Abstract
Superconductivity in a highly correlated kagome system has been theoretically proposed for years (refs. 1,2,3,4,5), yet the experimental realization is hard to achieve6,7. The recently discovered vanadium-based kagome materials8, which exhibit both superconductivity9,10,11 and charge-density-wave orders12,13,14, are nonmagnetic8,9 and weakly correlated15,16. Thus these materials are unlikely to host the exotic superconductivity theoretically proposed. Here we report the discovery of a chromium-based kagome metal, CsCr3Sb5, which is contrastingly featured with strong electron correlations, frustrated magnetism and characteristic flat bands close to the Fermi level. Under ambient pressure, this kagome metal undergoes a concurrent structural and magnetic phase transition at 55 K, with a stripe-like 4a0 structural modulation. At high pressure, the phase transition evolves into two transitions, possibly associated with charge-density-wave and antiferromagnetic spin-density-wave orderings. These density-wave-like orders are gradually suppressed with pressure and, remarkably, a superconducting dome emerges at 3.65–8.0 GPa. The maximum of the superconducting transition temperature, Tcmax = 6.4 K, appears when the density-wave-like orders are completely suppressed at 4.2 GPa, and the normal state exhibits a non-Fermi-liquid behaviour, reminiscent of unconventional superconductivity and quantum criticality in iron-based superconductors17,18. Our work offers an unprecedented platform for investigating superconductivity in correlated kagome systems.
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Acknowledgements
We thank Y. T. Song for the assistance with the low-temperature single-crystal X-ray diffraction measurements. This work is supported by the National Natural Science Foundation of China (grant nos. 12050003, 12004337, 12025408, 11921004, 12374142, 12304170, 12204298, 12274364 and 12274369), the National Key R&D Program of China (grant nos. 2022YFA1403202, 2023YFA1406100 and 2022YFA1403402), the Key R&D Program of Zhejiang Province, China (grant no. 2021C01002), the Strategic Priority Research Program of the Chinese Academy of Sciences (CAS) (grant nos. XDB33000000 and XDB33010100) and the Outstanding Member of Youth Promotion Association of CAS (grant no. Y2022004). The high-pressure experiments and the NMR experiments were carried out at the Cubic Anvil Cell and the High-field NMR stations of Synergic Extreme Condition User Facility, respectively.
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G.-H.C. coordinated the work, co-conceived the experiments with Y.L. and interpreted the results in discussion with J.-G.C., R.Z., J.-K.B., Y.L., X.-F.X. and C.C. The high-pressure experiments were performed by Z.-Y.L., P.-T.Y. and B.-S.W. under the leadership of J.-G.C.; J.-K.B. contributed to the structural analysis with help from J.-Y.Liu. The NMR measurement was done by Q.-X.S., J.L. and J.Y., supervised by R.Z. The theoretical calculations were made by L.-W.J., S.-Q.W., C.-C.X., H.J. and C.C. The crystals were grown by Y.L., W.-L.C., J.-Y.Lu. and C.-C.L. The ambient-pressure physical property measurements were done by Y.L., W.-Z.Y., Q.T., Z.R. and Z.-A.X. The paper was written by G.-H.C., J.-G.C., R.Z., J.-K.B., Y.L. and Z.-Y.L. All authors commented on the paper.
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Extended data figures and tables
Extended Data Fig. 1 Characterizations and electrical transport properties of CsCr3Sb5 crystals.
a, Optical photographs (left and middle top), an SEM image (right top), and the typical EDX spectrum. b,c, Reconstructed single-crystal XRD patterns of (hk0) and (h0l) reflection planes, respectively, at 298 K. d, XRD θ – 2θ scan at 300 and 20 K. e, The (00 l) reflections at different temperatures. f, Relative lattice parameter c/c300K as a function of temperature, showing a phase transition at T = 50 ± 5 K. g, ρab(T) data in heating and cooling modes. The inset shows the four-electrode measurement configuration. h,i, Magneto-resistivity as functions of field parallel (h) and perpendicular to (i) the crystallographic c axis. j, Magnetoresistance ([ρ(H)–ρ(0)]/ρ(0)%) versus T. k, Hall resistivity as a function of magnetic fields at various temperatures. l, Temperature dependence of Hall coefficient, RH. Anomalies at TMR ≈ THall ≈ 55 K are marked by the dashed vertical lines in j,l.
Extended Data Fig. 2 More information about Sb2 NMR spectra in CsCr3Sb5.
a, Crystal structure of CsCr3Sb5 showing distinct Sb1 and Sb2 sites. b, Frequency dependence of the peak frequencies in CsCr3Sb5 at 70 K obtained at fixed fields marked. Lines are linear fits which indicate the asymmetry parameter η is 0. c, 123Sb2 and 121Sb2 NMR lines at 60 K and under a magnetic field of μ0H0 = 25 T. d, 123Sb-NMR spectra at various temperatures under a magnetic field of 25 T parallel to the c axis. Solids lines are the Gaussian fit.
Extended Data Fig. 3 Information on the structural transition and modulations in CsCr3Sb5.
a-c, Reconstructed (hk0) planes of reflections at 40, 50, and 70 K, respectively, with unit vectors a* and b* marked. d-f, Reconstructed (0kl) planes of reflections at 40, 50, and 70 K, respectively. g, Reconstructed (\(\bar{1}\)kl) plane of reflections up to the highest resolution achieved by the data set. h, Illustration of monoclinic distortion in the reciprocal space to interpret the observed diffraction pattern in g. i, Possible group-subgroup graph with the number of twin domains based on the qualitative analysis on satellite and main reflections for the structural modulation at 40 K. j,l, (hk2) planes of XRD reflections at 55 and 70 K, respectively, for another piece of the crystal. k, A close-up of the marked area in j.
Extended Data Fig. 4 Close-ups of high-pressure ρ(T) data for CsCr3Sb5 crystals.
a-f, ρ(T) data obtained with a piston-type high-pressure cell. g-o, ρ(T) data measured using a cubic anvil cell (CAC). Evolution of the two characteristic temperatures of T1 and T2 (labelled in j), marked by the blue and olive dashed lines respectively, can be tracked.
Extended Data Fig. 5 χ′(T) and ρ(T) data at high pressures and/or under magnetic fields for CsCr3Sb5 crystals.
a, Temperature dependence of ac susceptibility χ‘ at zero field as well as under small magnetic fields for suppressing superconductivity of the reference material Pb. b,c, Close-ups of a highlighting the superconducting transitions at 5.05 GPa (b) and 7.07 GPa (c). d-i, Plots of ρ(T) at P = 3.8, 4.0, 4.1, 5.7, 7.0, and 8.0 GPa, respectively, from which the upper critical fields at zero temperature were extracted.
Extended Data Fig. 6 Temperature dependent normal-state resistivity under high pressures for CsCr3Sb5 crystals.
a, Bilogarithmic graph for the ρ(T) relations, which gives the power α in the formula ρ = ρ0’ + A’Tα. b, Plot of (ρ − ρ0) versus T2, which yields the coefficient A and ρ0 as described in the main text.
Extended Data Fig. 7 Superconducting critical temperature Tc versus Fermi temperature TF for various superconductors.
The TF values of CsCr3Sb5, CsV3Sb5, RbV3Sb5, and KV3Sb5 are estimated by the relation, A ∝ (m*/mb)2/TF2, where A is the coefficient of T -square resistivity and m*/mb is the electron-mass renormalization factor [K. Behnia, Ann. Phys. 534, 2100588 (2022)]. The electron-mass renormalization is estimated by comparing the theoretical and experimental the electronic specific-heat coefficient, m*/mb ≈ γexp/γbare, where γbare is obtained from the DFT calculations. At ambient pressure, the γexp/γbare value for CsCr3Sb5 is estimated to be 6, if the influence of DW-like order is roughly neglected. At the moderately high pressure, it generally tends to decrease to some extent because of pressure-induced band broadening. Here we adopt this “upper limit”, and then the TF values at 4–8 GPa for CsCr3Sb5 are estimated to be 150–400 K. The uncertainties of TF are not expected to exceed 100%, as marked by the vertical bar. The coefficient A of V-based kagome superconductors is obtained by fitting the low-T data using the formula ρ = ρ0 + AT2 (original data from [CsV3Sb5: B. R. Ortiz, et al., Phys. Rev. Lett. 125, 247002 (2020); RbV3Sb5: Q. Yin, et al., Chin. Phys. Lett. 38, 037403 (2021). KV3Sb5: B. R. Ortiz, et al., Phys. Rev. Mater. 5, 034801 (2021)]). And the m*/mb value for AV3Sb5 is about 1.0 (see the related data in Table 2). The TF data of other superconductors are taken from the references [T. Shibauchi, A. Carrington, and Y. Matsuda, Annu. Rev. Condens. Matter Phys. 5, 113 (2014)] and [Y. Cao et al., Nature 556, 44 (2018)]. The highlighted stripe region, where CsCr3Sb5 is located, is almost exclusively occupied by known unconventional superconductors.
Extended Data Fig. 8 Information on the DFT calculations for nonmagnetic hexagonal CsCr3Sb5.
a, Band structures obtained from DFT calculations and from Wannier downfolding result. b, Local axis definitions and real-space distributions of Cr-3d Wannier orbitals. c, Comparison of band structure with CsV3Sb5. The van Hove singularities and flat bands are highlighted with ellipses and transparent stripes, respectively. d, Comparison of band structure (top) and Fermi surface (bottom) at 0 and 5 GPa, respectively. Inside the red frame shows the main change in band structure. All the calculations were performed without spin-orbit interactions.
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Liu, Y., Liu, ZY., Bao, JK. et al. Superconductivity under pressure in a chromium-based kagome metal. Nature 632, 1032–1037 (2024). https://doi.org/10.1038/s41586-024-07761-x
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DOI: https://doi.org/10.1038/s41586-024-07761-x
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