234 research outputs found
Strong Light-Matter Coupling in Carbon Nanotubes as a Route to Exciton Brightening
We show that strong light-matter coupling can be used to overcome a long
standing problem that has prevented efficient optical emission from carbon
nanotubes. The luminescence from the nominally bright exciton states of carbon
nanotubes is quenched due to the fast nonradiative scattering to the dark
exciton state having a lower energy. We present a theoretical analysis to show
that by placing carbon nanotubes in an optical microcavity the bright exctonic
state may be split into two hybrid exciton-polariton states, while the dark
state remains unaltered. For sufficiently strong coupling between the bright
exciton and the cavity, we show that the energy of the lower polariton may be
pushed below that of the dark exciton. This overturning of the relative
energies of the bright and dark excitons prevents the dark exciton from
quenching the emission. Our resutls pave the way for a new approach to
band-engineering the properties of the nanoscale optoelectronic devices.Comment: 35 pages, 5 figures, 6 pages of supplementary materials, 1
supplementary figur
Caz reuşit de tratament chirurgical într-o singură intervenţie la criptorhidie abdominală bilaterală
Summary.
A successful case of surgical treatment of Bilateral Orchiopexie with Petrivalchi-Shumaher method
fixation" by way of a single intervention of an 18-year-old patient with the diagnosis Bilateral
Abdominal Cryptorchism". 85 patients were operated on according to this method
Excitons in narrow-gap carbon nanotubes
We calculate the exciton binding energy in single-walled carbon nanotubes
with narrow band gaps, accounting for the quasi-relativistic dispersion of
electrons and holes. Exact analytical solutions of the quantum relativistic
two-body problem are obtain for several limiting cases. We show that the
binding energy scales with the band gap, and conclude on the basis of the data
available for semiconductor nanotubes that there is no transition to an
excitonic insulator in quasi-metallic nanotubes and that their THz applications
are feasible.Comment: 11 pages, 3 figures. Several references and an additional appendix
adde
Combined Effect of Temperature Induced Strain and Oxygen Vacancy on Metal‐Insulator Transition of VO2 Colloidal Particles
Vanadium dioxide (VO2) is a promising material in the development of thermal and electrically sensitive devices due to its first order reversible metal-insulator transition (MIT) at 68 °C. Such high MIT temperature (TC) largely restricts its widespread application which could be enabled if a straightforward tuning mechanism were present. Here this need is addressed through a facile approach that uses the combined effects of temperature induced strain and oxygen vacancies in bulk VO2 colloidal particles. A simple thermal annealing process under varying vacuum is used to achieve phase transformation of metastable VO2(A) into VO2(M2), (M2+M3), (M1) and higher valence V6O13 phases. During this process, distinct multiple phase transitions including increased as well as suppressed TC are observed with respect to the annealing temperature and varied amount of oxygen vacancies respectively. The latent heat of phase transition is also significantly improved upon thermal annealing by increasing the crystallinity of the samples. This work not only offers a
facile route for selective phase transformation of VO2 as well as to manipulate the phase transition temperature, but also contributes significantly to the understanding of the role played by oxygen vacancies and temperature induced stress on MIT which is essential for VO2 based applications
Superlattice properties of carbon nanotubes in a transverse electric field
Electron motion in a (n,1) carbon nanotube is shown to correspond to a de
Broglie wave propagating along a helical line on the nanotube wall. This
helical motion leads to periodicity of the electron potential energy in the
presence of an electric field normal to the nanotube axis. The period of this
potential is proportional to the nanotube radius and is greater than the
interatomic distance in the nanotube. As a result, the behavior of an electron
in a (n,1) nanotube subject to a transverse electric field is similar to that
in a semiconductor superlattice. In particular, Bragg scattering of electrons
from the long-range periodic potential results in the opening of gaps in the
energy spectrum of the nanotube. Modification of the bandstructure is shown to
be significant for experimentally attainable electric fields, which raises the
possibility of applying this effect to novel nanoelectronic devices.Comment: 7 pages, 3 figure
Ionization degree of the electron-hole plasma in semiconductor quantum wells
The degree of ionization of a nondegenerate two-dimensional electron-hole
plasma is calculated using the modified law of mass action, which takes into
account all bound and unbound states in a screened Coulomb potential.
Application of the variable phase method to this potential allows us to treat
scattering and bound states on the same footing. Inclusion of the scattering
states leads to a strong deviation from the standard law of mass action. A
qualitative difference between mid- and wide-gap semiconductors is
demonstrated. For wide-gap semiconductors at room temperature, when the bare
exciton binding energy is of the order of T, the equilibrium consists of an
almost equal mixture of correlated electron-hole pairs and uncorrelated free
carriers.Comment: 22 pages, 6 figure
Levinson's theorem and scattering phase shift contributions to the partition function of interacting gases in two dimensions
We consider scattering state contributions to the partition function of a
two-dimensional (2D) plasma in addition to the bound-state sum. A partition
function continuity requirement is used to provide a statistical mechanical
heuristic proof of Levinson's theorem in two dimensions. We show that a proper
account of scattering eliminates singularities in thermodynamic properties of
the nonideal 2D gas caused by the emergence of additional bound states as the
strength of an attractive potential is increased. The bound-state contribution
to the partition function of the 2D gas, with a weak short-range attraction
between its particles, is found to vanish logarithmically as the binding energy
decreases. A consistent treatment of bound and scattering states in a screened
Coulomb potential allowed us to calculate the quantum-mechanical second virial
coefficient of the dilute 2D electron-hole plasma and to establish the
difference between the nearly ideal electron-hole gas in GaAs and the strongly
correlated exciton/free-carrier plasma in wide-gap semiconductors such as ZnSe
or GaN.Comment: 10 pages, 3 figures; new version corrects some minor typo
Widely tunable gain-switched operation of external cavity grating-coupled surface emitting laser
Widely tunable gain switching of a grating-coupled surface-emitting laser (GCSEL) has been demonstrated in a simple external cavity configuration for the first time. Pulse duration in range of 40-100ps and wavelength tuning over 100nm have been achieved. High power, tail-free optical pulses have been observed at 980nm
Spin-orbit terms in multi-subband electron systems: A bridge between bulk and two-dimensional Hamiltonians
We analyze the spin-orbit terms in multi-subband quasi-two-dimensional
electron systems, and how they descend from the bulk Hamiltonian of the
conduction band. Measurements of spin-orbit terms in one subband alone are
shown to give incomplete information on the spin-orbit Hamiltonian of the
system. They should be complemented by measurements of inter-subband spin-orbit
matrix elements. Tuning electron energy levels with a quantizing magnetic field
is proposed as an experimental approach to this problem.Comment: Typos noticed in the published version have been corrected and
several references added. Published in the special issue of Semiconductors in
memory of V.I. Pere
Levinson's Theorem for the Klein-Gordon Equation in Two Dimensions
The two-dimensional Levinson theorem for the Klein-Gordon equation with a
cylindrically symmetric potential is established. It is shown that
, where denotes
the difference between the number of bound states of the particle
and the ones of antiparticle with a fixed angular momentum , and
the is named phase shifts. The constants and
are introduced to symbol the critical cases where the half bound
states occur at .Comment: Revtex file 14 pages, submitted to Phys. Rev.
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