Superconductivity in PT doped BaFe2As2

Article abstract of DOI:10.1143/JPSJ.81.064704

A high-pressure method was used to systematically dope an antiferromagnetically metallic compound, BaFe₂As₂, with Pt, and the doped compounds were studied using X-ray diffraction, energy dispersive X-ray spectroscopy, and by measuring the electrical resistivity, magnetic susceptibility, and specific heat. This method significantly increased the Pt doping level for BaFe₂As₂, resulting in a revision of the phase diagram proposed for BaFe_2-xM_xAs₂. The Pt doped system showed a dome-like superconductivity region in the thermal phase diagram (0:02< x< 0:20) with an optimal Tc of 24.2K (x = 0:08), instead of the unusually wide superconductivity region found in early studies. Further doping beyond the optimal Pt concentration led to Tc continuously decreasing toward zero. The common superconducting feature of the 3d and 4d doped systems, BaFe_2-xM_xAs₂ (M = Co, Ni, Rh, or Pd), was confirmed for the 5d doped system, BaFe_2-xM _xAs₂, which suggested that the electron-phonon coupling was insignificant at the superconducting state.

Full text of DOI:10.1143/JPSJ.81.064704

Journal of the Physical Society of Japan 81 (2012) 064704
DOI: 10.1143/JPSJ.81.064704
FULL PAPERS
Superconductivity in Pt Doped BaFe2As2
ꢀ
1
1
Yanfeng GUO , Xia WANG2;3, Jun LI2;3, Shoubao ZHANG ,
y
2;3;4
1;2;4
Kazunari YAMAURA
, and Eiji TAKAYAMA-MUROMACHI
1International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science,
Tsukuba, Ibaraki 305-0044, Japan
2Department of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
3Superconducting Properties Unit, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
4JST, Transformative Research-Project on Iron Pnictides (TRIP), Tsukuba, Ibaraki 305-0044, Japan
(Received January 19, 2012; accepted March 28, 2012; published online May 11, 2012)
A high-pressure method was used to systematically dope an antiferromagnetically metallic compound, BaFe2As2,
with Pt, and the doped compounds were studied using X-ray diffraction, energy dispersive X-ray spectroscopy, and by
measuring the electrical resistivity, magnetic susceptibility, and specific heat. This method significantly increased the
Pt doping level for BaFe2As2, resulting in a revision of the phase diagram proposed for BaFe2ꢁxPtxAs2. The Pt doped
system showed a dome-like superconductivity region in the thermal phase diagram (0:02 < x < 0:20) with an optimal
Tc of 24.2 K (x ¼ 0:08), instead of the unusually wide superconductivity region found in early studies. Further doping
beyond the optimal Pt concentration led to Tc continuously decreasing toward zero. The common superconducting
feature of the 3d and 4d doped systems, BaFe2ꢁxMxAs2 (M ¼ Co, Ni, Rh, or Pd), was confirmed for the 5d doped
system, BaFe2ꢁxPtxAs2, which suggested that the electron–phonon coupling was insignificant at the superconducting
state.
KEYWORDS: superconductivity, BaFe2As2, impurity effects, high pressure synthesis, antiferromagnetic metals
There have been many theoretical suggestions about the role
1. Introduction
of the doping. Thus, the progress made in the experimental
Iron pnictide superconductors attract great interest in the studies on doping hold the promise of revealing the entire
condensed-matter-physics community because they capable nature of the SC. The 122-type series is a valuable subject to
of exhibiting superconductivity at Tc as high as 55 K.1,2) use to experimentally study the doping.
Doping compounds to a AFe2As2 host (A ¼ Ba, Sr, Ca, or
Chemical doping to the 122 compound has been
Eu) is of particular interest because carrier doping to cause conducted mostly with group VIII atoms.9–13) Regarding
superconductivity (SC) can be controlled in many ways the 3d and 4d atoms in the group, the doped BaFe2As2 shows
and sizable single crystals can be obtained without much fairly common electronic diagram features over the doping.
technical difficulty.3) In fact, the experimental results of However, the 5d atom Pt doped BaFe2ꢁxPtxAs2 shows
many independent studies are often consistent and contribute features that are very different from the common features
greatly to revealing the nature of the SC of iron pnictide. The for the 3d and 4d systems.19,20) In the early studies on
host, thus, has been intensively investigated in various ways; BaFe2ꢁxPtxAs2, SC appears over a remarkably wide doping
BaFe2As2 is antiferromagnetic (AFM) below ꢂ140 K,4) with range from x ¼ 0:02{0:50 (25 atomic mole %), with an
similar AFM features below ꢂ210 K for SrFe2As2 and almost invariable Tc of ꢂ25 K.20) In addition, an Ir doping
5)
ꢂ190 K for EuFe2As2.6) It has been argued the AFM order is study revealed that Tc was almost invariable at ꢂ24:2 K.12,21)
primarily the result of Fermi surface nesting between the Note that among the 5d metals, only Pt19,20) and Ir12,21) were
hole pockets around ꢀ and the electron pockets around M.7) successful at turning on the SC in the 122 compounds
It is also argued that this nesting can be disrupted by AFe2As2. However, it might be possible that the early results
chemical doping or physical squeezing, which weakens the were somewhat complicated by an inhomogeneous distribu-
magnetic order. The SC actually appears where the magnetic tion of Pt or Ir. Thus, a well controlled Pt or Ir doping is
order is weak or absent.8)
required to draw a much more solid picture of the SC of the
The chemical doping to AFe2As2 has been realized using 5d element doped 122 system. The expected results at least
a variety of d elements such as Co, Ni, Ru, Rh, Pd, Ir, provide us with useful information about electron–phonon
and Rb.9–13) Rather than the conventional carrier injection coupling in the SC state because the atomic masses are
picture, a recent theoretical study involving density func- entirely different.
tional calculations for BaFe2ꢁxXxAs2 (X ¼ Co, Ni, Cu, or
In this study, crystals of BaFe2ꢁxPtxAs2 (x ¼ 0{0:25)
Zn)14,15) suggested that the doping results in an electronic were grown under high-pressure (HP) conditions, and the SC
modification through amendment of the Fe2As2 layer. was investigated using electrical resistivity, magnetization,
Although the doping plays a significant role in establishing and specific heat measurements. The Pt distribution was
the SC,16) the doping features still need to be studied further. certainly improved by employing the HP method. The
For example, although the spin excitation and orbital results of this study significantly revised the earlier super-
fluctuation caused by the doping are probably essential to conducting phase diagram proposed for BaFe2ꢁxPtxAs2. It
the SC, their relationships to it remain quite vague.17,18) was confirmed that the common doping features for the 3d
and 4d doped BaFe2As2 were valid even for the 5d doped
ꢀE-mail: guo.yanfeng@nims.go.jp
yE-mail: yamaura.kazunari@nims.go.jp
system, which suggested that electron–phonon coupling in
the SC state was insignificant.
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J. Phys. Soc. Jpn. 81 (2012) 064704
FULL PAPERS
Y. GUO et al.
Fig. 1. (a) XRD patterns of polycrystalline BaFe1:8Pt0:2As2 prepared using HP and AP methods. (b) Normalized unit cell parameters of the
ꢁ
ꢁ
ꢁ 3
HP-prepared BaFe1ꢁxPtxAs2, where a0 ¼ 3:955ð1Þ A, c0 ¼ 12:95ð1Þ A, and V0 ¼ 202:6ð3Þ A (x ¼ 0). (c) XRD profile of a surface of a single crystal
BaFe1:8Pt0:2As2 (HP). (d) Photograph of a single crystal BaFe1:8Pt0:2As2 (HP) having the approximate dimensions of ꢂ0:6 ꢆ 0:5 ꢆ 0:02 mm3, and the
FWHM of the (008) peak.
The electrical resistivity, ꢂ, and specific heat, Cp, were
measured using a physical properties measurement system
2. Experimental
Polycrystalline BaFe2ꢁxPtxAs2 (x ¼ 0, 0.04, 0.06, 0.08, from Quantum Design. Electrical contacts were prepared
0.14, 0.16, 0.18, 0.20, and 0.25) was prepared as follows: from silver epoxy and Au wires at four locations on a pellet
binary precursors, BaAs and FeAs, were prepared from Ba for an electrical gauge current of 2 mA. The value of ꢂ was
pieces (scratched from a rod, 99.9%, Nilaco), Fe powder measured in a fixed magnetic field of 70 kOe or lower. Cp
(100 mesh 99.9%, Rare Metallic), and As powder (99.999%, was measured using a time-relaxation method between 2
High Purity Chem.). Each mixture was placed in an and 300 K in the apparatus. The magnetic susceptibility, ꢃ,
evacuated quartz tube and heated at 700 ꢃC for 10 h. and isothermal magnetization (M) were measured using a
Thereafter, a stoichiometric mixture of BaAs, FeAs, Fe, magnetic property measurement system from Quantum
and Pt (99.9%, High Purity Chem.) was placed in a boron Design. Loosely gathered powder was cooled to 2 K without
nitride cell. The nitride cell was sealed in a Ta capsule and applying a magnetic field, followed by warming to 300 K
then heated at 1300 ꢃC for 2 h in a belt-type HP apparatus, in an applied magnetic field of 10 or 0.01 kOe (zero-field
which maintained a constant applied pressure at 3 GPa cooling; ZFC). The sample was again cooled to 2 K in the
during the heating. The Ta capsule was quenched to room field (field cooling; FC). M was measured in the apparatus at
temperature before releasing the pressure. For a reference, 2, 5, 8, 12, 15, and 18 K in a field range of ꢄ70 kOe.
selected mixtures of BaFe2ꢁxPtxAs2 (x ¼ 0, 0.10, and 0.20)
3. Results and Discussion
were synthesized without applying the pressure (at ambient
pressure, AP). Each mixture was placed in an evacuated
XRD patterns of the polycrystalline BaFe1:8Pt0:2As2
quartz tube and heated at 1100 ꢃC for a total of 40 h with prepared under the HP and AP conditions are shown in
several intermediate grindings to secure a homogeneous Fig. 1(a). For the HP sample, the Bragg reflections were
reaction. Selected single crystals at x ¼ 0:04, 0.08, and 0.20 indexed very well using the ThCr2Si2-type structure model
were grown under the HP condition using a self-flux method (space group: I4=mmm),23) with the exception of tiny peaks
(6 h heating) with an additional 2 h of cooling to 1100 ꢃC.
for FeAs as an impurity. The pattern for the AP sample
Phase identification by powder x-ray diffraction (XRD) was shows additional small Bragg peaks, which indicate the
conducted at room temperature in a Rigaku diffractometer formation of PtAs2 as an additional impurity. The PtAs2
using Cu Kꢀ radiation from 5–100ꢃ in 2ꢁ. The lattice peaks were found to grow with an increase in the doping
parameters of the phases were refined using the software level. The size of the tetragonal unit cell of the HP sample
Powder X.22) True Pt and Fe compositions were investigated changed almost linearly against the Pt content [Fig. 1(b)],
using energy dispersive X-ray spectroscopy (EDX), and the Pt suggesting that the doped Pt does go into the crystal
distribution over a pellet was studied using a scanning electron structure, at least for x ꢅ 0:20. Because the Pt substitution
microscope (SEM), Inca Energy 300 (Oxford Instruments).
for Fe may be isovalent15) and the ionic size of Pt2þ is almost
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Y. GUO et al.
Fig. 2. (Color online) SEM images of polycrystalline BaFe1:8Pt0:2As2
prepared using (a) HP and (b) AP methods. The HP sample shows a rather
homogenous Pt distribution, while the AP sample shows many PtAs2-rich
clusters, as marked by small circles and ellipses.
Fig. 3. T dependence of ꢂ of non-doped polycrystalline host BaFe2As2
prepared using (a) HP and (b) AP methods (H ¼ 10 kOe). The dꢂ=dT and
dꢃ=dT curves are plotted against T inside the figures.
comparable to that of Fe2þ, the unit cell change was the present Pt doped superconductor, the ða=cÞ=ða0=c0Þ ratio
expected to be very small. The result apparently coincides is plotted against the Pt content in Fig. 1(b), which clearly
with this expectation. Although the substitution decreased shows a linear change. The optimal Tc of the present
the distance between the layers along the c-axis, it increased superconductor is observed at an x of 0.08–0.10 (shown
the average distance between the Fe atoms along the a-axis, later), where the ratio is fairly close to 1.005. Hence, the
which reflected different intra- and inter-plane bonding present result supports the argument.
natures. The observed anisotropy was qualitatively compar- SEM images of both the HP and AP prepared pellets
able to what was observed for the Co, Ni, or other d element of BaFe1:8Pt0:2As2 are compared in Figs. 2(a) and 2(b),
doped 122-type compounds, suggesting that the role of Pt in demonstrating contrasting Pt distributions. In the HP pellet,
the Pt is almost homogenously distributed on this scale,
the doping is not far from that of the other d elements.
To further investigate the Pt doping, the composition was while it is rather imperfectly distributed in the AP pellet. The
analyzed using EDX. Ten pieces of each of the selected region marked by ellipses and circles indicates a Pt rich area
HP samples (x ¼ 0:06, 0.10, 0.14, 0.20, and 0.25) were caused by the PtAs2 impurity. This observation is consistent
measured, and the averaged Fe to Pt composition ratios were with what was suggested by the XRD study. The SEM
estimated to be 1.944/0.059 (x ¼ 0:06), 1.897/0.102 (0.10), observation supports the successful Pt doping up to an x of
1.856/0.144 (0.14), 1.804/0.206 (0.20), and 1.744/0.242 0.20 under the HP condition.
(0.25). The true Pt composition was in good accord with
the starting composition, x, within one standard deviation. BaFe2As2 pellets measured in a zero magnetic field. The
The result again confirmed the incorporation of Pt in the anomalous drop at ꢂ150 K is suggestive of a correlation
Figure 3(a) shows ꢂðTÞ curves for both the AP and HP
lattice.
with a spin-density wave (SDW) or a structure transition, as
The XRD pattern of the surface of a BaFe1:8Pt0:2As2 observed in early studies.4) The characteristic temperature
crystal is shown in Fig. 1(c). Sharp peaks (002n, where n is (TD) was 142 and 144 K for the HP and AP pellets,
an integer), with nothing else visible in the XRD pattern, respectively, based on a dꢂ=dT analysis [inset for Fig. 3(a)].
indicated the crystal direction. The c-axis of the unit cell TD is fairly comparable with the temperatures reported
should be perpendicular to the surface. The full width at half previously,4) indicating that the present result is not very
maximum (FWHM) of the rocking curve of the main different from the earlier results. The data confirm that there
diffraction (008) was very sharp. It was 0.04ꢃ, as shown in was little difference in ꢂ between the HP and AP samples
Fig. 1(d), which suggests a good crystallization. A photo- (even the residual resistivity ratio, RRR, remains almost
graph of the crystal is shown in the inset of Fig. 1(d). The comparable). Figure 3(b) shows ꢃðTÞ curves for both
crystal size was approximately 0:6 ꢆ 0:5 ꢆ 0:02 mm3.
samples. A comparable anomaly, which is presumably
The Tc of iron-based superconductors is argued to tightly associated with the SDW, is detected: the dꢃ=dT analysis
correlate with the degree of distortion of the FeAs4 indicates peaks at 144 and 145 K for the HP and AP samples,
tetrahedron; the As–Fe–As bond angle of the optimal Tc respectively, which are fairly close to TD by ꢂ. The small
superconductors is close to the angle of the regular gap between the TD values of the HP and AP samples
tetrahedron (109.5ꢃ).24,25) Alternatively, the ða=cÞ=ða0=c0Þ might have been caused by the small degree of As non-
ratio is close to 1.005.26) To investigate the lattice feature of stoichiometry.
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Y. GUO et al.
Fig. 4. (Color online) ꢂ vs T for polycrystalline BaFe2ꢁxPtxAs2 (HP) for
(a) x ¼ 0{0:06 and (b) x ¼ 0:08{0:20.
Figure 4 shows ꢂðTÞ curves for the pellets of BaFe2ꢁx
-
PtxAs2 (x ꢅ 0:20) prepared under the HP condition. A clear
decrease in TD with increasing Pt content is observed up to
approximately x ¼ 0:08 [Fig. 4(a)], while the room tem-
perature ꢂ remains almost constant. In addition to TD,
another independent drop likely caused by the super-
conductivity appears at a much lower temperature (T <
25 K). The Tc evolution is seen in Fig. 4(b); Tc rises to
24.2 K at x ¼ 0:08 with a transition width of less than 1.5 K.
For the further analysis of the superconducting transition, the
x dependences of ꢃ and ꢂ are plotted in Figs. 5(a)–5(j). At
the lowest Pt content, the sample shows a weak diamagnetic
signal at T < 10 K (the shielding fraction at 2 K is 14% of
the full superconducting fraction). The fraction increased
with increasing Pt content, and the largest fractions of
79–92% were observed at x ¼ 0:08{0:14. Further Pt doping
decreased the fraction, as well as Tc. The highest Tc by ꢃ was
23.9 K (x ¼ 0:08), which is comparable to the Tc by ꢂ.
For further investigation, Cp was measured between 2 and
300 K over the entire range of HP-synthesized polycrystalline
pellets of BaFe2ꢁxPtxAs2 (0 ꢅ x ꢅ 0:20). The Pt concentra-
tion dependence of Cp vs T is plotted in Fig. 6(a). The room
temperature Cp values are almost x independent at 130–135
Fig. 5. ꢃ vs T and ꢂ vs T for polycrystalline BaFe2ꢁxPtxAs2 (HP). The ꢃ
was measured at H ¼ 10 Oe.
J molꢁ1 Kꢁ1), approaching the Dulong–Petit limit [natom
ꢆ
3R ¼ ꢂ125 J molꢁ1 Kꢁ1, where R is the molar gas constant
and natom is the number of atoms per mole]. A sharp peak is
observed between 100 and 140 K, depending on x, which
is likely the result of losing entropy during the SDW
transition.27) To see the peak clearly, an expanded Cp=T vs
T plot is provided in the inset of Fig. 6(a); the peak gradually
shifts toward the lower temperature with increasing x. The
peak top position is ꢂ135:4, 115.9, and 109.7 K at x ¼ 0,
0.02, and 0.04, respectively. The Pt concentration depend-
ence was almost consistent with the SDW transition found in
the ꢂ and ꢃ measurements. Unfortunately, the peak became
broader and smaller with further Pt doping (x ꢇ 0:06) and
disappeared or was masked by the lattice contribution (Clatt).
Fig. 6. (a) Cp vs T for polycrystalline BaFe2ꢁxPtxAs2 (HP) with x ¼
0:02{0:20. An expanded view close to the peaks at x ¼ 0, 0.02, and 0.04 is
shown in the inset. (b) ðCp ꢁ ClattÞ=T vs T for BaFe1:92Pt0:08As2. The inset
shows Cp=T vs T2 for BaFe1:92Zn0:08As2 collected at 0 and 9 T for use as a
lattice contribution reference.
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Y. GUO et al.
Table I. Scale factors A and B, ꢄ, and NðEFÞ of BaFe2ꢁxPtxAs2.
x in
BaFe2ꢁxPtxAs2
ꢄ
NðEFÞ
A
B
(mJ molꢁ1 Kꢁ2
)
(states eVꢁ1 f.u.ꢁ1
)
0.00
0.04
0.06
0.08
0.14
0.16
0.18
0.25
0.95
0.98
0.94
1.03
0.99
0.95
1.05
1.00
1.04
1.02
1.02
0.97
1.03
1.02
0.97
1.00
5.61
11.8
21.2
22.9
17.8
15.1
11.8
9.78
1.98
4.16
7.49
8.09
6.28
5.33
4.17
3.45
The Cp peak at Tc is often very broad in Fe-based
superconductors. Thus, a careful subtraction of Clatt was
essential to analyze the superconducting transition. In order
to correctly subtract Clatt from the total Cp, we employed the
Zn
Cp of the Zn-doped BaFe1:92Zn0:08As2 as a reference
because it shows neither SC nor SDW transitions.28) The
inset of Fig. 6(b) shows the CpZn=T vs T2 for BaFe1:92
Zn0:08As2. The small H dependence indicates insignificant
-
magnetic factors in CpZn. The Sommerfeld constant, ꢄZn
,
Zn
was estimated by fitting to a linear function (Cp ¼ ꢄZnT þ
ꢅZnT3, where ꢅZn is a coefficient), as shown. A ꢄZn of 22.1
mJ molꢁ1 Kꢁ2 was estimated. The Clatt of BaFe2ꢁxPtxAs2
(x ¼ 0{0:20) could then be calculated using the formula
Zn
Clatt ¼ ACp ðBTÞ ꢁ ꢄZnT, where A and B are variable
parameters. The total Cp of BaFe2ꢁxPtxAs2 (x ¼ 0{0:20)
then appears to fit well to the formula Cp ¼ Clatt þ ꢄT
between 40 and 130 K with a constraint regarding the
entropy conservation. The refined values of the parameters
used are summarized in Table I.
Fig. 7. Synthesis condition dependence of ꢂ vs T for BaFe1:8Pt0:2As2:
(a) AP polycrystalline pellet, (b) HP polycrystalline pellet, and (c) HP single
crystal.
The specific heat jump at Tc of the optimally doped
superconductor (x ¼ 0:08) is plotted in Fig. 6(b) after the
subtraction of Clatt. There is a sharp anomaly at 24.5 K (x ¼ 0:20). The estimated NðEFÞ systematically varied
(midpoint of the thermodynamical transition), which in- within the expected range in the theoretical calculations.30)
dicates Tc. In addition, the thermodynamical transition does
We next investigated the synthesis condition dependence
occur close to Tc by ꢂ and ꢃ. The specific heat jump, of the ꢂ of BaFe1:8Pt0:2As2. Figures 7(a)–7(c) show ꢂðTÞ
ꢂC=ꢄTc, was ꢂ1:55, which is slightly larger than the weak- curves for the AP- and HP-prepared polycrystals and a single
coupling BCS limit of 1.43. The transition features suggest crystal (HP) of BaFe1:8Pt0:2As2. The AP sample shows a
that the present compound was a strongly coupled super- manifest drop at 24 K, while the HP samples (both the
conductor.
polycrystal and single crystal) show drops at much lower
In addition to the analysis of the SC jump, ꢄ was roughly temperatures (8–12 K). It appears that the synthesis condi-
estimated by applying an extrapolation method above Tc: the tion had a significant impact on the superconducting state.
largest ꢄ of 22.9 mJ molꢁ1 Kꢁ2 was obtained at x ¼ 0:08, This motivated us to reinvestigate the superconducting phase
which is within the expected range of 20–30 mJ molꢁ1 Kꢁ2 diagram of BaFe2ꢁxPtxAs2. The net Pt content of the AP
from a LDA+DMFT study.29) This suggests that a large sample should be somewhat overestimated because of the
fraction of the Fermi surface was involved in the super- formation of a Pt rich impurity. Thus, the inhomogeneous
conducting transition. The Pt doping led to an increase in distribution of Pt was most likely responsible for the
ꢄ from 5.61 mJ molꢁ1 Kꢁ2 (x ¼ 0) to 22.9 mJ molꢁ1 Kꢁ2 significant Tc variation. However, the role of the PtAs2
(x ¼ 0:08) in the under-doped region, while in the over- ingrowth has not been well explained in relation to the SC,
doped region, the doped Pt continuously decreased ꢄ to but is left for a future work.
9.78 mJ molꢁ1 Kꢁ2 (x ¼ 0:25). Note that analogous beha-
A revised phase diagram for BaFe2ꢁxPtxAs2 is proposed in
viors regarding ꢄ have been observed for the other 122-type Fig. 8. The data were obtained solely from the HP-prepared
superconductors.30,31) The quasi-particle density of states samples. The revision is significant, especially in the over-
NðEFÞ for polarized spins can be calculated from the ꢄ doped area; the revised diagram no longer shows an
using the formula ꢄ ¼ ð1=3Þꢆ2kBNðEFÞð1 þ ꢇefÞ,32) where unusually wide SC area with an almost invariable Tc, as
ꢇef is the electron–phonon coupling constant. For the first indicated in early studies,20) but rather a dome-like SC area.
approximation, ꢇef is set to 0, then NðEFÞ increases from The optimal Tc was achieved using a Pt doping of 4 at. %,
1.98 (x ¼ 0) to 8.09 (x ¼ 0:08) states eVꢁ1 f.u.ꢁ1, as shown which was comparable to adding 0.08 extra electrons per Fe.
in Table I, which suggests an appreciable increase in NðEFÞ. This number is common to other 3d and 4d element doped
Further Pt doping decreased NðEFÞ to 3.45 states eVꢁ1 f.u.ꢁ1 BaFe2ꢁxMxAs2 superconductors.33) In addition, the coex-
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Y. GUO et al.
Fig. 8. (Color online) Revised T–x phase diagram for BaFe2ꢁxPtxAs2.
istence of an AFM order and superconductivity was
confirmed in the under-doped region (x ꢅ 0:04); further
investigation of the coexistence is left for future work. In
order to secure the present results, we carefully repeated the
same experiment: the composition x was investigated more
than two times by XRD and EDX using independently
synthesized samples. We conclude that the present results
are certainly reproducible.
Hereafter, we focus on the optimal SC of BaFe2ꢁxPtxAs2.
The isothermal magnetization of the polycrystalline
BaFe1:92Pt0:08As2 was measured, as presented in Fig. 9(a).
The magnetic hysteresis loops at various temperatures were
quite symmetric, suggesting a bulk pinning nature. The
critical current density (Jc) at 2 K was estimated to be
ꢂ2:8 ꢆ 105 A/cm2 via the Bean model.34) The Jc was nearly
Fig. 9. (Color online) (a) Isothermal magnetization of optimal super-
conductor BaFe1:92Pt0:08As2 (HP) measured at various temperatures.
(b) Expanded view of the ꢂðTÞ measured for various magnitudes of ꢈ0H
(0, 0.1, 1, 3, 5, 6, and 7 T), and (inset) ꢈ0Hc2 vs T deduced from the ꢂ data.
comparable to that for a single crystal of BaFe1:92Pt0:08As2 relatively small ꢉGL and ꢊ are good indications of strong
(3:4 ꢆ 105 A/cm2 at 2 K, our sample) as well as to the other coupling of the SC.
Jc reported for doped 122-type superconductors (ꢂ105{106
In the sꢄ-wave model, the SC should be extremely
A/cm2).35,36) The result thus indicates that the polycrystal- sensitive to disorders.40–43) However, the disorders asso-
line nature of BaFe2ꢁxPtxAs2 does not have a prominent ciated with the doped Pt seem to have a minor impact on the
impact on the SC. Note that a comparable conclusion was SC, along with those by Co and Ni.44) The small RRR for the
stated for the HP-prepared 1111-type superconductor.37) The present Pt-doped superconductor suggests that the impurity
result could alternately indicate that the polycrystalline scatterings, including the disorder were substantial. How-
nature is not a major source of the revision of the phase ever, the influence on the SC was weak. Recent impurity
diagram.
studies for 122-type superconductors found that even the
For further investigation of the optimal SC, the H strongest scattering center such as Zn does not decrease Tc as
dependence of the ꢂðTÞ of BaFe1:92Pt0:08As2 was measured, much as expected from the s -wave model, not only in the
ꢄ
as shown in Fig. 9(b). With an increase in the magnitude of optimally doped but also in the under- and over-doped
the applied H, Tc steadily decreased. However, even in the superconductors.45,46) Although a 5d atom has an electronic
highest magnetic field, the drop remained very sharp, state that is distinguishable from that of a 3d atom, the doped
suggesting that the upper critical field (Hc2) was very high, Pt seems to only play a role comparable to Ni and Co in the
as reported elsewhere.19,20) The Hc2ðTÞ plot against T in the present Fe-based 122-type superconductor. Further theore-
inset to Fig. 9(b) roughly estimates ꢈ0Hc2ð0Þ to be 90 T, tical studies for the impurity potential and band interactions
where ꢈ0 is the permeability constant, by an extrapolation of Pt in the conducting Fe2As2 layer are needed to reveal the
method using the Werthamer–Helfand–Hohenberg relation role of the disorder associated with the dopant.
ꢈ0Hc2ð0Þ ¼ ꢁ0:693ðdHc2=dTÞ
Tc.38) This estimation is
T¼Tc
slightly larger than that of a single crystal BaFe1:9Pt0:1As2
4. Conclusion
(65 T).19) The Hc2ð0Þ estimation allows us to measure the
In summary, 5d atom doping to the AFM metallic
superconducting coherence length (ꢉGL) via the single-band compound BaFe2As2 was studied. A systematic doping
Ginzburg–Landau relation, ꢉGL ¼ ½ꢃ0=2ꢆHc2ð0Þꢈ1=2, where control was successfully achieved using a HP method, and
ꢃ0 is the flux quantum. The ꢉGL for BaFe1:92Pt0:08As2 is the SC properties were characterized using XRD, EDX, and
ꢁ
roughly ꢂ25 A, which is nearly comparable to that for by measuring ꢂ, ꢃ, and Cp. The results confirmed the
39)
ꢁ
BaFe1:8Co0:2As2 (ꢂ30 A). The Ginzburg number, ꢊ, is existence of a dome-like SC region for x values of 0.02 to
calculated to be 4:84 ꢆ 10ꢁ4 using the formula ꢊ ¼ 0.20 in the thermal phase diagram of BaFe2ꢁxPtxAs2, as well
2
2
ðꢆꢇ0kBTcꢈ0=2ꢉGLꢃ0 Þ , where ꢇ0 is the London penetration as what was observed for the 3d and 4d doped BaFe2As2.
depth (ꢂ200 nm)37) and kB is the Boltzmann constant. The The optimal Tc of 24.2 K (x ¼ 0:08) was not far from the Tc
064704-6
#2012 The Physical Society of Japan

J. Phys. Soc. Jpn. 81 (2012) 064704
FULL PAPERS
Y. GUO et al.
(2010) 157004.
of the other doped BaFe2As2, which suggests that the
electron–phonon coupling was insignificant. Note that an
unusually wide SC area with an almost invariable Tc was
not observed, unlike the findings of earlier studies, which
suggest that an overestimation of the net Pt content might
have complicated the earlier results. Clearly, further doping
beyond the optimal Pt concentration continuously decreases
Tc toward zero. The present result thus significantly revises
the phase diagram proposed for BaFe2ꢁxPtxAs2, suggesting a
result resembling that of Pt-doped SrFe2As2 in which SC
emerged in a comparable dome-like region with an optimal
Tc of 16 K.47) Additional studies of the role played by the
disorders associated with Pt may further increase the optimal
Tc of the 122-type superconductor.
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Acknowledgments
We thank Dr. Miyakawa for the high-pressure experi-
ments and Mr. Shirako for the EDX study. This research was
supported in part by the World Premier International
Research Center (WPI) Initiative on Materials Nanoarchi-
tectonics from the Ministry of Education, Culture, Sports,
Science and Technology, Japan; Grants-in-Aid for Scientific
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#2012 The Physical Society of Japan