SEARCH · Searcharxiv

Results for “cond-mat.mtrl-sci”

Search indexed arXiv papers on artificial intelligence, large language models, computer vision and robotics. Read source abstracts and follow links to arXiv.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 361 records · Page 20Linked to original sources

arXiv2013

Electrical Detection of Spin Wave Resonance in a Permalloy Thin Strip

We investigated the microwave-induced DC response of spin wave resonance (SWR) in a permalloy thin strip via electrical detection. Our experimental results obtained by sweeping the external field reveal that: 1. the amplitude of SWR signals depend on the direction of external field and, 2. unlike the DC response of ferromagnetic resonance, SWR spectra are always anti-symmetrical. The spin dynamics are discussed based on these unusual signals in resonant condition.

cond-mat.mtrl-sci↗

arXiv2013

Simulation of light propagation in thin semiconductor films with non-local electron-photon interaction

The propagation of light in layered semiconductor media is described theoretically and simulated numerically within the framework of the non-equilibrium Green's function formalism as used for state-of-the-art nanodevice simulations, treating the non-local interaction of leaky photonic modes with the electronic states of thin semiconductor films on a non-equilibrium quantum statistical mechanics level of theory. For a diagonal photon self-energy corresponding to local coupling, the simulation results for a 500 nm GaAs slab under normal incidence are in excellent agreement with the predictions from the conventional transfer matrix method. The deviations of the local approximation from the result provided by the fully non-local photon self-energy for a 100 nm GaAs film are found to be small.

cond-mat.mes-hall↗

arXiv2013

How Channel Segregates Originates: The Flow of Accumulated Impurity Clusters in Solidifying Steels

The phenomenon, channel segregates (CS) as a result of gravity-driven flow due to density contrast occurred in the solid-liquid mushy zones1during solidification, often causes the severe destruction of homogeneity and even some fatal damages. Investigation on its mechanism sheds light on the understanding and controlling of the formation of solidifying metals,earth's core, igneous rock and sea ice. Until now, it still remains controversial what composes the density contrasts and, to what extent, how it affects channel segregates. Here, we show that in experimental 500kg and 100 ton commercial cast steel ingots CS originates from oxide Al2O3/MnS impurity clusters (OICs) initially nucleated from the oxide (Al2O3) particles, which induce an extra flow due to sharp density contrast between clusters and melt. The results uncover that, as OICs enrich and grow, their driven flow becomes stronger than the traditionally recognized inter-dendritic thermo-solutal convection, dominating the subsequent opening of the channels. This study extends the classical macrosegregation theory, highlights a significant technological breakthrough to control CS, and could quickly yield practical benefits to the worldwide manufacture of over 50 million tons of ingots, super-thick slab and heavy castings annually, as well as has general implications for the elaboration of other related natural phenomena.

cond-mat.mtrl-sci↗

arXiv2013

Structural and optical investigations of the iron-chalcogenide superconductor Fe$_{1.03}$Se$_{0.5}$Te$_{0.5}$ under high pressure

Iron-chalcogenide superconductor Fe$_{1.03}$Se$_{0.5}$Te$_{0.5}$ has been investigated under high pressure using synchrotron based x-ray diffraction and mid-infrared reflectance measurements at room temperature. Pressure dependence of the superconducting transition temperature (T$_c$) of the same sample has been determined by temperature-dependent resistance measurements up to 10 GPa. Although the high pressure orthorhombic phase ($\textit{Pbnm}$) starts emerging at 4 GPa, structural transition becomes clearly observable above 10 GPa. A strong correlation is observed between the Fe(Se,Te)$_{4}$ tetrahedral deformation in the tetragonal phase ($\textit{P4/nmm}$) and the sharp rise of T$_c$ up to $\sim$4 GPa, above which T$_c$ is found to be almost pressure independent at least up to 10 GPa. A subtle structural modification of the tetragonal phase is noticed above 10 GPa, suggesting a structural transition with possible Fe$^{2+}$ spin-state transition. The evolution with pressure of the optical conductivity shows that the Drude term increases systematically with pressure up to 5.4 GPa, indicating the evolution of the tetragonal phase towards a conventional metallic state. At further higher pressures the Drude term reduces drastically implying a poor metallic character of the high pressure orthorhombic phase. Our results suggest that occurrence of large onset T$_c$ above 4 GPa is due to a systematic increase of high pressure orthorhombic phase fraction.

cond-mat.mtrl-sci↗

arXiv2013

Magnetooptical properties of (Ga,Fe)N layers

Magnetooptical properties of (Ga,Mn)N layers containing various concentrations of Fe-rich nanocrystals embedded in paramagnetic (Ga,Fe)N layers are reported. Previous studies of such samples demonstrated that magnetization consists of a paramagnetic contribution due to substitutional diluted Fe ions as well as of ferromagnetic and antiferromagnetic components originating from Fe-rich nanocrystals, whose relative abundance can be controlled by the grow conditions. The nanocrystals are found to broaden and to reduce the magnitude of the excitonic features. However, the ferromagnetic contribution, clearly seen in SQUID magnetometry, is not revealed by magnetic circular dichroism (MCD). Possible reasons for differences in magnetic response determined by MCD and SQUID measurements are discussed.

cond-mat.mtrl-sci↗

arXiv2013

Spin Selection Rule-Based Sub-Millisecond Hyperpolarization of Nuclear Spins in Silicon

In this work, we devise a fast and effective nuclear spin hyperpolarization scheme, which is in principle magnetic field and temperature independent. We use this scheme to experimentally demonstrate polarizations of up to 66% for phosphorus donor nuclear spins in bulk silicon, which are created within less than 100 us in a magnetic field of 0.35 T at a temperature of 5 K. The polarization scheme is based on a spin-dependent recombination process via weakly-coupled spin pairs, for which the recombination time constant strongly depends on the relative orientation of the two spins. We further use this scheme to measure the nuclear spin relaxation time and find a value of approx. 100 ms under illumination, in good agreement with the value calculated for nuclear spin flips induced by repeated ionization and deionization processes.

cond-mat.mtrl-sci↗

arXiv2013

Laplacian-based generalized gradient approximations for the exchange energy

It is well known that in the gradient expansion approximation to density functional theory (DFT) the gradient and Laplacian of the density make interchangeable contributions to the exchange correlation (XC) energy. This is an arbitrary "gauge" freedom for building DFT models, normally used to eliminate the Laplacian from the generalized gradient approximation (GGA) level of DFT development. We explore the implications of keeping the Laplacian at this level of DFT, to develop a model that fits the known behavior of the XC hole, which can only be described as a system average in conventional GGA. We generate a family of exchange models that obey the same constraints as conventional GGA's, but which in addition have a finite-valued potential at the atomic nucleus unlike GGA's. These are tested against exact densities and exchange potentials for small atoms, and for constraints chosen to reproduce the SOGGA and the APBE variants of the GGA. The model reliably reproduces exchange energies of closed shell atoms, once constraints such the local Lieb-Oxford bound, whose effects depend upon choice of energy-density gauge, are recast in invariant form.

physics.chem-ph↗

arXiv2013

A molecular mechanism for the water-hydroxyl balance during wetting of TiO2

We show that the formation of the wetting layer and the experimentally observed continuous shift of the H2O-OH balance towards molecular water at increasing coverage on a TiO2(110) surface can be rationalized on a molecular level. The mechanism is based on the initial formation of stable hydroxyl pairs, a repulsive interaction between these pairs and an attractive interaction with respect to water molecules. The experimental data are obtained by synchrotron radiation photoelectron spectroscopy and interpreted with the aid of density functional theory calculations and Monte Carlo simulations.

cond-mat.mtrl-sci↗

arXiv2013

Intrinsic Magnetism of Grain Boundaries in Two-dimensional Metal Dichalcogenides

Grain boundaries (GBs) are structural imperfections that typically degrade the performance of materials. Here we show that dislocations and GBs in two-dimensional (2D) metal dichalcogenides MX2 (M = Mo, W; X = S, Se) can actually improve the material by giving it a qualitatively new physical property: magnetism. The dislocations studied all have a substantial magnetic moment of ~1 Bohr magneton. In contrast, dislocations in other well-studied 2D materials are typically non-magnetic. GBs composed of pentagon-heptagon pairs interact ferromagnetically and transition from semiconductor to half-metal or metal as a function of tilt angle and/or doping level. When the tilt angle exceeds 47° the structural energetics favor square-octagon pairs and the GB becomes an antiferromagnetic semiconductor. These exceptional magnetic properties arise from an interplay of dislocation-induced localized states, doping, and locally unbalanced stoichiometry. Purposeful engineering of topological GBs may be able to convert MX2 into a promising 2D magnetic semiconductor.

cond-mat.mtrl-sci↗

arXiv2013

Thermal transport due to quantum interference in magnetic tunnel junctions

We study the thermal transport in magnetic tunnel junctions. Thermal gradients across the tunneling barrier appear around the Fowler-Nordheim tunneling regime, due to the current-induced heat caused by quantum interference. Both thermovoltage and thermal temperature follow a linear response with the applied current, which is an evidence for a thermoelectric effect. By increasing the barrier transparency, the dynamics of thermoelectric properties is observed with the current. Accordingly, a large range of the Seebeck coefficient, 10 - 1000 μV/K, has been obtained in magnetic tunnel junctions.

cond-mat.mtrl-sci↗

arXiv2013

A First-Principles Study of CdSe Nanoclusters Capped by Thiol Ligands

A first-principles study of small CdnSen quantum dots (QDs) (n = 6, 13, and 33) has been performed for the study of QD-sensitized solar cells. We assessed the effects of the passivating thiol-radical ligands on the optimized structure, the energy gap, and on the absorption spectrum. The simplest thiol, methanethiol, and four other thiol type ligands, namely - cysteine (Cys), mercaptopropionic acid (MPA), and their reduced-chain analogues, were investigated. We have come to the following conclusions. (a) Thiol-radical ligands possessed greater effects on the structure and electronic properties of the CdSe QDs than thiol ligands alone. (b) The sulfur 3p orbitals were localized as the midgap states for the thiol-radical-ligated complex, which altered the absorption spectrum of bare Cd6Se6 by inducing a new lower energy absorption peak at 2.37 eV. (c) The thiol-radical-ligated complex was also found to be sensitive to the position and number of ligands. (d) Both the amine group on Cys and the carboxyl group on Cys and MPA showed a strong tendency to bond with the neighboring Cd atom, especially when the length of the ligand was reduced. This formation of Cd-N and Cd-O bonds resulted in smaller HOMO-LUMO gaps and a stronger binding between the ligands and the surface atoms of CdSe nanoclusters.

cond-mat.mtrl-sci↗

arXiv2013

Mechanical properties of hydrogen functionalized graphene allotropes

Molecular dynamics (MD) simulations have been performed to investigate the mechanical properties of hydrogen functionalized graphene allotropes (GAs) for H-coverage spanning the entire range (0-100%). Four allotropes (graphyne, cyclic graphene, octagonal graphene, and biphenylene) with larger unit lattice size than graphene are considered. The effect of the degree of functionalization and molecular structure on the Young's modulus and strength are investigated, and the failure processes of some new GAs are reported for the first time. We show that the mechanical properties of the hydrogenated GAs deteriorate drastically with increasing H-coverage within the sensitive threshold, beyond which the mechanical properties remain insensitive to the increase in H-coverage. This drastic deterioration arises both from the conversion of sp2 to sp3 bonding and easy rotation of unsupported sp3 bonds. Allotropes with different lattice structures correspond to different sensitive thresholds. The Young's moduli deterioration of fully hydrogenated allotropes can be up to 70% smaller than that of the corresponding pristine structure. Moreover the tensile strength shows an even larger drop of about 90% and higher sensitivity to H-coverage even if it is small. Our results suggest that the unique coverage-dependent deterioration of the mechanical properties must be taken into account when analyzing the performance characteristics of nanodevices fabricated from functionalized GAs.

cond-mat.mtrl-sci↗

arXiv2013

Tailoring the molecular structure to suppress extrinsic disorder in organic transistors

In organic field-effect transistors, the structure of the constituent molecules can be tailored to minimize the disorder experienced by charge carriers. Experiments on two perylene derivatives show that disorder can be suppressed by attaching longer core substituents - thereby reducing potential fluctuations in the transistor channel and increasing the mobility at low temperature - without altering the intrinsic transport properties.

cond-mat.mtrl-sci↗

arXiv2013

Physical improvements to a mesoscopic cellular automaton model for three-dimensional snow crystal growth

We motivate and derive the dynamical rules for a computationally feasible three-dimensional cellular automaton model of snow crystal growth. The model improves upon points of weak physical connections identified in other similar models which have produced morphological features observed in many snow crystal photographs. A systematic survey of the morphologies resulting from our model illustrates the degree to which these features persist in our results, and the trends that appear as model parameters are varied.

cond-mat.mtrl-sci↗

arXiv2013

Effect of solute segregation on shear-induced grain boundary motion

Atomic-scale simulations are performed to study the effect of solute segregation on the shear-induced motion of select grain boundaries in the classical $α$-Fe/C system. At shear rates larger than the solute diffusion rate, we observe a transition from coupled motion to sliding. Below a critical solute excess, the boundaries break away from the solute cloud and move in a coupled motion. At smaller shear rates, we observe extrinsic coupled motion at small stresses indicating that the coupling is aided by convective solute diffusion along the boundary. Our studies underscore the role of solutes in modifying the bicrystallography, temperature and rate dependence of shear accommodation at grain boundaries.

cond-mat.mtrl-sci↗

arXiv2013

Enhancing Graphene-derived Materials through Multimodal and Self-healable Interfaces

Recent studies have shown that graphene-derived materials not only feature outstandingly multifunctional properties, but also act as model materials to implant nanoscale structural engineering insights into their macroscopic performance optimization. Functionalizing the interfaces between graphene sheets by interlayer crosslinks has been proven to be an effective route to tune the mechanical properties. Here we explore the graphene-derived material with a layer-by-layer structure and multiple crosslinking mechanisms. The effects of multimodal and self-healable crosslinks are assessed in terms of interlayer loading transfer capability. The results show that the brick-and-motar hierarchy and synergetic effects from different crosslinks enable synergetic enhancement in the strength and toughness. The findings here could shed light on the development of high-performance paper-, fiber- or film-like macroscopic materials from monolayer nanosheets with nanoengineerable interfaces.

cond-mat.mtrl-sci↗

arXiv2013

Influence of solvation on the structural and capacitive properties of electrical double layer capacitors

We use molecular dynamics simulations to explore the impact of a non-ionic solvent on the structural and capacitive properties of supercapacitors based on an ionic liquid electrolyte and carbon electrodes. The study is focused on two pure ionic liquids, namely 1-butyl-3-methylimidazolium hexafluorophosphate and 1-butyl-3-methylimidazolium tetrafluoroborate, and their 1.5 M solutions in acetonitrile. The electrolytes, represented by coarse-grained models, are enclosed between graphite electrodes. We employ a constant potential methodology which allows us to gain insight into the influence of solvation on the polarization of the electrodes as well as the structural and capacitive properties of the electrolytes at the interface. We show that the interfacial characteristics, different for two distinct pure ionic liquids, become very similar upon mixing with acetonitrile.

cond-mat.mtrl-sci↗

arXiv2013

Carbon Nanotubes Production Using Arc Ignition Under Magnetic Field

We demonstrate that AC arc discharge under de-ionized water which has been recently used for carbon nanotubes (CNT) production can be modified to increase the quantity and quality of carbon nanotubes by applying a magnetic field. The carbon soot thus obtained was analyzed using SEM, TEM and Raman spectroscopy. The Raman scattering measurements showed interesting results. Radial breathing modes (RBM) were observed in the Raman data in the range 150 - 700 cm-1. The presence of RBM is good enough indication of the presence of the carbon nanotubes in the soot collected by us. The RBM data was analyzed further which indicated a superposition of two closely lying modes. It showed the distribution of tube diameter which depended on the application of magnetic field.

cond-mat.mtrl-sci↗