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Register a new userTemperature-dependent Raman scattering of natural and isotopically substituted PbS
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Lead sulfide is an important semiconductor that has found technological applications for over a century. Raman spectroscopy, a standard tool for the investigation and characterization of semiconductors, has limited application to this material because of the forbidden nature of its first order scattering and its opacity to visible lasers. Nevertheless, useful vibrational spectra from two-phonon processes are obtained with red lasers, probably because of a resonance in the concomitant electronic transitions. Here we report temperature dependent spectra, covering the 10-300 K range, for two samples with different sulfur isotopic compositions. The results are analyzed by comparison with ab initio calculations of the lattice dynamics of PbS and the corresponding densities of one and two-phonon states. Emphasis is placed on the analysis of the two phonon band centered at ~430 cm-1.
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We report a detailed magnetic, dielectric and Raman studies on partially disordered and biphasic double perovskite La2NiMnO6. DC and AC magnetic susceptibility measurements show two magnetic anomalies at TC1 ~ 270 K and TC2 ~ 240 K, which may indicate the ferromagnetic ordering of the monoclinic and rhombohedral phases, respectively. A broad peak at a lower temperature (Tsg ~ 70 K) is also observed indicating a spin-glass transition due to partial anti-site disorder of Ni2+ and Mn4+ ions. Unlike the pure monoclinic phase, the biphasic compound exhibits a broad but a clear dielectric anomaly around 270 K which is a signature of magneto-dielectric effect. Temperature-dependent Raman studies between the temperature range 12 K to 300 K in a wide spectral range from 220 cm-1 to 1530 cm-1 reveal a strong renormalization of the first as well as second-order Raman modes associated with the (Ni/Mn)O6 octahedra near TC1 implying a strong spin-phonon coupling. In addition, an anomaly is seen in the vicinity of spin-glass transition temperature in the temperature dependence of the frequency of the anti-symmetric stretching vibration of the octahedra.
Room temperature ferromagnetism was characterized for thin films of SrTi$_{0.6}$Fe$_{0.4}$O$_{3-{delta}}$ grown by pulsed laser deposition on SrTiO$_{3}$ and Si substrates under different oxygen pressures and after annealing under oxygen and vacuum conditions. X-ray magnetic circular dichroism demonstrated that the magnetization originated from Fe$^{2+}$ cations, whereas Fe$^{3+}$ and Ti$^{4+}$ did not contribute. Films with the highest magnetic moment (0.8 {mu}B per Fe) had the highest measured Fe$^{2+}$:Fe${^3+}$ ratio of 0.1 corresponding to the largest concentration of oxygen vacancies ({delta} = 0.19). Post-growth annealing treatments under oxidizing and reducing conditions demonstrated quenching and partial recovery of magnetism respectively, and a change in Fe valence states. The study elucidates the microscopic origin of magnetism in highly Fe-substituted SrTi$_{1-x}$Fe$_x$O$_{3-{delta}}$ perovskite oxides and demonstrates that the magnetic moment, which correlates with the relative content of Fe$^{2+}$ and Fe$^{3+}$, can be controlled via the oxygen content, either during growth or by post-growth annealing.
The most important bands for the evaluation of strain in graphene (the 2D and 2D prime modes) are investigated. It is shown that for Bernal-stacked bilayers, the two-phonon Raman features have three different components that can be assigned to processes originating solely from the top graphene layer, bottom graphene layer, and from a combination of processes originating both from the top and bottom layers. The individual components of the 2D and 2D prime modes are disentangled. The reported results enable addressing the properties of individual graphene layers in isotopically labelled turbostratic and Bernalstacked graphene systems.
High quality single crystals of Bi2Se3 were grown using a modified Bridgman technique, the detailed study were carried out using Raman spectroscopy and characterized by Laue diffraction and high resolution transmission electron microscopy. Polarized Raman scattering measurements were also carried out, and both the A1g and A2g phonon modes showed strong polarization effect, which is consistent with the theoretical prediction. The temperature dependent study (in the temperature range 83 K to 523 K of Raman active modes were reported and observed to follow a systematic red shift. The frequency of these phonon modes are found to vary linearly with temperature and can be explained by first order temperature co-efficient. The temperature co-efficient for A11g, E2g and A21g modes were estimated to be -1.44*10-2, -1.94*10-2 and -1.95*10-2cm-1/K respectively.
X-ray and neutron powder diffraction data as a function of temperature are analyzed for the colossal dielectric constant material CaCu3Ti4O12. The local structure is studied using atomic pair distribution function analysis. No evidence is found for enhanced oxygen displacement parameters suggesting that short-range octahedral tilt disorder is minimal. However, an unusual temperature dependence for the atomic displacement parameters of calcium and copper is observed. Temperature dependent modeling of the structure, using bond valence concepts, suggests that the calcium atoms become underbonded below approximately 260 K, which provides a rationale for the unusually high Ca displacement parameters at low temperature.
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