Thermoelectric Properties of Fe0.2Co3.8Sb12-xTex ... - Physics

Superconducting fluctuations, Anomalous Phonons and
Electronic excitations in iron-based superconductors
Pradeep Kumar, A. Bera, D. V. S. Muthu and A. K. Sood
Department of Physics, Indian Institute of Science, Bangalore, India
We will present our recent Raman studies on iron-based superconductors (FeBS) as a
function of temperature. In Ca 4 Al 2 O 5.7 Fe 2 As 2 superconductor, our spectroscopic studies
reveal that the phonon mode observed at ~ 230 cm -1 shows a jump in frequency by ~ 2 %
and linewidth increases by ~ 175 % at T o ~ 60 K. Below T o, anomalous softening of the
mode frequency and a large decreases by ~ 10 cm -1 in the linewidth are observed. These
precursor effects at T 0 ( ~ 2T c ), seen for the first time in FeBS, are attributed to
significant superconducting fluctuations possibly arising from a reduced coupling
between the two well separated ( ~ 15 Å ) Fe-As layers in the unit cell. Furthermore, a
large blue-shift of the mode frequency between 300K and 60K ( ~ 7% ) indicates strong
spin-phonon coupling [1].
In case of the 122 system i.e. Ca(Fe 0.97 Co 0.03 ) 2 As 2 , all three observed phonon mode show
strong renormalization effects below the structural as well as magnetic transition
temperature (T SM ~ 160K ) attributed to the spin-phonon and electron-phonon coupling.
We observed four very weak modes in the range 400-1000 cm -1 attributed to the
electronic Raman scattering. In addition, we have observed a broad Raman band
centering at ~ 3200 cm -1 signaling the existence of coupled orbital and magnetic
excitations owing to its anomalous temperature dependence [2].
We thank all our coauthors mentioned in the references below.
Reference:
[1] Pradeep Kumar et al., Appl. Phys. Lett. 100, 222602 ( 2012).
[2] Pradeep Kumar et al., To be submitted.