Doped Mott insulators exhibit some of the most intriguing quantum phases of matter, including quantum spin liquids, unconventional superconductors and non-Fermi liquid metals1,2,3. Such phases often arise when itinerant electrons are close to a Mott insulating state, and thus experience strong spatial correlations. Proximity between different layers of van der Waals heterostructures naturally realizes a platform for experimentally studying the relationship between localized, correlated electrons and itinerant electrons. Here we explore this relationship by studying the magnetic landscape of tantalum disulfide 4Hb-TaS2, which realizes an alternating stacking of a candidate spin liquid and a superconductor4. We report on a spontaneous vortex phase whose vortex density can be trained in the normal state. We show that time-reversal symmetry is broken in the normal state, indicating the presence of a magnetic phase independent of the superconductor. Notably, this phase does not generate ferromagnetic signals that are detectable using conventional techniques. We use scanning superconducting quantum interference device microscopy to show that it is incompatible with ferromagnetic ordering. The discovery of this unusual magnetic phase illustrates how combining superconductivity with a strongly correlated system can lead to unexpected physics.
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The data that support the findings of this study are available from the corresponding authors upon reasonable request.
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We thank H. Beidenkopf, Y. Dagan, A. Aurbach, E. Shimshoni, R. Ilan, F. de Juan and I. Sochnikov for discussions; and T. R. Devidas for assistance with the NbSe2 measurements. E.P., A.V.B. and B.K. were supported by the European Research Council grant number ERC-2019-COG-866236, the Israeli Science Foundation grant number ISF-1251/19, COST Action CA16218, the QuantERAERA-NET Cofund in Quantum Technologies, project number 731473 and the Pazy Research Foundation grant number 107-2018. E.B. was supported by the European Research Council grant number ERC-2019-COG-817799. J.R. was supported by the Israeli Science Foundation grant number ISF-994/19. A.K. was supported by Israeli Science Foundation grant number ISF-1263/21.
The authors declare no competing interests.
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Extended data figures and tables
(a) Local susceptibility maps taken at different temperatures below and above Tc. Below Tc, the diamagnetic response is homogeneous to within ±2% of the space-averaged signal, even at T = 2.6 K, close to Tc. At T = 2.8 K (>Tc) we detect no signal within our noise level, demonstrating that the system is completely normal within our field of view. (b) Local magnetometry images of the sample following a field cool at various temperatures. The images clearly show vortices below Tc, which are completely absent above Tc (note the change to the colour span at T = 2.8 K). Scale bars, 20 µm. (c) Local temperature dependence of the susceptibility taken at a representative single point on the sample. (d) Resistance measurements as a function of temperature. (e) Global magnetization measured after field cooling the sample with an external field of 100 Oe. All measurements show a sharp superconducting transition at ~ 2.7 K.
(a) The sample was first trained by field-cooling it through 3.6 K. It was then kept at 3 K for various amounts of time, before ZFC to 1.7 K and measuring the resulting vortex density. (b–d) SQUID images of the spontaneous vortices in the superconducting phase after waiting for (b) 3 min, (c) 6 min, and (d) 12 min at 3 K. The vortex density did not change as a function of time waited at the normal state. Scale bars, 30 µm.
The magnetization was measured at 2 K, showing no hysteresis when the field was swept from −7 T to 7 T.
(a) To demonstrate how the Meissner response of a NbSe2 flake can be used to probe external magnetic fields, we measured its magnetic signal at 4.2 K, at various magnetic fields. (b) The corresponding magnetic flux images at 4.2 K, showing signals due to the Meissner response. Note that both the presence of an external field and its polarity can be detected through the Meissner effect. When the field is turned off (scan #2) the signal disappears. (c) A “field cooling” protocol from 4.2 K to 1.7 K, like that used in Fig. 1. (d) The corresponding magnetic flux images. After the field is turned off at 1.7 K (scan #6), the Meissner response disappears, demonstrating that there is no remnant field in the system. (e) Line cuts showing the Meissner response from scans #4–6. The data from scan #5 (field on) is multiplied by 0.1.
The global magnetization of the sample as a function of the external field, taken at 3.2 K. The magnetic response was not hysteretic for fields up to 7 T.
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Persky, E., Bjørlig, A.V., Feldman, I. et al. Magnetic memory and spontaneous vortices in a van der Waals superconductor. Nature 607, 692–696 (2022). https://doi.org/10.1038/s41586-022-04855-2