Role of non-linear effects and standing waves in microwave spectroscopy: Corbino measurements on superconductors and VO2

Abstract

Broadband microwave spectroscopy can probe material properties in wide spectral and temperature ranges. The quality of such measurements crucially depends on the calibration, which also removes from the obtained spectra signatures of standing waves. Here we consider cryogenic Corbino-type reflection measurements on superconductors close to the critical temperature. We show that the non-linear sample response, which relates to sample heating, can lead to strong signatures of standing waves even in a well-calibrated Corbino spectrometer. We demonstrate our findings with microwave measurements as a function of frequency, power, and temperature and for different length of the microwave transmission line. Finally we note such non-linear effects beyond the case of superconductors by probing a VO 2 thin film at the insulator-metal transition. arXiv:1810.07784v3 [cond-mat.supr-con]

Kinetic planar resonators from strongly disordered ultra-thin MoC superconducting films investigated by transmission line spectroscopy

“…The temperature dependence of the resonances was fitted by numerically calculated complex conductivity of disordered superconductor 1 with finite Γ. The obtained Γ parameters relate reasonably to the ones expected from STS measurements, showing that this theory could be utilized to describe the electrodynamic response of highly disordered superconductors, and vice versa: this non-contact method could be used to estimate the Γ and T c of highly disordered films, avoiding the nontrivial calibration procedure required in similar spectroscopic techniques 36 , or the requirement of structuring special resonators on the films 37 . Moreover, the understanding of the nature of these resonances can help eliminate them in the required bandwidth by smart design, as for example in Ref.…”

Characterizing dielectric properties of ultra-thin films using superconducting coplanar microwave resonators

“…Microwave data have been acquired using a vector network analyzer (VNA) and a 4 He cryostat with variable temperature insert (VTI) to reach cryogenic temperatures; sample temperature T was 1.7 K unless stated otherwise. The output power of the VNA was chosen around −40 dBm to avoid possible regimes of non-linearity at lower powers due to two-level fluctuators 43 and at higher powers due to the superconductor 44,45 .…”

High-Strain-Induced Local Modification of the Electronic Properties of VO2 Thin Films

“…The reversible semiconductor-to-metal transition, often referred to as metal–insulator transition (MIT), can be triggered by various stimuli, such as heat, light, mechanical pressure, and electric and magnetic fields, which opens the possibility for realizing switching devices between well-defined off (insulating) and on (metallic) states. In order to better understand the microscopic origin of the MIT, VO 2 has been investigated with state-of-the-art (pump–probe) spectroscopy techniques employing radiation all across the electromagnetic spectrum from hard and soft X-rays, to XUV, infrared, THz, microwave, and radio frequency, as well as electron microscopy. , The interested reader is referred to the review by Shao et al Our current understanding is that the electronic phase transition of VO 2 is inherently coupled to an accompanying structural phase transition from a monoclinic crystal structure at low temperature to a tetragonal structure at high temperature. The monoclinic phase is defined by a characteristic dimerization and tilt motif of the vanadium ions, whereas the tetragonal phase is isostructural to rutile TiO 2 .…”