28 research outputs found
Magnetotransport properties of 8-Pmmn borophene: effects of Hall field and strain
The polymorph of 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
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
Topologically induced fractional Hall steps in the integer quantum Hall regime of
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 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 (D) topological insulator (TI) to thin
two-dimensional (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 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- lattice
We present electronic and transport properties of a zigzag nanoribbon made of
alpha- 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 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