Anisotropic Bond Research Articles

Application of quantum graph theory to metamaterial design: Negative refraction of acoustic waveguide modes

We leverage quantum graph theory to quickly and accurately characterize acoustic metamaterials comprising networks of interconnected pipes. Anisotropic bond lengths are incorporated in the model that correspond to space-coiled acoustic structures to exhibit dispersion spectra reminiscent of hyperbolic metamaterials. We construct two metasurfaces with embedded graph structure and, motivated by the graph theory, infer and fine-tune their dispersive properties to engineer nonresonant negative refraction of acoustic surface waves at their interface. Agreement between the graph model, full-wave simulations, and experiments bolsters quantum graph theory as a new paradigm for metamaterial design. Published by the American Physical Society 2024

Static magnetic order with strong quantum fluctuations in spin-1/2 honeycomb magnet Na2Co2TeO6

Kitaev interactions, arising from the interplay of frustration and bond anisotropy, can lead to strong quantum fluctuations and, in an ideal case, to a quantum-spin-liquid state. However, in many nonideal materials, spurious non-Kitaev interactions typically promote a zigzag antiferromagnetic order in the d-orbital transition-metal compounds. Here, by combining neutron scattering with muon-spin rotation and relaxation techniques, we provide mechanism insights into the exotic properties of Na2Co2TeO6, a candidate material of the Kitaev model. Below TN, the zero-field muon-spin relaxation rate becomes almost constant (~0.45 μs−1). We attribute this temperature-independent relaxation rate to the strong quantum fluctuations, as well as to the frustrated Kitaev interactions. As the magnetic field increases, neutron scattering data indicate a broader spin-wave excitation at the K-point. Therefore, quantum fluctuations seem not only robust but are even enhanced by the applied magnetic field. Our findings provide valuable hints for understanding the onset of the quantum-spin-liquid state in Kitaev materials.

A thermal–mechanical coupled bond–based peridynamic model for fracture of anisotropic materials

Thermal fracture analysis of anisotropic materials is a serious challenge in peridynamics (PD). This challenge arises from the need to incorporate anisotropy into both the thermal and mechanical constitutive relationships. While some previous research has explored anisotropy in mechanical field for specific materials, there has been a lack of emphasis on anisotropic materials in the thermal field. This gap creates a significant obstacle for the resolution of thermal fracture, a complex problem involving intricate thermal–mechanical coupling. To address this issue, an anisotropic bond–based PD (BB–PD) heat conduction equation was derived for the thermal analysis of anisotropic materials. In addition, a thermal–mechanical coupled BB–PD model with complete thermal–mechanical anisotropy was developed by integrating an anisotropic mechanical BB–PD model. Governed by a staggered coupling strategy, the pivotal parameters in the two physical fields can be timely updated, facilitating precise computation of the mechanical behavior in the thermal and mechanical fields. To validate the efficacy of the presented model, several numerical examples were performed, including the analysis of anisotropic heat conduction, the deformation analysis under purely mechanical loading, the deformation analysis in specific temperature fields, and two simulations of thermal fracture. The results show that the proposed model can accurately capture the temperature variation and deformation of anisotropic materials. Moreover, the proposed model demonstrates an excellent ability to predict thermal fracture.

Dimensional reduction of Kitaev spin liquid at quantum criticality

We investigate the fate of the Kitaev spin liquid (KSL) under the influence of an external magnetic field h in the [001] direction and upon tuning bond anisotropy of the Kitaev coupling Kz keeping Kx=Ky=K. Guided by density matrix renormalization group, exact diagonalization, and with insights from parton mean-field theory, we uncover a field-induced gapless-to-gapless Lifshitz transition from the nodal KSL to an intermediate gapless phase. The intermediate phase sandwiched between hc1 and hc2, which persists for a wide range of anisotropy Kz/K>0, is composed of weakly coupled one-dimensional quantum critical chains. This intermediate phase is a dimensional crossover, which asymptotically leads to the one-dimensional quantum Ising criticality characterized by the (1+1)D conformal field theory as the field reaches the phase transition at hc2. Beyond hc2 the system enters a partially polarized phase describable as effectively decoupled bosonic chains in which spin waves propagate along the one-dimensional zigzag direction. Our findings provide a comprehensive phase diagram and offer insights into the unusual physics of dimensional reduction generated by a uniform magnetic field in an otherwise two-dimensional quantum spin liquid. Published by the American Physical Society 2024

ScB2C2: The first high damage tolerant ultra-high temperature ceramic with hydrolysis resistance

The first high damage resistant ultra-high temperature ceramic ScB2C2 with excellent hydrolysis resistance was successfully prepared through boron carbon thermal reduction and hot pressing sintering using Sc2O3, B4C and graphite as raw materials. The Vickers hardness, flexural strength, fracture toughness, Young’s modulus, shear modulus, bulk modulus and Poisson’s ratio of as-prepared ScB2C2 ceramic are 6.0 ± 0.2 GPa, 246 ± 17 MPa, 4.11 ± 0.18 MPa·m1/2, 240 GPa, 101 GPa, 127 GPa and 0.18, respectively. In the a-b plane (B2C2 sheet) and along the c-axis direction, ScB2C2 exhibits extremely strong anisotropy of chemical bonds and elastic properties. The high residual strength, damage tolerance factor Dt and fracture toughness to strength ratio KIc/σf indicates that ScB2C2 has excellent damage tolerance. In addition, ScB2C2 displayed remarkable thermal shock resistance and hydrolysis resistance. These unique features endow ScB2C2 with significant potential for future ultra-high temperature applications.

Dilute suspensions of Janus rods: the role of bond and shape anisotropy.

Nanometer-sized clusters are often targeted due to their potential applications as nanoreactors or storage/delivery devices. One route to assemble and stabilize finite structures consists of imparting directional bonding patterns between the nanoparticles. When only a portion of the particle surface is able to form an inter-particle bond, finite-size aggregates such as micelles and vesicles may form. Building on this approach, we combine particle shape anisotropy with the directionality of the bonding patterns and investigate the combined effect of particle elongation and surface patchiness on the low density assembly scenario. To this aim, we study the assembly of tip-functionalised Janus hard spherocylinders by means of Monte Carlo simulations. By exploring the effects of changing the interaction strength and range at different packing fractions, we highlight the role played by shape and bond anisotropy on the emerging aggregates (micelles, vesicles, elongated micelles, and lamellae). We observe that shape anisotropy plays a crucial role in suppressing phases that are typical to spherical Janus nanoparticles and that a careful tuning of the interaction parameters allows promoting the formation of spherical micelles. These finite-size spherical clusters composed of elongated particles might offer more interstitials and larger surface areas than those offered by micelles of spherical or almost-spherical units, thus enhancing their storage and catalytic properties.

Assessing structure of Mg3Bi2−xSbx (0 ≤ x ≤ 2) at pressures below 40 GPa

Mg3Bi2–xSbx (0 ≤ x ≤ 2) have gained significant attention due to their potential in thermoelectric (TE) applications. However, there has been much debating regarding their structural properties and phase diagram as a function of pressure, which is crucial for understanding of their TE properties. Here, we investigate a unified phase diagram of Mg3(Bi,Sb)2 materials up to 40 GPa at room temperature using high-pressure X-ray diffraction. Two high-pressure phases with the structural transition succession of P3¯m1 → C2/m → P21/n are observed, which is valid for all Mg3Bi2–xSbx (0 ≤ x ≤ 2) compounds. We further explore the low-pressure phase P3¯m1 and report that alloying does not change the quasi-isotropic compression of the unit lattice parameters nor has effect on the anisotropic bond compressibility, as recently reported for the end-members. Our study presents a comprehensive picture of Mg3Bi2–xSbx as a function of pressure and chemical composition providing a solid foundation for the future experimental and theoretical studies searching for the most efficient TE compound in Mg3(Bi,Sb)2.

Oxide semiconductors for thermoelectric: The challenges and future

AbstractThe oxide thermoelectric (TE) ceramics have continued to attract the focus due to their high stability and low cost. However, their TE performance falls obviously backward with the typical TE compounds such as PbTe. Here, we revealed the reason for the difficulty of optimized performance is the large electronegativity difference between the metal and O ions in oxide, which results in strong electron localization and bond energy. The former leads to unfavorable electrical properties and the latter results in a high lattice thermal conductivity. Focusing on these issues, we briefly reviewed the strategies for electrical optimization including carrier concentration optimization, band gap tuning, and the density of state resonance, as well as the lattice thermal suppression strategies involving weakened bond, high entropy, grain size engineering, hierarchical architecture, textured polycrystal, and composite strategies for oxide TE ceramics. Finally, we proposed several possible perspectives for n‐type oxide TE ceramics, strategies for bond anisotropy, and predicted new TE oxides for further breakthroughs in oxide TE properties.

Bond properties and anisotropy performance of 3D-printed construction and demolition waste-based geopolymers: Effect of operational- and material-oriented parameters

This study investigated the bond performance of printed construction and demolition wastes (CDW)-based geopolymer, providing utilization of CDW in green materials through high tech-manufacturing methodologies. Considering the requirements of three-dimensional-additive manufacturing (3D-AM), the effects of operational and material-oriented parameters on mechanical performance were examined by conducting compressive strength, triplet shear, direct tensile, and diagonal tension tests. Anisotropy performance (perpendicular, lateral, and parallel), printing time intervals (0, 15, 30, 60-min.) between two consecutive layers, material aging time (0, 30, 60, 120-min.) of CDW-based materials, manufacturing methodologies (cast, horizontal- and vertical-printed) were evaluated on the printed specimen. The study showed that the tests conducted on masonry structures could be used on the 3D-printed structure as well. An anisotropic behavior in the compressive strength results was obtained by observing a decreasing trend in results from perpendicular to parallel and, parallel to lateral loading directions. Increasing printing time interval had a negative influence on the compressive strength of CDW-based geopolymers and anisotropy performance was mitigated in a 60-min printing time interval. Although bond strength decreased with raised printing time intervals, similar bond strength values were recorded for 30-min and 60-min printing time intervals. Similarly, increased material aging time caused decrement in bond strength, yet comparable bond strength was obtained for specimen printed with 60-min and 120-min aged mixture. According to the results, printing time intervals showed more prominent effects on bond strength compared to the material aging time. The test results indicate that vertical-printed specimens demonstrated similar mechanical performance with cast specimens, whereas horizontal-printed specimens exhibited slightly weaker performance compared to those.

Bending behavior of the cysteinyl bolaamphiphile nanobelt assembly induced by the anisotropic disulfide bond formation

In this study, we explore how the covalent bonds linking individual building block molecules can substantially alter the suprastructure of molecular self-assembly. We focus on the role of covalent bonding within the self-assembled structures of a cysteinyl bolaamphiphile which consists of two cysteinyl motifs connected by a central hydrophobic heptyl chain spacer. The cysteinyl bolaamphiphile molecules self-assemble into a nanobelt structure with the creation of disulfide bonds during assembly, which prompts anisotropic molecular ordering and subsequent bending of the nanobelts. A series of control experiments revealed the twofold contribution of disulfide bonds: they facilitate the bending of the nanobelt assembly and the formation of longer assembled structures. Molecular dynamics simulation study confirms that the anisotropic distribution of disulfide bonds causes the bending of a nanobelt assembly. Moreover, the simulation and experimental studies demonstrated an increase in the nanobelt curvature as more disulfide bonds are generated. These results highlight the previously unappreciated influence of covalent bonds in causing macroscopic deformation of self-assembled structures.

Anisotropic Non-oriented Bond Percolation in High Dimensions

We consider inhomogeneous non-oriented Bernoulli bond percolation on $$\mathbb {Z}^d$$ , where each edge has a parameter depending on its direction. We prove that, under certain conditions, if the sum of the parameters is strictly greater than 1/2, we have percolation in sufficiently high dimensions. The main tool is a dynamical coupling between models for different dimensions with different sets of parameters.

Orthorhombic Na2/3Cu0.1Mn0.9O2 cathode: Enhanced Na storage performances with the suppressed Mn–O bond anisotropy

Orthorhombic Mn-based layered oxides (P′2) have drawn much attention as promising cathode materials for sodium ion batteries (SIBs) with high specific capacities and rich resources. However, P′2-NaxMnO2 cathode suffers from inferior rate capability and poor cycling performances, owing to the complicated phase transition during de-/sodiation process. Herein, Cu-substitution P′2-Na2/3Cu0.1Mn0.9O2 is successfully prepared to mitigate the multiple phase transition by reducing the distortion of the MnO6 octahedron. Mn–O bond anisotropy is decreased and the spacing distance of Na layer is constricted by the modulation of the CuO6 octahedron, giving enhanced electronic conduction, validated by theoretical calculations and X-ray diffraction refinement. P′2-OP4 phase transition is replaced by reversible P′2-Z phase transition with slighter volume change, leading to the lower lattice strain in both c-axis direction and a-b plane, as probed by Operando/ex situ X-ray diffraction and synchrotron X-ray absorption spectra, which strengthens the stability of lattice skeleton. Consequently, P′2-Na2/3Cu0.1Mn0.9O2 exhibits outstanding rate capabilities (102 mAh/g at 750 mA g−1) and enhanced cycle performances (91 % at 150 mA g−1 after 100 cycles). This Cu-substitution strategy provides an effective method to design the structure-stable layered oxide cathodes for SIBs.

A promising thermoelectrics In4SnSe4 with a wide bandgap and cubic structure composited by layered SnSe and In4Se3

The wide-bandgap cubic-structure semiconductor In4SnSe4 can be regarded as a product of compositing two typical layered thermoelectric materials SnSe and In4Se3. Remarkably, In4SnSe4 inherited low thermal conductivity from its parent materials. To advance the potential thermoelectric property of In4SnSe4, we systematically investigated its crystal structure and the origin of the intrinsic low thermal conductivity. In4SnSe4 crystallized in a cubic phase (space group Pa3¯), with the lattice parameters of a = b = c = 12.66 Å. The anisotropy of InSe bonds in the lattice determined the complex structure of In4SnSe4 with 72 atoms in the primitive cell. More importantly, sound velocity and elastic properties unclosed the strong anharmonicity in In4SnSe4, which contributed greatly to the low thermal conductivity. With first-principles calculations, it was found that the lone-pair electrons from In+ mainly caused the anharmonicity in the lattice. Additionally, Br was proved to be an effective dopant for In4SnSe4 to improve the electrical transport properties. This work indicated that the complex wide-bandgap semiconductor In4SnSe4 with cubic phase and intrinsic low thermal conductivity was a new promising thermoelectric material with appropriate doping.

Active T1 transitions in cellular networks

In amorphous solids as in tissues, neighbor exchanges can relax local stresses and allow the material to flow. In this paper, we use an anisotropic vertex model to study T1 rearrangements in polygonal cellular networks. We consider two different physical realizations of the active anisotropic stresses: (i) anisotropic bond tension and (ii) anisotropic cell stress. Interestingly, the two types of active stress lead to patterns of relative orientation of T1 transitions and cell elongation that are different. Our work suggests that these two realizations of anisotropic active stresses can be observed in vivo. We describe and explain these results through the lens of a continuum description of the tissue as an anisotropic active material. We furthermore discuss the energetics of the dynamic tissue and express the energy balance in terms of internal elastic energy, mechanical work, chemical work and heat. This allows us to define active T1 transitions that can perform mechanical work while consuming chemical energy.Graphic abstract

Topological thermal Hall effect and magnonic edge states in kagome ferromagnets with bond anisotropy

The magnon band topology due to the Dzyaloshinskii–Moriya interaction (DMI) and its relevant topological thermal Hall effect has been extensively studied in kagome lattice magnets. In this theoretical investigation, we report a new mechanism for phase transitions between topological phases of magnons in kagome ferromagnets by tuning the anisotropic nearest-neighbor ferromagnetic interaction and DMI. Using the linear spin-wave theory, we calculate the Chern number and thermal Hall conductivity of magnons in low temperature regime. We show the magnon band structures and magnonic edge states in each topological phase. From the topological phase diagram, we find a sign reversal of the thermal Hall conductivity upon tuning the modulation factors. We explicitly demonstrate the correspondence of thermal Hall conductivity with the propagation direction of the magnonic edge states. Finally, we discuss candidate materials as experimental realizations of our theoretical model.

Examining the effect of flake orientation on the hydrophilicity of MoS2 by molecular simulations

The interaction of molybdenum disulfide (MoS2) with water and consequently the surface wettability are important to understand to efficiently use this layered material in semiconductor and lubricant applications. Using molecular dynamics simulations, we investigate the coupled effect of flake orientation (0° and 90°) and temperatures (300 K and 400 K) on the wettability of MoS2. Our results reveal that the horizontally oriented MoS2 is relatively less hydrophilic than the vertically oriented MoS2 due to the difference in the flake-orientation driven atomic interactions. An increase in temperature (≥400 K) transforms the horizontally oriented MoS2 to a relatively hydrophobic surface, while retaining the hydrophilicity in the vertically oriented flakes due to the bond anisotropy in the material, which impacts the surface free energy.

Polarization-dependent magnetic properties of periodically driven α−RuCl3

We study magnetic properties of a periodically driven Mott insulator with strong spin-orbit coupling and show some properties characteristic of linearly polarized light. We consider a $t_{2g}$-orbital Hubbard model driven by circularly or linearly polarized light with strong spin-orbit coupling and derive its effective Hamiltonian in the strong-interaction limit for a high-frequency case. We show that linearly polarized light can change not only the magnitudes and signs of the exchange interactions, but also their bond anisotropy even without the bond-anisotropic hopping integrals. Because of this property, the honeycomb-network spin system could be transformed into weakly coupled zigzag or step spin chains for the light field polarized along the $b$- or $a$-axis, respectively. Then, analyzing how the light fields affect several magnetic states in a mean-field approximation, we show that linearly polarized light can change the relative stability of the competing magnetic states, whereas such a change is absent for circularly polarized light. We also analyze the effects of both the bond anisotropy of nearest-neighbor hopping integrals and a third-neighbor hopping integral on the magnetic states and show that the results obtained in a simple model, in which the bond-averaged nearest-neighbor hopping integrals are considered, remain qualitatively unchanged except for the stability of zigzag states in the non-driven case and the degeneracy lifting of the zigzag or stripy states.

Colossal 3D Electrical Anisotropy of MoAlB Single Crystal.

3D anisotropic functional properties (such as magnetic, electrical, thermal, and optical properties, etc.) in a single material are not only beneficial to the multipurpose of a material, but also helpful to enrich the regulatory dimensionality of functional materials. Herein, a colossal 3D electrical anisotropy of layered MAB-phase MoAlB single crystal is introduced and dissected. Using high-temperature metal-solution method, high-quality MoAlB single crystals are obtained and a surprisingly strong out-of-plane (σa /σb = 1.43 × 105 , at 2 K) and in-plane (σa /σc = 12.12, at 2 K) electrical anisotropies are first observed. After a series of experimental and theoretical investigations, it is demonstrated that the 3D anisotropic crystal structure and chemical bond of MoAlB result in its 3D anisotropic phonon vibration and electronic structure, influence the corresponding electron-electron as well as electron-phonon interactions, and finally give rise to its colossal 3D anisotropy of electrical conductivity. This work experimentally and theoretically proves MoAlB single crystal possessing the 3D anisotropies of crystal structure, chemical bond, phonon vibration, electronic structure, and electrical transport, but also provides a promising platform for the future design of functionalized electronic devices as well as synthesis of new and large-sized in-plane anisotropic 2D material (MoBene).

Extent of frustration in the classical Kitaev- Γ model via bond anisotropy

In the pseudospin-$\frac{1}{2}$ honeycomb Mott insulators with strong spin-orbit coupling, there are two types of bond-dependent exchange interactions, named Kitaev ($K$) and $\Gamma$, leading to strong frustration. While the ground state of the Kitaev model is a quantum spin liquid with fractionalized excitations, the ground state of the $\Gamma$ model remains controversial. In particular, the phase diagram of the $K\Gamma$ model with ferromagnetic $K$ and antiferromagnetic $\Gamma$ interactions has been intensively studied because of its relevance to candidate materials such as $\alpha$-RuCl$_{3}$. Numerical studies also included the effects of tuning the bond strengths, i.e., $z$-bond strength different from the other bonds. However, no clear consensus on the overall phase diagram has been reached yet. Here we study the classical $K\Gamma$ model with the anisotropic bond strength using Monte Carlo simulations to understand the emerging phases out of competition between the two frustrated limits. We also address how the anisotropic bond strength affects the phase diagram and strength of quantum fluctuations. We found various large unit cell phases due to the competing frustrations, and analyzed their intrinsic degeneracy based on the symmetry of the Hamiltonian. Using the linear spin wave theory we showed that the anisotropic bond strength enhances quantum fluctuations in the $\Gamma$-dominant regime where a small reduced moment is observed. The implications of our findings in relation to the quantum model are also discussed.

Imaging Nanometer Phonon Softening at Crystal Surface Steps with 4D Ultrafast Electron Microscopy.

Step edges are an important and prevalent topological feature that influence catalytic, electronic, vibrational, and structural properties arising from modulation of atomic-scale force fields due to edge-atom relaxation. Direct probing of ultrafast atomic-to-nanoscale lattice dynamics at individual steps poses a particularly significant challenge owing to demanding spatiotemporal resolution requirements. Here, we achieve such resolutions with femtosecond 4D ultrafast electron microscopy and directly image nanometer-variant softening of photoexcited phonons at individual surface steps. We find large degrees of softening precisely at the step position, with a thickness-dependent, strain-induced frequency modulation extending tens of nanometers laterally from the atomic-scale discontinuity. The effect originates from anisotropic bond dilation and photoinduced incoherent atomic displacements delineated by abrupt molecular-layer cessation. The magnitude and spatiotemporal extent of softening is quantitatively described with a finite-element transient-deformation model. The high spatiotemporal resolutions demonstrated here enable uncovering of new insights into atomic-scale structure-function relationships of highly defect-sensitive, functional materials.