1,682 research outputs found
Berry phase theory of planar Hall effect in Topological Insulators
Negative longitudinal magnetoresistance, in the presence of an external
magnetic field parallel to the direction of an applied current, has recently
been experimentally verified in Weyl semimetals and topological insulators in
the bulk conduction limit. The appearance of negative longitudinal
magnetoresistance in topological semimetals is understood as an effect of
chiral anomaly, whereas it is not well-defined in topological insulators.
Another intriguing phenomenon, planar Hall effect - appearance of a transverse
voltage in the plane of applied co-planar electric and magnetic fields not
perfectly aligned to each other, a configuration in which the conventional Hall
effect vanishes, has recently been suggested to exist in Weyl semimetals. In
this paper we present a quasi-classical theory of planar Hall effect of a
three-dimensional topological insulator in the bulk conduction limit. Starting
from Boltzmann transport equations we derive the expressions for planar Hall
conductivity and longitudinal magnetoconductivity in topological insulators and
show the important roles played by the orbital magnetic moment for the
appearance of planar Hall effect. Our theoretical results predict specific
experimental signatures for topological insulators that can be directly checked
in experiments.Comment: 18 pages, 3 figure
Mirror anomaly and anomalous Hall effect in type-I Dirac semimetals
In addition to the well known chiral anomaly, Dirac semimetals have been
argued to exhibit mirror anomaly, close analogue to the parity anomaly of
()-dimensional massive Dirac fermions. The observable response of such
anomaly is manifested in a singular step-like anomalous Hall response across
the mirror-symmetric plane in the presence of a magnetic field. Although this
result seems to be valid in type-II Dirac semimetals (strictly speaking, in the
linearized theory), we find that type-I Dirac semimetals do not possess such an
anomaly in anomalous Hall response even at the level of the linearized theory.
In particular, we show that the anomalous Hall response continuously approaches
zero as one approaches the mirror symmetric angle in a type-I Dirac semimetal
as opposed to the singular Hall response in a type-II Dirac semimetal.
Moreover, we show that, under certain condition, the anomalous Hall response
may vanish in a linearized type-I Dirac semimetal, even in the presence of time
reversal symmetry breaking.Comment: 6 pages, 5 figure
Solar activity forecast with a dynamo model
Although systematic measurements of the solar polar magnetic field exist only
from mid 1970s, other proxies can be used to infer the polar field at earlier
times. The observational data indicate a strong correlation between the polar
field at a sunspot minimum and the strength of the next cycle, although the
strength of the cycle is not correlated well with the polar field produced at
its end. This suggests that the Babcock Leighton mechanism of poloidal field
generation from decaying sunspots involves randomness, whereas the other
aspects of the dynamo process must be reasonably ordered and deterministic.
Only if the magnetic diffusivity within the convection zone is assumed to be
high, we can explain the correlation between the polar field at a minimum and
the next cycle. We give several independent arguments that the diffusivity must
be of this order. In a dynamo model with diffusivity like this, the poloidal
field generated at the mid latitudes is advected toward the poles by the
meridional circulation and simultaneously diffuses towards the tachocline,
where the toroidal field for the next cycle is produced. To model actual solar
cycles with a dynamo model having such high diffusivity, we have to feed the
observational data of the poloidal field at the minimum into the theoretical
model. We develop a method of doing this in a systematic way. Our model
predicts that cycle 24 will be a very weak cycle. Hemispheric asymmetry of
solar activity is also calculated with our model and compared with
observational data.Comment: 17 pages, 18 figures, submitted to MNRA
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