arXiv subjects

Appalakondaiah Samudrala

Publications and source records attributed to Appalakondaiah Samudrala.

4 recordsLinked to original sources

arXiv2023

Modeling Interlayer Interactions and Phonon Thermal Transport in Silicene Bilayer

We develop an accurate interlayer pairwise potential derived from the \textit{ab-initio} calculations and investigate the thermal transport of silicene bilayers within the framework of equilibrium molecular dynamics simulations. The electronic properties are found to be sensitive to the temperature with the opening of the band gap in the $Γ$$\rightarrow$M direction. The calculated phonon thermal conductivity of bilayer silicene is surprisingly higher than that of monolayer silicene, contrary to the trends reported for other classes of 2D materials like graphene and hBN bilayers. This counterintuitive behavior of the bilayer silicene is attributed to the interlayer interaction effects and inherent buckling, which lead to a higher group velocity in the LA$_1$/LA$_2$ phonon modes. The thermal conductivity of both the mono- and bilayer silicene decreases with temperature as $κ\sim T^{-0.9}$ because of the strong correlations between the characteristic timescales of heat current autocorrelation function and temperature ($τ\sim T^{-0.75}$). The mechanisms underlying phonon thermal transport in silicene bilayers are further established by analyzing the temperature induced changes in acoustic group velocity.

cond-mat.mtrl-sci↗

arXiv2022

Electronic structure of lattice relaxed alternating twist tNG-multilayer graphene: from few layers to bulk AT-graphite

We calculate the electronic structure of AA'AA'...-stacked alternating twist N-layer (tNG) graphene for N = 3, 4, 5, 6, 8, 10, 20 layers and bulk alternating twist (AT) graphite systems where the lattice relaxations are modeled by means of molecular dynamics simulations. We show that the symmetric AA'AA'... stacking is energetically preferred among all interlayer sliding geometries for progressively added layers up to N=6. Lattice relaxations enhance electron-hole asymmetry, and reduce the magic angles with respect to calculations with fixed tunneling strengths that we quantify from few layers to bulk AT-graphite. Without a perpendicular electric field, the largest magic angle flat-band states locate around the middle following the largest eigenvalue eigenstate in a 1D-chain model of layers, while the density redistributes to outer layers for smaller magic twist angles corresponding to higher order effective bilayers in the 1D chain. A perpendicular electric field decouples the electronic structure into $N$ Dirac bands with renormalized Fermi velocities with distinct even-odd band splitting behaviors, showing a gap for N=4 while for odd layers a Dirac cone remains between the flat band gaps. The magic angle error tolerance estimated from density of states maxima expand progressively from $0.05^{\circ}$ in t2G to up to $0.2^{\circ}$ in AT-graphite, hence allowing a greater flexibility in multilayers. Decoupling of tNG into t2G with different interlayer tunneling proportional to the eigenvalues of a 1D layers chain allows to map tNG-multilayers bands onto those of periodic bulk AT-graphite's at different $k_z$ values. We also obtain the Landau level density of states in the quantum Hall regime for magnetic fields of up to 50~T and confirm the presence of nearly flat bands around which we can develop suppressed density of states gap regions by applying an electric field in N > 3 systems.

cond-mat.mes-hall↗

arXiv2022

Relaxation Effects in Twisted Bilayer Graphene: a Multi-Scale Approach

We present a multi-scale density functional theory (DFT) informed molecular dynamics and tight-binding (TB) approach to capture the interdependent atomic and electronic structures of twisted bilayer graphene. We calibrate the flat band magic angle to be at $θ_{\rm M} = 1.08^{\circ}$ by rescaling the interlayer tunneling for different atomic structure relaxation models as a way to resolve the indeterminacy of existing atomic and electronic structure models whose predicted magic angles vary widely between $0.9^\circ \sim 1.3^\circ$. The interatomic force fields are built using input from various stacking and interlayer distance dependent DFT total energies including the exact exchange and random phase approximation (EXX+RPA). We use a Fermi velocity of $\upsilon_{\rm F} \simeq 10^{6}$~m/s for graphene that is enhanced by about $\sim 15\%$ over the local density approximation (LDA) values. Based on this atomic and electronic structure model we obtain high-resolution spectral functions comparable with experimental angle-resolved photoemission spectra (ARPES). Our analysis of the interdependence between the atomic and electronic structures indicates that the intralayer elastic parameters compatible with the DFT-LDA, which are stiffer by $\sim$30\% than widely used reactive empirical bond order force fields, can combine with EXX+RPA interlayer potentials to yield the magic angle at $\sim 1.08^{\circ}$ without further rescaling of the interlayer tunneling.

cond-mat.mes-hall↗

arXiv2020

Ultra-high-resolution imaging of moiré lattices and superstructures using scanning microwave impedance microscopy under ambient conditions

Two-dimensional heterostructures with layers of slightly different lattice vectors exhibit a new periodic structure known as moire lattices. Moire lattice formation provides a powerful new way to engineer the electronic structure of two-dimensional materials for realizing novel correlated and topological phenomena. In addition, superstructures of moire lattices can emerge from multiple misaligned lattice vectors or inhomogeneous strain distribution, which offers an extra degree of freedom in the electronic band structure design. High-resolution imaging of the moire lattices and superstructures is critical for quantitative understanding of emerging moire physics. Here we report the nanoscale imaging of moire lattices and superstructures in various graphene-based samples under ambient conditions using an ultra-high-resolution implementation of scanning microwave impedance microscopy. We show that, quite remarkably, although the scanning probe tip has a gross radius of ~100 nm, an ultra-high spatial resolution in local conductivity profiles better than 5 nm can be achieved. This resolution enhancement not only enables to directly visualize the moire lattices in magic-angle twisted double bilayer graphene and composite super-moire lattices, but also allows design path toward artificial synthesis of novel moire superstructures such as the Kagome moire from the interplay and the supermodulation between twisted graphene and hexagonal boron nitride layers.

cond-mat.mes-hall↗