and K in different pressure ranges, we find that the calculated results of γ are in good agreement with the experimental data. Based on this modified model the thermal pressure P_th of NaCl is also investigated.     

Generalized Grüneisen parameters and low temperature limit of lattice thermal expansion of cadmium and zirconium

The generalized Grüneisen parameters (γ′ _j ) and (γ″ _j ) for cadmium and zirconium were calculated from the second- and third-order elastic constants to determine the low temperature limit of the volume thermal expansion of these metals of hexagonal symmetry. The low temperature limit of cadmium and zirconium was calculated to be positive values indicating a positive volume expansion down to 0 K even though many Grüneisen gammas were found to be negative.     

An atomistic model for the simulation of acoustic phonons, strain distribution, and Grüneisen coefficients in zinc-blende semiconductors

An accurate modeling of phonons, strain distributions, and Grüneisen coefficients is essential for the qualitative and quantitative design of modern nanoelectronic and nanooptoelectronic devices. The challenge is the development of a model that fits within an atomistic representation of the overall crystal yet remains computationally tractable. A simple model for introducing the anharmonicity of the interatomic potential into the Keating two-parameter valence-force-field model is developed. The new method is used for the calculation of acoustic phonon and strain effects in zinc-blende semiconductors. The model is fitted to the Grüneisen coefficients for long-wavelength acoustic phonons and reproduces the response to strain throughout the Brillouin zone in reasonable agreement with experiment.     

Quantification and improvement of the signal-to-noise ratio in a magnetic resonance image acquisition procedure

A procedure is developed to quantify and improve the signal-to-noise ratio (SNR) of magnetic resonance images. The image SNR is quantified using the correlation function of two independent acquisitions of an image. To test the performance of the quantification, SNR measurement data are fitted to theoretically expected curves. The proposed correlation technique is also used to improve the SNR by estimating the amplitude of the signal spectrum. The technique is applied to a set of MR images, and its performance in terms of gain in SNR, contrast-to-noise ratio (CNR), and resolution loss is compared to that of classical noise filters. The SNR as well as the CNR is improved significantly with minor loss of resolution. Finally, it is shown that the correlation technique can be implemented in a highly efficient way in almost any acquisition procedure of a magnetic resonance imaging system.     

Relation between the Grüneisen ratio and the pressure dependence of Poisson's ratio for metals

The Grüneisen ratio of crystalline solids is shown to be dependent on a parameter n whose values are characteristic of each solid, and can be determined by two independent ways: from experimental shock data and from the pressure derivative of Poisson's ratio. The determinations are made for several metals, using data on the pressure derivatives of polycrystalline elastic moduli or of the second order elastic constants measured on single crystals, and giving the pressure derivatives of Poisson's ratio by means of the Voigt-Reuss-Hill averaging procedure. The values of the parameter n deduced from shock data are found to be in good agreement with those deduced from the pressure derivatives of Poisson's ratio. Positive and negative values of parameter n correspond respectively to increasing and decreasing Poisson's ratio with increasing pressure. Discussion of the results is made using the linear and the quadratic relationships between shock velocity and particle velocity. It is shown that shock wave data cannot yield directly an accurate estimation of the derivative of the initial slope of the Hugoniot.     

Representation surfaces of Young''s modulus and Poisson''s ratio for BCC transition metals

 The general expressions for the compliance , Young's modulus E(h k l) and Poisson's ratio υ(h k l) along an arbitrary direction [h k l] are given for cubic crystals. The representation surfaces, for which the length of the radius vector in the [h k l] direction equals to E(h k l) or υ(h k l), are constructed for seven BCC transition metals Cr, Fe, Mo, Nb, Ta, V and W. Neglecting W, which is isotropic, both representation surfaces of Young's modulus and Poisson's ratio are spherical surfaces. The remaining BCC transition metals may be grouped into two classes. In the first group (Cr, Mo, Nb and V) with negative values of s_A, Young's modulus surface has eight depressed corners and six rounded protuberances at the centers of the faces. In the second group (Fe and Ta) with positive values of s_A, on the contrary, Young's modulus surface has eight rounded protuberance corners and six depressions at the centers of the faces. The contrary conclusions are obtained for Poisson's ratio.     

Huang–Rhys side-bands in the emission line of a single InAs quantum dot

We report the first direct observation of Huang–Rhys side-bands in the photoluminescence spectrum of a single InAs/GaAs quantum dot (QD). At low temperature (10 K) the single QD spectrum has a quasi-Lorentzian profile. At higher temperatures, we observe a strong deviation from a Lorentzian profile with the appearance of asymmetric side-bands which become symmetric above 70 K. We obtain an excellent agreement with theoretical calculations done in the framework of the Huang–Rhys formalism. We conclude that the zero-phonon linewidth is the relevant parameter for the observation of the low-energy acoustic phonon side-bands.     

Investigations of backscattering peaks and of the nature of the confining potential in open quantum dots

The properties of open quantum dots are examined in magneto-transport. The quantum dots are prepared from a two-dimensional electron system (2DES) in AlGaAs/GaAs by lateral gate structures. These quantum dots are open, i.e. they are still connected to the surrounding 2DES regions. The low magnetic field magnetoresistance shows peak structures. These structures can be related to semi-classical ballistic trajectories in the confining potential of a dot. The calculations of different confining potentials (abrupt “hard-wall” and parabolic “soft-wall”) are compared with the experimental results. The experiments are better described by a soft-wall potential.     

Real-time dynamics of the acoustically driven electron–hole transport in GaAs quantum wires

We have investigated the dynamics of photogenerated carriers in GaAs quantum wires during their transport by surface acoustic waves using spatially and time-resolved photoluminescence. This technique allows us to map in real time the spatial carrier distribution during the transport as well as to study the nature of radiative defect sites in the transport path.     

Magnetic resonance imaging as an adjunct to sonography in the evaluation of the female pelvis

Magnetic resonance imaging (MRI) of the pelvis is generally considered to be most beneficial in those cases where the pelvic sonogram is limited or equivocal. All cases that underwent both sonographic and MRI examinations at our institution for the evaluation of the female pelvis in the past two years were retrospectively reviewed. We reviewed the sonographic and MRI reports and the subsequent clinical management in the 41 cases that had both studies to assess whether MRI contributed to the clinical management decision. Both studies were interpreted independently based upon the known clinical and laboratory data available at the time. MRI was obtained in 21 cases because the sonogram was suboptimal or inconclusive. In the other 20 cases it was obtained for additional information, even though the sonogram was diagnostic. Of the 21 inconclusive sonographic studies, MRI established or clarified the diagnosis in all cases. Of the 20 studies where MRI was obtained for additional information, MRI added useful data that helped contribute to the clinical management of 11 patients. MRI is an important adjunct to pelvic sonography. It established, clarified, or added significant data in 78% of cases.     

Highly efficient radiative recombination of electron–hole pairs localized at compound semiconductor quantum dots embedded in Si

We have studied the optical properties of compound semiconductor quantum dots (CSQDs) embedded in Si. Both photoluminescence and electroluminescence spectra were found to be associated with an inhomogeneously broadened band in the near-infrared. A long decay lifetime of luminescence was observed, which is in support of an indirect transition in both k- and real-space. Strong localization of electron–hole pairs was found to occur due to a deep potential well created by the built-in electric dipole at the III–V/Si interface. A Si-based light-emitting diode with GaSb-CSQDs in the active layer showed a high value of quantum efficiency. Light amplification was also observed under pulsed laser excitation.     

An assessment of the safety of rapidly changing magnetic fields in the rabbit: Implications for NMR imaging

The effects of rapidly changing magnetic fields on the cardiac and respiratory functions of anaesthetised rabbits have been investigated. No changes were observed using pulsed fields with peak rates of change of 60 T/sec. The implications of these experiments for the safety of NMR imaging are discussed.     

Sensitivity-enhanced IPAP experiments for measuring one-bond C–C and C–H residual dipolar couplings in proteins

Sensitivity-enhanced 2D IPAP experiments using the accordion principle for measuring one-bond 13C'-13Calpha and 1Halpha-13Calpha dipolar couplings in proteins are presented. The resolution of the resulting spectra is identical to that of the decoupled HSQC spectra and the sensitivity of the corresponding 1D acquisitions are only slightly lower than those obtained with 3D HNCO and 3D HN(COCA)HA pulse sequences due to an additional delay 2Delta. For cases of limited resolution in the 2D 15N-1HN HSQC spectrum the current pulse sequences can easily be modified into 3D versions by introducing a poorly digitized third dimension, if so desired. The experiments described here are a valuable addition to the suites available for determination of residual dipolar couplings in biological systems.     

Mathematical modeling of two-photon thermal fields in laser–solid interaction

In this paper, we have developed an analytical model to study the temperature distributions in IR optical materials heated by laser pulses. Our model takes into account the two-photon absorption (TPA). The calculations are based on a three-dimensional model of heat diffusion in solids using the integral transform method. We find out the rigorous analytical expression of the thermal field when one considers both one- and two-photon absorption. The model is valid for any laser–solid system whose interaction can be described by the generalized Beer–Lambert law. Specific results are presented for an application of the model to ZnSe sample. We find out that TPA can produce detectable temperature variation.     

Predictability of SNR and reader preference in clinical MR imaging

Fifty-four independent scans were performed in two volunteers covering one anatomic region in each (the brain and knee) with the purpose of ascertaining the agreement between predicted and measured signal-to-noise ratios (SNR). Systematically varied parameters were number of excitations (NEX), field of view (FOV), section thickness (d_z), and the number of phase-encoding steps (N_y). Correlation coefficients of measured versus predicted SNR were 0.82 and 0.86, respectively, in the anatomies studied. Significantly improved correlations were found for data subpopulations in which NEX was held constant. To assess the criteria guiding reader preference, a blinded study was performed in which radiologists were asked to rate images from least to most desirable. In order to quantitatively determine the criteria for reader preference, plots of mean rating versus SNR, voxel volume, and an image quality index [IQI = SNR/(voxel volume)] were performed. The latter was found to be a better predictor of reader preference than either SNR or spatial resolution alone. The data suggests T₁-weighted scan protocols yielding SNR of approximately 20 are preferable with any excess SNR being traded for smaller voxel size or shorter scan times.     

Cardiac lipoma: Six-year follow-up with MRI characteristics, and a review of the literature

A total of 63 cardiac lipomas have been reported to date. Although most of these rare tumors cause no symptoms, a few can have a detrimental effect on myocardial function as well as displacing and encasing the coronary arteries. This case of a cardiac lipoma was initially seen in 1982. The lipoma was found to be nonresectable at surgery due to involvement of the coronary arteries. This neoplasm was characterized with magnetic resonance imaging in 1989 on a follow-up visit. Magnetic resonance imaging is shown to be superior to computed tomography for identifying the relationship of the lipoma to the coronary arteries. This is a significant factor in determining resectability.     

Effects of intrinsic anharmonicity in the Mie–Grüneisen equation of state and higher order corrections

The usual quasiharmonic Mie–Grüneisen (MG) equation of state is modified by the inclusion of ‘intrinsic anharmonicities’, which have been considered up to now primarily in the high temperature limit. A comparison with experimental data for the rare gas solids, Ar, Kr and Xe and for MgO reveals that the anharmonic contributions cannot be represented perfectly within the MG approximation. A small but significant modification of the MG approach is presented to estimate intrinsic anharmonic contributions within a mean-field approximation for the thermal part of the internal energy. This estimate results in reasonable interpolations to low temperatures, where quantum effects are dominant. The present approach is also compared with more restricted recent theoretical results.