Anisotropic Lattice Research Articles

Tuning magnetic and optical properties through strain in epitaxial LaCrO3 thin films

We report on the effect of epitaxial strain on magnetic and optical properties of perovskite LaCrO3 (LCO) single crystal thin films. Epitaxial LCO thin films are grown by pulsed laser deposition on proper choice of substrates to impose different strain states. A combined experimental and theoretical approach is used to demonstrate the direct correlation between lattice-strain and functional properties. The magnetization results show that the lattice anisotropy plays a critical role in controlling the magnetic behavior of LCO films. The strain induced tetragonality in the film lattice strongly affects the optical transitions and charge transfer gap in LCO. This study opens possibilities to tailor the functional properties of LCO and related materials by strain engineering in epitaxial growth.

Amplification and modulation effect of elliptical surface polaritons on a thermal diode

With the development of surface polaritons of two-dimensional metasurfaces with high mode density, considerable attention has been paid to their application in optimising the performance of the thermal diode; however, presently, the main focus is on isotropic metasurfaces. In this study, we investigated the amplification and modulation effect of elliptical surface polaritons on a near-field radiative thermal diode. Introducing black phosphorus (BP) in forward and reverse terminals of the thermal diode (comprising VO2 and Si) demonstrated prominent amplification of rectification performance (of ∼3.59 times) in the modified thermal diode than in the one without BP. Selecting different electron densities of BP and different doping densities of Si revealed the influence of the hybridisation effect of elliptical polaritons on thermal rectification and near-field radiative heat transfer. Finally, based on the in-plane intrinsic anisotropic lattice of BP, the modified thermal diode facilitated highly flexible manipulation of rectification performance through mechanical rotating, which to the best of our knowledge, has never been discussed in the field of near-field radiative diodes. Our findings provides significant insights to intensify asymmetric energy transfer and explore the radiative heat transfer mechanism of a coupled system comprising an anisotropic metasurface and an out-plane anisotropic material.

Casimir forces for the ideal Bose gas in anisotropic optical lattices: the effect of alternating sign upon varying dimensionality

We analyze the thermodynamic Casimir effect occurring in a gas of non-interacting bosons confined by two parallel walls with a strongly anisotropic dispersion inherited from an underlying lattice. In the direction perpendicular to the confining walls, the standard quadratic dispersion is replaced by the term |p| α with α ⩾ 2 treated as a parameter. We derive a closed, analytical expression for the Casimir force depending on the dimensionality d and the exponent α, and analyze it for thermodynamic states in which the Bose–Einstein condensate is present. For α ∈ {4, 6, 8, …} the exponent governing the decay of the Casimir force with increasing distance between the walls becomes modified and the Casimir amplitude Δ α (d) exhibits oscillations of sign as a function of d. Otherwise, we find that Δ α (d) features singularities when viewed as a function of d and α. Recovering the known previous results for the isotropic limit α = 2 turns out to occur via a cancellation of singular terms.

Thermodynamics of lattice vibrations in non-cubic crystals: the zinc structure revisited.

A phenomenological model of anisotropic lattice vibrations is proposed, using a temperature-dependent spectral cutoff and varying Debye temperatures for the vibrational normal components. The internal lattice energy, entropy and Debye-Waller B factors of non-cubic elemental crystals are derived. The formalism developed is non-perturbative, based on temperature-dependent linear dispersion relations for the normal modes. The Debye temperatures of the vibrational normal components differ in anisotropic crystals; their temperature dependence and the varying spectral cutoff can be inferred from the experimental lattice heat capacity and B factors by least-squares regression. The zero-point internal energy of the phonons is related to the low-temperature limits of the mean-squared vibrational amplitudes of the lattice measured by X-ray and γ-ray diffraction. A specific example is discussed, the thermodynamic variables of the hexagonal close-packed zinc structure, including the temperature evolution of the B factors of zinc. In this case, the lattice vibrations are partitioned into axial and basal normal components, which admit largely differing B factors and Debye temperatures. The second-order B factors defining the non-Gaussian contribution to the Debye-Waller damping factors of zinc are obtained as well. Anharmonicity of the oscillator potential and deviations from the uniform phonon frequency distribution of the Debye theory are modeled effectively by the temperature dependence of the spectral cutoff and Debye temperatures.

First-principles study of the lattice vibration, elastic anisotropy and thermodynamical properties of Tantalum Silicide with the different crystal structures

TaSi2 is the ultra-high-temperature ceramic material, the intrinsic brittleness hinders its high temperature applications. We design five possible structures (Cmcm-type, FdddZ-type, I4/mmm-type and P6222-type and P4332-type structures) by means of first-principles calculations. The structural influence on their lattice vibration, mechanical, electronic and thermal properties were investigated. Except for P4332-type structure, the rest of potential structures are structurally stable. Meanwhile, the I4/mmm-type structure presents the largest hardness among the four structures. As for the anisotropic features, P6222-type structure TaSi2 can be the potential isotropic material and Cmcm-type structure has the relatively obvious anisotropic features in consideration of the anisotropic indexes and the directional dependence of Young's modulus. The electron properties for the four possible structures were discussed by the PDOS and TDOS curves. The thermal behavior of Cmcm-type and FdddZ-type structures are similar in results, and exhibits the nearly coincident curves of the Debye temperature, heat capacity, entropy and enthalpy.

Anisotropic moir\xe9 optical transitions in twisted monolayer/bilayer phosphorene heterostructures

Moiré superlattices of van der Waals heterostructures provide a powerful way to engineer electronic structures of two-dimensional materials. Many novel quantum phenomena have emerged in graphene and transition metal dichalcogenide moiré systems. Twisted phosphorene offers another attractive system to explore moiré physics because phosphorene features an anisotropic rectangular lattice, different from isotropic hexagonal lattices previously reported. Here we report emerging anisotropic moiré optical transitions in twisted monolayer/bilayer phosphorenes. The optical resonances in phosphorene moiré superlattice depend sensitively on twist angle and are completely different from those in the constitute monolayer and bilayer phosphorene even for a twist angle as large as 19°. Our calculations reveal that the Γ-point direct bandgap and the rectangular lattice of phosphorene give rise to the remarkably strong moiré physics in large-twist-angle phosphorene heterostructures. This work highlights fresh opportunities to explore moiré physics in phosphorene and other van der Waals heterostructures with different lattice configurations.

Two-dimensional ferroelasticity in van der Waals \u03b2\u2019-In2Se3

Two-dimensional (2D) materials exhibit remarkable mechanical properties, enabling their applications as flexible and stretchable ultrathin devices. As the origin of several extraordinary mechanical behaviors, ferroelasticity has also been predicted theoretically in 2D materials, but so far lacks experimental validation and investigation. Here, we present the experimental demonstration of 2D ferroelasticity in both exfoliated and chemical-vapor-deposited β’-In2Se3 down to few-layer thickness. We identify quantitatively 2D spontaneous strain originating from in-plane antiferroelectric distortion, using both atomic-resolution electron microscopy and in situ X-ray diffraction. The symmetry-equivalent strain orientations give rise to three domain variants separated by 60° and 120° domain walls (DWs). Mechanical switching between these ferroelastic domains is achieved under ≤0.5% external strain, demonstrating the feasibility to tailor the antiferroelectric polar structure as well as DW patterns through mechanical stimuli. The detailed domain switching mechanism through both DW propagation and domain nucleation is unraveled, and the effects of 3D stacking on such 2D ferroelasticity are also discussed. The observed 2D ferroelasticity here should be widely available in 2D materials with anisotropic lattice distortion, including the 1T’ transition metal dichalcogenides with Peierls distortion and 2D ferroelectrics such as the SnTe family, rendering tantalizing potential to tune 2D functionalities through strain or DW engineering.

Phase diagram of the anisotropic triangular lattice Hubbard model

In a recent study [Phys. Rev. X 10, 021042 (2020)], we showed using large-scale density matrix renormalization group (DMRG) simulations on infinite cylinders that the triangular lattice Hubbard model has a chiral spin liquid phase. In this work, we introduce hopping anisotropy in the model, making one of the three distinct bonds on the lattice stronger or weaker compared with the other two. We implement the anisotropy in two inequivalent ways, one which respects the mirror symmetry of the cylinder and one which breaks this symmetry. In the full range of anisotropy, from the square lattice to weakly coupled one-dimensional chains, we find a variety of phases. Near the isotropic limit we find the three phases identified in our previous work: metal, chiral spin liquid, and 120$^\circ$ spiral order; we note that a recent paper suggests the apparently metallic phase may actually be a Luther-Emery liquid, which would also be in agreement with our results. When one bond is weakened by a relatively small amount, the ground state quickly becomes the square lattice N\'{e}el order. When one bond is strengthened, the story is much less clear, with the phases that we find depending on the orientation of the anisotropy and on the cylinder circumference. While our work is to our knowledge the first DMRG study of the anisotropic triangular lattice Hubbard model, the overall phase diagram we find is broadly consistent with that found previously using other methods, such as variational Monte Carlo and dynamical mean field theory.

Quantum critical phenomena in a spin- 12 frustrated square lattice with spatial anisotropy

We present a model compound with a spin-1/2 spatially anisotropic frustrated square lattice, in which three antiferromagnetic interactions and one ferromagnetic interaction are competing. We observe an unconventional gradual increase in the low-temperature magnetization curve reminiscent of the quantum critical behavior between gapped and gapless phases. In addition, the specific heat and electron spin resonance signals indicate one-dimensional characteristics. These results demonstrate quantum critical behavior associated with one dimensionalization caused by frustrated interactions in the spin-1/2 spatially anisotropic square lattice.

Ultrahigh thermal conductivity and strength in direct-gap semiconducting graphene-like BC6N: A first-principles and classical investigation

In recent years, graphene-like boron carbide and carbon nitride nanosheets have attracted remarkable attentions, owing to their semiconducting electronic nature and outstanding mechanical and heat transport properties. Graphene-like BC6N is an experimentally realized layered material and most recently has been the focus of numerous theoretical studies. Interestingly, the most stable form of BC6N monolayer remains unexplored and limited information are known concerning its intrinsic physical properties. Herein, on the basis of density functional theory (DFT) calculations we confirm that the most stable form of BC6N nanosheet shows a rectangular unitcell, in accordance with an overlooked experimental finding. We found that BC6N monolayer is a semiconductor with 1.19 eV HSE06-based direct gap and yields anisotropic and excellent absorption of visible light. First-principles results highlight that BC6N nanosheet exhibits anisotropic and ultrahigh tensile strength and lattice thermal conductivity, outperforming all other fabricated 2D semiconductors. We moreover develop classical molecular dynamic models for the evaluation of heat transport and mechanical properties of BC6N nanomembranes. The presented results in this work not only shed light on the most stable configuration of BC6N nanosheet, but also confirm its outstandingly appealing electronic, optical, heat conduction and mechanical properties, extremely motivating for further theoretical and experimental endeavors.

Magnetic ordering of the distorted kagome antiferromagnet Y3Cu9(OH)18[Cl8 (OH)] prepared via optimal synthesis

Experimental studies of high-purity kagome-lattice antiferromagnets (KAFM) are of great importance in attempting to better understand the predicted enigmatic quantum spin-liquid ground state of the KAFM model. However, realizations of this model can rarely evade magnetic ordering at low temperatures due to various perturbations to its dominant isotropic exchange interactions. Such a situation is for example encountered due to sizable Dzyaloshinskii-Moriya magnetic anisotropy in YCu$_3$(OH)$_6$Cl$_3$, which stands out from other KAFM materials by its perfect crystal structure. We find evidence of magnetic ordering also in the distorted sibling compound Y$_3$Cu$_9$(OH)$_{18}$[Cl$_8$(OH)], which has recently been proposed to feature a spin-liquid ground state arising from a spatially anisotropic kagome lattice. Our findings are based on a combination of bulk susceptibility, specific heat, and magnetic torque measurements that disclose a N\'eel transition temperature of $T_N=11$~K in this material, which might feature a coexistence of magnetic order and persistent spin dynamics as previously found in YCu$_3$(OH)$_6$Cl$_3$. Contrary to previous studies of single crystals and powders containing impurity inclusions, we use high-purity single crystals of Y$_3$Cu$_9$(OH)$_{18}$[Cl$_8$(OH)] grown via an optimized hydrothermal synthesis route that minimizes such inclusions. This study thus demonstrates that the lack of magnetic ordering in less pure samples of the investigated compound does not originate from the reduced symmetry of spin lattice but is instead of extrinsic origin.

Probing Sources of Capacity Fade in NMC622: An Operando xrd Study of NMC Batteries over Cycling

LiNi0.6Mn0.2Co0.2O2 (NMC622) is a highly promising Li-ion battery cathode due to its high energy density and excellent reversibility. However high potential charging which can lead to much greater energy density comes at the cost of compromised reversibility. It has been suggested that this loss of reversibility is a result of particle disintegration caused by the anisotropic lattice changes and elevated microstrain at high potentials. Operando X-ray diffraction was utilized to probe the influence of these factors on capacity fade in Li/NMC622 cells both before and after 100 cycles at 3-4.3 or 4.7 V. In addition, scanning electron microscopy (SEM) was performed to observe potential particle disintegration. This study allowed for the collection of structural information about the battery system in real time over the time frame in which capacity fade was observed. Initially, charging to 4.7 V led to increased lattice distortions when compared to 4.3 V; however, after cycling, the lattice distortions were identical independent of charging potential but charging to 4.7 V led to a substantial buildup of microstrain. Additionally, SEM showed that significant damage was done to the particle morphology at 4.7 V, while there were minimal changes at 4.3 V; showing that high potential elevated microstrain is likely a major factor in particle disintegration and capacity fade in this system. This study demonstrates important findings on the mechanisms of capacity fade in NMC622 batteries, which is crucial for the design of next generation NMC battery systems that can maximize both energy density and reversibility.

Anisotropic Thermal Conductivity of Nickel-Based Superalloy CM247LC Fabricated via Selective Laser Melting

Efforts to enhance thermal efficiency of turbines by increasing the turbine inlet temperature have been further accelerated by the introduction of 3D printing to turbine components as complex cooling geometry can be implemented using this technique. However, as opposed to the properties of materials fabricated by conventional methods, the properties of materials manufactured by 3D printing are not isotropic. In this study, we analyzed the anisotropic thermal conductivity of nickel-based superalloy CM247LC manufactured by selective laser melting (SLM). We found that as the density decreases, so does the thermal conductivity. In addition, the anisotropy in thermal conductivity is more pronounced at lower densities. It was confirmed that the samples manufactured with low energy density have the same electron thermal conductivity with respect to the orientation, but the lattice thermal conductivity was about 16.5% higher in the in-plane direction than in the cross-plane direction. This difference in anisotropic lattice thermal conductivity is proportional to the difference in square root of elastic modulus. We found that ellipsoidal pores contributed to a direction-dependent elastic modulus, resulting in anisotropy in thermal conductivity. The results of this study should be beneficial not only for designing next-generation gas turbines, but also for any system produced by 3D printing.

Anisotropic lattice expansion and enhancement of superconductivity induced by interstitial carbon doping in Rhenium

We report the effect of carbon doping on the structural and superconducting properties of elemental Re studied in a series of polycrystalline ReCx samples with x up to 0.10. Structural analyses indicate that the C atoms occupy the octahedral interstitial sites in the hexagonal close-packed Re lattice. This leads to an expansion of unit cell mainly along the a-axis and a shift of the binding energies of Re towards higher values. Furthermore, these ReCx samples are found to be weakly coupled, fully gapped, type-II superconductors. With increasing C content x, both Tc and zero temperature upper critical field Bc2(0) increase monotonically. At x = 0.10, Tc and Bc2(0) reach 2.65 K and 1.18 T, respectively, which are more than 50% and 60 times larger than the corresponding values of pure Re. Combined with theoretical calculations, we show that the enhancement in Tc induced by C doping is due to the increase in both the density of states at the Fermi level and electron-phonon coupling. Our results suggest that the introduction of interstitial atoms provides a viable way to tune the superconductivity in transition metal elements.

Effect of doping location induced anisotropy on thermophysical properties of dilute Fe2O3‐Y2O3‐ZrO2 solid solutions

AbstractMulti‐element doped zirconia ceramics are of interest as a satisfactory balance in low thermal conductivity, phase stability, and particularly fracture resistance dominated by non‐transformable tetragonal prime phase (t′) for the next generation thermal barrier coatings and catalysis supports. Compared with mass and radius of dopants, dopant location is rarely investigated due to the high complexity. The undersized transition metal oxides (Fe, Co, and Ni) within 2.0 mol% content were selected for doping 3Y‐TZP to improve thermophysical properties by distinctive doping locations. As shown by X‐ray diffraction and Raman spectra, the Fe3+/Fe2+ ions occupied both substitution and interstitial sites simultaneously, which significantly increased tetragonality and local anisotropy of FeY‐TZP polycrystal. The t′ phase transformation mechanism of the FeY‐TZP compounds quantitatively discussed regarding the coercive strain and stress. In opposition to prediction by Kopp's law, the coefficient of thermal expansion and specific heat capacity of the FeY‐TZP compounds unusally decreased, and the maximum reduction of −83% and −22.5% occured in the 1.0 mol% Fe doping. Cp‐T dependency of the specimens above 550 K strongly related with lattice anisotropy in addition to t′ phase content. Thermomechanical properties of the multicomponent zirconia are highly tunable with dopant locations, supplying a novel landscape for designing high‐entropy ceramics.

Microscale ZnO with controllable crystal morphology as a platform to study antibacterial action on Staphylococcus aureus.

Nano- and microcrystalline ZnO is an inexpensive, easily synthesized material with a multitude of applications. Its usefulness in the present and future stems from its exceptional optoelectronic, structural, and chemical characteristics as well as a broad range of production techniques. One application comes from its ability to inhibit bacterial growth. Despite the well-documented, vigorously studied antimicrobial action of ZnO particles, the most fundamental physical and chemical mechanisms driving growth inhibition are still not well identified. Particularly, the nature of interactions between ZnO surfaces and extracellular material is not totally clear. This is important given the anisotropic lattice of ZnO leading to two characteristically different lattice terminations: polar and nonpolar, polar being electrically charged with many defect sites and nonpolar being electrically neutral while remaining relatively defect-free. In this work, we employ a hydrothermal growth protocol that allows us to produce ZnO microcrystals with dependable control of morphology and, particularly, the relative abundances of polar and nonpolar free surfaces. This functions as a platform for our investigations into surface-surface interactions behind the antibacterial action of ZnO microcrystals. In our studies, we produced ZnO crystals comparable in size or larger than Staphylococcus aureus bacteria. This was done intentionally to ensure that the ZnO particles would not internalize into the bacterial cells. Our experiments were performed in conjunction with surface photovoltage studies of ZnO crystals to characterize electronic structure and charge dynamics that might be contributing to the antibacterial properties of our samples. We report on the interactions between ZnO microcrystalline surfaces and extracellular material of Staphylococcus aureus bacteria.

3D printing of dual phase-strengthened microlattices for lightweight micro aerial vehicles

The rapid advancement in CAD and 3D printing technology have brought the rise of mechanical metamaterials which inspired from nature and have optimized microstructural features to exhibit superior mechanical properties over conventional materials for various structural applications. Here, by adopting dual-phase strengthening mechanism in crystallography, we proposed a microlattice strengthening strategy which incorporates stretching-dominated octet-truss (OCT) units as the second phase particles into the diagonal planes of the bending-dominated body-centered cubic (BCC) lattice matrix, to form an anisotropic OCT-BCC lattice. The OCT-BCC dual-phase microlattice possess superior specific compressive strengths that are ~300% and 600% higher than BCC microlattices along its horizontal direction and longitudinal direction, respectively, accompanied with a significant increase in stiffness and energy absorption as well. Moreover, a large-scale OCT-BCC lattice metamaterial with dimensions up to 5.0 cm × 2.0 cm × 1.0 cm was successfully manufactured and integrated into a micro aerial vehicle (MAV). The metamaterial-integrated MAV has an airframe that is ~65% lighter than its bulk counterpart, resulting in a significant increase (~40%) in flight duration. This work not only provides an effective metamaterial enhancement design strategy, but also promotes the practical application of large-scale 3D printed metamaterial in the field of micro unmanned aerial vehicle.

Magnetoelastic transition and giant anisotropic magnetostriction effect in textured Mn<sub>1.9</sub>Co<sub>0.1</sub>Sb polycrystal

Mn2Sb-based alloys have extremely promising practical applications in many fields owing to their abundant and controllable magnetofuntional properties around the first-order magnetoelastic transition, as well as their low costs. However, only a few studies on the magnetostrain of these alloys have been reported. Herein, a highly textured Mn1.9Co0.1Sb polycrystal is fabricated by using the arc-melting technique, which produces a strong temperature gradient induced by rapid cooling during the arc-melting process, resulting in the highly selective orientation of crystal growth. X-ray diffraction and thermomagnetic measurements show that the grains grow in certain crystallographic directions. Metamagnetic transition is revealed by the isothermal magnetization curve measurements, which is observed to be more obvious at lower temperatures. The standard resistive strain-gauge method is adopted to measure the magnetostriction effect. Giant anisotropic magnetostrain values of up to 0.246% and −0.11% are obtained parallel and vertical to the texture of the alloy, respectively. This magnetostriction effect mainly originates from the anisotropic lattice distortion across the first-order magnetoelastic phase transition.

Modeling of crystal growth with density change induced flows by the anisotropic lattice Boltzmann scheme

The density change in solidification can evoke a kind of melt flow and affect the final crystal patterns. A novel model fitting into the anisotropic lattice Boltzmann scheme was developed to describe density change induced flows, in which the flow was straightly depicted by introducing a density gradient raised body force in the lattice Boltzmann equation. The effects of density ratios on crystal growth were numerically studied. The results show that the density change induced flows can significantly affect morphological transitions and there exists a competition between latent heat release and heat transport with mediation of the density change caused flows.

INFLUENCE OF ELECTRON IRRADIATION ON THE STRUCTURE EVOLUTION AND CREEP OF Zr1Nb ALLOY AT 380 °С E

The effect of electron irradiation on the evolution of the structure and creep of the samples Zr1Nb alloy of industrial production and samples obtained by the IPD method were studied. It was shown that irradiation with electrons with a dose not exceeding D = 5∙1019 сm–2 does not affect the plastic deformation mechanisms of the industrial Zr1Nb alloy, as a result of which its high thermomechanical and radiation stability is maintained. Irradiation by electrons with an energy of E = 10 MeV and doses of D = 6∙1017 and 5∙1019 сm–2 does not change the character of the deformation nanostructure of the Zr1Nb alloy, but initiates return processes in grain boundaries and boundary regions and cause the changes in crystalline anisotropy of GPU lattice, which leads to softening of the material during creep at 380 °C.