54 research outputs found
Time-dependent optical linewidth in fluctuating environments: Stochastic models
Time-resolved optical lineshapes are calculated using a second-order
inhomogeneous cumulant expansion. The calculation shows that in the
inhomogeneous limit the optical spectra are determined solely by two-time
correlation functions. Therefore, measurements of the Stokes-shift correlation
function and the inhomogeneous linewidth cannot provide information about the
heterogeneity lifetime for systems exhibiting dynamic heterogeneities. The
theoretical results are illustrated using a stochastic model for the optical
transition frequencies. The model rests on the assumption that the transition
frequencies are coupled to the environmental relaxation of the system. The
latter is chosen according to a free-energy landscape model for dynamically
heterogeneous dynamics. The model calculations show that the available
experimental data are fully compatible with a heterogeneity lifetime on the
order of the primary relaxation time.Comment: 19 pages, 4 figure
Nonresonant holeburning in the Terahertz range: Brownian oscillator model
The response to the field sequence of nonresonant hole burning, a
pump-wait-probe experiment originally designed to investigate slow relaxation
in complex systems, is calculated for a model of Brownian oscillators, thus
including inertial effects. In the overdamped regime the model predictions are
very similar to those of the purely dissipative stochastic models investigated
earlier, including the possibility to discriminate between dynamic homogeneous
and heterogeneous relaxation. The case of underdamped oscillations is of
particular interest when low-frequency excitations in glassy systems are
considered. We show that also in this situation a frequency selective
modification of the response should be feasable. This means that it is possible
to specifically address various parts of the spectrum. An experimental
realization of nonresonant holeburning in the Terahertz regime therefore is
expected to shed further light on the nature of the vibrations around the
so-called boson peak.Comment: 23 pages, 6 figure
Nonlinear response theory for Markov processes IV: The asymmetric double well potential model revisited
The dielectric response of non-interacting dipoles is discussed in the
framework of the classical model of stochastic reorientations in an asymmetric
double well potential (ADWP). In the nonlinear regime, this model exhibits some
pecularities in the static response. We find that the saturation behavior of
the symmetric double well potential model does not follow the Langevin function
and only in the linear regime the standard results are recovered. If a finite
asymmetry is assumed, the nonlinear susceptibilities are found to change the
sign at a number of characteristic temperatures that depend on the magnitude of
the asymmetry, as has been observed earlier for the third-order and the
fifth-order response. If the kinetics of the barrier crossing in the ADWP model
is described as a two-state model, we can give analytical expressions for the
values of the characteristic temperatures. The results for the response
obtained from a (numerical) solution of the Fokker-Planck equation for the
Brownian motion in a model ADWP behaves very similar to the two-state model for
high barriers. For small barriers no clearcut time scale separation between the
barrier crossing process and the intra-well relaxation exists and the model
exhibits a number of time scales. In this case, the frequency-dependent linear
susceptibility at low temperatures is dominated by the fast intra-well
transitions and at higher temperatures by the barrier crossing kinetics. We
find that for nonlinear susceptibilities the latter process appears to be more
important and the intra-well transitions play only role at the lowest
temperatures.Comment: 22 pages, 8 figures, revised version, accepted for publication in
Phys. Rev.
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