432 research outputs found
Canonical Transformation Approach to the Ultrafast Non-linear Optical Dynamics of Semiconductors
We develop a theory describing the effects of many-particle Coulomb
correlations on the coherent ultrafast nonlinear optical response of
semiconductors and metals. Our approach is based on a mapping of the nonlinear
optical response of the ``bare'' system onto the linear response of a
``dressed'' system. The latter is characterized by effective time-dependent
optical transition matrix elements, electron/hole dispersions, and interaction
potentials, which in undoped semiconductors are determined by the
single-exciton and two-exciton Green functions in the absence of optical
fields. This mapping is achieved by eliminating the optically-induced charge
fluctuations from the Hamiltonian using a Van Vleck canonical transformation.
It takes into account all many-body contributions up to a given order in the
optical fields as well as important Coulomb-induced quantum dynamics to all
orders in the optical field. Our approach allows us to distinguish between
optical nonlinearities of different origins and provides a physically-intuitive
interpretation of their manifestations in ultrafast coherent nonlinear optical
spectroscopy.Comment: 24 page
Femtosecond study of the interplay between excitons, trions, and carriers in (Cd,Mn)Te quantum wells
We present an absorption study of the neutral and positively charged exciton
(trion) under the influence of a femtosecond, circularly polarized, resonant
pump pulse. Three populations are involved: free holes, excitons, and trions,
all exhibiting transient spin polarization. In particular, a polarization of
the hole gas is created by the formation of trions. The evolution of these
populations is studied, including the spin flip and trion formation processes.
The contributions of several mechanisms to intensity changes are evaluated,
including phase space filling and spin-dependent screening. We propose a new
explanation of the oscillator strength stealing phenomena observed in p-doped
quantum wells, based on the screening of neutral excitons by charge carriers.
We have also found that binding heavy holes into charged excitons excludes them
from the interaction with the rest of the system, so that oscillator strength
stealing is partially blockedComment: 4 pages, 4 figure
Transverse sound in a magnetic field in UPt_3
We have propagated transverse sound in a magnetic field in the basal plane of UPt_3, with the polarization vector oriented both in the basal plane and perpendicular to it. We observe a strong anisotropy in the magnetic field dependence of the attenuation for the two polarizations. Using a simple phenomenological model, we can understand the low-temperature field dependence as a natural consequence of the anisotropy with temperature in zero field reported earlier [Phys. Rev. Lett. 56, 1078 (1986)]. However, for increasing temperatures there are significant deviations from this model. In no case do we find evidence for new superconducting phases in a magnetic field
Ultrafast pump-probe dynamics in ZnSe-based semiconductor quantum-wells
Pump-probe experiments are used as a controllable way to investigate the
properties of photoexcited semiconductors, in particular, the absorption
saturation. We present an experiment-theory comparison for ZnSe quantum wells,
investigating the energy renormalization and bleaching of the excitonic
resonances. Experiments were performed with spin-selective excitation and
above-bandgap pumping. The model, based on the semiconductor Bloch equations in
the screened Hartree-Fock approximation, takes various scattering processes
into account phenomenologically. Comparing numerical results with available
experimental data, we explain the experimental results and find that the
electron spin-flip occurs on a time scale of 30 ps.Comment: 10 pages, 9 figures. Key words: nonlinear and ultrafast optics,
modeling of femtosecond pump-probe experiments, electron spin-flip tim
Pump Built-in Hamiltonian Method for Pump-Probe Spectroscopy
We propose a new method of calculating nonlinear optical responses of
interacting electronic systems. In this method, the total Hamiltonian (system +
system-pump interaction) is transformed into a different form that (apparently)
does not have a system-pump interaction. The transformed Hamiltonian, which we
call the pump built-in Hamiltonian, has parameters that depend on the strength
of the pump beam. Using the pump built-in Hamiltonian, we can calculate
nonlinear responses (responses to probe beams as a function of the pump beam)
by applying the {\em linear} response theory. We demonstrate the basic idea of
this new method by applying it to a one-dimensional, two-band model, in the
case the pump excitation is virtual (coherent excitation). We find that the
exponent of the Fermi edge singularity varies with the pump intensity.Comment: 6 page
Exotic superconductivity in the coexistent phase of antiferromagnetism and superconductivity in CeCu2(Si0.98Ge0.02)2: A Cu-NQR study under hydrostatic pressure
We report a pressure () effect on CeCu(SiGe)
where an antiferromagnetic (AFM) order at 0.75 K coexists with
superconductivity below 0.4 K\@. At pressures exceeding
GPa, the AFM order is suppressed, which demonstrates that the sudden emergence
of AFM order due to the Ge doping is ascribed to the intrinsic lattice
expansion. The exotic superconductivity at GPa is found to evolve into
a typical heavy-fermion one with a line-node gap above GPa\@. We
highlight that the anomalous enhancement in nuclear spin-lattice relaxation
rate that follows a = const. behavior well below at =
0 GPa is characterized by the persistence of low-lying magnetic excitations,
which may be inherent to the coexistent state of antiferromagnetism and
superconductivity.Comment: 5 pages with 4 figures embedded in the text. To be published in J.
Phys. Soc. Jp
Polar type density of states in non-unitary odd-parity superconducting states of gap with point nodes
It is shown that the density of states (DOS) proportional to the excitation
energy, the so-called polar like DOS, can arise in the odd-parity states with
the superconducting gap vanishing at points even if the spin-orbit interaction
for Cooper pairing is strong enough. Such gap stuructures are realized in the
non-unitary states, F_{1u}(1,i,0), F_{1u}(1,varepsilon,varepsilon^{2}), and
F_{2u}(1,i,0), classified by Volovik and Gorkov, Sov. Phys.-JETP Vol.61 (1985)
843. This is due to the fact that the gap vanishes in quadratic manner around
the point on the Fermi surface. It is also shown that the region of quadratic
energy dependence of DOS, in the state F_{2u}(1,varepsilon,varepsilon^{2}), is
restricted in very small energy region making it difficult to distinguish from
the polar-like DOS.Comment: 5 pages, 3 figures, submitted to J. Phys.: Condens. Matter Lette
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