arXiv subjects

Hesam Askari

Publications and source records attributed to Hesam Askari.

18 recordsLinked to original sources

arXiv2025

Strain-induced Moiré Reconstruction and Memorization in Two-Dimensional Materials without Twist

Two-dimensional (2D) materials with a twist between layers exhibit a moiré interference pattern with larger periodicity than any of the constituent layer unit cells. In these systems, a wealth of exotic phases appear that result from moiré-dependent many-body electron correlation effects or non-trivial band topology. One problem with using twist to generate moiré interference has been the difficulty in creating high-quality, uniform, and repeatable samples due to fabrication through mechanical stacking with viscoelastic stamps. Here we show, a new method to generate moiré interference through the controlled application of layer-by-layer strain (heterostrain) on non-twisted 2D materials, where moiré interference results from strain-induced lattice mismatch without twisting or stacking. Heterostrain generation is achieved by depositing stressed thin films onto 2D materials to apply large strains to the top layers while leaving layers further down less strained. We achieve deterministic control of moiré periodicity and symmetry in non-twisted 2D multilayers and bilayers, with 97% yield, through varying stressor film force (film thickness X film stress) and geometry. Moiré reconstruction effects are memorized after the removal of the stressor layers. Control over the strain degree-of-freedom opens the door to a completely unexplored set of unrealized tunable moiré geometric symmetries, which may now be achieved in a high-yield and user-skill independent process taking only hours. This technique solves a long-standing throughput bottleneck in new moiré quantum materials discovery and opens the door to industrially-compatible manufacturing for 2D moiré-based electronic or optical devices.

cond-mat.mtrl-sci↗

arXiv2025

Scalable and deterministic construction of moir\'e superlattice in 2D materials using stressor films

Moir\'e superlattice in two-dimensional (2D) materials provides a powerful platform to engineer emergent electronic states, yet the construction of moir\'e superlattices remains lab-scale, involving much trial and error and with little control. Here, we demonstrate the construction of a heterostrain-induced moir\'e superlattice in transition metal dichalcogenides using a scalable process that deterministically induces strain to 2D materials. By applying patterned thin-film stressors and probing the resulting structures with scanning transmission electron microscopy, we directly resolve the induced heterostrain, lattice deformations, and stacking variations that produce the moir\'e superlattice. We find that uniaxial and biaxial heterostrain give rise to distinct moir\'e patterns, including stripes and distorted hexagonal patterns. With this approach, we create in-plane polar distortions and thus in-plane polarization at the domain boundaries of the moir\'e superlattice in MoS$_2$. The deterministic and scalable construction of moir\'e patterns using a well-established scalable process opens opportunities to design new moir\'e geometries in 2D materials.

cond-mat.mtrl-sci↗

arXiv2025

Low Speed Oblique Impact Behavior On Granular Media Across Gravitational Conditions; The role of cohesion

Analyses of impact provide rich insights from the evolution of granular bodies to their structural properties of the surface and subsurface layers of celestial bodies. Although chemical cohesive bonding has been observed in asteroid samples, and low-speed impact has been a subject of many studies, our understanding of the role of cohesion in these dynamics is limited, especially at small gravities such as those observed on asteroid surfaces. In this work, we use numerical discrete element method (DEM) and analytical dynamic resistive force theory (DRFT) modeling to examine the effect of cohesion on the outcome of the impact into loose granular media and explore scaling laws that predict impact behavior in the presence of cohesion under various gravitational conditions and cohesive strengths. We find that the effect of cohesion on the impact behavior becomes more significant in smaller gravitational acceleration, raising the need to scale the cohesion coefficient with gravity. We find that due to an insufficient understanding of confounding between cohesion and friction-induced quasi-static and inertial resistance, the outcomes of the DEM simulation models are incongruent with a suggested analytic model using Froude and Bond number scaling based on an additive contribution of frictional and inertial forces. Our study suggests that new dimensionless parameters and scaling are required to accurately capture the role of cohesion, given its ties to frictional behavior between the grain particles at different gravities.

cond-mat.soft↗

arXiv2023

Subsurface pulse, crater and ejecta asymmetry from oblique impacts into granular media

We carry out experiments of 104 m/s velocity oblique impacts into a granular medium (sand). Impact craters have nearly round rims even at a grazing angle of about $10^\circ$, however, the strength of seismic pulses excited by the impact is dependent upon impact angle, and the ratio between uprange and downrange velocity peaks can be as large as 5, particularly at shallow depths. Crater slope, an offset between crater center and impact site, crater volume, azimuthal variation in ejection angle, seismic pulse shapes and subsurface flow direction are also sensitive to impact angle, but to a much lower degree than subsurface pulse strength. Uprange and downrange pulse peak amplitudes can be estimated from the horizontal and vertical components of the momentum imparted to the medium from the projectile

astro-ph.EP↗

arXiv2023

Strain Engineering for High-Performance Phase Change Memristors

A new mechanism for memristive switching in 2D materials is through electric-field controllable electronic/structural phase transitions, but these devices have not outperformed status quo 2D memristors. Here, we report a high-performance bipolar phase change memristor from strain engineered multilayer 1T'-MoTe$_{2}$ that now surpasses the performance metrics (on/off ratio, switching voltage, switching speed) of all 2D memristive devices, achieved without forming steps. Using process-induced strain engineering, we directly pattern stressed metallic contacts to induce a semimetallic to semiconducting phase transition in MoTe2 forming a self-aligned vertical transport memristor with semiconducting MoTe$_{2}$ as the active region. These devices utilize strain to bring them closer to the phase transition boundary and achieve ultra-low ~90 mV switching voltage, ultra-high ~10$^8$ on/off ratio, 5 ns switching, and retention of over 10$^5$ s. Engineered tunability of the device switching voltage and on/off ratio is also achieved by varying the single process parameter of contact metal film force (film stress $\times$ film thickness).

physics.app-ph↗

arXiv2023

Froude number scaling unifies impact trajectories into granular media across gravitational conditions

The interactions of solid objects with granular media is countered by a resistance force that stems from frictional forces between the grains and the media's resistance to inertia imposed by the intruder. Earlier theories of granular intrusion have suggested an additive contribution of these two families of forces and had tremendous success in predicting resistive forces on arbitrary shaped objects. However, it remains unclear how these forces are influenced by gravitational conditions. We examine the role of gravity on surface impact behavior into cohesionless granular media using hundreds of soft-sphere discrete element simulations, we demonstrate that the outcome of impacts remain qualitatively similar under varying gravitational conditions if initial velocities are scaled with the Froude number, suggesting an underlying law. Using theoretical arguments, we provide reasoning for the observed universality and show that there is a hidden dependency in resistive forces into granular media on Froude number. Following the theoretical framework, we show that Froude number scaling precisely collapses impact trajectories across gravitational conditions, setting the foundation for explorations in granular behavior beyond Earth.

cond-mat.soft↗

arXiv2023

Variability estimation in a non-linear crack growth simulation model with controlled parameters using Designed Experiments testing

Variability in multiple independent input parameters makes it difficult to estimate the resultant variability in the system's overall response. The Propagation of Errors and Monte-Carlo techniques are two major methods to predict the variability of a system. However, in the former method, the formalism can lead to an inaccurate estimate for systems that have parameters varying over a wide range. For the latter, the results give a direct estimate of the variance of the response, but for complex systems with many parameters, the number of trials necessary to yield an accurate estimate can be very large to the point the technique becomes impractical. In this study, the effectiveness of the Tolerance Design method to estimate variability in complex systems is studied. We use a linear elastic 3 point bending beam model and a nonlinear extended finite elements crack growth model to test and compare the PE and MC methods with the TD method. Results from an MC estimate, using 10,000 trials, serve as a reference to validate the result in both cases. We find that the PE method works suboptimal for a coefficient of variance above 5% in the input variables. In addition, we find that the TD method works very well with moderately sized trials of designed experiment for both models. Our results demonstrate how the variability estimation methods perform in the deterministic domain of numerical simulations and can assist in designing physical tests by providing a guideline performance measure.

cond-mat.other↗

arXiv2023

An atomistic insight into moiré reconstruction in Twisted Bilayer Graphene beyond the magic angle

Twisted bilayer graphene exhibits electronic properties that are highly correlated with the size and arrangement of moiré patterns. While rigid rotation of two layers creates the topology of moiré patterns, local rearrangements of the atoms due to interlayer van der Waals interactions result in atomic reconstruction within the moiré cells. The ability to manipulate these patterns by controlling twist angle and/or externally applied strain provides a promising route to tune their properties. While this phenomenon has been extensively studied for angles close to or smaller than the magic angle (θm=1.1°), its extent for higher angles and how it evolves with strain is unknown and is believed to be mostly absent at high angles. We use theoretical and numerical analyses to resolve reconstruction in angles above θm using interpretive and fundamental physical measures. In addition, we propose a method to identify local regions within moiré cells and track their evolution with strain for a range of representative high twist angles. Our results show that reconstruction is actively present beyond the magic angle and its contribution to the evolution of the moiré cells is major. Our theoretical method to correlate local and global phonon behavior provides further validation on the role of reconstruction at higher angles. Our findings provide a better understanding of moiré reconstruction in large twist angles and the evolution of moiré cells in the presence of strain, that might be very crucial for twistronics-based applications.

cond-mat.mtrl-sci↗

arXiv2023

Moiré Engineering in 2D Heterostructures with Process-Induced Strain

We report deterministic control over moiré superlattice interference pattern in twisted bilayer graphene by implementing designable device-level heterostrain with process-induced strain engineering, a widely used technique in industrial silicon nanofabrication processes. By depositing stressed thin films onto our twisted bilayer graphene samples, heterostrain magnitude and strain directionality can be controlled by stressor film force (film stress x film thickness) and patterned stressor geometry, respectively. We examine strain and moiré interference with Raman spectroscopy through in-plane and moiré-activated phonon mode shifts. Results support systematic C$_{3}$ rotational symmetry breaking and tunable periodicity in moiré superlattices under the application of uniaxial or biaxial heterostrain. Experimental results are validated by molecular statics simulations and density functional theory based first principles calculations. This provides a method to not only tune moiré interference without additional twisting, but also allows for a systematic pathway to explore different van der Waals based moiré superlattice symmetries by deterministic design.

cond-mat.mes-hall↗

arXiv2022

Surface particle motions excited by a low velocity normal impact into a granular medium

In laboratory experiments, high speed videos are used to detect and track mm-size surface particle motions caused by a low velocity normal impact into sand. Outside the final crater radius and prior to the landing of the ejecta curtain, particle displacements are measured via particle tracking velocimetry and with a cross-correlation method. Surface particles rebound and are also permanently displaced with both peak and permanent displacements rapidly decaying as a function of distance from the crater center. The surface begins to move before most of the ejecta curtain has landed, but continues to move after the subsurface seismic pulse has decayed. Ray angles for surface and subsurface velocities are similar to those described by a Maxwell's Z-model. This implies that the flow field outside the crater excavation region is a continuation of the crater excavation flow. The ratio of final particle displacement to crater radius resembles that measured for other impact craters.

astro-ph.EP↗

arXiv2022

Sub-surface granular dynamics in the context of oblique, low-velocity impacts into angular granular media

Oblique, low-velocity impacts onto extraterrestrial terrain are an inevitable occurrence during space exploration. We conduct two-dimensional discrete simulations to model such impacts into a bed of triangular grains. Finite element method provides the basis for simulation, enabling the angular grain geometry. Our findings re-create the three classes of impact behavior previously noted from experiments: full-stop, rollout, and ricochet \citep*{Wright2020}. An application of Set Voronoi tessellation assesses packing fraction at a high resolution, revealing how grains shift relative to each other during an impact event. Calculation of Von Mises strain distributions then reveal how grains shift relative to the overall system, leading to the notion of the 'skin zone'. Intuition would suggest that the region of perturbed grains would grow deeper with higher velocity impacts, results instead show that increasing velocity may actually evoke a change in the grains' dissipative response that boosts lateral perturbation. Finally, we consider as a whole how sub-surface response could link with impactor dynamics to deepen our understanding of oblique, low-velocity impact events and help to improve mission outcomes.

astro-ph.EP↗

arXiv2022

Ricochets on Asteroids II: Sensitivity of laboratory experiments of low velocity grazing impacts on substrate grain size

We compare low velocity impacts that ricochet with the same impact velocity and impact angle into granular media with similar bulk density, porosity and friction coefficient but different mean grain size. The ratio of projectile diameter to mean grain length ranges from 4 in our coarsest medium to 50 in our finest sand. Using high speed video and fluorescent markers, we measure the ratio of pre- to post-impact horizontal and vertical velocity components, which we refer to as coefficients of restitution, and the angle of deflection caused by the impact in the horizontal plane. Coefficients of restitution are sensitive to mean grain size with the ratio associated with the horizontal velocity component about twice as large for our coarsest gravel as that for our finest sand. This implies that coefficients for hydro-static-like, drag-like and lift-like forces, used in empirical force laws, are sensitive to mean grain size. The coefficient that is most strongly sensitive to grain size is the lift coefficient which decreases by a factor of 3 between our coarsest and finest media. The deflection angles are largest in the coarser media and their size approximately depends on grain size to the 3/2 power. This scaling is matched with a model where momentum transfer takes place via collisions with individual grains. The dependence of impact mechanics on substrate size distribution should be considered in future models for populations of objects that impact granular asteroid surfaces.

astro-ph.EP↗

arXiv2021

Strain Engineering 2D MoS$_{2}$ with Thin Film Stress Capping Layers

We demonstrate a method to induce tensile and compressive strain into two-dimensional transition metal dichalcogenide (TMDC) MoS$_{2}$ via the deposition of stressed thin films to encapsulate exfoliated flakes. With this technique we can directly engineer MoS$_{2}$ strain magnitude by changing deposited thin film stress, therefore allowing variable strain to be applied on a flake-to-flake level. These thin film stressors are analogous to SiN$_{x}$ based stressors implemented in industrial CMOS processes to enhance Si mobility, suggesting that our concept is highly scalable and may be applied for large-scale integration of strain engineered TMDC devices. We choose optically transparent stressors to allow us to probe MoS$_{2}$ strain through Raman spectroscopy. Combining thickness dependent analyses of Raman peak shifts in MoS$_{2}$ with atomistic simulations, we can explore layer-by-layer strain transfer. MoS$_{2}$ on conventional substrates (SiO$_{2}$, MgO) show strain transfer into the top two layers of multilayer flakes with limited strain transfer to monolayers due to substrate adhesion. To mitigate this limitation, we also explore stressors on van der Waals heterostructures constructed of monolayer (1L) MoS$_{2}$ on hexagonal boron nitride (h-BN). This concept frees the 1L-MoS$_{2}$ allowing for a 0.85$\%$ strain to be applied to the monolayer with a corresponding strain induced bandgap change of 75 meV. By using thin films with higher stress, strain may be engineered to be even higher. Various stressors and deposition methods are considered, showing a stressor material independent transfer of strain that only depends on stressor film force with negligible defects induced into MoS$_{2}$ when thermal evaporation is used.

cond-mat.mes-hall↗

arXiv2020

Ricochets on Asteroids: Experimental study of low velocity grazing impacts into granular media

Spin off events and impacts can eject boulders from an asteroid surface and rubble pile asteroids can accumulate from debris following a collision between large asteroids. These processes produce a population of gravitational bound objects in orbit that can impact an asteroid surface at low velocity and with a distribution of impact angles. We present laboratory experiments of low velocity spherical projectiles into a fine granular medium, sand. We delineate velocity and impact angles giving ricochets, those giving projectiles that roll-out from the impact crater and those that stop within their impact crater. With high speed camera images and fluorescent markers on the projectiles we track spin and projectile trajectories during impact. We find that the projectile only reaches a rolling without slipping condition well after the marble has reached peak penetration depth. The required friction coefficient during the penetration phase of impact is 4-5 times lower than that of the sand suggesting that the sand is fluidized near the projectile surface during penetration. We find that the critical grazing impact critical angle dividing ricochets from roll-outs, increases with increasing impact velocity. The critical angles for ricochet and for roll-out as a function of velocity can be matched by an empirical model during the rebound phase that balances a lift force against gravity. We estimate constraints on projectile radius, velocity and impact angle that would allow projectiles on asteroids to ricochet or roll away from impact, finally coming to rest distant from their initial impact sites.

astro-ph.EP↗

arXiv2019

Strain-Based Room-Temperature Non-Volatile MoTe$_2$ Ferroelectric Phase Change Transistor

The primary mechanism of operation of almost all transistors today relies on electric-field effect in a semiconducting channel to tune its conductivity from the conducting 'on'-state to a non-conducting 'off'-state. As transistors continue to scale down to increase computational performance, physical limitations from nanoscale field-effect operation begin to cause undesirable current leakage that is detrimental to the continued advancement of computing. Using a fundamentally different mechanism of operation, we show that through nanoscale strain engineering with thin films and ferroelectrics (FEs) the transition metal dichalcogenide (TMDC) MoTe$_2$ can be reversibly switched with electric-field induced strain between the 1T'-MoTe$_2$ (semimetallic) phase to a semiconducting MoTe$_2$ phase in a field effect transistor geometry. This alternative mechanism for transistor switching sidesteps all the static and dynamic power consumption problems in conventional field-effect transistors (FETs). Using strain, we achieve large non-volatile changes in channel conductivity (G$_{on}$/G$_{off}$~10$^7$ vs. G$_{on}$/G$_{off}$~0.04 in the control device) at room temperature. Ferroelectric devices offer the potential to reach sub-ns nonvolatile strain switching at the attojoule/bit level, having immediate applications in ultra-fast low-power non-volatile logic and memory while also transforming the landscape of computational architectures since conventional power, speed, and volatility considerations for microelectronics may no longer exist.

physics.app-ph↗

arXiv2018

A Light-weight Vibrational Motor Powered Recoil Robot that Hops Rapidly Across Granular Media

A 1 cm coin vibrational motor fixed to the center of a 4 cm square foam platform moves rapidly across granular media (poppy seeds, millet, corn meal) at a speed of up to 30 cm/s, or about 5 body lengths/s. Fast speeds are achieved with dimensionless acceleration number, similar to a Froude number, up to 50, allowing the light-weight 1.4 g mechanism to remain above the substrate, levitated and propelled by its kicks off the surface. The mechanism is low cost and moves without any external moving parts. With 2 s exposures we photograph the trajectory of the mechanism using an LED blocked except for a pin-hole and fixed to the mechanism. Trajectories can exhibit period doubling phenomena similar to a ball bouncing on a vibrating table top. A two dimensional numerical model gives similar trajectories, though a vertical drag force is required to keep the mechanism height low. We attribute the vertical drag force to aerodynamic suction from air flow below the mechanism base and through the granular substrate. Our numerical model suggests that speed is maximized when the mechanism is prevented from jumping high off the surface. In this way the mechanism resembles a galloping or jumping animal whose body remains nearly at the same height above the ground during its gait.

cond-mat.soft↗

arXiv2017

A coin vibrational motor swimming at low Reynolds number

Low-cost coin vibrational motors, used in haptic feedback, exhibit rotational internal motion inside a rigid case. Because the motor case motion exhibits rotational symmetry, when placed into a fluid such as glycerin, the motor does not swim even though its vibrations induce steady streaming in the fluid. However, a piece of rubber foam stuck to the curved case and giving the motor neutral buoyancy also breaks the rotational symmetry allowing it to swim. We measured a 1 cm diameter coin vibrational motor swimming in glycerin at a speed of a body length in 3 seconds or at 3 mm/s. The swim speed puts the vibrational motor in a low Reynolds number regime similar to bacterial motility, but because of the vibration it is not analogous to biological organisms. Rather the swimming vibrational motor may inspire small inexpensive robotic swimmers that are robust as they contain no external moving parts. A time dependent Stokes equation planar sheet model suggests that the swim speed depends on a steady streaming velocity $V_{stream} \sim Re_s^{1/2} U_0$ where $U_0$ is the velocity of surface vibrations, and streaming Reynolds number $Re_s = U_0^2/(ων)$ for angular vibrational frequency $ω$ and fluid kinematic viscosity $ν$.

physics.flu-dyn↗

arXiv2015

Intrusion in heterogeneous materials: Simple global rules from complex micro-mechanics

The interaction of intruding objects with deformable materials is a common phenomenon, arising in impact and penetration problems, animal and vehicle locomotion, and various geo-space applications. The dynamics of arbitrary intruders can be simplified using Resistive Force Theory (RFT), an empirical framework originally used for fluids but works surprisingly well, better in fact, in granular materials. That such a simple model describes behavior in dry grains, a complex nonlinear material, has invigorated a search to determine the underlying mechanism of RFT. We have discovered that a straightforward friction-based continuum model generates RFT, establishing a link between RFT and local material behavior. Our theory reproduces experimental RFT data without any parameter fitting and generates RFT's key simplifying assumption: a geometry-independent local force formula. Analysis of the system explains why RFT works better in grains than in viscous fluids, and leads to an analytical criterion to predict RFT's in other materials.

cond-mat.soft↗