Search

Purpose The anisotropic surfaces of viscoelastic materials play a role in sliding friction; the purpose of this paper is to study the effect of the anisotropic surfaces on contact area and the friction coefficient. Design/methodology/approach A complex elastic modulus and an anisotropic power spectrum are used to compute the coefficient of friction based on the extension Persson theory which considers the partial contact and the variation in the roughness slopes. Findings The ratios of the relative contact area that varies with velocity are obtained with different angles and eccentricities, and the effect of the elastic modulus needs to be considered. The coefficients of the friction parallel to the direction of motion decrease as the angle increases, or as the eccentricity decreases. The friction coefficients in the vertical direction change irregularly when the angles or eccentricities increase. Originality/value An extension of Persson’s work considering the partial contact and the effective mean square slope of the roughness is applied to study sliding friction, and the effect of the elastic modulus on contact area is considered.

Using adjoint-based elastic reverse time migration, it is difficult to produce high-quality reflectivity images due to the limited acquisition apertures, band-limited source time function, and irregular subsurface illumination. Through iteratively computing the Hessian inverse, least-squares migration enables us to reduce the point-spread-function effects and improve the image resolution and amplitude fidelity. By incorporating anisotropy in the 2D elastic wave equation, we have developed an elastic least-squares reverse time migration (LSRTM) method for multicomponent data from the vertically transversely isotropic (VTI) media. Using the perturbed stiffness parameters [Formula: see text] and [Formula: see text] as PP and PS reflectivities, we linearize the elastic VTI wave equation and obtain a Born modeling (demigration) operator. Then, we use the Lagrange multiplier method to derive the corresponding adjoint wave equation and reflectivity kernels. With linearized forward modeling and adjoint migration operators, we solve a linear inverse problem to estimate the subsurface reflectivity models for [Formula: see text] and [Formula: see text]. To reduce the artifacts caused by data over-fitting, we introduce total-variation regularization into the reflectivity inversion, which promotes a sparse solution in terms of the model derivatives. To accelerate the convergence of LSRTM, we use source illumination to approximate the diagonal Hessian and use it as a preconditioner for the misfit gradient. Numerical examples help us determine that our elastic VTI LSRTM method can improve the spatial resolution and amplitude fidelity in comparison to adjoint migration.

Seismic anisotropy is an intrinsic elastic property. Appropriate accounting of anisotropy is critical for correct and accurate positioning seismic events in reverse time migration. Although the full elastic wave equation may serve as the ultimate solution for modeling and imaging, pseudoelastic and pseudoacoustic wave equations are more preferable due to their computation efficiency and simplicity in practice. The anisotropic parameters and their relations are not arbitrary because they are constrained by the energy principle. Based on the investigation of the stability condition of the pseudoelastic wave equations, we have developed a set of explicit formulations for determining the S-wave velocity from given Thomsen’s parameters [Formula: see text] and [Formula: see text] for vertical transverse isotropy and tilted transverse isotropy media. The estimated S-wave velocity ensures that the wave equations are stable and well-posed in the cases of [Formula: see text] and [Formula: see text]. In the case of [Formula: see text], a common situation in carbonate, a positive value of S-wave velocity is needed to avoid the wavefield instability. Comparing the stability constraints of the pseudoelastic- with the full-elastic wave equation, we conclude that the feasible range of [Formula: see text] and [Formula: see text] was slightly larger for the pseudoelastic assumption. The success of achieving high-accuracy images and high-quality angle gathers using the proposed constraints is demonstrated in a synthetic example and a field example from Saudi Arabia.

A small in-plane external uniaxial pressure has been widely used as an effective method to acquire single domain iron pnictide BaFe$_2$As$_2$, which exhibits twin-domains without uniaxial strain below the tetragonal-to-orthorhombic structural (nematic) transition temperature $T_s$. Although it is generally assumed that such a pressure will not affect the intrinsic electronic/magnetic properties of the system, it is known to enhance the antiferromagnetic (AF) ordering temperature $T_N$ ($, Comment: 11 pages, 4 figures, Supplementary information is available upon request

A developing frontier in condensed matter physics is the emergence of novel electromagnetic responses, such as topological and anomalous Hall effect (AHE), in ferromagnetic Weyl semimetals (FM-WSMs). Candidates of FM-WSM are limited to materials that preserve inversion symmetry and generate Weyl crossings by breaking time-reversal symmetry. These materials share three common features: a centrosymmetric lattice, a collinear FM ordering, and a large AHE observed when the field is parallel to the magnetic easy-axis. Here, we present CeAlSi as a new type of FM-WSM, where the Weyl nodes are stabilized by breaking inversion symmetry, but their positions are tuned by breaking time-reversal symmetry. Unlike the other FM-WSMs, CeAlSi has a noncentrosymmetric lattice, a noncollinear FM ordering, and a novel AHE that is anisotropic between the easy- and hard-axes. It also exhibits large FM domains that are promising for both device applications and an interplay between the Weyl nodes and FM domain walls., Preprint, 20 pages, 4 figures

After over 20 years of development in Daqingzijiang Oilfield, isolated sand bodies under the delta front and pro-delta in the Qingshankou Formation(qn)have become important oil and gas reservoir bodies. However, the cause for large amounts of isolated bar sand bodies in the lake-basin delta sediment system has not been reported in China. This article, through core observation, reveals plenty of evidence that there storm waves once existed. Combined with paleogeography and hydrodynamic force analysis, it describes the transformation effect of waves on delta sand bodies and on the formation mechanism of bar sand bodies. Based on a study on paleogeomorphology and the statistics of sand body distribution, we consider &lsquo, Storm waves conveying sand&rsquo, and &lsquo, landform controlling sand&rsquo, as the cause and distribution model of the delta&rsquo, s isolated bar sand body formation. We also think that the superposition of multiple bar sand bodies is the direct cause of the strong anisotropy in reservoirs and the complex relationship between oil and water in reservoirs. Most of these sand bodies have formed into lenticular lithologic hydrocarbon accumulations. On the basis of this integrated study on hydrocarbon accumulation, we set up an accumulation model of lenticular hydrocarbon accumulation involving the variables &lsquo, Surrounded by source rocks to generate hydrocarbon&rsquo, &lsquo, Driven by pressure difference&rsquo, Migration through multi pathways&rsquo, Accumulation by filtering water&rsquo

Currently, hydraulic slotted borehole technology for strengthening gas drainage is one of the main measures to control gas disasters in deep mines. In this paper, to analyze the evolution of gas flow field around the hydraulic slotted borehole, the borehole gas drainage was simulated based on the anisotropy of coal structure and the established multi-field (namely, coal deformation field, diffusion field and seepage field) coupling model. Then, the influences of characteristic parameters, including, initial diffusion coefficient, diffusion attenuation coefficient, initial gas pressure and initial permeability, on the gas production rate and reservoir parameters of hydraulic slotted borehole gas drainage were analyzed. Finally, the results were verified by comparing the model simulation with field tests in Yangliu Mine. The results show that the anisotropy of coal affects the effect of borehole gas drainage. The initial diffusion coefficient D0 and the diffusion attenuation coefficient λ have opposite effects on key parameters of coal. With a larger D0, it is easier for gas in matrix pores to diffuse into the fracture system, and accordingly the gas production rate and relative permeability grow. A larger λ makes it harder for gas in matrix pores to diffuse into fractures, thus decreasing the gas production rate and relative permeability. The initial gas pressure pm0 and the initial permeability kfx0 and kfy0 exert the same effect on the key parameters of coal. The larger their values are, the more easily the gas in fractures can be extracted under the same negative pressure of drainage, which contributes to the diffusion of gas from the matrix into the fracture system under the action of pressure difference. Through combining actual monitoring results of 3# and 4# drilling fields in Yangliu Mine and the results of numerical simulation, it is acquired that the gas production rate measured in situ coincides with the rate simulated by the model, verifying the feasibility and engineering applicability of the model.

In this paper, flow factors considering the roughness of the different orientation angles and different wavelengths are obtained from the derived form of the homogenization coefficient, which is calculated by a heterogeneous multiscale inverse approach. The influence of anisotropic roughness on the flow factors is investigated. When the angle is equal to 45 ° , the span flow is maximum. The flow factor curves happen to overlap when the wavelength ratio is less than 4 with double cosine roughness. To verify the flow factors, film pressures are calculated by a finite element heterogeneous multiscale method (FE-HMM) without considering the flow factors. Then, by comparing the results with solutions of a finite difference method, the FE-HMM method gets the results that are needed, and with a shorter time of calculation. The pressure curve is greatest when the orientation angle is 45 ° with a double cosine roughness, which is caused by the asperity number and the span flow. The pressure distributions of the cosine roughness and double cosine roughness are different as the orientation angle changes.

Antimony selenide (Sb2Se3) is a promising absorber material for thin film photovoltaics because of its attractive material, optical and electrical properties. In recent years, the power conversion efficiency (PCE) of Sb2Se3 thin film solar cells has gradually enhanced to 5.6%. In this article, we systematically studied the basic physical properties of Sb2Se3 such as dielectric constant, anisotropic mobility, carrier lifetime, diffusion length, defect depth, defect density and optical band tail states.We believe such a comprehensive characterization of the basic physical properties of Sb2Se3 lays a solid foundation for further optimization of solar device performance.

The elastic constants, formation enthalpies, elastic moduli, Poisson's ratios, acoustic velocities and Debye temperatures of D022-Ni3Nb phase at different pressures are determined using first-principles calculations. The effects of pressure on anisotropic elastic properties and minimum thermal conductivity are discussed in detail. It is found that the calculated equilibrium lattice parameters at 0 GPa well agree with the experimental results. The negative formation enthalpy indicates that Ni3Nb phase is thermodynamically stable in the ground state, and the stability becomes better as the pressure is increased. The bigger ratio of B/G (larger than 1.75) and positive Cauchy pressure indicate that Ni3Nb phase is ductile. Moreover, Ni3Nb phase exhibits high elastic anisotropy, and the elastic anisotropies become increasingly pronounced with the increase of pressure. Additionally, the anisotropic chemical bonds of Ni3Nb phase reduce the minimum thermal conductivity. The minimum thermal conductivity increases with the increased pressure.

Recent studies have demonstrated that extended imaging depth can be achieved using dual-axis optical coherence tomography (DA-OCT). By illuminating and collecting at an oblique angle, multiple forward scattered photons from large probing depths are preferentially detected. However, the mechanism behind the enhancement of imaging depth needs further illumination. Here, the signal of a DA-OCT system is studied using a Monte Carlo simulation. We modeled light transport in tissue and recorded the spatial and angular distribution of photons exiting the tissue surface. Results indicate that the spatial separation and offset angle created by the non-telecentric scanning configuration promote the collection of more deeply propagating photons than conventional on-axis OCT.

Transverse isotropy is characterized by a plane of isotropy and an axis of anisotropy. For rocks, the plane of isotropy is associated with the bedding plane, whereas the axis of anisotropy is the normal to the bedding plane. Transversely isotropic rocks are known to exhibit strength that depend on the orientation of the bedding plane relative to the direction of load. In this work, we present a constitutive framework for predicting the deformation and strength of transversely isotropic rocks. The model is based on anisotropic critical state plasticity with thermal softening. We conduct numerical simulations of boundary value problems to demonstrate the impact of bedding plane orientation on the deformation and strength of a transversely isotropic rock.

We present a continuum framework for coupled solid deformation–fluid flow in anisotropic elastoplastic porous media. A thermodynamic formulation of the coupled processes gives rise to an anisotropic Biot tensor that is a function of drained elastic tangent moduli tensor of the solid skeleton and the intrinsic bulk modulus of the solid constituent. Two effective stress measures emerge from the formulation, namely, σ ′ , which is energy-conjugate to the elastic strain, and σ ′ ′ , which is energy-conjugate to the plastic strain. For the special case of transverse isotropy that is commonly encountered in natural rocks, the Biot tensor can be expressed in terms of its normal and tangential components to the bedding plane, along with a microstructure tensor. Apart from its thermodynamic consistency, an advantage of this new formulation is that standard mixed finite element formulation can be employed to discretize the domain and solve initial boundary-value problems. We conduct plane strain simulations of coupled solid deformation–fluid flow in a transversely isotropic porous medium to demonstrate the impacts of material anisotropy, stress history, and the Biot tensor on the system response.

We investigate three aspects of material behavior in this ninth installment of the Cam-Clay series, namely, anisotropy, heterogeneity, and viscoplasticity. The main focus of the paper is creep in shale and how this time-dependent deformation behavior may be quantified across the scales, from nanometers to millimeters. Recognizing the highly heterogeneous nature of shale, we adopt a simplified representation of this material as a mixture of softer matter representing organics and clay, and harder matter representing the inorganic rock matrix. Due to the presence of bedding planes in the rock matrix, anisotropy in both the elastic and inelastic responses is assumed; however, the superimposed softer matter may be taken to be isotropic unless experimental evidence indicates otherwise. Viscoplasticity is considered for the first time in this series of work, in which both the Duvaut–Lions and Perzyna formulations are utilized. The two viscoplastic formulations are shown to predict very similar time-dependent deformation responses. The framework is used to interpret the results of multiscale triaxial laboratory creep tests in organic-rich Barnett shale. Time-dependent strain localization in the form of dilative shear bands is also shown to result from loading with varying strain rates, as well as from creep processes that accommodate the multiscale heterogeneity of shale.

Amplitude modes arising from symmetry breaking in materials are of broad interest in condensed matter physics. These modes reflect an oscillation in the amplitude of a complex order parameter, yet are typically unstable and decay into oscillations of the order parameter’s phase. This renders stable amplitude modes rare, and exotic effects in quantum antiferromagnets have historically provided a realm for their detection. Here we report an alternate route to realizing amplitude modes in magnetic materials by demonstrating that an antiferromagnet on a two-dimensional anisotropic triangular lattice (α-Na0.9MnO2) exhibits a long-lived, coherent oscillation of its staggered magnetization field. Our results show that geometric frustration of Heisenberg spins with uniaxial single-ion anisotropy can renormalize the interactions of a dense two-dimensional network of moments into largely decoupled, one-dimensional chains that manifest a longitudinally polarized-bound state. This bound state is driven by the Ising-like anisotropy inherent to the Mn3+ ions of this compound., Oscillations of the order parameter amplitude in magnetically ordered materials provide condensed matter analogues of the Higgs boson but in most cases they are unstable. Dally et al. show that the quasi-one-dimensional magnet α-Na0.9MnO2 supports stable amplitude excitations.

The Kitaev model is a spin model on a honeycomb lattice with bond-dependent anisotropic interactions whose ground state is an exactly solvable example of a quantum spin liquid. Interest in this model has been growing rapidly due to the potential for realization of this artificial model in insulating materials with strong spin-orbit interactions. One such example is the layered honeycomb material $\ensuremath{\alpha}$-RuCl${}_{3}$. Although this material orders magnetically rather than realizing a spin liquid ground state, there have been suggestions that suppression of the magnetic order through application of magnetic field could reveal a quantum spin liquid phase in this material. In this work, the authors examine the temperature-field phase diagram of $\ensuremath{\alpha}$-RuCl${}_{3}$ using neutron diffraction and bulk thermodynamic measurements. They found that the magnetic order disappears at magnetic fields above 8 Tesla, and a magnetic gap showing quantum critical scaling opens up in the high field phase.

Author(s): Wang, M; Valdivia, P; Yi, M; Chen, JX; Zhang, WL; Ewings, RA; Perring, TG; Zhao, Y; Harriger, LW; Lynn, JW; Bourret-Courchesne, E; Dai, P; Lee, DH; Yao, DX; Birgeneau, RJ