|dc.description.abstract||K-(BETS)2FeBr4 is a quasi-2D charge transfer organic metal with interesting electronic and magnetic
properties. It undergoes a transition to an antiferromagnetic (AF) state at ambient pressure at the
Neel temperature (T^^) = 2.5 K, as well as to a superconducting (SC) state at 1.1 K .
The temperature dependence of the electrical resistivity shows a small decrease at T;v indicating
the resistivity drops as a result of the onset of the ordering of Fe'*''" spins. A sharp drop in the
resistivity at 1.1 K is due to its superconducting transition. The temperature dependence of the
susceptibility indicates an antiferromagnetic spin structure with the easy axis parallel to the a-axis.
The specific heat at zero-field shows a large peak at about 2.4 K, which corresponds to the antiferromagnetic
transition temperature (Tat) and no anomaly is observed around the superconducting
transition temperature (1.1 K) demonstrating that the magnetically ordered state is not destroyed
by the appearance of another phase transition (the superconducting transition) in the 7r-electron
layers , .
This work presents an investigation of how the low frequency electromagnetic response is affected
by the antiferromagnetic and superconducting states, as well as the onset of strong correlation.
The location of the easy axis of three samples was determined and polarized thermal reflectance
measurements of these «-(BETS)2FeBr4 samples oriented with their vertical axis along the a- and
c axes were then carried out using a *He refrigerator cryostat and a Martin-Puplett type polarizing
interferometer at various temperatures (T = 0.5 K, 1.4 K. 1.9 K, 2.8 K) above and below the
superconducting state and/or antiferromagnetic state.
Comparison of the SC state to the normal state along the o- and c-axes indicates a rising thermal
reflectance at low frequencies (below 10 cm"' ) which may be a manifestation of the superconducting
energy gap. A dip-Hke feature is detected at low frequencies (below 15 cm"') in the thermal
reflectance plots which probe the antiferromagnetic state along the two axes, and may be due to the opening of a gap in the excitation spectrum as a result of the antiferromagnetism.
In another set of experiments, thermal reflectance measurements carried out along the a- and
c-axes at higher temperatures (10 K-80 K) show that the reflectivity decreases with increasing
temperature to 60 K (the coherence temperature) above which it increases again. Comparison of
the thermal reflectance plots along the a- and c-axes at higher temperatures reveals an anisotropy
between these two axes.
The Hagen-Rubens thermal reflectance plots corresponding to an average over the ac-plane were
calculated using experimental hterature resistivity values. Comparison of the Hagen-Rubens plots
with the experimental thermal reflectance along the a- and c-axes indicates that both exhibit the
general trend of a decrease in thermal reflectance with increasing frequency, however the calculated
Hagen-Rubens thermal reflectance at different temperatures is much lower than the experimental