arXiv subjects

Srivani Javvaji

Publications and source records attributed to Srivani Javvaji.

6 recordsLinked to original sources

arXiv2026

Atomistic substrate relaxation effects in the band gaps of graphene on hexagonal boron nitride

We assess the impact of atomistic substrate lattice relaxation schemes in the primary band gap at charge neutrality and the secondary valence band gap of graphene on hexagonal boron nitride (G/h-BN) as a function of twist angle. For zero twist angle, the primary gap decreases from $\sim 30$~meV in fully relaxed suspended G/h-BN bilayers, to $\sim 9$~meV when the remote h-BN substrate layer is kept rigid, and down to $\sim 3$~meV in completely rigid structures. In the presence of relaxations, the primary gap shows a maximum near $\sim 0.6^{\circ}$ coinciding with energetic stabilization due to alignment between the moiré pattern and the graphene lattice vectors, while the secondary valence band gap drops from $\sim 12$~meV down to zero beyond twist angles of $\sim 1^{\circ}$. A small but finite primary gap on the order of $\sim 1$~meV, with a mass sign favoring electronic occupation of carbon atop boron, persists across twist angles from $0^{\circ}$ to $30^{\circ}$ for all sliding configurations, and switches sign for twist angles between $30^{\circ}$ and $60^{\circ}$.

cond-mat.mes-hall↗

arXiv2024

Ab initio tight-binding Models for Mono- and Bilayer Hexagonal Boron Nitride (h-BN)

Hexagonal boron nitride ($\it h$-BN) exhibits dominant $π$-bands near the Fermi level, similar to graphene. However, unlike graphene, where tight-binding (TB) models accurately reproduce band edges near the $K$ and $K^{\prime}$ points in the Brillouin zone, a wider bandgap in $\it h$-BN necessitates capturing the band edges at both the $K$ and $M$ points for precise bandgap calculations. We present effective TB models derived from $\it ab initio$ calculations using maximally localized Wannier functions (MLWFs) centered on boron and nitrogen sites. These models consider hopping terms of up to four distant neighbors and achieve excellent agreement with $\it ab initio$ results near the $K$ and $M$ points. Furthermore, we compare the band structures from our simplified models with those obtained from $\it ab initio$ calculations and the full tight-binding model to assess their accuracy. To account for the effects of strains, we introduce fitting parametrizations that relate the hopping parameters of the effective TB model to the lattice constant and interlayer distance. Additionally, we utilize the two-center approximation to calculate the interlayer hopping energies based on the relative distances between sublattices to generalize the interlayer hopping parameters across different stacking configurations. We demonstrate the effectiveness of this method by comparing the electronic structure of zero-twist and twisted $\it h$-BN systems with $\it ab initio$ calculations.

cond-mat.mtrl-sci↗

arXiv2024

Moiré flat bands and antiferroelectric domains in lattice relaxed twisted bilayer hexagonal boron nitride under perpendicular electric fields

Local interlayer charge polarization of twisted bilayer hexagonal boron nitride (t2BN) is calculated and parametrized as a function of twist angle and perpendicular electric fields through tight-binding calculations on lattice relaxed geometries Lattice relaxations tend to increase the bandwidth of the nearly flat bands, where widths smaller than 1 meV are expected for angle less than 1.08 degree for parallel BN/BN alignment, and for angle less than 1.5 degree for the antiparallel BN/NB alignment. Local interlayer charge polarization maxima of 2.6 pC/m corresponding are expected at the AB and BA stacking sites of BN/BN aligned t2BN in the long moire period limit for angle less than 1 degree, and evolves non-monotonically with a maximum of 3.5 pC/m at angle equal to 1.6 degree before reaching 2 pC/m for angle equal to 6 degree. The electrostatic potential maxima due to the t2BN are overall enhanced by 20 percentage with respect to the rigid system assuming potential modulation depths of up to 300 mV near its surface. In BN/BN aligned bilayers the relative areas of the AB or BA local stacking regions can be expanded or reduced through a vertical electric field depending on its sign.

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↗

arXiv2019

Topological flat bands without magic angles in massive twisted bilayer graphenes

Twisted bilayer graphene (TBG) hosts nearly flat bands with narrow bandwidths of a few meV at certain {\em magic} twist angles. Here we show that in twisted gapped Dirac material bilayers, or massive twisted bilayer graphenes (MTBG), isolated nearly flat bands below a threshold bandwidth $W_c$ are expected for continuous small twist angles up to a critical $θ_c$ depending on the flatness of the original bands and the interlayer coupling strength. Narrow bandwidths of $W \lesssim $30 meV are expected for $θ\lesssim 3^{\circ} $ for twisted Dirac materials with intrinsic gaps of $\sim 2$ eV that finds realization in monolayers of gapped transition metal dichalcogenides (TMDC), silicon carbide (SiC) among others, and even narrower bandwidths in hexagonal boron nitride (BN) whose gaps are $\sim 5$ eV, while twisted graphene systems with smaller gaps of a few tens of meV, e.g. due to alignment with hexagonal boron nitride, show vestiges of the magic angles behavior in the bandwidth evolution. The phase diagram of finite valley Chern numbers of the isolated moire bands expands with increasing difference between the sublattice selective interlayer tunneling parameters. The valley contrasting circular dichroism for interband optical transitions is constructive near $0^{\circ}$ and destructive near $60^{\circ}$ alignments and can be tuned through electric field and gate driven polarization of the mini-valleys. Combining massive Dirac materials with various intrinsic gaps, Fermi velocities, interlayer tunneling strengths suggests optimistic prospects of increasing $θ_c$ and achieving correlated states with large $U/W$ effective interaction versus bandwidth ratios.

cond-mat.mes-hall↗

arXiv2018

Pressure Induced Compression of Flatbands in Twisted Bilayer Graphene

We investigate the bandwidth compression due to out of plane pressure of the moire flatbands near charge neutrality in twisted bilayer graphene for a continuous range of small rotation angles of up to $\sim2.5^{\circ}$. The flatband bandwidth minima angles are found to grow linearly with interlayer coupling ω and decrease with Fermi velocity. Application of moderate pressure values of up to 2.5 GPa achievable through a hydraulic press should allow accessing a flatband for angles as large as $\sim 1.5$^{\circ}$ instead of $\sim 1 \circ$ at zero pressure. This reduction of the moiré pattern length for larger twist angle implies an increase of the effective Coulomb interaction scale per moire cell by about 50% and enhance roughly by a factor of $\sim 2$ the elastic energy that resists the commensuration strains due to the moire pattern. Our results suggest that application of pressure on twisted bilayer graphene nanodevices through a hydraulic press will notably facilitate the device preparation efforts required for exploring the ordered phases near magic angle flatbands.

cond-mat.mes-hall↗