We examine the DC and radio frequency (RF) response of superconducting transmission line resonators comprised of very thin NbTiN films, [Formula: see text] in thickness, in the high-temperature limit, where the photon energy is less than the thermal energy. The resonant frequencies of these superconducting resonators show a significant nonlinear response as a function of RF input power, which can approach a frequency shift of [Formula: see text] in a [Formula: see text] span in the thinnest film. The strong nonlinear response allows these very thin film resonators to serve as high kinetic inductance parametric amplifiers.
Capacitance-voltage ( {C} - {V} ) characteristics and carrier transport properties of 2-D electron gases (2DEGs) in an undoped Si/SiGe heterostructure at {T}= {4} - {35} K are presented. Two capacitance plateaus due to density saturation of the 2DEG in the buried Si quantum well (QW) are observed and explained by a model of surface tunneling. The peak mobility at 4 K is 4.1 \times 10^{{5}} cm2/ \text{V}\cdot \text{s} and enhanced by a factor of 1.97 at an even lower carrier density compared to the saturated carrier density, which is attributed to the effect of remote carrier screening. At {T}\,\,=35 K, the mobility enhancement with a factor of 1.35 is still observed, which suggests the surface tunneling is still dominant.
Defects in materials are an ongoing challenge for quantum bits, so called qubits. Solid state qubits—both spins in semiconductors and superconducting qubits—suffer from losses and noise caused by two-level-system (TLS) defects thought to reside on surfaces and in amorphous materials. Understanding and reducing the number of such defects is an ongoing challenge to the field. Superconducting resonators couple to TLS defects and provide a handle that can be used to better understand TLS. We develop noise measurements of superconducting resonators at very low temperatures (20 mK) compared to the resonant frequency, and low powers, down to single photon occupation.
We investigate the thermoelectric transport properties of the half-filled lowest Landau level v=1/2 in a gated two-dimensional hole system in a strained Ge/SiGe heterostructure. The electron-diffusion dominated regime is achieved below 600 mK, where the diffusion thermopower Sxxd at v=1/2 shows a linear temperature dependence. In contrast, the diffusion-dominated Nernst signal Sxyd of v=1/2 is found to approach zero, which is independent of the measurement configuration (sweeping magnetic field at a fixed hole density or sweeping the density by a gate at a fixed magnetic field).
Terahertz (THz) photoconductive devices are used for generation, detection, and modulation of THz waves, and they rely on the ability to switch electrical conductivity on a subpicosecond time scale using optical pulses. However, fast and efficient conductivity switching with high contrast has been a challenge, because the majority of photoexcited charge carriers in the switch do not contribute to the photocurrent due to fast recombination. Here, we improve efficiency of electrical conductivity switching using a network of electrically connected nanoscale GaAs resonators, which form a perfectly absorbing photoconductive metasurface. We achieve perfect absorption without incorporating metallic elements, by breaking the symmetry of cubic Mie resonators. As a result, the metasurface can be switched between conductive and resistive states with extremely high contrast using an unprecedentedly low level of optical excitation. We integrate this metasurface with a THz antenna to produce an efficient photoconductive THz detector. The perfectly absorbing photoconductive metasurface opens paths for developing a wide range of efficient optoelectronic devices, where required optical and electronic properties are achieved through nanostructuring the resonator network.