Screening and transport in 2D semiconductor systems at low temperatures

Carrier type and density dependence of impact ionization characteristics in WSe2

The impact ionization process offers advantages in achieving low-power and high-speed switching in transistors and also provides high internal gain for photodetectors. We investigate the density dependence of both electron- and hole-initiated impact ionization in WSe2. We observe a record-low critical electric field for impact ionization and a large multiplication factor in WSe2 when the impact ionization is initiated by holes, particularly near a charge-neutral point. As the carrier density increases, the impact ionization decreases, which is attributed to the increase in Fermi energy and the weakening of the carrier-carrier interaction by increasing the screening effect. To understand the role of screening effects on the impact ionization, we examine the carrier scatterings of several scattering sources and the rate of change of the channel current with respect to the drain source voltage, ΔIDS/ΔVDS. We obtain the optimal conditions for impact ionization and apply these findings to fabricate avalanche photodetectors (APDs). This study proposes that the performance of low-power transistors or APDs, utilizing impact ionization, can be enhanced under optimized conditions by examining and controlling specific external parameters.

A criterion for strange metallicity in the Lorenz ratio

The Wiedemann-Franz (WF) law, stating that the Lorenz ratio L = κ/(Tσ) between the thermal and electrical conductivities in a metal approaches a universal constant L 0 = π 2 k B 2 / ( 3 e 2 ) at low temperatures, is often interpreted as a signature of fermionic Landau quasi-particles. In contrast, we show that various models of weakly disordered non-Fermi liquids also obey the WF law at T → 0. Instead, we propose using the leading low-temperature correction to the WF law, L(T) - L0 (proportional to the inelastic scattering rate), to distinguish different types of strange metals. As an example, we demonstrate that in a solvable model of a marginal Fermi-liquid, L(T) - L0 ∝ - T. Using the quantum Boltzmann equation (QBE) approach, we find analogous behavior in a class of marginal- and non-Fermi liquids with a weakly momentum-dependent inelastic scattering. In contrast, in a Fermi-liquid, L(T) - L0 is proportional to - T2. This holds even when the resistivity grows linearly with T, due to T - linear quasi-elastic scattering (as in the case of electron-phonon scattering at temperatures above the Debye frequency). Finally, by exploiting the QBE approach, we demonstrate that the transverse Lorenz ratio, Lxy = κxy/(Tσxy), exhibits the same behavior.

Chirality relaxation in low-temperature strongly Rashba-coupled systems

We study the relaxation dynamics of non-equilibrium chirality distributions of charge carriers in Rashba systems. We find that at low temperature inter-Rashba band transitions become suppressed due to the combined effect of the Rashba momentum split and the chiral spin texture of a Rashba system. Specifically, we show that momentum exchange between carriers and the phonon bath is effectively absent at temperatures where the momentum of thermal phonons is less than twice the Rashba momentum. This allows us to identify inter-carrier scattering as the dominant process by which non-equilibrium chirality distributions relax. We show that the magnitude of inter-carrier scattering is strongly influenced by the opposing spin structure of the Rashba bands. Finally, we provide an explicit result for the inter-band relaxation timescale associated with inter-carrier Coulomb scattering. We develop a general framework and assess its implications for GeTe, a bulk Rashba semiconductor with a strong Rashba momentum split.