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...

Materials with strongly correlated electrons often exhibit interesting physical properties. An example of these materials is the layered oxide perovskite Sr2RuO4, which has been intensively investigated due to its unusual properties. Whilst the debate on the symmetry of the superconducting state in Sr2RuO4 is still ongoing, a deeper understanding of the Sr2RuO4 normal state appears crucial as this is the background in which electron pairing occurs. Here, by using low-energy muon spin spectroscopy we discover the existence of surface magnetism in Sr2RuO4 in its normal state. We detect static weak dipolar fields yet manifesting at an onset temperature higher than 50 K. We ascribe this unconventional magnetism to orbital loop currents forming at the reconstructed Sr2RuO4 surface. Our observations set a reference for the discovery of the same magnetic phase in other materials and unveil an electronic ordering mechanism that can influence electron pairing with broken time reversal symmetry.

The discovery of 2D conductivity at the LaAlO 3 /SrTiO 3 interface has been linking, for over a decade, two of the major current research fields in materials science: correlated transition-metal-oxide systems and lowdimensional systems. Notably, despite the 2D nature of the interfacial electron gas, the samples are 3D objects with thickness in the mm range. This prevented researchers so far from adopting strategies that are only viable for fully 2D materials, or from effectively exploiting degrees of freedom related to strain, strain gradient and curvature. Here a method based on pure strain engineering for obtaining freestanding LaAlO 3 /SrTiO 3 membranes with micrometer lateral dimensions is demonstrated. Detailed transmission electron microscopy investigations show that the membranes are fully epitaxial and that their curvature results in a huge strain gradient, each layer showing a mixed compressive/tensile strain state. Electronic devices are fabricated by realizing ad hoc circuits for individual micromembranes transferred on silicon chips. The samples exhibit metallic conductivity and electrostatic field effect like 2D-electron systems in bulk heterostructures. The results open a new path for adding oxide functionalities into semiconductor electronics, potentially allowing for ultra-low voltage gating of a superconducting transistors, micromechanical control of the 2D electron gas mediated by ferroelectricity and flexoelectricity, and on-chip straintronics.

A simple approach is presented for designing complex oxide mesoscopic electronic devices based on the conducting interfaces of room temperature grown LaAlO3/SrTiO3 heterostructures. The technique is based entirely on methods known from conventional semiconductor processing technology, and we demonstrate a lateral resolution of ∼100 nm. We study the low temperature transport properties of nanoscale wires and demonstrate the feasibility of the technique for defining in-plane gates allowing local control of the electrostatic environment in mesoscopic devices.

Unconventional superconductivity was realized in systems comprising a monolayer of magnetic adatoms adsorbed on conventional superconductors, forming Shiba‐bands. Another approach to induce unconventional superconductivity and 2D Shiba‐bands was recently introduced, namely, by adsorbing chiral molecules (ChMs) on conventional superconductors, which act in a similar way to magnetic impurities as verified by conductance spectroscopy. However, the fundamental effect ChMs have on the strength of superconductivity has not yet been directly observed and mapped. In this work, local magnetic susceptometry is applied on heterostructures comprising islands of ChMs (α‐helix L‐polyalanine) monolayers adsorbed on Nb. It is found that the ChMs alter the superconducting landscape, resulting in spatially‐modulated weaker superconductivity. Surprisingly, the reduced diamagnetic response is located along the perimeter of the islands with respect to both their interior and the bare Nb. The authors suggest that topological edge‐states forming at the edges are the source of the reduced superconductivity, akin to the case of magnetic islands. The results pave new paths for the realization of topological‐superconductivity‐based devices with changing order parameter.