Showing 1–50 of 54 results for author: Tahir, M

Probing Interfacial Spin Dynamics and Temperature Dependent Asymmetry in Spin Pumping Across Ni80Fe20/Cu/Cr1.12Te2 Interfaces

Authors: Mahammad Tahir, Swati Pandey, Sourabh Manna, Rajdeep Singh Rawat, Rohit Medwal, Soumik Mukhopadhyay

Abstract: Spin transfers in magnetic multilayers offers a promising pathway toward ultrafast, energy-efficient spintronic devices. In this study, we investigate the interfacial spin pumping and temperature-dependent spin current exchange in a Cr1.12Te2/Cu/Ni80Fe20 (Py)(FM1/NM/FM2) trilayer structure. Using broadband and cryogenic ferromagnetic resonance (FMR) measurements, we investigate key magnetization d… ▽ More Spin transfers in magnetic multilayers offers a promising pathway toward ultrafast, energy-efficient spintronic devices. In this study, we investigate the interfacial spin pumping and temperature-dependent spin current exchange in a Cr1.12Te2/Cu/Ni80Fe20 (Py)(FM1/NM/FM2) trilayer structure. Using broadband and cryogenic ferromagnetic resonance (FMR) measurements, we investigate key magnetization dynamical parameters, including the effective Gilbert damping factor, effective magnetic fields, interfacial spin mixing conductance, and spin current density. Efficient spin angular momentum transfers from Py to Cr1.12Te2 are observed at room temperature. At lower temperatures, the enhanced linewidth reflects temperature dependent spin pumping effects occurring at distinct precession frequencies of the ferromagnetic layers. Notably, the absence of interfacial Damping indicates that spin pumping can be modulated by controlling the net spin current flow. These findings offer critical insight into temperature-dependent tunable spin transport mechanisms in magnetic multilayers, highlighting their potential for next-generation spintronic applications. △ Less

Submitted 30 January, 2026; originally announced January 2026.

Comments: 10 pages, 7 figures

Enhanced Spin Pumping and Magnetization dynamics in Ni$_{80}$Fe$_{20}$/MoS$_2$ stack via interface modification

Authors: Mahammad Tahir, Somya Diwakar, Subhakanta Das, Mukul Gupta, Rohit Medwal, Soumik Mukhopadhyay

Abstract: Materials with strong spin orbit coupling (SOC) are essential for realizing spin orbit torque (SOT) based magnetic memory devices. Transition metal dichalcogenides (TMDs) are promising candidates for such appli cations because of their inherently high SOC strength. In this study, we investigate the spin pumping effect at the interface between a monolayer of molybdenum disulfide (ML-MoS$_2$) and Ni… ▽ More Materials with strong spin orbit coupling (SOC) are essential for realizing spin orbit torque (SOT) based magnetic memory devices. Transition metal dichalcogenides (TMDs) are promising candidates for such appli cations because of their inherently high SOC strength. In this study, we investigate the spin pumping effect at the interface between a monolayer of molybdenum disulfide (ML-MoS$_2$) and Ni$_{80}$Fe$_{20}$ (Py) thin films using broadband ferromagnetic resonance (FMR) spectroscopy. FMR measurements reveal a notable enhancement in the effective Gilbert damping factor for the ML-MoS$_2$/Py (Pt = 0 nm) interface compared to the reference Py thin films, attributed to spin pumping across the ML-MoS$_2$/Py interface. To further quantify spin pumping efficiency, we introduce a high SOC platinum (Pt) interlayer at the ML-MoS$_2$/Py interface and systematically vary its thickness. This allows us to evaluate key spin transport parameters, including the enhancement in the effective Gilbert damping parameter, the effective spin mixing conductance that reflects the transfer of spin angular momentum from Py to ML-MoS$_2$ and the effective spin current density. △ Less

Submitted 14 May, 2025; originally announced May 2025.

Analytical gradients of random-phase approximation plus corrections from renormalized single excitations

Authors: Muhammad N. Tahir, Honghui Shang, Xinguo Ren

Abstract: The random-phase approximation (RPA) formulated within the adiabatic connection fluctuation-dissipation framework is a powerful approach to compute the ground-state energies and properties of molecules and materials. Its overall underbinding behavior can be effectively mitigated by a simple correction term, called renormalized single excitation (rSE) correction. Analytical gradient calculations of… ▽ More The random-phase approximation (RPA) formulated within the adiabatic connection fluctuation-dissipation framework is a powerful approach to compute the ground-state energies and properties of molecules and materials. Its overall underbinding behavior can be effectively mitigated by a simple correction term, called renormalized single excitation (rSE) correction. Analytical gradient calculations of the RPA energy have become increasingly available, enabling structural relaxations and even molecular dynamics at the RPA level. However, such calculations at the RPA+rSE level have not been reported, due to the lack of the rSE analytical gradient. Here, we present the first formulation and implementation of the analytical gradients of the rSE energy with respect to the nuclear coordinates within an atomic-orbital basis set framework, which allows us to assess the performance of RPA+rSE in determining the molecular geometries and energetics. It is found that the slight overestimation behavior of RPA for small covalently bonded molecules is strengthened by rSE, while such behavior for molecules bonded with purely dispersion interactions is corrected. We further applied the approach to the water clusters, and found that the energy difference between the low-energy isomers of water hexamers is almost unchanged when going from RPA to RPA+rSE geometries. For the bigger WATER27 test set, using the RPA+rSE geometries instead of the RPA ones leads to a slight reduction of the mean absolute error of RPA+rSE from 0.91 kcal/mol to 0.70 kcal/mol, at the complete basis set. △ Less

Submitted 12 May, 2025; originally announced May 2025.

Comments: 48 page, 3 figures and 5 tables in the main text

Roadmap on Advancements of the FHI-aims Software Package

Authors: Joseph W. Abbott, Carlos Mera Acosta, Alaa Akkoush, Alberto Ambrosetti, Viktor Atalla, Alexej Bagrets, Jörg Behler, Daniel Berger, Hannah Bertschi, Björn Bieniek, Jonas Björk, Volker Blum, Saeed Bohloul, Connor L. Box, Nicholas Boyer, Danilo Simoes Brambila, Gabriel A. Bramley, Kyle R. Bryenton, María Camarasa-Gómez, Christian Carbogno, Fabio Caruso, Sucismita Chutia, Michele Ceriotti, Gábor Csányi, William Dawson , et al. (181 additional authors not shown)

Abstract: Electronic-structure theory is the foundation of the description of materials including multiscale modeling of their properties and functions. Obviously, without sufficient accuracy at the base, reliable predictions are unlikely at any level that follows. The software package FHI-aims has proven to be a game changer for accurate free-energy calculations because of its scalability, numerical precis… ▽ More Electronic-structure theory is the foundation of the description of materials including multiscale modeling of their properties and functions. Obviously, without sufficient accuracy at the base, reliable predictions are unlikely at any level that follows. The software package FHI-aims has proven to be a game changer for accurate free-energy calculations because of its scalability, numerical precision, and its efficient handling of density functional theory (DFT) with hybrid functionals and van der Waals interactions. It treats molecules, clusters, and extended systems (solids and liquids) on an equal footing. Besides DFT, FHI-aims also includes quantum-chemistry methods, descriptions for excited states and vibrations, and calculations of various types of transport. Recent advancements address the integration of FHI-aims into an increasing number of workflows and various artificial intelligence (AI) methods. This Roadmap describes the state-of-the-art of FHI-aims and advancements that are currently ongoing or planned. △ Less

Submitted 20 April, 2026; v1 submitted 30 April, 2025; originally announced May 2025.

Comments: arXiv admin note: Includes articles arXiv:2502.02460, arXiv:2501.02550, arXiv:2411.01680, arXiv:2501.16091, arXiv:2411.04951

Enhancement of spin current to charge current conversion in Ferromagnet/Graphene interface

Authors: Mahammad Tahir, Subhakanta Das, Mukul Gupta, Rohit Medwal, Soumik Mukhopadhyay

Abstract: The use of graphene in spintronic devices is contingent on its ability to convert a spin current into a charge current. We have systematically investigated the spin pumping induced spin-to-charge current conversion at the Graphene/FM interface and the effect of interface modification through high spin orbit coupling (SOC) material (Pt) as an interlayer (IL) of varying thicknesses by using broadban… ▽ More The use of graphene in spintronic devices is contingent on its ability to convert a spin current into a charge current. We have systematically investigated the spin pumping induced spin-to-charge current conversion at the Graphene/FM interface and the effect of interface modification through high spin orbit coupling (SOC) material (Pt) as an interlayer (IL) of varying thicknesses by using broadband FMR spectroscopy. The spin mixing conductance is enhanced from $1.66 \times 10^{18}$ m$^{-2}$ to $2.72 \times 10^{18}$ m$^{-2}$ whereas the spin current density is enhanced from 0.135$\pm $0.003 to 0.242$\pm$0.004 MA/m$^{2}$ at the Graphene/FM interface due to the interface modification using high SOC material Pt as an interlayer. The spin current to charge current conversion efficiency turns out to be $\approx 0.003$ nm for the Graphene/FM interface. These findings support the idea that Graphene in combination with high SOC material (Pt) could be a potential candidate for spintronic applications, specifically for spin-torque-based memory applications. △ Less

Submitted 25 May, 2024; v1 submitted 25 April, 2024; originally announced April 2024.

Comments: 8 Pages, 6 figures

Efficient structural relaxation based on the random phase approximation: Applications to the water clusters

Authors: Muhammad N. Tahir, Honghui Shang, Jia Li, Xinguo Ren

Abstract: We report an improved implementation for evaluating the analytical gradients of the random phase approximation (RPA) electron-correlation energy based on atomic orbitals and the localized resolution of identity scheme. The more efficient RPA force calculations allow us to relax structures of medium-size water clusters. Particular attention is paid to the structures and energy orderings of the low-… ▽ More We report an improved implementation for evaluating the analytical gradients of the random phase approximation (RPA) electron-correlation energy based on atomic orbitals and the localized resolution of identity scheme. The more efficient RPA force calculations allow us to relax structures of medium-size water clusters. Particular attention is paid to the structures and energy orderings of the low-energy isomers of (H$_2$O)$_n$ clusters with $n=21$, 22, and 25. It is found that the energy ordering of the low-energy isomers of these water clusters are rather sensitive to how their structures are determined. For the five low-energy isomers of (H$_2$O)$_{25}$, the RPA energy ordering based on the RPA geometries is quite different from that based on the geometries relaxed by lower-level theories, in contrast with the situation of small water clusters like the water hexamer. The standard RPA underbinds the water clusters, and this underbinding behavior gets more pronounced as the complete basis set (CBS) limit is approached. The renormalized single excitation (rSE) correction remedies this underbinding, giving rise to a noticeable overbinding behavior at finite basis sets. However, as the CBS limit is approached, RPA+rSE yields an accuracy for the binding energies that is comparable to the best available double hybrid functionals, as demonstrated for the WATER27 testset. △ Less

Submitted 16 April, 2024; originally announced April 2024.

Valley-controlled transport in graphene/ WSe$_{2}$ heterostructures under an off-resonant polarized light

Authors: M. Zubair, P. Vasilopoulos, M. Tahir

Abstract: We investigate the electronic dispersion and transport properties of graphene/WSe$_{2}$ heterostructures in the presence of a proximity-induced spin-orbit coupling $λ_{v}$, sublattice potential $Δ$, and an off-resonant circularly polarized light of frequency $Ω$ that renormalizes $Δ$ to $\barΔ_{ηp} = Δ+ηp Δ_Ω $ with $η$ and $p$ the valley and polarization indices, respectively, and $ Δ_Ω $ the gap… ▽ More We investigate the electronic dispersion and transport properties of graphene/WSe$_{2}$ heterostructures in the presence of a proximity-induced spin-orbit coupling $λ_{v}$, sublattice potential $Δ$, and an off-resonant circularly polarized light of frequency $Ω$ that renormalizes $Δ$ to $\barΔ_{ηp} = Δ+ηp Δ_Ω $ with $η$ and $p$ the valley and polarization indices, respectively, and $ Δ_Ω $ the gap due to the off-resonant circularly polarized light. Using a low-energy Hamiltonian we find that the interplay between different perturbation terms leads to inverted spin-orbit coupled bands. At high $Ω$ we study the band structure and dc transport using the Floquet theory and linear response formalism, respectively. We find that the inverted band structure transfers into the direct band one when the off-resonant light is present. The valley-Hall conductivity behaves as an even function of the Fermi energy in the presence and absence of this light. At $Δ_Ω$ = $λ_{v}$ - $Δ$ a transition occurs from the valley-Hall phase to the anomalous Hall phase. In addition, the valley-Hall conductivity switches sign when the polarization of the off-resonant light changes. The valley polarization vanishes for $Δ_Ω$ = 0 but it is finite for $Δ_Ω$ $\neq$ 0 and reflects the lifting of the valley degeneracy of the energy levels, for $Δ_Ω \neq 0$, when the off-resonant light is present. The corresponding spin polarization, present for $Δ_Ω$ = 0, increases for $Δ_Ω$ $\neq$ 0. Further, pure $K$ or $K^{\prime}$ valley polarization is generated when $Δ_Ω$ changes sign. Also, the charge Hall conductivity is finite for $Δ_Ω\neq 0$ and changes sign when the handedness of the light polarization changes. △ Less

Submitted 5 July, 2023; originally announced July 2023.

Influence of interface-induced valley-Zeeman and spin-orbit couplings\\ on transport in graphene-on-WSe$_{2}$ heterostructures

Authors: M. Zubair, P. Vasilopoulos, M. Tahir

Abstract: We investigate the electronic dispersion and transport properties of graphene/WSe$_{2}$ heterostructures in the presence of a proximity induced spin-orbit coupling (SOC) using a low-energy Hamiltonian, with different types of symmetry breaking terms, obtained from a four-band, first and second nearest-neighbour tight-binding (TB) one. The competition between different perturbation terms leads to i… ▽ More We investigate the electronic dispersion and transport properties of graphene/WSe$_{2}$ heterostructures in the presence of a proximity induced spin-orbit coupling (SOC) using a low-energy Hamiltonian, with different types of symmetry breaking terms, obtained from a four-band, first and second nearest-neighbour tight-binding (TB) one. The competition between different perturbation terms leads to inverted SOC bands. Further, we study the effect of symmetry breaking terms on ac and dc transport by evaluating the corresponding conductivities within linear response theory. The scattering-independent part of the valley-Hall conductivity, as a function of the Fermi energy $E_{F}$, is mostly negative in the ranges $-λ_{R}\leqslant E_{F}$ and $E_{F}\geqslantλ_{R}$ when the strength $λ_{R}$ of the Rashba SOC increases except for a very narrow region around $E_{F}=0$ in which it peaks sharply upward. The scattering-dependent diffusive conductivity increases linearly with electron density, is directly proportional to $λ_{R}$ in the low- and high-density regimes, but weakens for $λ_{R}=0$. We investigate the optical response in the presence of a SOC-tunable band gap for variable $E_{F}$. An interesting feature of this SOC tuning is that it can be used to switch on and off the Drude-type intraband response. Furthermore, the ac conductivity exhibits interband responses due to the Rashba SOC. We also show that the valley-Hall conductivity changes sign when $E_F$ is comparable to $λ_R$ and vanishes at higher values of $E_F$. It also exhibits a strong dependence on temperature and a considerable structure as a function of the frequency. △ Less

Submitted 5 July, 2023; originally announced July 2023.

Interfacial magnetic anisotropy controlled spin pumping in Co60Fe20B20/Pt stack

Authors: Mahammad Tahir, Dhananjay Tiwari, Abhishek Juyal, Rohit Medwal, Soumik Mukhopadhyay

Abstract: Controlled spin transport in magnetic stacks is required to realize pure spin current-driven logic and memory devices. The control over the generation and detection of the pure spin current is achieved by tuning the spin to charge conversion efficiency of the heavy metal interfacing with ferromagnets. Here, we demonstrate the direct tunability of spin angular momentum transfer and thereby spin pum… ▽ More Controlled spin transport in magnetic stacks is required to realize pure spin current-driven logic and memory devices. The control over the generation and detection of the pure spin current is achieved by tuning the spin to charge conversion efficiency of the heavy metal interfacing with ferromagnets. Here, we demonstrate the direct tunability of spin angular momentum transfer and thereby spin pumping, in CoFeB/Pt stack, with interfacial magnetic anisotropy. The ultra-low thickness of CoFeB thin film tilts the magnetic easy axis from in-plane to out-of-plane due to surface anisotropy. The Ferromagnetic resonance measurements are performed to investigate the magnetic anisotropy and spin pumping in CoFeB/Pt stacks. We clearly observe tunable spin pumping effect in the CoFeB/Pt stacks with varying CoFeB thicknesses. The spin current density, with varying ferromagnetic layer thickness, is found to increase from 0.11 to 0.24 MA/m2, with increasing in-plane anisotropy field. Such interfacial anisotropy-controlled generation of pure spin current can potentially lead to next-generation anisotropic spin current-controlled spintronic devices. △ Less

Submitted 5 January, 2023; originally announced January 2023.

Comments: 9 Pages, 9 figures

Current-induced quasiparticle magnetic multipole moments

Authors: Muhammad Tahir, Hua Chen

Abstract: Magnetic ordering beyond the standard dipolar order has attracted significant attention in recent years, but it remains an open question how to effectively manipulate such nontrivial order parameters using external perturbations. In this context, we present a theory for Cartesian magnetic multipole moments and their currents created by electric currents based on a general gauge-invariant formula f… ▽ More Magnetic ordering beyond the standard dipolar order has attracted significant attention in recent years, but it remains an open question how to effectively manipulate such nontrivial order parameters using external perturbations. In this context, we present a theory for Cartesian magnetic multipole moments and their currents created by electric currents based on a general gauge-invariant formula for arbitrary-order spin magnetic multipole moments of Bloch wave packets. As a concrete example, we point out that the low-energy quasiparticles in phosphorene subject to a perpendicular electric field have a valley structure that hosts magnetic octupole moments. The quasiparticle magnetic octupole moments can be exhibited by an in-plane electric current and lead to accumulation of staggered spin densities at the corners of a rectangular-shaped sample. A current carrying the octupole moments can further be induced through nonlinear response. Our work paves the way to systematically searching for and utilizing quasiparticles with higher-order magnetic multipole moments in crystal materials. △ Less

Submitted 27 October, 2022; originally announced October 2022.

Journal ref: Phys. Rev. Lett. 131, 106701 (2023)

Localized resolution of identity approach to the analytical gradients of random-phase approximation ground-state energy: algorithm and benchmarks

Authors: Muhammad N. Tahir, Tong Zhu, Honghui Shang, Jia Li, Volker Blum, Xinguo Ren

Abstract: We develop and implement a formalism which enables calculating the analytical gradients of particle-hole random-phase approximation (RPA) ground-state energy with respect to the atomic positions within the atomic orbital basis set framework. Our approach is based on a localized resolution of identity (LRI) approximation for evaluating the two-electron Coulomb integrals and their derivatives, and t… ▽ More We develop and implement a formalism which enables calculating the analytical gradients of particle-hole random-phase approximation (RPA) ground-state energy with respect to the atomic positions within the atomic orbital basis set framework. Our approach is based on a localized resolution of identity (LRI) approximation for evaluating the two-electron Coulomb integrals and their derivatives, and the density functional perturbation theory for computing the first-order derivatives of the Kohn-Sham (KS) orbitals and orbital energies. Our implementation allows one to relax molecular structures at the RPA level using both Gaussian-type orbitals (GTOs) and numerical atomic orbitals (NAOs). Benchmark calculations show that our approach delivers high numerical precision compared to previous implementations. A careful assessment of the quality of RPA geometries for small molecules reveals that post-KS RPA systematically overestimates the bond lengths. We furthermore optimized the geometries of the four low-lying water hexamers -- cage, prism, cyclic and book isomers, and determined the energy hierarchy of these four isomers using RPA. The obtained RPA energy ordering is in good agreement with that yielded by the coupled cluster method with single, double and perturbative triple excitations, despite that the dissociation energies themselves are appreciably underestimated. The underestimation of the dissociation energies by RPA is well corrected by the renormalized single excitation correction. △ Less

Submitted 2 September, 2021; originally announced September 2021.

Comments: 15 pages, 1 figure, five tables

Berry curvature, orbital magnetization, and Nernst effect in biased bilayer WSe$_2$

Authors: Vassilios Vargiamidis, P. Vasilopoulos, M. Tahir, Neophytos Neophytou

Abstract: A valley-contrasting Berry curvature in bilayer transition metal dichalcogenides with spin-orbit coupling can generate valley magnetization when the inversion symmetry is broken, for example, by an electric field, regardless of time-reversal symmetry. A nontrivial Berry curvature can also lead to anomalous transport responses, such as the anomalous Hall effect and the anomalous Nernst effect. Appl… ▽ More A valley-contrasting Berry curvature in bilayer transition metal dichalcogenides with spin-orbit coupling can generate valley magnetization when the inversion symmetry is broken, for example, by an electric field, regardless of time-reversal symmetry. A nontrivial Berry curvature can also lead to anomalous transport responses, such as the anomalous Hall effect and the anomalous Nernst effect. Applied to a bilayer WSe$_2$, an electric field can tune the Berry curvature and orbital magnetic moment, which has important consequences for the orbital magnetization and the anomalous Nernst responses. The orbital magnetization and its two contributions, one due to the magnetic moment and one due to the Berry curvature, are calculated and interpreted in terms of opposite circulating currents of the bands in the two layers. The valley anomalous Nernst coefficient and spin Nernst coefficient are also calculated. We find that a finite electric field leads to peaks and dips in the Nernst responses that have the signs of the Berry curvatures of the bands and are proportional to their magnitudes; it also enhances the valley Nernst responses. These experimentally verifiable findings may be promising for caloritronic applications. △ Less

Submitted 14 December, 2020; originally announced January 2021.

Comments: 11 pages, 9 figures

Comparing particle-particle and particle-hole channels of random-phase approximation

Authors: Muhammad N. Tahir, Xinguo Ren

Abstract: We present a comparative study of particle-hole and particle-particle channels of random-phase approximation (RPA) for molecular dissociations of different bonding types. We introduced a \textit{direct} particle-particle RPA scheme, in analogy to the \textit{direct} particle-hole RPA formalism, whereby the exchange-type contributions are excluded. This allows us to compare the behavior of the part… ▽ More We present a comparative study of particle-hole and particle-particle channels of random-phase approximation (RPA) for molecular dissociations of different bonding types. We introduced a \textit{direct} particle-particle RPA scheme, in analogy to the \textit{direct} particle-hole RPA formalism, whereby the exchange-type contributions are excluded. This allows us to compare the behavior of the particle-hole and particle-particle RPA channels on the same footing. Our study unravels the critical role of exchange contributions in determining behaviors of the two RPA channels for describing stretched molecules. We also made an attempt to merge particle-hole RPA and particle-particle RPA into a unified scheme, with the double-counting terms removed. However, benchmark calculations indicate that a straightforward combination of the two RPA channels does not lead to a successful computational scheme for describing molecular dissociations. △ Less

Submitted 12 April, 2019; originally announced April 2019.

Comments: 13 pages, 9 figures

Journal ref: Phys. Rev. B 99, 195149 (2019)

Emergent flat band lattices in spatially periodic magnetic fields

Authors: M. Tahir, Olivier Pinaud, Hua Chen

Abstract: Motivated by the recent discovery of Mott insulating phase and unconventional superconductivity due to the flat bands in twisted bilayer graphene, we propose more generic ways of getting two-dimensional (2D) emergent flat band lattices using either 2D Dirac materials or ordinary electron gas (2DEG) subject to moderate periodic orbital magnetic fields with zero spatial average. Employing both momen… ▽ More Motivated by the recent discovery of Mott insulating phase and unconventional superconductivity due to the flat bands in twisted bilayer graphene, we propose more generic ways of getting two-dimensional (2D) emergent flat band lattices using either 2D Dirac materials or ordinary electron gas (2DEG) subject to moderate periodic orbital magnetic fields with zero spatial average. Employing both momentum-space and real-space numerical methods to solve the eigenvalue problems, we find stark contrast between Schrödinger and Dirac electrons, i.e., the former show recurring "magic" values of the magnetic field when the lowest band becomes flat, while for the latter the zero-energy bands are asymptotically flat without magicness. By examining the Wannier functions localized by the smooth periodic magnetic fields, we are able to explain these nontrivial behaviors using minimal tight-binding models on a square lattice. The two cases can be interpolated by varying the $g$-factor or effective mass of a 2DEG and by taking into account the Zeeman coupling, which also leads to flat bands with nonzero Chern numbers for each spin. Our work provides flexible platforms for exploring interaction-driven phases in 2D systems with on-demand superlattice symmetries. △ Less

Submitted 11 June, 2019; v1 submitted 29 August, 2018; originally announced August 2018.

Comments: 13 pages, 8 figures, with supplemental materials. Revised version

Journal ref: Phys. Rev. B 102, 035425 (2020)

Quantum magnetotransport in a bilayer MoS2: influence of a perpendicular electric field

Authors: M. Zubair, M. Tahir, P. Vasilopoulos, K. Sabeeh

Abstract: We first derive the energy dispersion of bilayer MoS$_{2}$ in the presence of a perpendicular electric field $E_z$. We show that the band gap and layer splitting can be controlled by the field $E_z$. Away from the $k$ point, the intrinsic SOC splitting increases in the conduction band but is weakly affected in the valence band. We then analyze the band structure in the presence of a perpendicular… ▽ More We first derive the energy dispersion of bilayer MoS$_{2}$ in the presence of a perpendicular electric field $E_z$. We show that the band gap and layer splitting can be controlled by the field $E_z$. Away from the $k$ point, the intrinsic SOC splitting increases in the conduction band but is weakly affected in the valence band. We then analyze the band structure in the presence of a perpendicular magnetic field $B$ and the field $E_z$, including spin and valley Zeeman terms, and evaluate the Hall and longitudinal conductivities. We discuss the numerical results as functions of the fields $B$ and $E_z$ for finite temperatures. The field $B$ gives rise to a significant spin splitting in the conduction band, to a beating in the Shubnikov-de Haas (SdH) oscillations when it's weak, and to their splitting when it's strong. The Zeeman terms and $E_{z}$ suppress the beating and change the positions of the beating nodes of the SdH oscillations at low $B$ fields and enhance their splitting at high $B$ fields. Similar beating patterns are observed in the spin and valley polarizations at low $B$ fields. Interestingly, a $90\%$ spin polarization and a $100\%$ square-wave-shaped valley polarization are observed at high $B$ fields. The Hall-plateau sequence depends on $E_z$. These findings may be pertinent to future spintronic and valleytronic devices. △ Less

Submitted 24 August, 2018; originally announced August 2018.

Comments: 14 figures 25 pages

Journal ref: Received 14 April 2017; published 7 July 2017

Magneto-optical properties of bilayer transition metal dichalcogenides

Authors: M. Zubair, M. Tahir, P. Vasilopoulos

Abstract: In transition metal dichalcogenides the spin-orbit interaction affects differently the conduction and valence band energies as functions of $k$ and the band gap is large. Consequently, when a perpendicular magnetic field $B$ is applied the conduction and valence band Landau levels are also different and this leads to a splitting of the interband optical absorption lines in both the absence and pre… ▽ More In transition metal dichalcogenides the spin-orbit interaction affects differently the conduction and valence band energies as functions of $k$ and the band gap is large. Consequently, when a perpendicular magnetic field $B$ is applied the conduction and valence band Landau levels are also different and this leads to a splitting of the interband optical absorption lines in both the absence and presence of an external electric field $E_{z}$. When $B$ and $E_{z}$ are present the peaks in the imaginary part of the Hall conductivity give two distinct contributions of opposite sign to the interband spectrum. The real part of the right- and left-handed interband conductivity, however, retains its two-peak structure but the peaks are shifted in energy and amplitude with respect to each other in contrast with graphene. The response of the intraband conductivity is significantly modified when the Fermi energy $E_{F}$ and the field $B$ are varied. Its optical spectral weight is found to increase with $E_{F}$ in contrast with the decrease observed in graphene. Further, the position and amplitude of the intraband response depends on the field $B$. The absorption peaks vary linearly with $B$ for all fields similar to bilayer graphene for low fields but in contrast with the high-field $\sqrt{B}$ dependence in it. △ Less

Submitted 24 August, 2018; originally announced August 2018.

Comments: 12 pages 12 figures

Journal ref: Phys. Rev. B 98, 155402 (2018)

Magneto-optical properties of topological insulator thin films with broken inversion symmetry

Authors: k. Rahim, A. Ullah, M. Tahir, K. Sabeeh

Abstract: We determine the optical response of ultrathin film topological insulators in the presence of a quantizing external magnetic field taking into account both hybridization between surface states, broken inversion symmetry and explicit time reversal symmetry breaking by the magnetic field. We find that breaking of inversion symmetry in the system, which can be due to interaction with a substrate or e… ▽ More We determine the optical response of ultrathin film topological insulators in the presence of a quantizing external magnetic field taking into account both hybridization between surface states, broken inversion symmetry and explicit time reversal symmetry breaking by the magnetic field. We find that breaking of inversion symmetry in the system, which can be due to interaction with a substrate or electrical gating, results in Landau level crossings which lead to additional optical transition channels that were previously forbidden. We show that by tuning the hybridization and symmetry breaking parameters, a transition from the normal to a topological insulator phase occurs with measurable signatures in both the longitudinal and optical Hall conductivity. △ Less

Submitted 12 April, 2017; v1 submitted 11 April, 2017; originally announced April 2017.

Magneto-optical transport properties of monolayer WSe2

Authors: M. Tahir, P. Vasilopoulos

Abstract: The recent experimental realization of a high quality WSe$% _{2} $ leads to the possibility of magneto-optical measurements and the manipulation of the spin and valley degrees of freedom. We study the influence of the very strong spin-orbit coupling and of the anisotropic lifting of the valley pseudospin degeneracy on its magnetotransport properties. The energy spectrum of WSe$_{2} $ is derived an… ▽ More The recent experimental realization of a high quality WSe$% _{2} $ leads to the possibility of magneto-optical measurements and the manipulation of the spin and valley degrees of freedom. We study the influence of the very strong spin-orbit coupling and of the anisotropic lifting of the valley pseudospin degeneracy on its magnetotransport properties. The energy spectrum of WSe$_{2} $ is derived and discussed in the presence of a perpendicular magnetic field $B$. Correspondingly we evaluate the magneto-optical Hall conductivity and the optical longitudinal conductivity as functions of the frequency, magnetic field, and Fermi energy. They are strongly influenced by the field $B$ and the strong spin splitting. The former exhibits valley polarization and the latter beatings of oscillations. The magneto-optical responses can be tuned in two different regimes: the mictrowave-to-terahertz regime and the visible-frequency one. The absorption peaks involving the $n=0$ LL appear in between these two regimes and show a magnetic control of the spin and valley splittings. We also evaluate the power absorption spectrum. △ Less

Submitted 23 July, 2016; originally announced July 2016.

Comments: 15 pages and 11 Figs. arXiv admin note: text overlap with arXiv:1505.06780

Journal ref: Phys. Rev. B 94, 045415 (2016)

Unconventional quantum Hall effect in Floquet topological insulators

Authors: M. Tahir, P. Vasilopoulos, U. Schwingenschlog

Abstract: We study an unconventional quantum Hall effect for the surface states of ultrathin Floquet topological insulators in a perpendicular magnetic field. The resulting band structure is modified by photon dressing and the topological property is governed by the low-energy dynamics of a single surface. An exchange of symmetric and antisymmetric surface states occurs by reversing the light's polarization… ▽ More We study an unconventional quantum Hall effect for the surface states of ultrathin Floquet topological insulators in a perpendicular magnetic field. The resulting band structure is modified by photon dressing and the topological property is governed by the low-energy dynamics of a single surface. An exchange of symmetric and antisymmetric surface states occurs by reversing the light's polarization. We find a novel quantum Hall state in which the zeroth Landau level undergoes a phase transition from a trivial insulator state, with Hall conductivity $σ_{yx}=0$ at zero Fermi energy, to a Hall insulator state with $σ_{yx}=e^2/2h$. These findings open new possibilities for experimentally realizing nontrivial quantum states and unusual quantum Hall plateaux at $(\pm1/2,\pm3/2,\pm5/2,...)e^2/h$. △ Less

Submitted 20 July, 2016; originally announced July 2016.

Comments: Accepted in J. Phys. Condensed Matter july 2016

Quantum capacitance in monolayers of silicene and related buckled materials

Authors: S. Nawaz, M. Tahir

Abstract: Silicene and related buckled materials are distinct from both the conventional two dimensional electron gas and the famous graphene due to strong spin orbit coupling and the buckled structure. These materials have potential to overcome limitations encountered for graphene, in particular the zero band gap and weak spin orbit coupling. We present a theoretical realization of quantum capacitance whic… ▽ More Silicene and related buckled materials are distinct from both the conventional two dimensional electron gas and the famous graphene due to strong spin orbit coupling and the buckled structure. These materials have potential to overcome limitations encountered for graphene, in particular the zero band gap and weak spin orbit coupling. We present a theoretical realization of quantum capacitance which has advantages over the scattering problems of traditional transport measurements. We derive and discuss quantum capacitance as a function of the Fermi energy and temperature taking into account electron-hole puddles through a Gaussian broadening distribution. Our predicted results are very exciting and pave the way for future spintronic and valleytronic devices. △ Less

Submitted 22 October, 2015; originally announced October 2015.

Comments: 11 pages and 4 figs, Accepted in Physica E. arXiv admin note: text overlap with arXiv:1105.1771 by other authors

Magneto-optical transport properties of monolayer phosphorene

Authors: M. Tahir, P. Vasilopoulos, F. M. Peeters

Abstract: The electronic properties of monolayer phosphorene are exotic due to its puckered structure and large intrinsic direct band gap. We derive and discuss its band structure in the presence of a perpendicular magnetic field. Further, we evaluate the magneto-optical Hall and longitudinal optical conductivities, as functions of temperature, magnetic field, and Fermi energy, and show that they are strong… ▽ More The electronic properties of monolayer phosphorene are exotic due to its puckered structure and large intrinsic direct band gap. We derive and discuss its band structure in the presence of a perpendicular magnetic field. Further, we evaluate the magneto-optical Hall and longitudinal optical conductivities, as functions of temperature, magnetic field, and Fermi energy, and show that they are strongly influenced by the magnetic field. The imaginary part of the former and the real part of the latter exhibit regular {\it interband} oscillations as functions of the frequency $ω$ in the range $\hslashω\sim 1.5-2$ eV. Strong {\it intraband} responses in the latter and week ones in the former occur at much lower frequencies. The magneto-optical response can be tuned in the microwave-to-terahertz and visible frequency ranges in contrast with a conventional two-dimensional electron gas or graphene in which the response is limited to the terahertz regime. This ability to isolate carriers in an anisotropic structure may make phosphorene a promising candidate for new optical devices. △ Less

Submitted 22 July, 2015; v1 submitted 25 May, 2015; originally announced May 2015.

Comments: 7 pages and 8 figures

Journal ref: Phys. Rev. B 92, 045420 (2015)

Off-resonant polarized light-controlled thermoelectric transport in ultrathin topological insulators

Authors: M. Tahir, P. Vasilopoulos

Abstract: We study thermoelectric transport in ultrathin topological insulators under the application of circularly polarized off-resonant light of frequency Ω and amplitude A. We derive analytical expressions for the band structure, orbital magnetization Morb, and the thermal (\k{appa}xy) and Nernst (αxy) conductivities. Reversing the light polarization from right to left leads to an exchange of the conduc… ▽ More We study thermoelectric transport in ultrathin topological insulators under the application of circularly polarized off-resonant light of frequency Ω and amplitude A. We derive analytical expressions for the band structure, orbital magnetization Morb, and the thermal (\k{appa}xy) and Nernst (αxy) conductivities. Reversing the light polarization from right to left leads to an exchange of the conduction and valence bands of the symmetric and antisymmetric surface states and to a sign change in Morb,αxy, and \k{appa}xy. Varying the sample thickness or A/Ω leads to a strong enhancement of Morb and αxy. These effects, accessible to experiments, open the possibility for selective, state-exchanged excitations under light and the conversion of heat to electric energy. △ Less

Submitted 20 April, 2015; originally announced April 2015.

Comments: 5 pages and 6 figs

Journal ref: Phys. Rev. B 91, 115311 (2015)

Electrically tunable magnetoplasmons in a monolayer of silicene or germanene

Authors: M. Tahir, P. Vasilopoulos

Abstract: We theoretically study electrically tunable magnetoplasmons in a monolayer of silicene or germanene. We derive the dynamical response function and take into account the effects of strong spin-orbit coupling (SOC) and of an external electric filed $E_z$ perpendicular to the plane of the buckled silicene/germanene. Employing the random-phase approximation we analyze the magnetoplasmon spectrum. The… ▽ More We theoretically study electrically tunable magnetoplasmons in a monolayer of silicene or germanene. We derive the dynamical response function and take into account the effects of strong spin-orbit coupling (SOC) and of an external electric filed $E_z$ perpendicular to the plane of the buckled silicene/germanene. Employing the random-phase approximation we analyze the magnetoplasmon spectrum. The dispersion relation has the same form as in a two-dimensional electron gas with the cyclotron and plasma frequencies modified due to the SOC and the field $E_z$. In the absence of SOC and $E_z$, our results agree well with recent experiments on graphene. The predicted effects could be tested by experiments similar to those on graphene and would be useful for future spintronics and optoelectronic devices. △ Less

Submitted 20 April, 2015; originally announced April 2015.

Comments: 8 pages and 6 figs

Journal ref: J. Phys.: Condens. Matter 27, 075303 (2015)

Tunable thermoelectricity in monolayers of MoS$_{2}$ and other group-VI dichalcogenides

Authors: M. Tahir, U. Schwingenschlogl

Abstract: We study the thermoelectric properties of monolayers of MoS$_{2}$ and other group-VI dichalcogenides under circularly polarized off-resonant light. Analytical expressions are derived for the Berry phase mediated magnetic moment, orbital magnetization, as well as thermal and Nernst conductivities. Tuning of the band gap by {\it off-resonant} light enhances the spin splitting in both the valence and… ▽ More We study the thermoelectric properties of monolayers of MoS$_{2}$ and other group-VI dichalcogenides under circularly polarized off-resonant light. Analytical expressions are derived for the Berry phase mediated magnetic moment, orbital magnetization, as well as thermal and Nernst conductivities. Tuning of the band gap by {\it off-resonant} light enhances the spin splitting in both the valence and conduction bands and, thus, leads to a dramatic improvement of the spin and valley thermoelectric properties. △ Less

Submitted 18 April, 2015; originally announced April 2015.

Comments: 10 pages and 6 figs

Journal ref: New J. Phys. 16 115003 (2014)

Photoinduced quantum spin and valley Hall effects and orbital magnetization in monolayer MoS2

Authors: M. Tahir, A. Manchon, U. Schwingenschlogl

Abstract: We theoretically demonstrate that 100\% valley-polarized transport in monolayers of MoS$_{2}$ and other group-VI dichalcogenides can be obtained using off-resonant circularly polarized light. By tuning the intensity of the off-resonant light the intrinsic band gap in one valley is reduced, while it is enhanced in the other valley, enabling single valley quantum transport. As a consequence, we pred… ▽ More We theoretically demonstrate that 100\% valley-polarized transport in monolayers of MoS$_{2}$ and other group-VI dichalcogenides can be obtained using off-resonant circularly polarized light. By tuning the intensity of the off-resonant light the intrinsic band gap in one valley is reduced, while it is enhanced in the other valley, enabling single valley quantum transport. As a consequence, we predict (i) enhancement of the longitudinal electrical conductivity, accompanied by an increase in the spin-polarization of the flowing electrons, (ii) enhancement of the intrinsic spin Hall effect, together with a reduction of the intrinsic valley Hall effect, and (iii) enhancement of the orbital magnetic moment and orbital magnetization. These mechanisms provide appealing opportunities to the design of nanoelectronics based on dichalcogenides. △ Less

Submitted 18 April, 2015; originally announced April 2015.

Comments: 10 pages and 2 figs

Journal ref: Phys. Rev. B 90, 125438 (2014)

Electrically Engineered Band Gap in Two-Dimensional Ge, Sn, and Pb: A First-Principles and Tight-Binding Approach

Authors: Thaneshwor P. Kaloni, Mohsen Modarresi, Muhammad Tahir, Mahmood Rezaee Roknabadi, Georg Schreckenbach, Michael S. Freund

Abstract: First-principles calculations were performed to investigate the electronic structure of two-dimensional (2-D) Ge, Sn, and Pb without and with the presence of an external electric field in combination with spin-orbit coupling. Tight-binding calculations based on four orbitals per atom and an effective single orbital are presented to match with the results obtained from first-principles calculations… ▽ More First-principles calculations were performed to investigate the electronic structure of two-dimensional (2-D) Ge, Sn, and Pb without and with the presence of an external electric field in combination with spin-orbit coupling. Tight-binding calculations based on four orbitals per atom and an effective single orbital are presented to match with the results obtained from first-principles calculations. In particular, the electronic band structure and the band splitting are investigated with both models. Moreover, the simple $k\cdot p$ model is also considered in order to understand the band splitting in the presence of an external electric field and spin-orbit coupling. A large splitting is obtained, which is expected to be useful for spintronic devices. The fair agreement between the first-principle, $k\cdot p$ model, and tight-binding approaches leads to a table of parameters for future tight-binding studies on hexagonal 2-D nanostructures. By using the tight binding parameters, the transport properties of typical 0-D triangular quantum dots between two semi-infinite electrodes in the presence of spin-orbit coupling are addressed. △ Less

Submitted 17 April, 2015; originally announced April 2015.

Comments: 23 pages, 6 figures, and 3 tables

Prediction of topological Nernst effect in silicene and similar 2D materials

Authors: I. Ahmed, M. Tahir, K. Sabeeh

Abstract: We consider Berry phase mediated Nernst effect in silicene. The low energy band structure of silicene consists of two valleys near the Dirac points, similar to graphene. The low energy transport properties of the quasiparticles can be described as Berry phase dependent phenomena. By contrast to graphene, silicene has strong spin-orbit interaction leading to opening of the gap in the energy spectru… ▽ More We consider Berry phase mediated Nernst effect in silicene. The low energy band structure of silicene consists of two valleys near the Dirac points, similar to graphene. The low energy transport properties of the quasiparticles can be described as Berry phase dependent phenomena. By contrast to graphene, silicene has strong spin-orbit interaction leading to opening of the gap in the energy spectrum and spin-splitting of the bands in each valley. If an electric field is applied perpendicular to the silicene sheet, it allows tunability of the gap. \ We show that this results in Berry-phase-supported spin and valley polarized Nernst effect when the system is subjected to a temperature gradient. The Nernst response can be used to create valley and spin polarization at the transverse edges of silicene sheet. The applied electric field also allows control of valley and spin polarization in silicene. The predicted valley and spin polarized Nernst effect in silicene is more general and applies to other two-dimensional (2D) buckled Dirac Fermion systems such as 2D germanium and tin. △ Less

Submitted 25 February, 2014; originally announced February 2014.

Quasi free-standing silicene in a superlattice with hexagonal boron nitride

Authors: T. P. Kaloni, M. Tahir, U. Schwingenschlögl

Abstract: We study a superlattice of silicene and hexagonal boron nitride by first principles calculations and demonstrate that the interaction between the layers of the superlattice is very small. As a consequence, quasi free-standing silicene is realized in this superlattice. In particular, the Dirac cone of silicene is preserved, which has not been possible in any other system so far. Due to the wide ban… ▽ More We study a superlattice of silicene and hexagonal boron nitride by first principles calculations and demonstrate that the interaction between the layers of the superlattice is very small. As a consequence, quasi free-standing silicene is realized in this superlattice. In particular, the Dirac cone of silicene is preserved, which has not been possible in any other system so far. Due to the wide band gap of hexagonal boron nitride, the superlattice realizes the characteristic physical phenomena of free-standing silicene. In particular, we address by model calculations the combined effect of the intrinsic spin-orbit coupling and an external electric field, which induces a transition from a metal to a topological insulator and further to a band insulator. △ Less

Submitted 29 October, 2013; originally announced October 2013.

Comments: 9 pages and 3 figures

Journal ref: Sci. Rep. 3, 3192 (2013)

Quantum capacitance of an ultrathin topological insulator film in a magnetic field

Authors: M. Tahir, K. sabeeh, U. Schwingenschlogl

Abstract: We present a theoretical study of the quantum magnetocapacitance of an ultrathin topological insulator film in an external magnetic field. The study is undertaken to investigate the interplay of the Zeeman interaction with the hybridization between the upper and lower surfaces of the thin film. Determining the density of states, we find that the electron-hole symmetry is broken when the Zeeman and… ▽ More We present a theoretical study of the quantum magnetocapacitance of an ultrathin topological insulator film in an external magnetic field. The study is undertaken to investigate the interplay of the Zeeman interaction with the hybridization between the upper and lower surfaces of the thin film. Determining the density of states, we find that the electron-hole symmetry is broken when the Zeeman and hybridization energies are varied relative to each other. This leads to a change in the character of the magnetocapacitance at the charge neutrality point. We further show that in the presence of both Zeeman interaction and hybridization the magnetocapacitance exhibits beating at low and splitting of the Shubnikov de Haas oscillations at high perpendicular magnetic field. In addition, we address the crossover from perpendicular to parallel magnetic field and find consistency with recent experimental data. △ Less

Submitted 6 February, 2013; originally announced February 2013.

Comments: 5 pages and 4 figures

Journal ref: Sci. Rep. 3, 1261 (2013)

Valley polarized quantum Hall effect and topological insulator phase transitions in silicene

Authors: M. Tahir, U. Schwingenschlogl

Abstract: Silicene is a buckled monolayer of silicon. Its electronic properties are distinct from both the conventional two dimensional electron gas and the famous graphene due to strong spin orbit interaction and the buckled structure. Silicene has the potential to overcome limitations encountered for graphene, in particular the zero band gap and weak spin orbit interaction. We find for silicene a valley p… ▽ More Silicene is a buckled monolayer of silicon. Its electronic properties are distinct from both the conventional two dimensional electron gas and the famous graphene due to strong spin orbit interaction and the buckled structure. Silicene has the potential to overcome limitations encountered for graphene, in particular the zero band gap and weak spin orbit interaction. We find for silicene a valley polarized quantum Hall effect and topological insulator phase transitions. We use the Kubo formalism to discuss the Hall conductivity and address the longitudinal conductivity for elastic impurity scattering in the first Born approximation. We show that the combination of an electric field with intrinsic spin orbit interaction leads to quantum phase transitions at the charge neutrality point. This provides a tool to experimentally tune the topological state of silicene. In contrast to graphene and other conventional topological insulators, the effects in silicene are experimentally accessible. Therefore, silicene constitutes a model system for exploring the spin and valley physics not accessible in graphene due to the small spin orbit interaction. △ Less

Submitted 9 January, 2013; v1 submitted 19 July, 2012; originally announced July 2012.

Comments: 4 pages 8 figures

Journal ref: Sci. Rep. 3, 1075 (2013)

Quantum spin/valley Hall effect and topological insulator phase transitions in silicene

Authors: M. Tahir, A. Manchon, K. Sabeeh, U. Schwingenschlogl

Abstract: We present a theoretical realization of quantum spin and quantum valley Hall effects in silicene. We show that combination of an electric field and intrinsic spin-orbit interaction leads to quantum phase transitions at the charge neutrality point. This phase transition from a two dimensional topological insulator to a trivial insulating state is accompanied by a quenching of the quantum spin Hall… ▽ More We present a theoretical realization of quantum spin and quantum valley Hall effects in silicene. We show that combination of an electric field and intrinsic spin-orbit interaction leads to quantum phase transitions at the charge neutrality point. This phase transition from a two dimensional topological insulator to a trivial insulating state is accompanied by a quenching of the quantum spin Hall effect and the onset of a quantum valley Hall effect, providing a tool to experimentally tune the topological state of silicene. In contrast to graphene and other conventional topological insulators, the proposed effects in silicene are accessible to experiments. △ Less

Submitted 16 June, 2012; originally announced June 2012.

Comments: 8 pages and 2 figures

Journal ref: Appl. Phys. Lett. 102, 162412 (2013)

Gap opening in the zeroth Landau level in gapped graphene: Pseudo-Zeeman splitting in an angular magnetic field

Authors: M. Tahir, K. Sabeeh

Abstract: We present a theoretical study of gap opening in the zeroth Landau level in gapped graphene as a result of pseudo-Zeeman interaction. The applied magnetic field couples with the valley pseudospin degree of freedom of the charge carriers leading to the pseudo-Zeeman interaction. To investigate its role in transport at the Charge Neutrality Point (CNP), we study the integer quantum Hall effect (QHE)… ▽ More We present a theoretical study of gap opening in the zeroth Landau level in gapped graphene as a result of pseudo-Zeeman interaction. The applied magnetic field couples with the valley pseudospin degree of freedom of the charge carriers leading to the pseudo-Zeeman interaction. To investigate its role in transport at the Charge Neutrality Point (CNP), we study the integer quantum Hall effect (QHE) in gapped graphene in an angular magnetic field in the presence of pseudo-Zeeman interaction. Analytical expressions are derived for the Hall conductivity using Kubo-Greenwood formula. We also determine the longitudinal conductivity for elastic impurity scattering in the first Born approximation. We show that pseudo-Zeeman splitting leads to a minimum in the collisional conductivity at high magnetic fields and a zero plateau in the Hall conductivity. Evidence for activated transport at CNP is found from the temperature dependence of the collisional conductivity. △ Less

Submitted 10 February, 2012; originally announced February 2012.

Comments: 20 pages, 4 figures, Accepted in J. Phys. Condensed matter

Inelastic electron transport through Quantum Dot coupled with an nano mechancial oscillator in the presence of strong applied magnetic field

Authors: M. Imran, B. Tariq, M. Tahir, K. Sabeeh

Abstract: In this study we explain the role of applied magnetic field in inelastic conduction properties of a Quantum Dot coupled with an oscillator . In the presence of strong applied magnetic field coulomb blockade effects become weak due to induced Zeeman splitting in spin degenerate eigen states of Quantum Dot.By contacting Quantum Dot by identical metallic leads tunneling rates of spin down and spin up… ▽ More In this study we explain the role of applied magnetic field in inelastic conduction properties of a Quantum Dot coupled with an oscillator . In the presence of strong applied magnetic field coulomb blockade effects become weak due to induced Zeeman splitting in spin degenerate eigen states of Quantum Dot.By contacting Quantum Dot by identical metallic leads tunneling rates of spin down and spin up electrons between Quantum Dot and electrodes will be symmetric. For symmetric tunneling rates of spin down and spin up electrons onto Quantum Dot, first oscillator get excited by spin down electrons and then spin up elctrons could excite it further. Where as average energy transferred to oscillator coupled with Quantum Dot by spin down electrons will further increase by average energy transferred by spin up electrons to oscillator. Here we have also discussed that with increasing Quantum Dot and electrodes coupling strength phononic side band peaks start hiding up, which happens because with increasing tunneling rates electronic states of Quantum Dot start gettting broadened. △ Less

Submitted 3 October, 2011; originally announced October 2011.

Electron transport through a coupled double dot molecule: role of inter-dot coupling, phononic and dissipative effects

Authors: M. Imran, B. Tariq, M. Tahir, K. Sabeeh

Abstract: In this work, we have investigated conduction through an artificial molecule comprising two coupled quantum dots. The question addressed is the role of inter-dot coupling on electronic transport. We find that the current through the molecule exhibits step-like features as a function of the voltage between the leads, where the step size increases as the inter-dot coupling is increased. These step-l… ▽ More In this work, we have investigated conduction through an artificial molecule comprising two coupled quantum dots. The question addressed is the role of inter-dot coupling on electronic transport. We find that the current through the molecule exhibits step-like features as a function of the voltage between the leads, where the step size increases as the inter-dot coupling is increased. These step-like features disappear with increasing tunneling rate from the leads, but we find that in the presence of coupling, this smooth behavior is not observed rather two kinks are seen in the current voltage curve. This shows that the resolution of the two levels persists if there is finite inter-dot coupling. Furthermore, we also consider the effects of electron-phonon interaction as well as dissipation on conduction in this system. Phononic side bands in the differential conductance survive for finite inter-dot coupling even for strong lead to molecule coupling. △ Less

Submitted 21 September, 2011; originally announced September 2011.

Time-dependent transport via a quantum shuttle

Authors: Muhammad Tahir, Angus MacKinnon

Abstract: We present a theoretical study of time-dependent transport via a quantum shuttle within the non-equilibrium Green's function technique. An arbitrary voltage is applied to the tunnel junction and electrons in the leads are considered to be at zero temperature. The transient and the steady state behavior of the system is considered here in order to explore the quantum dynamics of the shuttle device… ▽ More We present a theoretical study of time-dependent transport via a quantum shuttle within the non-equilibrium Green's function technique. An arbitrary voltage is applied to the tunnel junction and electrons in the leads are considered to be at zero temperature. The transient and the steady state behavior of the system is considered here in order to explore the quantum dynamics of the shuttle device as a function of time and applied bias. The properties of the phonon distribution of the oscillating dot coupled to the electrons are investigated using a non-perturbative approach. We derive a relation for the oscillator momentum charge density correlation function which is an interesting physical example for the visualization of shuttling phenomenon. We consider the crossover between the tunneling and shuttling regimes for different values of the key parameters as a function of applied bias and time. We also consider the energy transferred from the electrons to the oscillating dot as a function of time. This will provide useful insight for the design of experiments aimed at studying the quantum behavior of a shuttling device. △ Less

Submitted 27 May, 2011; originally announced May 2011.

Current noise of a resonant tunnel junction coupled to a nanomechanical oscillator

Authors: M. Tahir, A. MacKinnon

Abstract: We present a theoretical study of current noise of a resonant tunnel junction coupled to a nanomechanical oscillator within the non-equilibrium Green's function technique. An arbitrary voltage is applied to the tunnel junction and electrons in the leads are considered to be at zero temperature. The properties of the phonon distribution of the nanomechanical oscillator strongly coupled to the elect… ▽ More We present a theoretical study of current noise of a resonant tunnel junction coupled to a nanomechanical oscillator within the non-equilibrium Green's function technique. An arbitrary voltage is applied to the tunnel junction and electrons in the leads are considered to be at zero temperature. The properties of the phonon distribution of the nanomechanical oscillator strongly coupled to the electrons on the dot are investigated using a non-perturbative approach. An analytical calculations and numerical results for the current-voltage, shot noise and the corresponding Fano factor as a function of applied bias show significant features of the nanomechanical oscillator coupling dynamics. This will provide useful insight for the design of experiments aimed at studying the quantum behavior of an oscillator. △ Less

Submitted 20 May, 2010; originally announced May 2010.

Comments: 4 figures

Spin-orbit interaction effects on the magnetoplasmon spectrum of modulated two-dimensional electron gas

Authors: M. Tahir, K. Sabeeh

Abstract: We present a theoretical study of magnetoplasmon spectrum of a two-dimensional electron gas in the presence of Rashba spin-orbit interaction (RSOI), one-dimensional weak electric modulation and a perpendicular magnetic field. The intra-Landau-band magnetoplasmon spectrum is determined in the presence of spin-orbit interaction within the self consistent field approach at finite temperature. Due t… ▽ More We present a theoretical study of magnetoplasmon spectrum of a two-dimensional electron gas in the presence of Rashba spin-orbit interaction (RSOI), one-dimensional weak electric modulation and a perpendicular magnetic field. The intra-Landau-band magnetoplasmon spectrum is determined in the presence of spin-orbit interaction within the self consistent field approach at finite temperature. Due to Rashba effect, the spin of finite-momentum electrons feels a magnetic field perpendicular to the electron momentum in the inversion plane. The magnetoplasmon spectrum of the modulated two-dimensional electron gas (M2DEG) system is found to exhibit beating of Weiss oscillations due to Rashba effect which is the focus of this work. This effect is absent in the magnetoplasmon spectrum of M2DEG if Rashba spin-orbit interaction is not taken into account. In addition, our finite temperature theory ficilitates analysis of effects of temperature on the magnetoplasmon spectrum of M2DEG in the presence of RSOI. We find that the beating pattern is damped but continues to persist at a finite but low temperature. △ Less

Submitted 15 February, 2010; originally announced February 2010.

Comments: Accepted in Physica E

Time-dependent quantum transport in a resonant tunnel junction coupled to a nanomechanical oscillator

Authors: M. Tahir, A. MacKinnon

Abstract: We present a theoretical study of time-dependent quantum transport in a resonant tunnel junction coupled to a nanomechanical oscillator within the non-equilibrium Green's function technique. An arbitrary voltage is applied to the tunnel junction and electrons in the leads are considered to be at zero temperature. The transient and the steady state behavior of the system is considered here in ord… ▽ More We present a theoretical study of time-dependent quantum transport in a resonant tunnel junction coupled to a nanomechanical oscillator within the non-equilibrium Green's function technique. An arbitrary voltage is applied to the tunnel junction and electrons in the leads are considered to be at zero temperature. The transient and the steady state behavior of the system is considered here in order to explore the quantum dynamics of the oscillator as a function of time. The properties of the phonon distribution of the nanomechnical oscillator strongly coupled to the electrons on the dot are investigated using a non-perturbative approach. We consider both the energy transferred from the electrons to the oscillator and the Fano factor as a function of time. We discuss the quantum dynamics of the nanomechanical oscillator in terms of pure and mixed states. We have found a significant difference between a quantum and a classical oscillator. In particular, the energy of a classical oscillator will always be dissipated by the electrons whereas the quantum oscillator remains in an excited state. This will provide useful insight for the design of experiments aimed at studying the quantum behavior of an oscillator. △ Less

Submitted 24 January, 2010; originally announced January 2010.

Comments: 24 pages, 10 figures

Magnetotransport of a periodically modulated graphene monolayer

Authors: R. Nasir, K. Sabeeh, M. Tahir

Abstract: We have performed a detailed investigation of the electrical properties of a graphene monolayer which is modulated by a weak one dimensional periodic potential in the presence of a perpendicular magnetic field. The periodic modulation broadens the Landau Levels into bands which oscillate with B. The electronic conduction in this system can take place through either diffusive scattering or collis… ▽ More We have performed a detailed investigation of the electrical properties of a graphene monolayer which is modulated by a weak one dimensional periodic potential in the presence of a perpendicular magnetic field. The periodic modulation broadens the Landau Levels into bands which oscillate with B. The electronic conduction in this system can take place through either diffusive scattering or collisional scattering off impurities. Both these contributions to electronic transport are taken into account in this work. In addition to the appearance of commensurability oscillations in both the collisional and diffusive contributions, we find that Hall resistance also exhibits commensurability oscillations. Furthermore, the period and amplitude of these commensurability oscillations in the transport parameters and how they are affected by temperature are also discussed in this work. △ Less

Submitted 17 August, 2009; originally announced August 2009.

Comments: 16 pages, 7 figures

Magnetocapacitance of a graphene monolayer

Authors: M. Tahir, K. Sabeeh, A. MacKinnon

Abstract: We present a theoretical study of magnetocapacitance in a graphene monolayer at finite temperature taking into account the effects of disorder. The density of states (DOS) and magnetocapacitance found for graphene are compared to those found in standard two dimensional electron gas (2DEG) systems. The magnetic oscillations in DOS and magnetocapacitance are found to be enhanced and much more robust… ▽ More We present a theoretical study of magnetocapacitance in a graphene monolayer at finite temperature taking into account the effects of disorder. The density of states (DOS) and magnetocapacitance found for graphene are compared to those found in standard two dimensional electron gas (2DEG) systems. The magnetic oscillations in DOS and magnetocapacitance are found to be enhanced and much more robust with respect to temperature damping in monolayer graphene in comparison with a 2DEG. Furthermore, we find that there is a $π$ phase shift between magnetic oscillations in the two systems which can be attributed to Dirac electrons in graphene acquiring a Berry's phase as they traverse a closed path in a magnetic field. △ Less

Submitted 3 February, 2012; v1 submitted 29 August, 2008; originally announced August 2008.

Comments: Withdrawn due to some changes in other paper; the paper has been withdrawn due to further changes in the manuscript

Energy loss of charged particles in a two-dimensional Dirac plasma

Authors: Aqsa Arshad, Kashif Sabeeh, M. Tahir

Abstract: The stopping power and energy loss rate of charged particles traversing a two-dimensional Dirac plasma is investigated. The Dirac plasma considered here models a solid state system, recently realized graphene monolayer, where the conduction electrons obey the Dirac-like equation and exhibit a linear in momentum dispersion relation. Theoretical work presented here is based on the the dielectric r… ▽ More The stopping power and energy loss rate of charged particles traversing a two-dimensional Dirac plasma is investigated. The Dirac plasma considered here models a solid state system, recently realized graphene monolayer, where the conduction electrons obey the Dirac-like equation and exhibit a linear in momentum dispersion relation. Theoretical work presented here is based on the the dielectric response function and the dynamical structure function within the Random-Phase-Approximation (RPA). △ Less

Submitted 23 August, 2008; originally announced August 2008.

Comments: 9 pages, 5 figures, submitted in Physical Review B

The Magnetoplasmon spectrum of a weakly modulated two-dimensional electron gas system

Authors: M. Tahir, K. Sabeeh, A. MacKinnon

Abstract: The magnetoplasmon spectrum of a magnetically modulated two-dimensional electron gas (MM2DEG) is investigated. We derive the inter and intra Landau band magnetoplasmon spectrum within the self consistent field approach. The derivation is performed at zero temperature as well as at finite temperature. Results are presented for the inter and intra Landau band magnetoplasmon spectrum as a function… ▽ More The magnetoplasmon spectrum of a magnetically modulated two-dimensional electron gas (MM2DEG) is investigated. We derive the inter and intra Landau band magnetoplasmon spectrum within the self consistent field approach. The derivation is performed at zero temperature as well as at finite temperature. Results are presented for the inter and intra Landau band magnetoplasmon spectrum as a function of the inverse magnetic field. As a result of magnetic modulation, magnetic Weiss oscillations are found to occur in the magnetoplasmon spectrum. Furthermore, our finite temperature theory facilitates analysis of effects of temperature on the magnetoplasmon spectrum. The results are compared with those obtained for an electrically modulated 2DEG system. In addition, we derive and discuss the effects of simultaneous electric and magnetic modulations on the magnetoplasmon spectrum of 2DEG when the modulations are in phase as well as when they are out of phase. Magnetic oscillations are affected by the relative phase of the two modulations and position of the oscillations depends on the relative strength of the two modulations in the former case while we find complete suppression of Weiss oscillations for particular relative strength of the modulations in the latter case. △ Less

Submitted 12 December, 2009; v1 submitted 4 August, 2008; originally announced August 2008.

Comments: 19 pages, 6 figures, accepted in J. Phys. Condens. Matter

Journal ref: J. Phys.: Condens. Matter 22 015801 (2010)

Suppression of Weiss oscillations in the magnetoconductivity of modulated graphene monolayer

Authors: M. Tahir, K. Sabeeh

Abstract: We have investigated the electrical transport properties of Dirac electrons in a monolayer graphene sheet in the presence of both electric and magnetic modulations. The effects of the modulations on quantum transport when they are in phase and out of phase are considered. We present the energy spectrum and the bandwidth of the Dirac electrons in the presence of both the modulations. We determine… ▽ More We have investigated the electrical transport properties of Dirac electrons in a monolayer graphene sheet in the presence of both electric and magnetic modulations. The effects of the modulations on quantum transport when they are in phase and out of phase are considered. We present the energy spectrum and the bandwidth of the Dirac electrons in the presence of both the modulations. We determine the $σ_{yy}$ component of the magnetoconductivity tensor for this system which is shown to exhibit Weiss oscillations.Asymptotic expressions for $σ_{yy}$ are also calculated to better illustrate the effects of in-phase and out-of-phase modulations.We find that the position of the oscillations in magnetoconductivity depends on the relative strength of the two modulations. When the two modulations are out-of-phase there is complete suppression of Weiss oscillations for particular relative strength of the modulations. △ Less

Submitted 16 July, 2009; v1 submitted 17 July, 2008; originally announced July 2008.

Comments: 14 pages, 4 figures, Published in International journal of Modern Physics B

Journal ref: International Journal of Modern Physics B (IJMPB), Year: 2009 Vol: 23 Issue: 16 (30 June 2009) Page: 3445 - 3457

Temperature effects on magnetoplasmon spectrum of a weakly modulated graphene monolayer

Authors: M. Tahir, K. Sabeeh, A. MacKinnon

Abstract: In this work, we determine the effects of temperature on the magnetoplasmon spectrum of an electrically modulated graphene monolayer as well as the two-dimensional electron gas (2DEG). The intra-Landau-band magnetoplasmon spectrum within the Self Consistent Field (SCF) approach is investigated for both the aforementioned systems. Results obtained not only exhibit Shubnikov-de Hass (SdH) oscillatio… ▽ More In this work, we determine the effects of temperature on the magnetoplasmon spectrum of an electrically modulated graphene monolayer as well as the two-dimensional electron gas (2DEG). The intra-Landau-band magnetoplasmon spectrum within the Self Consistent Field (SCF) approach is investigated for both the aforementioned systems. Results obtained not only exhibit Shubnikov-de Hass (SdH) oscillations but also commensurability oscillations (Weiss oscillations). These oscillations are periodic as a function of inverse magnetic field. We find that both the magnetic oscillations, SdH and Weiss, have a greater amplitude and are more robust against temperature in graphene compared to conventional 2DEG. Furthermore, there is a $π$ phase shift between the magnetoplasmon oscillations in the two systems which can be attributed to Dirac electrons in graphene acquiring a Berry's phase as they traverse a closed path in a magnetic field. △ Less

Submitted 22 September, 2011; v1 submitted 11 July, 2008; originally announced July 2008.

Comments: 11 pages, 3 figures, Accepted in J. Phys. Condensed matter

Electronic structure of magnetically modulated graphene

Authors: K. Sabeeh, M. Tahir, A. MacKinnon

Abstract: We present a theoretical study of the electronic structure of magnetically modulated graphene. We consider monolayer graphene in the presence of a perpendicular magnetic field and a unidirectional weak magnetic modulation. The density of states and the bandwidth of the Dirac electrons in this system are determined. We have found magnetic Weiss oscillations in the bandwidth and the density of state… ▽ More We present a theoretical study of the electronic structure of magnetically modulated graphene. We consider monolayer graphene in the presence of a perpendicular magnetic field and a unidirectional weak magnetic modulation. The density of states and the bandwidth of the Dirac electrons in this system are determined. We have found magnetic Weiss oscillations in the bandwidth and the density of states. These oscillations are out of phase and larger in amplitude than the ones in the electrically modulated graphene. In addition, these oscillations are in phase and smaller in amplitude to those of magnetically modulated standard electron gas system. △ Less

Submitted 3 February, 2012; v1 submitted 29 April, 2008; originally announced April 2008.

Comments: withdrawn and will be submitted with revised version with different physics

Weiss oscillations in the magnetoconductivity of modulated graphene bilayer

Authors: M. Tahir, K. Sabeeh

Abstract: We present a theoretical study of Weiss oscillations in magnetoconductivity of bilayer graphene. Bilayer graphene in the presence of a perpendicular magnetic field and a unidirectional weak electric modulation is considered.We determine the $σ_{yy}$ component of the magnetoconductivity tensor for this system which is shown to exhibit Weiss oscillations. We show that Weiss oscillations in the mag… ▽ More We present a theoretical study of Weiss oscillations in magnetoconductivity of bilayer graphene. Bilayer graphene in the presence of a perpendicular magnetic field and a unidirectional weak electric modulation is considered.We determine the $σ_{yy}$ component of the magnetoconductivity tensor for this system which is shown to exhibit Weiss oscillations. We show that Weiss oscillations in the magnetoconductivity of bilayer graphene are enhanced and more robust with temperature compared to those in conventional two-dimensional electron gas systems whereas they are less robust with temperature compared to monolayer graphene. In addition, we also find phase differences of $π$ and $2π$ in the magnetoconductivity oscillations compared to monolayer graphene and conventional 2DEG system which arises due to the chiral nature of quasiparticles in graphene. △ Less

Submitted 29 April, 2008; v1 submitted 25 April, 2008; originally announced April 2008.

Comments: Submitted: 11 pages, 3 figures

Inter-band magnetoplasmons in mono- and bi-layer graphene

Authors: M. Tahir, K. Sabeeh

Abstract: Collective excitations spectrum of Dirac electrons in mono and bilayer graphene in the presence of a uniform magnetic field is investigated. Analytical results for inter-Landau band plasmon spectrum within the self-consistent-field approach are obtained. SdH type oscillations that are a monotonic function of the magnetic field are observed in the plasmon spectrum of both mono- and bi-layer graph… ▽ More Collective excitations spectrum of Dirac electrons in mono and bilayer graphene in the presence of a uniform magnetic field is investigated. Analytical results for inter-Landau band plasmon spectrum within the self-consistent-field approach are obtained. SdH type oscillations that are a monotonic function of the magnetic field are observed in the plasmon spectrum of both mono- and bi-layer graphene systems. The results presented are also compared with those obtained in conventional 2DEG. The chiral nature of the quasiparticles in mono and bilayer graphene system results in the observation of $π$ and $2π$ Berry's phase in the SdH- type oscillations in the plasmon spectrum. △ Less

Submitted 17 September, 2008; v1 submitted 15 April, 2008; originally announced April 2008.

Comments: 9 pages, 2 figures

Chiral tunneling through time-periodic potential in graphene

Authors: M. Ahsan Zeb, K. Sabeeh, M. Tahir

Abstract: Chiral tunneling through a harmonically driven potential barrier in graphene monolayer is considered in this work. Since the quasiparticles in this system are chiral in nature, tunneling is highly anisotropic, we determine the transmission probabilities for the central and sidebands as the incident angle of the electron beam is changed . Furthermore, we investigate how the transmission probabili… ▽ More Chiral tunneling through a harmonically driven potential barrier in graphene monolayer is considered in this work. Since the quasiparticles in this system are chiral in nature, tunneling is highly anisotropic, we determine the transmission probabilities for the central and sidebands as the incident angle of the electron beam is changed . Furthermore, we investigate how the transmission probabilities change as the width, amplitude and frequency of the oscillating barrier is changed. An interesting result of our study is that perfect transmission for normal incidence that has been reported for a static barrier persists for the oscillating barrier, manifestation of Klein tunneling in a time harmonic potential. △ Less

Submitted 23 August, 2008; v1 submitted 13 April, 2008; originally announced April 2008.

Comments: submitted in PRB; 17 pages, discussion of bilayer replaced with further analytical results for monolayer

Thermodynamic Properties of electrically modulated monolayer Graphene

Authors: R. Nasir, M. A. Khan, M. Tahir, K. Sabeeh

Abstract: Theoretical investigation of thermodynamic properties of electrically modulated monolayer graphene in the presence of a perpendicular magnetic field B is presented.This work is aimed at determining the modulation induced effects on the thermodynamic properties of electrically modulated graphene.The results obtained are compared with those of the conventional 2DEG. The one-dimensional periodic po… ▽ More Theoretical investigation of thermodynamic properties of electrically modulated monolayer graphene in the presence of a perpendicular magnetic field B is presented.This work is aimed at determining the modulation induced effects on the thermodynamic properties of electrically modulated graphene.The results obtained are compared with those of the conventional 2DEG. The one-dimensional periodic potential due to electric modulation lifts the degeneracy of the Landau Levels and converts them into bands whose width oscillates as a function of B. We find Weiss type oscillations for small values of B and dHvA type oscillations at larger values values of $B$. We find that the modulation induced effects on the thermodynamic properties are enhanced and less damped with temperature in graphene compared with conventional 2DEG system. △ Less

Submitted 21 April, 2008; v1 submitted 10 April, 2008; originally announced April 2008.

Comments: Submitted in PRB

Quantum transport in a resonant tunnel junction coupled to a nanomechanical oscillator

Authors: M Tahir, A MacKinnon

Abstract: We discuss the quantum transport of electrons through a resonant tunnel junction coupled to a nanomechanical oscillator at zero temperature. By using the Green's function technique we calculate the transport properties of electrons through a single dot strongly coupled to a single oscillator. We consider a finite chemical potential difference between the right and left leads. In addition to the… ▽ More We discuss the quantum transport of electrons through a resonant tunnel junction coupled to a nanomechanical oscillator at zero temperature. By using the Green's function technique we calculate the transport properties of electrons through a single dot strongly coupled to a single oscillator. We consider a finite chemical potential difference between the right and left leads. In addition to the main resonant peak of electrons on the dot, we find satellite peaks due to the creation of phonons. These satellite peaks become sharper and more significant with increasing coupling strength between the electrons and the oscillator. We also consider the energy transferred from the electrons to the oscillator. △ Less

Submitted 2 June, 2008; v1 submitted 26 November, 2007; originally announced November 2007.

Comments: Updated in response to referees' comments. Section IV amended including figure 3