Low-Energy Optical Conductivity of TaP: Comparison of Theory and Experiment

The ab-plane optical conductivity of the Weyl semimetal TaP is calculated from the band structure and compared to the experimental data. The overall agreement between theory and experiment is found to be best when the Fermi level is slightly (20 to 60 meV) shifted upwards in the calculations. This confirms a small unintentional doping of TaP, reported earlier, and allows a natural explanation of the strong low-energy (50 meV) peak seen in the experimental ab-plane optical conductivity: this peak originates from transitions between the almost parallel non-degenerate electronic bands split by spin-orbit coupling. The temperature evolution of the peak can be reasonably well reproduce by calculations using an analog of the Mott formula.

Self-similar transport, spin polarization and thermoelectricity in complex silicene structures

2D materials open the possibility to study Dirac electrons in complex self-similar geometries. The two-dimensional nature of materials like graphene, silicene, phosphorene and transition-metal dichalcogenides allow the nanostructuration of complex geometries through metallic electrodes, interacting substrates, strain, etc. So far, the only 2D material that presents physical properties that directly reflect the characteristics of the complex geometries is monolayer graphene. In the present work, we show that silicene nanostructured in complex fashion also displays self-similar characteristics in physical properties. In particular, we find self-similar patterns in the conductance, spin polarization and thermoelectricity of Cantor-like silicene structures. These complex structures are generated by modulating electrostatically the silicene local bandgap in Cantor-like fashion along the structure. The charge carriers are described quantum relativistically by means of a Dirac-like Hamiltonian. The transfer matrix method, the Landauer–Büttiker formalism and the Cutler–Mott formula are used to obtain the transmission, transport and thermoelectric properties. We numerically derive scaling rules that connect appropriately the self-similar conductance, spin polarization and Seebeck coefficient patterns. The scaling rules are related to the structural parameters that define the Cantor-like structure such as the generation and length of the system as well as the height of the potential barriers. As far as we know this is the first time that a 2D material beyond monolayer graphene shows self-similar quantum transport as well as that transport related properties like spin polarization and thermoelectricity manifest self-similarity.

Preparation and Characterization of Pristine PMMA and PVDF Thin Film using Solution Casting Process for Optoelectronic Devices

The PMMA and PVDF thin films were synthesized by solution casting process. The structural, spectroscopic, and morphological attributes of both the interface and surface of the film have been investigated. The structural properties of pristine PMMA and PVDF thin film were studied by XRD. The XRD spectra showed the detailed state of order and disorder of the system. From the XRD studies amorphous nature as well as the crystallanity (semi crystalline nature) of the polymer thin film were identified. SEM showed the surface morphology of the pristine PMMA and PVDF thin film. From the SEM image the size and the porous nature of the thin film were estimated. The UV showed the absorption characteristics of PMMA and PVDF thin films. From the UV-Visible spectra, we determined the direct and indirect optical energy gap of pristine PMMA and PVDF polymer samples by Devi's and Mott formula and Tauc's Expression. To the best of our knowledge, the direct and indirect energy gap in pristine PMMA and PVDF polymer thin films have been observed for the first time as no such report was found in literature survey.

How Accurate is Qcbed ? -- the Case of Rutile (TiO2)

An accuracy of better than 1% is needed to measure the changes in charge density due to bonding. Here we report an accuracy up to 0.025% (random error) obtained in rutile crystal structure factors measurement by QCBED. This error is the standard deviation in the mean value obtained from ten data sets. Systematic errors may be present. Figure 1 gives an example of the (200) refinement results. Table 1 lists several low order structure factor refinement results. The accuracy of the measured electron structure factors was 0.1-0.2% but after conversion to x-ray structure factors, the accuracy for low orders improved due to the Mott formula [1] For (110) and (101) reflections, the accuracy in x-ray structure factors became 0.025% and 0.048% respectively. This accuracy is equivalent to that of the X-ray single crystal Pendellosung method on silicon crystals [2].The experiments were done on a Leo 912 Omega TEM.

High precision measurement of Debye-Waller factors for NiAl

Motivation Quantitative convergent beam electron diffraction (CBED) is increasingly appreciated as a tool to determine bonding charge densities of crystalline materials. Simulated CBED patterns are fitted to experimental ones to derive the structure factors. These are converted by means of the Mott formula to yield the total charge density. Finally a neutral atom total charge density is subtracted and the difference is interpreted as the bonding charge density. Accounting for the temperature by a Debye-Waller factor (DWF) the g-th Fourier coefficient of the bonding charge density is then given by where u denotes the thermal root mean square atomic displacement. Here we have assumed thesimplest case of identical isotropic atomic vibrations for all atoms in the crystal. In order to estimate the error in due to the uncertainty in u we insert the results obtained by Fox and Tabbernor for NiAl at room temperature. They found differences between atomic and measured values of in the order of 2 percent.