arXiv subjects

Thaneshwor P. Kaloni

Publications and source records attributed to Thaneshwor P. Kaloni.

8 recordsLinked to original sources

arXiv2020

Site dependency of the magnetism for Mn adsorption on MgO/Ag(001): a combined DFT$+U$ and STM investigation

Theoretical and experimental investigation of the electronic and magnetic structure of transition metal atoms on an insulating interface with a metallic substrate at low temperatures is quite challenging. In this paper, we show a density functional theory plus Hubbard $U$ based protocol to study an Mn adatom on three symmetrically allowed absorption sites, namely on O, hollow (between two Mg and two O) and on Mg on MgO(001). We added a thick enough(bulk-like) metallic Ag slab beneath MgO for faithful replication of scanning tunneling microscopy (STM) experiments. Our study reveals that to determine the stable most binding site, we need to obtain a Hubbard $U$ value from the density functional theory(DFT) calculations, and our results show agreement with STM experiments. Our calculated Hubbard $U$ values for the three adatom sites are different. When Mn sits on O, it retains 2.4 $μ_B$ spin moment, close to its atomic spin moment. However, when Mn sits on other adatom sites, the spin moment decreases. Using the atom projected density of states, we find Mn on O atom shows very narrow crystal field splitting among \textit{d} orbitals, whereas on other adatom sites Mn \textit{d} orbitals show significant splitting. We calculate charge and spin densities and show vertical and horizontal propagation of charge and spin density varies widely between sites. Mn on Mg top shows an unusual feature, that Mn pushes Mg below, to make a coplanar of Mn geometry with the four oxygen atoms. In addition, we explained the reason for spin leaking down to the Ag layers. Mn on MgO/Ag does not show any spin-flip behavior in the STM, an unusual phenomenon, and we used our first principles-based approach to explain this unique observation.

cond-mat.mtrl-sci↗

arXiv2019

From atomic layer to the bulk: low-temperature atomistic structure, ferroelectric and electronic properties of SnTe films

SnTe hosts ferroelectricity that competes with its weak non-trivial band topology: in the high-symmetry rocksalt structure--in which its intrinsic electric dipole is quenched--this material develops metallic surface bands, but in its rhombic ground-state configuration--that hosts a non-zero spontaneous electric dipole--the crystalline symmetry is lowered and the presence of surface electronic bands is not guaranteed. Here, the type of ferroelectric coupling and the atomistic and electronic structure of SnTe films ranging from 2 to 40 ALs are examined on freestanding samples, to which atomic layers were gradually added. 4 AL SnTe films are antiferroelectrically-coupled, while thicker freestanding SnTe films are ferroelectrically-coupled. The electronic band gap reduces its magnitude in going from 2 ALs to 40 ALs but it does not close due to the rhombic nature of the structure. These results bridge the structure of SnTe films from the monolayer to the bulk.

cond-mat.mes-hall↗

arXiv2018

Water splits to degrade two-dimensional group-IV monochalcogenides in nanoseconds

The experimental exfoliation of layered group-IV monochalcogenides --semiconductors isostructural to black phosphorus-- using processes similar to those followed in the production of graphene or phosphorene has turned out unsuccessful thus far, as if the chemical degradation observed in black phosphorus was aggravated in these monochalcogenides. Here, we document a facile dissociation of water by these materials within ten nanoseconds from room-temperature Car-Parrinello molecular dynamics calculations under standard temperature and pressure conditions. These results suggest that humidity must be fully eradicated to exfoliate monolayers successfully, for instance, by placing samples in a hydrophobic solution during mechanical exfoliation. From another materials perspective, these two-dimensional materials that create individual hydrogen ions out of water without illumination may become relevant for applications in hydrogen production and storage.

cond-mat.mtrl-sci↗

arXiv2018

Exfoliation energy, quasi-particle bandstructure, and excitonic properties of selenium and tellurium atomic chains

Effects that are not captured by the generalized-gradient density-functional theory play a prominent effect on the structural binding, and on the electronic and optical properties of reduced-dimensional and weakly-bound materials. Here, we report the exfoliation energy of selenium and tellurium atomic chains with non-empirical van der Waals corrections, and their electronic and optical properties with the GW and Bethe-Salpeter formalisms. The exfoliation energy is found to be within 0.547 to 0.719 eV/u.c. for the selenium atomic chain, and 0.737 to 0.926 eV/u.c. for the tellurium atomic chain (u.c. stands for unit cell), depending on the approximation for the van der Waals interaction and the numerical tool chosen. The GW electronic bandgap turned out to be 5.22--5.47 (4.44--4.59) eV for the Se (Te) atomic chains, with the lowest bound obtained with the Godby-Needs (GB), and the upper bound to the Hybertsen-Louie (HL) plasmon-pole models (PPMs). The binding energy of the ground-state excitonic state ranges between 2.69 to 2.72 eV for selenium chains within the HL and GB PPM, respectively, and turned out to be 2.35 eV for tellurium chains with both approximations. The ground state excitonic wave function is localized within 50 Å along the axis for both types of atomic chains, and its energy lies within the visible spectrum: blue [2.50(GN)--2.78(HL) eV] for selenium, and yellow--green [2.09(GN)--2.28(HL) eV] for tellurium, which could be useful for LED applications in the visible spectrum.

cond-mat.mes-hall↗

arXiv2017

Tuning the ferro- to para-electric transition temperature and dipole orientation of group-IV monochalcogenide monolayers

Coordination-related, two-dimensional (2D) structural phase transitions are a fascinating and novel facet of two-dimensional materials with structural degeneracies. Nevertheless, a unified theoretical account of these transitions remains absent, and the following points are established through {\em ab-initio} molecular dynamics and 2D discrete clock models here: Group-IV monochalcogenide (GeSe, SnSe, SnTe, ...) monolayers have four degenerate structural ground states, and a 2D phase transition from a three-fold coordinated onto a five-fold coordinated structure takes place at finite temperature. On unstrained samples, the 2D phase transition requires lattice parameters to freely evolve. A fundamental energy scale permits understanding this transition. The transition temperature $T_c$ and the orientation of the in-plane intrinsic electric dipole can be controlled by moderate uniaxial tensile strain, and a modified discrete clock model describes the transition on strained samples. These results establish a general underlying theoretical background to understand structural phase transitions in 2D materials and their effects on material properties.

cond-mat.mes-hall↗

arXiv2016

Materials properties of out-of-plane heterostructures of MoS$_2$-WSe$_2$ and WS$_2$-MoSe$_2$

Based on first-principles calculations, the materials properties (structural, electronic, vibrational, and optical properties) of out-of-plane heterostructures formed from the transiti on metal dichalcogenides, specifically MoS$_2$-WSe$_2$ and WS$_2$-MoSe$_2$ were investigated. The heterostructures of MoS$_2$-WSe$_2$ and WS$_2$-MoSe$_2$ are found to be direct and ind irect band gap semiconductors, respectively. However, a direct band gap in the WS$_2$-MoSe$_2$ heterostructure can be achieved by applying compressive strain. Furthermore, the excitoni c peaks in both monolayer and bilayer heterostructures are calculated to understand the optical behavior of these systems. The suppression of the optical spectrum with respect to the c orresponding monolayers is due to interlayer charge transfer. The stability of the systems under study is confirmed by performing phonon spectrum calculations.

cond-mat.mtrl-sci↗

arXiv2015

Electrically Engineered Band Gap in Two-Dimensional Ge, Sn, and Pb: A First-Principles and Tight-Binding Approach

First-principles calculations were performed to investigate the electronic structure of two-dimensional (2-D) Ge, Sn, and Pb without and with the presence of an external electric field in combination with spin-orbit coupling. Tight-binding calculations based on four orbitals per atom and an effective single orbital are presented to match with the results obtained from first-principles calculations. In particular, the electronic band structure and the band splitting are investigated with both models. Moreover, the simple $k\cdot p$ model is also considered in order to understand the band splitting in the presence of an external electric field and spin-orbit coupling. A large splitting is obtained, which is expected to be useful for spintronic devices. The fair agreement between the first-principle, $k\cdot p$ model, and tight-binding approaches leads to a table of parameters for future tight-binding studies on hexagonal 2-D nanostructures. By using the tight binding parameters, the transport properties of typical 0-D triangular quantum dots between two semi-infinite electrodes in the presence of spin-orbit coupling are addressed.

cond-mat.mtrl-sci↗

arXiv2014

Large Enhancement and Tunable Band Gap in Silicene by Small Organic Molecule Adsorption

Adsorption of eight organic molecules (acetone, acetonitrile, ammonia, benzene, methane, methanol, ethanol, and toluene) onto silicene has been investigated using van der Waals density functional theory calculations (DFT-D). The calculated values of the adsorption energies vary from $-0.11$ eV to $-0.95$ eV. Quantitatively, these values are higher than the corresponding adsorption energies of the molecules adsorbed on graphene. In addition, electronic structure calculations have been performed. The obtained values of the band gap range from 0.006 eV to 0.35 eV for acetonitrile to acetone, respectively. Furthermore, the effective mass of the electron is estimated and found to be comparatively small, which is expected to result in high electron mobility. In addition, we study the effect of Li atoms doped in pristine and acetone adsorbed silicene. In particular, we focus on the variation of the adsorption energy with respect to the number of Li atoms in the systems. Our results suggest new approaches for the use of silicene molecular-based energy storage and conversion as well as electronic devices.

cond-mat.mes-hall↗