3,235 research outputs found
Localization of massless Dirac particles via spatial modulations of the Fermi velocity
The electrons found in Dirac materials are notorious for being difficult to
manipulate due to the Klein phenomenon and absence of backscattering. Here we
investigate how spatial modulations of the Fermi velocity in two-dimensional
Dirac materials can give rise to localization effects, with either full
(zero-dimensional) confinement or partial (one-dimensional) confinement
possible depending on the geometry of the velocity modulation. We present
several exactly solvable models illustrating the nature of the bound states
which arise, revealing how the gradient of the Fermi velocity is crucial for
determining fundamental properties of the bound states such as the zero-point
energy. We discuss the implications for guiding electronic waves in few-mode
waveguides formed by Fermi velocity modulation.Comment: 9 pages, 6 figure
Bielectron vortices in two-dimensional Dirac semimetals
Searching for new states of matter and unusual quasiparticles in emerging
materials and especially low-dimensional systems is one of the major trends in
contemporary condensed matter physics. Dirac materials, which host
quasiparticles which are described by ultrarelativistic Dirac-like equations,
are of a significant current interest from both a fundamental and applied
physics perspective. Here we show that a pair of two-dimensional massless
Dirac-Weyl fermions can form a bound state independently of the sign of the
inter-particle interaction potential, as long as this potential decays at large
distances faster than Kepler's inverse distance law. This leads to the
emergence of a new type of energetically-favourable quasiparticle: bielectron
vortices, which are double-charged and reside at zero-energy. Their bosonic
nature allows for condensation and may give rise to Majorana physics without
invoking a superconductor. These novel quasiparticles arguably explain a range
of poorly understood experiments in gated graphene structures at low doping.Comment: 9 pages, 2 figure
One-dimensional Coulomb problem in Dirac materials
We investigate the one-dimensional Coulomb potential with application to a
class of quasirelativistic systems, so-called Dirac-Weyl materials, described
by matrix Hamiltonians. We obtain the exact solution of the shifted and
truncated Coulomb problems, with the wavefunctions expressed in terms of
special functions (namely Whittaker functions), whilst the energy spectrum must
be determined via solutions to transcendental equations. Most notably, there
are critical bandgaps below which certain low-lying quantum states are missing
in a manifestation of atomic collapse.Comment: 7 pages, 5 figure
Massless Dirac fermions in two dimensions: Confinement in nonuniform magnetic fields
We show how it is possible to trap two-dimensional massless Dirac fermions in
spatially inhomogeneous magnetic fields, as long as the formed magnetic quantum
dot (or ring) is of a slowly decaying nature. It is found that a modulation of
the depth of the magnetic quantum dot leads to successive
confinement-deconfinement transitions of vortexlike states with a certain
angular momentum, until a regime is reached where only states with one sign of
angular momentum are supported. We illustrate these characteristics with both
exact solutions and a hitherto unknown quasi-exactly solvable model utilizing
confluent Heun functions.Comment: 7 pages, 3 figure
Optimal traps in graphene
We transform the two-dimensional Dirac-Weyl equation, which governs the
charge carriers in graphene, into a non-linear first-order differential
equation for scattering phase shift, using the so-called variable phase method.
This allows us to utilize the Levinson Theorem to find zero-energy bound states
created electrostatically in realistic structures. These confined states are
formed at critical potential strengths, which leads to us posit the use of
`optimal traps' to combat the chiral tunneling found in graphene, which could
be explored experimentally with an artificial network of point charges held
above the graphene layer. We also discuss scattering on these states and find
the zero angular momentum states create a dominant peak in scattering
cross-section as energy tends towards the Dirac point energy, suggesting a
dominant contribution to resistivity.Comment: 11 pages, 5 figure
Charged particle radiation damage in semiconductors. Part 14 - Study of radiation effects in lithium doped silicon solar cells
Lithium doped silicon solar cells under electron irradiation and determination of semiconductor parameter
Zero-energy states in graphene quantum dots and rings
We present exact analytical zero-energy solutions for a class of smooth
decaying potentials, showing that the full confinement of charge carriers in
electrostatic potentials in graphene quantum dots and rings is indeed possible
without recourse to magnetic fields. These exact solutions allow us to draw
conclusions on the general requirements for the potential to support fully
confined states, including a critical value of the potential strength and
spatial extent.Comment: 8 pages, 3 figures, references added, typos corrected, discussion
section expande
Evidencing a place for the hippocampus within the core scene processing network
Functional neuroimaging studies have identified several “core” brain regions that are preferentially activated by scene stimuli, namely posterior parahippocampal gyrus (PHG), retrosplenial cortex (RSC), and transverse occipital sulcus (TOS). The hippocampus (HC), too, is thought to play a key role in scene processing, although no study has yet investigated scene-sensitivity in the HC relative to these other “core” regions. Here, we characterised the frequency and consistency of individual scene-preferential responses within these regions by analysing a large dataset (n = 51) in which participants performed a one-back working memory task for scenes, objects, and scrambled objects. An unbiased approach was adopted by applying independently-defined anatomical ROIs to individual-level functional data across different voxel-wise thresholds and spatial filters. It was found that the majority of subjects had preferential scene clusters in PHG (max = 100% of participants), RSC (max = 76%), and TOS (max = 94%). A comparable number of individuals also possessed significant scene-related clusters within their individually defined HC ROIs (max = 88%), evidencing a HC contribution to scene processing. While probabilistic overlap maps of individual clusters showed that overlap “peaks” were close to those identified in group-level analyses (particularly for TOS and HC), inter-individual consistency varied across regions and statistical thresholds. The inter-regional and inter-individual variability revealed by these analyses has implications for how scene-sensitive cortex is localised and interrogated in functional neuroimaging studies, particularly in medial temporal lobe regions, such as the H
- …