Nonlinear Microwave Response Research Articles

Identification and modeling of microwave loss mechanisms in YBa/sub 2/Cu/sub 3/O/sub 7-X/

It has been proposed that the nonlinear microwave response of YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films is due to small- or large-angle grain boundaries which behave as weak links. To identify dominant loss mechanisms in the measured response of stripline resonators, the relative change in surface reactance and resistance, r=/spl Delta/X/sub s///spl Delta/R/sub s/, is used as a characteristic signature for the different mechanisms. We show that Meissner state and flux flow losses can be differentiated from hysteresis loss for a variety of films across a range of microwave powers, frequencies, and temperatures. In addition, a coupled-grain/RSJ model has been augmented to include Josephson currents up to and beyond the critical currents J/sub cj/ of the weak links. The model can account for a distribution of grain and boundary properties including I/sub c/R/sub n/ products, junction capacitances, and grain dimensions. The r-values predicted by the model are discussed and compared to measurements.

Frequency conversion in hysteretic HTS microwaveSQUIDs at 3 GHz

Two-tone excitation at 3 GHz of hysteretic YBa/sub 2/Cu/sub 3/O/sub 7/ microwave SQUIDs provide general information on the suitability of such devices for low-power signal processing, e.g. signal detection and frequency conversion. Intermodulation effects, which are expected to characterise the nonlinear dynamic microwave response of HTS films, are also extracted from the data.

Low-temperature nonlinear microwave response of incipient ferroelectrics

A review of investigations of microwave dielectric permittivity and dielectric nonlinearity of incipient ferroelectrics (SrTiO3, KTaO3, PbTe) is presented. Physical mechanisms of excess anharmonici...

Nonlinear microwave response of a YBaCuO

The review of modern situation concerning the studies of the generation of microwave harmonics in YBaCuO is given. Far from Tc the nonlinear response of ceramics YBaCuO has Josephson nature in a weak magnetic fields or is described in terms of the critical state model in a strong external magnetic field H. Near Tc, the signals of the second P2ω(T) and the third P3ω(T) harmonics have the form of spikes of generation. Yet the signal P2ω(H) shows a hysteresis and is proportional to the square of incident power P2ω∝P2ω while the signal P3ω∝P3ω and doesn't depend on the field H. Such behavior of the nonlinear response near Tc is typical of both ceramics and single crystals of YBaCuO. We use the Portis model to describe the signal P2ω and the model based on Eliashberg theory to describe the signal P3ω. The question about the model which can simultaneously explain the behavior of the second and the third harmonics remains open.

Nonlinear microwave response of superconductors

Superconductors (SC) have been investigated with microwaves by registering the nonlinear response (RNR) as a function of the modulation field. Equidistant narrow lines in a single YBaCuO crystal found by electron spin resonance (ESR) could be registered by RNR as absorption bands. Some narrow lines are present or absent depending on sweep direction and excursion. In granular SC, thin YBaCuO films showed critical (junction) fields H* which are smaller (3μT) than in bulk samples (5μT). They are as small as in BiSrCaCuO bulk. Using RNR with intermittent modulation, the measured complex hysteresis response indicates a spin glass type of structure of the SC network, and it demonstrates that the usual narrow band ESR registers only prepared states and not initial states corresponding to the scanning field.

Nonlinear microwave response in field-cooled high-Tc and type II superconductors

Abstract Experimental data on second-harmonic generation at microwave frequencies in field-cooled ceramic YBa2Cu3O7 and polycrystalline niobium are reported. Evidence is given that the signals are radiated by the component of the induced magnetization oscillating at the second-harmonic frequency of the driving field. It is shown that the experimental data are satisfactorily accounted for if the assumption is made that a rectification process of the microwave field is operated by superconductors in the critical state.

Nonlinear microwave response to scanning fields in high-T c oxides

Nonresonant microwave (MW) absorption is observed at the surface of high-Tc superconducting oxides in the presence of a static magnetic field and a scanning field. Nonlinear signals are obtained that show absorption troughs at the endpoints of the field scan superimposed on a hysteretic variation in absorption with field. The model of Portis et al. accounts quantitatively for the detailed shape of the observed signals. It enables the direct measurement of a critical field H*=(4π/c)Jcλ which depends on the critical current Jc and the static field penetration depth λ. In addition, the ratio λMW/λ is deduced from the shape of the troughs, with λMW the penetration depth of the MW fields. Typical values are H*≂5 μT and λMW≂3λ for a sintered YBaCuO sample at 77 K, and nearly the same H* but λMW≂λ at 4.2 K. Since the observed shapes also depend on the previous slow changes of the applied field, these investigations account for the memory effect observed with the usual resonance techniques.

Nonlinear microwave response to oscillating fields: Critical current and field penetration in high-Tc oxides

In the high- T c superconducting oxides, critical currents and magnetic field penetration can be measured using a novel technique: the nonlinear microwave response to scanned magnetic fields.

Magnetic resonance studies of itinerant antiferromagnetism in organic superconductors

The high conductivities of the Bechgaard salts, (TMTSF)2X, persist to low temperatures where various electronic instabilities occur. Nonlinear microwave loss and spin resonance experiments indicated a spin-density-wave transition at ≈ 12 K for X = PF6 and AsF6, as has been verified. Suppression of the SDW state by pressure leads to superconductivity but the SDW, SC intersection is obscure. For X = ClO4 at atmospheric pressure we observe nonlinear microwave response below 5.5 K and AFMR at 1.6 K clearly showing itinerant antiferromagnetism precedes superconductivity at 1.3 K. A residual spin resonance below the CDW transition of X = FSO3 may prove a useful probe as pressure induces SC near 2 K.