arXiv:0707.2799v1 [cond-mat.supr-con] 18 Jul 2007
High resolution X-ray scattering studies of structural phase transitions in underdoped
Y. Zhao,1B. D. Gaulin,1,2J. P. Castellan,1J. P. C. Ruff,1S. R. Dunsiger,1G. D. Gu,3and H. A. Dabkowska1
1Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, L8S 4M1, Canada.
2Canadian Institute for Advanced Research, 180 Dundas St. W., Toronto, Ontario, M5G 1Z8, Canada
3Physics Department, Brookhaven National Laboratory, Upton, New York 11973, USA
(Dated: February 1, 2008)
We have studied structural phase transitions in high quality underdoped La2−xBaxCuO4 single
crystals using high resolution x-ray scattering techniques. Critical properties associated with the
continuous High Temperature Tetragonal (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 three dimensional XY criticality, as expected
from theory. Power law behavior in the orthorhombic strain, 2(a-b)/(a+b), is observed over a
remarkably wide temperature range, spanning most of the MTO regime in the phase diagram. Low
temperature measurements investigating the Low Temperature Tetragonal (LTT, P42/ncm) phase,
below the strongly discontinuous MTO→LTT phase transition, in x=0.125 and x=0.095 samples
show that the LTT phase is characterized by relatively broad Bragg scattering, compared with
that observed at related wavevectors in the HTT phase. This shows that the LTT phase is either
an admixture of tetragonal and orthorhombic phases, or that it is orthorhombic with very small
orthorhombic strain, consistent with the “less orthorhombic” low temperature structure previously
reported in mixed La2−xSrx−yBayCuO4 single crystals. We compare the complex temperature-
composition phase diagram for the location of structural and superconducting phase transitions
in underdoped La2−xBaxCuO4 and find good agreement with results obtained on polycrystalline
PACS numbers: 61.10.Nz, 64.70.Kb
The complex interplay between spin, charge, and lat-
tice degrees of freedom in the quasi-two dimensional
copper-oxide high temperature superconductors have
been the subject of intense interest since the discov-
ery of superconductivity in the La2−xBaxCuO4 sys-
tem some 21 years ago1.
La2−xSrxCuO4display a fascinating series of structural,
magnetic and superconducting phase transitions as a
function of temperature2. While La2−xBaxCuO4was the
first layered cuprate high Tc superconductor to be dis-
covered, difficulties associated with the growth of high
quality single crystals have significantly limited its study.
As a result the La2−xBaxCuO4family is much less stud-
ied than the La2−xSrxCuO4family and other high tem-
perature superconductors which have an extended his-
tory of being grown and characterized in single crystal
form2, such as the YBa2Cu3O7−δ and Bi2Sr2CaCu2O8
Recently, significant progress has been made in grow-
ing the La2−xBaxCuO4family of materials in single crys-
tal form, and this has enabled several important new
studies of this and related systems7,8,9,10. It is there-
fore timely to perform high resolution structural studies
of these new single crystals, and to compare to previous
studies on La2−xBaxCuO4in polycrystalline form11,12.
One of the many interesting properties of the
La2−xBaxCuO4 family is the sequence of structural
phase transitions which this material displays on cool-
Both La2−xBaxCuO4 and
ing below room temperature for underdoped Ba con-
∼0.18).Previous studies on poly-
crystalline La2−xBaxCuO4 shows three different struc-
tures, which proceed from High Temperature Tetragonal
(HTT, I4/mmm), to Middle Temperature Orthorhombic
(MTO, Cmca) and finally to Low Temperature Tetrago-
nal (LTT, P42/ncm)11,12,13,14,15. The HTT→MTO and
the MTO→LTT phase transition temperatures are re-
ferred to as Td1and Td2, respectively. The HTT→MTO
transition is continuous, while the MTO→LTT transition
is known to be strongly discontinuous. These structures
are closely related to the magnetic and electronic prop-
erties of the La2−xBaxCuO4 and La2−xSrxCuO4 fam-
ilies.The phase diagram of the La2−xBaxCuO4 sys-
tem contains a dome of LTT phase, which is centred
around x=0.125. This Ba-concentration corresponds to a
steep depression of the superconducting TCas a function
of concentration, known as the 1/8 anomaly14,16. The
La2−xSrxCuO4system shows a much smaller ∼ 10% dip
in TC at x=0.125 and the absence of the LTT phase at
low temperatures17,18. The 1/8 anomaly within the LTT
phase also corresponds to strong incommensurate mag-
netic long range order at temperatures just below the
completion of the MTO-LTT phase transition7,9. Clearly,
the structural, magnetic, and superconducting properties
of the La2−xBaxCuO4 and La2−xSrxCuO4 systems are
The critical phenomena associated with the HTT-
MTO transition has been previously studied in pure
La2CuO4 as well as in La2−xSrxCuO4 in single crystal
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