BiS2 - based superconductivity in F-substituted NdOBiS2
Satoshi Demura,1,2 Yoshikazu Mizuguchi,1,3 Keita Deguchi,1,2 Hiroyuki Okazaki,1
Hiroshi Hara,1,2 Tohru Watanabe,1,2 Saleem James Denholme,1 Masaya Fujioka,1
Toshinori Ozaki,1 Hiroshi Fujihisa,4 Yoshito Gotoh,4 Osuke Miura,3 Takahide Yamaguchi,1
Hiroyuki Takeya,1 and Yoshihiko Takano1,2
E-mail : Demura.Satoshi@nims.go.jp
1National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
2University of Tsukuba, Graduate School of Pure and Applied Sciences, Tsukuba, Ibaraki
3Tokyo Metropolitan University, Graduate School of Science and Engineering, Hachioji,
Tokyo 192-0397, Japan
4National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5,
1-1-1, Higashi, Tsukuba 305-8565, Japan.
We have successfully synthesized a new BiS2-based superconductor NdOBiS2 with
F-doping. This compound is composed of superconducting BiS2 layers and blocking NdO
layers, which indicates that the BiS2 layer is the one of the common superconducting layers
like the CuO2 layer of cuprates or Fe-As layer of Fe-based superconductors. We can obtain
NdO1-xFxBiS2 with bulk superconductivity by a solid-state reaction under ambient pressure.
Therefore, NdO1-xFxBiS2 should be the suitable material to elucidate the mechanism of
superconductivity in the BiS2-layer.
Quite recently, Y. Mizuguchi et al. reported superconductivity in the novel BiS2-based
superconductor Bi4O4S3 with a superconducting transition temperature (Tc) of 8.6 K . This
material has a layered structure composed of superconducting BiS2 layers and blocking layers
of Bi4O4(SO4)1-x, where x indicates the defects of SO42- ions at the interlayer sites. The
stacking structure of the superconducting and blocking layer is analogous to those of high-Tc
cuprates [2-5] and Fe-based superconductors [6-14]. In both systems, their Tc can be enhanced
by changing the blocking layers. In order to enhance the Tc of the BiS2-based superconductor,
the investigation of exchanging the blocking layer will be of great interest.
Soon after the discovery of Bi4O4S3, a new BiS2-based superconductor LaO1-xFxBiS2 was
reported . This compound consists of the same superconducting layer but with different
blocking layers compared to that of Bi4O4S3. Furthermore, the superconductivity shows a
relatively high Tc of 10.6 K. This fact suggests that the BiS2 layer is the common
superconducting layer, and the blocking layer contributes to the enhancement of the Tc in this
system. LaO1-xFxBiS2 shows a small superconducting volume fraction for ambient pressure
but achieves bulk superconductivity under high pressure. This result suggests that high
pressure would promote F substitution. Therefore, chemical pressure for the exchange of La
by Nd possibly induces the promotion of F substitution. Here, we report that a new
BiS2-based superconductor NdO1-xFxBiS2 (x = 0.1 - 0.7) can be obtained by a solid-state
reaction under ambient pressure.
The polycrystalline samples of NdO1-xFxBiS2 (x = 0.1 - 0.7) were prepared by a
solid-state reaction. Mixtures of Bi grains, Bi2S3 grains, Nd2O3 powders, Nd2S3 powders, and
NdF3 powders with nominal compositions of NdO1-xFxBiS2 (x = 0.1 - 0.7) were ground,
pelletized, and sealed into an evacuated quartz tube. The tube was heated at 800 °C for 10 h.
The obtained samples were characterized by X-ray diffraction with Cu-Kα radiation using the
θ-2θ method. The temperature dependence of magnetization was measured by a
superconducting quantum interface device (SQUID) magnetometer with an applied field of 1
Oe. The resistivity measurements were performed using the four-terminal method from 300 to
2 K. In order to investigate an upper critical field of NdO1-xFxBiS2, the temperature
dependence of resistivity between 10 and 2 K was measured under a magnetic field of up to 7
Figure 1(a) shows the X-ray diffraction profile for the powdered samples of
NdO1-xFxBiS2 (x = 0.1 - 0.7). With the exception of a few minor peaks relating to impurity
phases, all of the peaks can be characterized to space group P4/nmm. The nominal x
dependence of the lattice constants a and c is summarized in Fig. 1(b) and (c). These lattice
constants were estimated from 2θ values and Miller indices. The a lattice constant exhibits
little change with increasing x while the c lattice constant dramatically decreases. The
decrease of the lattice parameter c indicates that F substitutes O since the ionic radius of F is
smaller than that of O.
Figure 2(a) shows the temperature dependence of magnetic susceptibility for
NdO1-xFxBiS2 (x = 0.1 - 0.7). A superconducting transition is observed for all samples. These
samples with x = 0.1 - 0.6 exhibit a high shielding volume fraction exceeding 90 % (2 K,
ZFC), indicating the appearance of bulk superconductivity in those samples. The x
dependence of Tc is plotted in Fig. 2(b). The Tc varies like a bell curve with increasing x. The
NdO0.7F0.3BiS2 sample exhibits the optimal Tc of all samples.
The temperature dependence of the resistivity for NdO0.7F0.3BiS2 is shown in Fig. 3.
Resistivity slightly decreases between 300 and 130 K. Below 130 K, resistivity gradually
increases and the superconducting transition appears around 6 K. This behavior, where the
resistivity increases with decreasing temperature is also observed in LaO1-xFxBiS2. The onset
and zero-resistivity temperatures are estimated to be Tconset = 5.6 K and Tczero = 5.2 K,
respectively. Figure 4(a) shows the temperature dependence of the resistivity from 10 to 2 K
under a magnetic field. The Tc of NdO0.7F0.3BiS2 decreases with increasing magnetic field.
The upper critical field (Bc2) and the irreversibility field (Birr) are plotted in Fig. 4(b). The
Bc2(0) was estimated to be 5.2 T with the data points at 0.4 ~ 2.0 T using the WHH theory,
which gives Bc2(0) = -0.69Tc(d Bc2/dT)|Tc .
In conclusion, the BiS2-based superconductor NdO1-xFxBiS2 (x = 0.1 - 0.7) has been
successfully synthesized by a solid-state reaction method. Chemical pressure promotes the F
substitution, which originated from the exchange of La by Nd. As a result, NdO1-xFxBiS2 with
bulk superconductivity can be obtained under ambient pressure. These results demonstrate
that the BiS2 layer is the common superconducting layer. Thus, if we synthesize materials with
the stacking structure consisting of the BiS2 layer and other blocking layers, new
superconductors with the BiS2-layer would be discovered.
This work was partly supported by a Grant-in-Aid for Scientific Research from the
Ministry of Education, Culture, Sports, Science and Technology (KAKENHI).
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(a) X-ray diffraction patterns of NdO1-xFxBiS2 (x = 0.1 - 0.7). Filled circles indicate the peaks
of the impurity phases. (b) and (c) show the nominal x dependence of the lattice constants a
and c , respectively.
(a) The temperature dependence of the magnetic susceptibility for NdO1-xFxBiS2 (x = 0.1 -
(b) The F concentration dependence of the superconducting transition temperature for
NdO1-xFxBiS2 (x = 0.1 - 0.7).
The temperature dependence of resistivity for NdO0.7F0.3BiS2 between 300 and 2 K.
(a) The temperature dependence of the resistivity from 10 to 2 K under magnetic fields
(b) Magnetic field – temperature phase diagram for NdO0.7F0.3BiS2. The dashed lines are liner
fits to the data.
Fig. 1(a). S. Demura
Intensity (arb. unit)
x = 0.1
x = 0.2
x = 0.3
x = 0.4
x = 0.5
x = 0.6
x = 0.7
Fig. 1(b). S. Demura
Fig. 1(c). S. Demura
Fig. 2(a) S. Demura
Magnetic susceptibility (emu / cm3)
● x = 0.7
▽ x = 0.6
□ x = 0.5
× x = 0.4
＋ x = 0.3
△ x = 0.2
○ x = 0.1
H = 1 Oe
Fig. 2(b) S.Demura
Fig. 3 S. Demura
Tc ～ 5.6 K
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Fig. 4 S. Demura