Plasma Instability and Amplified Mode Switching Effect in THz Field Effect Transistors with Grating Gate

We developed a theory of collective plasma oscillations in a dc current-biased field effect transistor with interdigitated dual grating gate and demonstrated a new mechanism of electron plasma instability in this structure. The instability in the plasmonic crystal formed in the transistor channel develops due to conversion of the kinetic energy carried by the drifting plasmons into electromagnetic energy. The conversion happens at the opposite sides of the gate fingers due to the asymmetry produced by the current flow and occurs through the gate finger fringing capacitances. The key feature of the proposed instability mechanism is the behavior of the plasma frequency peak and its width as functions of the dc current bias. At a certain critical value of the current, the plasma resonant peak with small instability increment experiencing redshift with increasing current changes to the blue shifting peak with large instability increment. This amplified mode switching (AMS) effect has been recently observed in graphene-interdigitated structures (S. Boubanga-Tombet et al., Phys. Rev. X 10, 031004 (2020)). The obtained theoretical results are in very good qualitative agreement with these experiments and can be used in future designs of the compact sources of THz EM radiation.Comment: 15 pages, 6 figure

Critical properties of two-dimensional Josephson junction arrays with zero-point quantum fluctuations

We present results from an extensive analytic and numerical study of a two-dimensional model of a square array of ultrasmall Josephson junctions. We include the ultrasmall self and mutual capacitances of the junctions, for the same parameter ranges as those produced in the experiments. The model Hamiltonian studied includes the Josephson, $E_J$, as well as the charging, $E_C$, energies between superconducting islands. The corresponding quantum partition function is expressed in different calculationally convenient ways within its path-integral representation. The phase diagram is analytically studied using a WKB renormalization group (WKB-RG) plus a self-consistent harmonic approximation (SCHA) analysis, together with non-perturbative quantum Monte Carlo simulations. Most of the results presented here pertain to the superconductor to normal (S-N) region, although some results for the insulating to normal (I-N) region are also included. We find very good agreement between the WKB-RG and QMC results when compared to the experimental data. To fit the data, we only used the experimentally determined capacitances as fitting parameters. The WKB-RG analysis in the S-N region predicts a low temperature instability i.e. a Quantum Induced Transition (QUIT). We carefully simulations and carry out a finite size analysis of $T_{QUIT}$ as a function of the magnitude of imaginary time axis $L_\tau$. We find that for some relatively large values of $\alpha=E_C/E_J$ ($1\leq \alpha \leq 2.25)$, the $L_\tau\to\infty$ limit does appear to give a {\it non-zero} $T_{QUIT}$, while for $\alpha \ge 2.5$, $T_{QUIT}=0$. We use the SCHA to analytically understand the $L_\tau$ dependence of the QMC results with good agreement between them. Finally, we also carried out a WKB-RG analysis in the I-N region and found no evidence of a low temperature QUIT, up to lowest order in ${\alpha}^{-1}$Comment: 39 pages, 18 postscript figures, to appear in Phys. Rev.