Piezoelectric polar nanoregions and relaxation-coupled resonances in relaxor ferroelectrics

摘要

indent It is a generally accepted fact that the unique dielectric propertiesof relaxor ferroelectrics are related to the formation of polar nanoregion(PNRs). Less well recognized is the corollary that, because they are polar andtherefore lack inversion symmetry, PNRs are also piezoelectric at the nanoscaleand can therefore behave as nanoresonators. Using the particular relaxorferroelectric K$_{t1-x}$Li$_{t x}$TaO$_{t 3}$ (KLT), we show that, whenelectrically excited into oscillation, these piezoelectric nanoresonators candrive macroscopic electro-mechanical resonances. Unexpectedly however, pairs ofcoupled resonances are observed, with one of the two exhibiting acharacteristic Fano-like lineshape. The complex resonance spectra can bedescribed equally well by two alternative but complementary models bothinvolving two resonances coupled through a relaxation: a purely classical onebased on two coupled damped harmonic oscillators and a semi-classical based ontwo discrete excitations coupled to each other through a continuum. Together,they provide complementary perspectives on the underlying physics of thesystem. Both reproduce the rapid evolution of the resonance spectrum acrossthree wide temperature ranges, including a phase transition range. In the hightemperature range, the coupling between modes is due to the collective $pi$relaxation of the lithium ions within PNRs and in the phase transition range to"heterophase relaxation" of the surrounding lattice between its hightemperature cubic and low temperature tetragonal phases. The coupling issuppressed in the intermediate range of the collective $pi/2$ relaxation ofthe lithium ions. Incidentally, the measured dielectric spectra are shown tobear a surprising but justifiable resemblance to the optical spectra of certainatomic vapors that exhibit electromagnetically induced transparency.