11,900 research outputs found
Double quantum dot as detector of spin bias
It was proposed that a double quantum dot can be used to be a detector of
spin bias. Electron transport through a double quantum dot is investigated
theoretically when a pure spin bias is applied on two conducting leads
contacted to the quantum dot. It is found that the spin polarization in the
left and right dots may be induced spontaneously while the intra-dot levels are
located within the spin bias window and breaks the left-right symmetry of the
two quantum dots. As a result, a large current emerges. For an open external
circuit an charge bias instead of a charge current will be induced in
equilibrium, which is believed to be measurable according to the current
nanotechnology. This method may provide a practical and whole electrical
approach to detect the spin bias (or the spin current) by measuring the charge
bias or current in a double quantum dot.Comment: 13 pages, 5 figure
Mode mixing induced by disorder in graphene PNP junction in a magnetic field
We study the electron transport through the graphene PNP junction under a
magnetic field and show that modes mixing plays an essential role. By using the
non-equilibrium Green's function method, the space distribution of the
scattering state for a specific incident modes as well the elements of the
transmission and reflection coefficient matrixes are investigated. All elements
of the transmission (reflection) coefficient matrixes are very different for a
perfect PNP junction, but they are same at a disordered junction due to the
mode mixing. The space distribution of the scattering state for the different
incident modes also exhibit the similar behaviors, that they distinctly differ
from each other in the perfect junction but are almost same in the disordered
junction. For a unipolar junction, when the mode number in the center region is
less than that in the left and right regions, the fluctuations of the total
transmission and reflection coefficients are zero, although each element has a
large fluctuation. These results clearly indicate the occurrence of perfect
mode mixing and it plays an essential role in a graphene PNP junction
transport
Double Andreev Reflections in Type-II Weyl Semimetal-Superconductor Junctions
We study the Andreev reflections (ARs) at the interface of the type-II Weyl
semimetal-superconductor junctions and find double ARs when the superconductor
is put in the Weyl semimetal band tilting direction, which is similar to the
double reflections of light in anisotropic crystals. The directions of the
double (retro and specular) ARs are symmetric about the normal due to the
hyperboloidal Fermi surface near the Weyl nodes, but with different AR
amplitudes depending on the direction and energy of the incident electron. When
the normal direction of the Weyl semimetal-superconductor interface is changed
from parallel to perpendicular with the tilt direction, the double ARs
gradually evolve from one retro-AR and one specular AR, passing through double
retro-ARs, one specular AR and one retro-AR, into one retro AR and one normal
reflection, resulting in an anisotropic conductance which can be observed in
experiments.Comment: 12 pages, 7 figure
Investigation of Stability and Reproducibility of Perovskite Solar Cells
Last decade has witnessed a remarkable increase in the efficiency of photovoltaic devices based on lead halide perovskite materials. However, the stability and reproducibility of perovskite solar cells remain two of the biggest challenges impeding their application as the next generation of low-cost and efficient photovoltaics. This thesis aims to investigating these two issues in three parts. First, we reveal the important role of microstructure in the degradation processes of methylammonium lead triiodide perovskite films in controlled oxygen atmospheres under continuous illumination. CH3NH3PbI3 films with small, irregular grains and high defect density degrade much faster and more severely than films with large uniform grains and better electronic properties. The second part of this thesis demonstrates that the presence of large-scale chemical compositional and electronic inhomogeneities in CH3NH3PbI3 films is a common issue, and suggests one way to mitigate this problem by improving the fabrication process to obtain a perovskite layer with a uniform surface composition and electronic properties. The last section aims to improve the reproducibility of triple cation perovskite (Cs0.05(MA0.17FA0.83)0.95Pb(I0.9Br0.1)3) devices by optimizing the antisolvent treatment procedure. Scanning electron microscopy and photovoltaic measurements elucidate the relationship between antisolvent type and dripping speed on perovskite film formation and device performance
Theory for electric dipole superconductivity with an application for bilayer excitons
Exciton superfluid is a macroscopic quantum phenomenon in which large
quantities of excitons undergo the Bose-Einstein condensation. Recently,
exciton superfluid has been widely studied in various bilayer systems. However,
experimental measurements only provide indirect evidence for the existence of
exciton superfluid. In this article, by viewing the exciton in a bilayer system
as an electric dipole, we provide a general theory for the electric dipole
superconductivity, and derive the London-type and Ginzburg-Landau-type
equations for the electric dipole superconductors. By using these equations, we
discover the Meissner-type effect and the electric dipole current Josephson
effect. These effects can provide direct evidence for the formation of the
exciton superfluid state in bilayer systems and pave new ways to drive an
electric dipole current.Comment: 10 pages, 5 figures, 1 Supplementary Informatio
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