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Center for Informatics

The importance of atomic structure to understand the superconducting β-Tungsten (W) thin films especially their critical transition temperature (Tc) and upper critical magnetic field Hc2(0) is reported. Here, Tc and Hc2(0) for a low thickness (~ 35 nm) film are found to be ~ 3.14 K and 10.72 T, whereas for a comparatively thicker (~ 60 nm) film, these are 1.38 K and 2.34 T, respectively. The β-phase is identified by grazing-incidence X-ray diffraction. The microstructure is probed by transmission electron microscopy, where high-resolution images manifest an improvement in crystallinity of the nanoscale grains in a disordered matrix with increasing film thickness. A drastic change in atomic structure corresponding to two different thicknesses is found to be associated with the change in oxygen concentration. Particularly, the existence of amorphous-W in the presence of β-W is revealed in 35 nm film, which is further supported by ab-initio calculations. Based on the atomic and microstructure, superconducting properties of the β-W films are analyzed and discussed. Graphical abstract: Correlation between the superconducting transition temperature and microstructure with varying β-tungsten film thickness [Figure not available: see fulltext.] © 2022, The Author(s), under exclusive licence to The Materials Research Society.

World University root node

Physics

(SoNS) School of Natural Sciences

Journal of Materials Research

Atomic scale microstructural insights of superconducting β-tungsten thin films

The A15 β phase of tungsten has recently attracted great interest for spintronic applications due to the finding of giant spin-Hall effect. As β phase is stabilized by oxygen, we have studied the electronic structure of O-doped β-W from first principles calculations. It is found that 20 at.% O-doping makes β phase lower in energy than α-W. These results are in good agreement with energy dispersive X-ray spectroscopy which also shows ~ 16.84 at.% O in 60 nm thick W films. The latter has predominantly β phase as confirmed by grazing incidence X-ray diffraction (XRD). The simulated XRD of bulk β having 15.79 at.% O also agrees with XRD results. Oxygen binds strongly on the surface and affects the Dirac fermion behavior in pure β-W. There is structural disorder, O-inhomogeneity, and higher density-of-states in O-doped β-W at EF compared with pure α. These results are promising to understand the properties of β-W. © 2022, The Author(s).

Fulltext

Scientific Reports

Crucial role of oxygen on the bulk and surface electronic properties of stable β phase of tungsten

We study the structural stability and electronic properties of A3B (A = Cr, Mo, and W; B = Al, Ga, In, Si, Ge, Sn, and Be) compounds using ab initio pseudopotential method with Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) for the exchange-correlation functional. It is found that Cr3Be, Cr3Al, Cr3Ga, Cr3Si, Cr3Ge, Mo3Be, Mo3Al, Mo3Ga, Mo3Si, Mo3Ge, and W3Al compounds favor the A15 phase with an exothermic formation energy. In some other cases, the A15 phase has the lowest energy, but the formation energy is endothermic. The stability of these compounds remains unaffected after considering spin-orbit coupling. The charge transfer, electronic density of states, and band structure in each case have been analyzed. The Bader charge analysis shows a large variation in the electronic charge on A atoms. Further we have studied the stability of A15 phase of W3M compounds with M, a transition metal such as Ru, Ta, Re, Os, Ir, and also Au, some of which have recently attracted attention for spintronics applications. A comparison of the results and the occurrence of Dirac fermion behavior in β-W as well as the effects due to the compound formation and spin-orbit coupling have been discussed. © 2022 Elsevier B.V.

Physica B: Condensed Matter

Ab initio study of the structural stability and Dirac fermion behaviour in A3B (A = Cr, Mo, W; B = Al, Ga, In, Si, Ge, Sn, Be) and W3M, M = Ru, Ta, Re, Os, Ir, Au compounds

Assemblies of clusters have been sought for a long time to synthesize new materials with unprecedented physical phenomena or to integrate desired functionalities for technological applications. However, except for some carbon fullerenes and ligated clusters, little progress has been made in achieving assemblies of other clusters due to their tendency for agglomeration. Here we study interaction in dimers of 34 well-studied endohedrally doped clusters (i.e., superatoms) and propose the criteria for such superatoms to be potential building blocks in terms of surface coordination, charge state, distribution of the highest occupied molecular orbital and electronic as well as atomic shell (double-shell) closure. From these results, the endohedrally doped Ti@Ge16 cage cluster stands out as a suitable building block to assemble solids and nanostructures with outstanding stabilities and diverse physical properties. We report here the finding of antiferromagnetic Mott insulator in metal intercalated two-dimensional crystal of such cluster. Our study provides essential knowledge for achieving stable cluster assemblies of different dimensionalities with precisely tailorable electronic structure for device applications.

Physical Review Materials

Searching for cluster Lego blocks for three-dimensional and two-dimensional assemblies

Recent experiments on chemical tuning of spacer cations (phenylethylammonium: PEA) with electronegative entities such as Cl or F in 2D-3D Ruddlesden-Popper (RP) perovskite layers show simultaneous improvement in their device performance and stability. In order to understand the role of such substitution on the properties and stability of 2D and 2D-3D layers of PEA2PbI4 with engineered spacer cations, here we present results of state-of-the-art ab initio calculations. It is found that Cl and F substitution in spacer cations does not affect much the inorganic framework but increases the stability of the layered compounds. This enhancement in stability is attributed to strong van der Waals interactions particularly in the case of Cl substitution. Moreover, a rotation of the phenyl ring does not significantly affect the electronic and optical properties of the 2D-3D RP perovskite layers. However, the chemical tuning of PEA spacer cations results in a moderate blue shift in the optical absorption spectra. Further study on F- and Cl-substituted PEA2PbBr4 layers show their better stability than in the case of iodides and the possibility of applications in blue light-emitting diodes. Our results are in very good agreement with the available experimental findings and pave the way toward a better understanding of the more stable RP perovskite layers for photovoltaic and optoelectronic applications. ©

ACS Applied Energy Materials

Effects of Cl and F Substitution in Phenylethylammonium Spacer Cations on Stability, Structure, and Optical Properties of 2D-3D Ruddlesden-Popper Perovskite Layers

We present results of a state-of-the-art computational study of the atomic and electronic structure of Cs2n+2MnMn′X6n+2 (M=Cu, Ag; M′=In, Bi; X=Cl, Br, I) layers with up to three-unit-cell thickness (n=3) as well as their bulk counterparts in the search for economical and stable halide double perovskites (HDPs) with a direct band gap and strong light absorption. Among the 24 layers we have studied, seven are found to be suitable for light emitting diodes, whereas one is proposed for solar cell applications. Our results on bulk Cs2AgM′X6 (M′=In, Bi; X=Cl, Br) HDPs agree with previous studies. Interestingly, we find that HDP layers as well as bulk are more stable than their respective lead-halide perovskites. The Cu-based HDPs are cost effective compared with those based on Ag and are found to be equally suitable as absorber materials with a lower band gap. However, bulk Cs2CuInX6 (X=Cl, Br) HDPs are found to be dynamically unstable. Our calculations show a combined effect of confinement and spin-orbit coupling in deciding the nature of the band gap for one (two)-unit-cell thick layers of Cs4AgBiX8 (Cs8Cu2Bi2X14). Furthermore, strong confinement effects are found to be crucial in making the thinnest layers of Cs4CuBiX8 (X=Cl, Br) direct band gap semiconductors. The reduction in the dimension from three to two dimensions introduces distortions in the layers of the HDPs, and it is more in the case of the Cu-based layers. The nature of the band gap becomes direct for very thin layers of Cs2n+2AgnBinX6n+2 (n=1) and Cs2n+2CunBinX6n+2 (n=1, 2) compared with indirect band gap in bulk. The HDP layers with a direct band gap and enhanced stability also have a strong absorption coefficient of the order of 105cm-1, making them interesting for solar cell applications. The effects of surfaces on the electronic structure of direct band gap layers of Cs2n+2CunBinBr6n+2 (n=1, 2) and Cs2n+2AgnInnCl6n+2 (n=3) are also shown with surface states. We hope that our results on HDPs will stimulate further research on these materials. © 2021 American Physical Society.

Direct band gap halide-double-perovskite absorbers for solar cells and light emitting diodes: Ab initio study of bulk and layers

In Chemo & Bioinformatics problems, machine learning algorithms have been found to be very useful. In these problems, both supervised and unsupervised techniques are frequently employed. For example, in protein function annotation, it is possible to extract different types of features including sequence-based, structure-based, expression profile-based, composition-based, etc. In these large compendia of features, many may be noisy and may not contain information related to the function annotation problem at hand. With the presence of these irrelevant and noisy features, the classifier or regressor may not be able to build an optimal model and the performance may not be very good. This is because the model finds it difficult to differentiate between signal and noise. With the increase in the information content of the data set, the prediction accuracy will also increase and provide better generalization. Over fitting will also reduce with increase in informative features. The computational complexity will decrease, and the speed of training and testing will increase manifold. In bioinformatics, model interpretability increases with the usage of a small subset of relevant features and may provide important domain information in the form of identifiable biomarkers. Due to the importance of feature selection, several methodologies are available for feature selection. In this chapter, we discuss in detail different classes of feature selection methods and algorithms and their applications in Chemo & Bioinformatics. Feature selection techniques can be broadly classified as filter, wrapper, and embedding techniques. © 2021 River Publishers.

Advanced Applications of Computational Mathematics

Variable Selection in Bioinformatics: Methods and Algorithms

Bioinformatics and Human Genomics Research

Protein function prediction: Human sequences and structures

We report the results of density functional theory calculations on the atomic and electronic structure of solids formed by assembling A2B2PN (A = Ge and Sn, B = Cl, Br, and I) inorganic double helices. The calculations have been performed using a generalized gradient approximation for the exchange-correlation functional and including van der Waals interactions. Our results show that the double helices crystallize in a monoclinic lattice with van der Waals type weak interactions between the double helices. In all cases except Ge2Cl2PN, the solids are stable with a binding energy between the double helices ranging from 0.06 eV per atom to 0.09 eV per atom and inter-double helices separation of more than 3.33 Å. All the solids are semiconducting. Further calculations have been done by using meta-GGA with a modified Becke-Johnson functional to obtain better band gaps, which are found to lie in the range of 0.91 eV to 1.49 eV. In the case of Ge2Br2PN the solid is a direct band gap semiconductor although the isolated double helix has an indirect band gap and it is suggested to be interesting for photovoltaic, and other optoelectronic applications. The charge transfer between the atoms has been studied using Bader charge analysis and the DDEC6 method in the CHARGEMOL program, which suggests charge transfer from the outer helix to the inner helix. This journal is © 2020 The Royal Society of Chemistry.