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At least 451 records · Page 25Linked to original sources

arXiv2013

Observation of plastoferrite character and semiconductor to metal transition in soft ferromagnetic Li0.5Mn0.5Fe2O4ferrite

We prepared Li0.5Mn0.5Fe2O4 ferrite through chemical reaction in highly acidic solution and subsequent sintering of chemical routed powder at temperatures > 800 0C. Surface morphology showed plastoferrite character for sintering temperature > 1000 0C. Mechanical softening of metal-oxygen bonds at higher measurement temperatures stimulated delocalization of charge carriers, which were strongly localized in A and B sites of the spinel structure at lower temperatures. The charge delocalization process has activated semiconductor to metallic transition in ac conductivity curves, obeyed by Jonscher power law and Drude equation, respectively. Metallic state is also confirmed by the frequency dependence of dielectric constant curves.

cond-mat.mtrl-sci↗

arXiv2013

Electron-Phonon Coupling in Wurtzite Semiconductors: Intervalley Scattering Selection Rules for Hexagonal GaN

Selection rules are presented for electron-phonon scattering in GaN with the wurtzite crystal structure. The results are obtained for the interband scattering between the lowest conduction band ($Γ$-valley) and the second conduction band ($U$-valley). These selection rules are derived based on the original group-theoretical analysis of the crystal vibrations in GaN, which included detailed compatibility relations for all phonon modes.

cond-mat.mtrl-sci↗

arXiv2013

Near-infrared studies of glucose and sucrose in aqueous solutions: water displacement effect and red shift in water absorption from water-solute interaction

We use near infrared spectroscopy to obtain concentration dependent glucose absorption spectra in their aqueous solutions in the near-infrared range (3800 - 7500 cm^{-1}). We introduce a new method to obtain reliable glucose absorption bands from aqueous glucose solutions without measuring the water displacement coefficients of glucose separately. Additionally, we are able to extract the water displacement coefficients of glucose, and this may give a new general method using spectroscopy techniques applicable to other water soluble materials. We also observe red shifts in the absorption bands of water in the hydration shell around solute molecules, which comes from contribution of the interacting water molecules around the glucose molecules in solutions. The intensity of the red shift get larger as the concentration increases, which indicates that as the concentration increases more water molecules are involved in the interaction. However, the red shift in frequency does not seem to depend significantly on the concentration up to our highest concentration. We also performed the same measurements and analysis with sucrose instead of glucose as solute and compare.

cond-mat.mtrl-sci↗

arXiv2013

Influence of ensemble boundary conditions (thermostat and barostat) on the deformation of amorphous polyethylene by molecular dynamics

Molecular dynamics simulations are increasingly being used to investigate the structural evolution of polymers during mechanical deformation, but relatively few studies focus on the influence of boundary conditions on this evolution, in particular the dissipation of both heat and pressure through the periodic boundaries during deformation. The research herein explores how the tensile deformation of amorphous polyethylene, modelled with a united atom method potential, is influenced by heat and pressure dissipation. The stress-strain curves for the pressure dissipation cases (uniaxial tension) are in qualitative agreement with experiments and show that heat dissipation has a large effect on the strain hardening modulus calculated by molecular dynamics simulations. The evolution of the energy associated with bonded and non-bonded terms was quantified as a function of strain as well as the evolution of stress in both the loading and non-loading directions to give insight into how the stress state is altered within the elastic, yield, strain softening, and strain hardening regions. The stress partitioning shows a competition between `tensile' Van der Waal's interactions and `compressive' bond stretching forces, with the characteristic yield stress peak clearly associated with the non-bonded stress. The lack of heat dissipation had the largest effect on the strain hardening regime, where an increase in the calculated temperature correlated with faster chain alignment in the loading direction and more rapid conformation changes. In part, these observations demonstrate the role that heat and pressure dissipation play on deformation characteristics of amorphous polymers, particularly for the strain hardening regime.

cond-mat.mtrl-sci↗

arXiv2013

Skin supersolidity slipperizing ice

Consistency between theory predictions and measurements and calculations revealed that the skin of ice, containing water molecules with fewer than four neighbours, forms a supersolid phase that is highly polarized, elastic, hydrophobic, with ultra-low density and high thermal stability. The supersolidity of skin sliperizes ice.

cond-mat.soft↗

arXiv2013

A critical assessment on Kassapoglou's statistical model for composites fatigue

Kassapoglou recently proposed a model for fatigue of composite materials which seems to suggest that fatigue SN curve can be entirely predicted on the basis of the statistical distribution of static strengths. The original abstract writes "Expressions for the cycles to failure as a function of R ratio are derived. These expressions do not require any curve fitting and do not involve any experimentally determined parameters. The fatigue predictions do not require any fatigue tests for calibration". These surprisingly ambitious claims and attractive results deserve careful scrutiny. We contend that the results seem to be due to a number of approximations and incorrect derivations, and one particular misleading assumption, which make the model not conform to a fatigue testing in a given specimen with resulting SN curve distribution. The quantitative agreement of some predictions (the scatter of distribution of fatigue lives being close to the mode value found in typical composites of aeronautical interest in the large Navy database) should not motivate any enthusiasm. It is believed that a proper statistical treatment of the fatigue process should not make wear-out constants disappear, and hence the SN curves would depend on them, and not just on scatter of static data. These serious concerns explain the large discrepancies found by 3 independent studies which tried to apply Kassapoglou's model to composite fatigue data, and to other well known results.

cond-mat.mtrl-sci↗

arXiv2013

Unified Model of Temperature Dependence of Core Losses in Soft Magnetic Materials Exposed to Nonsinusoidal Flux Waveforms and DC Bias Condition

Assuming that Soft Magnetic Material is a Complex System and expressing this feature by scaling invariance of the power loss characteristic, the unified model of the temperature dependence of Core Losses in Soft Magnetic Materials Exposed to Nonsinusoidal Flux Waveforms and DC Bias Condition has been constructed. In order to verify this achievement the appropriate measurement data concerning power losses and the all independent variables have been collected. The model parameters have been estimated and the power losses modeling has been performed. Comparison of the experimental values of power losses with their calculated values has showed good agreement.

cond-mat.mtrl-sci↗

arXiv2013

Dimensionality driven charge density wave instability in TiS$_2$

Density functional theory and density functional perturbation theory are used to investigate the electronic and vibrational properties of TiS$_2$. Within the local density approximation the material is a semi-metal both in the bulk and in the monolayer form. Most interestingly we observe a Kohn anomaly in the bulk phonon dispersion, which turns into a charge density wave instability when TiS$_2$ is thinned to less than four monolayers. Such charge density wave phase can be tuned by compressive strain, which appears to be the control parameter of the instability.

cond-mat.mes-hall↗

arXiv2013

Genuine driving voltage on polarization fatigue in (Pb,La)(Zr,Ti)O3 antiferroelectric thin films

The polarization fatigue in (Pb0.97La0.02)(Zr0.95Ti0.05)O3 (PLZT) antiferroelectric thin films deposited onto silicon wafers is studied by investigating the effect of the peak/average/effective cycling voltage through varying the waveform of the electrical excitation. Interestingly, it is found that the fatigue endurance of the film is determined by the effective voltage of the external driving excitation rather than by the peak or average voltages. Our results can be well explained in the framework of the local phase decomposition model and indicate that the effective voltage should be considered as the genuine driving voltage determining the polarization fatigue in PLZT antiferroelectric films.

cond-mat.mtrl-sci↗

arXiv2013

Existence of three-phase interlines on a cerium dioxide surface

The three-phase interline described by a statistical continuum limit (i.e. quasi-boundary) has been postulated to gain a deeper insight into the reduction of CeO2 to CeO1.940 in a LiCl-KCl eutectic melt. Fabrication of a CeO2 superstructure by a condensed-phase method provided a CeO2 (111) surface at the nanoscale, which allowed the three-phase interline to be identified given previously reported quantum confinement effects in quasi-stoichiometric CeO2 nanoparticles. Also, the CeO2 superstructure displays the same crystal lattice planes as a bulk CeO2 grain but the triply degenerate Raman-active peak of the grain is higher by a factor of ~ 2.5 with a wider full width at half maximum.

physics.chem-ph↗

arXiv2013

Chargeless spin current for switching and coupling of domain walls in magnetic nanowires

The demonstration of the generation and control of a pure spin current (without net charge flow) by electric fields and/or temperature gradient has been an essential leap in the quest for low-power consumption electronics. The key issue of whether and how such a current can be utilized to drive and control information stored in magnetic domain walls (DWs) is still outstanding and is addressed here. We demonstrate that pure spin current acts on DWs in a magnetic stripe with an effective spin-transfer torque resulting in a mutual DWs separation dynamics and picosecond magnetization reversal. In addition, long-range ($\sim$ mm) antiferromagnetic DWs coupling emerges. If one DW is pinned by geometric constriction, the spin current induces a dynamical spin orbital interaction that triggers an internal electric field determined by $\vec{E} \sim \hat{e}_{x} \cdot (\vec{n}_{1} \times \vec{n}_{2})$ where $\vec{n}_{1/2}$ are the effective DWs orientations and $\hat{e}_{x} $ is their spatial separation vector. This leads to charge accumulation or persistent electric current in the wire. As DWs are routinely realizable and tuneable, the predicted effects bear genuine potential for power-saving spintronics devices.

cond-mat.mes-hall↗

arXiv2013

Atomic mechanism of phase transition between metallic and semiconducting MoS2 single-layers

Structural transformation between metallic (1T) and semiconducting (2H) phases of single-layered MoS2 was systematically investigated by an in situ STEM with atomic precision. The 1T/2H phase transition is comprised of S and/or Mo atomic-plane glides, and requires an intermediate phase (α-phase) as an indispensable precursor. Migration of two kinds of boundaries (β and γ-boundaries) is also found to be responsible for the growth of the second phase. The 1T phase can be intentionally introduced in the 2H matrix by using a high dose of incident electron beam during heating the MoS2 single-layers up to 400~700°C in high vacuum and indeed controllable in size. This work may lead to the possible fabrication of composite nano-devices made of local domains with distinct electronic properties.

cond-mat.mtrl-sci↗

arXiv2013

Properties of Individual Dopant Atoms in Single-Layer MoS2: Atomic Structure, Migration, and Enhanced Reactivity

The differences in the behavior of Re (n-type) and Au (p-type) dopant atoms in single-layered MoS2 were investigated by in situ scanning transmission electron microscopy. Re atoms tend to occupy Mo sites, while Au atoms exist as adatoms and show larger mobility under the electron beam. Re substituted to Mo site showed enhanced chemical affinity, evidenced by agglomeration of Re adatoms around these sites. This may explain the difficulties in achieving a high compositional rate of homogeneous Re doping in MoS2. In addition, an in situ coverage experiment together with density functional theory calculations discovered a high surface reactivity and agglomeration of other impurity atoms such as carbon at the Re doped sites.

cond-mat.mtrl-sci↗

arXiv2013

Computational study of the structural, electronic and optical properties of bulk palladium nitrides

The atomic and electronic structures of Pd3N, PdN and PdN2 were investigated using ab initio density-functional theory (DFT). We studied cohesive energy vs. volume equation of states (EOS) for a set of reported and hypothetical structures. Obtained data was fitted to a third-order Birch-Murnaghan equation of state (EOS) so as to identify the energetically most stable phases and to determine their equilibrium structural parameters and stability and mechanical properties. Electronic properties were investigated by calculating the band diagrams and the total and partial density of states (DOS). Some possible pressure-induced phase transitions were tested. To derive the frequency-dependent optical spectra (i.e. absorption coefficient, reflectivity, refractive index, and energy-loss), we performed G_0W0 calculations within the random-phase approximation (RPA) to the dielectric tensor. Obtained results were compared with previous studies.

cond-mat.mtrl-sci↗

arXiv2013

Jahn-Teller distortion induced magnetic phase transition in cubic BaFeO$_{3}$

Using density functional theory (DFT) with local density approximation (LDA) and generalized gradient approximation (GGA) correlation functionals, the electronic and magnetic structures of cubic BaFeO$_{3}$ in the ferromagnetic (FM) and antiferromagnetic (AFM) states are studied. Our LDA/GGA and LDA$+U$/GGA$+U$ results show that cubic BFO has a FM ground state, in agreement with recent experimental works. Two types of Jahn-Teller (JT) distortions, denoted as JT1 and JT2, are considered. We find FM to ferrimagnetic (FIM) and FM to AFM magnetic phase transitionn in the JT1 and JT2 type of distortions, respectively. Larger strains are required for the FM-AFM transition as compared to the FM-FIM. DFT$+U$ calculations also show that the magnetic moments dramatically decrease at large strains due to strong overlapping between the Fe and O atoms. The origins of these transitions is discussed in terms of a competition between double exchange and superexchange interactions. Oxygen and Fe displacements are therefore responsible for the magnetic phase transitions and the reduction of the magnetic moments.

cond-mat.mtrl-sci↗

arXiv2013

Nonlinear hydrodynamic theory of crystallization

We present an isothermal fluctuating nonlinear hydrodynamic theory of crystallization in molecular liquids. A dynamic coarse-graining technique is used to derive the velocity field, a phenomenology, which allows a direct coupling between the free energy functional of the classical Density Functional Theory and the Navier-Stokes equation. Contrary to the Ginzburg-Landau type amplitude theories, the dynamic response to elastic deformations is described by parameter-free kinetic equations. Employing our approach to the free energy functional of the Phase-Field Crystal model, we recover the classical spectrum for the phonons and the steady-state growth fronts. The capillary wave spectrum of the equilibrium crystal-liquid interface is in a good qualitative agreement with the molecular dynamics simulations.

cond-mat.mtrl-sci↗

arXiv2013

Superconductivity in Graphene-Lithium

We present first-principles calculations on systems consisting of a few layers of graphene and lithium. In particular, we investigate the evolution of the electron-phonon coupling strength with increasing number of layers. We find that, for intercalated systems such as C6-Li-C6 or C6-Li-C6-Li-C6, the electron-phonon coupling is weak. However, for systems of equal number of graphene and lithium layers, such as C6-Li or C6-Li-C6-Li, the electron-phonon coupling is strong. We investigate the optimal configuration that yields the highest superconducting transition temperature.

cond-mat.supr-con↗

arXiv2013

\textit{Ab initio} calculation of transport and optical properties of aluminum: influence of simulation parameters

This work is devoted to the \textit{ab initio} calculation of transport and optical properties of aluminum. The calculation is based on the quantum molecular dynamics simulation, density functional theory and the Kubo-Greenwood formula. Mainly the calculations are performed for liquid aluminum at near-normal densities for the temperatures from melting up to 20000 K. The results on dynamic electrical conductivity, static electrical conductivity and thermal conductivity are obtained and compared with available reference and experimental data and the calculations of other authors. The influence of the technical parameters on the results is investigated in detail. The error of static electrical conductivity calculation is estimated to be about 20%; more accurate results require bigger number of atoms.

physics.comp-ph↗