Non-monotonic change of electronic properties by As substitution in LaFeP(O,F)

Article abstract of DOI:10.1016/j.physc.2010.04.004

The discovery of superconductivity at 28 K in fluorine doped LaFeAsO stimulated a great interest in iron arsenide superconductors. But many differences of physical properties exist between FeP and FeAs 1111 oxypnictide superconductors. To reveal its origin, we synthesized LaFeP_1-xAs _xO_0.9F_0.1, and investigated the x-dependence of the physical properties. In contrast to a monotonic change of lattice parameters, T_c and T-dependence of resistivity show non-monotonic change.

Full text of DOI:10.1016/j.physc.2010.04.004

Physica C 470 (2010) S298–S299
Contents lists available at ScienceDirect
Physica C
journal homepage: www.elsevier.com/locate/physc
Non-monotonic change of electronic properties by As substitution in LaFeP(O,F)
a
S. Saijo a, , S. Suzuki , S. Miyasaka a,b, S. Tajima a,b
*
a Department of Physics, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan
b JST, Transformative Research-Project on Iron Pinctides (TRIP), 5 Sanbancho, Chiyoda, Tokyo 102-0075, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Accepted 16 April 2010
Available online 24 April 2010
The discovery of superconductivity at 28 K in fluorine doped LaFeAsO stimulated a great interest in iron
arsenide superconductors. But many differences of physical properties exist between FeP and FeAs 1111
oxypnictide superconductors. To reveal its origin, we synthesized LaFeP1ÀxAsxO0.9F0.1, and investigated
the x-dependence of the physical properties. In contrast to a monotonic change of lattice parameters,
Tc and T-dependence of resistivity show non-monotonic change.
Keywords:
Iron pnictide superconductors
Resistivity
Ó 2010 Elsevier B.V. All rights reserved.
Antiferromagnetic fluctuation
1. Introduction
The lattice parameters were determined by powder X-ray dif-
fraction. It was also confirmed that the prepared samples were al-
When the superconductivity in LaFeP(O,F) was discovered in
2007 [1], it did not attract much attention because of its low Tc
(ꢀ5 K). However, after the discovery of higher Tc (ꢀ28 K) in
LaFeAs(O,F) [2], it has become crucially important to understand
the electronic state and the superconductivity mechanism in
LaFeP(O,F).
It is a puzzle why the value of Tc in LaFeP(O,F) is so low, com-
pared to that of LaFeAs(O,F). The electronic structures including
Fermi surfaces are quite similar in these two systems [3]. The
density of states at the Fermi level is expected to be larger in
LaFeP(O,F) than that in LaFeAs(O,F) in spite of the lower Tc in the
former material. Although the lattice parameters are different
between these two materials, it is not understandable that such
a small difference in lattice parameters plays an essential role in
determining Tc. In the present study, we approach this problem,
by investigating the solid solution system LaFeP1ÀxAsxO0.9F0.1 to
extract a key ingredient for high Tc superconductivity.
most of single phases. Electrical resistivity was measured using a
four-probe method.
3. Results and discussion
The lattice parameters a and c were found to increase linearly
with increasing As content (x). This proves that a solid solution
LaFeP1ÀxAsxO0.9F0.1 was successfully prepared.
Temperature dependences of electrical resistivity are shown in
Fig. 1. In all the samples, resistivity shows a sharp superconducting
transition, which enables us to determine Tc from zero resistivity
temperatures. The Tc values of both end materials were identical
to the previously reported ones [1,2]. As we see in the upper panel
of Fig. 2, Tc gradually increases with x up to 0.6, while it is saturated
or weakly decreased above x = 0.6.
In contrast to the monotonic change in lattice parameters, resis-
tivity changes with x in a complicated manner, as shown in Fig. 1.
We can discuss the resistivity change from the three viewpoints.
The first is power of T at low temperatures, the second is the resid-
ual resistivity, and the third is the slope at high temperatures.
2. Experiments
T-dependence of resistivity can be expressed as q(T) = q
0 + ATn.
Polycrystalline samples of LaFeP1ÀxAsxO0.9F0.1 (x = 0, 0.1, 0.2, 0.3,
0.4, 0.6, 0.8, and 1.0) were synthesized by solid state reaction. The
mixtures of LaAs, LaP, Fe2O3, LaF3, Fe in the stoichiometric ratio
were pressed into pellets in an Ar filled grove box. The pellets were
annealed in evacuated quartz tubes at 1100 for 40 h. Here, LaAs
(LaP) was obtained by reacting La chips and As pieces (P chips)
at 500 (400) for 10 h (15 h), then at 900 (700) for 15 h (10 h).
For x = 0, n is close to 2, indicating that the x = 0 end material is a
conventional Fermi liquid [4]. When x exceeds 0.2, the power (n)
rapidly decreases and reaches about unity at x = 0.6. Above
x = 0.6, n increases again and becomes close to 2. (See the lower
panel of Fig. 2.) The T-linear resistivity down to 30 K is similar to
the behavior of high-Tc cuprates, which strongly suggests that
the conduction mechanism is governed by a strong spin fluctuation
[5]. The x dependence of n indicates that the antiferromagnetic
fluctuation rapidly increases with x up to x = 0.6, followed by a
weak decrease above 0.6. At x = 1.0, antiferromagnetic fluctuation
* Corresponding author.
E-mail address: satsaijo@tsurugi.phys.sci.osaka-u.ac.jp (S. Saijo).
0921-4534/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2010.04.004

S. Saijo et al. / Physica C 470 (2010) S298–S299
S299
cannot be understood in terms of the disorder effect. There seems
no disorder effect on Tc, because Tc increases with introducing P in
LaFeAs(O,F).
The slope of
q(T) at high temperatures also shows a distinct
behavior in the intermediate composition region (0.4 < x < 0.8).
The slope increases with x and reaches the maximum value at
x ꢀ 0.8, followed by a small decrease at x = 1.0. In a conventional
metal, the high T slope of resistivity is proportional to the
electron–phonon coupling strength. In the present case, since spin
fluctuation dominates conduction mechanism, the slope of
may reflect other quantities. However, a rough tendency that the
slope is the largest near x = 0.8, which correlates with n and q0
q(T)
,
is very suggestive in relation to the electronic state basing on
superconductivity.
From the common x-dependences of all the three parameters as
well as Tc, we conclude that there is some critical change between
x = 0.2 and 0.4. Judging from the T-linear behavior of resistivity, we
can expect that the antiferromagnetic fluctuation is stronger in this
intermediate region. This may be linked to a rapid increase in Tc in
this region.
Fig. 1. Temperature dependence of resistivity for LaFeP1ÀxAsxO0.9F0.1 with various
xs.
The presence of a quantum critical point was suggested in the
BaFe2(As,P)2 system, based on a rapid change in n-value [6]. The
present result that Tc starts to increase when n becomes close to
unity is similar to the situation in BaFe2(As,P)2. However, in the
present case, both of the two phases are a superconducting phase,
while in the case of Ref. [6], a SDW phase is competing with a
superconducting phase. The present result indicates that two
superconducting phases are separated by some intermediate phase
where antiferromagnetic fluctuation is very strong. In order to clar-
ify the meaning of the observed unusual electronic change, further
studies in both experiments and theories are necessary.
30
20
10
LaFeAsxP1−xO0.9F0.1
0
2
4. Summary
The solid solution system of two superconductors LaFePO0.9F0.1
and LaFeAsO0.9F0.1 was successfully prepared over a whole range of
composition of As/P. In contrast to the x-linear changes of lattice
parameters, Tc shows a non-monotonic change. When a rapid in-
crease in Tc is observed at x ꢀ 0.3, the T-dependence of resistivity
changes from T2-behavior to T-linear one. Since the latter behavior
is an indication of strong spin fluctuation, the enhancement of spin
fluctuation can be considered to be linked to a rapid raise of Tc near
x = 0.3.
1
0
0.2
0.4
0.6
0.8
1
As content x
Fig. 2. As content (x) dependence of Tc (upper) and power of T in q(T) (lower).
is stronger than that for x = 0, although it is strongest not in the end
material (x = 1.0) but at x ꢀ 0.6.
References
Since the present samples are polycrystals, it is hard to discuss
the absolute value of resistivity. But we can see a rough tendency
in Fig. 1 that residual resistivity increases with x. There seems to
be a jump between x = 0.2 and 0.4. Partial replacement of P by As
may introduce scattering centers in a conduction path. However,
a larger residual resistivity even in the end material LaFeAs(O,F)
[1] Y. Kamihara et al., J. Am. Chem. Soc. 128 (2006) 31.
[2] Y. Kamihara et al., J. Am. Chem. Soc. 130 (2008) 3297.
[3] V. Vildsola et al., Phys. Rev. B78 (2008) 064518.
[4] S. Suzuki et al., J. Phys. Soc. Jpn. 78 (2009) 114712.
[5] T. Moriya, in: A. Fujimori, Y. Tokura (Eds.), Spectroscopy of Mott Insulators and
Correlated Metals, Springer, Berlin, 1995, pp. 66–79.
[6] S. Jiang et al., J. Phys. 21 (2009) 382203.