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.Comment: 4 pages, 3 figure

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.Comment: 11 pages, 11 figures. To appear in Journal of Physics: Condens. Matte

Topologically induced fractional Hall steps in the integer quantum Hall regime of MoS2MoS_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.Comment: 10 pages, 8 figures, accepted for publication in Nanotechnology (2016

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.Comment: 9 pages, 8 figures, accepted for publication in CARBON (2016

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.Comment: This is the published versio

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.Comment: 5 figures and 7 page

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.Comment: 5 figures and 7 pages. arXiv admin note: text overlap with arXiv:1204.563

Signature of tilted Dirac cones in Weiss oscillations of 8−Pmmn8-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.Comment: To appear in Physical Review

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.Comment: This is the published versio

Enhancement of thermoelectric performance of a nanoribbon made of alpha-T3\mathcal{T}_3 lattice

We present electronic and transport properties of a zigzag nanoribbon made of alpha-$\mathcal{T}_3$ lattice. Our particular focus is on the effects of the continous evolution of the edge modes ( from flat to dispersive) on the thermoelectric transport properties. Unlike the case of graphene nanoribbon, the zigzag nanoribbon of $\alpha-\mathcal{T}_3$ lattice can host a pair of dispersive (chiral) edge modes at the two valleys for specific width of the ribbon. Moreover, gap opening can also occur at the two valleys depending on the width. The slope of the chiral edge modes and the energy gap strongly depend on the relative strength of two kinds of hoping parameters present in the system. We compute corresponding transport coefficients such as conductance, thermopower, thermalconductivity and the thermoelectric figure of merits by using the tight-binding Green function formalism, in order to explore the roles of the dispersive edge modes. It is found that the thermopower and thermoelectric figure of merits can be enhanced significantly by suitably controlling the edge modes. The figure of merits can be enhanced by ten times under suitable parameter regime in comparison to the case of graphene. Finally, we reveal that the presence of line defect, close to the edge, can cause a significant impact on the edge modes as well as on electrical conductance. However, thermopower is relatively less sensitive to such defects.Comment: To appear in JPC