Quantum spin Hall insulators (QSHIs) and excitonic insulators (EIs) are prototypical topological and correlated states of matter, respectively. The topological phase transition between the two has attracted much theoretical interest but experimental studies have been hindered by the availability of tunable materials that can access such a transition. Here, by utilizing the interaction-enhanced g-factor and the flat moiré bands in twisted bilayer WSe2 (tWSe2), we realize tunable electron-like and hole-like Landau levels (LLs) in the opposite valleys of tWSe2 under a perpendicular magnetic field. At half-band-filling, which corresponds to electron-hole charge neutrality, periodic oscillations between QSHIs (for fully filled LLs) and EIs (for half-filled LLs) are observed due to the interplay between the cyclotron energy and the intervalley correlation; QSHIs with up to four pairs of helical edge states can be resolved. We further analyze the effect of Fermi surface nesting on the stability of EIs via electric field-tuning of the moiré band structure. Our results demonstrate a novel QSHI-to-EI topological phase transition and provide a comprehensive understanding of the fermiology of tWSe2.

Rhombohedral multilayer graphene has emerged as a promising platform for exploring correlated and topological quantum phases, enabled by its Berry-curvature-bearing flat bands. While prior work has focused on separated conduction and valence bands, we probe the extensive semimetallic regime of rhombohedral hexalayer graphene. We survey a rich phase diagram dominated by flavor-symmetry breaking and reveal an electric-field-driven band inversion through fermiology. Near this inversion, we find a superconducting-like state confined to a region with emergent electron and hole Fermi surfaces. In addition, two multiferroic orbital-magnetic phases are observed: a ferrovalley state near zero field and a ferroelectric state at large fields around charge neutrality. The latter shows electric-field-reversible magnetic hysteresis, consistent with a $\Delta P \cdot M$ multiferroic order parameter. Our fermiology analysis elucidates the correlated semimetal regime in rhombohedral graphene and underscores its potential to host diverse quantum phases.

Light-matter coupled Hamiltonians are central to cavity materials engineering and polaritonic chemistry, but are challenging to simulate with classical hardware due to the scaling of the Hilbert space with the number of quantum photon modes and matter complexity. Leveraging the fact that quantum computers naturally represent photonic modes efficiently, we present a novel approach to simulate quantum-electrodynamical (QED) systems on near-term quantum hardware. After developing the bosonic and mixed operators in the Qiskit Nature framework, we employ them to simulate a first-order Trotterized Hamiltonian for a spontaneous-emission problem of a two-level system in an optical cavity. We find that using a standing-waves photonic basis approach leads to fidelity issues due to hardware connectivity constraints and two-qubits gates errors. Hence, we propose using a localized photonic basis approach that enforces nearest-neighbor couplings, thanks to which we can map the Hamiltonian as a 1D qubit chain. We significantly reduce the noise and, by applying the zero-noise extrapolation error mitigation technique, we recover the accurate quantum dynamics. Finally, we also show that this approach is resilient when relaxing the 1D qubit chain approximation.

Attosecond light sources based on high-order harmonic generation (HHG) constitute to date the only table-top solution for producing coherent broadband radiation covering the spectral range from the extreme ultraviolet to the soft X-rays. The so-called emission cutoff can be extended towards higher photon energies by increasing the driving wavelength at the expense of conversion efficiency. An alternative route is to overdrive the process by using higher laser intensities, with the challenging requirement of interacting with higher plasma densities over short propagation distances. Here, we address this challenge by using a differentially pumped glass chip designed for optimal gas confinement over sub-mm lengths. By driving HHG with multicycle pulses at either 800 nm or 1500 nm, we demonstrate a cutoff extension by a factor of two compared to conventional phase matching approaches and surpassing the present record using multicycle fields. Our three-dimensional propagation simulations, in excellent agreement with the experiment, confirm that gas confinement is crucial since efficient phase matching of cutoff harmonics occurs only for short propagation lengths. Additionally, we show that the high photon energy component is not only temporally confined to the leading edge of the driving pulse, but also spatially confined in the near-field to an off-axis contribution due to reshaping of the driving field along propagation inside the medium. Our findings contribute to the fundamental understanding of HHG across different regimes.

The emergence of high transition temperature (Tc) superconductivity in strongly correlated materials remains a major unsolved problem in physics. High-Tc materials, such as cuprates, are generally complex and not easily tunable, making theoretical modelling difficult. Although the Hubbard model--a simple theoretical model of interacting electrons on a lattice--is believed to capture the essential physics of high-Tc materials, obtaining accurate solutions of the model, especially in the relevant regime of moderate correlation, is challenging. The recent demonstration of robust superconductivity in moiré WSe2, whose low-energy electronic bands can be described by the Hubbard model and are highly tunable, presents a new platform for tackling the high-Tc problem. Here, we tune moiré WSe2 bilayers to the moderate correlation regime through the twist angle and map the phase diagram around one hole per moiré unit cell (v = 1) by electrostatic gating and electrical transport and magneto-optical measurements. We observe a range of high-Tc phenomenology, including an antiferromagnetic insulator at v = 1, superconducting domes upon electron and hole doping, and unusual metallic states at elevated temperatures including strange metallicity. The highest Tc occurs adjacent to the Mott transition, reaching about 6% of the effective Fermi temperature. Our results establish a new material system based on transition metal dichalcogenide (TMD) moiré superlattices that can be used to study high-Tc superconductivity in a highly controllable manner and beyond.

Twisted bilayer transition metal dichalcogenide semiconductors, which support flat Chern bands with enhanced interaction effects, realize a platform for fractional Chern insulators and fractional quantum spin Hall (FQSH) insulators. A recent experiment has reported the emergence of a FQSH insulator protected by spin-Sz conservation at a moir\'e lattice filling factor {\nu}=3 in 2.1-degree twisted bilayer MoTe2. Theoretical studies have proposed both time-reversal symmetric and asymmetric ground states as possible candidates for the observed FQSH insulator, but the nature of the state remains unexplored. Here we report the observation of spontaneous time-reversal symmetry breaking at generic fillings in 2.1-degree twisted bilayer MoTe2 from {\nu}<1 all the way to {\nu}>6 except at {\nu}=2, 4, and 6. Although zero Hall response is observed at {\nu}=3 for magnetic fields higher than 20 mT, a finite anomalous Hall response accompanied by a magnetic hysteresis is observed at lower magnetic fields, demonstrating spontaneous time-reversal symmetry breaking. Our work shows the tendency towards ferromagnetism by doping the first three pairs of conjugate Chern bands in the material; it also sheds light on the nature of the FQSH insulator at {\nu}=3.

Chirality is a pervasive property of matter that underpins many important phenomena across physics, chemistry and biology. Given its broad significance, the development of protocols for rational control of chirality in solid state systems is highly desirable, especially if this effect can be tuned continuously and in two directions. Yet, this goal has remained elusive due to the absence of a universal conjugate field that couples linearly to this structural order. Here, we introduce the piezochiral effect, which enables control of chirality through mechanical strain. We first show by symmetry analysis that uniaxial strain induces chirality in a broad class of achiral crystals that host fragments of opposite chirality within each unit cell, an effect that has so far remained unrecognized. The strain-induced handedness can be tuned either by changing the strain direction or by switching between compressive and tensile strain. We experimentally verify this effect in AgGaS2, using measurements of the optical activity under strain. Our discovery establishes a new scheme for chirality control, with potential applications that range from spintronics to asymmetric catalysis, and enantioselective interactions in biosystems.

Novel theoretical developments have allowed to connect microscopic disorder in bosonic collective excitations to the signatures in two-dimensional terahertz spectroscopy (Gómez Salvador et al. 2025). Here, we employ this framework to analyze the recently measured Josepshon echoes in optimally doped La$_{2-x}$Sr$_x$CuO$_4$ (Liu et al. 2024). We consider the spatial gap inhomogeneities -observed in scanning tunneling microscopy- as input for the disorder in the superfluid density, and compute the resulting echo peaks. The excellent agreement supports the interpretation that the gap inhomogeneity arises solely from pairing gap fluctuations, with no evidence for non-superconducting competing local orders. Finally, we study the microscopic origin of the inelastic processes, contributing to the damping of the Josephson plasmon at low temperatures, and conclude that it can be attributed to nodal quasiparticles.

Topological Kondo insulators (TKIs) are topologically protected insulating states induced not by single-particle band inversions, but by the Kondo interaction between itinerant electrons and a lattice of local magnetic moments. Although experiments have suggested the emergence of three-dimensional (3D) TKIs in the rare earth compound SmB6, its two-dimensional (2D) counterpart has not been demonstrated to date. Here we report experimental evidence of a TKI in angle-aligned MoTe2/WSe2 moiré bilayers, which support a Kondo lattice with topologically nontrivial Kondo interactions. We prepare in a dual-gated device a triangular lattice Mott insulator in the MoTe2 layer Kondo-coupled to a half-filled itinerant band in the WSe2 layer. Combined transport and compressibility measurements show that the prepared state supports metallic transport at high temperatures and, at low temperatures, an insulating bulk with conducting helical edge states protected by spin-Sz conservation. The presence of Kondo singlets is further evidenced by their breakdown at high magnetic fields. Such behaviors are in stark contrast to the simple metallic state when the Mott insulator in the MoTe2 layer is depleted by gating. Our results open the door for exploring tunable topological Kondo physics in moiré materials.

Ultrafast switching of ferroic phases is an important research frontier, with significant technological potential. Yet, current efforts are meeting some key challenges, ranging from limited speeds in ferromagnets to intrinsic volatility of switched domains due to uncompensated depolarizing fields in ferroelectrics. Unlike these ferroic systems, ferroaxial materials host bistable states that do not break spatial-inversion or time-reversal symmetry, and are therefore immune to depolarizing fields. Yet, they are difficult to manipulate because external axial fields are not easily constructed with conventional methods. Here, we demonstrate ultrafast switching of ferroaxial order by engineering an effective axial field made up of circularly driven terahertz phonon modes. A switched ferroaxial domain remains stable for many hours and can be reversed back with a second terahertz pulse of opposite helicity. The effects demonstrated here may lead to a new platform for ultrafast information storage.

The interaction between electron spin and molecular chirality plays a fundamental role in quantum phenomena, with significant implications for spintronics and quantum computing. The chirality-induced spin selectivity (CISS) effect, where chiral materials preferentially transmit electrons of a particular spin, has sparked intense interest and debate regarding its underlying mechanism. Despite extensive research, the spatial distribution of spin polarization in chiral systems, the key evidence to reveal the spin scattering mechanism in CISS, has remained experimentally elusive particularly due to complications arising from spin-orbit coupling in metal electrodes typically used in such studies. Here we show, through reflective magnetic circular dichroism measurements on chiral tellurium nanowires with graphene electrodes, that current-induced spin polarization exhibits identical signs in both the nanowire and electrodes, distinct from the presumed spin filter scenario. The observed spin polarization scales linearly with current amplitude, aligns parallel to the current direction, reverses with chirality or current flow, and demonstrates spin relaxation lengths of several micrometers into graphene. Our findings provide the first direct visualization of spatial spin distribution in chiral devices. This work establishes a new paradigm for investigating spin-dependent phenomena in chiral materials and opens avenues for developing chirality-based spintronic and quantum devices.

Hysteretic gate responses of two-dimensional material heterostructures serve as sensitive probes of the underlying electronic states and hold significant promise for the development of novel nanoelectronic devices. Here we identify a new mechanism of hysteretic behavior in graphene/$h$BN/$\alpha$-$\mathrm{RuCl_3}$ charge transfer field effect devices. The hysteresis loop exhibits a sharp onset under low temperatures and evolves symmetrically relative to the charge transfer equilibrium. Unlike conventional flash memory devices, the charge transfer heterostructure features a transparent tunneling barrier and its hysteretic gate response is induced by the dynamic tuning of interfacial dipoles originating from quantum exchange interactions. The system acts effectively as a ferroelectric and gives rise to remarkable tunability of the hysteretic gate response under external electrical bias. Our work unveils a novel mechanism for engineering hysteretic behaviors via dynamic interfacial quantum dipoles.

We construct an interacting Wannier model for both AA-stacked and AB-stacked twisted SnSe2, revealing a rich landscape of correlated quantum phases. For the AA-stacked case, the system is effectively described by a three-orbital triangular lattice model, where each orbital corresponds to a valley and exhibits an approximate one-dimensional hopping structure due to a new momentum-space non-symmorphic symmetry. By exploring the interacting phase diagram using a combination of theoretical methods, including Hartree-Fock mean-field theory and exact solutions of the spin model in certain limits, we identify several exotic quantum phases. These include a dimerized phase with finite residual entropy, valence bond solids, and quantum paramagnetism. In the AB-stacked case, the system realizes an interacting kagome lattice model, where the Wannier orbitals associated with the three valleys form three sublattices. In the strong coupling regime, we use cluster mean-field methods to demonstrate the emergence of a classical spin liquid phase due to the frustrated lattice structure. The high tunability of the moiré system, which allows control over both the filling and interaction strength (via twist angle), renders twisted SnSe2 a versatile platform for realizing a wide range of exotic correlated quantum phases.

Carlo is a Monte Carlo simulation framework written in Julia. It provides MPI-parallel scheduling, organized storage of input, checkpoint, and output files, as well as statistical postprocessing. With a minimalist design, it aims to aid the development of high-quality Monte Carlo codes, especially for demanding applications in condensed matter and statistical physics. This hands-on user guide shows how to implement a simple code with Carlo and provides benchmarks to show its efficacy.

High-resolution spectroscopy allows one to probe weak interactions and to detect subtle phenomena. While such measurements are routinely performed on atoms and molecules in the gas phase, spectroscopy of adsorbed species on surfaces is faced with challenges. As a result, previous studies of surface-adsorbed molecules have fallen short of the ultimate resolution, where the transition linewidth is determined by the lifetime of the excited state. In this work, we conceive a new approach to surface deposition and report on Fourier-limited electronic transitions in single dibenzoterrylenes adsorbed onto the surface of an anthracene crystal. By performing spectroscopy and super-resolution microscopy at liquid helium temperature, we shed light on various properties of the adsorbed molecules. Our experimental results pave the way for a new class of experiments in surface science, where high spatial and spectral resolution can be combined.

Twisted bilayer graphene (tBLG) near the magic angle is a unique platform where the combination of topology and strong correlations gives rise to exotic electronic phases. These phases are gate-tunable and related to the presence of flat electronic bands, isolated by single-particle band gaps. This enables gate-controlled charge confinement, essential for the operation of single-electron transistors (SETs), and allows to explore the interplay of confinement, electron interactions, band renormalisation and the moiré superlattice, potentially revealing key paradigms of strong correlations. Here, we present gate-defined SETs in near-magic-angle tBLG with well-tunable Coulomb blockade resonances. These SETs allow to study magnetic field-induced quantum oscillations in the density of states of the source-drain reservoirs, providing insight into gate-tunable Fermi surfaces of tBLG. Comparison with tight-binding calculations highlights the importance of displacement-field-induced band renormalisation crucial for future advanced gate-tunable quantum devices and circuits in tBLG including e.g. quantum dots and Josephson junction arrays.

Light-dressed materials hold enormous potential for generating new electronic properties. The band structure resulting from light-dressing can exhibit starkly different quantum and topological phenomena. So far, optical control of charge within a light-dressed band structure has been elusive. Here, we demonstrate optical control of electrons in light-dressed graphene. By focusing circularly polarized femtosecond laser pulses at 1550 nm on monolayer graphene, we generate a Floquet topological insulator (FTI). With a phase-locked second harmonic field, we dynamically control electrons in this FTI state. For the first time, we observe photocurrent circular dichroism, the all-optical anomalous Hall effect, and FTI valley-polarized currents. The photocurrents show strong sub-cycle phase-sensitivity, opening the door to ultrafast control within topologically protected electronics (topotronics), spectroscopy, and attosecond physics in novel quantum materials.

Recent advances in the field of condensed-matter physics have unlocked the potential to realize and control emergent material phases that do not exist in thermal equilibrium. One of the most promising concepts in this regard is Floquet engineering, the coherent dressing of matter via time-periodic perturbations. However, the broad applicability of Floquet engineering to quantum materials is still unclear. For the paradigmatic case of monolayer graphene, the theoretically predicted Floquet-induced effects, despite a seminal report of the light-induced anomalous Hall effect, have been put into question. Here, we overcome this problem by using electronic structure measurements to provide direct experimental evidence of Floquet engineering in graphene. We report light-matter-dressed Dirac bands by measuring the contribution of Floquet sidebands, Volkov sidebands, and their quantum path interference to graphene's photoemission spectral function. Our results finally demonstrate that Floquet engineering in graphene is possible, paving the way for the experimental realization of the many theoretical proposals on Floquet-engineered band structures and topological phases.

Stimulated Raman scattering (SRS) is a powerful method for label-free imaging and spectroscopy of materials. Recent experiments have shown that quantum-enhanced Raman scattering can surpass the shot noise limit and improve the sensitivity substantially. Here, we introduce a full theory of quantum-enhanced SRS based on the framework of quantum metrology. Our results enable the assessment of quantum-enhancements of arbitrary measurement strategies and identify optimal measurement observables that extract maximal information about the signal. We use this to identify the optimal employment of squeezed states in SRS, highlighting the potential to improve quantum gains beyond those observed in recent experiments. Our work establishes the theoretical foundation for understanding and approaching the quantum limits of precision in SRS, and provide a tool to discuss nonlinear spectroscopy and imaging more broadly.