Abstract: We study the magnetic properties of platinum diselenide (PtSe2) intercalated with Ti, V, Cr, and Mn, using first-principle density functional theory (DFT) calculations and Monte Carlo (MC) simulations. First, we present the equilibrium position of intercalants in PtSe2 obtained from the DFT calculations. Next, we present the magnetic groundstates for each of the intercalants in PtSe2 along with their critical temperature. We show that Ti intercalants result in an in-plane AFM and out-of-plane FM groundstate, whereas Mn intercalant results in in-plane FM and out-of-plane AFM. V intercalants result in an FM groundstate both in the in-plane and the out-of-plane direction, whereas Cr results in an AFM groundstate both in the in-plane and the out-of-plane direction. We find a critical temperature of <0.01 K, 111 K, 133 K, and 68 K for Ti, V, Cr, and Mn intercalants at a 7.5% intercalation, respectively. In the presence of Pt vacancies, we obtain critical temperatures of 63 K, 32 K, 221 K, and 45 K for Ti, V, Cr, and Mn-intercalated PtSe2, respectively. We show that Pt vacancies can change the magnetic groundstate as well as the critical temperature of intercalated PtSe2, suggesting that the magnetic groundstate in intercalated PtSe2 can be controlled via defect engineering.

Abstract: Van der Waals (vdW) heterostructures continue to attract intense interest as a route of designing materials with novel properties that cannot be found in nature. Unfortunately, this approach is currently limited to only a few layers that can be stacked on top of each other. Here, we report a bulk vdW material consisting of superconducting 1H TaS2 monolayers interlayered with 1T TaS2 monolayers displaying charge density waves (CDW). This bulk vdW heterostructure is created by phase transition of 1T-TaS2 to 6R at 800 degrees C in an inert atmosphere. Its superconducting transition (T-c) is found at 2.6 K, exceeding the T-c of the bulk 2H phase. Using first-principles calculations, we argue that the coexistence of superconductivity and CDW within 6R-TaS2 stems from amalgamation of the properties of adjacent 1H and 1T monolayers, where the former dominates the superconducting state and the latter the CDW behavior.

Abstract: Electrenes, an atomically thin form of layered electrides, are very recent members of the 2D materials family. In this work, we employed first principle calculations to determine stable, exfoliatable, and application-promising 2D electrene materials among possible M2X compounds, where M is a group II-A metal and X is a nonmetal element (C, N, P, As, and Sb). The promise of stable electrene compounds for battery applications is assessed via their exfoliation energy, adsorption properties, and migration energy barriers toward relevant Li, Na, K, and Ca atoms. Our calculations revealed five new stable electrene candidates in addition to previously known Ca2N and Sr2N. Among these seven dynamically stable electrenes, Ba2As, Ba2P, Ba2Sb, Ca2N, Sr2N, and Sr2P are found to be very promising for either K or Na ion batteries due to their extremely low migration energy barriers (5-16 meV), which roughly demonstrates 105 times higher mobility than graphene and two to four times higher mobility than other promising 2D materials such as MXene (Mo2C).

Abstract: We present here a new interatomic potential parameter set to predict the thermal conductivity of zirconium trisulfide monolayers. The generated Tersoff-type force field is parameterized using data collected with first-principles calculations. We use non-equilibrium molecular dynamics simulations to predict the thermal conductivity. The generated parameters result in very good agreement in structural, mechanical, and dynamical parameters. The room temperature lattice thermal conductivity ( kappa) of the considered crystal is predicted to be kappa x x = 25.69Wm – 1K – 1 and kappa y y = 42.38Wm – 1K – 1, which both agree well with their corresponding first-principles values with a discrepancy of less than 5%. Moreover, the calculated kappa variation with temperature (200 and 400 K) are comparable within the framework of the accuracy of both first-principles and molecular dynamics simulations.

Abstract: Using the first-principles calculations, we explore the structural and novel electronic/optical properties of the C2N nanosheet. To this goal, we systematically investigate the affect of layer thickness, electrical field and strain on the electronic properties of the C2N nanosheet. By increasing the thickness of C2N, we observed that the band gap decreases. Moreover, by applying an electrical field to bilayer C2N, the band gap decreases and a semiconductor-to-metal transition can occur. Our results also confirm that uniaxial and biaxial strain can effectively alter the band gap of C2N monolayer. Furthermore, we show that the electronic and magnetic properties of C2N can be modified by the adsorption and substitution of various atoms. Depending on the species of embedded atoms, they may induce semiconductor (O, C, Si and Be), metal (S, N, P, Na, K, Mg and Ca), dilute-magnetic semiconductor (H, F, B), or ferro-magnetic-metal (Cl, Li) character in C2N monolayer. It was also found that the inclusion of hydrogen or oxygen impurities and nitrogen vacancies, can induce magnetism in the C2N monolayer. These extensive calculations can be useful to guide future studies to modify the electronic/optical properties of two-dimensional materials. (C) 2019 Elsevier Ltd. All rights reserved.

Abstract: The hydrogen interstitial and the substitutional AlZn, GaZn, and InZn are all shallow donors in ZnO and lead to n-type conductivity. Although shallow donors are expected to repel each other, we show by first-principles calculations that in ZnO these shallow donor impurities attract and form a complex, leading to a donor level deep in the band gap. This puts a limit on the n-type conductivity of (Al,Ga,In)-doped ZnO in the presence of hydrogen.

Abstract: The electronic structure of disk-shaped InP/InGaP self-assembled quantum dots is calculated within the effective-mass theory. The strain-dependent 6x6 multiband Hamiltonian for the valence band is simplified into an axially symmetric form. Both the continuum mechanical model, discretized by finite elements, and the isotropic model are used to calculate the strain distribution and their results are critically compared. The dependence of the electron and the hole energy levels on the dimensions of the quantum dot is investigated. We found that both the electron and hole energies are underestimated if the strain distribution is calculated by the isotropic elasticity theory. The agreement between the electron energies for the two approaches is better for thinner quantum dots. The heavy holes are confined inside the quantum dot, while the light holes are located outside the disk, but confined by the strain field near the edge of the disk periphery. We found that the (h) over bar /2 hole ground state crosses the 3 (h) over bar /2 ground state when the height of the quantum dot increases and becomes the ground state for sufficiently thick quantum disks. The higher hole levels exhibit both crossings between the states of the different parity and anticrossings between the states of the same parity.

Abstract: We propose a highly efficient numerical method to describe inhomogeneous superconductivity by using the kernel polynomial method in order to calculate the Greens functions of a superconductor. Broken translational invariance of any type (impurities, surfaces, or magnetic fields) can be easily incorporated. We show that limitations due to system size can be easily circumvented and therefore this method opens the way for the study of scenarios and/or geometries that were unaccessible before. The proposed method is highly efficient and amenable to large scale parallel computation. Although we only use it in the context of superconductivity, it is applicable to other inhomogeneous mean-field theories.

Abstract: The electronic and optical properties of InAs/GaSb core-shell nanowires are investigated within the effective mass k . p approach. These systems have a broken band gap, which results in spatially separated confinement of electrons and holes. We investigated these structures for different sizes of the InAs and GaSb core and shell radius. We found that for certain configurations, the conduction band states penetrate into the valence band states resulting in a negative band gap (E-g < 0), which leads to a conduction band ground state that lies below the valence band ground state at the Gamma point. For certain core-shell wires, only one conduction band state penetrates into the valence band and in this case, a minigap Delta opens up away from the Gamma point and as a consequence the electronic properties of the nanowire now depend on both E-g and Delta values.

Abstract: The spin interaction and the effective g factor of a magnetic exciton (ME) are investigated theoretically in a diluted magnetic semiconductor (DMS) quantum dot (QD), including the Coulomb interaction and the sp-d exchange interaction. At low magnetic field, the ME energy decreases rapidly with increasing magnetic field and saturates at high magnetic field for high Mn concentration. The ground state of the ME exhibits an interesting crossing behavior between sigma(+)-ME and sigma(-)-ME for low Mn concentration. The g(ex) factor of the ME in a DMS QD displays a monotonic decrease with increasing magnetic field and can be tuned to zero by an external magnetic field. (C) 2003 American Institute of Physics.

Abstract: Using the tight-binding formalism we show that in the recently experimentally realized AA-stacked graphite in essence two types of massless relativistic Dirac particles are present with a different effective speed of light. We also investigate how the electronic structure evolves from a single graphene sheet into AA-stacked graphite. It is shown that in contrast to AB-stacked graphene layers, the spectrum of AA-stacked graphene layers can be considered as a superposition of single-layer spectra and only particles with a linear spectrum at the Fermi energy around the K point are present. From the evolution of the band overlap we show that 6 multilayers of AA-stacked graphene already behave as AA-stacked graphite. The evolution of the effective speeds of light of the Dirac particles to their bulk values shows exactly the same behavior. The tight-binding parameters we use to describe AA-stacked graphite and multilayers of graphene are obtained by ab initio calculations.

Abstract: The effects of interface optical-phonon and confined slab LO-phonon modes on the polaron cyclotron-resonance frequency are investigated for a GaAs/AlAs quantum well. Using degenerate second-order perturbation theory, the polaron Landau levels are calculated and the polaron resonant region is investigated. In order to know the relative importance of the different resonant frequencies we present a full calculation of the magneto-optical absorption spectrum. At a fixed magnetic field we found four different peaks in the absorption spectrum. The relative oscillator strength of the different peaks changes with increasing magnetic field. For comparative purposes, the polaron Landau levels and cyclotron mass are also calculated using only the bulk LO-phonon modes. The influence of the finiteness of the confinement potential is investigated. We found that the interface-phonon modes influence the magnetopolaron resonance considerably near the optical-phonon frequencies for narrow wells. In the limit of zero magnetic field we recover our previous results and in the case of an infinite-barrier quantum well we are able to recover the results for a two- and three-dimensional system.

Abstract: We investigate theoretically the transport properties of Dirac fermions on the surface of a three-dimensional topological insulator. Dirac electrons can be totally reflected in front of a magnetic/electric p-n junction. For a p-n-p structure, multiple total internal reflections at the interfaces result in the bound states in the channel, which behaves like an electronic waveguide. This p-n-p like structure exhibits spin and momentum filtering features and could be used as a spin and/or charge diode.