Moiré patterns of twisted and scaled bilayers have recently emerged as a fertile source of quasiperiodic order in two-dimensional materials. Inspired by these systems, we introduce the near-coincidence method for generating quasiperiodic tilings of the plane. The method is intuitive—admitting pairs of nearly coincident points from superimposed layers—yet rigorous, as it maps naturally to the well-established cut-and-project formalism. It reproduces classical tilings, including the Ammann-Beenker, the Niizeki-Gähler, and the square and hexagonal Fibonacci tilings, and also reveals new tilings that are unlikely to arise from conventional constructions. The near-coincidence method is algorithmically simple and already realized in a web-based application that generates tilings from specified layer parameters and coincidence conditions. Future extensions include trilayer systems, where preliminary results yield dodecagonal order with square layers, and very small twist angles, where the method may capture the giant moiré patterns of bilayer and trilayer graphene.

We present a symmetry-resolved classification of two-dimensional spin-flip small-angle neutron scattering (SANS) patterns arising from dilute ensembles of spherical nanoparticles hosting magnetic vortex states. Based on a linear vortex ansatz with an axially symmetric distribution of vortex axes and the corresponding analytical expression for the orientationally averaged spin-flip SANS cross section, we show that the angular scattering patterns organize into four distinct symmetry regimes: a fourfold anisotropy corresponding to coherent field-aligned magnetization, vertical and horizontal twofold anisotropies associated with aligned and isotropically distributed vortex ensembles, and an isotropic ringlike condition separating the two twofold regimes. The corresponding symmetry boundaries are obtained analytically and define a compact symmetry landscape in the parameter space of vortex amplitude and vortex-axis distribution width. Comparison with a nonlinear vortex profile shows that these symmetry regions are robust with respect to the detailed radial structure of the vortex core. The angular anisotropies are therefore governed primarily by rotational symmetry and by the statistical distribution of vortex axes, providing a compact and model-transparent classification framework of experimental polarized SANS data.

Chromium thiophosphate (${\mathrm{CrPS}}_4$) is a long-known material: A layered semiconducting antiferromagnet. Its recently discovered gate-tunable metamagnetic phase transitions, the remarkable positive and oscillating magnetoresistance as a tunnel barrier, and its Fano-resonance luminescence, elusive among the multitude of ${\mathrm{Cr}}^{3+}$ compounds, call for revisiting the understanding of its electronic structure, especially regarding how it relates to magnetic order. Here, we employ X-ray magnetic circular dichroism, implemented in both absorption and resonant inelastic X-ray spectroscopies, together with quantum many-body calculations, to unveil the role of metal-ligand covalency in mediating the metamagnetic transitions in ${\mathrm{CrPS}}_4$, using crystal-field and charge-transfer excitations as fingerprints of the evolving magnetic order. We reveal the role of extended superexchange paths involving P and S atoms, coupling interactions between the Cr spins across the different magnetic phases—antiferromagnetic, canted, and ferromagnetic. Our results elucidate the electronic states involved in these phases and provide prescriptions for engineering the metamagnetic phase diagram of ${\mathrm{CrPS}}_4$.

We present a framework for characterizing higher-order topological phases directly from the one-particle density matrix, without any reference to an underlying Hamiltonian. Our approach extends the mode-shell correspondence, originally formulated for single-particle Hamiltonians, to Gaussian states subject to chiral constraints. In this correspondence, the mode index counts topological boundary modes, while the shell index quantifies the bulk topology in a region surrounding the modes, providing a bulk-boundary diagnostic. In one-dimensional topological insulators, the shell index reduces to the local chiral marker, recovering the winding number in the translation-invariant limit. We apply the mode-shell correspondence to a $C_4$-symmetric higher-order topological insulator with a chiral constraint and show that a fractional shell index implies that the higher-order phase is intrinsic. The one-particle density matrix is formulated in real space, so the mode-shell correspondence also applies to models without translation invariance. By introducing structural disorder into the $C_4$-symmetric higher-order insulator, we show that the mode-shell correspondence remains a meaningful diagnostic in amorphous structures. The mode-shell correspondence generalizes to interacting states with a gapped bulk spectrum in the one-particle density matrix, providing a practical and diverse route to characterize higher-order topology from the quantum state itself.

We investigate the behavior of interstitial ${}^8\mathrm{Li}^{+}$ implanted near the surface of $2H\text{−}{\mathrm{MoTe}}_2$ using $\beta$-detected nuclear magnetic resonance. We find that, unlike the muon, ${}^8\mathrm{Li}^{+}$ does not show any signature of induced magnetism. This result is consistent with density functional theory, which identifies the Li stopping site at the 2a Wyckoff position in the van der Waals gap and confirms the absence of detectable Li-induced electronic spin polarization. Both the spin-lattice relaxation and the resonance line shape show evidence of pronounced spin dynamics above $\sim 200\mathrm{K}$, reminiscent of local stochastic ${}^8\mathrm{Li}^{+}$ motion within a cage. The resonance line shape consists of quadrupolar satellites on top of a broad central peak. To better understand the interaction of ${}^8\mathrm{Li}^{+}$ with the host material, we employ a frequency-comb measurement by simultaneously exciting four frequencies corresponding to the first-order quadrupolar satellite transitions, ${\nu }_0\pm 3{\nu }_{\mathrm{comb}}$ and ${\nu }_0\pm {\nu }_{\mathrm{comb}}$, around the Larmor frequency ${\nu }_0$ as a function of ${\nu }_{\mathrm{comb}}$. This offers an enhanced sensitivity to the quadrupolar split portion of the line. Using this method, we find a small decrease in the quadrupolar frequency with increasing temperature, showing the typical behavior associated with thermally excited phonons and the absence of any magnetic response, in contrast to what was observed for other defects in $2H\text{−}{\mathrm{MoTe}}_2$.

We present optimal control strategies for the dc transport across a Josephson junction. Specifically, we consider a junction in which the Josephson coupling is driven parametrically, with either a bichromatic or a trichromatic driving protocol, and optimize the prefactor of the $1/\omega$ divergence of the imaginary part of the conductivity, which we refer to as the effective Josephson coupling $J_{\mathrm{eff}}$. We demonstrate that for an optimal bichromatic protocol an enhancement of $J_{\mathrm{eff}}$ by a factor of 70 can be reached, and for an optimal trichromatic protocol an enhancement by a factor of 135. This is motivated by pump-probe experiments that have demonstrated light-enhanced superconductivity along the $c$ axis of underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+\delta }$ (YBCO), where the junction serves as a minimal model for the $c$-axis coupling of superconducting layers. Therefore, the significant enhancement of superconductivity that we show for multifrequency protocols demonstrates that the advancement of pump-probe technology toward these strategies is highly desirable.

Strong directional disorder in local magnetic moments coupled to a Chern insulator gives rise to topological phases that cannot be realized in any clean system and are therefore genuinely disorder driven. We demonstrate this in a spinful Qi-Wu-Zhang model of a two-dimensional Chern insulator coupled to disordered classical spins of unit length. The topological phase diagram is computed numerically using two complementary approaches: twisted boundary conditions and the topological Hamiltonian technique. Our results show that strong disorder can act as a fundamental topological mechanism rather than merely a perturbation. For strong exchange coupling, tuning the mass parameter reveals a transition between phases with different Chern numbers $C$. Remarkably, this transition is driven by zeros, rather than poles, of the disorder-averaged Green’s function crossing the chemical potential, and has no analog in any clean system. We further identify a strong-coupling phase with $C=0$ that is nonetheless topologically nontrivial, characterized by a distinct Chern number $C^{\left(\mathrm{S}\right)}\ne 0$ over the manifold of classical spin configurations. This phase is also disorder driven, as $C^{\left(\mathrm{S}\right)}=0$ in the clean limit.

We propose a hybrid quantum system in which a magnet supporting nonreciprocal magnons, chiral magnons, or both mediates the dissipative and unidirectional coupling of spin qubits. By driving the qubits, the steady state of this qubit-qubit coupling scheme becomes the maximally entangled Bell state. We devise a protocol where the system converges to this entangled state and benchmark it including qubit decay and dephasing. The protocol is numerically tested on a hybrid system consisting of nitrogen-vacancy (NV) centers coupled to magnon surface modes of a yttrium iron garnet (YIG) film. We show that the dephasing time of the NV centers forms the bottleneck for achieving the entanglement of NV centers separated by a distance within the magnon coherence length. Our findings identify the key technological requirements and demonstrate a viable route toward steady-state entanglement of solid-state spins over distances of several microns using magnonic quantum networks, expanding the toolbox of magnonics for quantum information purposes.

We report the study of spin dynamics of ${\mathrm{Eu}}^{2+}$ in the antiferromagnetic axion topological insulator ${\mathrm{EuSn}}_2{\mathrm{As}}_2$ by means of antiferromagnetic resonance at 9.34 GHz. Below the Néel temperature ($T_{\mathrm{N}}$), two types of resonance modes, the conventional bulk antiferromagnetic resonance and additional surface ferromagnetic resonance, are observed. The latter turns out to be characteristic of the axion-insulating phase. Above $T_{\mathrm{N}}$, we prove the existence of a Kosterlitz-Thouless scenario that is relevant for the spin relaxation in the ${\mathrm{Eu}}^{2+}$ layers. The absence of Korringa relaxation indicates the strong confinement of the conduction electrons at the Fermi level to the SnAs layers.

The optical generation of an out-of-equilibrium spin population is a keystone process for quantum technologies and spintronics alike. Although this is an established technique for studying direct band-gap semiconductors, it has been proven limited in materials that possess weak oscillator strengths for the optical transitions. We address the problem by presenting an all-optical analog of the spin-pumping method. By applying this concept to a Ge-on-Si heterostructure, we observe luminescence from Si with a polarization degree as high as 9%. The progressive etching of the absorbing layer, assisted by magneto-optic experiments, allows us to ascertain that the polarized emission is determined by effective spin injection aided by the carrier lifetime shortening due to extended defects. These findings can facilitate the use of highly promising spin-dependent phenomena of Si, whose optical exploitation has been hampered by fundamental limitations due to its peculiar electronic structure.

We present a data-efficient framework for constructing general classical spin Hamiltonians by combining the spin-cluster expansion (SCE) with fully self-consistent noncollinear spin density functional theory (DFT). The key idea is to fit the SCE model to magnetic torques rather than to total energies. Because torques are site-resolved vectors, each spin configuration provides many informative regression targets, improving conditioning and substantially reducing the number of required DFT calculations, especially for large supercells. Applied to the B20-type chiral magnets ${\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x\mathrm{Ge}$ and ${\mathrm{Fe}}_{1-y}{\mathrm{Co}}_y\mathrm{Ge}$, the resulting SCE models determine full pairwise exchange tensors—including isotropic exchange, symmetric anisotropic exchange, and the Dzyaloshinskii-Moriya interaction—and predict the helical spin period via a micromagnetic mapping. The composition trends and the divergence of the period at the chirality sign-change point are well reproduced, in agreement with experiment. Moreover, the systematic nature of SCE enables controlled assessment of interaction order: As the training spin configurations become more disordered, the lowest-order model loses torque accuracy, whereas including higher-order interactions restores predictive power. These advances enable near-DFT-accurate spin models for finite-temperature magnetism and complex spin textures at modest computational cost, providing an extensible route to quantitative first-principles parametrization and predictive materials design.

Computing spectral functions in large, nonperiodic super-moiré systems remains an open problem due to the exceptionally large system size that must be considered. Here, we establish a tensor network methodology that allows computing momentum-resolved spectral functions of noninteracting and interacting super-moiré systems at an atomistic level. Our methodology relies on encoding an exponentially large tight-binding problem as an auxiliary quantum many-body problem, solved with a many-body kernel polynomial tensor network algorithm combined with a quantum Fourier transform tensor network. We demonstrate the method for one- and two-dimensional super-moiré systems, including super-moiré with nonuniform strain, interactions treated at the mean-field level, and quasicrystalline super-moiré patterns. Furthermore, we demonstrate that our methodology allows us to compute momentum-resolved spectral functions restricted to selected regions of a super-moiré, enabling direct imaging of position-dependent electronic structure and minigaps in super-moiré systems with nonuniform strain. Our results establish a powerful methodology to compute momentum-resolved spectral functions in exceptionally large super-moiré systems, providing a tool to directly model quantum twisting microscope experiments in twisted van der Waals heterostructures.

Using a linear hydrodynamic model, we theoretically investigate the interaction between penetrating electron beams and sub-5-nm metallic spherical nanoparticles. We derive an analytical expression of the electron energy loss probability that includes nonlocal effects in the response of the confined electron gas. Our study focuses on the longitudinal plasmon excitations—also known as confined bulk plasmons (CBPs)—which are inaccessible within local dielectric frameworks, and it shows that their excitation is highly sensitive to the impact parameter, the kinetic energy of the incident electron, and the nanoparticle size. In contrast to local approaches, our model predicts the emergence of several peaks associated with CBPs above the plasma frequency and a blueshift of their spectral envelope as the nanoparticle size decreases and the impact parameter of the electron beam increases from its center toward the surface. Moreover, it predicts a threshold impact parameter, corresponding to the minimum length of the electron's trajectory inside the nanoparticle required for efficient excitation of specific CBP modes. By exploiting a multipolar description of the CBPs, we identify the underlying selection rules governing their excitation by electron beams and correlate the observed blueshift at larger impact parameters with the preferential excitation of higher-order CBPs. The size-dependent dispersion of the CBPs further enhances this impact-parameter-dependent blueshift and also explains the decrease in the impact parameter threshold for smaller nanoparticles. Compared with the local response approximation, the hydrodynamic model captures these trends without ad hoc momentum cutoffs and with rapid multipolar convergence. Applied to sodium nanoparticles as a canonical free-electron system, the theory reproduces qualitative behaviors observed in experiments and ab initio calculations, providing practical guidance for the design and interpretation of CBP-sensitive electron energy loss spectroscopy measurements.

Circular dichroism in the angular distribution (CDAD) is the effect that the angular intensity distribution of photoemitted electrons depends on the handedness of the incident circularly polarized light. The origin of CDAD can be manifold, including intrinsic properties of the system under study, such as chirality, spin-orbit interaction, or quantum-geometrical properties, but CDAD can also originate from final-state effects influenced by the experimental geometry. For example, CDAD has been reported for achiral organic molecules at the interface to metallic substrates. For this latter case, we investigate two prototypical $\pi$-conjugated molecules, namely, tetracene and pentacene, whose frontier orbitals have a similar shape but exhibit distinctly different symmetries. By comparing experimental CDAD momentum maps with simulations within time-dependent density functional theory, we show how the final state of the photoelectron must be regarded as the source of the CDAD in such otherwise achiral and quantum-geometrically trivial systems. We gain additional insight into the mechanism by employing a simple scattering model for the final state, which allows us to decompose the CDAD signal into partial wave contributions.

Open quantum systems far from thermal equilibrium can exhibit remarkable physical phenomena including topological properties without a direct equilibrium counterpart. Along these lines, in periodically driven-dissipative systems within the effective non-Hermitian (NH) Hamiltonian approximation spectral winding numbers have been linked to intriguing nonreciprocal transport properties. Here, going beyond an NH Hamiltonian description, we introduce and study a microscopic lattice model of a driven open quantum system described by a Markovian quantum master equation, which exhibits the mentioned spectral winding within an NH approximation. By encompassing quantum jump processes in the topological analysis, we uncover a distinct jump-induced topological phase, which qualitatively corresponds to the richer nonreciprocal transport properties of the fully quantum model. In addition, we find that the NH skin effect, i.e., the accumulation of a macroscopic number of eigenstates at one end of the system, is already visible in the transient dynamics even for systems with periodic boundary conditions. Our results exemplify the subtle correspondence between NH topological properties and physical manifestations of Liouvillian topological properties in open quantum systems, thus providing a theoretical framework toward understanding unidirectional transport in quantum dissipative Floquet dynamics.

The excitonic insulator candidate ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$ has emerged as a promising platform to explore macroscopic quantum condensation and its collective excitations. Using polarization-resolved Raman spectroscopy, we identify a symmetry-selective electronic mode that softens and undergoes linewidth renormalization across the characteristic temperature $T^{*}\sim 100\mathrm{K}$. This mode, most prominent in the $B_{3g}$ channel, is assigned to vestigial collective excitations associated with excitonic correlations, as evidenced by its order-parameter-like temperature evolution and symmetry dependence. In parallel, a damped Drude-like response exhibits a suppressed scattering rate and maximum intensity at $T^{*}$, while phonon intensities display symmetry-dependent anomalies, signaling anisotropic band reconstructions and the spontaneous opening of a collective condensation gap. These observations provide compelling spectroscopic evidence for collective amplitude excitations in ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$, establishing this material as a bulk excitonic insulator.

We show that the standard textbook description of (restricted) Hartree-Fock theory for (Fermi) Hubbard-like models is in need of an update, and we present such an update allowing us to correct basic and established results in the condensed matter physics literature that are qualitatively wrong. Our update amounts to adding a test that reliably checks the thermodynamic stability of solutions of Hartree-Fock equations. This stability test makes it possible to detect, by simple means and with certainty, regions in phase space where the model exhibits mixed phases where two conventional phases coexist and translation invariance is broken in complicated ways; in such a mixed phase, unconventional physics is to be expected. Our results show that mixed phases are ubiquitous in Hubbard-like models in arbitrary dimensions.