Reduction of the field-dependent microwave surface resistance in YBa_2Cu_3O_7 with sub-micrometric BaZrO_3 inclusions as a function of BaZrO_3 concentration

TL;DR

This study investigates how sub-micrometric BaZrO3 inclusions in YBa2Cu3O7 thin films affect microwave surface resistance and vortex pinning, revealing optimal BaZrO3 concentrations for reduced microwave losses and enhanced vortex pinning.

Contribution

It demonstrates that BaZrO3 inclusions create deep pinning wells and significantly reduce microwave surface resistance in YBa2Cu3O7 films, with optimal effects at specific concentrations.

Findings

Losses decrease up to 5% BaZrO3 content, then increase.

Field-induced losses are non-monotonic with BaZrO3 concentration.

BaZrO3 inclusions enhance vortex pinning by raising energy barriers.

Abstract

In order to study the vortex pinning determined by artificially introduced pinning centers in the small-vortex displacement regime, we measured the microwave surface impedance at 47.7 GHz in the mixed state of YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ thin films, where sub-micrometric BaZrO$_3$ particles have been incorporated. As a function of the BaZrO$_3$ content, we observe that the absolute losses slightly decrease up to a BaZrO$_3$ content of 5%, and then increase. We found that the magnetic-field-induced losses behave differently, in that they are not monotonic with increasing BaZrO$_3$ concentration: at small concentration (2.5%) the field-induced losses increase, but large reduction of the losses themselves, by factors up to 3, is observed upon further increasing the BaZrO$_3$ concentration in the target up to 7%. Using measurements of both surface resistance and surface reactance we estimate vortex pinning-related parameters. We find that BaZrO$_3$ inclusions introduce deep and steep pinning wells. In particular, the minimum height of the energy barrier for single vortices is raised. At larger BaZrO$_3$ content (5% and 7%) the phenomenon is at its maximum, but it is unclear whether it shows a saturation or not, thus leaving room for further improvements.