99 research outputs found
Impact ionization fronts in Si diodes: Numerical evidence of superfast propagation due to nonlocalized preionization
We present numerical evidence of a novel propagation mode for superfast
impact ionization fronts in high-voltage Si -- structures. In
nonlinear dynamics terms, this mode corresponds to a pulled front propagating
into an unstable state in the regime of nonlocalized initial conditions. Before
the front starts to travel, field-ehanced emission of electrons from deep-level
impurities preionizes initially depleted base creating spatially nonuniform
free carriers profile. Impact ionization takes place in the whole high-field
region. We find two ionizing fronts that propagate in opposite directions with
velocities up to 10 times higher than the saturated drift velocity.Comment: 3 pages, 4 figure
Constraints on transmission, dispersion, and density of states in dielectric multilayers and stepwise potential barriers with arbitrary layer arrangement
Normal-incidence transmission and dispersion properties of optical
multilayers and one-dimensional stepwise potential barriers in the
non-tunneling regime are analytically investigated. The optical paths of every
constituent layer in a multilayer structure, as well as the parameters of every
step of the stepwise potential barrier, are constrained by a generalized
quarter-wave condition. No other restrictions on the structure geometry is
imposed, i.e., the layers are arranged arbitrarily. We show that the density of
states (DOS) spectra of the multilayer or barrier in question are subject to
integral conservation rules similar to the Barnett-Loudon sum rule but ocurring
within a finite frequency or energy interval. In the optical case, these
frequency intervals are regular. For the potential barriers, only non-periodic
energy intervals can be present in the spectrum of any given structure, and
only if the parameters of constituent potential steps are properly chosen.
Abstract The integral conservation relations derived analytically have also
been verified numerically. The relations can be used in dispersion-engineered
multilayer-based devices, e.g., ultrashort pulse compressors or ultracompact
optical delay lines, as well as to design multiple-quantum-well electronic
heterostructures with engineered DOS.Comment: 10 pages, 5 figures, to be submitted to PR
Tunable Emergent Heterostructures in a Prototypical Correlated Metal
At the interface between two distinct materials desirable properties, such as
superconductivity, can be greatly enhanced, or entirely new functionalities may
emerge. Similar to in artificially engineered heterostructures, clean
functional interfaces alternatively exist in electronically textured bulk
materials. Electronic textures emerge spontaneously due to competing
atomic-scale interactions, the control of which, would enable a top-down
approach for designing tunable intrinsic heterostructures. This is particularly
attractive for correlated electron materials, where spontaneous
heterostructures strongly affect the interplay between charge and spin degrees
of freedom. Here we report high-resolution neutron spectroscopy on the
prototypical strongly-correlated metal CeRhIn5, revealing competition between
magnetic frustration and easy-axis anisotropy -- a well-established mechanism
for generating spontaneous superstructures. Because the observed easy-axis
anisotropy is field-induced and anomalously large, it can be controlled
efficiently with small magnetic fields. The resulting field-controlled magnetic
superstructure is closely tied to the formation of superconducting and
electronic nematic textures in CeRhIn5, suggesting that in-situ tunable
heterostructures can be realized in correlated electron materials
Investigation of carrier confinement in direct bandgap GeSn/SiGeSn 2D and 0D heterostructures
Since the first demonstration of lasing in direct bandgap GeSn semiconductors, the research efforts for the realization of electrically pumped group IV lasers monolithically integrated on Si have significantly intensified. This led to epitaxial studies of GeSn/SiGeSn hetero- and nanostructures, where charge carrier confinement strongly improves the radiative emission properties. Based on recent experimental literature data, in this report we discuss the advantages of GeSn/SiGeSn multi quantum well and quantum dot structures, aiming to propose a roadmap for group IV epitaxy. Calculations based on 8-band k∙p and effective mass method have been performed to determine band discontinuities, the energy difference between Γ- and L-valley conduction band edges, and optical properties such as material gain and optical cross section. The effects of these parameters are systematically analyzed for an experimentally achievable range of Sn (10 to 20 at.%) and Si (1 to 10 at.%) contents, as well as strain values (−1 to 1%). We show that charge carriers can be efficiently confined in the active region of optical devices for experimentally acceptable Sn contents in both multi quantum well and quantum dot configurations
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