Searching for key parameter for determining Tc in Fe-based superconductors: Study of P/As substitution in RFe(P,As)(O,F) [R=La and Nd]

Article abstract of DOI:10.1016/j.jpcs.2010.10.059

We have investigated the relation between the temperature-dependence of resistivity and superconducting transition temperature T_c in RFeP_1-xAs_xO_0.90F_0.10 (x=01.0) (R=La and Nd). In contrast to the lin

Full text of DOI:10.1016/j.jpcs.2010.10.059

Journal of Physics and Chemistry of Solids 72 (2011) 414–417
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
Searching for key parameter for determining T in Fe-based superconductors:
c
Study of P/As substitution in RFe(P,As)(O,F) [R¼La and Nd]
a,b,Ã
a
a
a
a,b
Shigeki Miyasaka
, Akira Takemori , Satoshi Saijo , Shin-nosuke Suzuki , Setsuko Tajima
a
Department of Physics, Osaka University, Osaka 560-0043, Japan
b
JST, Transformative Research-Project on Iron Pnictides (TRIP), Tokyo 102-0075, Japan
a r t i c l e i n f o
a b s t r a c t
Available online 16 October 2010
We have investigated the relation between the temperature-dependence of resistivity and super-
conducting transition temperature T in RFeP1ꢀxAs 0.10 (x¼0–1.0) (R¼La and Nd). In contrast to the
linear change of the crystal structure with increasing x, the temperature dependence of resistivity and T
show non-monotonous x-dependence. When the As concentration x is increased, the temperature-
c
x 0.90
O F
Keywords:
D. Magnetic properties
D. Superconductivity
D. Transport properties
c
2
dependence of resistivity changes from T to T-linear, and T
and the Nd ones with xo0:60. The results indicate that the substitution of As for P induces the spin
fluctuation and resultantly enhances T . On the other hand, we could not find any relation between the
temperature-dependence of resistivity and T in the Nd samples with x40:60. This may suggest the
existence of other parameters for determining T besides the antiferromagnetic correlation in this system.
2010 Elsevier Ltd. All rights reserved.
c
distinctly increases in all the La compounds
c
c
c
&
1
. Introduction
for determining T
demonstrated that the bond angle between Fe and As/P is strongly
correlated with T [13]. According to their study, the optimum bond
c
in this system. The pioneering work by Lee et al.
Since the discovery of high-T
c
superconductivity in cuprates,
c
much experimental and theoretical effort has been made to explore
similar phenomena in other transition metal compounds with
layered structures. As a result, many superconducting systems,
angle of As/P–Fe–As/P is 109.471 which gives a regular tetrahedral
structure of As/P around Fe. However, it is not clear yet what
physical parameter is modified by this angle. On the other hand,
several theoretical investigations have indicated that the antifer-
romagnetic fluctuation plays an important role for the appearance
of superconductivity in the iron-based pnictide systems [14,15]. In
order to clarify the mechanism of the superconductivity in this
system, it is necessary to compare various physical properties of
2 2 2 4
2
y 2
such as YNi B C [1], Sr RuO [2], NaCoO H O [3] and so on, have
been found. Recently, superconductivity in the iron- and nickel-
based oxyphosphides, LaFePO [4] and LaNiPO [5] were reported.
These iron- and nickel-based oxyphosphides have not been
attracted much attention, because of their low superconducting
transition temperature, T
oxypnictides, however, Y. Kamihara and co-workers have found
a new superconductor, LaFeAs(O, F) with T
the research on this system have been intensified, and the super-
conductivity with higher critical temperature around 50 K have
c
. In the same family of iron-based
samples with different T
In this study, we have synthesized polycrystalline samples of
RFeP1ꢀxAs 0.10, where R¼La and Nd. Since F concentration is
fixed, we can apply a chemical pressure to the samples by this P/As
substitution. Both end members of RFePO0.90 0.10 and RFeAsO0.90
c
values.
c
¼26 K [6]. Since then,
x 0.90
O F
F
F
0.10
been discovered in other rare-earth systems, RFeAsO1ꢀx
F
x
and
are metallic in the whole temperature region, and show the transi-
RFeAsO1 (R¼rare earth elements) [7,8]. Moreover, many related
tion to the superconducting state at low temperatures. In RFeP1ꢀx
ꢀ
d
iron-based superconductors have been found, such as ðA,AuÞFe
2
As
2
As
ions and the P/As substitutions, because of the largely different T
respective end materials. We have investigated the resistivity proper-
ties for RFeP1ꢀxAs 0.10, where the chemical pressure is system-
x
O
0.90
F
0.10, we can control T
c
between 3 and 50 K by the change of R
(A¼Ca, Sr, and Ba, Au ¼ Na and K), A(Fe, Co)
2
As , AFe (As, P) , LiFeAs,
2
2
2
c
in the
FeSe and so on [9–12]. These iron-based compounds have a
common structure, 2-dimensional iron layers composed of the
x 0.90
O F
tetrahedral Fe(As, P)
4
.
atically changed, to search the important physical parameter for the
Since the superconductivity was discovered in the iron-based
pnictides [6], a lot of efforts have been paid to find a key parameter
superconductivity in this system.
2. Experiments
Ã
Corresponding author at: Department of Physics, Osaka University, Osaka 560-
Polycrystalline samples of RFeP1ꢀxAs
(R¼La and Nd) were synthesized by solid state reaction. RP and
x
O
0.90
F
0.10 (x ¼ 0–1.0)
0
043, Japan. Tel.: +81 6 6850 5584; fax: +81 6 6850 5585.
E-mail address: miyasaka@phys.sci.osaka-u.ac.jp (S. Miyasaka).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2010.10.059

S. Miyasaka et al. / Journal of Physics and Chemistry of Solids 72 (2011) 414–417
415
RAs, which were used as precursor, were obtained by reacting rare-
earth elements and P or As chips at 700 1C for 10 h and 900 1C for
5 h, respectively. The mixtures of RAs, RP, Fe , Fe and FeF in the
stoichiometric ratio were pressed into pellets, and they were
annealed at 1100 1C for 40 h. All reactions were performed in the
evacuated silica tubes, and the procedures in a highly pure Ar filled
grove box.
The samples were ground and checked by powder X-ray
diffraction using Cu Ka1 radiation at room temperature. All diffrac-
tion peaks of respective samples can be assigned to the calculated
R
FeP 1-xAs O0.90F0.10
x
8.70
8.60
1
2
O
3
2
4
4
4
3
.10
.05
.00
.95
c axis
a axis
x 0.90
Bragg peaks for the tetragonal RFeP1ꢀxAs O F0.10, indicating that
there is no impurity. The in-plane (a) and out-of-plane lattice
8
.50
constants (c) of these systems with the As concentration of x¼0–
1
.0 are obtained by using least squares fitting of the measured peak
3
positions of the powder X-ray diffraction data in 2
Electrical resistivity was measured using a conventional four-probe
method.
y ¼ 202100 .
8
8
8
.50
.40
.30
In this study, we have not determined the F concentration by the
chemical analysis. However, all the samples were synthesized by
the same procedure, and their solid state reaction were performed
in the sealed quartz tubes. So, the F concentrations in the present
systems are expected not to deviate largely from the nominal ones.
4
3
3
.00
.95
.90
c axis
a axis
In RFeAsO1ꢀy
suppressed by the F-doping [16]. The values of lattice constants are
very small in the FeAs end member of RFeP1ꢀxAs
x¼1.0, indicating that the F concentration is almost y¼0.10. In other
end member of RFePO0.90
y
F , the lattice constants of a and c are distinctly
x 0.90
O F0.10 with
F
0.10 (x¼0), the lattice constants estimated
by X-ray analysis indicates that the F concentration is almost similar
to the nominal one [17]. Moreover, the lattice constants linearly
0
0.2
0.4
0.6
0.8
1.0
x 0.90
increase with the As content x in RFeP1ꢀxAs O F0.10. These results
suggest that the actual F concentrations may be almost equal to the
x (As content)
nominal ones in the present systems.
Fig. 1. As content (x) dependence of lattice constants of a- and c-axes, indicated by
x 0.90 x 0.90
closed and open circles for LaFeP1ꢀxAs O F0.10 (a) and NdFeP1ꢀxAs O F0.10 (b).
Dashed lines are guide for eyes.
3
. Results and discussion
Fig. 1 presents the As-concentration (x) dependence of lattice
constants of a- and c-axes. All lattice constants linearly increase
with As concentration. The result proves that a solid solution of
R¼Nd system, when As content is increased from x¼0 to x¼0.60. As
well as the La system, the power n of the Nd one has minimum value
of about 1 around x¼0.60. In both systems, the n slightly increases,
but the value is still below 1.3 in the region of x40:60. The T-linear
RFeP1ꢀxAs
x
O
0.90
F
0.10 with R¼La and Nd has been successfully
prepared, and the crystal structure have been continuously chan-
ged with x.
c
resistivity is very similar to the behavior of high-T cuprates, which
strongly suggests that the conduction mechanism is governed by a
strong spin fluctuation [18]. The x dependence of power n indicates
that the chemical pressure, induced by the P/As substitution, rapidly
increases antiferromagnetic fluctuation with x up to x¼0.60. In the
region of x¼0.60–1.00, the strength of antiferromagnetic fluctuation
In contrast of the linearly structural change with As concentra-
tion x, the resistivity behaviors for these systems do not depend
monotonically on x. Figs. 2(a) and (b) represent the temperature-
dependent resistivity for R¼La and Nd compounds with various As
concentrations, respectively. The resistivity for all the samples
shows metallic behavior. In both systems, the resistivity for the
n
seems to be almost constant. As shown in Fig. 3(b), the coefficient of T
of low temperature resistivity, A is enhanced around x¼0.60 with the
increase of x in both systems. This behavior of A may be also related
with the enhancement of the antiferromagnetic fluctuation.
phosphorous end member with x¼0 (RFePO0.90
F0.10) has low
magnitude and shows small temperature dependence. With
increasing x, the residual resistivity and the temperature depen-
dence are rapidly enhanced around x¼0.40, and they have max-
imum at x¼0.60–0.80. The x-dependence of resistivity is non-
monotonic but systematic. The systematic change with x in
resistivity was observed in both systems, which indicates that a
grain boundary effect on resistivity is not serious as far as we
discuss a relative change of resistivity with x.
c
The x dependence of the critical transition temperature T seems
to be related to the above-mentioned resistivity behaviors rather
than the monotonous change of crystal structure with x. Fig. 3(c)
presents the x dependence of T
samples, the resistivity shows a sharp transition to the super-
c
for R¼La and Nd systems. In all the
conducting state, which enables us to determine T
resistivity temperatures. In the R¼La system,
c
from zero
c
gradually
T
The temperature dependence of resistivity can be expressed as
increases with x up to x¼0.60, while it is saturated or weakly
decreased above x¼0.60. As well as La system, the T of the R¼Nd
one is monotonically enhanced up to x¼0.60 by the substitution of
against x is
n
r
ðTÞ ¼ r0 þA at low temperatures, where r0 is residual resistivity, A
c
n
the coefficient of T , and n the power of temperature (T) in the low-T
resistivity ð
r
ðTÞÞ. Fig. 3(a) shows the x dependence of the power of
As for P. In the R¼Nd system, the increasing ratio of T
c
temperature in resistivity, n. For x¼0 in both of R¼La and Nd systems,
n is close to 2, indicating that the oxyphosphide end materials with
x¼0 are a conventional Fermi liquid. [17] In the R¼La system, the
power (n) rapidly decreases around x¼0.20 and reaches about unity
at x¼0.60. On the other hand, the n almost linearly decreases in the
slightly suppressed around x ¼ 0.60–0.80, and enhanced again
above x¼0.80. In all the samples with R¼La and those with R¼Nd
c
below x¼0.60, T distinctly correlated with the power of tempera-
ture in low-temperature resistivity n, i.e., the samples with high T
show the T-linear resistivity at low temperatures.
c

4
16
S. Miyasaka et al. / Journal of Physics and Chemistry of Solids 72 (2011) 414–417
RFeP1-xAs O
F0.10
RFeP1-x As O0.90F0.10
x
0.90
x
8
6
4
2
x=0.80
R=La
R=Nd
2
1
1
.0
.5
.0
0
0
.60
.40
1.00
n
1
2
8
4
0
ρ=ρ +AT
x=0
.20
0
0
9
x=0.60
0
.80
0.40
0
0
6
3
4
0
.20
1.00
20
x=0
0
100
200
300
0
0.2
0.4
0.6
0.8
1
Temperature (K)
x (As content)
Fig. 2. Temperature dependence of resistivity for LaFeP1ꢀxAs
x 0.90
O F0.10 (a) and
Fig. 3. As content (x) dependence of power of temperature in resistivity n (a), the
coefficient of Tn term of resistivity A (b), and T
NdFeP1ꢀxAs 0.10 (b) with various As concentration of x.
x 0.90
O F
c
(c) in RFeP1ꢀxAs 0.10 (R¼La and
x 0.90
O F
Nd). Solid and dashed lines are guide for eyes.
Fig. 4 presents the plot of the power n v.s. T
samples with R¼La and Nd. All the La compounds show the almost
linear relationship between n and T . Similar relation between n
and T
is also observed in the region of x ¼ 0–0.60 in R¼Nd system.
This distinct relation between the power n and T suggests that the
antiferromagnetic fluctuation causes the superconducting state
with high T . Similar relation between T and the power n has been
observed in other Fe-based pnictides [19]. In contrast, the power n
and T has no clear relation in the Nd samples with x40:60. The
behavior of the power n and T is still open to question in the Nd
c
for respective
RFeP1-xAs O0.90F
0.10
x
c
c
2
1
.0
.5
.0
R=La
R=Nd
c
c
c
c
c
compounds with x40:60. This result may indicate the existence of
the key parameters besides the antiferromagnetic fluctuation,
c
which determine T in the iron-based pnictide superconductors.
4
. Conclusion
We have investigated the P/As substitution effect in RFeP1ꢀx
1
As
x
O
0.90
F
0.10 (x¼0–1.0) (R¼La and Nd). The lattice constants in these
0
20
40
systems linearly increase with As concentration of x, indicating that the
P/As substitution induces the chemical pressure. In contrast with the
linear change of the crystal structure, the resistivity behaviors and the
Tc
Fig. 4. Correlation of
c
T and the power of temperature in resistivity n in
RFeP1ꢀxAs
guide for eyes.
x
O
0.90
F
0.10 (R¼La and Nd) with various x. Solid and dashed lines are
c
superconducting transition temperature T show non-monotonous x-
dependence. The temperature-dependence of resistivity changes from
2
T to T-linear with the As concentration x, indicating the increase of spin
fluctuation. In the R¼La system, T
concentration x up to x¼0.60, while it is saturated or weakly decreased
in the region of x40:60. T
of the R¼Nd one is monotonically
c
continuously increases with As
enhanced up to x¼0.60. In the R¼Nd system, the increasing ratio of
against x is slightly suppressed around x¼0.60–0.80, and enhanced
again above x¼0.80. The results indicate that the substitution of As for
T
c
c

S. Miyasaka et al. / Journal of Physics and Chemistry of Solids 72 (2011) 414–417
417
P induces the strong antiferromagnetic fluctuation and enhances T
c
in
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temperature-dependence of resistivity and T
x40:60. This may suggest the existence of other parameters for
determining T in the Fe-based superconducting systems.
c
in the Nd samples with
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