arXiv subjects

L. Varani

Publications and source records attributed to L. Varani.

8 recordsLinked to original sources

arXiv2019

Spatial dispersion of the high-frequency conductivity of two-dimensional electron gas subjected to a high electric field: collisionless case

We present the analysis of high-frequency (dynamic) conductivity with the spatial dispersion, $\sigma (\omega, {\bf q})$, of two-dimensional electron gas subjected to a high electric field. We found that at finite wavevector, ${\bf q}$, and at high fields, the high-frequency conductivity shows following peculiarities: strong non-reciprocal dispersion; oscillatory behavior; a set of frequency regions with negative $\sigma'$; non-exponential decay of $\sigma'$ and $\sigma''$ with frequency (opposite to the Landau damping mechanism). We illustrate the general results by calculations of spectral characteristics of particular plasmonic heterostructures on the basis of III-V semiconductor compounds. We conclude that the detailed analysis of the spatial dispersion of the dynamic conductivity of 2DEG subjected to high electric fields is critically important for different THz applications.

physics.app-ph↗

arXiv2019

THz frequency- and wavevector-dependent conductivity of low-density drifting electron gas in GaN. Monte Carlo calculations

We report the results of Monte Carlo simulation of electron dynamics in stationary and space- and time-dependent electric fields in compensated GaN samples. We have determined the frequency and wavevector dependencies of the dynamic conductivity, $\sigma_{\omega,q}$. We have found that the spatially dependent dynamic conductivity of the drifting electrons can be negative under stationary electric fields of moderate amplitudes, $2..5$ kV/cm. This effect is realized in a set of frequency windows. The low-frequency window with negative dynamic conductivity is due to the Cherenkov mechanism. For this case the time-dependent field induces a {\it traveling wave} of the electron concentration in real space and a {\it standing wave} in the energy/momentum space. The higher frequency windows of negative dynamic conductivity are associated with the optical phonon transient time resonances. For this case the time-dependent field is accompanied by oscillations of the electron distribution in the form of the {\it traveling} waves in both the real space and the energy/momentum space. We discuss the optimal conditions for the observation of these effects. We suggest that the studied negative dynamic conductivity can be used to amplify electromagnetic waves at the expense of energy of the stationary field and current.

cond-mat.mtrl-sci↗

arXiv2012

Wave excitations of drifting two-dimensional electron gas under strong inelastic scattering

We have analyzed low-temperature behavior of two-dimensional electron gas in polar heterostructures subjected to a high electric field. When the optical phonon emission is the fastest relaxation process, we have found existence of collective wave-like excitations of the electrons. These wave-like excitations are periodic in time oscillations of the electrons in both real and momentum spaces. The excitation spectra are of multi-branch character with considerable spatial dispersion. There are one acoustic-type and a number of optical-type branches of the spectra. Their small damping is caused by quasi-elastic scattering of the electrons and formation of relevant space charge. Also there exist waves with zero frequency and finite spatial periods - the standing waves. The found excitations of the electron gas can be interpreted as synchronous in time and real space manifestation of well-known optical-phonon-transient-time-resonance. Estimates of parameters of the excitations for two polar heterostructures, GaN/AlGaN and ZnO/MgZnO, have shown that excitation frequencies are in THz-frequency range, while standing wave periods are in sub-micrometer region.

cond-mat.mes-hall↗

arXiv2011

THz emission induced by optical beating in nanometer-length field-effect-transistors

Experimental results of direct measurement of resonant monochromatic terahertz emission optically excited in InGaAs transistor channels are presented. The emission is attributed to two-dimensional plasma waves excited by photogeneration of electron-hole pairs in the channel at the frequency $f_0$ of the beating of two cw-laser sources. The presence of resonances for the radiation emission in the range of $f_0\pm 10$ GHz (with $f_0$ from 0.3 up to 0.5 THz) detected by a Si-bolometer is found. Numerical results support that such a high quality of the emission resonances can be explained by the approach of an instability in the transistor channel.

cond-mat.mes-hall↗

arXiv2011

Voltage controlled terahertz transmission through GaN quantum wells

We report measurements of radiation transmission in the 0.220--0.325 THz frequency domain through GaN quantum wells grown on sapphire substrates at room and low temperatures. A significant enhancement of the transmitted beam intensity with the applied voltage on the devices under test is found. For a deeper understanding of the physical phenomena involved, these results are compared with a phenomenological theory of light transmission under electric bias relating the transmission enhancement to changes in the differential mobility of the two-dimensional electron gas.

cond-mat.mes-hall↗

arXiv2010

Room Temperature Coherent and Voltage Tunable Terahertz Emission from Nanometer-Sized Field Effect Transistors

We report on reflective electro-optic sampling measurements of TeraHertz emission from nanometer-gate-length InGaAs-based high electron mobility transistors. The room temperature coherent gate-voltage tunable emission is demonstrated. We establish that the physical mechanism of the coherent TeraHertz emission is related to the plasma waves driven by simultaneous current and optical excitation. A significant shift of the plasma frequency and the narrowing of the emission with increasing channel's current are observed and explained as due to the increase of the carriers density and drift velocity.

cond-mat.mes-hall↗

arXiv2000

Common Origin of Quantum Regression and Quantum Fluctuation Dissipation Theorems

It is shown that the quantum fluctuation dissipation theorem can be considered as a mathematical formulation in the spectral representation of Onsager hypothesis on the regression of fluctuations in physical systems. It is shown that the quantum fluctuation dissipation theorem can be generalized to an arbitrary stationary state.

cond-mat.stat-mech↗

arXiv1999

Injection statistics simulator for dynamic analysis of noise in mesoscopic devices

We present a model for electron injection from thermal reservoirs which is applied to particle simulations of one-dimensional mesoscopic conductors. The statistics of injected carriers is correctly described from nondegenerate to completely degenerate conditions. The model is validated by comparing Monte Carlo simulations with existing analytical results for the case of ballistic conductors. An excellent agreement is found for average and noise characteristics, in particular, the fundamental unities of electrical and thermal conductances are exactly reproduced.

cond-mat.mes-hall↗