Abstract
Iron-based superconductors (FeSCs) are a fascinating family of materials in which several electronic bands and strong antiferromagnetic (AFM) correlations are key ingredients for competing ground states1,2,3,4,5,6, including antiferromagnetism, electronic nematicity and unconventional superconductivity. FeTe, unlike its superconducting isostructural counterpart FeSe, has long been considered an AFM metal sans superconductivity7,8,9. Here we use molecular-beam epitaxy (MBE) to grow FeTe films and perform post-growth annealing under a Te flux. By performing spin-polarized scanning tunnelling microscopy and spectroscopy (STM/S), we demonstrate that the AFM order in as-grown FeTe films is induced by interstitial Fe atoms that disrupt the ideal 1:1 stoichiometry. Notably, the removal of these interstitial Fe atoms through Te annealing yields stoichiometric FeTe films that show no AFM order and instead exhibit robust superconductivity with a critical temperature of about 13.5 K. This superconducting state is further confirmed by the observation of Cooper-pair tunnelling, zero electrical resistance and the Meissner effect. Therefore, our results demonstrate that stoichiometric FeTe is inherently a superconductor, overturning a long-held view that it is an AFM metal. This work clarifies the origin of superconductivity in FeTe-based heterostructures10,11,12,13,14,15 and demonstrates the importance of stoichiometry control in understanding the competition between antiferromagnetism and superconductivity in FeSCs.
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The data that support the findings of this article are openly available at Zenodo61 (https://doi.org/10.5281/zenodo.17944465).
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Acknowledgements
We thank M. H. W. Chan, Y. Ge, A. J. Grutter, L. Y. Kong, C. X. Liu, Z. Q. Mao, N. Samarth, Z. Y. Wang, X. X. Wu, X. D. Xu, B. H. Yan and J. Zhu for helpful discussions. This work is primarily supported by a DOE grant (DE-SC0023113), including the MBE growth and STM/S measurements. The atomic force microscopy and XRD measurements are supported by the ONR Award (N000142412133). The electrical transport measurements are supported by the ARO award (W911NF2210159) and a NSF grant (DMR-2241327). The STEM measurements are supported by the Penn State MRSEC for Nanoscale Science (DMR-2011839). The MFM measurements are supported by a DOE grant (DE-SC0018153). J.Y. acknowledges the support from the startup funds at the University of Florida. P.J.H. acknowledges the support from the NSF grant (DMR-2231821). C.-Z.C. acknowledges the support from the Gordon and Betty Moore Foundation’s EPiQS Initiative (GBMF9063).
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C.-Z.C. conceived and designed the experiment. Z.-J.Y., Z.W., B.X., S.P., H.R. and C.-Z.C. performed the MBE growth. Z.-J.Y., H.R., P.X. and C.-Z.C. conducted electrical transport measurements. Z.W., B.X., S.P., P.X., J.S., V.G. and C.-Z.C. performed all STM/S measurements. Z.-J.Y., P.X. and C.-Z.C. performed the atomic force microscopy and XRD measurements. Y.-T.C. and W.W. performed MFM measurements. N.D., K.D.H. and D.R.H. performed STEM measurements. J.Y. and P.J.H. provided theoretical support. Z.-J.Y., W.W., J.Y., P.J.H. and C.-Z.C. analysed the data and wrote the manuscript, with input from all authors.
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Extended data figures and tables
Extended Data Fig. 1 Spin-polarized STM measurements on an as-grown 40-UC FeTe film under opposite magnetic fields.
a, Schematic of spin-polarized STM measurements on FeTe films. Some iron atoms from the surface of FeTe attached to the apex of the PtIr tip make it spin-polarized. Its polarization direction can be controlled by applying an external magnetic field μ0H. b,c, STM images (20 × 20 nm2) of the same region in Fig. 1a on the as-grown 40-UC FeTe film at μ0H = +4 T (b) and μ0H = −4 T (c) (Vs = −15 mV, Is = 5 nA and T = 4.2 K). The STM images in b and c are measured using magnetic Fe-functionalized tips from the surface of FeTe. Under opposite magnetic fields, the spin polarization of the STM tip is reversed, causing the double-stripe pattern to shift by a half-period (aTe). d, Magnetic contrast image, obtained by subtracting b from c, which highlights the bicollinear AFM order of the as-grown 40-UC FeTe film.
Extended Data Fig. 2 STM/S results of a single interstitial Fe atom in FeTe films.
a–i, STM images (5 × 5 nm2) of the 40-UC FeTe film with a single interstitial Fe atom measured under different Vs. a, Vs = 3.5 V and Is = 1 nA. b, Vs = 1 V and Is = 9 nA. c, Vs = 500 mV and Is = 9 nA. d, Vs = 50 mV and Is = 5 nA. e, Vs = 15 mV and Is = 1 nA. f, Vs = −15 mV and Is = 1 nA. g, Vs = −50 mV and Is = 5 nA. h, Vs = −500 mV and Is = 5 nA. i, Vs = −2 V and Is = 9 nA. j, Colour plot of dI/dV spectra along the red arrow direction in e (that is, the [100] direction) across a single interstitial Fe atom (Vs = 15 mV, Is = 2 nA and Vmod = 0.1 mV). k, Colour plot of dI/dV spectra along the green arrow direction in e (that is, the [110] direction) across a single interstitial Fe atom (Vs = 15 mV, Is = 2 nA and Vmod = 0.1 mV). Several impurity states are resolved near the interstitial Fe atom. The STM images in a–i are acquired using a spin-polarized tip at T = 4.2 K, whereas the dI/dV spectra are obtained using a PtIr tip at T = 310 mK. Scale bars, 1 nm (a–i).
Extended Data Fig. 3 Distribution of interstitial Fe atoms on the as-grown 40-UC FeTe film.
STM image (20 × 20 nm2) of the same region in Fig. 1a on the as-grown 40-UC FeTe film (Vs = 3.5 V and Is = 5 nA). The STM image is acquired using a spin-polarized tip at T = 4.2 K. Scale bar, 2 nm.
Extended Data Fig. 4 STM images of the 40-UC FeTe film after four-cycle Te-annealing treatments (that is, cycle IV).
a, STM image (20 × 20 nm2) measured using a lower Vs (Vs = −10 mV and Is = 1 nA). Inset, FT image. b, STM image (20 × 20 nm2) of the same region in a measured using a higher Vs (Vs = 3.5 V and Is = 1 nA). The red spots in b represent the interstitial Fe atoms. Inset, the corresponding overlay of the STM images in a and b. The red (blue) areas in b, inset represent the AFM (non-AFM) region shown in a. The black spots in b, inset represent the interstitial Fe atoms in b. The STM images in a and b are acquired using a spin-polarized tip at T = 4.2 K. Scale bars, 2 nm (a,b); 1 Å−1 (a, inset); 2 nm (b, inset).
Extended Data Fig. 5 Surface morphologies of 40-UC FeTe films after each Te-annealing cycle.
a–f, STM images (500 × 500 nm2) of 40-UC FeTe films: as-grown FeTe (a) (Vs = 1.5 V and Is = 50 pA), cycle I (b) (Vs = 3.5 V and Is = 50 pA), cycle II (c) (Vs = 1.5 V and Is = 20 pA), cycle III (d) (Vs = 1.5 V and Is = 20 pA), cycle IV (e) (Vs = 1.5 V and Is = 20 pA) and cycle V (f) (Vs = 1.5 V and Is = 20 pA). The STM images are acquired using a PtIr tip at T = 4.2 K. g–l, Atomic force microscopy images (5 × 5 μm2) of 40-UC FeTe films: as-grown FeTe (g), cycle I (h), cycle II (i), cycle III (j), cycle IV (k) and cycle V (l). The as-grown 40-UC FeTe film exhibits atomically flat terraces with square pyramidal structures. During the Te-annealing treatments, these square terraces gradually expand and their edges become rounded, suggesting the formation of new FeTe layers. Scale bars, 100 nm (a–f); 1 μm (g–l).
Extended Data Fig. 6 Abrikosov vortices of a stoichiometric 40-UC FeTe film.
a–o, The Abrikosov vortices of a stoichiometric 40-UC FeTe film measured at different μ0H and Vbias (42 × 42 nm2, Vs = 15 mV, Is = 1 nA and Vmod = 2 mV). The image in i is identical to Fig. 3c. All STM/S measurements are performed using a PtIr tip at T = 310 mK. Scale bars, 4 nm.
Extended Data Fig. 7 Josephson STM/S results of a stoichiometric 40-UC FeTe film.
a, Schematics of the tunnelling processes at different Vbias. The blue (red) areas represent the occupied (empty) states and the dashed lines indicate the chemical potential EF. For |eVbias| > (Δ + Δtip), electron tunnelling occurs between the tip and the sample. When eVbias is close to zero, Cooper-pair tunnelling dominates. b, Typical dI/dV spectrum (Vs = 15 mV, Is = 2 nA and excitation voltage Vmod = 0.1 mV) of a stoichiometric 40-UC FeTe film measured using a superconducting Nb tip at T = 310 mK with RN = 7.5 MΩ. c, Enlarged dI/dV spectrum in b. Two pairs of dI/dV peaks have been observed at ±Δtip and ±(Δ + Δtip), in which Δtip is the superconducting gap size of the Nb tip. See Supplementary Information Section I.3 for further discussion about non-zero spectral features between ±Δtip and ±(Δ + Δtip).
Extended Data Fig. 8 Rxx–T curves of 40-UC FeTe films after each Te-annealing cycle.
a–f, T-dependent Rxx of 40-UC FeTe films: as-grown (a), after cycle I (b), cycle II (c), cycle III (d), cycle IV (e) and cycle V (f). Insets, enlarged Rxx–T curves for 2 K ≤ T ≤ 20 K. The Rxx–T curves in a and f are reused from Fig. 4a.
Extended Data Fig. 9 dI/dV spectra of 40-UC FeTe films after each Te-annealing cycle.
a–f, Typical dI/dV spectra of 40-UC FeTe films: as-grown (a) (Vs = 20 mV, Is = 1 nA and Vmod = 0.2 mV), after cycle I (b) (Vs = 15 mV, Is = 0.3 nA and Vmod = 0.5 mV), cycle II (c) (Vs = 15 mV, Is = 1 nA and Vmod = 0.2 mV), cycle III (d) (Vs = 15 mV, Is = 1 nA and Vmod = 0.2 mV), cycle IV (e) (Vs = 15 mV, Is = 1 nA and Vmod = 0.2 mV) and cycle V (f) (Vs = 15 mV, Is = 2 nA and Vmod = 0.2 mV). All STM/S measurements are performed using a PtIr tip at T = 4.2 K. To better visualize and compare the dI/dV spectra measured under different setpoints, the dI/dV spectra in a–f are normalized by (dI/dV)norm = (dI/dV)/(Is/Vs), which removes the dependence on STM/S setpoint parameters Vs and Is.
Extended Data Fig. 10 Numerical calculations on impurity-induced bicollinear AFM order in FeTe.
a–f, Left, calculated magnetization mR over 24 × 24 Fe square lattices with N = 0 (a), N = 7 (b), N = 14 (c), N = 21 (d), N = 28 (e) and N = 35 (f). mR is defined as the difference between the spin-up and spin-down electron probabilities at position R. Right top, FT images of mR. Right bottom, distribution of impurities within the 24 × 24 Fe square lattices, denoted by the magnetic impurity term SR, which is chosen randomly (Methods). All calculations are performed with VR = 5 eV and |SR | = 0.5 eV.
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Yan, ZJ., Wang, Z., Xia, B. et al. Stoichiometric FeTe is a superconductor. Nature (2026). https://doi.org/10.1038/s41586-026-10321-0
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DOI: https://doi.org/10.1038/s41586-026-10321-0
