281 research outputs found
Excitonic effects on the two-color coherent control of interband transitions in bulk semiconductors
Quantum interference between one- and two-photon absorption pathways allows
coherent control of interband transitions in unbiased bulk semiconductors;
carrier population, carrier spin polarization, photocurrent injection, and spin
current injection may all be controlled. We extend the theory of these
processes to include the electron-hole interaction. Our focus is on photon
energies that excite carriers above the band edge, but close enough to it so
that transition amplitudes based on low order expansions in are
applicable; both allowed-allowed and allowed-forbidden two-photon transition
amplitudes are included. Analytic solutions are obtained using the effective
mass theory of Wannier excitons; degenerate bands are accounted for, but
envelope-hole coupling is neglected. We find a Coulomb enhancement of two-color
coherent control process, and relate it to the Coulomb enhancements of one- and
two-photon absorption. In addition, we find a frequency dependent phase shift
in the dependence of photocurrent and spin current on the optical phases. The
phase shift decreases monotonically from at the band edge to 0 over an
energy range governed by the exciton binding energy. It is the difference
between the partial wave phase shifts of the electron-hole envelope function
reached by one- and two-photon pathways.Comment: 31 pages, 4 figures, to be published in Phys. Rev.
Electron correlation effects and magnetic ordering at the Gd(0001) surface
Effects of electron correlation on the electronic structure and magnetic
properties of the Gd(0001) surface are investigated using of the full-potential
linearized augmented plane wave implementation of correlated band theory
("LDA+U"). The use of LDA+U instead of LDA (local density approximation) total
energy calculations produces the correct ferromagnetic ground state for both
bulk Gd and the Gd surface. Surface strain relaxation leads to an 90 %
enhancement of the interlayer surface-to-bulk effective exchange coupling.
Application of a Landau-Ginzburg type theory yields a 30 % enhancement of the
Curie temperature at the surface, in very good agreement with the experiment.Comment: revised version: minor typos correcte
Spontaneous emission from large quantum dots in nanostructures: exciton-photon interaction beyond the dipole approximation
We derive a rigorous theory of the interaction between photons and spatially
extended excitons confined in quantum dots in inhomogeneous photonic materials.
We show that, beyond the dipole approximation, the radiative decay rate is
proportional to a non-local interaction function, which describes the
interaction between light and spatially extended excitons. In this regime,
light and matter degrees of freedom cannot be separated and a complex interplay
between the nanostructured optical environment and the exciton envelope
function emerges. We illustrate this by specific examples and derive a series
of important analytical relations, which are useful for applying the formalism
to practical problems. In the dipole limit, the decay rate is proportional to
the projected local density of optical states and we obtain the strong and weak
confinement regimes as special cases.Comment: 14 pages, 4 figure
Poboljšan postupak sinteze nekih novih 1,3-diaril-2-propen-1-ona koristeći PEG-400 kao reciklirajuće otapalo i njihovo antimikrobno vrednovanje
A simple and convenient route is described for the synthesis of novel hetero 1,3-diaryl-2-propen-1-ones (chalcones) by using recyclable poly PEG-400 as an alternative reaction solvent. The reaction is clean with excellent yield, shorter reaction time and reduces the use of volatile organic compounds (VOCs). All the synthesized compounds were evaluated for their antimicrobial activities against several pathogenic representatives.Opisana je jednostavna i pogodna metoda sinteze novih hetero 1,3-diaril-2-propen-1-ona (kalkona) koristeći poli(etilenglikol) (PEG-400) kao alternativno otapalo. Reakcija je jednoznačna, a uporaba hlapljivih organskih otapala je smanjena. Iskorištenja na produktima su visoka, a reakcijska vremena kraća. Svi sintetizirani spojevi testirani su na antimikrobno djelovanje na nekoliko patogenih mikroorganizama
Topological crystalline insulator states in Pb(1-x)Sn(x)Se
Topological insulators are a novel class of quantum materials in which
time-reversal symmetry, relativistic (spin-orbit) effects and an inverted band
structure result in electronic metallic states on the surfaces of bulk
crystals. These helical states exhibit a Dirac-like energy dispersion across
the bulk bandgap, and they are topologically protected. Recent theoretical
proposals have suggested the existence of topological crystalline insulators, a
novel class of topological insulators in which crystalline symmetry replaces
the role of time-reversal symmetry in topological protection [1,2]. In this
study, we show that the narrow-gap semiconductor Pb(1-x)Sn(x)Se is a
topological crystalline insulator for x=0.23. Temperature-dependent
magnetotransport measurements and angle-resolved photoelectron spectroscopy
demonstrate that the material undergoes a temperature-driven topological phase
transition from a trivial insulator to a topological crystalline insulator.
These experimental findings add a new class to the family of topological
insulators. We expect these results to be the beginning of both a considerable
body of additional research on topological crystalline insulators as well as
detailed studies of topological phase transitions.Comment: v2: published revised manuscript (6 pages, 3 figures) and
supplementary information (5 pages, 8 figures
Spin-induced optical second harmonic generation in the centrosymmetric magnetic semiconductors EuTe and EuSe
Spectroscopy of the centrosymmetric magnetic semiconductors EuTe and EuSe
reveals spin-induced optical second harmonic generation (SHG) in the band gap
vicinity at 2.1-2.4eV. The magnetic field and temperature dependence
demonstrates that the SHG arises from the bulk of the materials due to a novel
type of nonlinear optical susceptibility caused by the magnetic dipole
contribution combined with spontaneous or induced magnetization. This
spin-induced susceptibility opens access to a wide class of centrosymmetric
systems by harmonics generation spectroscopy.Comment: 5 pages, 3 figures, submitted to PR
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