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Artificial heavy fermions in a van der Waals heterostructure

Abstract

Heavy-fermion systems represent one of the paradigmatic strongly correlated states of matter1,2,3,4,5. They have been used as a platform for investigating exotic behaviour ranging from quantum criticality and non-Fermi liquid behaviour to unconventional topological superconductivity4,5,6,7,8,9,10,11,12. The heavy-fermion phenomenon arises from the exchange interaction between localized magnetic moments and conduction electrons leading to Kondo lattice physics, and represents one of the long-standing open problems in quantum materials3. In a Kondo lattice, the exchange interaction gives rise to a band with heavy effective mass. This intriguing phenomenology has so far been realized only in compounds containing rare-earth elements with 4f or 5f electrons1,4,13,14. Here we realize a designer van der Waals heterostructure where artificial heavy fermions emerge from the Kondo coupling between a lattice of localized magnetic moments and itinerant electrons in a 1T/1H-TaS2 heterostructure. We study the heterostructure using scanning tunnelling microscopy and spectroscopy and show that depending on the stacking order of the monolayers, we can reveal either the localized magnetic moments and the associated Kondo effect, or the conduction electrons with a heavy-fermion hybridization gap. Our experiments realize an ultimately tunable platform for future experiments probing enhanced many-body correlations, dimensional tuning of quantum criticality and unconventional superconductivity in two-dimensional artificial heavy-fermion systems15,16,17.

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Fig. 1: Artificial heavy-fermion heterostructure.
Fig. 2: Kondo resonance in a 1T/1H-TaS2 vertical heterostructure.
Fig. 3: Heavy-fermion hybridization gap in a 1H/1T-TaS2 vertical heterostructure.

Data availability

All of the data supporting the findings are available from the corresponding authors upon request.

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Acknowledgements

This research made use of the Aalto Nanomicroscopy Center (Aalto NMC) facilities and was supported by the European Research Council (ERC-2017-AdG no. 788185 “Artificial Designer Materials”), the Academy of Finland (Academy professor funding nos. 318995 and 320555, Academy postdoctoral researcher no. 309975, Academy research fellow nos. 331342 and 336243) and the Jane and Aatos Erkko Foundation. We acknowledge the computational resources provided by the Aalto Science-IT project.

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Contributions

V.V., J.L.L., S.K. and P.L. conceived the experiment, V.V., M.A., S.C.G. and S.K. carried out the sample growth and the low-temperature STM experiments. V.V. analysed the STM data. G.C. and J.L.L. developed the theoretical model. V.V., J.L.L. and P.L. wrote the manuscript with input from all co-authors.

Corresponding authors

Correspondence to Jose L. Lado or Peter Liljeroth.

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Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature thanks Milan Allan, Stefan Kirchner and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Fig. 1 Grid spectroscopy measurement of a 1T-TaS2/1H-TaS2 heterostructure.

a, STM image of a 1T/1H-TaS2 heterostructure (V = 200 mV and I = 10 pA). b, Tunneling spectra across the red line shown in (a), starting from top left. We observe a Kondo peak located on each of the CDW centers. c, dI/dV map at V = 0 mV measured on the area shown in (a) showing the Kondo lattice. Due to a Kondo peak, there is a higher dI/dV intensity on each of the CDW centers. Middle and bottom rows: dI/dV maps at given bias voltages, all measured on the area shown in (a) and using the same colour scale as in (c). Each of the dI/dV curves was normalized by dividing it by its mean value. Single tunneling spectra on different CDW centers and on a different area are shown in Extended Data Fig. 2.

Extended Data Fig. 2 Single tunneling spectra in the centre of different CDW unit cells.

Left: STM image of a 1T-TaS2/1H-TaS2 heterostructure (V = 200 mV and I = 20 pA). Right: tunneling spectra measured at locations highlighted by the same-coloured dots on the STM image, spectra are vertically offset for clarity.

Extended Data Fig. 3 Fitting of the Kondo resonances.

Left: Temperature dependence of a Kondo resonance in tunneling spectroscopy (black lines), and their respective fits to the Fano lineshape (red lines). The spectra are vertically offset for clarity. Right: Table of the fit parameters.

Extended Data Fig. 4 Thermal broadening of the Kondo resonance.

Tunneling spectroscopy of a Kondo resonance at 300 mK (blue), 18 K (green), and a simulated tunneling spectrum at 18 K (red), where the 300 mK spectrum was taken as the density of states.

Extended Data Fig. 5 Tunneling spectra of 1H-TaS2 on different substrates.

Tunneling spectroscopy of 1H-TaS2 on HOPG (purple line), bilayer 1H-TaS2 (blue line) and 1H-TaS2 on monolayer 1T-TaS2 (green line). The spectra are measured on positions highlighted by the same-coloured dots in the STM image on the left. The spectra exhibit a dip around the Fermi level, but while 1H-TaS2 on HOPG (purple line) and bilayer 1H-TaS2 (blue line) have finite zero-bias conductance, only the spectroscopy of 1H/1T-TaS2 (green line) exhibits a heavy-fermion gap with zero conductance at zero bias.

Extended Data Fig. 6 Examples of different 1H/1T-TaS2 heterostructures.

STM images (left) and corresponding tunneling spectra measured on top of a heterostructure (right). Purple dots highlight the positions, where the tunneling spectra were taken. All the heterostructures exhibit a heavy-fermion gap with zero conductance at zero bias and approximately the same gap width, regardless of the island size.

Extended Data Fig. 7 Tunneling spectroscopy across middle of the 1H/1T-TaS2 island.

a, STM image of a 1H/1T-TaS2 vertical heterostructure (V = 900 mV and I = 20 pA). b, Tunneling spectra across the red line shown in (a). c, Average tunneling spectrum from the spectra across line in (b).

Extended Data Fig. 8 Spatial dependence of the heavy-fermion gap.

a, STM image of a 1H-TaS2/1T-TaS2 heterostructure (V = 50 mV and I = 500 pA). Larger scale topographic image is shown in Extended Data Fig. 9e. b, Tunneling spectra across the red line shown in (a). c, dI/dV map at V = 0 mV measured on the area shown in (a). Middle and bottom rows: dI/dV maps at given bias voltages, all measured on the area shown in (a) and using the same colour scale as in (c).

Extended Data Fig. 9 High-resolution STM images of different heterostructures.

a, c, e, g, Large area STM images. b, STM image of a 1H-TaS2 on HOPG (V = 50 mV and I = 500 pA). Inset shows fast Fourier transform of the image. d, STM image of a monolayer 1T-TaS2 on HOPG (V = 1 V and I = 20 pA). f, STM image of a 1H/1T-TaS2 vertical heterostructure (V = 50 V and I = 500 pA). Inset shows fast Fourier transform of the image. h, STM image of a 1T/1H-TaS2 vertical heterostructure (V = 0.3 V and I = 50 pA). 1H-TaS2 on HOPG exhibits a strong 3×3 CDW, while 1H-TaS2 on 1T-TaS2 shows no signs of CDW. Both 1T-TaS2 on HOPG and 1T-TaS2 on 1H-TaS2 exhibit a strong √13×√13 CDW.

Extended Data Fig. 10 Magnetic field dependence of a heavy-fermion gap.

Tunneling spectroscopy of a heavy-fermion gap measured on a 1H/1T-TaS2 vertical heterostructure at different applied magnetic fields, the spectra are vertically offset for clarity.

Supplementary information

Supplementary Information

This file contains Supplementary Sections 1–3: detailed description of the theoretical model; discussion on the comparison with the magnetic field dependence of natural heavy-fermion compounds; discussion on the potential further probes of the heavy-fermion regime.

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Vaňo, V., Amini, M., Ganguli, S.C. et al. Artificial heavy fermions in a van der Waals heterostructure. Nature 599, 582–586 (2021). https://doi.org/10.1038/s41586-021-04021-0

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