Chemical pressure effect in CeFeAs1-xPxO 0.95F0.05

Article abstract of DOI:10.1016/j.jpcs.2010.10.046

We investigate the chemical pressure effect due to P doping in the CeFeAs_1-xP_xO_0.95F_0.05 (0≤x≤0.4) system. The compound CeFeAsO_0.95F_0.05 without P doping is on the boundary between antiferromagnet (AFM) and superconductor. The AFM order of Ce³ local moments causes a significant reentrance behavior in both resistivity and magnetic susceptibility. Upon P doping, T_c increases and reaches a maximum of 21.3 K at x=0.15, and then it is suppressed to lower temperatures. Meanwhile, the AFM order of Ce³ ions remains nearly the same in the whole doping range (0≤x≤0.4). Our experimental results suggest a competition between superconductivity and Kondo effect in the Ce 1111 system.

Full text of DOI:10.1016/j.jpcs.2010.10.046

Journal of Physics and Chemistry of Solids 72 (2011) 430–433
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Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
Chemical pressure effect in CeFeAs1ꢀxPxO0.95 0.05
F
Ã
Yongkang Luo, Xiao Lin, Han Han, Shuai Jiang, Yuke Li, Jianhui Dai, Guanghan Cao, Zhu-an Xu
Department of Physics, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e i n f o
a b s t r a c t
Available online 14 October 2010
We investigate the chemical pressure effect due to P doping in the CeFeAs1ꢀxPxO0.95F0.05 ð0rxr0:4Þ
system. The compound CeFeAsO0.95F0.05 without P doping is on the boundary between antiferromagnet
(AFM) and superconductor. The AFM order of Ce3+ local moments causes a significant reentrance behavior
in both resistivity and magnetic susceptibility. Upon P doping, Tc increases and reaches a maximum of
21.3 K at x¼0.15, and then it is suppressed to lower temperatures. Meanwhile, the AFM order of Ce3+ ions
remains nearly the same in the whole doping range ð0rxr0:4Þ. Our experimental results suggest a
competition between superconductivity and Kondo effect in the Ce 1111 system.
Keywords:
D. Superconductivity
D. Magnetic properties
& 2010 Elsevier Ltd. All rights reserved.
1. Introduction
P doping range for this system. We proposed a scenario of
competition between Ruderman–Kittel–Kasuya–Yosida (RKKY)
Since the recent discovery of 26 K superconductivity (SC) in
layered rare earth oxy-pnictide compound LaFeAsO1ꢀxFx [1], great
efforts have been made in searching new high Tc superconductors
[2–6]. In all these rare earth oxy-pnictides, there is a structural
phase transition below about 150 K, accompanied with a stripe-
type antiferromagnetic (AFM) ordering associated with Fe2+ ions
[7–9]. Chemical doping of Fꢀ on O2ꢀ site suppresses both structure
transition and AFM order by introducing electron-type charge
carriers to the FeAs-layers, resulting in the appearance of SC. On the
other hand, the studies on the effect of high pressure on
superconductivity [10–13], have revealed the doping (e.g. the
F content) dependence of pressure effect, as summarized in Ref.
[14]. Namely, the high pressure will enhance Tc for the underdoped
samples, and affect Tc only slightly for the optimally Fꢀ doped
samples, but suppress Tc for overdoped samples. This doping
dependence of pressure effect was also seen in 122 system,
and is very different from the case of high Tc cuprate super-
conductors [15].
interaction and Kondo scale, and the absence of SC was attributed
to the strong Ce-4f spin fluctuation near the two critical points.
In this paper, we report the investigation of chemical pressure
effect on CeFeAsO0.95F0.05 by P doping on As site. Our results point
to an on-the-boundary superconductor of CeFeAsO0.95F0.05 with
Tc¼13.3 K, accompanied by a reentrance caused by AFM order of
Ce3+ ions at TN¼2.8 K. Furthermore, the chemical pressure depen-
dence of Tc is also dome-like, i.e., Tc is firstly enhanced with
increasing P doping, reaches a maximum of 21.3 K at x ¼0.15, and
finally decreases. However, the AFM order temperature (TN)
corresponding to Ce3+ ions does not change very much with
P content x. Combined with previous reports [20,11,13], our
results provide the evidence for the competition between SC and
Kondo effect.
2. Experimental
Chemical pressure induced by P doping on As site has been
proved to be effective to induce SC in pnictide parent compounds,
both in 1111 [16] and 122 [17–19] systems. We have previously
systematically studied the phase diagram of CeFeAs1ꢀxPxO [20],
where two quantum critical points were proposed [21], corres-
We have synthesized a series of CeFeAs1ꢀxPxO0.95F0.05 (x¼0,
0.05, 0.1, 0.15, 0.2, 0.3, and 0.4) polycrystalline samples by solid
state reaction method. Ce, Fe, As, P, CeF3 and CeO2 of high purity
ðZ99:95%Þ were used as the starting materials. Firstly, CeAs (or
CeP) was presynthesized by reacting Ce discs and As (or P) powders
at 1323 K for 72 h. FeAs (or FeP) was presynthesized by reacting Fe
and As (or P) powders at 973 K for 20 h. Secondly, powders of CeAs,
CeP, CeO2, CeF3, FeAs and FeP were weighted according to the
stoichiometric ratio, throughly ground, and pressed into a pellet
under a pressure of 600 MPa in an Argon filled glove box. The pellet
was sealed into an evacuated quartz tube, which was then slowly
heated to 1448 K and kept at that temperature for 50 h.
ponding to the disappearance of Fe2+ AFM order at xc ¼ 0:37 and of
1
Ce3+ magnetism at xc ¼ 0:92, respectively, and furthermore, a
2
third critical point standing for the evolution of AFM to ferro-
magnetic (FM) ordering of Ce3+ at xc coincides to be close to xc .
3
1
However, no SC was observed down to 2 K in the whole
Ã
Powder X-ray diffraction (XRD) was performed at room tem-
Corresponding author.
E-mail address: zhuan@zju.edu.cn (Z.-a. Xu).
perature using a D/Max-rA diffractometer with Cu-K radiation and
a
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2010.10.046

Y. Luo et al. / Journal of Physics and Chemistry of Solids 72 (2011) 430–433
431
a graphite monochromator. Lattice parameters were refined by a
least-squares fit using at least 30 XRD peaks. Quantum Design
Physical Property Measurement System (PPMS-9) was used to
perform the resistivity measurement. The dc magnetization mea-
surement was performed in a Quantum Design Magnetic Property
Measurement System (MPMS-5) under magnetic field H¼10 Oe for
the superconducting samples or 1000 Oe for measuring normal
state susceptibility.
3. Results and discussion
The room-temperature XRD patterns of the representative
CeFeAs1ꢀxPxO0.95F0.05 samples (x¼0, 0.2 and 0.4) are shown in
Fig. 1. For all the compounds, no obvious trace of impurity phases
can be detected, and all the XRD peaks can be well indexed based on
a tetragonal cell with the space group of P4/nmm (no. 129),
indicating that all the samples are of high purity. Fig. 1(b) shows
the variations of lattice constants a and c with P doping content x.
Upon P doping, both a and c decrease monotonously. Further more,
the lattice constants of the F and P co-doped systems are smaller
than those of CeFeAs1ꢀxPxO with identical x, suggesting the
successful fluorine and phosphor doping. It can also be found
from Fig. 1(b) that the c-axis shrinks more quickly than the a-axis
does. This means that the chemical pressure could be anisotropic.
In Fig. 2, we present the temperature dependent resistivity of
CeFeAs1ꢀxPxO0.95F0.05. As is shown in Fig. 2(b), resistivity of
CeFeAsO0.05F0.05 decreases almost linearly as temperature
decreases until an obvious upturn occurs below 100 K, which is
reminiscent of the suppressed Fe2+ AFM order and/or structure
Fig. 2. Temperature dependent resistivity of CeFeAs1ꢀxPxO0.95F0.05: (a) resistivity of
CeFeAsO0.95F0.05 under magnetic field
m0H of 0, 0.1, 1, 3 and 5 T; (b)
r-T curves of
CeFeAs1ꢀxPxO0.95F0.05; (c) the enlarged plot of
r-T curves around the super-
conducting transition region.
distortion. When temperature is further cooled down,
rðTÞ shows a
distinct decrease around 13.3 K and a sharp peak at 2.8 K.
Intuitively the peak around 2.8 K is related to the Ce-4f AFM
transition, similar to that of parent compound CeFeAsO [22]. This
Ce-4f AFM order will be further confirmed by the temperature
dependence of susceptibility whereafter (Fig. 3(b)). The measure-
ment of magnetoresistance confirms that the drop around 13.3 K is
due to SC transition (seeing Fig. 2(a)). The magnetic field broadens
the SC transition region, and suppresses the SC transition
temperature Tocnset (defined as the temperature at which resisti-
vity drops to 90% of the normal state) to a lower temperature. Upon
5% P doping, SC transition in resistivity becomes much more
obvious, and Tocnset increases to 17.4 K, but the resistance does
Fig. 3. (a) Temperature dependence of magnetic susceptibility of CeFeAs1ꢀx
PxO0.95F0.05 (x¼0, 0.05, 0.15, 0.3), measured under H¼10 Oe, in ZFC (solid symbols)
and FC (open symbols) protocols. Inset: enlarged plot of CeFeAsO0.95F0.05 to show
the AFM transitions of Ce3+ ions and the kink induced by superconductivity. (b)
and 1=
ðTÞ of CeFeAsO0.95F0.05 measured under H¼ 1000 Oe, to show the Curie–
Weiss behavior.
wðTÞ
w
not drop to zero due to the reentrance caused by the Ce3+ AFM
order. As xZ0:1, zero resistance is finally observed, and Tconset
reaches a maximum of 21.3 K at x¼0.15. Tocnset starts to decrease as
x40:15.
Fig. 1. Room-temperature XRD patterns of selected CeFeAs1ꢀxPxO0.95F0.05 (x¼0,
0.2, and 0.4). Inset: Variations of lattice constants a (squares) and c (circles) with
P doping content x. The solid symbols stand for data of CeFeAs1ꢀxPxO0.95F0.05, while
the open ones correspond to CeFeAs1ꢀxPxO.

432
Y. Luo et al. / Journal of Physics and Chemistry of Solids 72 (2011) 430–433
one hand, in the CeFeAs1ꢀxPxO system, we have demonstrated in
our previous study that P doping weakens Fe2+ AFM correlation,
which will lead to the presence of xc [20]; but on the other hand,
1
inter-layer Kondo coupling (labeled by JK) and density of state at
Fermi level (denoted by N(EF)) are simultaneously increased, which
will consequently result in the sign-change of exchange interaction
Jf¼JRKKY + J0f , where jJRKKY j ꢁ jJK NðEFÞj [25] and J0 o0, are the RKKY
2
f
interaction and the exchange interaction in the absence of Kondo
coupling, respectively, and is responsible to the evolution of
AFM–FM of Ce-4f ground state at around xc . SC was expected to
3
be observed enclosing xc , however since xc is coincidently close to
1
3
xc , the strong Ce-4f spin fluctuation could make it difficult for
1
conducting electrons to form Cooper pairs. In the case of the
CeFeAs1ꢀxPxO0.95F0.05 system, however, according to the phase
diagram figured by the study of neutron scattering [26], 5% F doping
should suppress Fe2+ AFM transition to about 50 K, thus intuitively,
xc would shift leftward to a much lower P doping level. It should be
1
pointed out that the upturn on
AFM transition is hardly seen as x40:15, therefore it is mostly
r
ðTÞ curve associated to the Fe2+
w
Fig. 4. Phase diagram of CeFeAs1ꢀxPxO0.95F0.05 ð0rxr0:4Þ. Tocnset and Tc were
possible that the critical point xc locates in the regime of xo0:15.
1
obtained from the resistivity (onset point) and susceptibility (upturn point on
Furthermore, according to our results, the turning point xc should
3
dw=dT-T plot) measurements respectively, while TN was determined by the peak
persist to the regime with x larger than 0.4. This separation of the
points xc and xc results in quenching of Fe2+ AFM order at a region
position in the 2T curve. Lines are only guides to eyes.
w
1
3
where Kondo effect is not prominent, and in this region enclosing
The temperature dependence of dc magnetic susceptibility of
CeFeAs1ꢀxPxO0.95F0.05 is shown in Fig. 3. For the compound
CeFeAsO0.95F0.05 without P doping, the Meissner effect is not
obvious, and only a kink in susceptibility around 13 K can be
found, confirming that it is on the boundary between AFM order of
Fe ions and superconductor. Upon P doping, the diamagnetic
screening effect becomes larger, and the volume fraction of
magnetic shielding reaches to over 70% at the optimal P doping
level x¼0.15. To investigate the normal state properties
of CeFeAs1ꢀxPxO0.95F0.05, we performed the susceptibility measure-
ment under H¼1000 Oe, and the result is presented in Fig. 3(b). The
kink related to the SC transition observed under H¼ 10 Oe is now
xc , SC should dominate. Therefore the effect of chemical pressure is
1
to enhance SC; as P is further doped, Kondo effect is more and more
evident, and thus SC is suppressed. The peculiar relation between
SC and Kondo effect still needs to be clarified, and calls on a further
investigation.
4. Conclusion
In summary, we have investigated the effect of chemical
pressure on CeFeAsO0.95F0.05 by P doping on the As site. We found
that the compound CeFeAsO0.95F0.05 without P doping is on the
boundary between AFM phase of Fe ions and superconductor,
where the AFM order of Ce3+ moment causes a reentrant behavior.
In contrast to the previous reports on the effect of physical pressure
on fluorine-doped system [11,13] where Tc is initially suppressed
by pressure in the fluorine optimal-doped and over-doped regions,
Tc of the CeFeAs1ꢀxPxO0.95F0.05 system initially increases with
increasing chemical pressure, reaches a maximum of 21.3 K at
x¼0.15, and is finally suppressed to lower temperatures. Combined
with previous reports, our studies suggest a competition between
SC and Kondo effect, and highlight the importance of inter-layer
Kondo physics in pnictides.
totally suppressed, and the
behavior above 150 K, with the fitted effective magnetic moment of
2:48
B, very close to the magnetic moment of free Ce3+ ions
w2T curve exhibits a good Curie–Weiss
m
ð2:54mBÞ calculated by the Hund’s rule. As temperature is cooled
down to 2.8 K, a peak related to the AFM order of Ce3+ ions can be
observed. We define the Ce-AFM transition temperature TN as the
peak position. It should be pointed out that in the whole P doping
region ð0rxr0:4Þ, the position of Ce3+ AFM order does not change
much on the
increasing P content x.
w2T curve, i.e., TN remains nearly same with
We summarize the phase diagram of SC and magnetism in
CeFeAs1ꢀxPxO0.95F0.05 in Fig. 4. Chemical pressure introduced by
P doping on the As site increases Tocnset to a maximum of 21.3 K at
x¼0.15 and the Tc–x curve is also dome-like. In contrast to the
variation of Tc with x, TN, the AFM order temperature of Ce3+ ions,
only varies slightly with P doping content x. The chemical pressure
enhancing SC could certainly be explained to be the consequence of
the increase of the carrier concentration, however, there is still a
problem: How does Kondo effect affect SC? Or in other words, why
cannot this P doping chemical pressure induce SC in CeFeAs1ꢀxPxO
as other rare earth oxy-pnictides, but can enhance SC in
Acknowledgments
This work is supported by the National Science Foundation of
China, and the National Basic Research Program of China (Grant
nos. 2007CB925001 and 2009CB929104).
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