Soon after the discovery of high-temperature superconductors in perovskite copper-oxides (cuprates)2,3, another type of high-temperature perovskite superconductors was found in bismuth-oxide Ba1-xKxBiO3, (bismuthates). Such a finding ignited a new excitement in the community 4-6. Although they possess the same perovskite structure, the superconductivity in these materials develops from different ordered states upon hole doping. The superconductivity in cuprates emerges in the proximity of an antiferromagnetic (AFM) ordered state7, while the superconductivity in bismuthates appears near a charge-density-wave (CDW) ordered state8-10. It suggests that the suppressed ordered state of the AFM or the CDW is closely associated with the emergence of superconductivity in these superconductors. In the past few decades, many experimental works on the perovskite cuprates and bismuthates have been reported8-23. However, understanding their high-temperature superconductivity in a unified way still stands as a grand challenge due to the seemingly diversified underlying physics of electron correlations, which fundamentally determines the mechanism of the high-temperature superconductivity24,25. Therefore, searching for the universal trends through the same tuning method to investigate the different types of high-Tc superconducting oxides with a similar crystal structure is expected to be a crucial step toward a better understanding their universal mechanism of superconductivity.
Pressure is an effective method to tune superconductivity beyond chemical doping, because it can dramatically manipulate the crystal and corresponding electronic structures by compressing the lattice without adding the complexity of chemistry. As a result, it has been widely adopted as an independent control parameter in the studies on superconductivity. The pressure-induced variations of the superconducting transition temperature (Tc) have been observed in many of the layered-perovskite cuprates26-33. Intriguingly, a pressure-induced quantum phase transition from a superconducting state to an insulating-like state was found recently in cuprate superconductors1, which presents universally in the samples - regardless of doping levels and numbers of the copper-oxide plane in a unit cell. These experimental results renewed the traditional knowledge that applying pressure usually enhances the bandwidth and makes the material more metallic34. The central questions for this surprising phenomenon are: What is the intrinsic root of driving such a QPT from a superconducting to an insulating-like state, and what is the determining factor in stabilizing high-Tc superconductivity? In this study, we performed the combined measurements of high-pressure transport, x-ray diffraction and absorption for the perovskite bismuthate Ba1-xKxBiO3, hoping to reveal some possible common physics of these two hole-doped superconducting perovskite oxides.
The single crystals were grown by a modified electrochemical method [see Supplementary Information - SI]. The single crystal diffraction measurements on one of our superconducting samples, Ba0.6K0.4BiO3, indicate that it crystallizes in a cubic unit cell at room temperature with lattice parameters a = 4.340 Å and space group Pm-3m (No.221), as shown in Fig.1a-1d. The diffraction spots for the (0kl), (h0l) and (hk0) zones demonstrate that the sample is of high quality. Since Ba1-xKxBiO3 is sensitive to air and moisture, we loaded each of the samples into diamond anvil cell (DAC) in a glovebox, then took the DAC from the glovebox and performed high-pressure resistance measurements.
As shown in Fig.1e, the plot of resistance versus temperature measured at 1.2 GPa shows a sudden drop at ~ 31 K and subsequently reaches a zero-resistance state. Applying a magnetic field, the drop shifts to the lower temperature, signaling a superconducting transition. The superconducting transition was confirmed by the measurements of the magnetic moment in zero-field cooling (ZFC) and field-cooling (FC) modes, which show diamagnetic throws at ~ 31 K (Fig.1f). These results are in accordance with the results reported previously35- 38.
Next, we conducted the high-pressure resistance measurements on the superconducting Ba1-xKxBiO3 samples with different doping concentrations. Figure 2 shows the results of resistance versus temperature obtained at different pressures for the x=0.40 sample with Tc = 31 K (Fig.2a), the x=0.43 sample with Tc = 28.5 K (Fig. 2b), the x=0.52 sample with Tc = 18.1 K (Fig.2c) and the x=0.58 sample with Tc = 14.1 K (Fig.2d). It is found that these samples exhibit the same high-pressure behavior: Tc displays slight variation up to the first critical pressure (Pc1) and then exhibits a monotonous decrease with increasing pressure until the second critical pressure (Pc2) where the superconducting state is fully suppressed, and subsequently an insulating state appears. We repeated the measurements on new samples and found that the results were reproducible (see SI).
The pressure-Tc phase diagrams for the measured samples with different doping concentrations are summarized in Fig.3a, which is established based on Tc versus pressure for each of the samples (Fig.3b-3e). It is seen that more doping reinforces the decrease of the Tc value, until a universal quantum phase transition from a superconducting (SC) state to an insulating (I) state. Intriguingly, it can be seen that the higher the doping concentration, the higher the critical pressure required for driving the QPT (Fig. 3f).
It is generally believed that the emergence of the QPT in materials is associated with the change in crystal and electronic structures. We performed high-pressure synchrotron X-ray diffraction measurements at 300 K for the x=0.40 sample at beamline 4W2 at the Beijing Synchrotron Radiation Facility to clarify the possible microscopic origin. No structural phase transition is found in the pressure range up to 37.4 GPa (see SI), confirming that the observed QPT in Ba1-xKxBiO3 is not associated with any pressure-induced structural phase transition. Furthermore, we note that, although the declining rate of the lattice parameter a varies slightly in the pressure range covering the entire superconducting regime, the response of Tc to pressure displays a drastic change (Fig.4 a and 4b), i.e., Tc exhibits slight variation below Pc1, while in the range between Pc1 and Pc2 displays a monotonous decrease, where the lattice constant dependence of pressure shows a slower decreasing tendency. Interestingly, the robust superconductivity below Pc1 in the compressed bismuthate superconductors is entirely different from what is seen in the copper oxide superconductors, whose Tc is sensitive to the applied pressure1,39. Such an unusual superconducting behavior observed in the compressed bismuthate superconductors has been only found in some elemental and alloy superconductors40-42. This is the first time to discover the Tc ‘plateau’ in pressurized high-temperature superconductors, the underlying physics of which calls for further investigations. In order to focus on the QPT, a key theme of this study, we will report our investigation on the exotic Tc ‘plateau’ in a separated paper43.
To understand the observed phenomenon, we plotted the pressure dependence of resistance (R/R1.2 GPa) that was measured just above the superconducting transition temperature in Fig.4c. We found that R/R1.2 GPa began to increase at the pressure above Pc1, where Tc starts to decrease (Fig.4a). Upon further compression, the resistance rises rapidly (see inset of Fig.4c), in the meantime, Tc declines dramatically. This suggests that the insulating phase starts at Pc1, and then gets more and more prevailed by elevated pressure between Pc1 and Pc2. When pressure is close to Pc2, superconducting puddles immerse in an insulating background, giving rise to a weak link superconductivity. Our measurements of the superconducting transition under different magnetic fields support the existence of such a weak link superconductivity - we found that the sample subjected to a pressure between Pc1 and Pc2 loses its zero-resistance state, and its onset Tc can be destroyed by a relatively small magnetic field (see SI).
To investigate the origin of the QPT found in the bismuthate superconductors, we conducted high-pressure X-ray absorption measurements for the x=0.40 and 0.52 samples at beamlines of Dynamic and 15U at Shanghai Synchrotron Radiation Facility, respectively. Representative LIII-edge X-ray absorption spectra of the two samples collected at different pressures can be found in SI. The ambient-pressure result indicates that the two Ba1-xKxBiO3 samples bear a mixed valence state, in good agreement with the results reported44-46. To show the pressure-induced Tc change with the evolution of the mean valence (n) of Bi ions in Ba1-xKxBiO3, we extracted pressure dependence of n for the x=0.40 sample (Fig.4d). It is found that n increases monotonously from 3.8+ at ambient pressure to 5+ at ~Pc2, where the superconducting-insulating transition takes place. Further increasing pressure up to 41.7 GPa, the value of n maintains at 5+. Similar results are also observed from the x=0.52 sample (see SI). Considering the unsteady nature of the perovskite crystal structure of the Ba1-xKxBiO3 superconductors, we propose that the transformation from a mixed valence state to a single-valence state of Bi ions is associated with the superconducting-insulating transition. Thus, a possible root for the transition could be suggested that applying pressure distorts the Bi-O bond and thus imposes the electron configuration of the Bi ions from Bi+3(5d106s26p0) to Bi+5(5d106s06p0) 47-49. It is noted that, when Bi+3 ions no longer exist in the sample, the system is compelled to enter an anomalous insulating state, instead of a metallic state as predicted by the band structure theory8. This implies that the electronic state of Bi+3 plays a vital role in stabilizing superconductivity.
The observed quantum criticality phenomenon in the compressed Ba1-xKxBiO3 superconductors is similar to what is seen in the compressed cuprate superconductors 1. To investigate the possibility of the pressure-induced valence change in bismuth-bearing cuprates, we performed high-pressure X-ray absorption measurements on the hole-doped superconducting Bi2Sr2CaCu2O8+d up to 48.2 GPa, where the sample moves in an insulating-like state1. The pressure dependence of main valence (n) of Bi ions shows that, unlike the high-pressure behavior observed from Ba1-xKxBiO3, no dramatic valence change was found (see SI). The deep-inside physics of the pressure-induced insulating state in these two types of superconductors calls for further investigations.
The high-pressure phenomenon found in both hole-doped copper- and bismuth-oxide perovskite superconductors is highly unusual, renovating our knowledge about the high-Tc superconductors that they ought to turn a metallic state after the superconductivity is fully suppressed34. Our results indicate that the hole-doping effect on these superconducting materials is damaged by applying pressure. Moreover, an important commonality is also revealed from the high-pressure behavior of these two types of superconducting oxides - the heavier the hole doping, the higher the pressure required to peter out the hole-doping effect on stabilizing the superconductivity and finally realize the QCT. However, the QPT found in these two superconducting oxides may have different origins. One of the possible scenarios for the observed QPT is proposed here. For cuprates, the pressure may compel the electrons from sites of the apical oxygen to move to the oxygen sites in the superconducting CuO2 plane of cuprates7, as a result of occupying the hole sites in the plane. Such a pressure-induced ‘traffic re-jam’ of electrons may make the material to be insulated again16. While for the bismuthates, the pressure-induced valence change possibly counteracts the doping effect on stabilizing the superconductivity, finally expediting the system to enter the insulating state.
In order to check our proposed scenario, we performed pressure-releasing experiments on the compressed Ba0.57K0.43BiO3 sample. We found that the compressed sample in the insulating state recovered to its superconducting state, initially at 13.7 GPa, and reached a zero-resistance state at 5.4 GPa [see SI]. More significantly, we found that releasing the pressure makes the mean valence of Bi ions recovered from 5+ where the sample is in an insulating state to 3.8+ where the sample is in a superconducting state (Fig. 4d). The phenomenon of the reversible superconductivity is like what is found in the perovskite cuprate superconductors1, indicating that the pressure-induced insulating state in both of the oxides is metastable. These results further support our explanation for the microscopic physics of the superconducting-insulating transition: the pressure-induced valence change from Bi+3 to Bi+5 is the fundamental root for realizing the superconducting-insulating transition in Ba1-xKxBiO3. The consequence of decompression releases the pressure-induced metastable distortion of the Bi-O bond50 and thus allows the insulating state to return to the superconducting state.
The universal high-pressure behavior about the QPT and their peculiarities found in these two types of perovskite superconducting oxides highlight that the high-Tc superconductivity of the perovskite superconductor are generally driven by the interplays among the unstable lattice, orbital, spin, and charge degrees of freedom in the superconducting ground state. As no detailed information in available literature could appropriately explain the observed high-pressure behaviors found in these hole-doped perovskite superconducting oxides in a unified way, it deserves further investigations with other advanced experimental probes and sophisticated theoretical studies.