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Transmission-Line Model for Materials with Spin-Momentum Locking
摘要： We provide a transmission-line representation for channels exhibiting spin-momentum locking (SML) that can be used for both time-dependent and steady-state transport analysis on a wide variety of materials with spin-orbit coupling such as topological insulators, Kondo insulators, transition metals, semimetals, oxide interfaces, and narrow band-gap semiconductors. This model is based on a time-dependent four-component di?usion equation obtained from the Boltzmann transport equation assuming linear response and elastic scattering in the channel. We classify all electronic states in the channel into four groups (U+, D+, U?, and D?) depending on the spin index [up (U), down (D)] and the sign of the x component of the group velocity (+, ?) and assign an average electrochemical potential to each of the four groups to obtain the four-component di?usion equation. For normal metal channels, the model decouples into the well-known transmission-line model for charge and a time-dependent version of the Valet-Fert equation for spin. We ?rst show that, in the steady-state limit, our model leads to simple expressions for charge-spin interconversion in SML channels in good agreement with existing experimental data on diverse materials. We then use the full time-dependent model to study spin-charge separation in the presence of SML, a subject that has been controversial in the past. Our model shows that the charge and spin signals travel with two distinct velocities, resulting in well-known spin-charge separation, which is expected to persist even in the presence of SML. However, our model predicts that the lower velocity signal is purely spin, while the higher velocity signal is largely charge with an additional spin component proportional to the degree of SML, which has not been noted before. Finally, we note that our model can be used within standard circuit simulators such as SPICE to obtain numerical results for complex geometries.
关键词： spin-orbit coupling,spin-charge separation,transmission-line model,topological insulators,charge-spin interconversion,spin-momentum locking
更新于2025-09-23 15:21:01
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Resonant Tunneling Diode (RTD) Terahertz Active Transmission Line Oscillator with Graphene-Plasma Wave and Two Graphene Antennas
摘要： This study describes the design of a resonant tunneling diode (RTD) oscillator (RTD oscillator) with a RTD-gated-graphene-2DEF (two dimensional electron fluid) and demonstrates the functioning of this RTD oscillator through a transmission line simulation model. Impedance of the RTD oscillator changes periodically when physical dimension of the device is of considerable fraction of the electrical wavelength. As long as impedance matching is achieved, the oscillation frequency is not limited by the size of the device. An RTD oscillator with a graphene film and negative differential resistance (NDR) will produce power amplification. The positive electrode of the DC power supply is modified and designed as an antenna. So, the reflected power can also be radiated to increase RTD oscillator output power. The output analysis shows that through the optimization of the antenna structure, it is possible to increase the RTD oscillator output to 22 mW at 1.9 THz and 20 mW at 6.1 THz respectively. Furthermore, the RTD oscillator has the potential to oscillate at 50 THz with a matching antenna.
关键词： transmission line model,RTD-gated-graphene-2DEF,resonant tunneling diode (RTD),graphene-plasma,two-dimensional electron fluid (2DEF),resonant tunneling diode oscillator (RTD oscillator),terahertz
更新于2025-09-11 14:15:04