925 research outputs found
Competition between the inter-valley scattering and the intra-valley scattering on magnetoconductivity induced by screened Coulomb disorder in Weyl semimetals
Recent experiments on Weyl semimetals reveal that charged impurities may play
an important role. We use a screened Coulomb disorder to model the charged
impurities, and study the magneto-transport in a two-node Weyl semimetal. It is
found that when the external magnetic field is applied parallel to the electric
field, the calculated longitudinal magnetoconductivity shows positive in the
magnetic field, which is just the negative longitudinal magnetoresistivity
(LMR) observed in experiments. When the two fields are perpendicular to each
other, the transverse magnetoconductivities are measured. It is found that the
longitudinal (transverse) magnetoconductivity is suppressed (enhanced)
sensitively with increasing the screening length. This feature makes it hardly
to observe the negative LMR in Weyl semimetals experimentally owing to a small
screening length. Our findings gain insight into further understanding on
recently actively debated magneto-transport behaviors in Weyl semimetals.
Furthermore we studied the relative weight of the inter-valley scattering and
the intra-valley scattering. It shows that the former is as important as the
latter and even dominates in the case of strong magnetic fields and small
screening length. We emphasize that the discussions on inter-valley scattering
is out of the realm of one-node model which has been studied.Comment: 14 pages, 5 figure
Single or multi-flavor Kondo effect in graphene
Based on the tight-binding formalism, we investigate the Anderson and the
Kondo model for an adaom magnetic impurity above graphene. Different impurity
positions are analyzed. Employing a partial wave representation we study the
nature of the coupling between the impurity and the conducting electrons. The
components from the two Dirac points are mixed while interacting with the
impurity. Two configurations are considered explicitly: the adatom is above one
atom (ADA), the other case is the adatom above the center the honeycomb (ADC).
For ADA the impurity is coupled with one flavor for both A and B sublattice and
both Dirac points. For ADC the impurity couples with multi-flavor states for a
spinor state of the impurity. We show, explicitly for a 3d magnetic atom,
, (,), and (,) couple
respectively with the , , and representations (reps) of group in ADC case. The basses for
these reps of graphene are also derived explicitly. For ADA we calculate the
Kondo temperature.Comment: 11 pages, 1 fures, 2 tables, accepted by EP
Laser-assisted spin-polarized transport in graphene tunnel junctions
Keldysh nonequilibrium Green's function method is utilized to study
theoretically the spin polarized transport through a graphene spin valve
irradiated by a monochromatic laser field. It is found that the bias dependence
of the differential conductance exhibits successive peaks corresponding to the
resonant tunneling through the photon-assisted sidebands. The multi photon
processes originate from the combined effects of the radiation field and the
graphene tunneling properties, and are shown to be substantially suppressed in
a graphene spin valve which results in a decrease of the differential
conductance for a high bias voltage. We also discussed the appearance of a
dynamical gap around zero bias due to the radiation field. The gap width can be
tuned by changing the radiation electric field strength and the frequency. This
leads to a shift of the resonant peaks in the differential conductance. We also
demonstrate numerically the dependencies of the radiation and spin valve
effects on the parameters of the external fields and those of the electrodes.
We find that the combined effects of the radiation field, the graphene, and the
spin valve properties bring about an oscillatory behavior in the tunnel
magnetoresistance (TMR), and this oscillatory amplitude can be changed by
scanning the radiation field strength and/or the frequency.Comment: 31 pages, 5 figures, corrected version to the paper in J. Phys.:
Condens. Matter 24 (2012) 26600
Magnon Spin Photogalvanic Effect in Collinear Ferromagnets
We propose a spin photogalvanic effect of magnons with broken inversion
symmetry. The dc spin photocurrent is generated via the Aharonov-Casher effect,
which includes the Drude, Berry curvature dipole, shift, injection, and
rectification components with distinct quantum geometric origin. Based on a
symmetry classification, we uncover that there exist linearly polarized (LP)
magnon spin photocurrent responses in the breathing kagome-lattice ferromagnet
with Dzyaloshinskii-Moriya interaction, and the circularly polarized (CP)
responses due to the symmetry breaking by applying a uniaxial strain. We
address that the topological phase transitions can be characterized by the spin
photocurrents. This study presents a deeper insight into the nonlinear
responses of light-magnon interactions, and suggests a possible way to generate
and control the magnon spin current in real materials
Nonlinear photocurrents from radio to infrared region in the WTe monolayer: A quantum kinetics study
Second-order nonlinear photocurrents, which refer to DC current [i.e.,
photogalvanic effect (PGE)] and second harmonic current [i.e., second harmonic
generation (SHG)] induced by light, are important physical phenomena in
nonlinear optics. The PGE (SHG) related to linearly and circularly polarized
light are called the linear and circular PGE (LPGE and CPGE) [linear and
circular SHG (LSHG and CSHG)], respectively. In this work, we use the quantum
kinetics under relaxation time approximation to study the dependence of
second-order nonlinear photocurrents on Fermi level and frequency under
different out-of-plane electric fields in WTe monolayer from radio to
infrared region. We find that the maximum frequency at which the Berry
curvature dipole mechanism for the nonlinear Hall effect plays a major role is
about 1 THz. In radio and microwave regions, two large peaks of nonlinear
conductivities occur when the Fermi level is equal to the energy corresponding
to gap-opening points. In terms of frequency, in radio region, LPGE and SHG
conductivities maintain a large constant while the CPGE conductivity
disappears. In microwave region, LPGE and SHG start to decrease with increasing
frequency while the CPGE is large. In 125-300 THz region and in y direction,
the presence of DC current without the disturbance of second harmonic current
under circularly polarized light may be useful for fabricating new
optoelectronic devices. Moreover, we illustrate that when calculating the
nonlinear photocurrents of practical materials, the theories in the clean limit
fail and it is necessary to use a theory that considers scattering effects. We
also point out that for materials with femtosecond-scale relaxation times and
complex energy band structures, the quantum kinetics is more accurate than the
semi-classical Boltzmann equation method. Besides, phenomenological expressions
of PGE and SHG are provided
Effect of Impurities and Effective Masses on Spin-Dependent Electrical Transport in Ferromagnet-Normal Metal-Ferromagnet Hybrid Junctions
The effect of nonmagnetic impurities and the effective masses on the
spin-dependent transport in a ferromagnet-normal metal-ferromagnet junction is
investigated on the basis of a two-band model. Our results show that impurities
and the effective masses of electrons in two ferromagnetic electrodes have
remarkable effects on the behaviors of the conductance, namely, both affect the
oscillating amplitudes, periods, as well as the positions of the resonant peaks
of the conductance considerably. The impurity tends to suppress the amplitudes
of the conductance, and makes the spin-valve effect less obvious, but under
certain conditions the phenomenon of the so-called impurity-induced resonant
tunneling is clearly observed. The impurity and the effective mass both can
lead to nonmonotonous oscillation of the junction magnetoresistance (JMR) with
the incident energy and the thickness of the normal metal. It is also observed
that a smaller difference of the effective masses of electrons in two
ferromagnetic electrodes would give rise to a larger amplitude of the JMR.Comment: Revtex, 10 figure
Symmetry Dictionary on Charge and Spin Nonlinear Responses for All Magnetic Point Groups with Nontrivial Topological Nature
Recently, charge or spin nonlinear transport with nontrivial topological
properties in crystal materials has attracted much attention. In this paper, we
perform a comprehensive symmetry analysis for all 122 magnetic point groups
(MPGs) and provide a useful dictionary for charge and spin nonlinear transport
from Berry curvature dipole, Berry connection polarization and Drude term with
nontrivial topological nature. The results are obtained by making a full
symmetry investigation on matrix representations of six nonlinear response
tensors. We further identify every MPG that can accommodate two or three of the
nonlinear tensors. The present work gives a solid theoretical basis for overall
understanding the second-order nonlinear responses in realistic materials.Comment: 5 pages, 2 figure
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