Abstract
Recent experimental observations have showed some signatures of superconductivity close to 80 K in La3Ni2O7 under pressure and have raised the hope of achieving high-temperature superconductivity in bulk nickelates. However, a zero-resistance state—a key characteristic of a superconductor—was not observed. Here we show that the zero-resistance state does exist in single crystals of La3Ni2O7−δ using a liquid pressure medium at up to 30 GPa. We also find that the system remains metallic under applied pressures, suggesting the absence of a metal–insulator transition proximate to the superconductivity. Moreover, analysis of the normal state T-linear resistance reveals a link between this strange-metal behaviour and superconductivity. The association between strange-metal behaviour and high-temperature superconductivity is very much in line with other classes of unconventional superconductors, including the cuprates and Fe-based superconductors. Further investigations exploring the interplay of strange-metal behaviour and superconductivity, as well as possible competing electronic or structural phases, are essential to understand the mechanism of superconductivity in this system.
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Acknowledgements
We acknowledge fruitful discussions with C. Cao, H. Lin, F. Steglich and G. Zhang. We also thank S. Zhang and Y. Peng for sharing their unpublished single-crystal X-ray diffraction results, as well as Z. Dong, Y. Wang and Z. Chen for sharing their unpublished transmission electron microscopy results. Work at Zhejiang University was supported by the National Key R&D Program of China (grant nos. 2022YFA1402200 and 2023YFA1406100), the Key R&D Program of Zhejiang Province, China (grant no. 2021C01002), the National Natural Science Foundation of China (grant nos. 12034017, 12374151, 12174332, 12204408 and 12222410) and the Zhejiang Provincial Natural Science Foundation of China (grant no. LR22A040002). L.J. was supported by ‘the Fundamental Research Funds for the Central Universities’ (grant no. 226-2024-00039). Work at Sun Yat-sen University was supported by the National Natural Science Foundation of China (grant no. 12174454), Guangdong Basic and Applied Basic Research Funds (grant no. 2021B1515120015), Guangzhou Basic and Applied Basic Research Funds (grant no. 2024A04J6417) and Guangdong Provincial Key Laboratory of Magnetoelectric Physics and Devices (grant no. 2022B1212010008).
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H.Y. and L.J. conceived the experiments. The single crystals were provided by H.S., M.H. and M.W. Y.Z., D.S., Y.H., Z.S., K.Y., J.Z., Z.Y., Y.X., Y.S. and R.L. performed the transport, Laue, scanning electron microscope and energy-dispersive X-ray measurements. M.S., Y.Z., L.J. and H.Y. wrote the paper with input from all authors.
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Extended data
Extended Data Fig. 1 Resistance near the structural phase transition.
Resistance of La3Ni2O7−δ sample S1-1 at 13.7 GPa as a function of temperature from 280 K down to 2 K. The black (red) arrow denotes the data taken upon cooling (warming). At high temperatures there is both a weak anomaly and hysteresis between warming and cooling, which might be associated with a structural phase transition. Note that in Ref. 7 a change of room temperature crystal structure is observed above 10 GPa, and therefore this anomaly may correspond to a transition between the corresponding structural phases. The inset shows the sample configuration in the DAC after pressure loading. The resistance was measured using a four-probe method. The yellow and red arrows point to the gold electrode and the pressure-transmitting medium Daphne oil 7373, respectively. The black region in the middle is the sample, and the rubies and silver epoxy paste are underneath the sample.
Extended Data Fig. 2 R(T) curves at 16.6 GPa under different currents and I-V curves at different pressures and temperatures.
a, Resistance of sample S1-2 at 16.6 GPa in the vicinity of the superconducting transition, measured with different excitation currents. For currents below 31.6 μA, the resistance curve remains unaffected by the current magnitude. b, I-V curves of sample S1-2 at 1.5 K under various pressures. c, I-V curves of sample S1-2 at various temperatures under a pressure of 16.6 GPa. The critical current at 1.5 K is around 3 mA, and the sample has a cross-sectional area of 70 μm × 5 μm, allowing us to estimate a critical current density of 850 A/cm2.
Extended Data Fig. 3 Resistance of La3Ni2O7−δ single crystal samples under pressure.
We measured the R(T) curves of seven La3Ni2O7−δ single crystal samples under pressure. a-d, Four samples exhibited zero resistance at high pressures, and energy-dispersive x-ray analysis (EDX) measurements (Extended Data Fig. 4) show that three of these samples are more homogeneous and close to the stoichiometric composition (S1-1, S1-2, S1-3), while EDX of sample S2-1 was not measured as it broke upon releasing the pressure. For sample S1-3, in order to measure the Hall resistivity at high pressures (Extended Data Fig. 8), we used a 500-μm-diameter culet, which results in a more uneven pressure distribution near its maximum pressure of 20 GPa, leading to a broader superconducting transition at 18.9 GPa for this sample. e-g, The three samples (S2-2, S2-3, S2-4) with compositions far from 3:2:7 with large δ did not show zero resistance.
Extended Data Fig. 4 Scanning electron microscope images and chemical compositions of different La3Ni2O7−δ single crystal samples.
a-f, Scanning electron microscope (SEM) images were taken of six of the small samples that were measured in a DAC. For each sample, we used EDX to check the chemical composition. We randomly selected various positions (marked with blue solid dots) for chemical composition analysis, where the La content was normalized to 3. The final results are listed in the tables below the SEM images, where the mean values are also given, as well as the standard deviation in parentheses. On average, the atomic ratios of La:Ni:O for samples S1-1, S1-2, and S1-3 (a-c) are close to the stoichiometric values of 3:2:7, with smaller standard deviations indicating that those samples are more homogeneous. However, the atomic ratios of S2-2, S2-3, and S2-4 (d-f) deviate much more from stoichiometry, with larger variations in the compositions between different points across the sample. For S2-3, we also include a ‘selected average’ where the regions with Ni ≫ 2 are excluded from the statistics.
Extended Data Fig. 5 Possible origin of the extrinsic insulating behaviour.
We conducted measurements on samples S2 and S2-1 (panels a-b) which were subjected to thermal treatment (heated for 2 hours at 400 K) and sample S3 (panel c) which had no thermal treatment. After the thermal treatment, as shown in a and b, the R(T) curves (black left axis) at 1.2 GPa and 7.1 GPa both exhibit weakly insulating behaviour. However, it can be seen that the phase angle (red right axis) gradually deviates from zero with decreasing temperature, indicating that the measured resistance is not the intrinsic signal. At the same time, we found that the contact resistance between the silver epoxy paste and sample increased gradually from a few ohms at 300 K, to several tens of kiloohms at 2 K, which explains the sizeable deviation in the phase angle. In contrast, for the sample without heat treatment the contact resistance between the sample and silver epoxy paste remains at the level of ohms and does not change with temperature. As shown in c, as the temperature decreases the phase angle remains nearly zero, and the R(T) curve exhibits metallic behaviour. This suggests that the additional heat treatment may lead to oxygen vacancies, resulting in the formation of an insulating layer on the surface and an increase of the contact resistance upon cooling. This phenomenon has also been observed in Y1Ba2Cu3O7−δ33.
Extended Data Fig. 6 Temperature dependence of the upper critical fields at two pressures.
Upper critical field of sample S1-1 measured at a, 20.5 GPa, and b, 26.6 GPa. The red and black points in both panels represent the upper critical fields determined by \({T}_{c}^{\,{0}}\) and \({T}_{c}^{\tt{onset}}\), respectively, which are determined from the construction shown in Fig. 1. The solid lines show the results of fitting with a Ginzburg-Landau model, and the green region between red and black solid lines represents the region where there is an intermediate state or a thermally activated flux flow. By fitting Hc2(T) determined from \({T}_{c}^{\,{0}}\) with a Ginzburg-Landau model, we obtained Hc2(0) of 23 T and 19 T for 20.5 GPa and 26.6 GPa, respectively. By fitting Hc2(T) determined from \({T}_{c}^{\tt{onset}}\), respective zero-temperature values of 97 T and 83 T are obtained.
Extended Data Fig. 7 Crystal structure, Laue measurements, and magnetic susceptibility of single crystalline La3Ni2O7−δ.
a, Crystal structure. b, c, d, X-ray Laue pattern corresponding to the [001] direction, where simulations of the orthorhombic structure with space group Amam are shown by red spots. e, f, g Temperature dependence of the magnetic susceptibility χ(T) of samples S1-S3 measured in a magnetic field of 0.4 T applied perpendicular to the c axis. The temperature dependence of the magnetic susceptibility of the three samples are also displayed in this figure for fields perpendicular to the c-axis, which show similar behaviour, and also correspond well to the data in Ref. 29 for this field direction.
Extended Data Fig. 8 Hall resistivity ρxy (H) at 80 K under various pressures.
ρxy(H) for La3Ni2O7−δ sample S1-3 from 7.5 GPa to 19.4 GPa, where the red dashed lines show fits to a linear magnetic field dependence.
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Zhang, Y., Su, D., Huang, Y. et al. High-temperature superconductivity with zero resistance and strange-metal behaviour in La3Ni2O7−δ. Nat. Phys. (2024). https://doi.org/10.1038/s41567-024-02515-y
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DOI: https://doi.org/10.1038/s41567-024-02515-y
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