Magnetski ovisan odziv visokotemperaturnih supravodiča na mikrovalno zračenje

Sustavno je proučavana temperaturna i magnetska ovisnost apsorpcije mikrovalova u keramičkom uzorku i monokristalu visokotemperatumog supravodiča YBa_2Cu_3O_7-δ . Magnetska ovisnost mikrovalne apsorpcije mjerena je pri raznim temperaturama na istoj apsolutnoj skali apsorpcije. Za razliku od ovisnosti apsorpcije u monokristalu, u slučaj u keramičkog uzorka razlikuj u se dva područja: područje slabih magnetskih polja (do 10 mT) , u kojem dominiraj u procesi u intergranulamom mediju , i područje jačih polja (iznad 10 mT) , gdje dominiraju procesi u supravodljivim zrnima. Fenomeni mikrovalne apsorpcije u slabim magnetskim poljima objašnjeni su pomoću modela mreže Josephsonovih spojeva u magnetskom polju . Taj model vrlo dobro opisuj e sve pojave opažene kako izravnim mjerenjem tako i mjerenjem, pomoću modulacije magnetskog polja u prvom i drugom harmoniku fazne detekcije. Magnetska ovisnost mikrovalne apsorpcije u jakim poljima pripisuj e se viskoznom gušenju gibanj a virova. Za opis temperaturne i magnetske ovisnosti površinskog otpora predložen je model efektiv ne kompleksne vodljivosti za supravodiče II. vrste u miješanom stanju, pomoću kojeg je određeno i kritično polje H_c2 za keramički uzorak na temperaturama iznad 77 K. Primjenom modulacije magnetskog polja opažena je neobična histereza koja nema analogona u signalu izravno mjerene apsorpcije. Pokazano je da je ta histereza posljedica neharmoničnosti sile zapinjanja.Temperature and magnetic field dependence of microwave absorption in single crystals and ceramic samples of high-Tc superconductor YBa_2Cu_30_7-δ, is studied systematically. Magnetic field dependence of microwave absorption is measured at various temperatures on the same absolute absorption scale. Ceramic sample, contrary to the monocrystal, exhibits two absorption regions: low magnetic field region (up to 10 mT) dominated by the processes in intergranular medium and high magnetic field region (above 10 mT) dominated by the processes in superconductive grains, respectively. Microwave absorption phenomena in low magnetic field region are explained by the network of Josephson junctions. All the phenomena observed by both direct and modulated microwave absorption in first and second harmonic lock-in detection are well described by this model. Magnetic field dependence of microwave absorption in the high field region is ascribed to the viscous damping of vortex motion. The model of effective complex conductivity for the mixed state of type II supercoductors is proposed for the explanation of temperature and magnetic field dependence of the surface resistance. The model is used for determination of the upper critical field H_c2 for the ceramic sample at temperatures above 77 K. It is shown that the modulated absorption signal exhibits an unusual hysteresis, which has no analogon in the direct absorption signal. The hysteresis originates from the anharmonicity of the pinning force