By means of first-principles calculations, the structural stability, mechanical properties and electronic structure of the newly synthesized incompressible Re2C, Re2N, Re3N and an analogous compound Re3C have been investigated. Our results agree well with the available experimental and theoretical data. The proposed Re3C is shown to be energetically, mechanically and dynamically stable and also incompressible. Furthermore, it is suggested that the incompressibility of these compounds is originated from the strong covalent bonding character with the hybridization of 5d orbital of Re and the 2p orbital of C or N, and a zigzag topology of interconnected bonds, e.g., Re-Re, Re-C or Re-N bonding.
Based on density functional theory, we have systematically studied the structural stability, mechanical properties and chemical bonding of the transition metal borides M3B4 (M=Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) for the first time. All the present studied M3B4 have been demonstrated to be thermodynamically and mechanically stable. The bulk modulus, shear modulus, Youngs modulus, Poissons ratio, microhardness, Debye temperature and anisotropy have been derived for ideal polycrystalline M3B4 aggregates. In addition, the relationship between Debye temperature and microhardness has been discussed for these isostructral M3B4. Furthermore, the results of the Cauchy pressure, the ratio of bulk modulus to shear modulus, and Poissons ratio suggest that the valence electrons of transition metals play an important role in the ductility of M3B4. The calculated total density of states for M3B4 indicates that all these borides display a metallic conductivity. By analyzing the electron localization function, we show that the improvement of the ductility in these M3B4 might attribute to the decrease of their angular bonding character.
Transition-metal-oxide based resistance random access memory is a promising candidate for next-generation universal non-volatile memories. Searching and designing appropriate new materials used in the memories becomes an urgent task. Here, a new structure with the TaO2 formula was predicted using evolutionary algorithms in combination with first-principles calculations. This new structure having a triclinic symmetry (T-TaO2) is both energetically and dynamically more favorable than the commonly believed rutile structure (R-TaO2). Our hybrid functional calculations show that T-TaO2 is a semiconductor with a band gap of 1.0 eV, while R-TaO2 is a metallic conductor. This large difference in electrical property makes TaO2 a potential candidate for resistance random access memory (RRAM). Furthermore, we have shown that T-TaO2 is actually a Peierls distorted R-TaO2 phase and the transition between these two structures is via a directional displacement of Ta atoms. The energy barrier for the reversible phase transition from R-TaO2 to T-TaO2 is 0.19 eV/atom and the other way around is 0.23 eV/atom, suggesting low power consumption for the resistance switch. The present findings provide a new mechanism for the resistance switch and will also stimulate experimental work to fabricate tantalum oxides based RRAM.
Left- and right-handed helical modes statistical absolute equilibria appear textit{separately}. If both chiral sectors present, one can dominate around its positive pole, which is relevant to the nearly maximally helical (force free for magnetic field) states of turbulence. Pure magnetodynamics (PMD, or electron magnetohydrodynamics --- EMHD), pure hydrodynamics (PHD), and, single-fluid and two-fluid MHDs are studied. Relevant documented data and issues of cascade properties, and, helical and nonhelical dynamos are revisited. We also discuss new scenarios, such as PMD inverse magnetic helicity and forward energy cascades, and, the continuous transition from completely-inverse to partly-inverse-and-partly-forward and to completely-forward energy transfers in PHD.
In this paper, we propose an efficient implementation of combining Dynamical Mean field theory (DMFT) with electronic structure calculation based on the local density approximation (LDA). The pseudo-potential-plane-wave method is used in the LDA part, which makes it possible to be applied to large systems. The full loop self consistency of the charge density has been reached in our implementation which allows us to compute the total energy related properties. The procedure of LDA+DMFT is introduced in detail with a complete flow chart. We have also applied our code to study the electronic structure of several typical strong correlated materials, including Cerium, Americium and NiO. Our results fit quite well with both the experimental data and previous studies.
Biomolecular logic systems processing biochemical input signals and producing digital outputs in the form of YES/NO were developed for analysis of physiological conditions characteristic of liver injury, soft tissue injury and abdominal trauma. Injury biomarkers were used as input signals for activating the logic systems. Their normal physiological concentrations were defined as logic-0 level, while their pathologically elevated concentrations were defined as logic-1 values. Since the input concentrations applied as logic 0 and 1 values were not sufficiently different, the output signals being at low and high values (0, 1 outputs) were separated with a short gap making their discrimination difficult. Coupled enzymatic reactions functioning as a biomolecular signal processing system with a built-in filter property were developed. The filter process involves a partial back-conversion of the optical-output-signal-yielding product, but only at its low concentrations, thus allowing the proper discrimination between 0 and 1 output values.
Although 2011 marks the 50th anniversary of Nb3Sn as the first high field superconductor, real understanding of its upper critical field behavior {mu}0Hc2 is incomplete. Here we show surprising {mu}0Hc2 data on highly homogeneous bulk samples examined both by small-current, transport and by volumetric-averaging specific heat and the reversible magnetization techniques, which exhibit identical upper critical field {mu}0Hc2(0.3 K) ~ 29{pm} 0.2 T with or without undergoing the cubic to tetragonal transition, a result in strong contrast to widely used multiple-source data compilations that show a strong depression of {mu}0Hc2(0K) from 29 T to 21.4 T in the tetragonal state.
Variation of the geometrical and electronic properties of the gold materials in different dimensions has been investigated by $ab$ $initio$ method, taking into account the spin-orbit (SO) interaction. It is found that SO effects in different dimensional Au materials depend greatly on fundamental symmetry and dimensionality. For single walled gold nanotubes (SWGNTs), SO interaction decreases significantly the conducting channel number of achiral SWGNT (4, 0), and leads to spin splitting at Fermi level of chiral SWGNT, indicating that quasi-1D SWGNT can be a good candidate for the spin-electron devices. Furthermore, our results suggest that cage cluster might be synthesizable experimentally by taking gold tube structure as parent material.
The radial-breathing-like phonon modes (RBLMs) of the double-walled carbon nanotubes are studied in a simple analytical model, in which the interaction force constants (FCs) can be obtained analytically from the continuous model. The RBLMs frequencies are obtained by solving the dynamical matrix, and their relationship with the tube radii can be obtained analytically, offering a powerful experimental tool for determining precisely the radii of the multi-walled carbon nanotubes.
With the empirical bond polarizability model, the nonresonant Raman spectra of the chiral and achiral single-wall carbon nanotubes (SWCNTs) under uniaxial and torsional strains have been systematically studied by textit{ab initio} method. It is found that both the frequencies and the intensities of the low-frequency Raman active modes almost do not change in the deformed nanotubes, while their high-frequency part shifts obviously. Especially, the high-frequency part shifts linearly with the uniaxial tensile strain, and two kinds of different shift slopes are found for any kind of SWCNTs. More interestingly, new Raman peaks are found in the nonresonant Raman spectra under torsional strain, which are explained by a) the symmetry breaking and b) the effect of bond rotation and the anisotropy of the polarizability induced by bond stretching.
