237 research outputs found
Doorway States and Billiards
Whenever a distinct state is immersed in a sea of complicated and dense
states, the strength of the distinct state, which we refer to as a doorway, is
distributed in their neighboring states. We analyze this mechanism for 2-D
billiards with different geometries. One of them is symmetric and integrable,
another is symmetric but chaotic, and the third has a capricious form. The fact
that the doorway-state mechanism is valid for such highly diverse cases, proves
that it is robust.Comment: 7 pages, 6 figures, Accepted in Proceedings of "Symmetries in
Nature", Symposium in Memoriam Marcos Moshinsk
Schematic baryon models, their tight binding description and their microwave realization
A schematic model for baryon excitations is presented in terms of a symmetric
Dirac gyroscope, a relativistic model solvable in closed form, that reduces to
a rotor in the non-relativistic limit. The model is then mapped on a nearest
neighbour tight binding model. In its simplest one-dimensional form this model
yields a finite equidistant spectrum. This is experimentally implemented as a
chain of dielectric resonators under conditions where their coupling is
evanescent and good agreement with the prediction is achieved.Comment: 17 pages, 15 figure
Some remarks on particle size effects on the abrasion of a range of Fe based alloys
The low-stress three body abrasion behaviour of a range of steels was investigated. The tests were carried out in a rubber wheel tester (according to ASTM G65-94, reapproved in 2000) at room temperature. The abrasive particles used were angular alumina particles of four different sizes. The results showed that, in general, the smaller particles (50 8m and 125 8m average size) caused more damage. With these particles, observations of surface morphology indicarted a more intense cutting and ploughing action, leading to more damage, whereas bigger particles i.e. larger 250 8m and 560 8m particles produced less damage, and their action involved more plastic deformation type wear. The 304 SS had a lower abrasion resistance than the 310 SS. For the austentic and ferritic steels the subsurface deformation was larger for impact with the coarser particles. Variations in substrate hardness had no effect on the abrasive behaviour observed. On the whole, the hardest steel (mild steel in martensitic condition) showed the higher extent of damage, irrespective of particle size
First experimental realization of the Dirac oscillator
We present the first experimental microwave realization of the
one-dimensional Dirac oscillator, a paradigm in exactly solvable relativistic
systems. The experiment relies on a relation of the Dirac oscillator to a
corresponding tight-binding system. This tight-binding system is implemented as
a microwave system by a chain of coupled dielectric disks, where the coupling
is evanescent and can be adjusted appropriately. The resonances of the finite
microwave system yields the spectrum of the one-dimensional Dirac oscillator
with and without mass term. The flexibility of the experimental set-up allows
the implementation of other one-dimensional Dirac type equations.Comment: 6 figures, 5 page
Probing the Dark Exciton in Monolayer MoS by Quantum Interference in Second Harmonic Generation Spectroscopy
We report resonant second harmonic generation (SHG) spectroscopy of an
hBN-encapsulated monolayer of MoS. By tuning the energy of the excitation
laser, we identify a dark state transition (D) that is blue detuned by +25 meV
from the neutral exciton X. We observe a splitting of the SHG spectrum into
two distinct peaks and a clear anticrossing between them as the SHG resonance
is tuned through the energy of the dark exciton D. This observation is
indicative of quantum interference arising from the strong two-photon
light-matter interaction. We further probe the incoherent relaxation from the
dark state to the bright excitons, including X and localized excitons LX,
by the resonant enhancement of their intensities at the SHG-D resonance. The
relaxation of D to bright excitons is strongly suppressed on the bare substrate
whilst enabled when the hBN/MoS/hBN heterostructure is integrated in a
nanobeam cavity. The relaxation enabled by the cavity is explained by the
phonon scattering enhanced by the cavity phononic effects. Our work reveals the
two-photon quantum interference with long-lived dark states and enables the
control through nanostructuring of the substrate. These results indicate the
great potential of dark excitons in 2D-material based nonlinear quantum
devices
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