Extreme Anisotropy Research Articles

Micromechanics of kink-band formation in bimetallic layered composites

Bimetallic layered composites with ultrathin layers possess high strength and hardness, and excellent shock resistance and radiation damage tolerance. However, they are prone to deformation localization and readily form kink-bands when subjected to layer parallel compression. Following this, we carried out extensive plane strain finite element finite deformation analyses to understand and rationalize the experimentally observed kink-band formation in these composites. In the calculations, both constituent materials were assumed to follow a rate-independent isotropic elastic-plastic constitutive relation with an ad-hoc thickness dependent yield strength. Our results demonstrate that in these composites, layer refinement, together with the strength differential between the layers of the constituent materials, is sufficient to trigger kink-banding. Importantly, this phenomenon occurs even in the absence of elastically stiff layers, geometrical imperfections (such as waviness of the layers), and extreme plastic anisotropy within the layers. Additionally, we performed parametric studies to investigate the individual effects of layer thickness, strength differential between the two constituent materials, and strain-hardenability of the materials on kink-band formation. The outcomes of our parametric studies reveal that the strain-hardenability of the constituent materials stabilizes the formation of kink-bands. Furthermore, it leads to a transition from the abrupt formation of through-width kink-bands to the onset and propagation of stable inclined wedge-shaped kink-bands, similar to the experimental observations.

Cracking and associated volumetric expansion of NMC811 secondary particles

Secondary particles comprising a large number of nickel-rich single crystal primary particles are extensively used as storage particles in cathodes of lithium-ion batteries. It is well-established that crack formation in secondary particles is an important degradation mode that contributes to decline in battery performance. Recent X-ray tomographic observations suggest that, at very low C-rates, concentration gradients of lithium within an NMC811 secondary particle are negligible yet cracking still occurs. Additionally, during delithiation the primary particles shrink yet a volumetric expansion of the secondary particle occurs. These observations are explained by a numerical model of distributed cracking due to the extreme anisotropy of lithiation strain of primary particles. The incompatible deformation from grain to grain induces large self-stresses even in the absence of spatial gradients in the lithium concentration. The stress state is sufficient to drive a dynamic catastrophic fracture event, and the associated kinetic energy acquired by the primary particles moves them apart (akin to an explosive event) with the carbon and binder domain surrounding each secondary particle restricting the outward motion of the primary particles. It is predicted that a volume expansion of the secondary particles on the order of 20 % accompanies cracking, in agreement with recently reported observations.

Experimental Observation of Highly Anisotropic Elastic Properties of Two-Dimensional Black Arsenic.

Anisotropic two-dimensional layered materials with low-symmetry lattices have attracted increasing attention due to their unique orientation-dependent mechanical properties. Black arsenic (b-As), with the puckered structure, exhibits extreme in-plane anisotropy in optical, electrical, and thermal properties. However, experimental research on mechanical properties of b-As is very rare, although theoretical calculations predicted the exotic elastic properties of b-As, such as the anisotropic Young's modulus and negative Poisson's ratio. Herein, experimental observations on highly anisotropic elastic properties of b-As were demonstrated using our developed in situ tensile straining setup based on the effective microelectromechanical system. The cyclic and repeatable load-displacement curves proved that Young's modulus along the zigzag direction was ∼1.6 times greater than that along the armchair direction, while the anisotropic ratio of ultimate strain reached ∼2.5, attributed to the hinge structure in the armchair direction. This study could provide significant insights into the design of novel anisotropic materials and explore their potential applications in nanomechanics and nanodevices.

Extreme Polarization Anisotropy in Resonant Third-Harmonic Generation from Aligned Carbon Nanotube Films.

Carbon nanotubes (CNTs) possess extremely anisotropic electronic, thermal, and optical properties owing to their 1D character. While their linear optical properties have been extensively studied, nonlinear optical processes, such as harmonic generation for frequency conversion, remain largely unexplored in CNTs, particularly in macroscopic CNT assemblies. In this work, macroscopic films of aligned and type-separated (semiconducting and metallic) CNTs are synthesized and polarization-dependent third-harmonic generation (THG) from the films with fundamental wavelengths ranging from 1.5 to 2.5µm is studied. Both films exhibited strongly wavelength-dependent, intense THG signals, enhanced through exciton resonances, and third-order nonlinear optical susceptibilities of 2.50×10-19 m2 V-2 (semiconducting CNTs) and 1.23×10-19 m2 V-2 (metallic CNTs), respectively are found, for 1.8µm excitation. Further, through systematic polarization-dependent THG measurements, the values of all elements of the susceptibility tensor are determined, verifying the macroscopically 1D nature of the films. Finally, polarized THG imaging is performed to demonstrate the nonlinear anisotropy in the large-size CNT film with good alignment. These findings promise applications of aligned CNT films in mid-infrared frequency conversion, nonlinear optical switching, polarized pulsed lasers, polarized long-wave detection, and high-performance anisotropic nonlinear photonic devices.

Strong Coupling and Extreme Anisotropy in Infrared Polaritonic Media.

Strong Coupling and Extreme Anisotropy in Infrared Polaritonic Media.

Elucidating the extreme anisotropy in the J-integral value of commercially pure titanium

Extensive multivariant twinning and higher GND-density in TD-RD than RD-ND orientation cause extreme J-integral anisotropy in CP-Ti. Furthermore, c-axis nearly parallel to crack propagation exhibits dominating fluted voids by extensive prismatic slip, while c-axis perpendicular to crack propagation shows flat and fluted voids by dominating multivariant twinning and slip in determining CP-Ti fracture toughness.

Full mode-converting transmission between longitudinal and bending waves in plates and beams

Two major challenges are often encountered in wave manipulation involving coupled bending and longitudinal modes. First, the coupling phenomena are generally governed by cubic equations, which are critically complicated for theoretical exploration. Second, there is a significant impedance mismatch between these two modes, which hinders the transmodal energy transferring. In this work, we successfully overcome these challenges, and propose an interference theory that can realize a nearly full mode-converting transmission between bending and longitudinal modes. Specifically, we find an alternative way in which the coupling phenomena can be described using only a portion of the wave characteristics, and solution of the cubic equation is no more required. Besides, we consider locally resonant metamaterials that exhibit extreme anisotropy in effective mass density. The proposed theory consists of four conditions, namely, the interference condition, impedance condition, polarization condition, and supplementary condition. Requirements on the effective material properties are derived from these conditions, and locally resonant metamaterials are designed accordingly. The theory, applicable not only for thin plates, but also for Euler-Bernoulli beams, offers a new way to realize free manipulation of wave energy in elastic systems.

Mechanical properties of re-entrant chiral anisotropic honeycomb

In this paper, we proposed a honeycomb structure with enhanced anisotropy in the orthogonal direction with a re-entrant chiral hybrid deformation mechanism. The analytical formulas for the equivalent Poisson's ratio and normalized Young's modulus were derived based on the theory of the Timoshenko beam under small static deformations and verified by experiments and finite element simulations. Parametric analysis demonstrated the extreme anisotropy of this structure in the orthogonal direction, achieving a Poisson's ratio −10 or lower in one direction, while remaining nearly zero in the other direction. This property allowed the structure to be designed into a tubular stent that remained stable along its axial length when subjected to a compressive load in the radial direction. However, the stent underwent significant contraction in the radial direction when a compressive load was applied along the axial direction. This stent is expected to have biomedical applications to solve problems associated with cardiovascular obstruction, esophageal obstruction, and airway stenosis.

Bulk ReSe2: Record High Refractive Index and Biaxially Anisotropic Material for All-Dielectric Nanophotonics

We show that bulk rhenium diselenide, ReSe2, is characterized by a record high value of the refractive index exceeding 5 in the near-infrared frequency range. We use back focal plane reflection spectroscopy to extract the components of the ReSe2 permittivity tensor and reveal its extreme biaxial anisotropy. We also demonstrate the good agreement between the experimental data and the predictions of the density functional theory. The combination of the large refractive index and giant optical anisotropy makes ReSe2 a promising material for all-dielectric nanophotonics in the near-infrared frequency range.

Observation of directional leaky polaritons at anisotropic crystal interfaces

Extreme anisotropy in some polaritonic materials enables light propagation with a hyperbolic dispersion, leading to enhanced light-matter interactions and directional transport. However, these features are typically associated with large momenta that make them sensitive to loss and poorly accessible from far-field, being bound to the material interface or volume-confined in thin films. Here, we demonstrate a new form of directional polaritons, leaky in nature and featuring lenticular dispersion contours that are neither elliptical nor hyperbolic. We show that these interface modes are strongly hybridized with propagating bulk states, sustaining directional, long-range, sub-diffractive propagation at the interface. We observe these features using polariton spectroscopy, far-field probing and near-field imaging, revealing their peculiar dispersion, and – despite their leaky nature – long modal lifetime. Our leaky polaritons (LPs) nontrivially merge sub-diffractive polaritonics with diffractive photonics onto a unified platform, unveiling opportunities that stem from the interplay of extreme anisotropic responses and radiation leakage.

Absence of Ginzburg-Landau mechanism for vestigial order in the normal phase above a two-component superconductor

A two-component superconductor may hypothetically support a vestigial order phase above its superconducting transition temperature, with rotational or time-reversal symmetry spontaneously broken while remaining nonsuperconducting. This has been suggested as an explanation for the observed normal state nematicity of the nematic superconductor ${M}_{x}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$. We examine the condition for this vestigial order to occur within Ginzburg-Landau theory with order parameter fluctuations, on both the nematic and chiral sides of the theory. Contrary to prior theoretical results, we rule out a large portion of parameter space for possible vestigial order. We argue that very extreme anisotropy is one prerequisite for the formation of a stable vestigial phase via this mechanism, which is likely not met in real materials.

The Bulk van der Waals Layered Magnet CrSBr is a Quasi-1D Material.

Correlated quantum phenomena in one-dimensional (1D) systems that exhibit competing electronic and magnetic order are of strong interest for the study of fundamental interactions and excitations, such as Tomonaga-Luttinger liquids and topological orders and defects with properties completely different from the quasiparticles expected in their higher-dimensional counterparts. However, clean 1D electronic systems are difficult to realize experimentally, particularly for magnetically ordered systems. Here, we show that the van der Waals layered magnetic semiconductor CrSBr behaves like a quasi-1D material embedded in a magnetically ordered environment. The strong 1D electronic character originates from the Cr-S chains and the combination of weak interlayer hybridization and anisotropy in effective mass and dielectric screening, with an effective electron mass ratio of mXe/mYe ∼ 50. This extreme anisotropy experimentally manifests in strong electron-phonon and exciton-phonon interactions, a Peierls-like structural instability, and a Fano resonance from a van Hove singularity of similar strength to that of metallic carbon nanotubes. Moreover, because of the reduced dimensionality and interlayer coupling, CrSBr hosts spectrally narrow (1 meV) excitons of high binding energy and oscillator strength that inherit the 1D character. Overall, CrSBr is best understood as a stack of weakly hybridized monolayers and appears to be an experimentally attractive candidate for the study of exotic exciton and 1D-correlated many-body physics in the presence of magnetic order.

Field direction dependent quantum-limit magnetoresistance of correlated Dirac electrons in perovskite CaIrO3

Correlated-electron perovskite ${\mathrm{CaIrO}}_{3}$ shows the topologically protected line node near the Fermi level, wherein the high-mobility Dirac electrons reach the quantum limit (QL) with the one-dimensionally (1D) dispersive $n=0$ Landau level (LL) state at a moderate magnetic field. In the QL, the longitudinal magnetoresistance (MR) shows an extreme anisotropy against the field $(B)$ direction in spite of nearly isotropic Dirac band dispersions. The resistivity for $B\ensuremath{\parallel}a$ shows an insulating behavior with the MR ratio exceeding $2000%$ around 18 T, whereas the $B\ensuremath{\parallel}c$ MR always remains metallic. This is accounted for in terms of the large difference of Fermi velocity of the $n=0$ LL between $B\ensuremath{\parallel}a$ and $B\ensuremath{\parallel}c$ due to the $B$ direction dependent $5d$ orbital electron hopping, which triggers the different instability toward the charge density wave formation for the 1D $(\ensuremath{\parallel}B)$ dispersive $n=0$ LL.

Afterglow Polarization from Off-axis Gamma-Ray Burst Jets

As we further our studies on gamma-ray bursts (GRBs), via both theoretical models and observational tools, more and more options begin to open for exploration of its physical properties. As GRBs are transient events primarily dominated by synchrotron radiation, it is expected that the synchrotron photons emitted by GRBs should present some degree of polarization throughout the evolution of the burst. Whereas observing this polarization can still be challenging due to the constraints on observational tools, especially for short GRBs, it is paramount that the groundwork is laid for the day we have abundant data. In this work, we present a polarization model linked with an off-axis spreading top-hat jet synchrotron scenario in a stratified environment with a density profile n(r) ∝ r −k . We present this model's expected temporal polarization evolution for a realistic set of afterglow parameters constrained within the values observed in the GRB literature for four degrees of stratification k = 0, 1, 1.5, and 2 and two magnetic field configurations with high extreme anisotropy. We apply this model and predict polarization from a set of GRBs exhibiting off-axis afterglow emission. In particular, for GRB 170817A, we use the available polarimetric upper limits to rule out the possibility of an extremely anisotropic configuration for the magnetic field.

Extreme Optical Anisotropy in the Type-II Dirac Semimetal NiTe2 for Applications to Nanophotonics

Compared to artificial metamaterials, where nano-fabrication complexities and finite-size inclusions can hamper the desired electromagnetic response, several natural materials like van der Waals crystals hold great promise for designing efficient nanophotonic devices in the optical range. Here, we investigate the unusual optical response of NiTe$_2$, a van der Waals crystal and a type-II Dirac semimetal hosting Lorentz-violating Dirac fermions. By {\it ab~initio~} density functional theory modeling, we show that NiTe$_2$ harbors multiple topological photonic regimes for evanescent waves (such as surface plasmons) across the near-infrared and optical range. By electron energy-loss experiments, we identify surface plasmon resonances near the photonic topological transition points at the epsilon-near-zero (ENZ) frequencies $\approx 0.79$, $1.64$, and $2.22$ eV. Driven by the extreme crystal anisotropy and the presence of Lorentz-violating Dirac fermions, the experimental evidence of ENZ surface plasmon resonances confirm the non-trivial photonic and electronic topology of NiTe$_2$. Our study paves the way for realizing devices for light manipulation at the deep-subwavelength scales based on electronic and photonic topological physics for nanophotonics, optoelectronics, imaging, and biosensing applications.

Interplay of turbulence and proton-microinstability growth in space plasmas

Numerous prior studies have shown that as proton beta increases, a narrower range of proton temperature anisotropy values is observed. This effect has often been ascribed to the actions of kinetic microinstabilities because the distribution of observational data aligns with contours of constant instability growth rates in the beta-anisotropy plane. However, the linear Vlasov theory of instabilities assumes a uniform background in which perturbations grow. The established success of linear-microinstability theories suggests that the conditions in regions of extreme temperature anisotropy may remain uniform for a long enough time so that the instabilities have the chance to grow to sufficient amplitude. Turbulence, on the other hand, is intrinsically nonuniform and nonlinear. Thin current sheets and other coherent structures generated in a turbulent plasma may quickly destroy the uniformity. It is, therefore, not a-priori obvious whether the presence of intermittency and coherent structures favors or disfavors instabilities. To address this question, we examined the statistical distribution of growth rates associated with proton temperature-anisotropy driven microinstabilities and local nonlinear time scales in turbulent plasmas. Linear growth rates are, on average, substantially less than the local nonlinear rates. However, at the regions of extreme values of temperature anisotropy, near the “edges” of the populated part of the proton temperature anisotropy-parallel beta plane, the instability growth rates are comparable or faster than the turbulence time scales. These results provide a possible answer to the question as to why the linear theory appears to work in limiting plasma excursions in anisotropy and plasma beta.

Topological Lifshitz transition in twisted hyperbolic acoustic metasurfaces

Acoustic metamaterials and metasurfaces have been explored in the past few years to realize a wide range of extreme responses for sound waves. As one remarkable phenomenon, extreme anisotropy and hyperbolic sound propagation are particularly challenging to realize compared to electromagnetic waves because of the scalar nature of airborne acoustics. In parallel, moiré superlattices and the rapidly expanding domain of twistronics have shown that large anisotropy combined with tailored geometrical rotations can enable tantalizing emerging phenomena, such as tailored phase transitions in metamaterials. Connecting these areas of research, here, we explore the realization of acoustic hyperbolic metasurfaces and their combination to drive topological phase transitions from hyperbolic to elliptic sound propagation. The transition point occurring at a specific rotation angle between two acoustic metasurfaces supports highly directional canalization of sound, opening exciting opportunities for twisted acoustics metasurfaces for robust surface wave guiding and steering.

Phonon anharmonicity in exfoliated black arsenic flakes

Black arsenic (b-As) with extreme in-plane anisotropy, incredible ambient stability, and excellent transport performance has unlimited potential for device applications. Uncovering phonon–phonon (ph–ph) interactions is a critical step in understanding its transport, especially thermal transport properties. Here, we report on the phonon anharmonicity in an exfoliated b-As flake using temperature-dependent polarized Raman spectroscopy from 80 to 300 K. Polarization of Ag2 mode helps identifying the crystalline orientation of the flake. A nonlinear redshift of peak frequencies of Ag1, B2g, and Ag2 modes is observed along the given crystalline orientation. Contributions from intrinsic ph–ph interactions are extracted: cubic-phonon decay dominates the ph–ph scattering processes at low temperature, while cubic- and quartic-phonon decays together do at high temperature. It is further confirmed by the nonlinear full-width-at-half-maxima broadening. Our findings disclose the phonon anharmonicity in b-As and give insight for better understanding and utilizing its thermodynamic properties.

Quantum oscillations of the j=3/2 Fermi surface in the topological semimetal YPtBi

The bismuth-based half-Heusler materials host a nontrivial topological band structure, unconventional superconductivity, and large spin-orbit coupling in a system with very low electron density. In particular, the inversion of $p$-orbital-derived bands with an effective angular momentum $j$ of up to 3/2 is thought to play a central role in anomalous Cooper pairing in the cubic half-Heusler semimetal YPtBi, which is thought to be the first ``high-spin'' superconductor. Here, we report an extensive study of the angular dependence of quantum oscillations (QOs) in the electrical conductivity of YPtBi, revealing an anomalous Shubnikov--de Haas effect consistent with the presence of a coherent $j=3/2$ Fermi surface. The QO signal in YPtBi manifests an extreme anisotropy upon rotation of the magnetic field from the [100] to [110] crystallographic direction, where the QO amplitude vanishes. This radical anisotropy for such a highly isotropic system cannot be explained by trivial scenarios involving changes in effective mass or impurity scattering, but rather is naturally explained by the warping feature of the $j=3/2$ Fermi surface of YPtBi, providing direct proof of active high angular momentum quasiparticles in the half-Heusler compounds.

Unidirectionally excited phonon polaritons in high-symmetry orthorhombic crystals.

Advanced control over the excitation of ultraconfined polaritons—hybrid light and matter waves—empowers unique opportunities for many nanophotonic functionalities, e.g., on-chip circuits, quantum information processing, and controlling thermal radiation. Recent work has shown that highly asymmetric polaritons are directly governed by asymmetries in crystal structures. Here, we experimentally demonstrate extremely asymmetric and unidirectional phonon polariton (PhP) excitation via directly patterning high-symmetry orthorhombic van der Waals (vdW) crystal α-MoO3. This phenomenon results from symmetry breaking of momentum matching in polaritonic diffraction in vdW materials. We show that the propagation of PhPs can be versatile and robustly tailored via structural engineering, while PhPs in low-symmetry (e.g., monoclinic and triclinic) crystals are largely restricted by their naturally occurring permittivities. Our work synergizes grating diffraction phenomena with the extreme anisotropy of high-symmetry vdW materials, enabling unexpected control of infrared polaritons along different pathways and opening opportunities for applications ranging from on-chip photonics to directional heat dissipation.