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Suppression of superconductivity in layered Bi4O4S3 by Ag doping

We report X-ray diffraction, magnetization and transport measurements for polycrystalline samples of the new layered superconductor Bi4-xAgxO4S3 (0<x<0.2). The superconducting transition temperature (TC) decreases gradually and finally suppressed for x>0.10. Accordingly, the resistivity changes from a metallic behavior for x<0.1 to a semiconductor-like behavior for x>0.1. The analysis of Seebeck coefficient shows there are two types of electron-like carriers dominate at different temperature regions, indicative of a multiband effect responsible for the transport properties. The suppression of superconductivity and the increased resistivity can be attributed to a shift of the Fermi level to the lower-energy side upon doping, which reduces the density of states at EF. Further, our result indicates the superconductivity in the parent Bi4O4S3 is intrinsic and the dopant Ag prefers to enter the BiS2 layers, which may essentially modify the electronic structure.


I. INTRODUCTION
To study the layered materials is one of the strategies for exploring new superconductors. The discoveries of superconductivity with high superconducting transition temperature (T C ) in cuprates and oxypnictides have given us good examples. 1,2 In those compounds, the CuO 2 layer or the Fe 2 An 2 (An=P, As, Se, Te) layer plays a central role in determining the superconductivity. Very recently, Mizuguchi et al., reported the superconductivity in layered Bismuth oxy-sulfide Bi 4 O 4 S 3 with T C ~ 4.5 K. 3 Structurally, Bi 4 O 4 S 3 is composed of a stacking of Bi 4 O 4 (SO 4 ) spacer layers and two BiS 2 layers. 3 Band structure calculation indicates the Fermi level lies within the bands mainly originating from the Bi 6p orbitals within the BiS 2 layer. So the BiS 2 layer was believed to be a basic unit as the CuO 2 layer in cuprates or the Fe 2 An 2 layer in oxypnictides. 3 Since the discovery of superconductivity, Bi 4 O 4 S 3 has attracted increasing attention because it could stimulate extensive investigations on the BiS 2 -based layered compounds for exploring new high-T C superconductors. [4][5][6][7][8][9] The Hall coefficient measurements indicate an exotic multi-band behavior and the charge carriers are mainly electron-type, 4 which is consistent with our Seebeck coefficient measurement. 5 Our magnetization measurement implies that Bi 4 O 4 S 3 is a typical type-II superconductor. 5 Although the characterizations based on the polycrystalline samples with impurities, such as Bi and Bi 2 O 3 , indicated a bulk superconducting behavior, [6][7] it is questionable whether the superconductivity in Bi 4 O 4 S 3 is intrinsic because the almost pure sample synthesized by high pressure doesn't exhibit superconductivity down to 2K. 8 Further, chemical doping that usually serves as a powerful way to study the underlying physics of superconductivity, however, has not been reported up to date. Here we report our systematical structural, magnetic, electrical and thermal transport measurements for Ag doped Bi 4-x Ag x O 4 S 3 (0≤x≤0.2). The lattice shrinks gradually with Ag doping. The T C is reduced and finally suppressed for x>0.1. Accompanying the suppression of superconductivity, the resistivity is increased remarkably and behaves as a semiconductor for x>0.1, attributable to a shift of the Fermi energy (E F ) demonstrated by the analysis of Seebeck coefficient. For all samples, the Seebeck coefficient is linearly temperature dependent at both low-and high-temperature regions with different slopes, indicating a multiband behavior. Moreover, our results demonstrate that the superconductivity in Bi 4 O 4 S 3 is intrinsic and the dopant Ag enters the BiS 2 layer rather than the impurity Bi phase.

II. EXPERIMENTAL DETAILS
The Ag, Bi, S and Bi 2 O 3 powders with high purity in stoichiometric ratio were fully mixed and ground, and then pressed into pellets. 5 The pellets were sealed in an evacuated silica tube. The tube was heated at 450°C for 10 hours, and then at 510 °C for another 10 hours. The obtained pellets were ground again and the above treatment was repeated. The room-temperature crystal structure and lattice constants were determined by powder X-ray diffraction (XRD) (Philips X'pert PRO) with Cu K α radiation. The electronic and thermal transport measurements were performed in a Quantum Design physical property measurement system (PPMS), and the magnetization measurement was performed on a superconducting quantum interference device (SQUID) system.

III. RESULT AND DISCUSSION
In Fig. 1(a), the XRD patterns are shown for the polycrystalline Bi 4-x Ag x O 4 S 3 (0≤x≤0. 2) samples. The Bragg diffractions can be indexed using the tetragonal structure with the space group of I4/mmm. 3 The diffractions from the impurity Bi were also observed and one of them was marked by the asterisk. The diffractions from the Bi 4-x Ag x O 4 S 3 show a dependence on the Ag concentration x, while those from Bi don't. For instance, the (110) peak of Bi 4-x Ag x O 4 S 3 shifts clearly towards the higher angles as x increases, as seen in Fig. 1(b). As shown in Fig. 1(c), however, the position and intensity of the Bi peak are unchangeable with x. This indicates that the dopant Ag indeed enters the Bi 4 O 4 S 3 lattice. The lattice parameters for Bi 4-x Ag x O 4 S 3 are obtained by a standard Rietveld refinement for the powder XRD patterns using the Rietica software. 10 As a result, both the lattice constant a (=b) and c decrease with increasing Ag content, as plotted in Fig.   2.
The temperature dependence of the resistivity ρ(T) for Bi 4-x Ag x O 4 S 3 was measured by a standard four-probe method under a zero magnetic field. As displayed in Fig. 3(a), the metallic ρ(T) shape at x=0 changes gradually to a semiconducting one when x > 0.10, accompanying a increase of the entire resistivity. Meanwhile, the superconducting transition temperature is reduced with increasing x and non superconductivity can be found down to 2K for x>0.1 (Fig. 3(b)). The magnetic susceptibility 4πχ(T) is displayed in Fig. 4. In good agreement with the resistivity result, the superconducting transition temperature decreases as x increases. Simultaneously, the shielding volume fraction, determined as the value of −4πχ at 2 K, reduces from -0.45 emu/(cm 3 Oe) for x = 0 to -3×10 -4 emu/(cm 3 Oe) for x = 0.7, above which the superconductivity, even if exists, lost its bulk characteristics. The superconducting transition temperatures defined by different ways will be discussed in the following text.
The Seebeck coefficient S(T) is plotted in Fig. 5(a) for selected samples. The S(T) is negative in the entire temperature range up to 320K, indicating the major carriers are electron-like for all samples. As shown in the inset of Fig. 5(a), the Seebeck coefficient at 320K, S 320K , increases as x increases and a sharp enhancement happens at ~ x=0.10, consistent with the increasing resisitivity and the appearance of the semiconductor-like behavior for x>0.1 (Fig. 3). Generally, the S(T) consists of three different parts, the diffusion term, the spin-dependent scattering term and the phonon-drag term due to electron-phonon interaction. 11 The spin-dependent contribution is probably very weak since there are no magnetic elements in Bi 4-x Ag x O 4 S 3 . The phonon-drag effect gives ~ T 3 dependence for T«Θ D , and ~1/T for T ≥ Θ D (where Θ D is the Debye temperature), and thus a phonon-drag peak in S(T) at ~ Θ D /5. 11 The Θ D is 192K for the parent sample derived from heat capacity measurement. 6 However, the expected peak structure at ~ Θ D /5 = 38K is absent and the high-temperature S(T) doesn't obey the ~1/T expression, indicating the phonon contribution is considerably weak in the current samples. Usually, the diffusion term gives rise to a linearly temperature dependent S(T) at low temperatures, 11  at both low and high temperatures cross at some temperatures, marked as T*. The T* is found to be almost constant for x<0.10 within the error bars, but reduces rapidly above x=0.10, as shown in the inset of Fig. 5(b). Therefore, it indicates there exist two types of predominant electron-like charge carriers, one is active at low temperatures, the other at high temperatures. 12 As x increases, the high-temperature charge carriers get more and more dominant, while the low-temperature one becomes weaker, shifting the T* to the lower temperatures. This behavior is possibly related with the p x and p y orbitals consisting the band structure near the E F . 3 This result agrees in principle with the multiband feature of the Hall coefficient R H (T) data. 4 We are now focusing on the low-temperature linearly temperature dependent S(T) since the superconductivity occurs at the low temperatures. Given a single band mode with an energy-independent relaxation time, the slope dS/dT can be written as where k B is the Boltzman constant, E F the Fermi energy corresponds to band maximum or minimum. The E F can be estimated roughly using the slope of the low-temperature linear S(T), dS/dT. So, a linear fitting was applied to all data available below 50K. The value of dS/dT is found to increase rapidly with increasing x when x < 0.05, and then saturates for x ≥ 1.0 (see Fig. 6). It Therefore, the suppression of superconductivity is mainly attributable to the modification of electronic structure, for instance, the shift of E F mentioned above, though detailed mechanisms need further efforts, particularly from the theoreticians.

IV. CONCLUSION
In summary, polycrystalline Bi 4-x Ag x O 4 S 3 (0≤x≤0.2) samples were prepared, and physical properties investigated. The superconductivity is suppressed by x=0. 10