Journal of Modern Physics, 2011, 2, 463-471
doi:10.4236/jmp.2011.26056 Published Online June 2011 (
Copyright © 2011 SciRes. JMP
Superconductivity Modulated by Binary Doping in
Chunqing Qu1,2, Zhiyong Liu1, Yuming Lu1, Changzhao Chen1, Chuanbing Cai1, Aihua Fang3,
Fuqiang Huang3, Mingfeng Wang2, Xiaoming Xie2
1Research Center for Superc ond ucto rs an d A ppl i ed Tec hnologies, Physics Department,
Shanghai University , Shanghai, China
2State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and
Information Technology, Chinese Academy of Sciences, Shanghai, China
3Shanghai Institu te of Ceramics, Chinese Academy of Sciences, Shanghai, China
Received February 12, 2011; revised March 17, 2011; accepted April 12, 2011
Binary doping effect is studied for the Fe-based superconductors of Nd1-xBaxFeAsO1-2xF2x (x = 0.02, 0.05, 0.1,
0.15, 0.2). The X-ray diffractions show that the c-axis lattice constant decreases monotonously with the dop-
ing content, in contrast to the little change in the a-axis. Temperature dependences of electric resistivity and
magnetic susceptibility reveal that the superconductivity for the studied system emerges at x = 0.1, and en-
hances together with Hc2(0) as the doping content x increases further. In case of x = 0.2, the superconducting
critical temperature reaches as high as 50 K, which is the first demonstration of superconductivity with a
high fluorine-doping induced by both electron and hole doping in this family. Negative Hall coefficient (RH)
indicates that electron-type carriers are dominated in the present samples. The complicated temperature de-
pendence of RH, is believed to arise from a multiband effect together with a complicated scattering, espe-
cially at the temperature near the Tc.
Keywords: Binary Doping, Iron-Based Superconductor, Flux Pinning, Scattering
1. Introduction
The fantastic discovery of superconductivity with the
transition temperature Tc of around 26 K [1] in the new
class of iron-arsenide has attracted wide interest in the
world. Among the family of Fe-based superconductors,
the LaFeAsO1-y (FeAs-1111) exhibits quite high critical
temperatures, such as Tc = 55 K [2] in the case of elec-
tron doping like SmFeAsO1-xFx, and Tc = 25 K [3] in the
case of hole doping like La1-xSrxFeAsO.
It is well known that there are other types of iron-
pnictide superconductors, among which the FeAs1-y
(FeAs-11), a type of Fe-chalcogenide superconductor,
attracts a lot of attentions due to its simple structure and
composition [4]. It is reported that the FeAs-11 exhibits
the zero resistance transition temperature of 8 K for the
PbO-type α-FeSe compound [5], while the LixFeAs
(FeAs-111), as an infinite layered structure [6,7] may
exhibit the Tc of 18 K without SDW phases.
In contrast, the FeAs-1111 involves two types of par-
ent compounds [1], i.e., LnFeAsO (Ln is rare earths) and
AFeAsF (A is positive bivalent ion), both exhibiting the
tetragonal space group P4/nmm at 300 K. The first one
shows an anomaly of magnetic phase transition near 150
K, which is believed to be caused by the spin-density-
wave (SDW) instability [8]. This kind of Fe-based su-
perconductor is characterized as electron-doped [9-13]
and its Tc may be enhanced to be 55 ~ 56 K via replacing
lanthanum with other rare-earth elements. [2,9-14] The
second parent compound such as SrFeAsF being of
ZrCuSiAs-tpye structures [15,16], shows an SDW and/or
structural transition at about 173 K, and its positive Hall
coefficient (RH) suggesting that the conduction of this
parent phase is dominated by hole-like charge carriers.
According to existing investigations, the parent com-
pounds of both LnFeAsO and AFeAsF show the SDW
but no superconductivity, and they can evolve into su-
perconductors as a consequence of the normative sym-
metry breaking due to the element doping, such as
Gd1-xThxFeAsO [13] SmFe0.9Co0.1AsO [17], CeFeAs1-x
PxO [18], and LaFeAsO1-xFx [1]. Similar phenomena also
exists in FeAs-122 system, such as Ca1-xNaxFe2As2 [19],
Ba(Fe0.961Ru0.039)2As2 [20], EuFe2(As0.7P0.3)2 [21] and so
With respect to the FeAs-1111 system, the fluorine
doping or oxygen deficiency related to the oxygen-site is
frequently studied [22,23], disclosing that they are effec-
tive to produce the superconductivity. For FeAs-11 sys-
tem, it is reported that the clean superconducting phase
exists only in those samples with intentional Se defi-
ciency [24,25], and single crystal samples are hardly ob-
tained with a stoichiometric composition of 1:1. All of
these suggest that the superconductivity may emerge or
be enhanced by element substitution due to normative
symmetry breaking in electronic structure, charge or
stress etc.
Apart from one element or one site substitution, it is
also reported that the second element doping at other site
is applicable to modify the normative symmetry and then
the superconductivity [26-28], such as Ce1xGdx
FeAsO0.84F0.16 [26], and SmFe1xRuxAsO0.85F0.15 [27].
Compared with the case of one site doping, two ele-
ment substitutions or doping may exhibit more unique
phenomenon. In the present paper, we focus on the
FeAs-1111 system and study its doping effect using bi-
nary substitution, namely Nd1-xBaxFeAsO1-2xF2x. This spe-
cial system with both electron-doping and hole-doping
allow more breaking of original symmetry, and this may
give rise to complicate modification for superconducting
and magnetic behaviors. In the flowing sections, the
structural and electromagnetic characteristics regarding
this system are systemically investigated, to understand
its distinct performances and underlying mechanism due
to binary doping.
2. Experimental
Polycrystalline bulks with nominal compositions of
Nd1-xBaxFeAsO1-2xF2x (x = 0.02, 0.05, 0.1, 0.15, and 0.2)
were synthesized by conventional solid-state reaction.
Firstly, the intermediate phases of Fe2As, NdAs and FeO
were prepared by the reactions of the mixed commercial
powders of Nd (99.5%), Fe (99.9%), As (99.999%) and
Fe2O3 (99.9%), sealed in a vacuum quartz tube, annealed
at 600˚C for 6 h and then 850˚C for 12 h.
Subsequently, the resultant precursors together with
BaF2 powder in stoichiometry of Nd1-xBaxFeAsO1-2xF2x
were ground for 15 min. All the weighing and mixing
procedures were performed in a glove box filled with
high purity Ar gas. After that, the mixtures pressed to
into pellets in diameter of 8mm, were sealed in a vacuum
quartz tube, and then heated up to 1160˚C by a rate of
230˚C/h. For the reference, the pellets are divided into
two sorts, with and without surface wrapping by Ta
pieces [29] to test the effect of preventing fluorine-vola-
tilizing during the solid-state reaction.
Note that the present precursors with and without Ta
wrapping were maintained at the temperature of 1160˚C
for 10 h only, rather than several ten hours reported by
others. This is purposely selected as we believe that it is
critical to shorten the heating time to avoid the fluo-
rine-volatilizing at high temperatures, and to increase the
synthesis efficiency as well.
The crystal structure and phase purity of all studied
samples were characterized by X-ray diffraction (XRD).
The superconducting properties were measured by using
a physical property measurement system (Quantum De-
sign PPMS) in magnetic fields up to 9 T. Magnetic and
transport performances are evaluated by a series of char-
acteristic curves including the temperature dependences
of resistivity, dc susceptibility, and the Hall coefficient
etc. For the magnetization measurements, rectangular
specimens were prepared nearly in the same dimension
of about 6 × 2 × 1 mm3, and the applied magnetic fields
were normal to the long axis direction of the specimen.
3. Result and Discussion
3.1. Microstructure, Lattice Constants and Their
Doping Dependences
The XRD patterns for the samples with various doping
contents of 0.02 - 0.2 are shown in Figure 1. It is clear
that the main peaks for all studied samples can be in-
dexed to the FeAs-1111 phase with the tetragonal
ZrCuSiAs-type structure, regardless of the slight shift of
main peaks and several weak peaks corresponding with
the impurity phase of NdOF
The doping dependence of the lattice constants calcu-
lated from the XRD data are presented in Figure 2.
There is little change in the lattice constants of a-axis as
the doping content increases. This may result from the
contrary contributions to the in-plane lattice from two
elements doping, since the ionic radius of Ba2+ is larger
than that of Nd3+, while the ionic radius of F is smaller
than that of O2–. In contrast, the c-axis lattice constants
decrease monotonously with the doping content. Such a
reduction probably arises from the decrease in layer dis-
tance between FeAs and NdO which may take places due
to chemical pressure variation along the c-axis [30]. Note
that this is unlike the case of hole-like system such as
Pr1-xSrxFeAsO [31] and Nd1-xSrxFeAsO [32], where the
c-axis lattices increase monotonously with increasing the
doping content of Sr. It is thus observed that the c-axis
lattice increase with the doping content is mostly present
Copyright © 2011 SciRes. JMP
Figure 1. (Colour online) X-ray diffraction patterns for the
Nd1-xBaxFeAsO1-2xF2x compounds with various doping con-
tents from x = 0.02 (bottom) to x = 0.2 (top).
Figure 2. (Colour online) Doping content dependence of
a-axis and c-axis lattice constants.
in hole-like doping FeAs-1111 system, while c-axis de-
crease with the doping mostly in electron-like doping.
With respect to the chemical pressure change, it is sup-
posed that a further increase in Tc may emerge with more
BaF2 chemically doped into the FeAs-1111 system [33].
3.2. Phase Diagram Regarding
Superconductivity and SDW
Characteristic transition temperatures are evaluated by
both R-T curve and dc susceptibility measurements.
Figure 3(a) shows the R-T curves for the samples with
various doping contents prepared by solid-state reaction
without Ta wrapping. To test the stability of the samples,
the R-T curves for all samples are measured twice, i.e.,
immediately after growing and after five months in air,
which show a similar transport behavior, suggesting no
obvious performance degradation with time going. The
inset of Figure 3(a), shows two R-T curves for the same
sample of x = 0.15, measured before and after five
months to give a direct comparison.
Figure 3(b) shows the superconducting critical tem-
perature as well as its transition width, identified from
the R-T curves in Figure 3(a). It is revealed that the pre-
sent samples exhibit the superconductivity as x is more
than 0.1, being as high as 50 K at x = 0.2. Moreover, the
superconducting transition width ΔT, determined by the
temperatures corresponding to 90% and 10% of the nor-
mal-state resistivity, decreases with increasing doping
content, being as low as 1.86 K at x = 0.2.
Figure 3. (colour online) (a) Temperature dependence of
resistivity for the studied samples with x = 0.05, 0.1, 0.15,
and 0.2. Inset shows two R-T curves for the same sample of
x = 0.15, measured before and after 5 months, respectively;
(b) The doping content dependence of Tonset and ΔT (deter-
mined by the difference of and .
Copyright © 2011 SciRes. JMP
466 C. Q. QU ET AL.
To further confirm the transport results, the tempera-
ture dependences of dc susceptibility are measured by
ZFC (zero field cooling) and FC (field cooling) modes
with an excitation field of 100oe. Figure 4 shows the
temperature dependences of the volume magnetic sus-
ceptibility for two typical samples of x = 0.15 and x =
0.2, demonstrating their onset superconducting critical
temperatures of 44.2 K and 48.8 K, respectively.
Note that the Tc measured by dc susceptibility is a lit-
tle lower than the values measured by transport due to
percolation effect. They are, however, close to the values
of dominated by the 50% of the normal state re-
sistivity, which is frequently observed for Fe-base su-
perconductors. Moreover, the width of magnetic transi-
tion is generally larger than that of resistivity, which is
due to the magnetic signal out of statistic results for
whole grains, being more sensitive than the transport.
Apart from superconducting transition temperatures,
another characteristic temperature Ts regarding the phase
transition of SDW, can be identified as well based on the
temperature dependences of resistance and dc suscepti-
bility. For the sample of x = 0.05, no superconductivity is
observed, while a SDW transition temperature of 117 K
is clearly demonstrated.
Figure 5 shows a phase diagram with respect to the
doping content dependence of characteristic temperatures,
including the SDW transition temperature Ts and the su-
perconducting transition temperature Tc for all studied
samples. It is revealed that the samples with and without
Ta wrapping exhibit similar dependence of the doping
content. The superconductivity appears at x > 0.1, and it
still exists at a high doping content, e.g. x = 0.2. While
the Ts is hardly affected by wrapping condition, the Tc for
the sample without wrapping is generally high than the
sample with wrapping. This implies that a different
wrapping way will lead to a different Tc, regardless of
identical composition and heating route, This is probably
due to suppressed fluorine-evaporating as well as F dop-
ing content in the case of wrapping, which reminds us of
selecting the samples only without Ta wrapping, which
give a relatively high Tc, for more meaningful discus-
sions below.
For the samples of x = 0.15 and x = 0.2, a clear super-
conducting transition is observed due to effective fluo-
rine doping, and a metallic characteristic in normal state
with a good linear dependence of R-T. The sample of x =
0.1, however, shows a little complicated temperature
dependence of resistance, as this compound lies the
boundary of the superconducting area in the phase dia-
gram. An upturn peak of resistance occurs at the tem-
peratures close to Tc, followed by a semiconductor-like
behavior, and then a metallic characteristic as the tem-
perature increases.
Figure 4. (Colour online) Temperature dependences of ZFC
and FC magnetization curves for the samples of x = 0.15 (a)
and x = 0.2 (b).
Figure 5. (Colour online) Phase diagram with respect to the
superconducting and SDW Transition, determined by the
doping dependence of characteristic temperatures for the
samples with or without Ta wrapping treatment.
Copyright © 2011 SciRes. JMP
Similar behavior was also observed in NdFe1-xRhxAsO
[34] and it was assumed that such a semiconductor-like
behavior actually was one of natures of this family,
which was unambiguously proved by band structure cal-
culations as well as photoemission spectroscopy experi-
ments [35]. For the present case, as x increases, both the
Tc and ΔT are improved due to the increase of BaF2 dop-
ing content, and then a more rigid metallic characteristic
that eventually hides the semiconductor-like nature at all.
This assumption is in consistent with the observation of
the residual resistance ratio (RRR = R(300 K)/R(Tc),
which is enhanced from 1.93 to 5.5 as the BaF2 doping
content of x increases from 0.1 to 0.2.
3.3. Characteristic Magnetic Fields
Figure 6 shows the temperature dependences of resistiv-
ity under different magnetic fields for the sample of x =
0.15 and 0.2. Based on the magnetotransport measure-
ments, two types of characteristic magnetic fields, irre-
versibility fields (Hirr) and upper critical fields (Hc2) are
Figure 6. (Colour online) Temperature dependences of re-
sistance under various magnetic fields for two samples of
x = 0.15 (a) and x = 0.2 (b).
determined by the criterion of 10% and 90% of nor-
mal-state resistivity, respectively. Note that similar
evaluation are widely applied to YBaCuO, MgB2, the
F-doped LaFeAsO and Sr-doped PrFeAsO polycrystal-
line samples [31,36,37].
Figure 7(a) shows the relations between the reduced
temperature and characteristic magnetic fields. It is re-
vealed that both Hirr and Hc2 are enhanced with increas-
ing x from 0.1 to 0.2. In case of x = 0.2, the Hirr(T) curve
is closer to Hc2(T) curve, which implies that the stronger
flux pinning effect hinders the dissipating resistance due
to flux jumping.
Figure 7(b) shows the slopes of Hc2(T) near Tc for
several superconducting samples with higher doping
content of x. By using the Werthamer-Helfand Hohen-
berg (WHH) theory, the zero temperature upper critical
field Hc2(0) can be evaluated by the formula below,
00.693 dc
 
Taking Tc = 50 K for x = 0.2 sample, one can get the
Hc2(0) of 163 T. As well, Hc2(0) is calculated to be 110 T,
and 43 T for x = 0.15 and x = 0.1, respectively. Note that
these Hc2(0) values are actually lower than that of
SmFeAs(OF) [38], and FeAs-42622 family [16]. How-
ever, the slope of 2
T for the present sample of
x = 0.2 is much larger than that of hole doped
Pr1-xSrxFeAsO [31], and also than that of electron doped
LaFeASO1-xFx) [36,37]. Obviously, the values of upper
critical field in the case of the binary doping are much
more sensitive than that of single element doping. This
suggests that the present system with both electron-doping
and hole-doping gives rise to more breaking of original
symmetry, and then more modification for supercon-
ducting behaviors. A direct mechanism may arise from
the increased quasiparticle density of states (DOS) near
the Fermi level due to both electron and hole doping.
3.4. Hall Effect and Dominant Carriers
To get more information about the conducting carriers,
the Hall coefficient (RH) of all samples are measured
using the five-probe technique. The deliberated binary
doping probably allow more symmetry breaking of the
electronic structure, compared with the cases of sole hole
doping or electron doping, and this will give rise to the
more disorders of internal structure of the compound.
Figure 8 shows the temperature dependences of RH for
the samples of x = 0.05, 0.10, and 0.20. The negative
Hall coefficient RH indicates that the electron-type carri-
ers dominate the conduction in the present samples, al-
though the binary doping is purposely designed. This is
Copyright © 2011 SciRes. JMP
468 C. Q. QU ET AL.
Figure 7. (Colour online) (a) Reduced temperature de-
pendence of Hirr and Hc2 for x = 0.1, 0.15, 0.2. (b) Doping
content dependence of μ0dHd90%/dT and Hc2(0); the inset
shows the Doping content dependence of μ0dHd10%/dT.
reasonable if one recall the difference in doping level
between Oxygen and Neodymium site. The former is
double of the later, leading to the breaking of normative
system in electron structure. This is in agreement with
the observation of the c-axis lattices, which decreases
monotonously with increasing the doping content, unlike
the case of hole-doped FeAs-1111 system, as discussed
in the prior section.
It is well known that the Hall coefficient RH is nearly
independent of temperature for conventional metals [31],
Whereas, a strong temperature dependence of RH occurs
in high-temperature oxide superconductors, and it is re-
garded as one of the exotic properties. For the present
FeAs-based superconductor, the SDW transition may
occur at 117 K, while its superconductivity does not
emerge at all, such as in the sample of x = 0.05. For this
Figure 8. (Colour online) Temperature dependence of Hall
coefficiency for the samples of x = 0.05, 0.1, 0.15, 0.2. It is
seen that the RH has a sharp transition near the Tc, and the
electron-type carriers are dominated for all the studied
composition, the temperature dependence of RH is com-
paratively simple, i.e., the absolute value decreases mo-
notonously with increasing temperature.
A band structure calculation for LaFePO indicates that
all the d-orbital energy levels for five Fe atoms in
FeAs-1111 superconductors are not fully occupied, and
the cross of the Fermi level EF, leads to five Fermi sur-
faces [39]. Therefore, it is believed that the temperature
dependence of RH arises from the multiband effects or
magnetic skew scattering mechanism alternatively. Note
that the scattering of conduction electrons from local
moments is asymmetric due to spin-orbital coupling. A
rough estimated from the relation of n = 1/(e*RH), indi-
cates the carrier density for our present samples is nearly
around 1.6 × 1022 cm–3 at the normal state, being 10
times larger than that of LaFeAsO0.9F0.1-y [37].
For further increase of BaF2 doping, temperature de-
pendence of RH become stronger, although the monoto-
nous trend still exists. For the present samples of x = 0.1
and x = 0.15, the temperature dependence of RH exhibits
a similar behavior. As the temperature increases, the RH
jumps from a large negative value to a small constant,
which actually corresponds to the phase transition around
In the case of x = 0.2, the monotonic change disap-
pears and the temperature dependence of RH becomes
complicated. Nevertheless, a common feature remains
for such a sample, i.e. the RH undergoes a dramatic tran-
sition near the superconducting critical temperature. The
carrier density slow decrease to 1.2 × 1022 cm–3 at 60 K
from the normal state, when it turns into SC state, the
Copyright © 2011 SciRes. JMP
is system.
. Summary
mperature near the Tc for such a binary doping
. Acknowledgements
demy of Sciences are gratefully
knowledged as well.
. References
No. 11, 2008, pp. 3296-
value rapid decline to 0.255 × 1022 cm–3at 40 K, while
the further low temperature makes the value dramatically
shot up to 6.25 × 1024 cm–3. At the temperature below 30
K, the value of RH is close to zero. As the temperature
increases, it becomes a large negative value firstly, and
then upturns to be a small constant up to 240 K, and fi-
nally goes to increase until 300 K.
The above temperature dependence appears unusual. It,
however, is not hard to understand if one recalls the
multi-band effect together with a complicated scattering
[3,14,40-42], by which a similar scenario in the hole-like
Pr1-xSrxFeAsO [31] can be explained well.
It is believed that the present samples follow a strong
multi-band effect with a large variation of charge carrier
densities as well, especially at the high doping content
where the nominal composition of F reaches as high as
40%. These factors suggest that the resultant RH is
probably weighted by the sum of the contributions from
each band. Assuming that the scattering rate of each
band has different temperature dependence, it is reason-
able to see the weighted sum change with temperature. In
reality, both the effects are able to make collective con-
tributions to the Hall signal in the present Nd1-xBax
FeAsO1-2xF2x since the electrons are dominant carries in
Effect of binary doping on the compound of Nd1-xBax
FeAsO1-2xF2x has been investigated with respect to dis-
tinct structural and superconducting properties. It is re-
vealed that the c-axis lattice constants decrease monoto-
nously with increasing doping content. In case of x > 0.1,
the magnetic order of SWD may be destroyed and su-
perconductivity takes places, which are confirmed by the
measurements of electrical resistivity, and dc susceptibil-
ity, and Hall coefficient. The onset superconducting
transition temperature can reach 50 K at x = 0.2. The
negative Hall coefficient at the studied temperature range
suggests that the electron-type charge carriers are domi-
nated, regardless of both electron and hole doping pur-
posely designed. The complicated temperature depend-
ence of RH, is believed to be attributed to a multiband
effect together with a complicated scattering, especially
at low te
This work is partly sponsored by the Ministry of Science
and Technology of China (973 Projects, No. 2011CBA
00105 and 863 Projects, No. 2009AA03Z204), the Sci-
ence and Technology Commission of Shanghai Munici-
pality (No. 10dz1203500), and Shanghai Leading Aca-
demic Discipline Project (No. S30105). And the supports
from the Knowledge Innovation Program of the Chinese
Academy of Sciences (Physical Properties and Mecha-
nism of Iron based Superconductors), the Open Project
of State Key Laboratory of Functional Materials for In-
formatics, Chinese Aca
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