arXiv subjects

Anton V. Velichko

Publications and source records attributed to Anton V. Velichko.

5 recordsLinked to original sources

arXiv2021

Defect-Free Axially-Stacked GaAs/GaAsP Nanowire Quantum Dots with Strong Carrier Confinement

Axially-stacked quantum dots (QDs) in nanowires (NWs) have important applications in fabricating nanoscale quantum devices and lasers. Although their performances are very sensitive to crystal quality and structures, there is relatively little study on defect-free growth with Au-free mode and structure optimisation for achiving high performances. Here, we report a detailed study of the first self-catalyzed defect-free axially-stacked deep NWQDs. High structural quality is maintained when 50 GaAs QDs are placed in a single GaAsP NW. The QDs have very sharp interfaces (1.8~3.6 nm) and can be closely stacked with very similar structural properties. They exhibit the deepest carrier confinement (~90 meV) and largest exciton-biexciton splitting (~11 meV) among non-nitride III-V NWQDs, and can maintain good optical properties after being stored in ambient atmosphere for over 6 months due to excellent stability. Our study sets a solid foundation to build high-performance axially-stacked NWQD devices that are compatible with CMOS technologies.

physics.app-ph↗

arXiv2000

Phenomenological Model of the Nonlinear Microwave Response of a Superconductor Containing Weak Links

A phenomenological model is proposed which describes the effect of both dc and rf magnetic fields, $H$, on the microwave surface impedance, Z_s=R_s+jX_s, of a superconductor containing weak link. Two cases are considered; a weak link between two grains, shunted by another grain, and a non-shunted weak link. In both cases the dependences of R_s and X_s on applied field H were found to be anomalous. Under certain conditions, the values of Z_s(H) can fall below the zero-field values. Comparison with experiment is performed, and very good qualitative and (in some cases) quantitative agreement is found.

cond-mat.supr-con↗

arXiv2000

Anomalous Features in Surface Impedance of Y-Ba-Cu-O Thin Films: Dependence on Frequency, RF and DC Fields

Two high-quality Y-Ba-Cu-O thin films on MgO substrates have been investigated using the coplanar resonator technique at 8 and 16 GHz. Both films exhibit an anomalous decrease in their surface impedance, Zs as a function of microwave field, Hrf. In zero dc field, Hrf-dependences of Rs and Xs for both the samples are uncorrelated, and only one of the quantities, Rs or Xs, displays anomalous behavior. Here, application of relatively weak (~5 mT) dc magnetic fields, Hdc can produce a correlated decrease of Rs(Hrf) and Xs(Hrf). The dependences of Zs on Hdc in both low and high microwave power regimes were found to be non-monotonic. The frequency dependence of Rs ~ fn, 1.7<n<2.5, remained the same upon the transition from low to high microwave power ranges. The consequences of the reported findings for microwave device applications are briefly discussed

cond-mat.supr-con↗

arXiv1999

Unusual features in the nonlinear microwave surface impedance of Y-Ba-Cu-O thin films

Striking features have been found in the nonlinear microwave (8 GHz) surface impedance $Z_s=R_s + jX_s$ of high-quality YBaCuO thin films with comparable low power characteristics [$R_{res}\sim 35--60 μΩ$ and $λ_L(15 K)\sim 130--260 nm$]. The surface resistance $R_s$ is found to increase, decrease, or remain independent of the microwave field $H_{rf}$ (up to 60 mT) at different temperatures and for different samples. However, the surface reactance $X_s$ always follows the same functional form. Mechanisms which may be responsible for the observed variations in $R_s$ and $X_s$ are briefly discussed.

cond-mat.supr-con↗

arXiv1999

Modelling the Nonlinear High-Frequency Response of a Short Josephson Junction under Two-Frequency Irradiation

The nonlinear response of a short Josephson Junction (JJ), being irradiated simultaneously with two high-frequency signals, has been studied in the framework of the nonlinear Resistively-Shunted Junction (RSJ) Model. One of the signals, hereafter referred to as "probe signal", has a small amplitude $I_{pr}<I_c$ ($I_c$ is the critical current of the JJ) and frequency $f_{pr}$, and is used to monitor the response of the junction to the other high-power signal with amplitude $I_{pm}$ and frequency $f_{pm}$, hereafter referred to as "pump signal". Varying the frequency ratio $f_{pm}/f_{pr}$ from 0.5 to 100, and the current amplitude of the probe signal from 0.01 to 0.9 of $I_c$, we found that the dependence of the junction impedance at the frequency $f_{pr}$, $Z_s^{f_{pr}}$, versus $I_{pm}$ preserves its general features, independent of $f_{pm}/f_{pr}$ and $I_{pr}/I_c$ values. At the same time, some particular features, like negative values of $Re(Z_s^{f_{pr}})$ and "fine" structure of the steps in $Z_s^{f_{pr}}(I_{pm})$ are observed for $f_{pm}/f_{pr}<1$ and for particular values of $I_{pr}/I_c$. In general, the behavior of $Z_s^{f_{pr}}(I_{pm})$ is rather different from that predicted by the nonlinear RSJ model for a short JJ in the regime of single-frequency irradiation, when one and the same signal plays the roles of the pump and the probe signals simultaneously. Possible applications of the model are briefly discussed.

cond-mat.supr-con↗