SVIBOR - Project code: 1-03-063

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Project code: 1-03-063


MAGNETIC RESONANCE OF SUPERCONDUCTORS AND FERROELECTRICS


Main researcher: DULČIĆ, ANTONIJE (10872)


Assistants
Type of research: basic
Duration from: 01/01/91. to 12/31/93.
Papers on project (total): 46
Papers on project quoted in Current Contents: 22
Institution name: Institut "Ruđer Bošković", Zagreb (98)
Department/Institute: Department for Experimental Physics
Address: Bijenička 54, pp 1016
City: 10000 - Zagreb, Croatia
Communication
Fax: 385 (041) 434-467
E-mail: DULCICetOLIMP.IRB.HR
Phone: 385 (01) 425-646
Summary: The project is oriented towards investigations of magnetic and microwave properties of high temperature superconductors, and dynamics of crystal lattice in the vicinity of a ferroelectric transition. Over the last five years, the work on the project has been carried out with the aim to determine how the magnetic flux lattice in the mixed state of a superconductor responds to alternating magnetic fields. For this purpose, several related methods were used. Electron spin resonance (ESR) of a paramagnetic probe on the surface of the superconductor served for the detection of subcritical and critical currents induced under the conditions of sweeping magnetic field. In a single crystal YBa2Cu3O7, we observed a transition from the regime of flux creep to the regime of flux flow when the field sweep rate exceeded 30T/s. Using paramagnetic probe on the surface of a thin superconducting film, we have studied the inhomogeneity of the magnetic field. We have deduced an expresion for an effective microwave conductivity in the mixed state by taking into account the electrodynamics in the Meissner state, and the oscillations of the flux lattice. The resulting theoretical expression for the microwave resisistance was used in order to determine the upper critical field Hc2 as a fit parameter to the experimental curves. The advantage of this new method for the determination of Hc2 consists in the posibility to determine even the values of Hc2 which are much beyond the maximum field that can be reached by a conventional laboratory magnet. In a single crystal YBa_2Cu_3O_7 at the temperature of 72K we determined Hc2=40T even though the magnetic dependence of the microwave resistance was measured only up to 2.25T. Near Tc, fluctuation conductivity was studied and scaling property was established. When a modulation field in the frequency range of 1-100kHz was applied along with the microwave field, one could observe modulated microwave absorption which, after a lock-in detection, showed a signal with two components. We have elaborated a model of oscillations of a magnetic flux line in a pinning potential. Dynamic fluctuation of polarization was studied in detail in the vicinity of the phase transition in a ferroelectric single crystal KH2PO4 using paramagnetic probe SO_4(3-) and ENDOR spectroscopy. The relevance of these investigations is seen in the fact that the obtained results provide a better insight into the relationship between the local dynamics of the particular ions in the crystal to the total macroscopic fluctuation of the polarization. The results indicate that the previously known models, which were taking into account only the relative displacement of K(+) and P(5-) ions, were not satisfactory, so that a more subtile model was needed for the appearance of the spontaneous polarization.

Keywords: superconductivity, microwave resistance, mixed state, ferroelectrics, electron spin resonance (ESR), electron nuclear double resonance (ENDOR)

Research goals: As the main aims of the project over the past five years, one could name the determination of the effective microwave conductivity in the mixed state of superconductors, and a critical examination of the previously known models for the appearance of the spontaneous polarization in ferroelectrics. Based on the effective microwave conductivity, one could proceed further towards the development of a method for the determination of the upper critical field, which would surpass the previously known methods in precision and in reachability of high values of Hc2 which exceed by far the maximum field of conventional laboratory magnets. The determination of Hc2(T) is important because it yields the coherence length, which is one of the fundamental parameters of the superconducting state. Furthermore, the aim of the project was to combine three fields (static, modulating, and microwave) in order to have the possibility to investigate the dynamics of flux lines in the pinning potential. Thus, the modulation field would serve to move the flux lines in the potential well, while the microwave field would play the role of a detector of instantaneous state. This method would have a further goal to investigate the mechanism of thermally activated flux creep and the examination of the anharmonicity of the pinning potential. These investigations represent the fundamental background for the technologically very important question of irreversibility and the behavior of the critical currents. In the field of ferroelectrics, the aim was to provide a closer clarification of the relationship between the local dynamics of individual ions in the crystal and the total macroscopic fluctuation of the polarization, in particular for ferroelectrics with hydrogen bonds. All of the aims set forward were completed, and the results are discussed in sections on the description of work, and in the abstract.

COOPERATION - INSTITUTIONS


  1. Name of institution: Universitaet Stuttgart
    Type of institution: University/Faculty
    Type of cooperation: Joint publishing of scientific papers
    City: Stuttgart, Njemačka

  2. Name of institution: West Virginia University
    Type of institution: University/Faculty
    Type of cooperation: Joint publishing of scientific papers
    City: Morgantown, USA

OTHER ACHIEVEMENTS


  1. Name: Microwave magnetoresistance in single crystal high-Tc superconductors (pozvano predavanje na 26th Congress AMPERE on Magnetic Resonance, Atena, Grčka, 9-12.9.1992.)
    Authors: A. Dulčić

  2. Name: Determination of Hc_2 upper critical field from microwave absorption measurements, pozvano predavanje na XV Conference on Radio and Microwave Spectroscopy, Poznan, Poljska, 26-30.04.1993.
    Authors: A. Dulčić
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