arXiv subjects

Indrajit Maity

Publications and source records attributed to Indrajit Maity.

22 records · Page 2Linked to original sources

arXiv2019

Phonons in Twisted Transition Metal Dichalcogenide Bilayers ("Twistnonics"): Ultra-soft Phasons, and a transition from Superlubric to Pinned Phase

The tunability of the interlayer coupling by twisting one layer with respect to another layer of two-dimensional materials provides a unique way to manipulate the phonons and related properties. We refer to this engineering of phononic properties as "Twistnonics". We study the effects of twisting on low-frequency shear (SM) and layer breathing (LBM) modes in transition metal dichalcogenide (TMD) bilayer using atomistic classical simulations. We show that these low-frequency modes are extremely sensitive to twist and can be used to infer the twist angle. We find unique "ultra-soft" phason modes (frequency $\lesssim 1\ \mathrm{cm^{-1}}$, comparable to acoustic modes) for any non-zero twist, corresponding to an \textit{effective} translation of the moir{é} lattice by relative displacement of the constituent layers in a non-trivial way. Unlike the acoustic modes, the velocity of the phason modes is quite sensitive to twist angle. As twist angle decreases, ($θ\lesssim 3^{\circ},\ \gtrsim 57^{\circ}$) the ultra-soft modes represent the acoustic modes of the "emergent" soft moir{é} scale lattice. Also, new high-frequency SMs appear, identical to those in stable bilayer TMD ($θ= 0\degree/60\degree$), due to the overwhelming growth of stable stacking regions in relaxed twisted structures. Furthermore, we find remarkably different structural relaxation as $θ\to 0^{\circ}$, $\to 60^{\circ}$ due to sub-lattice symmetry breaking. Our study reveals the possibility of an intriguing $θ$ dependent superlubric to pinning behavior and of the existence of ultra-soft modes in \textit{all} two-dimensional (2D) materials.

cond-mat.mtrl-sci↗

arXiv2019

Evolution of high-frequency Raman modes and their doping dependence in twisted bilayer MoS2

Twisted van der Waals heterostructures unravel a new platform to study strongly correlated quantum phases. The interlayer coupling in these heterostructures is sensitive to twist angles ($θ$) and key to controllably tune several exotic properties. Here, we demonstrate a systematic evolution of the interlayer coupling strength with twist angle in bilayer $\mathrm{MoS_{2}}$ using a combination of Raman spectroscopy and classical simulations. At zero doping, we show a \textit{monotonic} increment of the separation between the $\mathrm{A_{1g}}$ and $\mathrm{E^{1}_{2g}}$ mode frequencies as $θ$ decreases from $10^{\circ} \to 1^{\circ}$, which saturates to that for a bilayer at small twist angles. Furthermore, using doping-dependent Raman spectroscopy we reveal $θ$ dependent softening and broadening of the $\mathrm{A_{1g}}$ mode, whereas the $\mathrm{E^{1}_{2g}}$ mode remains unaffected. Using first principles-based simulations we demonstrate large (weak) electron-phonon coupling for the $\mathrm{A_{1g}}$ ($\mathrm{E^{1}_{2g}}$) mode explaining the experimentally observed trends. Our study provides a non-destructive way to characterize the twist angle, the interlayer coupling and establishes the manipulation of phonons in twisted bilayer $\mathrm{MoS_{2}}$ (twistnonics).

cond-mat.mtrl-sci↗

arXiv2019

Kolmogorov-Crespi Potential For Multilayer Transition Metal Dichalcogenides: Capturing Structural Transformations In Moiré Superlattices

We develop parameters for the interlayer Kolmogorov-Crespi (KC) potential to study structural features of four transition metal dichalcogenides (TMDs): MoS$_2$, WS$_2$, MoSe$_2$ and WSe$_2$. We also propose a mixing rule to extend the parameters to their heterostructures. Moiré superlattices of twisted bilayer TMDs have been recently shown to host shear solitons, topological point defects and ultraflatbands close to the valence band edge. Performing structural relaxations at the DFT level is a major bottleneck in the study of these systems. We show that the parametrized KC potential can be used to obtain atomic relaxations in good agreement with DFT relaxations. Furthermore, the moiré superlattices relaxed using DFT and the proposed forcefield yield very similar electronic band structures.

cond-mat.mtrl-sci↗

arXiv2018

Temperature Dependent Layer Breathing Modes in Two Dimensional Materials

Relative out of plane displacements of the constituent layers of two dimensional materials gives rise to unique low frequency breathing modes. By computing the height-height correlation functions in momentum space, we show that, the layer breathing modes (LBMs) can be mapped consistently to vibrations of a simple linear chain model. Our calculated thickness dependence of LBM frequencies for few layer (FL) graphene and molybdenum disulphide (MoS$_{2}$) are in excellent agreement with available experiments. Our results show a redshift of LBM frequency with increase in temperature, which is a direct consequence of anharmonicities present in the interlayer interaction. We also predict the thickness and temperature dependence of LBM frequencies for FL hexagonal boron nitride (hBN). Our study provides a simple and efficient way to probe the interlayer interaction for layered materials and their heterostructures, with the inclusion of anharmonic effects.

cond-mat.mtrl-sci↗