255 research outputs found
Transient response under ultrafast interband excitation of an intrinsic graphene
The transient evolution of carriers in an intrinsic graphene under ultrafast
excitation, which is caused by the collisionless interband transitions, is
studied theoretically. The energy relaxation due to the quasielastic acoustic
phonon scattering and the interband generation-recombination transitions due to
thermal radiation are analyzed. The distributions of carriers are obtained for
the limiting cases when carrier-carrier scattering is negligible and when the
intercarrier scattering imposes the quasiequilibrium distribution. The
transient optical response (differential reflectivity and transmissivity) on a
probe radiation and transient photoconductivity (response on a weak dc field)
appears to be strongly dependent on the relaxation and recombination dynamics
of carriers.Comment: 9 pages, 8 figure
Depletion of carriers and negative differential conductivity in an intrinsic graphene under a dc electric field
The heating of carriers in an intrinsic graphene under an abrupt switching
off a dc electric field is examined taking into account both the energy
relaxation via acoustic and optic phonons and the interband
generation-recombination processes. The later are caused by the interband
transitions due to optical phonon modes and thermal radiation. Description of
the temporal and steady-state responses, including the nonequilibrium
concentration and energy as well as the current-voltage characteristics, is
performed. At room temperature, a nearly-linear current-voltage characteristic
and a slowly-varied concentration take place for fields up to -- 20 kV/cm.
Since a predominant recombination of high-energy carriers due to optical phonon
emission at low temperatures, a depletion of concentration takes place below --
250 K. For lower temperatures the current tends to be saturated and a negative
differential conductivity appears below -- 170 K in the region of fields -- 10
V/cm.Comment: 8 pages, 10 figures, extended versio
Spectral and polarization dependencies of luminescence by hot carriers in graphene
The luminescence caused by the interband transitions of hot carriers in
graphene is considered theoretically. The dependencies of emission in mid- and
near-IR spectral regions versus energy and concentration of hot carriers are
analyzed; they are determined both by an applied electric field and a gate
voltage. The polarization dependency is determined by the angle between the
propagation direction and the normal to the graphene sheet. The characteristics
of radiation from large-scale-area samples of epitaxial graphene and from
microstructures of exfoliated graphene are considered. The averaged over angles
efficiency of emission is also presented.Comment: 6 pages, 5 figure
Thermal-radiation-induced nonequilibrium carriers in an intrinsic graphene
We examine an intrinsic graphene connected to the phonon thermostat at
temperature T under irradiation of thermal photons with temperature T_r, other
than T. The distribution of nonequilibrium electron-hole pairs was obtained for
the cases of low and high concentration of carriers. For the case when the
interparticle scattering is unessential, the distribution function is
determined by the interplay of intraband relaxation of energy due to acoustic
phonons and interband radiative transitions caused by the thermal radiation.
When the Coulomb scattering dominates, then the quasi-equilibrium distribution
with effective temperature and non-equilibrium concentration, determined
through balance equations, is realized. Due to the effect of thermal radiation
with temperature concentration and conductivity of carriers in
graphene modify essentially. It is demonstrated, that at the negative
interband absorption, caused by the inversion of carriers distribution, can
occur, i.e. graphene can be unstable under thermal irradiation.Comment: 5 pages, 4 figure
Coherent oscillations of electrons in tunnel-coupled wells under ultrafast intersubband excitation
Ultrafast intersubband excitation of electrons in tunnell-coupled wells is
studied depending on the structure parameters, the duration of the infrared
pump and the detuning frequency. The temporal dependencies of the photoinduced
concentration and dipole moment are obtained for two cases of transitions: from
the single ground state to the tunnel-coupled excited states and from the
tunnel-coupled states to the single excited state. The peculiarities of
dephasing and population relaxation processes are also taken into account. The
nonlinear regime of the response is also considered when the splitting energy
between the tunnel-coupled levels is renormalized by the photoexcited electron
concentration. The dependencies of the period and the amplitude of oscillations
on the excitation pulse are presented with a description of the nonlinear
oscillations damping.Comment: 8 pages, 12 figure
Rabi oscillations under ultrafast excitation of graphene
We study coherent nonlinear dynamics of carriers under ultrafast interband
excitation of an intrinsic graphene. The Rabi oscillations of response appear
with increasing of pumping intensity. The photoexcited distribution is
calculated versus time and energy taking into account the effects of energy
relaxation and dephasing. Spectral and temporal dependencies of the response on
a probe radiation (transmission and reflection coefficients) are considered for
different pumping intensities and the Rabi oscillations versus time and
intensity are analyzed.Comment: 6 pages, 6 figure
Gauged N=3, D=4 supergravity: A new web of marginally connected vacua
We analyze the vacuum structure of N=3,D=4 supergravity coupled to 9 vector multiplets with gauge group SO(3)×SU(3). Aside from the central N=3 AdS4 vacuum at the origin, on which the supermultiplet structure reproduces the massless sector of M-theory compactified on N0,1,0, we find a rich structure of AdS4 vacua preserving N=0,1,2,3 supersymmetry. These new vacua are arranged in a manifold spanned by scalar fields corresponding to exactly marginal deformations of the dual CFT. This manifold has the form T3/K, where K is a discrete subgroup of the gauge group: N=3,2 and 1 vacua correspond, respectively, to a point, a line and a surface in the three-dimensional vacuum manifold. We study RG flows from the central N=3 vacuum and elaborate on the possible higher dimensional origin of the new vacua. For the reader's convenience we also provide a review of the embedding tensor formulation of D=4, N=3 gauged supergravities. In particular we provide formulas involving the fermion shift tensors and mass matrices in N=3 theories, which can be applied to a generic gauging
Electronic states in heterostructures formed by ultranarrow layers
Low-energy electronic states in heterosrtuctures formed by ultranarrow layer
(single or several monolayers thickness) are studied theoretically. The host
material is described within the effective mass approximation and effect of
ultranarrow layers is taken into account within the framework of the transfer
matrix approach. Using the current conservation requirement and the inversion
symmetry of ultranarrow layer, the transfer matrix is written through two
phenomenological parameters. The binding energy of localized state, the
reflection (transmission) coefficient for the single ultranarrow layer case,
and the energy spectrum of superlattice are determined by these parameters.
Spectral dependency of absorption in superlattice due to photoexcitation of
electrons from localized states into minibands is strongly dependent on the
ultranarrow layers characteristics. Such a dependency can be used for
verification of the transfer matrix parameters.Comment: 7 pages, 7 figure
Synthesis of Improved Catalytic Materials for High-Temperature Water-gas Shift Reaction
In this investigation, we report the preparation and characterization of Co-, Cu- and Mn-substituted iron oxide catalytic materials supported on activated carbon. Co-precipitation method and low temperature treatment were used for their synthesis. The influence of chemical composition, stoichiometry, particle size and dispersity on their catalytic activity was studied. Samples were characterized in all stages of their co-precipitation, heating and spend samples after catalytic tests. The obtained results from room and low temperature Mössbauer spectroscopy were combined with analysis of powder X-ray diffraction patterns (XRD). They revealed the preparation of nano-sized iron oxide materials supported on activated carbon. Relaxation phenomena were registered also for the supported phases. The catalytic performance in the water-gas shift reaction was studied. The activity order was as follows: Cu0.5Fe2.5O4 > Co0.5Fe2.5O4 > Mn0.5Fe2.5O4. Catalytic tests demonstrated very promising results and potential application of studied samples due to their cost-effective composition
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