Higher-order topological insulators (HOTIs) represent a novel class of topological materials, characterised by the emergence of topological boundary modes at dimensions two or more lower than those of bulk materials. Recent experimental studies [1–4] have identified conducting channels at the hinges of HOTIs, although their topological nature remains unexplored. In this study, we investigated Shapiro steps in Al‒WTe₂‒Al proximity Josephson junctions (JJs) under microwave irradiation and examined the topological properties of the hinge states in WTe₂. Specifically, we analysed the microwave frequency dependence of the absence of the first Shapiro step in hinge-dominated JJs, which is consistent with a contribution from a 4π-periodic current‒phase relationship characteristic of topological JJs. These findings encourage further research into topological superconductivity in superconducting hybrid devices based on HOTIs.

Josephson junctions are essential devices in superconducting electronics and quantum computing hardware. Here we predict electrical control of the supercurrent in composite superconductor-insulator-ferroelectric-insulator-superconductor (S-I-FE-I-S) Josephson junctions. Inversion symmetry broken by unequal dielectric barrier thicknesses and/or potentials converts ferroelectric polarization reversal into a substantial change of the critical current. Using a WKB approximation, we model the nonvolatile switching of the critical current with on-off efficiency that is tunable by thicknesses and potential barriers of the insulating layers, as well as the thickness and dielectric constant of the ferroelectric layer. We also derive a compact linear expression for the critical current valid for small polarizations. Our results identify ferroelectric Josephson junctions as electrically programmable superconducting current switches for cryogenic memory and logic applications.

Crystalline solids can become band insulators due to fully filled bands, or Mott insulators due to strong electronic correlations. While Mott insulators can theoretically occur in systems with an even number of electrons per unit cell, distinguishing them from band insulators experimentally has remained a longstanding challenge. In this work, we present a unique momentum-resolved signature of a dimerized Mott-insulating phase in the experimental spectral function of Nb₃Br₈: the top of the highest occupied band along the out-of-plane direction k_z has a momentum-space separation Δk_z = 2π/d, whereas that of a band insulator is less than π/d, where d is the average interlayer spacing. Identifying Nb₃Br₈ as a Mott insulator is crucial to understand its role in the field-free Josephson diode effect. Moreover, our method could be extended to other van der Waals systems where tuning interlayer coupling and Coulomb interactions can drive a band- to Mott-insulating transition.

By combining ab initio downfolding with cluster dynamical mean-field theory, we study the degree of correlations in monolayer, bilayer, and bulk breathing-mode kagome van der Waals materials Nb₃(F, Cl, Br, I)₈. Our new material-specific many-body model library shows that in low-temperature bulk structures the Coulomb correlation strength steadily increases from I to Br, Cl, and F, allowing us to identify Nb₃I₈ as a weakly correlated insulator whose gap is only mildly affected by the local Coulomb interaction. Nb₃Br₈ and Nb₃Cl₈ are strongly correlated insulators, whose gaps are significantly influenced by Coulomb-induced vertex corrections. Nb₃F₈ is a prototypical bulk Mott insulator whose gap is initially opened by strong correlation effects. Angle-resolved photoemission spectroscopy measurements comparing Nb₃Br₈ and Nb₃I₈ allow us to experimentally confirm these findings by revealing spectroscopic footprints of the degree of correlation. Our calculations further uncover how the thickness and the stacking affect the degree of correlations and predict that the entire material family can be tuned into correlated charge transfer or Mott-insulating phases upon electron or hole doping. Our magnetic property analysis based on our model parameter library additionally confirms that interlayer magnetic interactions likely drive the lattice phase transition to the low-temperature structures. The accompanying bilayer hybridization through interlayer dimerization yields magnetic singlet-like ground states in the Cl, Br, and I compounds. We further prove that all low-temperature compounds are dynamically stable and that electron-phonon coupling to the low-energy subspace is suppressed. Our findings establish Nb₃X₈ as a robust, versatile, and tunable class for van der Waals-based Coulomb and Mott engineering with a rich phase diagram and allow us to speculate on the symmetry-breaking effects necessary for the recently observed Josephson diode effect in NbSe₂/Nb₃Br₈/NbSe₂ heterostructures.

2D ferroelectric (FE) materials have opened new opportunities in non-volatile memories, computation and non-linear optics due to their robust polarization in the ultra-thin limit and inherent flexibility in device integration. Recently, interest has grown in the use of 2D FEs in electro-optics, demanding the exploration of their electronic and optical properties. In this work, the discovery of an unprecedented anomalous thickness-dependent change in refractive index, as large as δn ∼ 23.2%, is reported in the 2D ferrielectric CuInP₂S₆, far above the ultra-thin limit, and at room temperature. It is also shown that the anomalous behavior in CuInP₂S₆ may be generalizable to other ferroelectric materials such as LiNbO₃. Furthermore, CuInP₂S₆ exhibits a giant birefringence in the blue-ultraviolet regime, with a maximum |n_OOP − n_IP| ∼ 1.24 at t ∼ 22 nm and λ = 339.5 nm, which is, to the best of our knowledge, the largest of any known material in this wavelength regime. Changes in the optical constants of CuInP₂S₆ are related to changes in the Cu(I) FE polarization contribution, inducing changes in its ionic mobility, and opening the door to electronic control of its optical response for use in photonics and electro-optics.

In solid materials, non-trivial topological states, electron correlations and magnetism are central ingredients for realizing quantum properties, including unconventional superconductivity, charge and spin density waves and quantum spin liquids. The kagome lattice, made up of corner-sharing triangles, can host these three ingredients simultaneously and has proved to be a fertile platform for studying diverse quantum phenomena including those stemming from the interplay of these ingredients. This Review introduces the fundamental properties of the kagome lattice and discusses the complex phenomena observed in several materials systems, including the intertwining of charge order and superconductivity in some kagome metals, the modulation of magnetism and topology in some kagome magnets, and the combination of symmetry breaking and Mott physics in ‘breathing’ kagome insulators. The Review also highlights open questions in the field and future research directions in kagome systems.

Materials with Kagome nets are of particular importance for their potential combination of strong correlation, exotic magnetism, and electronic topology. KV3Sb5 was discovered to be a layered topological metal with a Kagome net of vanadium. Here, we fabricated Josephson Junctions of K1-xV3Sb5 and induced superconductivity over long junction lengths. Through magnetoresistance and current versus phase measurements, we observed a magnetic field sweeping direction-dependent magnetoresistance and an anisotropic interference pattern with a Fraunhofer pattern for in-plane magnetic field but a suppression of critical current for out-of-plane magnetic field. These results indicate an anisotropic internal magnetic field in K1-xV3Sb5 that influences the superconducting coupling in the junction, possibly giving rise to spin-triplet superconductivity. In addition, the observation of long-lived fast oscillations shows evidence of spatially localized conducting channels arising from edge states. These observations pave the way for studying unconventional superconductivity and Josephson device based on Kagome metals with electron correlation and topology.

There is an ongoing interest in kagome materials because they offer tunable platforms at the intersection of magnetism and electron correlation. Herein, we examine single crystals of new kagome materials, LnxCo3(Ge1-ySny)3 (Ln = Y, Gd; y = 0.11, 0.133), which were produced using the Sn flux-growth method. Unlike many of the related chemical analogues with the LnM6X6 formula (M = transition metal and X = Ge, Sn), the Y and Gd analogues crystallize in a hybrid YCo6Ge6/CoSn structure, with Sn substitution. While the Y analogue displays temperature-independent paramagnetism, magnetic measurements of the Gd analogue reveal a magnetic moment of 8.48 μB, indicating a contribution from both Gd and Co. Through anisotropic magnetic measurements, the direction of Co-magnetism can be inferred to be in plane with the kagome net, as the Co contribution is only along H//a. Crystal growth and structure determination of YxCo3(Ge,Sn)3 and GdxCo3(Ge,Sn)3, two new hybrid kagome materials of the CoSn and YCo6Ge6 structure types. Magnetic properties, heat capacity, and resistivity on single crystals are reported.

The superconducting analogue to the semiconducting diode, the Josephson diode, has long been sought with multiple avenues to realization being proposed by theorists^1–3. Showing magnetic-field-free, single-directional superconductivity with Josephson coupling, it would serve as the building block for next-generation superconducting circuit technology. Here we realized the Josephson diode by fabricating an inversion symmetry breaking van der Waals heterostructure of NbSe₂/Nb₃Br₈/NbSe₂. We demonstrate that even without a magnetic field, the junction can be superconducting with a positive current while being resistive with a negative current. The ΔI_c behaviour (the difference between positive and negative critical currents) with magnetic field is symmetric and Josephson coupling is proved through the Fraunhofer pattern. Also, stable half-wave rectification of a square-wave excitation was achieved with a very low switching current density, high rectification ratio and high robustness. This non-reciprocal behaviour strongly violates the known Josephson relations and opens the door to discover new mechanisms and physical phenomena through integration of quantum materials with Josephson junctions, and provides new avenues for superconducting quantum devices.

The Kagome lattice is an important fundamental structure in condensed matter physics for investigating the interplay of electron correlation, topology, and frustrated magnetism. Recent work on Kagome metals in the AV3Sb5 (A = K, Rb, and Cs) family has shown a multitude of correlation-driven distortions, including symmetry breaking charge density waves and nematic superconductivity at low temperatures. Here, we study the new Kagome metal Yb0.5Co3Ge3 and find a temperature-dependent kink in the resistivity that is highly similar to the AV3Sb5 behavior and is commensurate with an in-plane structural distortion of the Co Kagome lattice along with a doubling of the c-axis. The symmetry is lower below the transition temperature, with a breaking the in-plane mirror planes and C6 rotation, while gaining a screw axis along the c-direction. At very low temperatures, anisotropic negative magnetoresistance is observed, which may be related to anisotropic magnetism. This raises questions about the types of the distortions in Kagome nets and their resulting physical properties including superconductivity and magnetism.

It has been suggested that certain antiferromagnetic topological insulators contain axion quasiparticles (AQs), and that such materials could be used to detect axion dark matter (DM). The AQ is a longitudinal antiferromagnetic spin fluctuation coupled to the electromagnetic Chern-Simons term, which, in the presence of an applied magnetic field, leads to mass mixing between the AQ and the electric field. The electromagnetic boundary conditions and transmission and reflection coefficients are computed. A model for including losses into this system is presented, and the resulting linewidth is computed. It is shown how transmission spectroscopy can be used to measure the resonant frequencies and damping coefficients of the material, and demonstrate conclusively the existence of the AQ. The dispersion relation and boundary conditions permit resonant conversion of axion DM into THz photons in a material volume that is independent of the resonant frequency, which is tuneable via an applied magnetic field. A parameter study for axion DM detection is performed, computing boost amplitudes and bandwidths using realistic material properties including loss. The proposal could allow for detection of axion DM in the mass range between 1 and 10 meV using current and near future technology.