ICQM faculty member publishes an article in Physical Review Letters
reporting "Nematic Crossover in BaFe2As2 under Uniaxial Stress"
Time：2015-11-10ClickTimes：
In the phase diagram of iron-based superconductors, superconductivity is
commonly situated near the vanishing point of the phase boundary between
tetragonal and orthorhombic crystal structures. This commonality has invoked
major research efforts to elucidate the nature of the so-called nematic phase,
in which the discrete C4 rotational symmetry is lowered into C2 but the lattice
translational symmetry is intact. The exact onset temperature of spontaneous
nematicity, Tnem and, in particular, whether Tnem is simply the orthorhombic
structural transition temperature Ts or is even higher, has been a subject of
intense research interest and debate.
Recently, Prof. Yuan Li and his PhD student Xiao Ren, undergraduate students
Lian Duan, Yuwen Hu and Jiarui Li published a paper titled ‘Nematic Crossover in
BaFe2As2 under Uniaxial Stress’ in Physical Review Letters 115, 197002 (2015).
The work was done in collaboration with Prof. Pengcheng Dai’s group at Rice
University, USA and the Institute of Physics, Chinese Academy of Sciences, where
high-quality single crystals of BaFe2As2 were synthesized. Through systematic
measurement of Eg phonon splitting as a function of temperature using
high-resolution Raman spectroscopy, in both freestanding and uniaxially-stressed
BaFe2As2 single crystals, it is revealed that (1) modest uniaxial stress on the
order of a few MPa changes the phase transition at Ts into a crossover that
spans a considerable temperature range above Ts, and (2) a hard-to-avoid
distribution of residual stress of the same order of magnitude remains in
nominally freestanding sample, causing similar crossover phenomena that can be
easily mistaken as a separate phase transition above Ts. The results indicate a
lack of genuine phase transition at T > Ts.
Compared to previous experimental attempts on resolving this issue, most of
which required the use of single-domain samples produced under external stress,
this is the first time to use Raman scattering, which can detect spontaneous
point-group symmetry breaking without resorting to single-domain samples, to
investigate the phase boundary of the nematic order in detail. The work provides
strong evidence for a non-negligible influence of stress, either externally
applied or residual in realistic samples, on the phenomena that are seemingly
indicative of C4 symmetry breaking, hence establishing the existence of only one
true nematic phase transition in the phase diagram.
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Raman spectroscopy can detect point-group symmetry breaking without
resorting to single-domain samples. In the above figure, it is shown
that upon application of a uniaxial stress on a BaFe2As2 crystal, phonon
energy in Raman spectra obtained near the transition temperature Ts
changes significantly, contrary to a commonly used assumption that the
uniaxial stress only ‘detwins’ the sample without altering the physics.
The observed behavior is typically found when a sharp phase transition
becomes a broad crossover under a polarizing field.
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The work was supported by the NSF of China (Grant No. 11374024) and the NBRP
of China (Grant No. 2013CB921903).
