Yu. Talanov

Origin of the irreversible microwave absorption versus the state of vortex matter in Bi2Sr2CaCu2Ox single crystals

Abstract We have investigated properties of hysteretic microwave absorption (MWA) by the Bi2Sr2CaCu2Ox single crystals in the temperature range from 10 K to Tc. It was found that the shape of the MWA hysteresis loop and its dependence on the magnetic field modulation amplitude undergo essential changes with temperature. The analysis has shown that near 50 K, the origin of the signal detected by electron spin resonance spectrometer with magnetic field modulation changes. It is due to crossover to another pinning type. At T

Critical state of an YBa2Cu3Ox strip in a perpendicular magnetic field as revealed by a scanning ESR probe

Abstract The distribution of a magnetic field in a superconducting sample is studied by a scanning ESR microprobe. The main attention is focused to the penetration of a perpendicular magnetic field into an YBaCuO single crystal strip. The field profile in the interior of the sample is mapped from the data on the ESR-signal shift of the spin probe measured at different points over the plane surface. The penetration of the magnetic flux is shown to be delayed and this gives rise to the formation of steep fronts in the profile of the field near the center of the strip. The profile observed is highly dependent on the magnetic history. The demagnetizing effects and the distribution of the current are discussed. The analysis of the experimental data obtained gives evidence that the critical state picture differs considerably from the well known Bean model.

ESR investigation of the superconducting critical state in YBaCuO single crystals

Abstract The mixed state of superconducting Yba 2 Cu 3 O x single crystals has been studied by the observation of the electron spin resonance (ESR) of a paramagnetic probe on their surface. To investigate the distribution of the magnetic field at the vicinity and on the surface of a superconductor the evolution of the ESR spectrum was observed upon elevating the probe above the crystal by means of a buffer layer of varying thickness placed in between. The field inhomogeneity was found to extend to a distance of the order of the crystal size. The spatial scale of inhomogeneity on the superconductor surface was of the same magnitude. From the dependences of ESR signal parameters on the cooling temperature the features of the distribution of magnetic flux inside a sample have been revealed and the values of the critical current density have been determined on the base of the Beans model of the critical state of a superconductor.

Peculiarities of the vortex dynamics in YBa2Cu3Ox single crystals as revealed by irreversible microwave absorption

Abstract We report the results of a wide experimental study of the irreversible modulated microwave absorption as a function of temperature, applied magnetic field and modulation amplitude in the YBa2Cu3Ox single crystals. To analyze the experimental data the model of the microwave power dissipation by the flux lines has been developed taking into account thermal fluctuations, the distributions of currents and vortices over a sample. We have obtained the information about regimes of vortex motion in the different areas of the H–T phase diagram and estimated the values of flux flow viscosity and critical current density.

Electron spin resonance study of the local properties of the microscopic phase separation in La1 − x Ca x MnO3 single-crystal films

Electron spin resonance is studied in films of La1−xCaxMnO3 manganites with compositions in the vicinity of the calcium content x = 0.5, in which the phase separation is most clearly pronounced. It is found that the La0.5Ca0.5MnO3 manganite undergoes different types of phase separation: (i) at temperatures above the Curie point, ferromagnetic regions exist in the paramagnetic phase; and (ii) at temperatures below the Néel point, antiferromagnetic microregions coexist with ferromagnetic microregions. Two types of ferromagnetic domains with different magnetization orientations are revealed In the temperature range between the Curie and Néel points.

Microwave absorption study of polycrystalline SmO1-xFxFeAs

Measurements of nonresonance microwave absorption have been used to determine the position of the irreversibility line and to estimate the critical current density for the three SmO1?xFxFeAs samples with fluorine concentrations x = 0.06, 0.08, and 0.1. The irreversibility lines of all samples are characterized with a sharp slope, indicating a strong pinning up to Tc. A weak field dependence of the critical current density and an estimate of parameters characterizing the vortex matter allow us to suggest the presence of additional pinning centers in underdoped samples. It is likely that these pinning centers are nanoscale inclusions of a possibly magnetic phase occurring in the fluorine concentration range of 0.06?0.08.

Detection of vortex excitations in a Bi2Sr2Ca2Cu3O10 crystal at temperatures above the critical temperature by EPR of the surface layer

Vortex excitations have been detected at temperatures both below and above the critical temperature when investigating local magnetic fields on the surface of a Bi2Sr2Ca2Cu3O10 single crystal by means of an electron paramagnetic resonance (EPR) probe. A thin layer of a diphenyl picrylhydrazyl organic radical deposited on the crystal surface is used as the EPR probe. A narrow EPR signal makes it possible to detect weak distortions of the magnetic field appearing at T ≿ Tc. The analysis of the temperature dependences of the resonance field and the EPR linewidth is thebasis of the assumption of the vortex nature of magnetic excitations in this temperature range.

Shift of the surface-barrier part of the irreversibility line due to columnar defects in Bi2Sr2CaCu2O8 thin films

Abstract In the present work we report the results of studying the influence of the uranium-ion irradiation of the Bi 2 Sr 2 CaCu 2 O 8 thin films on the high-temperature part (close to T c ) of their irreversibility line. We studied irreversible properties of the films by measuring the hysteresis of nonresonant microwave absorption. The results have revealed the shift of irreversibility line towards low-temperatures and magnetic fields. The effect is most significant for the films irradiated with large doses, B Φ >1 T. This fact is in good agreement with the theoretical prediction by Koshelev and Vinokur of suppression of surface-barrier by columnar defects.

Hysteresis of the magnetic properties and the irreversibility line in Bi2Sr2CaCu2Oy layered superconductors

Measurements of the irreversibility line in Bi2Sr2CaCu2Oy single crystals are made by three experimental methods: by dc magnetization measurements, microwave absorption measurements, and registration of the harmonics in the high-frequency response of the superconductor. It is found that the position of the irreversibility line on the magnetic phase diagram depends substantially on the method of measurement and the frequency used. Analysis of the results shows that the discrepancy in the irreversibility lines is due to both a difference in the mechanisms of hysteresis of the magnetic properties and to the different scales of the measurement times. It is shown that the position of the irreversibility line is determined at low temperatures by the limiting value of the vortex creep rate for the given method of registration and at high temperatures by the time required for two-dimensional vortices to overcome the surface barrier. It is concluded that the irreversibility line is a dynamic phase transition. The ...

Scanning ESR-microprobe studies of the flux distribution in type-II superconductors

A new method for the study of the distribution of the magnetic field in a superconducting sample is developed. The field profile in the interior of the sample is mapped from the data on the spin probe ESR-signal shift measured at different points over the plane surface. The main attention is focused to the penetration of a perpendicular magnetic field into single crystal strips. The penetration of magnetic flux into an YBCO crystal is shown to be delayed and this gives rise to formation of the steep fronts in the profile of the field near the center of the strip. The profile observed differs considerably from the well-known Bean picture. It is highly dependent on the magnetic history. For the case of a Bi:2212 rectangular crystal, the crossover of the magnetic field profile from a dome shape, typical for a geometrical barrier (T > 30 K), to a bulk pinning picture (T < 25 K) was observed upon variation of temperature. The analysis of the field distribution made it possible to determine the temperature dependence of the lower critical field.