Doped Mott insulators exhibit some of the most intriguing quantum phases of matter, including quantum spin-liquids, unconventional superconductors, and non-Fermi liquid metals [1][2][3] . Such phases often arise when itinerant electrons are close to a Mott insulating state, and thus experience strong spatial correlations 4,5 . 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 4Hb-TaS2, which realizes an alternate stack of a candidate spin liquid and a superconductor 6 . 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 above Tc, indicating the presence of a magnetic phase independent of the superconductor. Strikingly, this phase does not generate detectable magnetic signals. We use scanning superconducting quantum interference device (SQUID) microscopy to show that it is incompatible with ferromagnetic ordering. The discovery of this new form of hidden magnetism illustrates how combining superconductivity with a strongly correlated system can lead to new, unexpected physics.
Main textIn Van der Waals heterostructures, proximity between layers of different materials can be exploited to generate new states of matter [7][8][9] , or to use one layer in order to probe the properties of the other 10,11 . Indeed, correlated insulating, superconducting, nematic and time-reversal symmetry broken states emerge when uncorrelated electronic systems are stacked together [12][13][14][15][16] . Heterostructures involving strongly correlated systems as their constituents therefore hold promise to realize new phases or find new coupling mechanisms between the layers. A particularly interesting set of ground states to pair is a superconductor and a Mott insulator. Unconventional superconductivity often emerges when a Mott insulator is destroyed by doping 2,3,12 , but how these two phases interact when stacked as individual building blocks remains to be explored. This combination is naturally realized in 4Hb-TaS2, in which two 2D structures of TaS2, octahedral (1T) and trigonal prismatic (2H), are alternatingly stacked 17 . In bulk form, 2H-TaS2 is a superconductor with Tc = 0.7 K 18 , while 1T-TaS2 is a correlated insulator 19 with electrons localized on a triangular lattice, predicted to host a quantum spin liquid ground state 20,21 . This suggestion is supported by muon spin relaxation experiments [22][23][24] and nuclear magnetic resonance measurements 23,25 which show an absence of longrange magnetic order. Furthermore, although resistivity clearly shows an insulating behavior, residual specific heat 22,24 and thermal conductivity 26 , and scanning tunneling microscopy measurements 27,28 indicate gapless neutral excitations. Combined, these results point towards a gapless spin liquid ground state in bulk 1...
