Optické vlastnosti vysokoteplotních supravodičů a manganátů

Mgr. Adam Dubroka, Ph.D.

Doctoral thesis

Optical properties of high-temperature superconductors and manganites

Abstract:

The thesis concerns three topics from the field of high-temperature superconductors and manganese oxides (manganites). First, we examine the infrared response of artificial epitaxial layered heterostructures composed of YBa2Cu3O7 and La0.67Ca0.33MnO3 at nanometer scale. We have found that the far infrared response of conducting and superconducting electrons in these heterostructures is weak and the spectra display an unexpectedly strong mid infrared band. This band exhibits signatures of metalinsulator transition and superconductivity-induced changes. The observations are in a qualitative agreement with results of model calculations of the response of a heterogeneous medium. The heterogeneity is probably caused by irregularities in the samples that do not strongly suppress superconductivity or metalinsulator transition but create the lateral dielectric contrast. In the second part, we use Raman and infrared spectroscopy to study a series of manganites La1ySryMn1xMxO3 in which the manganese ions have been partially substituted by other transition metal elements. Broad bands in the Raman spectra and the dc resistivity extracted from the infrared reflectivity suggest that electronic properties of these compounds are mainly influenced by the radius of the substituent. The Cu-substituted compounds exhibit an anomaly suggesting that in this case, additionally, electronic and/or magnetic properties of the substituent play an important role. The Raman spectra display an additional Ag-like mode which we attribute to the local breathing mode of oxygen atoms in a close vicinity of the substituent. In the last part, we study superconductivity-induced changes of the infrared response along the c-axis of multilayer superconductors. Our model calculations of the coupling between phonons and the superconducting condensate explain the spectral changes of the in-phase and out-of-phase vibrations of planar oxygens observed in the trilayer superconductors Tl2Ba2Ca2Cu3O10 and Bi2Sr2Ca2Cu3O10. We discuss also the infrared response of the normal and superconducting electrons and propose a picture which may account for the temperature development of the spectra.

Kľúčové slová

Keywords, infrared spectroscopy, Raman spectroscopy, hightemperature superconductors, manganites, superlattices, phonon anomalies Klicova slova, infraervená spektroskopie, Ramanova spektroskopie, vysokoteplotní supravodie, manganáty, supermížky, fononové anomálie

Jazyk práce: Czech

Datum vytvoření / odevzdání či podání práce: 20. 7. 2006

Obhajoba závěrečné práce

• Obhajoba proběhla 20. 7. 2006
• Vedúci: prof. RNDr. Josef Humlíček, CSc.

Masaryk University

Faculty of Science

Doctoral programme / odbor:
Physics / Solid Matter Physics