199 research outputs found
Existence of an upper limit on the density of excitons in carbon nanotubes by diffusion-limited exciton-exciton annihilation: Experiment and theory
Through an investigation of photoemission properties of highly-photoexcited
single-walled carbon nanotubes, we demonstrate that there is an upper limit on
the achievable excitonic density. As the intensity of optical excitation
increases, all photoluminescence emission peaks arising from different
chirality single-walled carbon nanotubes showed clear saturation in intensity.
Each peak exhibited a saturation value that was independent of the excitation
wavelength, indicating that there is an upper limit on the excitonic density
for each nanotube species. We propose that this saturation behavior is a result
of efficient exciton-exciton annihilation through which excitons decay
non-radiatively. In order to explain the experimental results and obtain
excitonic densities in the saturation regime, we have developed a model, taking
into account the generation, diffusion-limited exciton-exciton annihilation,
and spontaneous decays of one-dimensional excitons. Using the model, we were
able to reproduce the experimentally obtained saturation curves under certain
approximations, from which the excitonic densities were estimated. The validity
of the model was confirmed through comparison with Monte Carlo simulations.
Finally, we show that the conventional rate equation for exciton-exciton
annihilation without taking into account exciton diffusion fails to fit the
experimentally observed saturation behavior, especially at high excitonic
densities.Comment: 5 figures, 1 tabl
Is the optical conductivity of heavy fermion strange metals Planckian?
Strange metal behavior appears across a variety of condensed matter settings and beyond, and achieving a universal understanding is an exciting prospect. The beyond-Landau quantum criticality of Kondo destruction has had considerable success in describing the behavior of strange metal heavy fermion compounds, and there is some evidence that the associated partial localization-delocalization nature can be generalized to diverse materials classes. Other potential overarching principles at play are also being explored. An intriguing proposal is that Planckian scattering, with a rate of kBT/ℏ, leads to the linear temperature dependence of the (dc) electrical resistivity, which is a hallmark of strange metal behavior. Here we extend a previously introduced analysis scheme based on the Drude description of the dc resistivity to optical conductivity data. When they are well described by a simple (ac) Drude model, the scattering rate can be directly extracted. This avoids the need to determine the ratio of charge carrier concentration to effective mass, which has complicated previous analyses based on the dc resistivity. However, we point out that strange metals typically exhibit strong deviations from Drude behavior, as exemplified by the “extreme” strange metal YbRh2Si2. This calls for alternative approaches, and we point to the power of strange metal dynamical (energy-over-temperature) scaling analyses for the inelastic part of the optical conductivity. If such scaling extends to the low-frequency limit, a strange metal relaxation rate can be estimated, and may ultimately be used to test whether strange metals relax in a Planckian manner
Dicke Superradiance in Solids
Recent advances in optical studies of condensed matter have led to the
emergence of phenomena that have conventionally been studied in the realm of
quantum optics. These studies have not only deepened our understanding of
light-matter interactions but also introduced aspects of many-body correlations
inherent in optical processes in condensed matter systems. This article is
concerned with superradiance (SR), a profound quantum optical process predicted
by Dicke in 1954. The basic concept of SR applies to a general -body system
where constituent oscillating dipoles couple together through interaction with
a common light field and accelerate the radiative decay of the system. In the
most fascinating manifestation of SR, known as superfluorescence (SF), an
incoherently prepared system of inverted atoms spontaneously develops
macroscopic coherence from vacuum fluctuations and produces a delayed pulse of
coherent light whose peak intensity . Such SF pulses have been
observed in atomic and molecular gases, and their intriguing quantum nature has
been unambiguously demonstrated. Here, we focus on the rapidly developing field
of research on SR in solids, where not only photon-mediated coupling but also
strong Coulomb interactions and ultrafast scattering exist. We describe SR and
SF in molecular centers in solids, molecular aggregates and crystals, quantum
dots, and quantum wells. In particular, we will summarize a series of studies
we have recently performed on quantum wells in strong magnetic fields. These
studies show that cooperative effects in solid-state systems are not merely
small corrections that require exotic conditions to be observed; rather, they
can dominate the nonequilibrium dynamics and light emission processes of the
entire system of interacting electrons.Comment: 23 pages, 26 figure
Magneto-reflection spectroscopy of monolayer transition-metal dichalcogenide semiconductors in pulsed magnetic fields
We describe recent experimental efforts to perform polarization-resolved
optical spectroscopy of monolayer transition-metal dichalcogenide
semiconductors in very large pulsed magnetic fields to 65 tesla. The
experimental setup and technical challenges are discussed in detail, and
temperature-dependent magneto-reflection spectra from atomically thin tungsten
disulphide (WS) are presented. The data clearly reveal not only the valley
Zeeman effect in these 2D semiconductors, but also the small quadratic exciton
diamagnetic shift from which the very small exciton size can be directly
inferred. Finally, we present model calculations that demonstrate how the
measured diamagnetic shifts can be used to constrain estimates of the exciton
binding energy in this new family of monolayer semiconductors.Comment: PCSI-43 conference (Jan. 2016; Palm Springs, CA
Exciton Diamagnetic Shifts and Valley Zeeman Effects in Monolayer WS and MoS to 65 Tesla
We report circularly-polarized optical reflection spectroscopy of monolayer
WS and MoS at low temperatures (4~K) and in high magnetic fields to
65~T. Both the A and the B exciton transitions exhibit a clear and very similar
Zeeman splitting of approximately 230~eV/T (), providing
the first measurements of the valley Zeeman effect and associated -factors
in monolayer transition-metal disulphides. These results complement and are
compared with recent low-field photoluminescence measurements of valley
degeneracy breaking in the monolayer diselenides MoSe and WSe. Further,
the very large magnetic fields used in our studies allows us to observe the
small quadratic diamagnetic shifts of the A and B excitons in monolayer WS
(0.32 and 0.11~eV/T, respectively), from which we calculate exciton
radii of 1.53~nm and 1.16~nm. When analyzed within a model of non-local
dielectric screening in monolayer semiconductors, these diamagnetic shifts also
constrain and provide estimates of the exciton binding energies (410~meV and
470~meV for the A and B excitons, respectively), further highlighting the
utility of high magnetic fields for understanding new 2D materials.Comment: 9 pages, 5 figure
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