325 research outputs found
Strain-engineered Majorana Zero Energy Modes and {\phi}0 Josephson State in Black Phosphorus
We develop a theory for strain control of Majorana zero energy modes and
Josephson effect in black phosphorus (BP) devices proximity coupled to a
superconductor. Employing realistic values for the band parameters subject to
strain, we show that the strain closes the intrinsic band gap of BP, however
the proximity effect from the superconductor reopens it and creates Dirac and
Weyl nodes. Our results illustrate that Majorana zero energy flat bands connect
the nodes within the band-inverted regime in which their associated density of
states is localized at the edges of the device. In a ferromagnetically mediated
Josephson configuration, the exchange field induces super-harmonics into the
supercurrent phase relation in addition to a {\phi}0 phase shift, corresponding
to a spontaneous supercurrent, and strain offers an efficient tool to control
these phenomena. We analyze the experimental implications of our findings, and
show that they can pave the way for creating a rich platform for studying
two-dimensional Dirac and Weyl superconductivity
Symmetry of superconducting correlations in displaced bilayers of graphene
Using a Green's function approach, we study phonon-mediated superconducting
pairing symmetries that may arise in bilayer graphene where the monolayers are
displaced in-plane with respect to each other. We consider a generic coupling
potential between the displaced graphene monolayers, which is applicable to
both shifted and commensurate twisted graphene layers; study intralayer and
interlayer phonon-mediated BCS pairings; and investigate AA and AB(AC) stacking
orders. Our findings demonstrate that at the charge neutrality point, the
dominant pairings in both AA and AB stackings with intralayer and interlayer
electron-electron couplings can have even-parity -wave class and odd-parity
-wave class of symmetries with the possibility of invoking equal-pseudospin
and odd-frequency pair correlations. At a finite doping, however, the AB (and
equivalently AC) stacking can develop pseudospin-singlet and pseudospin-triplet
-wave symmetry, in addition to -wave, -wave, -wave, and their
combinations, while the AA stacking order, similar to the undoped case, is
unable to host the -wave symmetry. When we introduce a generic coupling
potential, applicable to commensurate twisted and shifted bilayers of graphene,
-wave symmetry can also appear at the charge neutrality point. Inspired by a
recent experiment where two phonon modes were observed in a twisted bilayer
graphene, we also discuss the possibility of the existence of two-gap
superconductivity, where the intralayer and interlayer phonon-mediated BCS
picture is responsible for superconductivity. These analyses may provide a
useful tool in determining the superconducting pairing symmetries and mechanism
in bilayer graphene systems
Pseudocanalization regime for magnetic dark-field hyperlens
Hyperbolic metamaterials (HMMs) are the cornerstone of the hyperlens, which
brings the superresolution effect from the near-field to the far-field zone.
For effective application of the hyperlens it should operate in so-called
canalization regime, when the phase advancement of the propagating fields is
maximally supressed, and thus field broadening is minimized. For conventional
hyperlenses it is relatively straightforward to achieve canalization by tuning
the anisotropic permittivity tensor. However, for a dark-field hyperlens
designed to image weak scatterers by filtering out background radiation
(dark-field regime) this approach is not viable, because design requirements
for such filtering and elimination of phase advancement i.e. canalization, are
mutually exclusive. Here we propose the use of magnetic (-positive and
negative) HMMs to achieve phase cancellation at the output equivalent to the
performance of a HMM in the canalized regime. The proposed structure offers
additional flexibility over simple HMMs in tuning light propagation. We show
that in this ``pseudocanalizing'' configuration quality of an image is
comparable to a conventional hyperlens, while the desired filtering of the
incident illumination associated with the dark-field hyperlens is preserved
Acousto-optical phonon excitation in cubic piezoelectric slabs and crystal growth orientation effects
Symmetry analysis of strain, electric and magnetic fields in the -class of topological insulators
Based on group theoretical arguments we derive the most general Hamiltonian
for the -class of materials including terms to third
order in the wave vector, first order in electric and magnetic fields, first
order in strain and first order in both strain and wave vector. We determine
analytically the effects of strain on the electronic structure of
. For the most experimentally relevant surface
termination we analytically derive the surface state spectrum, revealing an
anisotropic Dirac cone with elliptical constant energy counturs giving rise to
different velocities in different in-plane directions. The spin-momentum
locking of strained is shown to be modified and for
some strain configurations we see a non-zero spin component perpendicular to
the surface. Hence, strain control can be used to manipulate the spin degree of
freedom via the spin-orbit coupling. We show that for a thin film of
the surface state band gap induced by coupling between
the opposite surfaces changes opposite to the bulk band gap under strain.
Tuning the surface state band gap by strain, gives new possibilities for the
experimental investigation of the thickness dependent gap and optimization of
optical properties relevant for, e.g., photodetector and energy harvesting
applications. We finally derive analytical expressions for the effective mass
tensor of the BiSe class of materials as a function of strain and
electric field
Dynamic Coupling of Piezoelectric Effects, Spontaneous Polarization, and Strain in Lattice-Mismatched Semiconductor Quantum-Well Heterostructures
A static and dynamic analysis of the combined and self-consistent influence of spontaneous polarization, piezoelectric effects, lattice mismatch, and strain effects is presented for a three-layer one-dimensional AlN/GaN wurtzite quantum-well structure with GaN as the central quantum-well layer . It is shown that, contrary to the assumption of Fonoberov and Balandin [J. Appl. Phys. 94, 7178 (2003); J. Vac. Sci. Technol. B 22, 2190 (2004)], even in cases with no current transport through the structure, the strain distributions are not well captured by minimization of the strain energy only and not, as is in principle required, the total free energy including electric and piezoelectric coupling and spontaneous polarization contributions. Furthermore, we have found that, when an ac signal is imposed through the structure, resonance frequencies exist where strain distributions are even more strongly affected by piezoelectric-coupling contributions depending on the amount of mechanical and electrical losses in the full material system
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