Van der Waals heterostructures have risen as a tunable platform to combine different electronic orders, due to the flexibility in stacking different materials with competing symmetry broken states. Among them, van der Waals ferromagnets such as CrI3
and superconductors as NbSe2 provide a natural platform to engineer novel phenomena at ferromagnet-superconductor interfaces. In particular, NbSe2 is well known for hosting strong spin-orbit coupling effects that influence the properties of the superconducting state. Here we put forward a ferromagnet/NbSe2/ferromagnet heterostructure where the interplay between Ising superconductivity in NbSe2 and magnetism controls the magnetic alignment of the heterostructure. In particular, we show that the interplay between spin-orbit coupling and superconductivity allows controlling magnetic states in van der Waals materials. Our results show how hybrid van der Waals ferromagnet/superconductor heterostructure can be used as a tunable materials platform for superconducting spin-orbitronics.
Electronic flat bands represent a paradigmatic platform to realize strongly correlated matter due to their associated divergent density of states. In common instances, including electron-electron interactions leads to magnetic instabilities for repul
sive interactions and superconductivity for attractive interactions. Nevertheless, interactions of Kondo nature in flat band systems have remained relatively unexplored. Here we address the emergence of interacting states mediated by Kondo lattice coupled to a flat band system. Combining dynamical mean-field theory and tensor networks methods to solve flat band Kondo lattice models in one and two dimensions, we show the emergence of a robust underscreened regime leading to a magnetically ordered state in the flat band. Our results put forward flat band Kondo lattice models as a platform to explore the genuine interplay between flat band physics and many-body Kondo screening.
Janus dichalcogenide multilayers provide a paradigmatic platform to engineer electronic phenomena dominated by spin-orbit coupling. Their unique spin-orbit effects stem from local mirror symmetry breaking in each layer, which induces a colossal Rashb
a spin-orbit effect in comparison with the conventional dichalcogenide counterparts. Here we put forward twisted dichalcogenide bilayers as a simple platform to realize spin-orbit correlated states. We demonstrate the emergence of flat bands featuring strong spin-momentum locking and the emergence of non-collinear symmetry broken states when interactions are included. We further show that the symmetry broken states can be controlled by means of a magnetic substrate, strongly impacting the non-collinear magnetic texture of the moire unit cell. Our results put forward twisted Janus multilayers as a powerful platform to explore spin-orbit correlated physics, and highlighting the versatility of magnetic substrates to control unconventional moire magnetism.
Topological superconductors represent one of the key hosts of Majorana-based topological quantum computing. Typical scenarios for one-dimensional topological superconductivity assume a broken gauge symmetry associated to a superconducting state. Howe
ver, no interacting one-dimensional many-body system is known to spontaneously break gauge symmetries. Here, we show that zero modes emerge in a many-body system without gauge symmetry breaking and in the absence of superconducting order. In particular, we demonstrate that Majorana zero modes of the symmetry-broken superconducting state are continuously connected to these zero-mode excitations, demonstrating that zero-bias anomalies may emerge in the absence of gauge symmetry breaking. We demonstrate that these many-body zero modes share the robustness features of the Majorana zero modes of symmetry-broken topological superconductors. We introduce a bosonization formalism to analyze these excitations and show that a ground state analogous to a topological superconducting state can be analytically found in a certain limit. Our results demonstrate that robust Majorana-like zero modes may appear in a many-body systems without gauge symmetry breaking, thus introducing a family of protected excitations with no single-particle analogs.
Quasiperiodicity has long been known to be a potential platform to explore exotic phenomena, realizing an intricate middle point between ordered solids and disordered matter. In particular, quasiperiodic structures are promising playgrounds to engine
er critical wavefunctions, a powerful starting point to engineer exotic correlated states. Here we show that systems hosting a quasiperiodic modulation of antiferromagnetism and spin-singlet superconductivity, as realized by atomic chains in twisted van der Waals materials, host a localization-delocalization transition as a function of the coupling strength. Associated with this transition, we demonstrate the emergence of a robust quasiperiodic critical point for arbitrary incommensurate potentials, that appears for generic relative weights of the spin-singlet superconductivity and antiferromagnetism. We show that the inclusion of residual electronic interactions leads to an emergent spin-triplet superconducting state, that gets dramatically enhanced at the vicinity of the quasiperiodic critical point. Our results put forward quasiperiodicity as a powerful knob to engineer robust superconducting states, providing an alternative pathway towards artificially designed unconventional superconductors.
The advent of topological phases of matter revealed a variety of observed boundary phenomena, such as chiral and helical modes found at the edges of two-dimensional (2D) topological insulators. Antichiral states in 2D semimetals, i.e., copropagating
edge modes on opposite edges compensated by a counterpropagating bulk current, are also predicted, but, to date, no realization of such states in a solid-state system has been found. Here, we put forward a procedure to realize antichiral states in twisted van der Waals multilayers, by combining the electronic Dirac-cone spectra of each layer through the combination of the orbital moire superstructure, an in-plane magnetic field, and inter-layer bias voltage. In particular, we demonstrate that a twisted van der Waals heterostructure consisting of graphene/two layers of hexagonal boron nitride [(hBN)$_2$]/graphene will show antichiral states at in-plane magnetic fields of 8 T, for a rotation angle of 0.2$^{circ}$ between the graphene layers. Our findings engender a controllable procedure to engineer antichiral states in solid-state systems, as well as in quantum engineered metamaterials.
Twisted van der Waals materials have become a paradigmatic platform to realize exotic correlated states of matter. Here, we show that a twisted dichalcogenide bilayer (WSe$_2$) encapsulated between a magnetic van der Waals material (CrBr$_3$) feature
s flat bands with tunable valley and spin flavors. We demonstrate that, when electron-electron interactions are included, spin-ferromagnetic and valley-ferromagnetic states emerge in the flat bands, stemming from the interplay between correlations, intrinsic spin-orbit coupling and exchange proximity effects. We show that the specific symmetry broken state is controlled by the relative alignment of the magnetization of the encapsulation, demonstrating the emergence of correlated states controlled by exchange bias. Our results put forward a new van der Waals heterostructure where symmetry broken states emerge from a genuine interplay between twist engineering, spin-orbit coupling and exchange proximity, providing a powerful starting point to explore exotic collective states of matter.
Quantum spin-liquids are strongly correlated phases of matter displaying a highly entangled ground state. Due to their unconventional nature, finding experimental signatures of these states has proven to be a remarkable challenge. Here we show that t
he effects of local impurities can provide strong signatures of a Dirac quantum spin-liquid state. Focusing on a gapless Dirac quantum spin-liquid state as realized in NaYbO$_2$, we show that single magnetic impurity coupled to the quantum spin-liquid state creates a resonant spinon peak at zero frequency, coexisting the original Dirac spinons. We explore the spatial dependence of this zero-bias resonance, and show how different zero modes stemming from several impurities interfere. We finally address how such spinon zero-mode resonances can be experimentally probed with inelastic spectroscopy and electrically-driven paramagnetic resonance with scanning tunnel microscopy. Our results put forward impurity engineering as a means of identifying Dirac quantum spin-liquids with scanning probe techniques, highlighting the dramatic impact of magnetic impurities in a macroscopically entangled many-body ground state.
Twisted graphene multilayers have demonstrated to yield a versatile playground to engineer controllable electronic states. Here, by combining first-principles calculations and low-energy models, we demonstrate that twisted graphene trilayers provide
a tunable system where van Hove singularities can be controlled electrically. In particular, it is shown that besides the band flattening, bulk valley currents appear, which can be quenched by local chemical dopants. We finally show that in the presence of electronic interactions, a non-uniform superfluid density emerges, whose non-uniformity gives rise to spectroscopic signatures in dispersive higher energy bands. Our results put forward twisted trilayers as a tunable van der Waals heterostructure displaying electrically controllable flat bands and bulk valley currents.
Twisted graphene bilayers provide a versatile platform to engineer metamaterials with novel emergent properties by exploiting the resulting geometric moir{e} superlattice. Such superlattices are known to host bulk valley currents at tiny angles ($alp
haapprox 0.3 ^circ$) and flat bands at magic angles ($alpha approx 1^circ$). We show that tuning the twist angle to $alpha^*approx 0.8^circ$ generates flat bands away from charge neutrality with a triangular superlattice periodicity. When doped with $pm 6$ electrons per moire cell, these bands are half-filled and electronic interactions produce a symmetry-broken ground state (Stoner instability) with spin-polarized regions that order ferromagnetically. Application of an interlayer electric field breaks inversion symmetry and introduces valley-dependent dispersion that quenches the magnetic order. With these results, we propose a solid-state platform that realizes electrically tunable strong correlations.
