arXiv subjects

SK Firoz Islam

Publications and source records attributed to SK Firoz Islam.

33 records · Page 2Linked to original sources

arXiv2018

Driven conductance of an irradiated semi-Dirac material

We theoretically investigate the electronic and transport properties of a semi-Dirac material under the influence of an external time dependent periodic driving field (irradiation) by means of Floquet theory. We explore the inelastic scattering mechanism between different side-bands, induced by irradiation, by using Floquet scattering matrix approach. The scattering probabilities between two nearest side-bands depend monotonically on the strength of the amplitude of the irradiation. The external irradiation induces gap into the band dispersion which is strongly dependent on the angular orientation of momentum. Although, the high frequency limit indicates that the gap opening does not occur in an irradiated semi-Dirac material, a careful analysis of the full band structure beyond this limit reveals that gap opening indeed appears for higher values of momentum (away from the Dirac point). Furthermore, the angular dependent dynamical gap is also present which cannot be captured within the high frequency approximation. The contrasting features of irradiated semi-Dirac material, in comparison to irradiated graphene, can be probed via the behavior of conductance. The latter exhibits the appearance of non-zero conductance dips due to the gap opening in Floquet band spectrum. Moreover, by considering a nanoribbon geometry of such material, we also show that it can host a pair of edge modes which are fully decoupled from the bulk, which is in contrast to the case of graphene nanoribbon where the edge modes are coupled to the bulk. We also investigate that if the nanoribbon of this material is exposed to the external irradiation, decoupled edge modes penetrate into the bulk.

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arXiv2018

Probing the edge states in a zigzag phosphorene nanoribbon via RKKY exchange interaction

Phosphorene is an anisotropic puckered two-dimensional ($2$D) hexagonal lattice of phosphorus atoms. The edge modes in a zigzag phosphorene nanoribbon (ZPNR) are quasi-flat in nature and fully isolated from the bulk states, which are unique in comparison to the other hexagonal lattices like graphene, silicene etc. We theoretically investigate the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction between two magnetic impurities placed on the nanoribbon, and extract the signatures of the flat edge states via the behavior of it. Due to the complete separation of the edge states from the bulk, we can isolate the edge mode contribution via the RKKY interaction from that of the bulk by tuning the external gate potential when both the impurities are placed at the same edge. The bulk induced RKKY interaction exhibits very smooth oscillation with the distance between the two impurities, whereas for edge modes it fluctuates very rapidly. We also explore the effect of tensile strain both in absence and presence of gate voltage and reveal that the RKKY interaction strength can be boosted under suitable doping, when both the impurities are within the bulk.

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arXiv2018

Magnetotransport properties of 8-Pmmn borophene: effects of Hall field and strain

The polymorph of ${\rm 8-Pmmn}$ borophene is an anisotropic Dirac material with tilted Dirac cones at two valleys. The tilting of the Dirac cones at two valleys are in opposite direction, which manifests itself via valley dependent Landau levels in presence of an in-plane electric field (Hall field). The sensitivity of the Landau levels on valley index causes valley polarized magnetotransport properties in presence of a Hall field, which is in contrast to the monolayer graphene with isotropic non-tilted Dirac cones. The longitudinal conductivity and Hall conductivity are evaluated by using linear response theory in low temperature regime. An analytical approximate form of longitudinal conductivity is also obtained to reveal how the tilting of the Dirac cones affects the frequency of longitudinal conductivity oscillation (Shubnikov-de Hass oscillation). On the other hand, Hall conductivity exhibits graphene-like plateau excepts the appearance of valley dependent steps which is purely attributed to the Hall field induced lifting valley degeneracy in Landau levels. Another noticeable point is that if the real magnetic field is replaced by the strain induced pseudo magnetic field then the electric field looses its ability to cause valley polarized transport.

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arXiv2017

Signature of tilted Dirac cones in Weiss oscillations of $8-Pmmn$ borophene

Polymorph of 8-Pmmn borophene exhibits anisotropic tilted Dirac cones. In this work, we explore the consequences of the tilted Dirac cones in magnetotransport properties of a periodically modulated borophene. We evaluate modulation induced diffusive conductivity by using linear response theory in low temperature regime. The application of weak modulation (electric/magnetic or both) gives rise to the magnetic field dependent non-zero oscillatory drift velocity which causes Weiss oscillation in the longitudinal conductivity at low magnetic field. The Weiss oscillation is studied in presence of an weak spatial electric, magnetic and both modulations individually. The tilting of the Dirac cones gives rise to additional contribution to the Weiss oscillation in longitudinal conductivity. Moreover, it also enhances the frequency of the Weiss oscillation and modifies its amplitude too. Most remarkably, It is found that the presence of out-of phase both i.e., electric and magnetic modulations can cause a sizable valley polarization in diffusive conductivity. The origin of valley polarization lies in the opposite tilting of the two Dirac cones at two valleys.

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arXiv2017

Enhancement of crossed Andreev reflection in normal-superconductor-normal junction of thin topological insulator

We theoretically investigate the subgapped transport phenomena through a normal-superconductor-normal (NSN) junction made up of ultra thin topological insulator with proximity induced superconductivity. The dimensional crossover from three dimensional ($3$D) topological insulator (TI) to thin two-dimensional ($2$D) TI introduces a new degree of freedom, the so-called hybridization or coupling between the two surface states. We explore the role of hybridization in transport properties of the NSN junction, especially how it affects the crossed Andreev reflection (CAR). We observe that a rib-like pattern appears in CAR probability profile while examined as a function of angle of incidence and length of the superconductor. Depending on the incoming and reflection or transmission channel, CAR probability can be maneuvered to be higher than $97\%$ under suitable coupling between the two TI surface states along with appropriate gate voltage and doping concentration in the normal region. Coupling between the two surfaces also induces an additional oscillation envelope in the behavior of the angle averaged conductance, with the variation of the length of the superconductor. The behavior of co-tunneling (CT) probability is also very sensitive to the coupling and other parameters. Finally, we also explore the shot noise cross correlation and show that the behavior of the same can be monotonic or non-monotonic depending on the doping concentration in the normal region. Under suitable circumstances, shot noise cross correlation can change sign from positive to negative or vice versa depending on the relative strength of CT and CAR.

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arXiv2017

Amplification of Cooper pair splitting current in a graphene based Cooper pair beam splitter geometry

Motivated by the recent experiments [Scientific reports 6, 23051 (2016), Phys. Rev. Lett. 114, 096602 (2015)], we theoretically investigate Cooper pair splitting current in a graphene based Cooper pair beam splitter geometry. By considering the graphene based superconductor as an entangler device, instead of normal (2D) BCS superconductor, we show that the Cooper pair splitting current mediated by Crossed Andreev process is amplified compared to its normal superconductor counterpart. This amplification is attributed to the strong suppression of local normal Andreev reflection process (arising from the Cooper pair splitting) from the graphene based superconductor to lead via the same quantum dot, in comparison to the usual 2D superconductor. Due to the vanishing density of states at the Dirac point of undoped graphene, a doped graphene based superconductor is considered here and it is observed that Cooper pair splitting current is very insensitive to the doping level in comparison to the usual 2D superconductor. The transport process of non-local spin entangled electrons also depends on the type of pairing i.e. whether the electron-hole pairing is on-site, inter-sublattice or the combination of both. The inter-sublattice pairing of graphene causes the maximum non-local Cooper pair splitting current, whereas presence of both pairing reduces the Cooper pair splitting current.

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arXiv2017

Valley-polarized magnetoconductivity and particle-hole symmetry breaking in a periodically modulated $α$-$\mathcal{T}_3$ lattice

We explore the transport properties of a periodically modulated $α$-$\mathcal{T}_3$ lattice in the presence of a perpendicular magnetic field. The effect of the Berry phase on electrical conductivity oscillation, so-called Weiss oscillation, caused by the modulation induced non-zero drift velocity of charge carriers is investigated. Employing linear response theory within the low temperature regime, we analyze Weiss oscillation as a function of the external magnetic field for both electrically and magnetically modulated $α$-$\mathcal{T}_3$ lattice numerically as well as analytically. The Berry phase makes this hexagonal lattice structure behave differently than other two-dimensional fermionic systems. It causes a significant valley polarization in magnetoconductivity. Most interestingly, the combined effect of both modulations breaks the particle-hole symmetry and causes a smooth transition from even (odd) to odd (even) filling fraction corresponding to the density of states peaks by means of the Berry phase.

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arXiv2016

A scheme to realize the quantum spin-valley Hall effect in monolayer graphene

Quantum spin Hall effect was first predicted in graphene. However, the weak spin orbit interaction in graphene meant that the search for quantum spin Hall effect in graphene never fructified. In this work we show how to generate the quantum spin-valley Hall effect in graphene via quantum pumping by adiabatically modulating a magnetic impurity and an electrostatic potential in a monolayer of strained graphene. We see that not only exclusive spin polarized currents can be pumped in the two valleys in exactly opposite directions but one can have pure spin currents flowing in opposite directions in the two valleys, we call this novel phenomena the quantum spin-valley Hall effect. This means that the twin effects of quantum valley Hall and quantum spin Hall can both be probed simultaneously in the proposed device. This work will significantly advance the field of graphene spintronics, hitherto hobbled by the lack of spin-orbit interaction. We obviate the need for any spin orbit interaction and show how graphene can be manipulated to posses features exclusive to topological insulators.

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arXiv2016

Topologically induced fractional Hall steps in the integer quantum Hall regime of $MoS_2$

The quantum magnetotransport properties of a monolayer of molybdenum disulfide are derived using linear response theory. Especially, the effect of topological terms on longitudinal and Hall conductivity is analyzed. The Hall conductivity exhibits fractional steps in the integer quantum Hall regime. Further complete spin and valley polarization of the longitudinal conductivity is seen in presence of these topological terms. Finally, the Shubnikov-de Hass oscillations are suppressed or enhanced contingent on the sign of these topological terms.

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arXiv2014

Beating pattern in quantum magnetotransport coefficients of spin-orbit coupled Dirac fermions in gated silicene

We report theoretical study of magnetotransport coefficients of spin-orbit coupled gated silicene in presence and absence of spatial periodic modulation. The combined effect of spin-orbit coupling and perpendicular electric field manifests through formation of regular beating pattern in Weiss and SdH oscillations. Analytical results, in addition to the numerical results, of the beating pattern formation are provided. The analytical results yield a beating condition which will be useful to determine the spin-orbit coupling constant by simply counting the number of oscillation between any two successive nodes. Moreover, the numerical results of modulation effect on collisional and Hall conductivities are presented.

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arXiv2014

Thermoelectric properties of an ultra-thin topological insulator

Thermoelectric coefficients of an ultra-thin topological insulator are presented here. The hybridization between top and bottom surface states of a topological insulator plays a significant role. In absence of magnetic field, thermopower increases and thermal conductivity decreases with increase of the hybridization energy. In presence of magnetic field perpendicular to the ultra-thin topological insulator, thermoelectric coefficients exhibit quantum oscillations with inverse magnetic field, whose frequency is strongly modified by the Zeeman energy and phase factor is governed by the product of the Lande g-factor and the hybridization energy. In addition to the numerical results, the low-temperature approximate analytical results of the thermoelectric coefficients are also provided. It is also observed that for a given magnetic field these transport coefficients oscillate with hybridization energy, whose frequency depends on the Lande g-factor.

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arXiv2013

In-plane electric field effect on a spin-orbit coupled two-dimensional electron system in presence of magnetic field

The effect of in-plane electric field on Landau level spacing, spin splitting energy, average spin polarization and average spin current in the bulk as well as at the edges of a two-dimensional electron system with Rashba spin-orbit coupling are presented here. The spin splitting energy for a particular magnetic field is found to be reduced by the external in-plane electric field. Unlike the case of a two-dimensional electron system without Rashba spin-orbit interaction, here the Landau level spacing is electric field dependent. This dependency becomes stronger at the edges in comparison to the bulk. The average spin polarization vector rotates anti-clockwise with the increase of electric field. The average spin current also gets influenced significantly by the application of the in-plane electric field.

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arXiv2013

Modulation effect on the spin Hall resonance

The effect of a weak electrical modulation on spin Hall resonance is presented here. In presence of the magnetic field normal to the plane of the motion of electron, the Landau levels are formed which get broadened due to the weak modulation. The width of the Landau levels broadening are periodic with the inverse magnetic field. There is a certain magnetic field for which the crossing of Landau levels between spin-up and spin-down branches takes place. This gives rise to the resonance in the spin Hall conductivity (SHC). The Landau levels broadening or the energy correction due to the modulation removes the singularity appears at the resonance field in SHC, leading to the suppression of SHC accompanied by two new peaks around this point. The separation of these two peaks increases with the increase of the modulation period. Moreover, we find that the height of the two peaks are also modulation period dependent.

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arXiv2012

Magnetotransport properties of a magnetically modulated two-dimensional electron gas with the spin-orbit interaction

We study the electrical transport properties of a two-dimensional electron gas with the Rashba spin-orbit interaction in presence of a constant perpendicular magnetic field $(B_0 \hat z)$ which is weakly modulated by ${\bf B_1} = B_1 \cos (q x) \hat z$, where $B_1 \ll B_0 $ and $q = 2 π/a $ with $a$ is the modulation period. We obtain the analytical expressions of the diffusive conductivities for spin-up and spin-down electrons. The conductivities for spin-up and spin-down electrons oscillate with different frequencies and produce beating patterns in the amplitude of the Weiss and Shubnikov-de Haas oscillations. We show that the Rashba strength can be determined by analyzing the beating pattern in the Weiss oscillation. We find a simple equation which determines the Rashba spin-orbit interaction strength if the number of Weiss oscillations between any two successive nodes is known from the experiment. We compare our results with the electrically modulated 2DEG with the Rashba interaction. For completeness, we also study the beating pattern formation in the collisional and the Hall conductivities.

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arXiv2011

Thermodynamic properties of a magnetically modulated graphene

The effect of magnetic modulation on thermodynamic properties of a graphene monolayer in presence of a constant perpendicular magnetic field is reported here. One-dimensional spatial electric or magnetic modulation lifts the degeneracy of the Landau levels and converts into bands and their band width oscillates with magnetic field leading to Weiss-type oscillation in the thermodynamic properties. The effect of magnetic modulation on thermodynamic properties of a graphene sheet is studied and then compared with electrically modulated graphene and magnetically modulated conventional two-dimensional electron gas (2DEG). We observe Weiss-type and de Haas-van Alphen (dHvA) oscillations at low and high magnetic field, respectively. There is a definite phase difference in Weiss-type oscillations in thermodynamic quantities of magnetically modulated graphene in compare to electrically modulated graphene. On the other hand, the phase remains same and amplitude of the oscillation is large when compared with the magnetically modulated 2DEG. Explicit asymptotic expressions of density of states and the Helmholtz free energy are provided to understand the phase and amplitude of the Weiss-type oscillations qualitatively. We also study thermodynamic properties when both electric and magnetic modulations are present. The Weiss-type oscillations still exist when the modulations are out-of-phase.

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