Step-like Features Research Articles

Analysis of the correlation between micro-mechanical fields and fatigue crack propagation path in nodular cast iron

The morphology of the fatigue crack in nodular cast iron has great influence on the material performance under cyclic loading. In contrast to previous phenomenological studies, the present work attempts to unravel a relation between the crack propagation path and the micro-mechanical fields developing at the microstructural level. To this end, a fatigue crack in a compact tension specimen is imaged with X-ray tomography and a new segmentation procedure based on digital volume correlation is used to distinguish the part of the crack growing in the matrix from the graphite nodules. The micromechanical fields surrounding the tip of the notch as well as the tip of the fully developed fatigue crack are estimated via 3D microstructure-resolved finite element models, whose size is maintained small compared to the specimen by using boundary conditions measured in-situ with digital image correlation. The numerical fields are compared to the location of the crack nucleation site as well as to the crack growth direction. A weak correlation is found between the nucleation site and the equivalent plastic strain, suggesting that nucleation is controlled by plasticity occurring at a lower scale. On the other hand, a clear correlation is revealed between the crack growth direction and the direction perpendicular to the maximum principal stress. Based on this result, a mechanism is proposed to explain the step-like features characterizing the shape of the fatigue crack in nodular cast iron.

On the Strength of Hair across Species

Summary The hair of mammals varies in diameter across species from ∼60 μm in humans to over ∼400 μm in giraffes and elephants. Here, we establish that the tensile strength of hair generally decreases with increasing diameter. Although there is a commonality in the morphology of the hair in such differing species as they all possess a cortex surrounded by cuticles, there are also significant differences in the hierarchical structure that contribute to the observed differences in strength. In this work, the internal structure of the hair from different species is examined, with some unique peculiarities in javelina and capybara hair uncovered. The dimensional dependence of hair strength is compared using Weibull predictions. The good correlation of the strength of hair using such Weibull analysis suggests that the failure strength is dictated by the probability distribution of flaws within the cortex.

Analyses of structure, electronic and multiferroic properties of Bi1-xNdxFeO3 (x = 0, 0.05, 0.10, 0.15, 0.20, 0.25) system

Magnetoelectric multiferroics are of considerable interest due to the fascinating science and potential applications. In this study, multiferroic Bi1-xNdxFeO3 (0 ≤ x ≤ 0.25) samples have been prepared by a two-stage solid-state reaction method and correlation between various properties has been studied. It can be seen that Nd3+ substitution destabilized the polar R3c symmetry due to chemical strain effects arising from the size mismatch between Bi3+ and Nd3+ cations which triggered partial structural transformation to triclinic structure (P1symmetry). Raman scattering experiments elucidated slight changes in covalency of Bi–O bond and softening of modes corresponding to oxygen motion suggests a significant destabilization of FeO6 octahedra. The increase in magnetization has been attributed to the suppression of the spiral spin structure and variation in Fe–O–Fe bond angle due to the strain induced in the unit cell by Nd3+ substitution. The step-like feature in magnetization at the low field has been attributed to charge melting in connection with manganites. An anomalous decrease in coercivity with decrease in temperature has been observed for Bi0.75Nd0.25FeO3 sample which can be explained on the basis that the decrease in effective magnetic anisotropy and is an indicator of presence of magnetoelectric coupling. The bandgap increases with increase in Nd3+ content which is attributed to the decrease in interatomic lattice parameters which resulted in increased binding forces of valance electrons.

A new method for evaluating rock mass quality of slopes based on interval continuous mathematical models

Rock mass classification has been playing an important role in the analyses of civil engineering in recent years. The quality evaluation of slope rock mass becomes essential for slope stability assessment. Neither continuous expression and comparable function are rarely taken into consideration in rock mass classification, nor is the shortage in the previous quality evaluation of slop rock mass. Through constructing the benchmark intervals of evaluation indicators; mathematical models between quantized value and relative quantized value which is a new proposed parameter; and linear weighted function, the interval continuous mathematical models (ICMM) for quality evaluation of slope rock mass are presented. The application result shows that ICMM is effective for improving the comparability of rock mass quality of slopes by unifying evaluation indicators. On the other hand, the ICMM realizes the continuous expression of rock mass quality of slopes by removing the step-like features caused by the pre-set rating criteria of each evaluation indicator and rock mass quality classification, guiding the follow-up construction and protection of slopes, and ensuring the balance between economy and safety. Therefore, the ICMM is more scientific and reasonable than extant classification systems for evaluating rock mass quality of slopes.

Exploring the CMB power suppression in canonical inflation models

There exists some evidence of a suppression in power in the CMB multipoles around 0l∼ 2–30. If taken seriously, this is in tension with the simplest inflationary models driven by a single scalar field with a standard type of slowly varying potential function V(ϕ). Such potential functions generate a nearly scale invariant spectrum and so they do not possess the requisite suppression in power. In this paper we explore if canonical two-derivative inflation models, with a step-like feature in the potential, can improve agreement with data. We find that improvement can be made when one utilizes the standard slow-roll approximation formula for the power spectrum. However, we find that in order to have a feature in the power spectrum that is sufficiently localized so as to not significantly disrupt higher l or lower l, the potential's step-like feature must be so sharp that the standard slow-roll approximations break down. This leads us to perform an exact computation of the power spectrum by solving for the Bunch-Davies mode functions numerically. We find that the corresponding CMB multipoles do not provide a good agreement with the data. We conclude that, unless there is fine-tuning, canonical inflation models do not fit this suppression in the data.

Magnetoelectric control of topological phases in graphene

Topological antiferromagnetic (AFM) spintronics is an emerging field of research, which involves the topological electronic states coupled to the AFM order parameter known as the N$\acute{\rm e}$el vector. The control of these states is envisioned through manipulation of the N$\acute{\rm e}$el vector by spin-orbit torques driven by electric currents. Here we propose a different approach favorable for low-power AFM spintronics, where the control of the topological states in a two-dimensional material, such as graphene, is performed via the proximity effect by the voltage induced switching of the N$\acute{\rm e}$el vector in an adjacent magnetoelectric AFM insulator, such as chromia. Mediated by the symmetry protected boundary magnetization and the induced Rashba-type spin-orbit coupling at the interface between graphene and chromia, the emergent topological phases in graphene can be controlled by the N$\acute{\rm e}$el vector. Using density functional theory and tight-binding Hamiltonian approaches, we model a graphene/Cr2O3 (0001) interface and demonstrate non-trivial band gap openings in the graphene Dirac bands asymmetric between the K and K' valleys. This gives rise to an unconventional quantum anomalous Hall effect (QAHE) with a quantized value of $2e^2/h$ and an additional step-like feature at a value close to $e^2/2h$, and the emergence of the spin-polarized valley Hall effect (VHE). Furthermore, depending on the N$\acute{\rm e}$el vector orientation, we predict the appearance and transformation of different topological phases in graphene across the $180^{\circ}$ AFM domain wall, involving the QAHE, the valley-polarized QAHE and the quantum VHE (QVHE), and the emergence of the chiral edge state along the domain wall. These topological properties are controlled by voltage through magnetoelectric switching of the AFM insulator with no need for spin-orbit torques.

Integer quantum Hall conductivity and longitudinal conductivity in silicene under the electric field and magnetic field

We investigate the integer Hall conductivity and longitudinal conductivity of silicene under the magnetic field, electric field, and exchange field in this paper. We focus not only on the case of low-temperature and δ-function impurities (i.e., independent of the scattering momentum), which only exist the intra-Landau level transition, but also on the case of inter-Landau level transition which also with the non-elastic scattering. The resulting longitudinal conductivity is very different from the intra-Landau level one at low-temperature. The expressions of the Hall conductivity, longitudinal conductivity, valley contributed Hall conductivity, and the spin or valley Hall conductivity are deduced in this paper. We also compute the dynamical polarization under the magnetic field which is an important quantity related to many exciting and novel properties of the two-dimension materials, including the screened Coulomb scattering due to the charged impurities. The polarization function here is also related to the Landau level index under the magnetic field, and shows a step-like feature rather than the logarithmically divergent as which appears in the zero-magnetic field case, and it is also naturally related to the conductivity in silicene. The generalized Laguerre polynomial is used in both the longitudinal conductivity and dynamical polarization function under magnetic field. Our results are also valid for the silicene-like materials like the genmanene, MoS2.

Analysis of the Correlation between Micro-Mechanical Fields and Fatigue Crack Propagation Path in Nodular Cast Iron

The morphology of the fatigue crack in nodular cast iron has great influence on the material performance under cyclic loading. In contrast to previous phenomenological studies, the present work attempts to unravel a relation between the crack propagation path and the micro-mechanical fields developing at the microstructural level. To this end, a fatigue crack in a compact tension specimen is imaged with X-ray tomography and a new segmentation procedure based on digital volume correlation is used to distinguish the part of the crack growing in the matrix from the graphite nodules. The micromechanical fields surrounding the tip of the notch as well as the tip of the fully developed fatigue crack are estimated via 3D microstructure-resolved finite element models, whose size is maintained small compared to the specimen by using boundary conditions measured in-situ with digital image correlation. The numerical fields are compared to the location of the crack nucleation site as well as to the crack growth direction. A weak correlation is found between the nucleation site and the equivalent plastic strain, suggesting that nucleation is controlled by plasticity occurring at a lower scale. On the other hand, a clear correlation is revealed between the crack growth direction and the direction perpendicular to the maximum principal stress. Based on this result, a mechanism is proposed to explain the step-like features characterizing the shape of the fatigue crack in nodular cast iron.

Spin-Order-Induced Ferroelectricity and Magnetoelectric Effect in LiCuFe2(VO4)3

Mixed spin chain compounds, ACuFe2(VO4)3 (A= Li,Na), reach magnetically ordered state at TN ~ 11 K (Li) or ~ 9 K (Na) and experience further transformation of magnetic order at T* ~ 7 K (Li) or ~ 5 K (Na), evidenced in magnetic susceptibility chi and specific heat Cp measurements. While no anomaly has been detected in dielectric property of NaCuFe2(VO4)3, the step-like feature precedes a sharp peak in permittivity epsilon at TN in LiCuFe2(VO4)3. These data suggest the spin-order-induced ferroelectricity in Li compound and no such thing in Na compound. On the contrary, the Moessbauer spectroscopy study suggests similarly wide distribution of hyperfine field in between T* and TN for both the compounds. The first principles calculations also provide similar values for magnetic exchange interaction parameters in both compounds. These observations lead us to conclude on the crucial role of alkali metals mobility within the channels of the crystal structure needed to be considered in explaining the improper multiferroicity in one compound and its absence in other.

Photonic Hall effect

In this work, we report on the emergence of a photonic Hall effect (PHE) system within a narrow filtered background of a one-dimensional defective optical dielectric structure with graphene under the influence of a constant magnetic field regime. It is observed that at low temperature and relatively strong applied magnetic fields, electromagnetic defective transmission spectra corresponding to the two decoupled right- and left-handed polarized modes possess a step-like transmission feature which is referred to as “quantum Hall defect modes” (QHD modes or QHDs) in this paper. Tunable growing transitional transmission steps for QHDs with increasing magnetic field intensity were shown to be possible. Observation of sensitive magneto-transmission oscillations to the thermal excitations in the last plateaus slowly ascending toward unity is another special feature noted in this work. The results of this study, which is carried out based on rapid standard calculations for the transfer matrix approach is supplied with commercial simulations marking the first PHE system, promise an proper candidate for new photonic applications, especially new tunable magneto-based lenses and photonic magneto-thermal sensors.

Factors driving stable growth of He clusters in W: first-principles study

The evolution of helium (He) bubbles is responsible for the surface morphology variation and subsequent degradation of the properties of plasma-facing materials (PFMs) in nuclear fusion reactors. These severe problems unquestionably trace back to the behavior of He in PFMs, which is closely associated with the interaction between He and the matrix. In this paper, we decomposed the binding energy of the He cluster into three parts, those from W–W, W–He, and He–He interactions, using density functional theory. As a result, we clearly identified the main factors that determine a steplike decrease in the binding energy with increasing number of He atoms, which explains the process of self-trapping and athermal vacancy generation during He cluster growth in the PFM tungsten. The three interactions were found to synergetically shape the features of the steplike decrease in the binding energy. Fairly strong He–He repulsive forces at a short distance, which stem from antibonding states between He atoms, need to be released when additional He atoms are continuously bonded to the He cluster. This causes the steplike feature in the binding energy. The bonding states between W and He atoms in principle facilitate the decreasing trend of the binding energy. The decrease in binding energy with increasing number of He atoms implies that He clusters can grow stably.

Optoelectronic properties of defective MoS2 and WS2 monolayers

We theoretically explore the effect of metal and disulphur vacancies on electronic and optical properties of MoS2 and WS2 monolayers based on a Slater-Koster tight-binding model and including the spin-orbit coupling. We show that the vacancy defects create electronic flat bands by shifting the Fermi level towards the valence band, indicating that both types of vacancies may act as acceptor sites. The optical spectra of the pristine monolayers show step-like features corresponding to the transition from spin split valence band to the conduction band minimum, whereas the defective monolayers exhibit additional peaks in their spectra arising from induced midgap states in their band structures. We find that Mo and W vacancies contribute mostly in the low-energy optical spectrum, while the S2 vacancies enhance the optical conductivity mainly in the visible range of the spectrum. This suggest that depending on the type of vacancy, the atomic defects in MoS2 and WS2 monolayers may increase the efficiency of solar cells used in photovoltaic systems.

Anisotropic magnetic properties of the triangular plane lattice material TmMgGaO4

The crystal growth, structure, and basic magnetic properties of TmMgGaO4 are reported. The Tm ions are located in a planar triangular lattice consisting of distorted TmO6 octahedra, while the Mg and Ga atoms randomly occupy intermediary bilayers of M-O triangular bipyramids. The Tm ions are positionally disordered. The material displays an antiferromagnetic Curie Weiss theta of ∼−20 −25 K, with no clear ordering visible in the magnetic susceptibility down to 1.8 K; the structure and magnetic properties suggest that ordering of the magnetic moments is frustrated by both structural disorder and the triangular magnetic motif. Single crystal magnetization measurements indicate that the magnetic properties are highly anisotropic, with large moments measured perpendicular to the triangular planes. At 2 K, a broad step-like feature is seen in the field-dependent magnetization perpendicular to the plane on applied field near 2 T.

The initial stages of atomic force microscope based local anodic oxidation of silicon

In this paper, the initial stages of local anodic oxidation (LAO) process initiated by AFM probe are studied on the wide (∼100μm) terraces of the atomic-smooth Si (111) surface when creating dense array of local oxidation points. The dependence of LAO points height on the value of voltage initiating the oxidation is found to have a pronounced step-like feature with a characteristic period of 0.7 ± 0.1 nm. The presented analysis shows for the first time the realization of the step-layer mechanism of anodic oxide growth on the Si (111) surface.

Late time sky as a probe of steps and oscillations in primordial Universe

The standard model of cosmology with nearly Gaussian, isotropic, scale invariant and adiabatic initial conditions describes the cosmological observations well. However, the study of any deviation from the mentioned conditions will open up a new horizon to the physics of early universe. In this work, we study the effect of the oscillatory and step-like features in potentials of inflationary models in late time large scale structure observations. Mainly we study the matter power spectrum, number density of the structures, dark matter halo bias and specifically CMB lensing. We show that the oscillatory models can introduce some degeneracy with late time effects on BAO scale. We also conclude that high frequency oscillatory models which are favored by Planck data do not have significant effect on the non linear structure formation. Finally we show that inflationary models with step functions which deviates from the standard model in small scales l ⩽ 1 Mpc can be constrained by future experiments via CMB lensing. We propose the idea that CMB lensing is a bias independent observation which can be used as a small scale physics probe due to distribution of the lenses in low redshifts. Meantime this model can alter the prediction of the cosmological model for the number density of small structures and can be used as a probable explanation for galactic scale crisis of ΛCDM.

Effects of uniaxial pressure on the quantum tunneling of magnetization in a high-symmetry Mn12 single-molecule magnet

The symmetry of single-molecule magnets dictates their spin quantum dynamics, influencing how such systems relax via quantum tunneling of magnetization (QTM). By reducing a system's symmetry, through the application of a magnetic field or uniaxial pressure, these dynamics can be modified. We report measurements of the magnetization dynamics of a crystalline sample of the high-symmetry $[\mathrm{M}{\mathrm{n}}_{12}{\mathrm{O}}_{12}{({\mathrm{O}}_{2}\mathrm{CM}\mathrm{e})}_{16}{(\mathrm{M}\mathrm{e}\mathrm{OH})}_{4}]\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{M}\mathrm{e}\mathrm{OH}$ single-molecule magnet as a function of uniaxial pressure applied either parallel or perpendicular to the sample's ``easy'' magnetization axis. At temperatures between 1.8 and 3.3 K, magnetic hysteresis loops exhibit the characteristic steplike features that signal the occurrence of QTM. After applying uniaxial pressure to the sample in situ, both the magnitude and field position of the QTM steps changed. The step magnitudes were observed to grow as a function of pressure in both arrangements of pressure, while pressure applied along (perpendicular to) the sample's easy axis caused the resonant-tunneling fields to increase (decrease). These observations were compared with simulations in which the system's Hamiltonian parameters were changed. From these comparisons, we determined that parallel pressure induces changes to the second-order axial anisotropy parameter as well as either the fourth-order axial or fourth-order transverse parameter, or to both. In addition, we find that pressure applied perpendicular to the easy axis induces a rhombic anisotropy $E\phantom{\rule{0.16em}{0ex}}\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}D/2000\phantom{\rule{0.16em}{0ex}}\mathrm{per}\phantom{\rule{0.16em}{0ex}}\mathrm{kbar}$ that can be understood as deriving from a symmetry-breaking distortion of the molecule.

Interacting fermions in rotation: chiral symmetry restoration, moment of inertia and thermodynamics

We study rotating fermionic matter at finite temperature in the framework of the Nambu-Jona-Lasinio model. In order to respect causality the rigidly rotating system must be bound by a cylindrical boundary with appropriate boundary conditions that confine the fermions inside the cylinder. We show the finite geometry with the MIT boundary conditions affects strongly the phase structure of the model leading to three distinct regions characterized by explicitly broken (gapped), partially restored (nearly gapless) and spontaneously broken (gapped) phases at, respectively, small, moderate and large radius of the cylinder. The presence of the boundary leads to specific steplike irregularities of the chiral condensate as functions of coupling constant, temperature and angular frequency. These steplike features have the same nature as the Shubnikov-de Haas oscillations with the crucial difference that they occur in the absence of both external magnetic field and Fermi surface. At finite temperature the rotation leads to restoration of spontaneously broken chiral symmetry while the vacuum at zero temperature is insensitive to rotation (“cold vacuum cannot rotate”). As the temperature increases the critical angular frequency decreases and the transition becomes softer. A phase diagram in angular frequency-temperature plane is presented. We also show that at fixed temperature the fermion matter in the chirally restored (gapless) phase has a higher moment of inertia compared to the one in the chirally broken (gapped) phase.

Quantum-interference-enhanced thermoelectricity in single molecules and molecular films

We provide a brief overview of recent measurements and predictions of thermoelectric properties of single-molecules and porous nanoribbons and discuss some principles underpinning strategies for enhancing their thermoelectric performance. The latter include (a) taking advantage of steep slopes in the electron transmission coefficient T ( E ) , (b) creating structures with delta-function-like transmission coefficients and (c) utilising step-like features in T ( E ) . To achieve high performance, we suggest that the latter may be the most fruitful, since it is less susceptible to inhomogeneous broadening. For the purpose of extrapolating thermoelectric properties of single or few molecules to monolayer molecular films, we also discuss the relevance of the conductance-weighted average Seebeck coefficient. Nous procédons à un bref survol des mesures et prédictions récentes concernant les propriétés thermoélectriques de molécules individuelles ou de nanorubans poreux, puis nous discutons quelques-uns des principes sous-jacents aux stratégies visant à augmenter leurs performances thermoélectriques. On relèvera parmi ces dernières (a) l'utilisation de pentes élevées du coefficient de transmission électronique T ( E ) , (b) la création de structures avec des pics de transmission et (c) l'exploitation de ces derniers. Pour atteindre de hautes performances, nous suggérons que cette dernière approche puisse être la plus fructueuse, puisqu'elle est moins susceptible de présenter des élargissements inhomogénes. Afin d'extrapoler les propriétés thermoélectriques d'une ou de quelques molécules à des films moléculaires monocouche, nous discutons aussi la pertinence de l'utilisation d'une moyenne du coefficient Seebeck pondérée par la conductance.

Theory of Hall effect in two-dimensional giant Rashba systems

The weak-field Hall conductivity of disordered two-dimensional systems with strong Rashba spin-orbit interaction is studied in a self-consistent Born approximation. Explicit numerical results are obtained for scatterers with a Gaussian potential and for charged impurities. The singular behavior associated with a conelike crossing band appears only in the case of scatterers with a long-range Gaussian potential, which do not cause mixing with the outer band. In the case of more realistic scatterers such as charged impurities, the singularity is completely removed except the presence of a weak steplike feature. The Hall conductivity associated with the spin-Zeeman energy is also strongly reduced by interband mixing and generally remains much smaller than the orbital contribution.

Giant spontaneous magnetization jumps in LaFe12B6

Unexpected giant metamagnetic transitions consisting of a succession of extremely sharp magnetization steps separated by plateaus are reported at low temperature in LaFe12B6. Under certain combinations of the external parameters (temperature and magnetic field), the time dependence of the magnetization displays an unusual step-like feature. It is demonstrated that this giant spontaneous jump in magnetization takes place after a silent time and shows a striking resemblance with the phenomenon of an “incubation time” found in martensitic-like transformations.