Condensed Matter

Recently, van der Waals heterostructure has attracted interest both theoretically and experimentally for their potential applications in photoelectronic devices, photovoltaic devices, plasmonic devices and photocatalysis. Inspired by this, we design a lepidocrocite-type TiO2/GaSe heterostructure. Via first-principles simulations, we show that such a heterostructure is a direct bandgap semiconductor with a strong and broad optical absorption, ranging from visible light to UV region, exhibiting its potential application in photoelectronic and photovoltaic devices. With the planar-averaged electron density difference and Bader charge analysis, the heterostructure shows a strong capacity of enhancing the charge redistribution especially at the interface, prolonging the lifetime of excitons, and hence improving photocatalytic performance. By applying biaxial strain and interlayer coupling, the heterostructure exhibits a direct-indirect bandgap transition and shows a potential for mechanical sensors due to the smooth and linear variation of bandgaps. Furthermore, our result indicates that a lower interlayer distance leads to a stronger charge redistribution. The calculation of irradiating ultrafast on the heterostructure further reveals a semiconductor-metal transition for the heterostructure. Moreover, we find an enhanced induced plasmonic current in the heterostructure under both x-polarized and z-polarized laser, which is beneficial to plasmonic devices designs. Our research provides valuable insight in applying the lepidocrocite-type TiO2/GaSe heterostructure in photoelectronic, photovoltaic, photocatalytic, mechanical sensing and plasmonic realms.

The interface plays a critical role in the mechanical properties of composites. In the present work, a novel phase-field based cohesive zone model (CZM) is proposed for the cracking simulation. The competition and interaction between the bulk and interfacial cracking are taken into consideration directly in both displacement- and phase-field. A modified family of degradation functions is utilized to describe traction-separation law in the CZM. Finite element implementation of the present CZM was carried out with a completely staggered algorithm. Several numerical examples, including a single bar tension test, a double cantilever beam test, a three-point bending test, and a single fiber-reinforced composite test, are carried out to validate the present model by comparison with existing numerical and experimental results.The present model shows its advantage on modeling interaction between bulk and interfacial cracking. 1. Introduction

First principles density functional theory (DFT) simulations of antiferroelectric (AFE) PbZrO 3 and PbHfO 3 reveal a dynamical instability in the phonon spectra of their purported low temperature Pbam ground states. This instability doubles the c -axis of Pbam and condenses five new small amplitude phonon modes giving rise to an 80-atom Pnam structure. Compared with Pbam , the stability of this structure is slightly enhanced and highly reproducible as demonstrated through using different DFT codes and different treatments of electronic exchange & correlation interactions. This suggests that Pnam is a new candidate for the low temperature ground state of both materials. With this finding, we bring parity between the AFE archetypes and recent observations of a very similar AFE phase in doped or electrostatically engineered BiFeO 3 .

In this review we introduce computer modelling and simulation techniques which are used for ferrimagnetic materials. We focus on models where thermal effects are accounted for, atomistic spin dynamics and finite temperature macrospin approaches. We survey the literature of two of the most commonly modelled ferrimagnets in the field of spintronics--the amorphous alloy GdFeCo and the magnetic insulator yttrium iron garnet. We look at how generic models and material specific models have been applied to predict and understand spintronic experiments, focusing on the fields of ultrafast magnetisation dynamics, spincaloritronics and magnetic textures dynamics and give an outlook for modelling in ferrimagnetic spintronics.

We propose a solution to the longstanding permalloy problem ??why the particular composition of permalloy, Fe 21.5 Ni 78.5 , achieves a dramatic drop in hysteresis, while its material constants show no obvious signal of this behavior. We use our recently developed coercivity tool to show that a delicate balance between local instabilities and magnetic material constants are necessary to explain the dramatic drop of hysteresis at 78.5% Ni. Our findings are in agreement with the permalloy experiments and, more broadly, provide theoretical guidance for the discovery of novel low hysteresis magnetic alloys.

Vibro-tactile feedback is, by far the most common haptic interface in wearable or touchable devices. This feedback can be amplified by controlling the wave propagation characteristics in devices, by utilizing phenomena such as structural resonance. However, much of the work in vibro-tactile haptics has focused on amplifying local displacements in a structure by increasing local compliance. In this paper, we show that engineering the resonance mode shape of a structure with embedded localized mass amplifies the displacements without compromising on the stiffness or resonance frequency. The resulting structure, i.e., a tuned mass amplifier, produces higher tactile forces (7.7 times) compared to its counterpart without a mass, while maintaining a low frequency. We optimize the proposed design using a combination of a neural network and sensitivity analysis, and validate the results with experiments on 3-D printed structures. We also study the performance of the device on contact with a soft material, to evaluate the interaction with skin. Potential avenues for future work are also presented, including small form factor wearable haptic devices and remote haptics.

Electron-phonon ( e -ph) interactions are pervasive in condensed matter, governing phenomena such as transport, superconductivity, charge-density waves, polarons and metal-insulator transitions. First-principles approaches enable accurate calculations of e -ph interactions in a wide range of solids. However, they remain an open challenge in correlated electron systems (CES), where density functional theory often fails to describe the ground state. Therefore reliable e -ph calculations remain out of reach for many transition metal oxides, high-temperature superconductors, Mott insulators, planetary materials and multiferroics. Here we show first-principles calculations of e -ph interactions in CES, using the framework of Hubbard-corrected density functional theory (DFT+ U ) and its linear response extension (DFPT+ U ), which can describe the electronic structure and lattice dynamics of many CES. We showcase the accuracy of this approach for a prototypical Mott system, CoO, carrying out a detailed investigation of its e -ph interactions and electron spectral functions. While standard DFPT gives unphysically divergent and short-ranged e -ph interactions, DFPT+ U is shown to remove the divergences and properly account for the long-range Fröhlich interaction, allowing us to model polaron effects in a Mott insulator. Our work establishes a broadly applicable and affordable approach for quantitative studies of e-ph interactions in CES, a novel theoretical tool to interpret experiments in this broad class of materials.

Microscopic model of the interaction of spins with a microwave field in a random-anisotropy magnet has been developed. Numerical results show that microwave absorption occurs in a broad range of frequencies due to the distribution of Imry-Ma domains on sizes and effective anisotropy. That distribution is also responsible for the weak dependence of the absorption on the damping. At a fixed frequency of the ac-field the spatial pattern of the amplitude of spin oscillations exhibits maxima corresponding to the oscillations of resonant magnetic domains. The dependence of the peak absorption frequency on the ratio of the magnitude of per-spin random anisotropy to the strength of the ferromagnetic exchange agrees with the scaling derived from the Imry-Ma argument. The effect noticeably increases in low-dimensional systems, suggesting that materials comprised of microscopic amorphous leaves and wires can be promising candidates for enhanced microwave absorption.

The tremendous diversity of zeolite frameworks makes ab initio simulations of their structure, stability, reactivity and, properties virtually impossible. To enable large-scale reactive simulations of zeolites with ab initio quality, we trained neural network potentials (NNP) with the SchNet architecture on a structurally diverse DFT database. This database was iteratively extended by active learning to cover the configuration space from low-density zeolites to high-pressure silica polymorphs including low-energy equilibrium configurations and high-energy transition states. The resulting reactive NNPs model equilibrium structures, vibrational properties, and phase transitions at high temperatures such as thermal zeolite collapse in excellent agreement with both DFT and experiments. The novel NNPs allowed revision of a zeolite database containing more than 330 thousand hypothetical zeolites previously generated employing analytical force fields. NNP structure optimizations revealed more than 20 thousand additional hypothetical frameworks in the thermodynamically accessible range of zeolite synthesis. Additionally, the obtained zeolite database provides vital input for future machine learning studies on the structure, stability, reactivity and properties of hypothetical and existing zeolites.

Density functionals with asymptotic corrections to the long-range potential are recognized as entry-level methods for electronic structure calculations on molecules that can sustain charge transfer. Even though charge transfer is also extremely common in Materials-Science, the application of such methods has been rare. We implement CAM-B3LYP within the VASP framework. Results obtained for 8 representative materials: aluminum, diamond, graphene, silicon, NaCl, MgO, 2D h-BN and 3D h-BN, indicate that CAM-B3LYP predictions embody mean-absolute deviations (MAD) compared to HSE06 that are reduced by a factor of 4 for lattice parameters, 4 for quasiparticle band gaps, 3 for the lowest optical excitation energies, and 6 for exciton binding energies; the only property for which HSE06 performed better was the band-gap of silicon. CAM-B3LYP also appears to provide similar or improved accuracy compared to ab initio G0W0 and Bethe-Salpeter equation (BSE) approaches. The CAM-B3LYP implementation in VASP was verified by comparison of optimized geometries and reaction energies for isolated molecules taken from the ACCDB database, evaluated in large periodic unit cells, to analogous results obtained using Gaussian-basis sets by Gaussian-16. Using standard GW pseudopotentials and energy cutoffs for the plane-wave calculations and the aug-cc-pV5Z basis set for the atomic-basis ones, the MAD in energy for 1738 chemical reactions was 0.34 kcal mol-1 (0.015 eV), whilst for 1291 bond lengths this was 0.0036 ?. An approximate scheme for the speedy implementation of CAM-B3LYP into other electronic-structure codes for materials-science applications was considered and shown to be less accurate, but nevertheless useful. Analytical functional derivatives for CAM-B3LYP were also obtained and implemented, ca. halving computational cost compared to use of standard numerical derivatives.