Low-Temperature Enhancement in the Upper Critical Field of Underdoped LaO1−xFxBiS2 (x = 0.1–0.3)

Abstract

Recently, a series of new superconductors containing BiS2 layers has been discovered.1–6) The BiS2-based superconductors have many similarities with cuprate7) and Fe-based superconductors.8,9) The superconductivity is introduced by carrier (electron) doping into their insulating parent compounds. They have layered crystal structure and the superconducting BiS2 planes could be sandwiched by various charge-reservoir spacer layers, e.g., Bi4O4(SO4)1 x and LnO1 xFx (Ln = rare earth elements). Because of these features, it is expected that there exist a variety of BiS2based superconductors, which will provide an opportunity to investigate exotic superconducting properties. For LaO1 xFxBiS2, the phase diagram of the superconducting transition temperature Tc vs x reported until now down to 2K is shown in Fig. 1 for samples prepared by both the conventional solid state reaction method (as-grown) and the high-pressure-annealing method10) (circles and squares). Tc seems to have a dome structure with a maximum at x 0:5. It has been found that Tc increases up to around 10K in applied pressure or after annealing at high pressure,10,11) although the origin of the Tc enhancement has not been clarified yet. In this article, we investigate the superconducting properties of the as-grown samples in the underdoped region (x < 0:4), which have not been studied yet because temperatures below 1K is needed. Polycrystalline samples of LaO1 xFxBiS2 were prepared by the conventional solid-state reaction method as described in Ref. 2. The obtained samples were characterized by X-ray diffraction with Cu Ki radiation using the 2 –a method. Samples with typical weight of 0.1–0.2 g have been used for ac susceptibility measurements. The real part 0 and imaginary part 00 of ac susceptibility were measured by a mutual inductance method down to 0.15K using a commercial dilution refrigerator. The amplitude of the ac field was 0.3Oe-rms. Figure 2 shows the temperature dependences of 0 and 00 measured in zero field for samples with different F concentrations. We have confirmed that superconducting phase continues down to x 1⁄4 0:1. The data indicate the superconducting volume fraction is larger than 40% for all samples, reflecting the bulk nature of the superconductivity. The value of Tc is defined as the temperature at which 0 reaches 0.1% of the expected full Meissner signal. The obtained data are also plotted (solid triangles) in Fig. 1. Combining with the present data, it becomes clearer that Tc of as-grown samples also has a dome structure with the maximum at x 1⁄4 0:5 and decreases with decreasing F concentration down to x 1⁄4 0:2. On the contrary, Tc at x 1⁄4 0:1 is higher than that at x 1⁄4 0:2. Although we have not clarified the reason for this anomaly, this may be associated with structural instability which probably exists in low x region. Our recent single-crystal structure analysis of LaOBiS2 indicates that the crystal structure is subtly distorted from the tetragonal P4=nmm.12) Note that the existence of anharmonic ferroelectric soft phonons and one-dimensional charge density wave instability have been pointed out by first-principles calculations.13,14) Through the electron–phonon counpling, the superconducting properties in the low x region may be affected by the lattice instability. We have performed the ac susceptibility measurements also in applied fields to determine the superconducting phase boundary. Figure 3 shows the temperature dependence of 0Hc2 for various x determined by temperature and field dependent measurements of 0. The most salient feature in Fig. 3 is that Hc2ðTÞ shows a extraordinary upward curvature with decreasing temperature, especially for x 1⁄4 0:3. This highly unusual behavior is completely different from that of the standard model by Werthamer–Helfand–Hohenberg (WHH);15,16) the dashed lines in Fig. 3 are fits to the data near TcðH 1⁄4 0TÞ. The slope jdð 0Hc2Þ=dTjT1⁄4Tc is 10.082, 14