arXiv subjects

Sajid Sekh

Publications and source records attributed to Sajid Sekh.

8 recordsLinked to original sources

arXiv2026

Kagome edge states under lattice termination, spin-orbit coupling, and magnetic order

We study the edge state properties of a two-dimensional kagome lattice using a tight-binding approach, focusing on the role of lattice termination, spin-orbit coupling, and magnetic order. In the pristine limit, we show that the existence of localized edge states is highly sensitive to boundary geometry, with certain terminations completely suppressing edge modes. Kane-Mele spin-orbit coupling opens a bulk gap and stabilizes topologically protected helical edge states, yielding a robust $\mathbb{Z}_2$ insulating phase that is insensitive to termination details. In contrast, the combined effect of a Zeeman field and Rashba spin-orbit coupling drives the system into Chern insulating phases, with Chern numbers consistent with the number of chiral edge modes. We further demonstrate that non-coplanar magnetic textures generate multiple Chern phases through finite scalar spin chirality, with Kane-Mele coupling strongly tuning the topological gaps. Our results provide important insights into the tunability of edge states in the kagome lattice, which can be key to designing materials with novel electronic properties and topological phases.

cond-mat.mtrl-sci↗

arXiv2025

Electronic and magnetic properties of the NdNiO$_2$/SrTiO$_3$ thin films

The hole-doped NdNiO$_2$ layer deposited on the SrTiO$_{3}$ surface exhibits unconventional superconductivity. Here, we present a systematic study of the electronic and magnetic properties of the NdNiO$_2$ superconductor using the density functional theory (DFT). The strong local Coulomb interactions in the Ni($3d)$ and Nd($4f$) states are included within the DFT+$U$ method. The effect of Sr doping on the electronic band structure and density of states was studied for the NdNiO$_2$ thin films deposited on the SrTiO$_3$ (001) surface. The results obtained for the uncapped thin films were compared with the calculations for the NdNiO$_2$ films capped by the SrTiO$_3$ layer. We have found significant changes in the electronic structure and magnetic properties of the thin films compared to the bulk crystal.

cond-mat.mtrl-sci↗

arXiv2025

Topological in-gap chiral edge states in superconducting Haldane model with spin-orbit coupling

Topological superconductivity is currently one of the prime interests, given the properties of its exotic nature of chiral edge states. A broken time-reversal symmetry (TRS) is an essential ingredient in the recipe of a chiral edge state. The Haldane model is one of the many factors that can break TRS in a system. Thus, we explore the possibility of topological superconductivity in the Haldane model under the influence of a conventional superconductor. The edge states originating from such recipes mostly remain outside the superconducting gap. Contrary to this, in the presence of spin-orbit coupling, the edge modes lie within the superconducting gap, and can lead to a gapless state for some range of parameters. Moreover, we use band inversion and projection on the real-space lattice to confirm the topological and chiral nature of the obtained edge states.

cond-mat.supr-con↗

arXiv2025

Low temperature phase of AuSn$_{4}$ induced by the van der Waals interactions

AuSn$_{4}$ is the example of an compound that exhibits topological properties. Recent XRD measurements reveal an ambiguous nature of the crystal structure, as it can be realized with either Aea2 or Ccca symmetry. Motivated by this, we analyze the dynamical stability of the compound. We discuss the role of van der Waals (vdW) corrections within the ab initio calculation. Interestingly, our main result indicates that AuSn$_{4}$ can be unstable with both Aea2 and Ccca symmetries, due to the soft modes in the phonon spectra. From the soft mode analyses we find dynamically stable Pc structure. This structure has always smaller energy than Aea2 or Ccca crystal and it stays independent of the vdW correction. We also show that the comparison of theoretical electronic properties with the experimental ARPES measurements. Our findings may be valuable in the future investigations of AuSn$_{4}$-like compounds.

cond-mat.mtrl-sci↗

arXiv2024

Circular dichroism as a probe for topology in three-dimensional semimetals

Higher-pseudospin fermions, associated with multiple band-crossings in topological semimetals, are the condensed matter analogues of higher-spin fermions in high-energy physics. In this paper, we demonstrate that analyzing the response of a circular drive is an effective way to detect the topology of the lowest-energy Bloch band, as it can be connected to a frequency-dependent probe. The dichroic response exhibits circular dichroism due to the differential excitation rates by the left- and right-circular orientations of a time-periodic drive, induced on a filled band, because of the geometrical properties of the Bloch bands. Our analytical approximation reveals that the dichroic response is quantized for isotropic systems, when the frequency of the drive is above a critical value, and thus correctly infers the ground-state Chern number. We demonstrate this through explicit numerical computations by considering three kinds of semimetals with pseudospin values of 1/2, 1, and 3/2, respectively, and all having linear dispersions. Furthermore, we investigate the effects of tilt and anisotropy on the systems, and find that although tilt does not have any effect on the response, the presence of anisotropy can drastically hamper the quantization. Our scheme thus provides an important methodology for designing future experiments to detect the topology of band structures.

cond-mat.mes-hall↗

arXiv2023

Magneto-optical conductivity in the type-I and type-II phases of Weyl/multi-Weyl semimetals

Magneto-optical conductivity is a very widely studied transport coefficient, useful to understand and characterize the behaviour of materials under magnetic fields. Using the Kubo formula, we compute the components of the conductivity tensor $σ_{μν}$ transverse to a uniform magnetic field $\mathbf B$, for a single node of a multi-Weyl semimetal (with monopole charge $J$ equal to two or three). We also include the results for a Weyl semimetal (with $J=1$), and identify peaks in the conductivity profile which were not reported in earlier studies. In our analysis, we explore how the tilting of the Weyl/multi-Weyl cone affects $σ_{μν}$, focussing on both type-I and type-II phases. All these systems have a linear-in-momentum dispersion along the tilting axis, which is chosen to align with $\mathbf B$. In the type-II phases, open Fermi pockets appear as artifacts of the low-energy effective continuum models, which ignore higher-order momentum terms of the actual bandstructures. Hence, we supplement the linear power term with a cubic term, which closes the Fermi pockets, thus eliminating any need for an ad hoc cutoff for the momentum integrals. Our results reveal that the absorptive parts of $σ_{μν}$ display multiple peaks as functions of the frequency, whose locations are determined by an energy scale $\sim |\mathbf B|^{J/2} $.

cond-mat.mes-hall↗

arXiv2023

Floquet transmission in Weyl/multi-Weyl and nodal-line semimetals through a time-periodic potential well

In mesoscopic physics, the application of a time-periodic drive leads to novel transport behaviour, which is absent in the static regimes. Here we consider a quantum pumping protocol, such that the quasiparticles of Weyl/multi-Weyl and nodal-line semimetals are subjected to a time-periodic rectangular potential well. The presence of an oscillating potential of frequency $ω$ creates equispaced Floquet side-bands with spacing $\hbar ω$. As a result, a Fano resonance is observed when the difference in the Fermi energy (i.e., the energy of the incident quasiparticle), and the energy of one of the (quasi)bound state levels of the well, coincides with the energy of an integer number of photons (each carrying energy quantum $\hbar ω$, equal to the side-band spacing). Using the Floquet theory and the scattering matrix approach, in the zero-temperature non-adiabatic pumping limit, we find characteristic Fano resonance patterns in the transmission coefficients, which depend on the nature of the dispersion. The inflection points in the pumped shot noise spectra also serve as a proxy for the corresponding Fano resonances. Therefore, we also numerically evaluate the pumped shot noise. Finally, we correlate the existence of the Fano resonance points to the (quasi)bound states of the well, by explicitly calculating the bound states of the static well (which are a subset of the bound states of the driven system). Since we consider semimetals with anisotropic dispersions, all the features observed depend on the orientation of the potential well. We believe that our results will serve as a guide for future experiments investigating quantum transmission in nonequilibrium settings.

cond-mat.mes-hall↗

arXiv2022

Magnus Hall effect in three-dimensional topological semimetals

Magnus Hall effect (MHE) is a non-linear Hall effect requiring no external magnetic field, which can be observed when an in-built electric field couples to the Berry curvature of the bandstructure, producing a current in the transverse direction. In this paper, we explore MHE in the context of various three-dimensional semimetals, incorporating various features like tilt, anisotropy, and multi-fold degeneracy. We numerically calculate the Magnus Hall conductivity tensors and transport coefficients, within the framework of the Boltzmann transport theory. Although MHE was originally predicted for two-dimensional materials with time-reversal symmetry (TRS), we show that a finite MHE response is possible in materials without TRS. If TRS is preserved, broken inversion symmetry is needed to prevent the cancellation of the MHE contributions while summing over the Brillouin zone. The amount of tilt of the node of a semimetal greatly affects the transport coefficients. In presence of anisotropic dispersions, we find that the MHE features differ depending on the directions of measurements (as expected). To demonstrate these dependencies, our investigations include Weyl, multi-Weyl, multi-fold, and nodal-line semimetals. Our analysis is of great importance for transport measurements in experiments involving non-linear Hall effects.

cond-mat.mes-hall↗