arXiv subjects

Ayan Mondal

Publications and source records attributed to Ayan Mondal.

4 recordsLinked to original sources

arXiv2026

Commensurate moiré superlattices in anisotropically strained twisted bilayer graphene

We investigate how anisotropic strain reorganizes commensurate moiré superlattices and electronic structure in twisted bilayer graphene (TBG) across a finite range of reference twist angles. Motivated by experiments showing robust moiré phenomenology under angular disorder and heterostrain (Kapfer et al., Science 381,677 (2023)), we construct commensurate strained supercells generated by a general anisotropic deformation of the top graphene layer of TBG. The results show that anisotropic strain does not generically destroy the electronic structure of nearby pristine moiré systems; rather, its effect depends sensitively on whether the strained commensurate geometry remains two dimensional or crosses over toward a quasi one dimensional regime. This provides a geometric perspective on the persistence of moiré electronic features over a finite window of twist angle and heterostrain. Within this framework, the allowed strained configurations naturally separate into tilted two dimensional moiré patterns and quasi one dimensional stripe like patterns. We find that several such strained two dimensional solutions occur near a given pristine twist angle, and that nearby solutions retain triangular like AA-region localization, comparable low energy bandwidths, and a low field Hofstadter spectrum close to the unstrained system. In contrast, quasi one dimensional strained configurations show stronger dimensional reduction, reduced Dirac point multiplicity, stripe like spatial localization, and stronger Hofstadter splitting.

cond-mat.mes-hall↗

arXiv2025

Magneto transport of pressure induced flatbands in large angle twisted bilayer graphene

Twisted bilayer graphene (TBG) exhibits flat electronic bands at the so-called magic angle ($\sim 1.1^\circ$), leading to strong electron correlations and emergent quantum phases such as superconductivity and correlated insulating states. However, beyond the magic angle, the band structure generally remains dispersive, diminishing interaction-driven phenomena. In this work, we explore the equivalence between pressure-induced flatbands and the magic-angle flatband in large-angle TBG by systematically analyzing the role of interlayer coupling modifications under perpendicular pressure. We show that pressure-induced flatbands exhibit spatial localization similar to magic-angle TBG, with charge density concentrated in the AA-stacked regions. Furthermore, the Hall conductivity and magneto-transport properties under an external magnetic field reveal that these pressure-induced flatbands share key signatures with the quantum Hall response of magic-angle TBG. The obtained Hofstadter spectrum shows four consistent low-energy gaps across all twist angles under pressure, which align with the calculated Hall conductivity plateaus. Our findings suggest that pressure offers an alternative pathway to engineer flat electronic bands and correlated states in TBG, extending the landscape of tunable moiré materials beyond the constraints of the magic angle.

cond-mat.mes-hall↗

arXiv2025

Evidence of spin-phonon-charge coupling in quasi-one-dimensional Ising spin chain system $α$-CoV$_2$O$_6$

The quasi-one-dimensional Ising spin chain system $α$-CoV$_2$O$_6$ is considered to exhibit fascinating magnetic properties at lower temperatures. We comprehensively study magnetic properties and lattice dynamics using a combination of x-ray diffraction (XRD), DC magnetization, specific heat, temperature-dependent XRD and Raman scattering measurements, along with the theoretical charge density map calculations. The DC magnetization and specific heat confirm the antiferromagnetic (AFM) long-range order (LRO) at Neel temperature T$_N$ = 15 K as well as the short-range ordering (SRO) below 100 K. Temperature-dependent Raman measurement has been performed in the temperature range of 5 K to 180 K, revealing spin-phonon coupling (SPC) below 100 K, which is well above the T$_N$ and is attributed to SRO. The SPC is calculated for different vibrational modes. The temperature-dependent XRD supports the key finding of magnetoelastic coupling at T$_N$, which is strongly correlated to the magnetoelectric phenomena. The renormalization of Raman modes and lattice anomalies near T$_N$ illustrate spin-lattice coupling via magnetoelastic and spin-phonon interactions leading to the interplay between spin, charge, and phonon degrees of freedom in $α$-CoV$_2$O$_6$. The presence of spin-charge coupling is confirmed by the theoretical charge density maps, which show that the formation of electric dipoles between the Co and O atoms in the AFM state results from the p-d hybridization.

cond-mat.mtrl-sci↗

arXiv2023

Magnetotransport properties of a twisted bilayer graphene in the presence of external electric and magnetic field

We extensively investigate the electronic and transport properties of a twisted bilayer graphene when subjected to both an external perpendicular electric field and a magnetic field. Using a basic tight-binding model, we show the flat electronic band properties as well as the density of states (DOS), both without and with the applied electric field. In the presence of an electric field, the degeneracy at the Dirac points is lifted where the non-monotonic behavior of the energy gap exists, especially for twist angles below 3$^\circ$. We also study the behavior of the Landau levels (LL) spectra for different twist angles within a very low energy range. These LL spectra get modified under the influence of the external electric field. Moreover, we calculate the dc Hall conductivity ($σ_{xy}$) for a very large system using the Kernel Polynomial Method (KPM). Interestingly, $σ_{xy}$ makes a transition from a half-integer to an integer quantum Hall effect, \textit{i.e.} the value of $σ_{xy}$ shifts from $\pm 4(n+1/2) (2e^2/h)$ ($n$ is an integer) to $\pm 2n (2e^2/h)$ around a small twist angle of $θ=2.005^\circ$. At this angle, $σ_{xy}$ acquires a Hall plateau at zero Fermi energy. However, the behavior of $σ_{xy}$ remains unaltered when the system is exposed to the electric field, particularly at the magic angle where the bands in both layers can hybridize and strong interlayer coupling plays a crucial role.

cond-mat.mes-hall↗