arXiv subjects

F. T. Vasko

Publications and source records attributed to F. T. Vasko.

At least 37 records · Page 2Linked to original sources

arXiv2010

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.

cond-mat.mes-hall↗

arXiv2010

Anisotropic conductivity of doped graphene due to short-range non-symmetric scattering

The conductivity of doped graphene is considered taking into account scattering by short-range nonsymmetric defects, when the longitudinal and transverse components of conductivity tensor appear to be different. The calculations of the anisotropic conductivity tensor are based on the quasiclassical kinetic equation for the case of monopolar transport at low temperatures. The effective longitudinal conductivity and the transverse voltage, which are controlled by orientation of sample and by gate voltage (i.e. doping level), are presented.

cond-mat.mes-hall↗

arXiv2009

Electrooptics of graphene: field-modulated reflection and birefringence

The elecrooptical response of graphene due to heating and drift of carriers is studied theoretically. Real and imaginary parts of the dynamic conductivity tensor are calculated for the case of effective momentum relaxation, when anisotropic contributions are small. We use the quasiequilibrium distribution of electrons and holes, characterized by the effective temperature of carriers and by concentrations, which are controlled by gate voltage and in-plane electric field. The geometry of normal propagation of probe radiation is considered, spectral and field dependences of the reflection coefficient and the relative absorption are analyzed. The ellipticity degree of the reflected and transmitted radiation due to small birefringence of graphene sheet with current have also been determined.

cond-mat.mes-hall↗

arXiv2009

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.

cond-mat.mes-hall↗

arXiv2009

Negative lateral conductivity of hot electrons in a biased superlattice

Nonequilibrium electron distribution in a superlattice subjected to a homogeneous electric field (biased superlattice with equipopulated levels) is studied within the tight-binding approximation, taking into account the scattering by optical and acoustic phonons and by lateral disorder. It is found that the distribution versus the in-plane kinetic energy depends essentially on the ratio between the Bloch energy and the optical phonon energy. The in-plane conductivity is calculated for low-doped structures at temperatures 4.2 K and 20 K. The negative conductivity is found for bias voltages corresponding to the Bloch-phonon resonance condition.

cond-mat.mtrl-sci↗

arXiv2009

Hot carriers in an intrinsic graphene

Heating of carriers in an intrinsic graphene under dc electric field is considered taking into account the intraband energy relaxation due to acoustic phonon scattering and the interband generation-recombination transitions due to thermal radiation. The distribution of nonequilibrium carriers is obtained for the cases when the intercarrier scattering is unessential and when the carrier-carrier Coulomb scattering effectively establishes the quasiequilibrium distribution with the temperature and the density of carriers that are determined by the balance equations. Because of an interplay between weak energy relaxation and generation-recombination processes a very low threshold of nonlinear response takes place. The nonlinear current-voltage characteristics are calculated for the case of the momentum relaxation caused by the elastic scattering. Obtained current-voltage characteristics show low threshold of nonlinear behavior and appearance of the second ohmic region, for strong fields.

cond-mat.mes-hall↗

arXiv2008

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 $T_r\neq T$ concentration and conductivity of carriers in graphene modify essentially. It is demonstrated, that at $T_r>T$ the negative interband absorption, caused by the inversion of carriers distribution, can occur, i.e. graphene can be unstable under thermal irradiation.

cond-mat.mtrl-sci↗

arXiv2008

Nonequilibrium carriers in an intrinsic graphene under interband photoexcitation

We study nonequilibrium carriers (electrons and holes) in an intrinsic graphene at low temperatures under far- and mid-infrared (IR) radiation in a wide range of its intensities. The energy distributions of carriers are calculated using a quasiclassic kinetic equation which accounts for the energy relaxation due to acoustic phonons and the radiative generation-recombination processes associated with thermal radiation and the carrier photoexcitation by incident radiation. It is found that the nonequilibrium distributions are determined by an interplay between weak energy relaxation on acoustic phonons and generation-recombination processes as well as by the effect of pumping saturation. Due to the effect of saturation, the carrier distribution functions can exhibit plateaus around the pumping region at elevated intensities. As shown, at sufficiently strong mid-IR pumping, the population inversion can occur below the pumping energy. The graphene dc conductivity as a function of the pumping intensity exhibits a pronounced nonlinearity with a sub-linear region at fairly low intensities and a saturation at a strong pumping. However, an increase in the pumping intensity in very wide range leads only to a modest increase in the carrier concentration and, particularly, the dc conductivity. The graphene conductivity at mid-IR irradiation exhibit strong sensitivity to mechanisms of carrier momentum relaxation.

cond-mat.mtrl-sci↗

arXiv2007

Voltage and temperature dependencies of conductivity in gated graphene

The resistivity of gated graphene is studied taking into account electron and hole scattering by short- and long-range structural imperfections the characteristics of disorder were taken from the scanning tunneling microscopy data and by acoustic phonons. The calculations are based on the quasiclassical kinetic equation with the normalization condition fixed by surface charge. The gate-voltage and temperature effects on the resistance peak, which is centered at the point of intrinsic conductivity, are found to be in agreement with the transport measurements.

cond-mat.mtrl-sci↗

arXiv2007

Transient quantum evolution of 2D electrons under photoexcitation of a deep center

We have considered the ballistic propagation of the 2D electron Wigner distribution, which is excited by an ultrashort optical pulse from a short-range impurity into the first quantized subband of a selectively-doped heterostructure with high mobility. Transient ionization of a deep local state into a continuum conduction c-band state is described. Since the quantum nature of the photoexcitation, the Wigner distribution over 2D plane appears to be an alternating-sign function. Due to a negative contribution to the Wigner function, the mean values (concentration, energy, and flow) demonstrate an oscillating transient evolution in contrast to the diffusive classical regime of propagation.

cond-mat.other↗

arXiv2007

Complex permittivity of a biased superlattice

Intersubband response in a superlattice subjected to a homogeneous electric field (biased superlattice with equipopulated levels) is studied within the tight-binding approximation, taking into account the interplay between homogeneous and inhomogeneous mechanisms of broadening. The complex dielectric permittivity is calculated beyond the Born approximation for a wide spectral region. A detectable gain below the resonance is obtained for the low-doped $GaAs$-based biased superlattice in the THz spectral region. Conditions of the stimulated emission regime for metallic and dielectric waveguide structures are discussed.

cond-mat.other↗

arXiv2006

Transient magnetoconductivity of photoexcited electrons

Transient magnetotransport of two-dimensional electrons with partially-inverted distribution excited by an ultrashort optical pulse is studied theoretically. The time-dependent photoconductivity is calculated for GaAs-based quantum wells by taking into account the relaxation of electron distribution caused by non-elastic electron-phonon interaction and the retardation of the response due to momentum relaxation and due to a finite capacitance of the sample. We predict large-amplitude transient oscillations of the current density and Hall field (Hall oscillations) with frequencies corresponding to magnetoplasmon range, which are initiated by the instability owing to the absolute negative conductivity effect.

cond-mat.mtrl-sci↗

arXiv2006

Electron energy spectrum and density of states for non-symmetric heterostructures in an in-plane magnetic field

Modifications of spin-splitting dispersion relations and density of states for electrons in non-symmetric heterostructures under in-plane magnetic field are studied within the envelope function formalism. Spin-orbit interactions, caused by both a slow potential and the heterojunction potentials (which are described by the boundary conditions) are taken into account. The interplay between these contributions and the magnetic field contribution to the spin-splitting term in the Hamiltonian is essential when energy amount resulting from the Zeeman and spin-orbit coupling are of the same order. Such modifications of the energy spectra allow us to separate the spin-orbit splitting contributions due to a slow potential and due to the heterojunctions. Numerical estimates for selectively-doped heterojunction and quantum well with narrow-gap region of electron localization are performed.

cond-mat.mtrl-sci↗

arXiv2005

Transient response of photoexcited electrons: negative and oscillating current

Time-dependent current of the electrons excited in the conduction band after ultrafast interband photogeneration is studied theoretically. The transient photocurrent is calculated for the nonlinear regime of response to a stationary electric field. The response demonstrates transient absolute negative conductivity when the electrons are excited slightly below the optical phonon energy, while the periodic oscillations of the electric current appear after formation of the streaming distribution. The quenching of these peculiarities by the elastic scattering of electrons is also considered.

cond-mat.mtrl-sci↗

arXiv2005

Modulation of intersubband IR absorption under intense THz irradiation

We analyze the modification of the intersubband absorption of electrons in quantum wells under intense THz irradiation. An expression for the induced current is obtained, based on the adiabatic approach and the resonant approximation. We predict the occurrence of a significant fine structure as well as the broadening and shift of the absorption under THz pump in a MW/cm$^2$ intensity range.

cond-mat.mtrl-sci↗

arXiv2005

Temporal stimulated intersubband emission of photoexcited electrons

We have studied the transient evolution of electrons distributed over two levels in a wide quantum well, with the two levels below the optical phonon energy, after an ultrafast interband excitation and cascade emission of optical phonons. If electrons are distributed near the top of the passive region, a temporal negative absorption appears to be dominant in the intersubband response. This is due to the effective broadening of the upper level state under the optical phonon emission. We have then considered the amplification of the ground mode in a THz waveguide with a multiquantum well placed at the center of the cavity. A huge increase of the probe signal is obtained, which permits the temporal stimulated emission regime of the photoexcited electrons in the THz spectral region.

cond-mat.mtrl-sci↗

arXiv2003

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.

cond-mat.mtrl-sci↗

arXiv2003

Gain without inversion in a biased superlattice

Intersubband transitions in a superlattice under homogeneous electric field is studied within the tight-binding approximation. Since the levels are equi-populated, the non-zero response appears beyond the Born approximation. Calculations are performed in the resonant approximation with scattering processes exactly taken into account. The absorption coefficient is equal zero for the resonant excitation while a negative absorption (gain without inversion) takes place below the resonance. A detectable gain in the THz spectral region is obtained for the low-doped $GaAs$-based superlattice and spectral dependencies are analyzed taking into account the interplay between homogeneous and inhomogeneous mechanisms of broadening.

cond-mat.dis-nn↗