3,706 research outputs found
Optical transitions and energy relaxation of hot carriers in Si nanocrystals
Dynamics of hot carriers confined in Si nanocrystals is studied theoretically
using atomistic tight binding approach. Radiative, Auger-like and
phonon-assisted processes are considered. The Auger-like energy exchange
between electrons and holes is found to be the fastest process in the system.
However the energy relaxation of hot electron-hole pair is governed by the
single optical phonon emission. For a considerable number of states in small
nanocrystals single-phonon processes are ruled out by energy conservation law.Comment: 3 pages, 4 figure
Energy transfer processes in Er-doped SiO2 sensitized with Si nanocrystals
We present a high-resolution photoluminescence study of Er-doped SiO2
sensitized with Si nanocrystals (Si NCs). Emission bands originating from
recombination of excitons confined in Si NCs and of internal transitions within
the 4f-electron core of Er3+ ions, and a band centered at lambda = 1200nm have
been identified. Their kinetics have been investigated in detail. Based on
these measurements, we present a comprehensive model for energy transfer
mechanisms responsible for light generation in this system. A unique picture of
energy flow between subsystems of Er3+ and Si NCs is developed, yielding truly
microscopic information on the sensitization effect and its limitations. In
particular, we show that most of the Er3+ ions available in the system are
participating in the energy exchange. The long standing problem of apparent
loss of optical activity of majority of Er dopants upon sensitization with Si
NCs is clarified and assigned to appearance of a very efficient energy exchange
mechanism between Si NCs and Er3+ ions. Application potential of SiO2:Er
sensitized by Si NCs is discussed in view of the newly acquired microscopic
insight.Comment: 30 pages 13 figure
Dispersion and damping of zone-boundary magnons in the noncentrosymmetric superconductor CePt3Si
Inelastic neutron scattering (INS) is employed to study damped spin-wave
excitations in the noncentrosymmetric heavy-fermion superconductor CePt3Si
along the antiferromagnetic Brillouin-zone boundary in the low-temperature
magnetically ordered state. Measurements along the (1/2 1/2 L) and (H H 1/2-H)
reciprocal-space directions reveal deviations in the spin-wave dispersion from
the previously reported model. Broad asymmetric shape of the peaks in energy
signifies strong spin-wave damping by interactions with the particle-hole
continuum. Their energy width exhibits no evident anomalies as a function of
momentum along the (1/2 1/2 L) direction, which could be attributed to
Fermi-surface nesting effects, implying the absence of pronounced commensurate
nesting vectors at the magnetic zone boundary. In agreement with a previous
study, we find no signatures of the superconducting transition in the magnetic
excitation spectrum, such as a magnetic resonant mode or a superconducting spin
gap, either at the magnetic ordering wavevector (0 0 1/2) or at the zone
boundary. However, the low superconducting transition temperature in this
material still leaves the possibility of such features being weak and therefore
hidden below the incoherent background at energies ~0.1 meV, precluding their
detection by INS
New high magnetic field phase of the frustrated chain compound LiCuVO
Magnetization of the frustrated chain compound LiCuVO, focusing
on high magnetic field phases, is reported. Besides a spin-flop transition and
the transition from a planar spiral to a spin modulated structure observed
recently, an additional transition was observed just below the saturation
field. This newly observed magnetic phase is considered as a spin nematic
phase, which was predicted theoretically but was not observed experimentally.
The critical fields of this phase and its dM/dH curve are in good agreement
with calculations performed in a microscopic model (M. E. Zhitomirsky and H.
Tsunetsugu, preprint, arXiv:1003.4096v2).Comment: 5 pages, 4 figure
- …