Results 1 - 10 of 4187
Results 1 - 10 of 4187. Search took: 0.022 seconds
|Sort by: date | relevance|
[en] We investigate the origin of the volume change and magnetoelastic interaction observed at the magnetic first-order transition in the magnetocaloric system La(Fe1-xSix)13 by means of first-principles calculations combined with the fixed-spin moment approach. Here, we find that the volume of the system varies with the square of the average local Fe moment, which is significantly smaller in the spin disordered configurations compared to the ferromagnetic ground state. The vibrational density of states obtained for a hypothetical ferromagnetic state with artificially reduced spin-moments compared to a nuclear inelastic X-ray scattering measurement directly above the phase transition reveals that the anomalous softening at the transition essentially depends on the same moment-volume coupling mechanism. Finally, in the same spirit, the dependence of the average local Fe moment on the Si content can account for the occurrence of first- and second-order transitions in the system.
[en] Insulating, conducting, and superconducting phases can be identified through their characteristic low-energy response. We discuss experiments that employ ultrashort coherent THz pulses and direct field-resolved detection to probe time-varying correlations of charge carriers in semiconductors and superconductors. These experiments detect 1s 2p intra-exciton resonances of photoexcited e h gases in GaAs quantum wells. Stimulated emission from intra-excitonic transitions is observed in Cu2O, between 3p and 2s exciton states. Finally, we discuss the formation kinetics of Cooper pairs from nonequilibrium quasiparticles as observed in the transient THz response of the high-TC superconductor Bi-2212
[en] The development of more accurate and sensitive diagnostic techniques is a key factor in efforts to improve cancer survival rates. The technique of infrared aperture fibre scanning near-field optical microscopy (IR-SNOM),together with radiation from the infrared free-electron laser (IR-FEL) on ALICEat Daresbury Laboratory (UK), has been used to obtain IR images of a crypt-like feature and the surrounding tissue; the tissue was taken from a patient with oesophageal adenocarcinoma and with a history of Barrett’s oesophagus. We have shown that the DNA signal is enhanced relative to other contributions in the region of the crypt, and the glycoprotein signal shows a less pronounced increase in the region of the crypt. The Amide II signal is found to be anti-correlated with the DNA and glycoprotein profiles. The absorbance of the Amide II signal is found to differ for three different types of cancer tissue. In conclusion, high-resolution IR images of the crypt reveal additional structure that would not be resolved in diffraction-limited techniques.
[en] In this paper, we study the interplay of fluctuations and superconductivity in BaFe2As2 (Ba-122) compounds with Ba and Fe substituted by K (p doping) and Co (n doping), respectively. To this end, we measured electronic Raman spectra as a function of polarization and temperature. We observe gap excitations and fluctuations for all doping levels studied. The response from fluctuations is much stronger for Co substitution and, according to the selection rules and the temperature dependence, originates from the exchange of two critical spin fluctuations with characteristic wave vectors (±π,0) and (0,±π). At 22% K doping (p = 0.22), we find the same selection rules and spectral shape for the fluctuations but the intensity is smaller by a factor of 5. Since there exists no nematic region above the orthorhombic spin-density-wave (SDW) phase, the identification of the fluctuations via the temperature dependence is not possible. The gap excitations in the superconducting state indicate strongly anisotropic near-nodal gaps for Co substitution which make the observation of collective modes difficult. The variation with doping of the spectral weights of the A1g and B1g gap features does not support the influence of fluctuations on Cooper pairing. Thus, the observation of Bardasis–Schrieffer modes inside the nearly clean gaps on the K-doped side remains the only experimental evidence for the relevance of fluctuations for pairing.
[en] The atomic structure of the characteristic defects (Mg-rich hexagonal pyramids) in p-doped bulk and MOCVD GaN:Mg thin films grown with Ga polarity was determined at atomic resolution by direct reconstruction of the scattered electron wave in a transmission electron microscope. Small cavities were present inside the defects, confirmed also with positron annihilation. The inside walls of the cavities were covered by GaN of reverse polarity compared to the matrix. Defects in bulk GaN:Mg were almost one order of magnitude larger than in thin films. An exchange of Ga and N sublattices within the defect compared to the matrix lead to a 0.6 ± 0.2 (angstrom) displacement between the Ga sublattices of these two areas. A /3 shift with change from AB stacking in the matrix to BC within the entire pyramid was observed. Annealing of the MOCVD layers lead to slight increase of the defect size and an increase of the photoluminescence intensity. Positron annihilation confirms presence of vacancies of different sizes triggered by the Mg doping in as-grown samples and decrease of their concentration upon annealing at 900 and 1000 C
[en] We study the structural and electronic properties of isolated single-wall carbon nanotubes (SWNTs) under hydrostatic pressure using a combination of theoretical techniques: continuum elasticity models, classical molecular dynamics simulations, tight-binding electronic structure methods, and first-principles total energy calculations within the density-functional and pseudopotential frameworks. For pressures below a certain critical pressure Pc, the SWNTs' structure remains cylindrical and the Kohn-Sham energy gaps of semiconducting SWNTs have either positive or negative pressure coefficients depending on the value of (n,m) with a distinct 'family' (of the same n-m) behavior. The diameter and chirality dependence of the pressure coefficients can be described by a simple analytical expression. At Pc, molecular-dynamics simulations predict that isolated SWNTs undergo a pressure-induced symmetry-breaking transformation from a cylindrical shape to a collapsed geometry. This transition is described by a simple elastic model as arising from the competition between the bond-bending and PV terms in the enthalpy. The good agreement between calculated and experimental values of Pc provides a strong support to the 'collapse' interpretation of the experimental transitions in bundles
[en] Measurements of the EPR spectrum of seven scheelite structure crystals doped with Mn2+ ions are performed in the temperature range from 4.2 to 350 K. Experimental data of a preceding paper are completed. The temperature dependent zero field splitting is discussed in terms of implicit and explicit contributions. Being the most important one the temperature effect of b20 is described with an Einstein model where the impurity force constant is related to regular cationic vibrations by a Born-Mayer potential. (author)
[en] The lattice reflectivity spectra of CuGaS2, CuInS2, and CuGasub(x)Insub(1-x)S2 (x = 0.30, 0.50, 0.80) were analyzed by the Kramers-Kronig method. Phonon frequencies were determined by the energy position of extrema in the spectral dependence of the modulus of the dielectric function. A composition dependence was found of infrared active phonon frequencies of the CuGasub(x)Insub(1-x)S2 solid solutions
[en] Measurements of the surface resistance derivative dR/dH in dependence on magnetic field strength H are presented as observed at pure superconducting niobium single crystals. At crystals not annealed after preparation procedure the measured impedance spectra do not represent the phase transitions appearing at critical fields indicating thus the existence of surface layers with properties different from the bulk of pure niobium. At annealed crystals the critical magnetic field strengths are marked as characteristic discontinuities in the spectra. The spectra observed at recently annealed crystals being in Meissner state can be interpreted by means of magnetic surface states. (author)