Search results for "density [astrophysics]"
showing 10 items of 561 documents
Magnetic field dependent thermal conductance in La0.67Ca0.33MnO3
2015
Abstract Using the differential 3 ω technique we measured the low-temperature out-of-plane thermal conductance of heteroepitaxial thin film La0.67Ca0.33MnO3 (LCMO). The magnetic field dependence of the thermal conductance reached values of up to 23%. The effect was observed to be largest in the vicinity of the metal–insulator transition, since the enhancement in thermal conductance is triggered by the colossal magnetoresistance effect increasing the electronic contribution to the thermal conductance. The point of the maximal change was adjusted by post-annealing the samples in an oxygen atmosphere. Samples with a higher transition temperature and lower epitaxial strain displayed a lower mag…
Temperature dependence of x-ray absorption spectra in the ferromagnetic Heusler alloysMn2VAlandCo2FeAl
2010
We investigate the temperature dependence of the spin-resolved unoccupied density of states (DOS) in ferromagnetic ${\text{Co}}_{2}\text{FeAl}$ and ferrimagnetic ${\text{Mn}}_{2}\text{VAl}$ epitaxial films on MgO(100) using x-ray magnetic circular dichroism. We observe an unexpected strong temperature dependence of the DOS beyond the change expected from the Fermi distribution function. An increase in spectral weight is observed for majority states below the Fermi energy in the case of ${\text{Mn}}_{2}\text{VAl}$ and for minority states above the Fermi energy in the case of ${\text{Co}}_{2}\text{FeAl}$. Reduced atomic order near the interface suppresses the unexpected temperature dependence…
Novel Carbon Nanotubes Rolled from 6,6,12-Graphyne: Double Dirac Points in 1D Material
2017
Two kinds of novel carbon nanotubes, namely, (N, 0) and (0, N) 6,6,12-graphyne nanotubes (6,6,12-GNTs), are constructed by rolling up the rectangular 6,6,12-graphyne sheets along two different sides into cylinders. The mechanical and electronic properties of 6,6,12-GNTs with varied N from 3 to 20 are investigated by using density functional theory. Unlike the single-wall carbon nanotubes, the Young’s moduli of 6,6,12-GNTs do not remain constant in the case of (N, 0), but the (0, N) tubes possess almost the same one around 0.32 TPa. The band structures and density of states are also exhibited in this work. When the tube sizes N are bigger than four, Dirac points appear at Fermi level in the …
First-principles calculations of perovskite thin films
2002
Abstract The results of the electronic structure calculations for different surface terminations of SrTiO3 (1 0 0) perovskite thin films are discussed. These calculations are based on ab initio Hartree–Fock method with a posteriori electron correlation corrections and density functional theory with a number of different exchange-correlation functionals, including hybrid (B3PW, B3LYP) exchange techniques. Results are compared with previous ab initio plane-wave local density approximation and classical shell model calculations. Calculated considerable increase of the Ti–O chemical bond covalency nearby the surface is confirmed by experimental data. We predict also the band-gap reduction, espe…
Disorder-induced vibrational anomalies from crystalline to amorphous solids
2021
The origin of boson peak -- an excess of density of states over Debye's model in glassy solids -- is still under intense debate, among which some theories and experiments suggest that boson peak is related to van-Hove singularity. Here we show that boson peak and van-Hove singularity are well separated identities, by measuring the vibrational density of states of a two-dimensional granular system, where packings are tuned gradually from a crystalline, to polycrystals, and to an amorphous material. We observe a coexistence of well separated boson peak and van-Hove singularities in polycrystals, in which the van-Hove singularities gradually shift to higher frequency values while broadening th…
Electronic Properties of 6H Hexagonal SrMnO3
2015
SrMnO3 ceramic was prepared by conventional high-temperature sintering method. Dielectric studies were performed within the temperature range 125–300 K at the frequency 10–400 kHz. The electronic properties of stoichiometric and oxygen defected SrMnO3 crystal in hexagonal 6H- P63/mmc symmetry at 0 K were calculated in ab initio procedure based on density functional theory using the Spanish Initiative for Electronic Simulations with Thousands of Atoms (SIESTA) code. The lattice parameters and density of states were obtained. The ideal 6H-SrMnO3 crystal has the band gap energy about 0.2–0.3 eV at 0 K. Contribution of an exchange-correlation energy into the total energy is about 32%, a magneti…
Optimum Carrier Concentration in n-Type PbTe Thermoelectrics
2014
Taking La- and I-doped PbTe as an example, the current work shows the effects of optimizing the thermoelectric figure of merit, zT, by controlling the doping level. The high doping effectiveness allows the carrier concentration to be precisely designed and prepared to control the Fermi level. In addition to the Fermi energy tuning, La-doping modifies the conduction band, leading to an increase in the density of states effective mass that is confirmed by transport, infrared reflectance and hard X-ray photoelectron spectroscopy measurements. Taking such a band structure modification effect into account, the electrical transport properties can then be well-described by a self-consistent single…
Proposal for a Dual Spin Filter Based on [VO(C 3 S 4 O) 2 ] 2–
2018
Polynuclear magnetic molecules often present dense electronic transmission spectra with many overlapping conduction spin channels. Single-metal complexes display a sparser density of states, which in the presence of a fixed external magnetic field makes them interesting candidates for spin filtering. Here we perform a DFT study of a family of bis- and tris-dithiolate vanadium complexes sandwiched between Au(111) electrodes and demonstrate that [VO(C3S4O)2]2– can behave as a dual spin filter. This means that an external electrical stimulus can switch between the selective transmission of spin-up and spin-down carriers. By using an electrostatic gate, we show that the onset for the spin-up co…
Influence of the intermediate density-of-states occupancy on open-circuit voltage of bulk heterojunction solar cells with different fullerene accepto…
2010
Electron density of states (DOS) and recombination kinetics of bulk heterojunction solar cells consisting of a poly(3-hexylthiophene) (P3HT) donor and two fullerene acceptors, either [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) or 4,4′-dihexyloxydiphenylmethano[60]fullerene (DPM6), have been determined by means of impedance spectroscopy. The observed difference of 125 mV in the output open-circuit voltage is attributed to significant differences of the occupancy of the DOS in both fullerenes. Whereas DPM6 exhibits a full occupation of the electronic band, occupancy is restricted to the tail of the DOS in the case of PCBM-based devices, implying a higher rise of the Fermi level in the D…
A quantitative electron-microscopic study of a linear polyethylene fraction crystallized at different temperatures
1981
Replica and thin-section electron microscopy was performed on a linear polyethylene fraction (Mw = 1.89 × 105, Mn = 1.79 × 10 5) which was either isothermally crystallized or quenched at difference temperatures. The results are numerically analyzed in such a manner so as to give the distribution of the total long spacing and of the crystallite and amorphous thicknesses. The quantitative information about the maximum and minimum values for these parameters at each crystallization temperature yields important clues as to possible molecular processes involved. Qualitative information regarding general morphology, molecular tilt with respect to the lamellar surface, and possible crystallographi…