Phonon anomalies and charge dynamics in Fe_{1-x}Cu_{x}Cr_{2}(4)

A detailed investigation of phonon excitations and charge carrier dynamics in single crystals of Fe_{1-x}Cu_{x}Cr_{2}S_{4} (x = 0, 0.2, 0.4, 0.5) has been performed by using infrared spectroscopy. In FeCr_{2}S_{4} the phonon eigenmodes are strongly affecte

4

R

ν (cm-1)

FIG.3:(Color online)Reflectivity R vs.wave number for

Fe1−x Cu x Cr2S4with x=0.2(upper panel)and x=0.5

(lower panel)at T=5K(open circles)and T=300K

(open triangles).The dashed lines represent results offits as

described in the text.

results for x=0.4are very similar to those obtained

for x=0.5and,hence,only the data for x=0.5is

shown and discussed.The reflectivity of both samples,

x=0.2and0.5,shows a Drude-like contribution due

to the presence of free charge carriers,while FeCr2S4can

be described as an insulator.The highest Drude-like con-

ductivity is found for x=0.2and the phonon modes are

on the verge of being fully screened.For both compounds

the internal mode d at∼120cm−1(see Fig.1for the

pure compound)can hardly be detected.Focusing on the

group of external modes,a new mode e appears close to

350cm−1,while on increasing Cu concentration x mode

a at380cm−1becomes considerably reduced in intensity.

Without an accompanying lattice dynamical calculation

one cannot decide,if this new mode represents an im-

purity mode due to the doping with Cu or a symmetry

change.There are reports in literature15,38claiming the

reduction of symmetry to F