High resolution X-ray scattering studies of structural phase transitions in underdoped La$_{2-x}$Ba$_x$CuO$_4$

摘要

We have studied structural phase transitions in high quality underdopedLa$_{2-x}$Ba$_x$CuO$_4$ single crystals using high resolution x-ray scatteringtechniques. Critical properties associated with the continuous High TemperatureTetragonal (HTT, $I4/mmm$) to Middle Temperature Orthorhombic (MTO, $Cmca$)phase transition were investigated in single crystal samples with x=0.125,0.095, and 0.08 and we find that all behavior is consistent with threedimensional XY criticality, as expected from theory. Power law behavior in theorthorhombic strain, 2(a-b)/(a+b), is observed over a remarkably widetemperature range, spanning most of the MTO regime in the phase diagram. Lowtemperature measurements investigating the Low Temperature Tetragonal (LTT,$P4_{2}/ncm$) phase, below the strongly discontinuous MTO$\to$LTT phasetransition, in x=0.125 and x=0.095 samples show that the LTT phase ischaracterized by relatively broad Bragg scattering, compared with that observedat related wavevectors in the HTT phase. This shows that the LTT phase iseither an admixture of tetragonal and orthorhombic phases, or that it isorthorhombic with very small orthorhombic strain, consistent with the ``lessorthorhombic" low temperature structure previously reported in mixedLa$_{2-x}$Sr$_{x-y}$Ba$_y$CuO$_4$ single crystals. We compare the complextemperature-composition phase diagram for the location of structural andsuperconducting phase transitions in underdoped La$_{2-x}$Ba$_x$CuO$_4$ andfind good agreement with results obtained on polycrystalline samples.