Block sparsity promoting algorithm for efficient construction of cluster expansion models for multicomponent alloys.

Multicomponent alloys are gaining significance as drivers of technological breakthroughs especially in structural and energy storage materials. The vast configuration space of these materials prohibit computational modeling using first-principles based methods alone. The cluster expansion (CE) method is the most widely used tool for modeling configurational disorder in alloys. CE relies on machine learning algorithms to train Hamiltonians and uses first-principles calculated data as training sets. In this paper we present a new compressive sensing-based algorithm for the efficient construction of CE Hamiltonians of multicomponent alloys. Our algorithm constructs highly sparse and physically reasonable models from a carefully selected small training set of alloy structures. Compared to conventional fitting algorithms, the algorithm achieves more than 50% reduction in the training set size. The resultant sparse models can sample the configuration space at least 3 × faster. We demonstrate this algorithm on 4 different alloy systems, namely Ag-Au, Ag-Au-Cu, Ag-Au-Cu-Pd and (Ge,Sn)(S,Se,Te).The sparse CE models for these alloys can rapidly reproduce known ground state orderings and order-disorder transitions. Our method can truly enable high-throughput multicomponent alloy thermodynamics by reducing the cost associated with model construction and configuration sampling.

Computational design of novel MAX phase alloys as potential hydrogen storage media combining first principles and cluster expansion methods.

Finding a suitable material for hydrogen storage under ambient atmospheric conditions is challenging for material scientists and chemists. In this work, using a first principles based cluster expansion approach, the hydrogen storage capacity of the Ti2AC (A = Al, Ti, Cr, Mn, Fe, Co, Ni, Cu, and Zn) MAX phase and its alloys was studied. We found that hydrogen is energetically stable in Ti-A layers in which the tetrahedral site consisting of one A atom and three Ti atoms is energetically more favorable for hydrogen adsorption than other sites in the Ti-A layer. Ti2CuC has the highest hydrogen adsorption energy than other Ti2AC phases. We find that the 83.33% Cu doped Ti2AlxCu1-xC alloy structure is both energetically and dynamically stable and can store 3.66 wt% hydrogen under ambient atmospheric conditions, which is higher than that stored by both Ti2AlC and Ti2CuC phases. These findings indicate that the hydrogen capacity of the MAX phase can be significantly improved by doping an appropriate atom species.

A phase field model combined with a genetic algorithm for polycrystalline hafnium zirconium oxide ferroelectrics.

Ferroelectric hafnium zirconium oxide (HZO) thin films show significant promise for applications in ferroelectric random-access memory devices, ferroelectric field-effect transistors, and ferroelectric tunneling junctions. However, there are shortcomings in understanding ferroelectric switching, which is crucial in the operation of these devices. Here a computational model based on the phase field method is developed to simulate the switching behavior of polycrystalline HZO thin films. Furthermore, we introduce a novel approach to optimize the effective Landau coefficients describing the free energy of HZO by combining the phase field model with a genetic algorithm. We validate the model by accurately simulating switching curves for HZO thin films with different ferroelectric phase fractions. The simulated domain dynamics during switching also shows amazing similarity to the available experimental observations. The present work also provides fundamental insights into enhancing the ferroelectricity in HZO thin films by controlling the grain morphology and crystalline texture. It can potentially be extended to improve the ferroelectric properties of other hafnia based thin films.

Is There a Lower Size Limit for Superconductivity?

The ultimate lower size limit for superconducting order to exist is set by the "Anderson criterion"-arising from quantum confinement-that appears to be remarkably accurate and universal. We show that carefully grown, phase-pure, nanocrystalline bcc-Ta remains superconducting (with ordering temperature, TC ≈ 0.9 K) down to sizes 40% below the conventional estimate of the Anderson limit of 4.0 nm. Further, both the TC and the critical magnetic field exhibit an unusual, nonmonotonic size dependence, which we explain in terms of a complex interplay of quantum size effects, surface phonon softening, and lattice expansion. A quantitative estimation of TC within first-principles density functional theory shows that even a moderate lattice expansion allows superconductivity in Ta to persist down to sizes much lower than the conventional Anderson limit, which can be traced to anomalous softening of a phonon due to its coupling with electrons. This appears to indicate the possibility of bypassing the Anderson criterion by suitable crystal engineering and obtaining superconductivity at arbitrarily small sizes, an obviously exciting prospect for futuristic quantum technologies. We take a critical look at how the lattice expansion modifies the Anderson limit, an issue of fundamental interest to the study of nanoscale superconductivity.

Two-Dimensional Rectangular and Honeycomb Lattices of NbN: Emergence of Piezoelectric and Photocatalytic Properties at Nanoscale.

Using first-principles calculations, we predict that monolayered honeycomb and rectangular two-dimensional (2D) lattice forms of NbN are metastable and naturally derivable from different orientations of its rocksalt structure. While the rectangular form is shown to retain the metallic and superconducting (SC) properties of the bulk, spectacularly contrasting properties emerge in the honeycomb form of NbN: it exhibits (a) semiconducting electronic structure suitable for valleytronics and photocatalysis of water splitting, (b) piezoelectricity with a spontaneous polarization originating from a rare sd(2)-sp(2) type hybridization, and (c) a wide gap in its phonon spectrum making it suitable for use in hot carrier solar cells. Our work demonstrates how low coordination numbers and associated strong bonding stabilize 2D nanoforms of covalently bonded solids and introduce novel functionalities of technological importance.