arXiv subjects

Bheema Lingam Chittari

Publications and source records attributed to Bheema Lingam Chittari.

At least 19 recordsLinked to original sources

arXiv2026

Time reversal symmetry broken quantum spin hall effect in pseudospin-1 Dirac-Rashba system

The Quantum spin Hall (QSH) phase is conventionally understood to be protected by time-reversal symmetry (TRS). Here, we theoretically investigated the fate of the QSH phase in a pseudospin-1 fermionic $α-\mathcal{T}_3$ system in the presence of a TRS-breaking ferromagnetic exchange field and spin-nonconserving Rashba spin-orbit coupling. Despite broken TRS, the QSH phase survives over a finite parameter regime and is characterised by a non-zero projected spin-Chern number $C_σ(σ= \uparrow, \downarrow)$, protected by a spin-spectral gap. In the absence of Rashba coupling, the QSH phase remains robust up to an $α$-dependent critical exchange field. Rashba SOC qualitatively reshapes the phase diagram by driving transitions into two distinct quantum anomalous Hall (QAH) phases: a $C=2$ phase, irrespective of $α$-values, and a $C=1$ phase for $α\neq 0,1$, which is further identified as a valley-polarized QAH phase arising from a single valley. Rotating the magnetization to in-plane gaps out the first-order helical edge states and gives rise to second-order topological insulator (SOTI) phases that host localized corner states in suitable finite geometry. We further identify a topological phase transition between two different SOTI phases, mediated by nanoribbon edge states at an exchange field equal to $α$. These results establish spin-resolved topology in a higher pseudospin system as well as the $α-\mathcal{T}_3$ lattice as a versatile platform for engineering and controlling multiple topological phases through magnetic exchange and spin-orbit coupling.

cond-mat.mes-hall↗

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↗

arXiv2026

Layer-Dependent Orbital Magnetization in Graphene-Haldane Heterostructures

Rhombohedral multilayer graphene (RMG) proximity-coupled to a Haldane substrate provides a platform to investigate the interplay between band topology, layer number, and electric-field control of orbital magnetism. Using a tight-binding model and the modern theory of orbital magnetization, we study the layer-dependent magnetic response in bilayer, trilayer, tetralayer and pentalayer graphene under Haldane proximity. While monolayer graphene develops a global topological gap with quantized magnetization slope, multilayer systems remain metallic due to protected low-energy bands associated with unperturbed sublattices. Despite the absence of a global gap, finite valley-contrasting Berry curvature produces non-trivial layer-dependent Chern numbers. We decompose the total orbital magnetization into self-rotation ($M_{\mathrm{SR}}$) and center-of-mass ($M_C$) contributions, revealing their distinct behaviors across doping and applied interlayer bias. In bilayer graphene, magnetization remains negative and monotonic. Remarkably, trilayer and tetralayer graphene display a bias-induced sign reversal of orbital magnetization beyond critical thresholds ($Δ\simeq -55$ meV for 3LG, $-50$ meV for 4LG) in the hole-doped regime, a feature completely absent in the bilayer. It is further established in the case of 5LG, that the magnetization reversal is independent of the topological transition, and depends on the direction of bias and hole doping. The effect persists across both hole and electron doping, demonstrating that layer count serves as a key tuning parameter for orbital magnetism. Our findings establish topologically proximitized multilayer graphene as a versatile platform for electric-field-manipulable orbitronic and valleytronic devices.

cond-mat.mes-hall↗

arXiv2025

Near room temperature magnetoelectric response and tunable magnetic anisotropy in the two-dimensional magnet 1T-CrTe2

Magnets with controllable magnetization and high critical temperature are essential for practical spintronics devices, among which the two-dimensional 1T-CrTe2 stands out because of its high experimental critical temperature up to about 300K down to the single layer limit. By using ab initio density functional theory, we investigate the magnetic properties of monolayer and bilayer 1T-CrTe2 and demonstrate that the magnetic properties, such as the magnetocrystalline anisotropy, critical Curie temperature and magnetizations, can be influenced by strain or electric fields.

cond-mat.mtrl-sci↗

arXiv2025

Tunable Interfacial Thermal Conductance in Graphene/Germanene van der Waals Heterostructure using an Optimized Interlayer Potential

Accurately modeling interfacial thermal transport in van der Waals heterostructures is challenging due to the limited availability of interlayer interaction potentials. We develop a pairwise interlayer potential for graphene/germanene van der Waals heterostructure using the binding energy obtained from ab-initio density functional theory calculations and use it to calculate the interfacial thermal conductivity. Our calculations reveal that the interfacial thermal conductivity shows superior tunability with external strain. The phonon density of states calculations show a blueshift in the phonon spectra with an applied compressive strain in the direction of heat flow, increasing the interfacial thermal conductance to $\sim$136% of the unstrained value. In contrast, a tensile strain is found to cause an opposite effect, reducing the conductance to $\sim$70% of the unstrained value. Moreover, due to increased availability of phonons for heat transfer, both temperature and interaction strength are found to correlate positively with the interfacial thermal conductance for both directions of heat flow.

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arXiv2025

Valley-polarized Quantum Anomalous Hall and Topological Metal Phase in Rashba induced pseudospin-1 lattice

We study the topological properties of Rashba spin-orbit coupling and exchange coupling induced pseudospin-$1$ system Dice lattice under the influence of a staggered electric potential and magnetization. The band structure and topological phases of the system are investigated and compared with the pseudospin-$\frac{1}{2}$ system honeycomb lattice. Under individual influence of the staggered electric field and magnetization, the system undergoes a distinct phase transition: (i) a staggered electric potential drives the system from a quantum anomalous Hall $(C_n = 2)$ to a valley polarized quantum anomalous Hall phase $(C_n = -1)$ associated with edge modes with a flip in the chirality; while (ii) a staggered magnetization changes the system to a topological metal associated with unconventional antichiral edge bands, from a topological insulator. These results are further supported by calculations of the Chern phase diagrams, Hall conductance, zigzag, and armchair edge states. Our findings enhance the understanding of new topological phases in the 2D pseudospin-$1$ system and open up a new platform to explore the anti-chiral edge states.

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

Gate and Carriers tunable Valley Imbalance in Topological Proximitized Rhombohedral Trilayer Graphene

We investigated the electronic structure, Fermi surface topology and the emergence of valley imbalance in rhombohedral trilayer graphene (RTG) induced by the topological proximity and the electric fields. We show that, a strong proximity strength isolates the unperturbed low energy bands at the charge neutrality and the isolated topological bands show metallic nature under the influence of applied electric fields. Our calculations indicate that valley-resolved metallic states with a finite Chern number $|C| =$3 can appear near charge neutrality for appropriate electric fields and second-nearest-neighbor strengths. The Fermi surface topology of these metallic bands greatly influenced by the applied electric fields and carrier doping. The valley imbalance lead to the dominant carriers of either $e^-$ or $h^+$ Fermi surface pockets and the choice of carriers is subjected to the direction of electric fields. The gate-tunable and carrier-induced valley imbalance in topologically proximated rhombohedral trilayer graphene may have potential applications toward the realization of superconductivity.

cond-mat.mes-hall↗

arXiv2024

Orbital Hall Conductivity in a Graphene/Haldane and Haldane/Haldane Bilayers

We investigate the orbital Hall conductivity in bilayer graphene (G/G) by modifying one or both the layers as Haldane type ($\rm G/ \tilde G$ : Graphene/Haldane and $\rm \tilde G/ \tilde G$ : Haldane/Haldane) with the inclusion of next nearest neighbour (NNN) hopping strength ($t_2$) and flux ($ϕ$). It is observed that the low energy bands of $\rm G/ \tilde G$ and $\rm \tilde G/ \tilde G$ are isolated with a gap at charge neutrality with the next nearest neighbour (NNN) hopping term $t_2e^{\pm iϕ}$. The time reversal (\textit{TR}) symmetry breaking with $t_2e^{\pm iϕ}$ induces a large orbital magnetic moment ($\vb{m}_n(\vb{k})$) for the $n^{th}$ band in $\rm G/ \tilde G$ and $\rm \tilde G/ \tilde G$ bilayers. This \textit{TR} symmetry breaking, modulated by the $t_2$ strength, leads to the emergence of {\it Orbital Ferromagnetism} and {\it Valley Orbital Magnetism} within the BZ for the Haldane single layer as well for both $\rm G/ \tilde G$ and $\rm \tilde G/ \tilde G$. We show that for the applied longitudinal electric fields, the intrinsic angular momentum ($L^z$) gives the orbital current ($\mathcal{J}^{z,orb}$) along a transverse direction and generates the orbital Hall conductivity (OHC). We further show that the orbital magnetic polarity leads the Haldane single layer to {\it Orbital Chern Insulator} with the quantized OHC in the gap over the occupied bands. Moreover, the accumulation of orbital magnetic moment of the bands in Haldane graphene bilayer shows {\it Quantum Orbital Hall Insulator} and {\it Orbital Chern Insulators}. Similarly, we show that in the hetero-bilayers, one of the layers of the Haldane type generates the orbital magnetism and induces the OHC. We conclude that the isolated bands in Haldane graphene bilayers with external stimuli are of an orbital nature and have various quantum orbital Hall phases.

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arXiv2024

Experimental and Computational Insights Into the Magnetic Anisotropy and Magnetic Behaviour of Layered Room-Temperature Ferromagnet Cr$_{1.38}$Te$_2$

We investigate the structural, magnetocrystalline anisotropy, critical behaviour, and magnetocaloric effect in the layered room-temperature monoclinic ferromagnet Cr$_{1.38}$Te$_2$. The critical behavior is studied by employing various techniques such as the modified Arrott plot (MAP), the Kouvel-Fisher method (KF), and the critical isothermal analysis (CI) around the Curie temperature ($T_C$) of 316 K. The derived critical exponents are self-consistent and obey the rescaling analysis. The Monte-Carlo simulations reproduce the experimentally obtained critical exponents. However, the derived critical exponents do not suggest any single universality class of the magnetic interactions. On the other hand, the renormalization group (RG) theory suggests 3D-Ising type long-range exchange interactions [$J(r)$], decaying with distance ($r$) as $J (r) = r^{-(d+σ)}= r^{-4.73}$. Further, magnetocrystalline anisotropy energy density (K$_u$) is found to be temperature dependent. The ground state magnetic easy-axis ($b$-axis) is identified by analyzing the magnetocrystalline anisotropy energy (MAE) using the density functional theory calculations. Maximum entropy change -$ΔS_{m}^{max}$$\approx$2.51 J/kg-K is found near the $T_C$.

cond-mat.mtrl-sci↗

arXiv2024

Endless Dirac nodal lines and high mobility in kagome semimetal Ni3In2Se2 single crystal

Kagome-lattice crystal is crucial in quantum materials research, exhibiting unique transport properties due to its rich band structure and the presence of nodal lines and rings. Here, we investigate the electronic transport properties and perform first-principles calculations for Ni$_{3}$In$_{2}$Se$_{2}$ kagome topological semimetal. First-principle calculations indicate six endless Dirac nodal lines and two nodal rings with a $π$-Berry phase in the Ni$_{3}$In$_{2}$Se$_{2}$ compound. The temperature-dependent resistivity is dominated by two scattering mechanisms: $s$-$d$ interband scattering occurs below 50 K, while electron-phonon ($e$-$p$) scattering is observed above 50 K. The magnetoresistance (MR) curve aligns with the theory of extended Kohler's rule, suggesting multiple scattering origins and temperature-dependent carrier densities. A maximum MR of 120\% at 2 K and 9 T, with a maximum estimated mobility of approximately 3000 cm$^{2}$V$^{-1}$s$^{-1}$ are observed. The Ni atom's hole-like d$_{x^{2}-y^{2} }$ and electron-like d$_{z^{2}}$ orbitals exhibit peaks and valleys, forming a local indirect-type band gap near the Fermi level (E$_{F}$). This configuration enhances the motion of electrons and holes, resulting in high mobility and relatively high magnetoresistance.

cond-mat.mtrl-sci↗

arXiv2023

Topological properties of nearly flat bands in bilayer $α-\mathcal{T}3$ lattice

We study the effect of Haldane flux in the bilayer $α$-$\mathcal{T}_3$ lattice system, considering possible non-equivalent, commensurate stacking configurations with a tight-binding formalism. The bilayer $α$-$\mathcal{T}_3$ lattice comprises six sublattices in a unit cell, and its spectrum consists of six bands. In the absence of Haldane flux, threefold band crossings occur at the two Dirac points for both valence and conduction bands. The introduction of Haldane flux in a cyclically stacked bilayer $α$-$\mathcal{T}_3$ lattice system separates all six bands, including two low-energy, corrugated nearly flat bands, and assigns non-zero Chern numbers to each band, rendering the system topological. We demonstrate that the topological evolution can be induced by modifying the hopping strength between sublattices with the scaling parameter $α$ in each layer. In the dice lattice limit ($α= 1$) of the Chern-insulating phase, the Chern numbers of the three pairs of bands, from low energy to higher energies, are $\pm 2$, $\pm 3$, and $\pm 1$. Interestingly, a continuous change in the parameter $α$ triggers a topological phase transition through band crossings between the two lower energy bands. These crossings occur at different values for the conduction and valence bands and depend further on the next nearest neighbor (NNN) hopping strength. At the transition point, the Chern numbers of the two lower conduction and valence bands change discontinuously from $\pm 2$ to $\pm 5$ and $\pm 3$ to $0$, respectively, while leaving the Chern number of the third band intact.

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↗

arXiv2023

Topological flat bands in rhombohedral tetralayer and multilayer graphene on hexagonal boron nitride moire superlattices

We show that rhombohedral four-layer graphene (4LG) nearly aligned with a hexagonal boron nitride (hBN) substrate often develops nearly flat isolated low energy bands with non-zero valley Chern numbers. The bandwidths of the isolated flatbands are controllable through an electric field and twist angle, becoming as narrow as $\sim10~$meV for interlayer potential differences between top and bottom layers of $|Δ|\approx 10\sim15~$meV and $θ\sim 0.5^{\circ}$ at the graphene and boron nitride interface. The local density of states (LDOS) analysis shows that the nearly flat band states are associated to the non-dimer low energy sublattice sites at the top or bottom graphene layers and their degree of localization in the moire superlattice is strongly gate tunable, exhibiting at times large delocalization despite of the narrow bandwidth. We verified that the first valence bands' valley Chern numbers are $C^{ν=\pm1}_{V1} = \pm n$, proportional to layer number for $n$LG/BN systems up to $n = 8$ rhombohedral multilayers.

cond-mat.mes-hall↗

arXiv2022

Nearly flat bands in twisted triple bilayer graphene

We investigate the electronic structure of alternating-twist triple Bernal-stacked bilayer graphene (t3BG) as a function of interlayer coupling $ω$, twist angle $θ$, interlayer potential difference $Δ$, and top-bottom bilayers sliding vector $\boldsymbolτ$ for three possible configurations AB/AB/AB, AB/BA/AB, and AB/AB/BA. The parabolic low-energy band dispersions in a Bernal-stacked bilayer and gap-opening through a finite interlayer potential difference $Δ$ allows the flattening of bands in t3BG down to $\sim 20$~meV for twist angles $θ\lesssim 2^{\circ}$ regardless of the stacking types. The easier isolation of the flat bands and associated reduction of Coulomb screening thanks to the intrinsic gaps of bilayer graphene for finite $Δ$ facilitate the formation of correlation-driven gaps when it is compared to the metallic phases of twisted trilayer graphene under electric fields. We obtain the stacking dependent Coulomb energy versus bandwidth $U/W \gtrsim 1$ ratios in the $θ$ and $Δ$ parameter space. We also present the expected $K$-valley Chern numbers for the lowest-energy nearly flat bands.

cond-mat.mes-hall↗

arXiv2021

Modulating Curie Temperature and Magnetic Anisotropy in Nanoscale Layered Cr_{2}Te_{3} Films: Implications for Room-Temperature Spintronics

Nanoscale layered ferromagnets have demonstrated fascinating two-dimensional magnetism down to atomic layers, providing a peculiar playground of spin orders for investigating fundamental physics and spintronic applications. However, strategy for growing films with designed magnetic properties is not well established yet. Herein, we present a versatile method to control the Curie temperature (T_{C}) and magnetic anisotropy during growth of ultrathin Cr_{2}Te_{3} films. We demonstrate increase of the TC from 165 K to 310 K in sync with magnetic anisotropy switching from an out-of-plane orientation to an in-plane one, respectively, via controlling the Te source flux during film growth, leading to different c-lattice parameters while preserving the stoichiometries and thicknesses of the films. We attributed this modulation of magnetic anisotropy to the switching of the orbital magnetic moment, using X-ray magnetic circular dichroism analysis. We also inferred that different c-lattice constants might be responsible for the magnetic anisotropy change, supported by theoretical calculations. These findings emphasize the potential of ultrathin Cr_{2}Te_{3} films as candidates for developing room-temperature spintronics applications and similar growth strategies could be applicable to fabricate other nanoscale layered magnetic compounds.

cond-mat.mtrl-sci↗

arXiv2021

Stacking and gate tunable topological flat bands, gaps and anisotropic strip patterns in twisted trilayer graphene

Trilayer graphene with a twisted middle layer has recently emerged as a new platform exhibiting correlated phases and superconductivity near its magic angle. A detailed characterization of its electronic structure in the parameter space of twist angle $θ$, interlayer potential difference $Δ$, and top-bottom layer stacking $τ$ reveals that flat bands with large Coulomb energy vs bandwidth $U/W > 1$ are expected within a range of $\pm 0.2^{\circ}$ near $θ\simeq1.5^{\circ}$ and $θ\simeq1.2^{\circ}$ for $τ_{\rm AA}$ top-bottom layer stacking, between a wider $1^{\circ} \sim 1.7^{\circ}$ range for $τ_{\rm AB}$ stacking, whose bands often have finite valley Chern numbers thanks to the opening of primary and secondary band gaps in the presence of a finite $Δ$, and below $θ\lesssim 0.6^{\circ}$ for all $τ$ considered. The largest $U/W$ ratios are expected at the magic angle $\sim 1.5^{\circ}$ when $|Δ| \sim 0$~meV for AA, and slightly below near $\sim 1.4^{\circ}$ for finite $|Δ| \sim 25$~meV for AB stackings, and near $θ\sim 0.4^{\circ}$ for both stackings. When $τ$ is the saddle point stacking vector between AB and BA we observe pronounced anisotropic local density of states (LDOS) strip patterns with broken triangular rotational symmetry. We present optical conductivity calculations that reflect the changes in the electronic structure introduced by the stacking and gate tunable system parameters.

cond-mat.mes-hall↗

arXiv2021

Magnetoelectric Response of Antiferromagnetic Van der Waals Bilayers

We predict that antiferromagnetic bilayers formed from van der Waals (vdW) materials, like bilayer CrI$_3$, have a strong magnetoelectric response that can be detected by measuring the gate voltage dependence of Faraday or Kerr rotation signals, total magnetization, or anomalous Hall conductivity. Strong effects are possible in single-gate geometries, and in dual-gate geometries that allow internal electric fields and total carrier densities to be varied independently. We comment on the reliability of density-functional-theory estimates of interlayer magnetic interactions in van der Waals bilayers, and on the sensitivity of magnetic interactions to pressure that alters the spatial separation between layers.

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