Effects of simultaneous carrier doping in the charge reservoir and conducting layers of superconducting CeO0.9F0.1Fe 1-xCoxAs

Article abstract of DOI:10.1016/j.physc.2010.07.008

Electron-doping of the semimetal (CeOFeAs) by either fluorine (max T _c ～ 43 K) or cobalt (max T_c ～ 11 K) leads to superconductivity. Here we show the effect of transition metal (Co) substitution at the iron site on the superconducting properties of CeO_0.9F _0.1FeAs (T_c ～ 38 K) to understand the interplay of charge carriers in both the rare earth-oxygen and Fe-As layers. Simultaneous doping of equivalent number of charge carriers in both layers leads to a T _c of 9.8 K which is lower than the T_c obtained when either the conducting layer (FeAs) or charge reservoir layer (CeO) is individually doped. This suggests a clear interplay between the two layers to control the superconductivity. Resistivity upturn and negative magnetoresistance are observed with Co doping that is interpreted in the gamut of Kondo effect. Hall coefficient and thermoelectric power indicate increased carrier concentration with cobalt doping in CeO_0.9F_0.1FeAs. The rf penetration depth both for CeO_0.9F_0.1Fe_0.95Co _0.05As and CeO_0.9F_0.1FeAs show an exponential temperature dependence with gap values of ～1.6 and 1.9 meV respectively.

Full text of DOI:10.1016/j.physc.2010.07.008

Physica C 470 (2010) 1928–1932
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Physica C
journal homepage: www.elsevier.com/locate/physc
Effects of simultaneous carrier doping in the charge reservoir and conducting
layers of superconducting CeO0.9F0.1Fe1ꢀxCoxAs
b,
S.J. Singh a, J. Prakash b, S. Patnaik a, , A.K. Ganguli
*
**
a School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110 067, India
b Department of Chemistry, Indian Institute of Technology, New Delhi 110 016, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Electron-doping of the semimetal (CeOFeAs) by either fluorine (max Tc ꢁ 43 K) or cobalt (max Tc ꢁ 11 K)
leads to superconductivity. Here we show the effect of transition metal (Co) substitution at the iron site
on the superconducting properties of CeO0.9F0.1FeAs (Tc ꢁ 38 K) to understand the interplay of charge car-
riers in both the rare earth-oxygen and Fe–As layers. Simultaneous doping of equivalent number of
charge carriers in both layers leads to a Tc of 9.8 K which is lower than the Tc obtained when either
the conducting layer (FeAs) or charge reservoir layer (CeO) is individually doped. This suggests a clear
interplay between the two layers to control the superconductivity. Resistivity upturn and negative mag-
netoresistance are observed with Co doping that is interpreted in the gamut of Kondo effect. Hall coeffi-
cient and thermoelectric power indicate increased carrier concentration with cobalt doping in
CeO0.9F0.1FeAs. The rf penetration depth both for CeO0.9F0.1Fe0.95Co0.05As and CeO0.9F0.1FeAs show an
exponential temperature dependence with gap values of ꢁ1.6 and 1.9 meV respectively.
Ó 2010 Elsevier B.V. All rights reserved.
Received 24 June 2010
Accepted 22 July 2010
Available online 25 July 2010
Keywords:
Iron-based superconductor
Electronic transport and superconducting
gap
1. Introduction
vanishes at x > 0.06 leading to superconductivity [4]. Most strik-
ingly, it has been shown that instead of F at O sites, Co doping at
Superconductivity in the ferropnictides has provided a novel
opportunity for a deeper understanding of the mechanism of high
temperature superconductivity in the presence of competing mag-
netic and superconducting correlations. Consequently, there have
been several efforts to draw a parallel between the high Tc cuprates
and ferropnictides [1,2]. It has now been established that unlike
cuprates, where the ground state is a Mott Insulator with superex-
change linked local moments at copper sites, the oxypnictides are
(semi)metallic with effectively very weak magnetism as evidenced
by a spin density wave correlation. Band structure calculations and
photoelectron spectroscopic studies have revealed itinerant charac-
teristics of Fe 3d electrons [2]. Further, amongst the ferropnictides,
the Ce(O/F)FeAs (max Tc ꢁ 43 K, x = 0.2) system is unique because of
striking similarity [3] of the electronic phase diagram with cuprates,
notably a dome-shaped Tc dependence on carrier doping [4]. The
parent oxypnictide, CeOFeAs, undergoes a structural phase transi-
tion from tetragonal (P4/nmm) to orthorhombic (Cmma) structure
around 155 K followed by an antiferromagnetic ordering of spins
in the Fe sublattice at ꢁ140 K. From detailed neutron scattering
Fe sites in LnOFeAs can also induce superconductivity [5,6]. This
is instructive because LaOCoAs is a metal that becomes a ferromag-
net at 66 K [7]. All the above properties indicate that magnetic
fluctuations play a critical role in the stabilization of the supercon-
ducting state in ferropnictides. To get further insight into the elec-
tronic correlation effects of the FeAs layer, we have chosen to dope
the iron sites with cobalt in the superconducting composition
(CeO0.9F0.1FeAs) where the AFM (SDW) state has already been sup-
pressed by F doping. In the undoped CeOFeAs, the nearest neighbor
and next nearest neighbor interactions are superexchange con-
trolled and lead to a frustrated stripelike AFM state [4]. Wang et
al. [5] reported the superconductivity in cobalt-doped LnOFeAs
(Ln = La and Sm) where cobalt doping at Fe sites leads to a double
exchange interaction between Fe and Co sites [5]. Thus the role of
Co doping in optimally doped CeO0.9F0.1FeAs is two fold; to over-
dope carriers onto the FeAs (conducting) layers and to introduce
disorder in the Fe antiferromagnetic order (dormant due to F dop-
ing). In this paper we report studies on resistivity, magnetoresis-
tance, Hall effect, thermoelectric power and rf penetration depth
of such a optimally doped series of oxypnictide specimen undergo-
ing simultaneous addition of charge carriers with local magnetic
moments.
measurements it is now established that upon F doping in CeO1ꢀx
-
FxFeAs, both AFM ordering and the structural phase transition
2. Experimental
* Corresponding author. Tel.: +91 11 26704783; fax: +91 11 26717537.
** Corresponding author.
Polycrystalline samples with nominal compositions of CeO0.9F0.1
Fe1ꢀxCoxAs with ‘x’ = 0.05, 0.10 and 0.15 were synthesized by a two
-
E-mail addresses: spatnaik@mail.jnu.ac.in (S. Patnaik), ashok@chemistry.iitd.ernet.
in (A.K. Ganguli).
0921-4534/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2010.07.008

S.J. Singh et al. / Physica C 470 (2010) 1928–1932
1929
step solid state reaction method [8] using high purity Ce, CeO2,
(i)
CeF3, Co3O4, As and FeAs as starting materials. FeAs was obtained
by reacting Fe chips and As powder at 800 °C for 24 h. The raw
materials (all with purities better than 99.9%) were taken accord-
ing to stoichiometric ratio and then sealed in evacuated silica am-
poules (10ꢀ4 torr) and heated at 950 °C for 24 h. The powder was
then compacted (5 tonnes) and the disks were wrapped in Ta foil,
sealed in evacuated silica ampoules and heated at 1150 °C for 48 h.
All chemicals were handled in a nitrogen-filled glove box. The
resulting samples were characterized by powder X-ray diffraction
x = 0.15
x = 0.1
(PXRD) with Cu Ka radiation. The lattice parameters were obtained
using a least squares fit to the observed d values.
x = 0.05
Resistivity, Hall effect and rf susceptibility measurements were
carried out using a cryogenic 8 T cryogen-free magnet in conjunc-
tion with a variable temperature insert (VTI). Standard four probe
technique was used for transport measurements. The external
magnetic field (0–3 T) was applied perpendicular to the probe cur-
rent direction and the data were recorded during the warming cy-
cle with heating rate of 1 K/min. The inductive part of the magnetic
susceptibility was measured using a tunnel diode based rf penetra-
tion depth technique [9]. The sample was kept inside an inductor
that formed a part of an LC circuit of an ultrastable oscillator
(ꢁ2.3 MHz). A change in the magnetic state of the sample results
in a change in the inductance of the coil and is reflected as a shift
in oscillator frequency which is measured by an Agilent 53131A
counter. The thermoelectric power measurement of each sample
was carried out by bridge geometry where a temperature gradient
was created across a 2 mm by 3 mm rectangular disk.
10
20
30
40
50
60
2θ
4.000
3.998
3.996
3.994
3.992
3.990
3.988
3.986
3.984
8.610
8.605
8.600
8.595
8.590
8.585
8.580
(ii)
3. Results and discussion
Fig. 1(i) shows the PXRD patterns of CeO0.9F0.1Fe1ꢀxCoxAs
(x = 0.05, 0.10 and 0.15) compounds. The observed reflections
could be satisfactorily indexed on the basis of a tetragonal ZrCu-
SiAs type structure. Fig. 1(ii) shows the variation of lattice param-
eters as a function of Co content. With increasing Co content (x),
the ‘a’-axis goes down marginally while the c-axis shrinks signifi-
cantly indicating successful incorporation of cobalt ions (the ionic
size of cobalt(II) is smaller than that of iron(II)).
0.04
0.06
0.08
0.10
0.12
0.14
0.16
Cobalt concentration
Fig. 1. (i) Powder X-ray diffraction patterns (PXRD) of CeO0.9F0.1Fe1ꢀxCoxAs
(x = 0.05, 0.10 and 0.15). (ii) The variation of the lattice parameters (a and c) on
the Co content (x) for CeO0.9F0.1Fe1ꢀxCoxAs.
The zero field resistivity plots between 1.6 K and 300 K for
CeO0.9F0.1Fe1ꢀxCoxAs (Fig. 2) show metallic behavior in the normal
state. The sample without Co doping (CeO0.9F0.1FeAs) shows the
superconducting transition at around 38.5 K [10]. With Co doping,
the onset of superconducting transition decreased to 23.4 and 9.8 K
for (a) CeO0.9F0.1Fe0.95Co0.05As and (b) CeO0.9F0.1Fe0.9Co0.1As respec-
tively (Fig. 2). The diamagnetic behavior of these superconductors
was confirmed from the susceptibility studies which are shown in
Fig. 7. On increasing the Co content (x = 0.15) further the Tc de-
creases dramatically and only a steep drop in the resistivity around
4 K (Fig. 3d) was observed (zero resistance state could not be
achieved down to 1.6 K). The residual resistivity value (RRR =
CeO0.9F0.1FeAs (a)
6
5
4
3
2
1
0
c
a
CeO0.9F0.1Fe0.95Co0.05As (b)
CeO0.9F0.1Fe0.9Co0.1As (c)
CeO0.9F0.1Fe0.85Co0.15As (d)
b
d
40
30
20
10
0
q300/q25) is 5.8 and 2.11 for x = 0.05 and 0.10 compositions respec-
0.00
0.08
x
0.16
300
tively (RRR = 4.34 for CeO0.9F0.1FeAs [10]) indicate increase in dis-
order with Co doping. It is to be noted that while in the
compositions without fluorine CeOFe1ꢀxCoxAs, both the transition
0
50
100
150
200
250
Temperature (K)
temperature and
q(Tc) always decreased with increasing x > 0.01
Fig. 2. The temperature dependence of resistivity (q) as a function of temperature
for CeO0.9F0.1Fe1ꢀxCoxAs (x = 0, 0.05, 0.10 and 0.15). Inset shows the dependence of
transition temperature (Tc) with Co content (x).
[6], on the contrary we observe that
q
(Tc) first increases with Co
doping but reduces drastically for x ꢁ 0.15 and shows dome like
behavior (optimal dopant concentration exists). This is reminiscent
of distinctly different transport behavior in the metallic regions of
LaO1ꢀxFxFe1ꢀyCoyAs (x = 0.1) [11]. Various studies have shown that
optimal Tc in all oxypnictide superconductors depends on the pres-
ence of ideal (undistorted) FeAs4 tetrahedra (bond angle of 109°
though the interlayer distance between CeO/F and FeAs layers de-
creases by doping Co in place of Fe. Our results are in line with
what is seen in LaO(Fe/Co)As, where the Fe–As–Fe bond angle in-
creases with Co doping [5]. This reinforces the understanding that
Fe–As–Fe bond angles play a dominant role in determining the
280) [4]. The decrease in Tc on Co doping at Fe site in CeO0.9F0.1
Fe1ꢀxCoxAs may be related to increase in the Fe–As–Fe angle
-

1930
S.J. Singh et al. / Physica C 470 (2010) 1928–1932
0.6
(a) 0.6
(b)
CeO0.9F0.1Fe0.95Co0.05As
CeO0.9F0.1FeAs
0.5
0.4
0.3
0.2
0.1
0.4
0.2
0.0
40
60
80
100
20
40
60
80
100
T (K)
T (K)
0.70
1.342
1.340
1.338
1.336
1.334
(c)
(d)0.650
H = 0 T
H = 1 T
H = 3 T
CeO0.9F0.1Fe0.9Co0.1As
0.645
0.65
0.60
0.55
0.640
0.635
0.630
0.625
2.922
2.916
2.910
2.904
2.898
2.892
H = 0 T
H = 1 T
H = 3 T
3
6
9
12
15
T (K)
CeO0.9F0.1Fe0.85Co0.15As
10
15
18
21
T (K)
24
27
30
10
100
100
T (K)
T (K)
Fig. 3. The normalized resistivity with respect to temperature variation for CeO0.9F0.1Fe1ꢀxCoxAs series, in figure (c) and (d) temperature scale in log(T). Inset of figure (c) and
(d) show the negative magnetoresistance for x = 0.10 and x = 0.15. Arrows show the minimum point (Tmin) and the red dashed lines are linear fit. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of this article.)
optimal Tc, more than the decreasing distances between the reser-
voirs and conducting layers.
hole and electron pockets, the transport is dominated by electrons.
We find that the slope of xy vs. H decreases with increasing cobalt
q
Fig. 3 shows the evolution of the temperature dependence of
normal state resistivity as a function of Co doping. We find a strik-
ing increase in q(Tc) with cobalt doping for x < 0.15 and a resistivity
concentration. This indicates that carrier concentration increases
with the substitution of cobalt (in place of iron) in this optimally
doped superconductor CeO0.9F0.1FeAs.
minimum followed by an upturn as shown in Fig. 3a–d. When we
increased the Co concentration in CeO0.9F0.1FeAs, this upturn tem-
perature Tmin shifted to lower temperature. Weak localization or
Kondo effect could be invoked towards explaining this upturn
[12]. As plotted in the inset of Fig. 3c and d we also observe sign
change in magnetoresistance (MR) that give credence to the inter-
pretation that the resistivity upturn is due to spin flip scattering
between the charge carriers and local Co moments. Further, the
x-axis in 3c and 3d are set to log scale and the dashed line indicates
robust log(1/T) behavior as expected for a Kondo system. The
width of log(T) area decreases with increasing Co fraction. Such
behavior has also been seen in Ni doped LaOFeAs [13] and Rh
doped NdFeAsO systems [14].
The thermopower (S) as a function of temperature for the par-
ent compound CeOFeAs is shown in Fig. 5. In the high temperature
region (200–300 K), S was observed to be negative (suggesting
electrons to be the majority charge carriers), and its value in-
creased linearly as a function of temperature. A linear dependence
of the thermopower (S) is a signature of metallic behavior which
reaches a minimum (Tmin) around 170 K for the parent compound
CeOFeAs as that reported previously [17]. On further cooling, the
thermopower increases. The Co-doped compositions (CeO0.9F0.1-
Fe1ꢀxCoxAs) also show a negative value of the thermopower
(Fig. 5). For CeO0.9F0.1FeAs (without Co doping) as shown in
Fig. 5, the Seebeck coefficient varies from ꢀ22
lV/K at 300 K to a
value of ꢁꢀ104 V/K at ꢁ60 K. The Co-doped samples have similar
l
To understand the effective carrier concentration in this opti-
mally doped series of a multiband superconductor (upon addition
of Co), in Figs. 4 and 5 we plot Hall and thermoelectric power data,
respectively. While for a metal the variation of Hall coefficient (RH)
with temperature remains nearly invariant with temperature, it
exhibits temperature dependent behavior for a multiband material
such as MgB2 [15] or materials with non-Fermi liquid behavior
such as the cuprates [16]. As shown in Fig. 4a, we find that the
room temperature value but a much weaker minima and then
sharp increase as the temperature is lowered further. Thus the
absolute value of thermopower decreases with increasing Co dop-
ing. In a minimal two band model [18], the conductivity and ther-
mopower depend on hole and electron contributions, and therefore
while it is certain that with cobalt doping the conduction mecha-
nism is dominated by electrons, the sharp increase in thermopow-
er is due to contribution of holes in the over-doped regime.
To obtain the upper critical field (Hc2), we have compared the
temperature dependence of the resistivity under varying magnetic
transverse resistivity (
qxy) remains negative up to room tempera-
ture for all the compositions. This indicates that inspite of multiple

S.J. Singh et al. / Physica C 470 (2010) 1928–1932
1931
(a)
1.8
3
2
1
0
T = 40 K
dH /dT|T=T
= - 1.56 T/K
0.0
-0.7
-1.4
-2.1
-2.8
-3.5
1.2
0.6
0.0
14
16
18
20
22
T (K)
H = 0 T
H = 1 T
H = 2 T
H = 3 T
CeO0.9F0.1FeAs
CeO0.9 0.1Fe0.95Co0.05As
F
CeO0.9F0.1Fe0.9Co0.1As
CeO0.9 0.1
F
Fe0.85Co0.15As
10
15
20
25
0
1
2
3
4
Temperature (K)
μ0H (T)
Fig. 6. Temperature dependence of the electrical resistivity of CeO0.9F0.1Fe0.95-
Co0.05As with varying magnetic fields. Inset shows the temperature dependence of
upper critical field (j) and irreversibility field ( ) as a function of temperature.
(b)
-2
-4
CeO0.9F0.1Fe0.95Co0.05As is shown in inset of Fig. 6. The zero field
upper critical field Hc2(0) is calculated using the Werthamer–
Helfand–Hohenberg (WHH) formula [19]. Using the value of tran-
sition temperature (Tc) of 23.4 K, we find Hc2 = 25.3 T. This value is
smaller than the reported Hc2 value (ꢁ94 T) of CeO0.9F0.1FeAs [10].
-6
Using the value of Hc2(0) we can also calculate the mean field
1=2
-8
Ginzberg–Landau coherence length nab ¼ ð/0=2
p
Hc2
Þ
. Using
U0 = 2.07 ꢃ 10ꢀ7 G cm2 and the Hc2 values, we obtain a coherence
length of ꢁ36 Å for CeO0.9F0.1Fe0.95Co0.05As composition. This value
is higher than that reported for sample CeO0.9F0.1FeAs [10] and
indicates that impurity scattering in the superconducting state
does not increase with Co doping.
CeO0.9F0.1FeAs
CeO0.9F0.1Fe0.95Co0.05As
160 200
-10
40
80
120
240
Temperature (K)
The dependence of the penetration depth is directly related to
the anisotropy of the superconducting energy gaps [20]. Both in
the fluorine-doped or oxygen-deficient FeAs based superconduc-
tors, most of the reports indicate a fully gapped Fermi surface
(FS). Measurements of the London penetration depth, k(T), on
NdO0.9F0.1FeAs [21], La0.8Th0.2OFeAs [22] and SmO1ꢀxFyFeAs [23]
show an exponential temperature dependence of k(T) at low tem-
peratures. In Fig. 7 we plot the penetration depth for the x = 0.0 and
0.05 composition. We have considered penetration depth data up
Fig. 4. (a) Hall resistivity as a function of field and (b) Hall coefficient as a function
of temperature with 10% error bar for CeO0.9F0.1FeAs (j) and CeO0.9F0.1Fe0.95
-
Co0.05As ( ). The straight line in figure (a) is a guide to eye and shows linearity.
35
0
2000
b
a
-35
-70
-105
1500
a
b
0.0
0.2
0.4
1000
500
0
T/Tc
0
100
200
300
T (K)
Fig. 5. The variation of thermoelectric power (S) with respect to temperature for
CeO0.9F0.1Fe1ꢀxCoxAs (x = 0, 0.05, 0.10 and 0.15) with respect to parent compound
CeOFeAs.
0
10
20
30
40
T (K)
fields for CeO0.9F0.1FeAs and CeO0.9F0.1Fe0.95Co0.05As. From Fig. 6 it
is clear that the Tc (onset) shifts weakly with magnetic field, but
the zero resistivity temperature shifts much more rapidly to lower
temperatures. Using a criterion of 90% and 10% of normal state
Fig. 7. Dependence of penetration depth for (a) CeO0.9F0.1FeAs and (b) CeO0.9F0.1
Fe0.95Co0.05As up to the transition temperature attesting to the onset of bulk
diamagnetism at Tc. The inset shows variation of penetration depth, k(T) for both
the samples. The red and blue lines show the exponential fitting. (For interpretation
of the references to colour in this figure legend, the reader is referred to the web
version of this article.)
-
D
resistivity (qn), the upper critical field (Hc2) and the irreversibility
field Hꢂ(T) were calculated. The H–T phase diagram for sample

1932
S.J. Singh et al. / Physica C 470 (2010) 1928–1932
to 0.3Tc to ensure validity of the low temperature BCS expression
Acknowledgements
for an isotropic s-wave state [20]
rffiffiffiffiffiffiffiffiffiffiffiffiffi
A K G and S P thank D S T and U G C, Govt. of India for financial
support. J P and S J S thank CSIR, Govt. of India, for fellowships.
p
D
2T
ð0Þ
D
k ¼ kð0Þ ꢃ
exp½ꢀ
D
ð0Þ=Tꢄ
ð1Þ
where
Dk(T) is the difference between penetration depth at the
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k(0) and (0) are the zero temperature values of penetration depth
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4. Conclusions
We have successfully synthesized cobalt-doped CeO0.9F0.1
Fe1ꢀxCoxAs (x = 0.05, 0.1, and 0.15) compounds to study the inter-
play of charge carriers in both charge reservoir and charge carrier
layers. The substitution of cobalt ion at the iron site in CeO0.9F0.1
Fe1ꢀxCoxAs compounds suppresses the transition temperature (Tc)
and upper critical field (Hc2). Our penetration depth analysis indi-
cates that both CeO0.9F0.1FeAs and CeO0.9F0.1Fe0.95Co0.05As follow
S wave pairing symmetry with gap values of 1.9 and 1.6 meV
respectively. We see increase in coherence length with increased
disorder in the FeAs layer. The Hall and thermopower data show
increase in charge carrier density on increasing the cobalt substitu-
tion at iron site.
-
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